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Active control of run-up noise from propeller aircraft Germain, Pierre 2000

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ACTIVE CONTROL OF RUN-UP NOISE F R O M PROPELLER AIRCRAFT By PIERRE G E R M A I N B.A.Sc. (Civil Engineering), The University of Ottawa, 1993 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF T H E REQUIREMENTS FOR T H E D E G R E E OF MASTER OF APPLIED SCIENCE in T H E FACULTY OF G R A D U A T E STUDIES (Department of Mechanical Engineering) We accept this thesis as conforming to the required standard T H E UNIVERSITY OF BRITISH COLUMBIA May 2000 © Pierre Germain, 2000 In p resent ing this thesis in partial fu l f i lment of the requ i rements for an advanced degree at the Univers i ty of Brit ish C o l u m b i a , I agree that the Library shall make it freely available fo r re ference and study. I further agree that pe rm i s s i on fo r extens ive c o p y i n g of this thes is fo r scho lar l y p u r p o s e s may b e granted by the h e a d of my depa r tmen t or by his o r her representat ives. It is u n d e r s t o o d that c o p y i n g or pub l i ca t i on of this thesis for f inancial gain shall no t be a l l owed w i t hou t my wr i t ten pe rm i s s i on . Depa r tmen t of The Univers i ty of British C o l u m b i a Vancouve r , C a n a d a Da te Zj^/ZlOOO DE-6 (2/88) ABSTRACT Engine run-ups are part of the regular maintenance schedule at Vancouver Airport. The noise generated by the run-ups propagates into neighbouring communities, disturbing the residents. This research focuses on controlling run-up noise from propeller aircraft. It is well known that passive control measures - such as aircraft enclosures and acoustic barriers - can only attenuate sound at high and mid frequencies. Conversely, Active Noise Control ( A N C ) a new technology involving using noise to reduce noise, is an inexpensive alternative to enclosures. Propeller aircraft generate tonal noise that is highly compatible with A N C . Tests on a Beechcraft 1900D found that the fundamental frequency of 112 H z , as wel l as the first three harmonics, generated high noise levels in a community 3 km away from the run-up site. The insertion loss of an existing blast fence at the run-up site was measured, and found only to be efficient above 200 Hz . Computer simulations for different arrangements of A N C systems aimed at reducing run-up noise in residential areas were performed. Large triangular zones of local attenuation o f 10 dB or more were obtained when 9 or more control channels were used. Increases of noise were predicted outside of these areas, but these were minimized as more control channels were employed. Using an A N C system in conjunction with a barrier, such as the existing blast fence, was shown to provide attenuation at all frequencies. A N C experiments were conducted in an anechoic chamber using 1 and 3 control channels. The fundamental and first harmonic of the Beechcraft 1900D noise were significantly attenuated with A N C . The experimental data correlated well with the theoretical predictions, validating the simulations. The results from both the computer simulations and experiments indicated the great potential of controlling run-up noise with multi-channel A N C systems. However, implementing a real A N C system at Vancouver Airport would require further evaluating the required size o f the quiet zone as well as selecting the number of control sources and the distance from the primary source to the control sources accordingly. TABLE OF CONTENTS Abstract i i Table of Contents i i i List of Tables vi List of Figures vii Acknowledgements xi CHAPTER 1: INTRODUCTION 1 1.1 Airport Noise 1 1.2 Aircraft Noise 3 1.3 Sound Sources 5 1.4 Outdoor Aircraft-Noise Propagation 6 1.5 Receivers 8 1.6 Conventional Run-Up Noise Control Measures 9 1.7 Active Noise Control 11 1.7.1 Active Noise Barriers 16 1.8 Thesis Outline 18 CHAPTER 2 : RUN-UP NOISE M E A S U R E M E N T A N D ANALYSIS .... 19 2.1 Run-Up Noise Measurements 19 2.2 Run-Up Noise Data Analysis 22 2.2.1 Noise Directivity Patterns 23 2.2.2 Fundamental Frequencies and Harmonics 27 2.2.3 Coherence Analysis 29 2.2.4 Existing Blast-Fence Insertion Loss 31 iii CHAPTER 3: ACTIVE NOISE CONTROL THEORY 33 3.1 Introduction 33 3.2 Single-Channel Local-Control Theory 34 3.2.1 Free-field condition 34 3.2.2 Half-space condition 36 3.3 Multiple Control-Source A N C Theory 39 3.4 Active Noise-Barrier Theory 42 3.4.1 Effect of Rig id Ground on Control Efficiency 44 3.5 Multi-pole Source Generation 45 CHAPTER 4: ACTIVE NOISE CONTROL COMPUTER SIMULATIONS 47 4.1 Introduction 47 4.2 A N C System Design 48 4.3 Single-Channel Local Control 49 4.3.1 Single-Channel Control Simulations 50 4.3.1.1 Effect of increasing primary-to-control-source distance (r p s) 50 4.3.1.2 Effect of increasing control-source-to-error-microphone distance (rse) 56 4.3.1.3 Effect of increasing control-source and error-microphone height. 60 4.4 Mult iple Control Sources 64 4.4.1 Multiple-Channel Local-Control Simulations 64 4.4.2 Effect of Primary-to-Control-Source Distance (r p s) on Quiet-Zone Size 67 4.4.3 Effect o f Number o f Control Channels on Quiet-Zone Size 68 4.4.4 Effect on Harmonics 69 4.5 Active Noise Barrier Simulations 77 4.6 Multi-pole Source A N C Simulations 80 CHAPTER 5: ACTIVE NOISE CONTROL EXPERIMENTS 86 5.1 A N C Experiment Setup 86 5.2 Pure-Tone Attenuation Measurements 89 5.2.1 100-Hz Tone Attenuation Measurements 90 5.2.2 400-Hz Tone Attenuation Measurements 93 iv 5.3 Run-Up Noise Attenuation Measurements 96 5.3.1 Single-Channel A N C Experiments 96 5.3.2 3-Channel A N C Experiments 97 5.4 Summary 103 CHAPTER 6: IMPLEMENTATION OF ACTIVE NOISE CONTROL A T THE V A N C O U V E R INTERNATIONAL AIRPORT 104 6.1 Introduction 104 6.2 Possible Location of A N C Systems 104 6.3 A N C Equipment 112 6.4 Implementation of A N C for Different Aircraft 114 CHAPTER 7: CONCLUSION 115 7.1 Accomplishments 115 7.2 Future Work 117 Bibliography 119 Appendix A : Spectra Measured at Position 1 122 Appendix B: Spectra Measured at Position 2 136 Appendix C: Spectra Measured at Position 3 150 Appendix D: Spectra Measured at Position 4 155 Appendix E: Blast Fence Insertion Loss 170 Appendix F: Current Sea Island Zoning and Geographic Coordinates 182 LIST OF TABLES Table 2.1. Comparison of theoretical and experimental Blade Passing Frequencies 28 Table 4.1. Optimal ranges for a 9-channel system, r p s = 20 m and r s e= 40 m 70 Table 4.2. Attenuations provided with an A N C systems optimized for £2 (r s s = 2.0 m). . . 76 Table 5.1 Optimum range of r s s for a 3-channel system with r p s=1.4 m, r s e = l . l m 101 vi LIST OF FIGURES Figure 1.1 Map o f Y V R and sources o f complaints due to run-up noise 2 Figure 1.2 Examples o f broadband noise (top) and tonal noise (bottom) 4 Figure 1.3 Bending o f sound rays during: a) temperature inversion or downwind condition; b) temperature lapse or upwind condition 7 Figure 1.4. Example o f wind and temperature variations with altitude 7 Figure 1.5. Parameters used for Maekawa's expression 9 Figure 1.6. Run-up pen for B1 - B Aircraft, designed for 60 d B A at 3000m 10 Figure 1.7 Typical Hush House 11 Figure 1.8. Young's Principle o f Interference 12 Figure 1.9. Basic Active Noise Control setup 13 Figure 1.10. Wavefront matching and quiet zone creation 14 Figure 1.11. Effect on size o f quiet zone depending on primary-control source distance and control-error mic source distance: a) primary near control source and control source near error mic; b) primary far from control source and control source near error mic; c) primary close to control source and control source far from error mic; d) primary far from control source and control source far from error mic. 15 Figure 1.12. Multiple-Input Multiple-Output A N C System 16 Figure 1.13. Active Noise Barrier Setup 17 Figure 2.1. Beechcraft 1900D twin-engined propeller aircraft 20 Figure 2.2. Noise measurement set-up 21 Figure 2.3. Map o f Y V R showing microphone positions and blast fence (aircraft denoted by triangle) 21 Figure 2.4. Existing blast fence at Vancouver International Airport.. . . . 22 Figure 2.5. Total sound pressure level directivity pattern o f the Beechcraft 1900D at Position 1 25 Figure 2.6. B P F sound pressure level directivity pattern o f the Beechcraft 1900D at Position 1 25 Figure 2.7. Total sound pressure level directivity pattern o f the Beechcraft 1900D at Position 2 ..26 Figure 2.8. B P F sound pressure level directivity pattern o f the Beechcraft 1900D at Position 2 26 Figure 2.9. Coherence analysis results for the 105° heading, for the 0-350 H z range 31 Figure 3.1. Single-channel local-control setup in free field 34 vii Figure 3.2. Point source over a reflective surface 37 Figure 3.3. Single-channel local-control system in half space 38 Figure 3.4. Multiple-channel A N C system 40 Figure 3.5. Active Noise Barrier 42 Figure 3.6. Active Noise Barrier parameters 43 Figure 3.7. Four propagation paths o f sound diffraction over the barrier on reflective ground.. 45 Figure 3.8. Directivity pattern o f a dipole sound source 46 Figure 3.9. Directivity pattern o f a quadrupole sound source ....46 Figure 4.1. Single-channel control with r s e= r p s = 5 m, for free-field (top) and half-space (bottom) conditions 52 Figure 4.2. Single-channel control with r s e = r p s = 10 m, for free-field (top) and half-space (bottom) conditions 53 Figure 4.3. Single-channel control with r s e= r p s = 20 m, for free-field (top) and half-space (bottom) conditions 54 Figure 4.4. Single-channel control with rse= r p s = 40 m, for free-field (top) and half-space (bottom) conditions 55 Figure 4.5 Definition o f quiet zone and quiet-zone angle 9 q z 56 Figure 4.6. Single-channel control with r p s = 20 m, r ^ 5 m for free-field (top) and half-space (bottom) conditions 57 Figure 4.7. Single-channel control with r p s = 20 m, r ^ 40 m for free-field (top) and half-space (bottom) conditions 58 Figure 4.8. Single-channel control with r p s = 20 m, r ^ 100 m for free-field (top) and half-space (bottom) conditions 59 Figure 4.9. Single-channel control with primary source at y= 0 m, control source at y=20 m and error microphone at y=60 m, showing sound level increase localized around control source, and maximum attenuation around the error microphone 60 Figure 4.10. Single-channel control with r p s = 20 m, r ^ 40 m, hp= 2 m, hc=lv= 0 m for free-field (top) and half-space (bottom) conditions 61 Figure 4.11. Alignment o f an A N C system. 62 Figure 4.12. Single-channel control with r p s = 20 m, r ^ 40 m, hp=2 m, hc= .4 m and he= 8 m, for free-field (top) and half-space (bottom) conditions 63 Figure 4.13. 3^channel control with r p s=20 m, r s e = 40 m, h p= h<f= he= 2 m and r s s values close to rss-max = 9.17 m (top) and r^-mi,, = 6.17 m (bottom) for free-field conditions .65 Figure 4.14. 9-channel control with r p s=20 m, r s e = 40 m, h ^ hc= he= 2 m and r ^ 2.9 m. .66 Figure 4.15. 21-channel control with r p s=20 m, r ^ 40 m, hp= hc= he= 2 m and r s s= 2.6 m. 66 viii Figure 4.16. 61-channel control with r p s=20 m, r ^ 40 m, hp= hc= he= 2 m and r s s= 1.7 m. 67 Figure 4.17. Quiet-zone angle vs. r p s = r s e 68 Figure 4.18. Quiet-zone angle vs. number o f control sources 69 Figure 4.19. 9-channel attenuation o f f o with r , ^ 20 m, r s e= 40 m and r s s = 2.9 m 70 Figure 4.20. 9-channel attenuation o f f i with rps= 20 m, r ^ 40 m and r s s = 2.9 m 71 Figure 4.21. 9-channel attenuation o f fi with rps= 20 m, r ^ 40 m and r^ = 2.9 m. 71 Figure 4.22. 9-channel attenuation o f f3 with r p s= 20 m, r ^ 40 m and r^ = 2.9 m. 72 Figure 4.23. 9-channel attenuation o f fi with rps= 20 m, r ^ 40 m and r s s = 2.9 m 72 Figure 4.24. 9-channel attenuation o f fo with rps= 20 m, r s e= 40 m and r s s = 2.0 m. 73 Figure 4.25. 9-channel attenuation o f f i with r ^ 20 m, r ^ 40 m and r^ = 2.0 m. 74 Figure 4.26. 9-channel attenuation o f fi with rps= 20 m, r ^ 40 m and r^ = 2.0 m. 74 Figure 4.27. 9-channel attenuation o f f3 with rps= 20 m, r ^ 40 m and r^ = 2.0 m. 75 Figure 4.28. 9-channel attenuation o f fj with r ^ 20 m, r s e= 40 m and r s s = 2.0 m 75 Figure 4.29. A L P from a 9-channel system using r^ = 2.0 m and a L P F at 500 Hz , for l^H 1900D HH free-field 1 1 half-space 77 Figure 4.30. Existing Blast Fence at Vancouver International Airport 78 Figure 4.31. 3-channel A N B with ^ = 2 0 m, ^=40 m and ^=6.2 m. 79 Figure 4.32. 9-channel A N B with r p s=20 m, ^=40 m and r s s=2.9 m. 79 Figure 4.33. 21-channel A N B with r p s=20 m, ^=40 rss=2.6 m. 80 Figure 4.34. Sound radiation patterns: a) two monopoles in phase (left) and out-of phase(right); b) two dipoles in phase(left) and out-of phase(right); and c) two quadrupoles in phase(left) and out-of phase(right) 82 Figure 4.35. 9-channel attenuation o f two monopoles in phase (top), and out-of-phase (bottom) 83 Figure 4.36. 9-channel attenuation o f two dipoles in phase (top), and out-of-phase (bottom) 84 Figure 4.37. 9-channel attenuation o f two quadrupoles in phase (top), and out-of-phase (bottom) 85 Figure 5.1. 3-channel A N C setup in anechoic chamber 88 Figure 5.2. 3-channel A N C setup in anechoic chamber. From back to front: primary source, control sources, error microphones and receiver microphone 88 Figure 5.3. Horizontal (top) and vertical (bottom) grids set up for receiver measurements. 89 Figure 5.4. Single-channel A N C results at 100 H z for horizontal plane (top) and vertical plane (bottom): (a) experimental (b) theoretical (white areas around the primary source denote high levels beyond the colourbar scale) 91 ix Figure 5.5. 3-channel A N C results at 100 H z for horizontal plane (top) and vertical plane (bottom): (a) experimental, (b) theoretical (white areas around the primary source denote high levels beyond the colourbar scale) 92 Figure 5.6. Single-channel A N C results at 400 H z for horizontal plane (top) and vertical plane (bottom): (a) experimental, (b) theoretical (white areas around the primary source denote high levels beyond the colourbar scale) 94 Figure 5.7. 3-channel A N C results at 400 H z for horizontal plane (top) and vertical plane (bottom): (a) experimental, (b) theoretical (white areas around the primary source denote high levels beyond the colourbar scale) 95 Figure 5.8. Spectra from single-channel A N C experiment, 0.15 m behind error microphone before control, after control 97 Figure 5.9. A N C results at fo for horizontal plane (top) and vertical plane (bottom): (a) single-channel, (b) 3-channel 98 Figure 5.10. A N C results at fi for horizontal plane (top) and vertical plane (bottom): (a) single-channel, (b) 3-channel 99 Figure 5.11. A N C results at for horizontal plane (top) and vertical plane (bottom): (a) single-channel, (b) 3-channel 100 Figure 5.12. Spectra from 3-channel A N C experiment, 0.15 m behind error microphone before control, after control 102 Figure 5.13. Spectra from 3-channel A N C experiment, 1.15 m behind error microphone before control, after control 102 Figure 6.1. Run-up locations at the Vancouver International Airport 106 Figure 6.2. Implementation of a 61-channel A N C system at Button 08. Expected change in noise level: • <-10dB,CZI -10to0dB,L~Z] 0 t o + 4 d B 107 Figure 6.3. Implementation of a 35-channel A N C system at Button 08. Expected change in noise level: E Z I < - 1 0 d B , C Z ] - 1 0 t o 0 d B , C Z ] - 1 0 t o + 6 d B 109 Figure 6.4. Estimate o f 61-channel r p s vs. Gqz curve 110 Figure 6.5. Implementation o f a 35-channel A N C system at the A i r B C hangar. Expected change in noise level: CZ1 <-10dB, CZ1 - l O t o O d B , ! I - 1 0 t o + 4 d B . I l l ACKNOWLEDGEMENTS First and foremost, I would like to thank my thesis supervisor, Dr. Murray Hodgson, for his ongoing support, assistance and encouragement throughout my research. His expertise of acoustics is invaluable, and his teachings have provided me with knowledge of acoustics beyond my expectations. I would also like to thank Dr. Jingnan Guo, for contributing his expertise of A N C to this research, and helping me with the A N C theory, simulations, and experiments. I would like to express a word of gratitude to the Vancouver Airport Authority ( V A A ) for their contribution to the funding of this research, and notably Mark Cheng, for his collaboration throughout the project, including his organization of and help with the run-up noise measurements. I must also thank Edward Haboly and Natsumi Valerga o f the V A A , as wel l as Doug Kennedy of B K L Consultants for their co-ordination of and help with the run-up noise measurements. Thanks also to Central Mountain A i r for providing the aircraft for the run-up noise measurements. Lastly, but not least, I would like to thank my parents Jean-Pierre and Suzanne Germain for their ongoing love, encouragement and support throughout this, and all other endeavours I have undertaken. XI - CHAPTER 1 -INTRODUCTION 1.1 A i r p o r t Noise Noise generated by airport operations is a common source o f nuisance for nearby residents. The main sources of complaints from residents l iving near airports are aircraft takeoffs, landings, fly-overs and engine run-ups. The first three activities involve aircraft in flight, whereas engine run-ups involve stationary aircraft. Apart from directing flight paths away from communities as much as possible, and limiting the number o f night-time flights, there is little that airport management can do in terms of reducing in-flight noise. The mitigation of in-flight aircraft noise must be addressed at the source; that is by the aircraft engine designer. On the other hand, engine run-up noise is an issue for which airports can investigate noise-control measures, since the aircraft is stationary. Engine run-ups are a normal part of aircraft maintenance. They involve revving the engines at different power settings (idle, above idle, full power) in order to confirm proper operation after maintenance. Transport Canada has defined standards requiring that each aircraft be tested before being put back into service [1]. Since these aircraft are in use during the day, for the most part, run-ups must be performed mostly at night, generating complaints from nearby communities. 1 A l l major airports around the world have to deal with aircraft run-up noise complaints. The Vancouver International Airport ( Y V R ) , one of Canada's largest, is no exception, due to its proximity to several communities. A map of the airport and surrounding areas, as well as sources of complaints due to run-up noise are shown in Figure 1.1. These are mainly from South Vancouver and north Richmond. Aircraft noise issues are addressed by the Vancouver International Airport Authority ( V A A ) , which publishes annual reports of their noise management summaries Figure 1.1 Map of YVR and sources of complaints due to run-up noise. 2 and strategies. Mitigating engine run-up noise is among these strategies, which include scheduling run-ups during the daytime as much as possible and orientating the aircraft during run-ups so that the strongest directionality of the noise is directed towards the Pacific Ocean to the west. The V A A ' s duties also involve the investigation of conventional noise-control technologies, as wel l as the latest developments in noise control, for application to run-up noise. One of the newest technologies is active noise control ( A N C ) , which has become one of the most widely researched topics in acoustics since the 1980's. In 1998, the V A A approached the University of British Columbia to investigate the feasibility of using A N C to further manage run-up noise. This thesis is the result of that research, which was funded by the Natural Science and Engineering Research Counci l ( N S E R C ) , and the V A A . A literature review was performed in order to acquire knowledge on aircraft noise, its propagation into the environment, conventional run-up noise-control methods and, finally, A N C . Several papers were studied in order to examine the feasibility o f using A N C techniques to control run-up noise. 1.2 Ai rc ra f t Noise Different types of aircraft engines radiate different noise characteristics [2]. The two main types are jet and propeller engines. Jet aircraft produce broadband noise, which is characterized by a range of frequencies having equal, or near-equal, sound levels. This type of noise can be compared to the sound of a waterfall, or what is commonly referred to as 'static' from a television set or radio. Propeller noise is characterized as tonal noise, which means that sharp peaks dominate the frequency spectrum. These peaks consist of a fundamental frequency, and integer multiples of the fundamental, called 'harmonics'. For propeller noise, the fundamental frequency is also called the 'blade-passing frequency' (BPF). For an exposed propeller, this can be calculated as: B P F = N x V H z (1.1) 60 3 where: N is the number of rotating blades, V is the speed of rotation (in R P M ) . This type of noise can be compared to the sound of a lawnmower or chainsaw; it is a more 'piercing' noise that tends to be more annoying than broadband noise. Examples of broadband and tonal noise spectra are shown in Figure 1.2. Run-up noise at the Vancouver Airport was the subject of a study in 1992 [3]. Among the findings of this report was that propeller aircraft - more specifically the D H C -& a> co <D o (O O lO c j c M c o c o c o ^ - ^ - m i f l c o t O f - i ^ i ^ o o c o c n a J o o o Frequency (Hz) 105 100 95 90 ~ 85 oo 80 75 70 65 O C O U ) * - i f i O > C O C O O c x t s : c o r ^ » - ( D O i o _ „ _ . _ . . . Frequency (Hz) Figure 1.2 Examples of broadband noise (top) and tonal noise (bottom) spectra. 8 ' s - were at the top of the list o f noise complaints by aircraft type. This trend has continued over the years, especially since jet-engine design has advanced much more rapidly in terms of minimizing noise generation than has propeller engine design. Jet engines have the benefit o f being enclosed; noise can be attenuated using sound-absorbing materials or, more recently, using active noise control [4] . On the other hand, propeller aircraft have exposed propellers, for which the challenge of minimizing noise generation is imposed solely on blade design. It is therefore easy to understand why propeller-aircraft noise is of greater concern. This thesis focuses on the feasibility of using active noise control for propeller aircraft, although general conclusions w i l l be made in regards to controlling run-up noise from all types of aircraft. 1.3 Sound Sources The most basic representation of a sound source is that of a monopole point source, which radiates spherical wavefronts. This assumes that all points on the sphere are in phase; therefore the radiation is essentially the same in all directions (omnidirectional). Point sources, along with other types of sources such as line or plane sources, have simple directivity patterns. In reality, sound sources radiate in more irregular patterns. In Chapter 2 we w i l l explain how propeller aircraft noise can be represented as monopole sources (same directivity in all directions), dipole sources (strong directivity in two opposite directions) or quadrupole sources (strong directivity in four opposing directions). Directivity of a source w i l l play a key role when we discuss outdoor aircraft noise propagation, which is presented in the next section. In directions in which the source radiation is particularly high, noise levels w i l l be particularly high at any given distance, and levels w i l l remain above prevailing background noise levels at greater distances. 5 1.4 Outdoor Aircraft-Noise Propagation Outdoor sound propagation is a widely researched topic in acoustics. The inverse-square law, also known as spherical divergence, describes how the sound pressure level decreases as the distance from the sound source increases. However, there are many other factors that influence the propagation of sound outdoors, such as topography, ground impedance and meteorological conditions [5,6]. The effect of ground impedance is dependent on frequency [7]. A t very low frequencies (<100 Hz) , the ground can be considered as a nearly perfect reflector. Therefore, whereas high frequencies of sound propagating over a grassy field may be strongly absorbed, lower frequencies are little affected. This is the case at Vancouver Airport, which has relatively flat grassy fields around its perimeter. Atmospheric absorption, which is a function of air temperature and relative humidity, can produce significant attenuations of high-frequency sounds even at small distances [8]. The effects on low frequencies are significantly less, and attenuations are only noticeable at large distances from the source. This phenomenon is also called molecular absorption, and it depends on the concentration of water vapour in the air. The phenomenon of atmospheric absorption becomes more complex once it is combined with temperature and wind effects. Temperature and wind variations with altitude produce refraction phenomena, which affect the sound levels and spectra at,large distances. When temperatures near the ground surface are greater than those at higher elevations, such as may be the case in the daytime, a temperature lapse condition occurs. Since sound travels faster in warmer air than in cooler air, the effect of a temperature lapse is an upward bending of the sound rays. This phenomenon also occurs when sound travels in an upwind direction. A shadow region is created at some distance away from the source; sound levels can be 10 to 15 dB lower than the levels measured in the absence of a temperature lapse or upwind condition [6] (see Figure 1.3(b)). The opposite effect occurs in cases where the temperature o f the air increases with elevation (termed a "temperature inversion") or when the sound travels in a downwind direction. The sound rays refract downwards in these cases, focusing sound into areas far away from the source which would have been unaffected otherwise 6 (see Figure 1.3(a)). A temperature inversion results in less attenuation at long distances at night (after sunset) than during the day. Figure 1.4 shows an example o f how temperature and wind gradients vary with elevation. Source (a) Sound speed increasing with altitude. Shadow near ground (b) Sound speed decreasing with altitude. Figure 1.3. Bending of sound rays during: a) temperature inversion or downwind condition; b) temperature lapse or upwind condition 96 72 a: sz CTl 48 'a> X 24 12 ? 06:45 pm (10/24/85) 0 1 2 3 4 5 670 90 110 12 13 14 15 16 17 Windspeed, Wind Direction, Temperature, V (m/s) * (deg) T (° C) Figure 1.4. Example of wind and temperature variations with altitude 1 With respect to run-up noise at airports, temperature and wind effects play a major role in directing noise into the nearby communities. For instance, in the case o f a temperature inversion or downwind condition (Figure 1.3(a)), run-up noise may not be audible at a small distance from the airport, but may be at a larger distance. Turbulent winds also modify sound propagation, but in a more complex manner. The vortices and eddies created by turbulence scatter the sound energy, causing sound levels to fluctuate, in some cases by as much as 10 dB. Turbulence effects rarely increase levels; however they can re-direct sound into shadow zones in the case of a temperature lapse, reducing the attenuation [9]. Despite their significance, in this thesis the meteorological conditions are assumed neutral, wind and gradient effects are not considered. 1.5 Receivers A receiver can be something (a microphone) or someone (a human ear) at a distance from the source that 'captures' or 'hears' the sound pressure generated by the source. The sound pressure at a distance r from the source is calculated by the inverse-square law, also known as spherical divergence, and is represented by the expression: *< r > = T - T (1-2) 4nr where p is the sound pressure, po is the density of air, c is the speed of sound, and W is the sound power of the point source. The sound-pressure level is represented by the decibel (dB), and is calculated using the expression: I , = 1 0 1 o g (1.3) where p0 is a reference sound pressure of 2 x 10"5 Pa. When discussing sound levels heard by humans, the sound-pressure level is often discussed in terms of the 'A-weighted' decibel (dBA). A-weighting is a correction which accounts for the non-linearity of human hearing, and it is used in noise regulations at all levels of government. Since this research deals with community annoyance issues, sound pressure levels w i l l be discussed in units of d B A for the most part. 1.6 Conventional Run-Up Noise Control Measures Traditional methods of attenuating noise consist of passive methods, which include sound-absorbing materials and barriers. Both of these methods are very efficient at attenuating mid and high frequencies, but not so efficient at low frequencies. The lowest range of frequencies that can be attenuated by an absorber is highly dependent on the thickness of the material - the thicker the material, the lower the frequency it can absorb. This means that, in order to attenuate very low frequencies, a large thickness o f material is required, resulting in excessively high construction costs. Barriers attenuate sound by blocking the transmission path from the source to the receiver. They are not effective at low frequencies, however, since the longer wavelengths are transmitted through the barrier and result in less diffraction of sound. In practice barriers, such as roadside barriers, have been used to shield noise from residential areas. The attenuation provided by an acoustic barrier is quantified by the insertion loss (IL). The most commonly-used method for estimating the insertion loss o f a barrier is Maekawa's asymptotic expression [10] where 8 = r+ro - R is the path-length difference (as shown in Figure 1.5) and X is the wavelength of the sound. It can be seen from this expression that the insertion loss o f a barrier is inversely related to the wavelength of the sound; the higher the frequency of the sound, the larger the insertion loss of the barrier. JZ, = 13+ 10 log W, 7V> 1 where N is Fresnel's zone number for the barrier, expressed as (1.4) (1.5) Barrier Receiver Figure 1.5. Parameters used for Maekawa 's expression 9 A t airports, attempts at reducing run-up noise with passive measures have come in the form of structures, namely run-up pens and hush houses. Run-up pens have been used in situations where maximum noise reductions of the order of 12 or 18 d B A are required in the far field, depending on aircraft engine mounting and pen design [11]. Run-up pens generally consist of acoustically designed walls that surround the aircraft on three sides, without roofs but with additional panels situated on top of the walls and angled upwards. Typically, a run-up pen is designed to attenuate noise emitted from a particular type o f aircraft. Such structures have been built for North American Rockwell in Palmdale, California for the B - 1 B aircraft (see Figure 1.6) and the Deutsche Aerospace Airbus in Hamburg, Germany. British Airways at Heathrow Airport in London, England acquired a run-up pen that could accommodate the Boeing 747 as well as all Airbus Industry's aircraft. In 1997, Chicago O'Hare airport also acquired a run-up pen, that w i l l accommodate up to B747-400 sized aircraft. When higher noise reductions of the order of 20 or even 40 dB are required, a complete Hush-House must be built. This is a completely enclosed structure, with a long corridor called an augmenter tube in which the engine exhaust is directed away from the facility and into the atmosphere via a deflector ramp (see Figure 1.7). The United States A i r Force ( U S A F ) has used hush-houses in an effort to reduce the impact on the NOISHIEU) P A W L S \ r*3BhlBUD* ARSOflBtMS KASTOERjeCTOR PANEL2 SDH WAUL ftWHS H'B AIRCRAFT Figure 1.6. Run-up pen for Bl-B Aircraft, designed for 60 dBA at 3000m 10 EXHAUST DEFLECTOR Figure 1.7 Typical Hush House. community of the noise associated with high-power testing of military aircraft and uninstalled jet engines [12]. The need for noise control was also triggered by the fact that U S A F personnel were found to be suffering hearing loss. These structures have been successful in meeting the noise specifications of 80 d B A at 250 feet radially from the exhaust of the engine, as well as at 250 feet radially from the exit o f the augmenter tube. Although properly designed run-up pens and hush-houses are successful at reducing run-up noise in neighbouring areas, the costs associated with building such structures can be of the order of $20 to $30 mill ion. Furthermore, the effects of low-frequency noise diffracting over and around the pens, possibly causing annoyance in nearby communities, is often ignored. 1.7 Act ive Noise Con t ro l Although passive control methods are effective for certain applications, they can only effectively control sound in the mid to high frequency range. The wavelength of low-frequency sound is longer; therefore, passive control treatments need to be excessively bulky and heavy in order to be effective. A s A N C is effective at controlling 11 low-frequency sound, it is currently being studied extensively and is considered to be a low-cost alternative to passive control methods. In the field of acoustics the term "active" implies that electronics are involved. For A N C systems, sensors (microphones), controllers (signal sources) and speakers are used to detect, quantify, recreate and cancel a particular sound source. H o w does A N C work? According to Young's principle of destructive interference a sound wave with a given amplitude and phase can be cancelled by superimposing another sound wave of the same frequency, with the same amplitude but opposite phase (meaning that their phases differ by 180°)[13]. This is illustrated in Figure 1.8. Although Lord Rayleigh first introduced the concept of A N C in 1896 [14], it was not put to use until 1936 when Paul Lueg first patented a system to cancel plane waves in a duct [15]. A N C has been intensively studied for different applications only in the past 20 years with the fast development of computer technology, from headphones for aircraft pilots to transformer and lawnmower noise. The basic A N C setup is shown in Figure 1.9. The primary source to be controlled is measured by a detection sensor (microphone), which sends a reference signal to the controller. The controller reads this signal then reproduces it, with a 180° shift in phase, and sends it out via a control source (loudspeaker). In some cases, more than one control source can be used to optimize the noise reduction obtained with the A N C system. A n error sensor is placed in the area where sound reduction is required. This sensor is also called a monitor sensor since it monitors the performance o f the controller and provides Figure 1.8. Young's Principle of Interference 12 x x detection sensor \ \ i primary noise] ) 1 control source error sensor V1?, error signal Figure 1.9. Basic Active Noise Control setup continuous feedback o f information to the controller. This allows for the correction o f any fluctuations in the sound field, which the controller cannot account for, such as those due to changes in temperature and humidity. In a sense, it picks up the "errors" o f the controller and relays this information back to the controller, which then corrects itself. There are two types o f setups used for A N C systems [16,17]. The first system, shown in Figure 1.9, is called feedforward A N C . The controller reproduces the reference signal with a 180° phase shift, and sends it to the control source. This results in a cancellation o f the primary source as detected by the detection sensor. The second system is called feedback A N C . In a feedback system, a detection sensor is not used. The controller reproduces the signal from the error sensor with a 180° phase shift, and sends it to the control source to cancel out the total sound field arriving at the error sensor. A hybrid feedforward/feedback system may also be used, for particular applications [18]. When an A N C system is used, it creates a "zone o f quiet"[17]. The size o f this zone varies with the distance between the primary source and the detection sensor, the detection sensor and the control source, and between the detection sensor and the error microphone. Locating the primary source and the control source i n close proximity to each other can result i n what is called global control, which results in an attenuation o f the total sound power o f the original sound source. A substantial reduction of the total sound-power output can only be achieved i f the control source(s) are separated from the primary source by a distance which is less than one quarter of the wavelength at the frequency o f interest [13]. 13 When such an arrangement is not possible, or when noise reduction is only required in a specified area, a local control strategy is used. Such a control strategy produces conical areas o f noise reduction, with the area o f greatest reduction located around the error sensor. The size o f the quiet zone is dependent on the relative positions o f the primary source, the secondary source and the error microphone. The wavefronts o f the primary and secondary sources must be equal in amplitude and opposite in phase at the locations around the error microphone for cancellation to occur. This is called 'wavefront matching' (see Figure 1.10). The size and shape of the conical quiet zone depend on the distance between the primary and secondary sources (see Figure 1.11). A n attempt at using global control when dealing with noise emanating from aircraft w i l l be difficult, since the control speakers would have to be located very close to the aircraft engines in order to achieve the desired control. Since this may present safety problems, a local control strategy would likely be the preferred option. Figure 1.10. Wavefront matching and quiet zone creation 14 a) zone efmaxumtmreductum secondary source /^^j / n*tT°Pk0*€ primary source quiet zone b) zone of maximum reduction \ error microphone secondary source \ ((g)) \ cjliT \ quietzone primary source , zone of maximum reduction / secondary source \ /error mtcropaone f^A Kami,,, ((§))< viiiiii1 primary source quietzone ""^ X^  d) zan« A/- maximum refaction \ error itacropHooe secondary source \ / \ quietzone prunary source Figure 1.11. Effect on size of quiet zone depending on primary-control source distance and control-error mic source distance: a) primary near control source and control source near error mic; b) primary far from control source and control source near error mic; c) primary close to control source and control source far from error mic; d) primary far from control source and control source far from error mic. When evaluating an A N C system, one should start by examining the efficiency of a single-input, single-output (SISO) system, such as the one presented in Figure 1.9, to see i f it would be sufficient for the task. For cases where it would not be sufficient in achieving the desired control, a multiple-input, multiple-output (MTMO) system can be implemented. In a M T M O system, several control sources and error microphones are used, spaced equally in a line (see Figure 1.12) or in a plane. This arrangement wi l l result in a larger quiet zone than would a SISO system. 15 Figure 1.12. Multiple-Input Multiple-Output ANC System As mentioned before, the presence o f a hard (reflective) surface below the aircraft plays a role in the propagation of aircraft noise. This w i l l also affect an A N C system, as a reflective surface may reduce the system's effectiveness [13,19]. Guo[20] found that the effect of the presence o f a nearby reflective surface is to significantly reduce the size of the quiet zone. This w i l l have to be taken into consideration when assessing A N C for run-up noise. 1.7.1 Active Noise Barriers A N C has also been used in conjunction with barriers [20,21], with the error microphones placed on top o f the barrier and the control sources positioned between the primary source and the barrier. The A N C system attenuates sound that would normally diffract over the top o f the barrier. The philosophy behind such a setup, as was explained in a previous section, is that while a barrier's insertion loss is very effective at controlling mid and high frequencies, it is less effective at controlling low frequencies. The active system on top o f the barrier may solve this problem, by attenuating the low frequencies that tend to diffract over the top of the barrier. The optimal arrangements of the positions o f the control sources and error microphones are the same as when they are arranged in a line for an A N C system without the barrier. The optimal spacing between adjacent control sources (and thus error 16 microphones) is dependent on the wavelength of the frequency to be attenuated [16] (see Figure 1.13). When the spacings of the control sources and error microphones are within the optimal range, the extra sound attenuation due to the control system can be more than 10 dB in a large area behind the barrier [20,21]. It has been shown that the effect o f a reflective surface on either side o f the barrier can dramatically alter these results, since the reflections from the ground interfere with the sound field, hindering the A N C process. It has been found that the efficiency of the control system can be significantly increased i f a non-reflective treatment is applied to the ground on the source side o f the barrier [21]. Researchers have attempted to use A N C to control noise from different sources, including aircraft and automobile cabins, headsets, heating and ventilation ducts, lawnmowers, leaf blowers/vacuum, and transformers. Although al l o f these experiments were quite successful and, in the case o f the A N C headset, have become commercialized, al l o f these deal with situations where global control can be achieved; little research has been conducted with respect to local control, underlining the importance of the research presented in this thesis. 17 1.8 Thesis Outl ine In Chapter 2, we w i l l discuss run-up noise measurements on a Beechcraft 1900D propeller aircraft, performed at the Vancouver Airport in the summer of 1999. A n analysis of the resulting spectra w i l l be presented, as well as a coherence analysis o f the run-up noise measured at two different locations. A t the end of the chapter, measurements of the insertion loss of an existing blast fence at the Vancouver Airport w i l l be presented. In Chapter 3, we w i l l present A N C theory, developing expressions that were used for the simulations in Chapter 4. Expressions for free-field and half-space A N C systems, single- and multiple-control source systems,, and active noise barriers w i l l be presented. We w i l l also present how to model dipole and quadrupole sources. Chapter 4 w i l l present the results of computer simulations performed using M A T L A B 5.2. Simulations for different arrangements of single- and multiple-control source systems w i l l be shown for both free-field and half-space assumptions, using a single monopole source. This w i l l be followed by multi-pole source simulations, using source radiation patterns that better resemble those from actual propellers. The last part of the chapter w i l l present active noise barrier simulations. Chapter 5 w i l l present the results of A N C experiments conducted in an anechoic chamber, simulating free-field conditions. Experiments using 100-Hz and 400-Hz pure tones, as wel l as the run-up noise recorded at the Vancouver Airport, w i l l be presented. These results w i l l be compared with computer simulations of the same set-up, to attempt at validating the theory. Chapter 6 w i l l discuss the feasibility of using an actual A N C system at the Vancouver Airport. Possible locations of A N C systems, as well as the expected performance and attenuation results, w i l l be presented. Conclusions of this research, summarizing the results from the previous chapters, and recommendations for future work w i l l be discussed in Chapter 7. 18 - CHAPTER 2 -RUN-UP NOISE MEASUREMENT AND ANALYSIS 2.1 R u n - U p Noise Measurements Measurements of aircraft run-up noise were carried out in July 1999 in order to study propeller-aircraft noise characteristics. A Beech-1900D twin-engined turboprop aircraft (see Figure 2.1) was provided by Central Mountain A i r for the noise measurements. The aircraft performed engine run-ups at full power, during which both four-bladed propellers rotated at approximately 1700 rpm. Four microphone positions were used to measure the resulting sound pressure level (see Figure 2.2). Two Bruel & Kjaer 2230 free-field microphones, positioned at approximately 73 m (Position 1) and 98 m (Position 2) away from the aircraft captured the near-field run-up noise. A Bruel & Kjaer 4165 free-field microphone was positioned in a community north of the airport, approximately 3 km (Position 3) from the aircraft (see Figure 2.3). A fourth microphone (Bruel & Kjaer 2230) was positioned on the opposite side of the blast fence to the aircraft, again at a distance of 73 m (Position 4). The aircraft performed 12 full-power run-ups, each for a duration of one minute, rotating by 30° in between run-ups. Run-up noise signals, along with the ambient noise, idle-engine noise and calibration tones were recorded on portable Teac D A T recorders. The recordings were then analyzed in the laboratory using a Larson Davis 2800 Real-Time Analyzer. Spectra for all aircraft headings are presented in the appendices, with both 1/3-octave-band and narrow-band resolution. 19 In order to conduct a proper investigation of the compatibility of aircraft run-up noise with an A N C system, certain noise characteristics had to be studied. First, the noise levels generated by the run-ups were analyzed, in order to study the radiation directivity of the Beech- 1900D and the level attenuations with distance. Second, the fundamental frequencies and harmonics of the rotating-blade noise were studied. Third, the outdoor propagation of the aircraft noise was investigated. Fourth, a coherence analysis of the noise was performed using the data collected at Positions 1 and 2. Finally, the insertion loss of an existing blast fence (see Figure 2.4) located near the north runway of the airport was estimated, from the differences between the levels measured at Positions 1 and 4. Figure 2.1. Beechcraft 1900D twin-engined propeller aircraft 2 0 73m 25m Blast Fence Position 4 Figure 2.2. Noise measurement set-up. Figure 2.3. Map of YVR showing microphone positions and blast fence (aircraft denoted by triangle) 21 Figure 2.4. Existing blast fence at Vancouver International Airport 2.2 R u n - U p Noise Data Analysis In this section, the analysis of the fundamental frequencies, levels, and spectra of the run-up noise measured at the four receiver positions is discussed. The spectra were analyzed with both 1/3-octave-band and narrow-band (by F F T analysis) resolution. The 1/3-octave-band spectra are presented in the appendices in both unweighted linear and A -weighted decibels, while the narrow-band spectra are presented in A-weighted decibels only. The 1/3-octave-band spectra enabled a very wide range of frequencies to be observed. The range from 12.5 H z to 20 kHz, which covers the human-hearing range o f 20 H z to 20 kHz , was chosen for the purpose of this study. The F F T analysis provided more details of the sound field captured by the microphone, clearly displaying its fundamental frequencies and related harmonics. A 0 to 1250 H z range, which displays the fundamental, as well as approximately 10 harmonics, for each spectrum was chosen for this study. The noise spectra at microphone Positions 1, 2, 3 and 4 are presented in Appendices A , B , C and D , respectively. The headings refer to the direction of the nose 2 2 of the aircraft relative to geographic north. Appendices A and B contain the spectra for Positions 1 and 2, respectively, including 11 headings, the ambient noise and the idle-engine noise. The spectrum for the 345° heading was deemed unusable due to excessive noise generated by wind in the microphones. Appendix C contains the spectra captured by the microphone at Position 3 for the ambient noise and three headings; the spectra at all other headings were unusable due to excessive background noise which masked the aircraft noise. Appendix D contains the spectra for the microphone at Position 4, all 12 headings, as wel l as the ambient noise and the idle-engine noise. 2.2.1 Noise Directivity Patterns A n effective way to illustrate the radiation pattern of a particular noise source is to generate noise directivity patterns, which show the sound pressure levels at a constant radius from the center of the source. The sound pressure levels measured at Positions 1 and 2 enabled such directivity patterns to be generated. The total noise levels measured at Position 1, 73 m away from the aircraft, varied with aircraft headings between 100 and 109 d B A . These levels were wel l above the idle-engine noise of 83 d B A and the ambient noise level of 77 d B A . The microphone at Position 2 (98 m away) measured levels between 94 and 108 d B A , above the idle-engine and ambient noise levels of 79 d B A and 78 d B A , respectively. Using the levels measured at these two positions, noise directivity patterns were generated. The total noise and B P F contours measured at Position 1 are shown in Figures 2.4 and 2.5, respectively. The total noise and B P F directivity patterns measured at Position 2 are shown in Figures 2.6 and 2.7, respectively. In order to analyze these directivity patterns, it is of interest to try to understand the radiation pattern o f propellers, by studying the mechanisms that generate noise from a spinning propeller. These mechanisms include thickness noise, loading noise and quadrupole noise [2,26,27]. Thickness noise arises from the transverse periodic displacement of the air by the volume of a passing blade element. This mechanism creates linear noise characteristics, meaning that it is the mechanism responsible for creating harmonics at integer multiples 23 of the fundamental or blade-passing frequency (BPF). Thickness noise can be represented by a monopole source distribution and becomes important at high rotational speeds. Loading noise is a combination of thrust and torque (or lift and drag) components which result from the pressure field that surrounds each blade as a consequence o f its motion. This type of noise can be represented as a dipole and is an important mechanism at low to moderate speeds. Quadrupole noise arises from turbulent airflow over the blade sections, and can be used to account for all o f the viscous and propagation effects not represented by thickness and loading sources. This creates nonlinear noise characteristics, meaning that it w i l l generate tones which are not at integer multiples of the B P F . Quadrupole noise is the main component of aerodynamic noise arising from turbulent airflow. A propeller operating during a run-up encounters a great deal of nonuniform inflow, including naturally occurring turbulence from the atmosphere and ground vortices and wakes from various parts of the aircraft [2]. Since run-up propeller speeds vary from one aircraft to another, we cannot generalize as to which mechanism w i l l govern the generated run-up noise. B y observing Figures 2.5 and 2.8, we see that at Position 1 (73 m away) there is a strong directionality towards the right of the aircraft from the 105° to 165° headings, and towards the 45° and 255° headings in both cases. A t Position 2, the directionality towards the right of the aircraft is still prevalent, as is the directionality towards the 45° heading. A t Position 2 there is also a stronger directionality towards the 315° heading than at Position 1. None of these radiation patterns are symmetrical, which could be due to ground reflection, reflection from the blast fence, or the two propellers spinning out-of-phase with each other. A dipole from the left propeller appears to be present at Position 2, but not at Position 1. This could indicate that radiation patterns are sensitive to distance from the aircraft, which is probably due to the turbulent nature of the wind blast generated by the propellers during a run-up. A n inclined dipole appears to be present for the two B P F contours, with the null shifting from the 195° heading at Position 1 to the 225° heading at Position 2. The directivity appears to rotate as the receiver position moves further away from the aircraft. 24 75 75 105 Figure 2.8. BPF sound pressure level directivity pattern of the Beechcraft 1900D at Position 2. 26 From these results, it is difficult to pinpoint exactly which noise mechanism is governing the radiation patterns, since no clear monopole, dipole or quadrupole radiation patterns can be seen. It is important to note that these patterns are generally associated with in-flight noise, making the analysis more difficult when attempting to study run-up noise directivity. In addition, the turbulent airflow over the blades, and reflections from the ground and blast fence, w i l l distort the directivity, further increasing the complexity of the analysis. 2.2.2 Fundamental Frequencies and Harmonics A s was introduced in Chapter 1, the fundamental frequency of the noise generated by a rotating propeller is called the blade passing frequency (BPF), and it can be calculated from a simple formula (see Equation 1.1). During the measurements, the two four-bladed propellers of the Beech- 1900D rotated at a speed of approximately 1700 rpm. Using Equation (1.1), a B P F of 113.3 H z is obtained. The microphone at Position 1 was in the best position to study the B P F of the propeller noise, as it was the closest microphone to the aircraft and corresponded to an unshielded sound path. Table 2.1 displays the fundamental frequencies, observed from the 0-1250 H z F F T analysis of the noise captured by this microphone. We can see that the measured B P F ' s for most aircraft headings were within 1% of the theoretical value of 113.3 Hz . In fact, the spectra at all but two headings revealed fundamental frequencies o f 112.5 Hz . Observing the spectra in the Appendices, it can be seen that many harmonics appear as sharp peaks at multiples of the fundamental frequency of 112.5 H z - i.e. at 225, 337.5, 450 Hz , and so on. In Appendix A , strong harmonic components for Position 1 are seen, with the fundamental and first harmonic having lower levels due to the A -weighting. For the most part, the harmonics decay at a rate of 1-2 d B A per harmonic -approximately the same rate of decay as the idle-engine noise. 27 Aircraft Fundamental Deviation from Heading Frequency Theoretical BPF (degrees from north) (BPF) (Hz) (%) 75 112.5 -1 105 107.8 -5 135 107.8 -5 165 112.5 -1 195 112.5 -1 225 112.5 -1 255 112.5 -1 285 112.5 -1 315 112.5 -1 345 N / A N / A 15 112.5 -1 45 112.5 -1 Table 2.1. Comparison of theoretical and experimental blade passing frequencies Harmonics with levels as high as the fundamental B P F are a characteristic o f propellers operating under static conditions - i.e. when the aircraft is stationary as during run-ups. When the aircraft is in motion or in flight, the harmonics drop off very rapidly -by as much as 8 dB per harmonic. This results in lower total noise levels when the aircraft is in motion. The high levels of the harmonics during a run-up (static conditions) are created by nonuniform inflow to the propellers, including naturally occurring turbulence in the atmosphere, ground vortices, and wakes from fuselages, wings, nacelles or test stands [2]. In Appendix B , it can be seen that the total levels at Position 2 are lower by 2-4 d B A than at Position 1. This represents significantly less attenuation than the 39 dB predicted by spherical divergence, indicating very strong directivity for the Beech- 1900D run-up noise. The distance between these two positions was only 25 m; therefore the lower levels at Position 2 were relatively unaffected by atmospheric absorption in the 0-1250 H z range. In Appendix C, it is seen that the total levels at Position 3, which was approximately 3 km away from the aircraft, are 37-42 d B A lower than at Position 1. The 28 effects o f attenuation with distance, atmospheric absorption and temperature could have all contributed to the reduction of sound level measured at Position 3. Again these attenuations are significantly less than the 81 dB expected with spherical divergence. The spectra at 015 0 and 045 °, which must be compared with the headings of 225 0 and 255 ° for Positions 1 and 2, respectively, display sharp peaks at the fundamental and the first three harmonics only. The average rate o f decay is 8 dB/harmonic between the first, second and third harmonics, after which it is closer to 4 dB/harmonic. The two different slopes reflect those of the ambient noise spectra; seeing as the run-up and ambient noise levels were very close to each other at this position, it is not surprising that the shape of the run-up noise spectrum resembled that of the ambient-noise spectrum. The noise levels at Position 3 (see Appendix C) were recorded between the hours of 8 and 10 pm on a warm summer evening; therefore the temperature-lapse condition would have been present. The levels would likely have been higher had the measurements been conducted in the middle of the night, which is in a temperature-inversion condition, when most complaints occur. 2.2.3 Coherence Analysis A s was explained in Chapter 1, i f an A N C system is to attenuate noise effectively, the error signal must continuously send 'correction' signals to the controller, to account for fluctuations in the sound field. A way to quantify how much fluctuation occurs is by performing a coherence analysis. I f the reference signal is strongly correlated with the control signal sent to the control source, it is deemed 'coherent', and the need for an error signal is minimized. A coherence analysis was therefore performed, in order to quantify the randomness of the recorded run-up noise and determine i f a qualified reference signal is available [28]. The results from this analysis are also relevant to the A N C experiments that w i l l be presented in Chapter 5. Coherence is a statistical measure that determines how similar the sound pressures at different positions in space are to each other. In acoustics, it is the direct measure o f exactly to what extent two functions (e.g. X and Y ) are linearly related, the functions being two random sound-pressure fluctuations [16]. The degree of coherence is 29 represented by values between 0 and 1(1 representing perfect coherence) that indicate to what extent function X corresponds to function Y at each frequency. For this run-up noise study, the functions are the noise signals as measured at two different points in space; this is called the 'auto-correlation'- the degree to which the noise correlates with itself at the two points. The noise data recorded at Positions 1 and 2 were used for this analysis. The noise data measured at Positions 1 and 2 were recorded simultaneously on the left and right channels of the D A T recorder. These were input to a computer and digitized into functions X and Y . The method used for the coherence analysis was Welch's averaged periodogram method. The functions X and Y were divided into overlapping sections, then windowed to a given length. The squared magnitude of the Discrete Fourier Transforms (DFTs) of the sections of X and the sections of Y were averaged to form P x x and Pyy, the Power Spectral Densities of X and Y , respectively. The products of the DFTs of the sections of X and Y were then averaged to form P x y , the Cross Spectral Density of X and Y . The coherence C x y is given by: 2 C x y = ^ (2.1) P P xx yy This analysis was performed for various headings, and similar results were obtained at all headings. The coherence for the 105° heading is shown in Figure 2.9. The results in Figure 2.9 show a coherence value of 0.7 to 1.0 for the 105 0 heading, indicating a very good correlation between the two positions. The 0-350 H z range was chosen in order to display the fundamental frequency (112 Hz) and the first harmonic (224 Hz) , which are the frequencies that w i l l l ikely be selected for active control in the computer simulations (Chapter 5): The coherence graph shows a dip in the 112-Hz region, while the coherence appears to be stronger (close to 1.0) in the 224-Hz region. This would indicate that an A N C system optimized for the 224-Hz wavelength might be more effective than a system optimized for the 112-Hz wavelength. 30 Ol i i i i i i I 0 50 100 150 200 250 300 350 F r e q u e n c y (Hz) Figure 2.9. Coherence-analysis results for the 105° heading, for the 0-350 Hz range 2.2.4 Existing Blast-Fence Insertion Loss The microphone at Position 4 was positioned 73 m away from the aircraft, on the other side of the blast fence, so that the levels could be compared with those measured at Position 1, thus estimating the blast-fence insertion loss. Since Positions 1 and 4 were at 90 ° to each other relative to the aircraft, one must compare the spectrum at Position 4 with the spectrum at Position 1 for a 90° difference in headings. For example, one would compare the spectrum at 165° at Position 1 with the spectrum at 0 7 5 0 at Position 4 in order to determine the insertion loss for the aircraft at a heading of 075 °. Appendix E shows the apparent insertion loss provided by the blast fence, for every heading of the aircraft. In Chapter 1, we discussed barriers as noise-control measures, and how they are effective only at mid- and high frequencies, depending on the thickness and type of material used in their construction. They are relatively ineffective at low frequencies, since the longer sound wavelengths either transmit through the barrier or diffract over the top of the barrier. In the case 31 of the blast fence at Vancouver International Airport, low frequencies can also pass in between the upper slats, providing additional transmission paths for the aircraft noise to the other side (see Figure 2.4). These phenomena can be observed by looking at the insertion loss graphs in Appendix E . Observing the insertion loss graphs, we notice that a similar pattern emerges at every heading. The blast fence appeared to provide a total insertion loss of 8-20 dB, with the average being around 12 dB. Most of the attenuation appears in the 500-8000 H z range, with an attenuation of at least 8 dB for all headings and as much as 24 dB in the case of the heading o f 225 °. A small "dip" at 400 Hz , where less attenuation occurs, can be seen for most headings. A more noticeable "dip", at which little or no insertion loss occurs, exists at 160 H z for all headings. One of these dips could correspond to the critical frequency of the blast fence, at which the fence is vibrating at its own natural frequency. In between these "dips" we notice less attenuation than at higher frequencies. Below 160 Hz , even lower attenuation, ranging between 0-12 dB, with the average around 6 dB, is obtained. If more attenuation is required, an Active Noise Barrier can be designed, in order to increase the low-frequency insertion loss of the blast fence, by positioning error microphones at the top of the blast fence [29]. The extra attenuation provided by an Active Noise Barrier w i l l be simulated in Chapter 4. 32 - CHAPTER 3 -ACTIVE NOISE CONTROL THEORY 3.1 Introduction In this chapter, we w i l l derive A N C expressions, which w i l l be used in the M A T L A B programming of the A N C simulations of Chapter 4. Researchers in active noise control have developed different algorithms; the most commonly used are those developed by Nelson & Elliott [16]. The theory presented in this chapter is based on the 'filtered-X least-mean-squared feedforward' algorithms, first introduced by Nelson & Elliott, then further studied by Guo [30]. Two separate sound-field conditions w i l l be considered: free-field and half-space. The free-field condition assumes that no reflective surfaces are present (or that the surfaces are completely absorptive), whereas the half-space condition assumes that a totally reflective surface is present below the control system. Both conditions are presented since, in reality, the ground below the aircraft, which may include asphalt, grass or other materials, w i l l reflect some sound energy, especially at low frequencies. In this case, the situation is closer to a half-space condition. The ground can only be considered as a perfect reflector at frequencies below 100 H z [16]; therefore it is best to investigate both free-field and half-space conditions, and deduce that the results from the realistic situation w i l l lie somewhere between the two. 33 Expressions for single-channel, multiple-channel and active noise barrier theory w i l l be presented. For the most part, the primary and control sources are assumed in the theory to be single monopole sources, which radiate spherically as defined by the inverse-square law (see Equation 1.2). However, we w i l l also investigate multi-pole sources, which more closely model the radiation from actual propellers. The modeling of multi-pole sources w i l l be presented in this chapter. 3.2 Single-Channel Local-Control Theory 3.2.1 Free-field condition A single-channel control system is shown in Figure 3.1. In Figure 3.1, qp is the strength of the primary source, qs is the strength of the secondary source, rse is the distance from the control (secondary) source to the error microphone, rpe is the distance from the primary source to the error microphone,.and rps is the distance from the primary source to the secondary source. The sound-power outputs of the primary source, the control source and the whole system are, respectively [16,31]: E primary source control source error sensor Figure 3.1. Single-channel local-control setup in free field 34 W s = \ ^ s \ 2 + \ ^ s K P % \ ( 3 1 ) Wp=^Z0\qp\2+^Rc(qpZ^s] (3-2) WT =Wp+Ws =\z\qp\2 +\qs\2)+Ue(qpZ;sqs +qXPqP) * (3-3) where Z 0 = co2po /4nc0 is the characteristic impedance of air, Zsp = Zps = Z0 [sin kr I krps + i cos kr I krps ] is the acoustic transfer impedance between the primary source and the secondary source, and k = 2nlX is the wavenumber. When a single-channel local-control strategy is employed, a zone of attenuation is created around the error microphone when the primary source, control source and error microphone are arranged in a straight line, as in Figure 3.1. This linear arrangement was found to be optimal by Guo [30], in comparison to a triangular arrangement where the error sensor is at an angle to the primary-secondary source line. The aim of a strategy involving local: sound-pressure attenuation is to drive the total sound pressure at the error microphone to a minimum (ideally to zero). When this is achieved, the sound pressure in an area around the error microphone is also attenuated, and a quiet zone is created. The strength of the control source and the total sound-power output are, respectively [30]: WT=W0 2k, ' ,2 V rpe ^ perps c o s * ( r p e - r « ) s i n A r J ps (3.5) The quiet zone is defined as the area in which the primary sound pressure is attenuated by 10 dB or more [32,33]. In local control, this attenuation occurs at the expense of an increase o f sound pressure in other areas. The size of the quiet zone is dependent on the relative positions of the primary source, the secondary source and the error microphone. The wavefronts of the primary and secondary sources must be equal in 35 amplitude and opposite in phase at the location of the error microphone for cancellation to occur. This is called 'wavefront matching', as introduced in Chapter 1 (see Figure 1.10). A s was seen in Figure 1.10, a triangular quiet zone manifests itself as longer arc lengths as the primary-source wavefronts coincide with the wavefronts o f the control source. In a 3-dimensional free field, the quiet zone is conical, with the zenith at the control source. The width of this quiet zone can be adjusted by varying the distance between the primary and control sources. Bringing the two sources closer w i l l widen the quiet zone, whereas increasing the distance between the two sources w i l l result in a narrowing of the quiet zone. 3.2.2 Half-space condition When a sound source is located near a reflective surface, the reflected sound interferes with the direct sound from the primary source. This w i l l affect the performance of an A N C system, since the control source must now attenuate the direct sound as wel l as a portion of, or all of, the reflected sound. Different methods have been used to simulate the effect of a reflective surface on the propagation of sound. The method used in the half-space computer simulations was the image-source method, in which the direct sound from a copy or 'image' of the primary source replaces the reflective surface [31,34,35] (see Figure 3.2). In Figure 3.2, q is the strength of the point source, qi = C,q is the strength of the imaginary source, Cr is the reflection coefficient determined by the characteristics of the reflective surface, and h is the height of the point source above the reflective surface. The sound pressure at the receiver position then becomes and the sound-power output of the point source above a reflective surface is given as (3.6) Wh=W0{\ + Rp,p), (3.7) 36 noise source image source 9i Figure 3.2. Point source over a reflective surface where W0 = jZQ\q\ is the power output of the point source without the reflective surface and R , = Re(iCre~'2hp /2khp). For a rigid surface, Cr — 1, and the power output becomes Wh = W0(l+smc(2kh)) (3.8) where sinc(2&/z) is defined as sin(2kh)/(2kh). Figure 3.3 shows an A N C system above a reflective surface. qp and qs are the strengths o f the primary and secondary sources, E is the position of the error sensor. rse is the distance from the secondary source to the error microphone, rpe is the distance from the primary source to the error microphone, and rps is the distance from the primary source to the secondary source. rs-e ,rp-e, rp-s are the distances from the secondary (control) source to the error microphone, from the primary source to the error microphone, and from the primary source to the secondary source through the reflective surface, respectively. hp, hs and he are the heights of the primary source, secondary source and error sensor above the reflective surface, respectively. The acoustic transfer impedances between the primary source and the error sensor, between the secondary source and the error sensor and between the primary source and the secondary source become: 37 primary source control source Is image primary source image secondary source Figure 3.3. Single-channel local-control system in half space Zps = iZc vCr V tope •«V. \ kr, + Cr \ krse Pe J kr, , s e / ( -ikrm v K ->kr„: \ • + Cr-kr (3.9) (3.10) (3.11) Ps J In local control, the sound pressures at the error microphones are chosen as the 'cost functions', meaning that the objective is to minimize the total squared sound pressures at those positions. In half space, the required source strength of the control source becomes: qs = Aqp, (3.12) where A = -Zpe/Zse is the ratio of the acoustic transfer impedance between the primary source and the error microphone, and the acoustic transfer impedance between the 38 secondary source and the error microphone. The total sound power radiated by both the primary and secondary sources in half-space becomes WTh = W0[l + Rp,p + 2 ( s i n c ^ +Re( iC r e" f l r ' - / krp,s])Re(A) + \A\2 (1 +R,5) , (3.13) where R p - P i s defined in Equation (3.7) and Rs,s = Re(iCre  i2h' I2khs). The total sound pressure at any position in upper space is P - Zpiqp + Zsiqs where Z pi — iZ0 + Cr ->kr„; \ (e* r>i e h Cr pi J (3.14) (3.15) (3.16) rpi,  rp'i, rsi and rs-i are the distances from the observation point to the primary source, the image primary source, the secondary source and the image secondary source, respectively. 3.3 Mul t i p l e Control-Source A N C Theory A multiple control-source A N C system is shown in Figure 3.4. I f M primary sources and N secondary control sources are distributed in free space, their corresponding source strengths are, respectively: q p = k . ^ / , 2 > - - - ^ p W ] r , (3-17) fls=[qsi,qs2,---,qsN]T, (3.18) where T denotes the transpose. The sound pressure at an observation position in the space can be expressed as: P = Pp + Ps = Zpqp + Zsqs, (3.19) 39 fps —rs Figure 3.4. Multiple-channel ANC system where Pp and Ps are the complex sound pressures at the particular field-point position due to the primary sources and the secondary sources, respectively. Zp and Zs are row vectors of the acoustic transfer impedances from the primary sources, and from the secondary sources, to the observation position, respectively. If L error microphones are used, the complex pressures at the L error microphones are P = Zpeqp + Zseqs. The sum of the squared sound pressures at the microphone positions is selected as the 'cost function' / for local control: J= P"P = (Zpeqp + Zseqsf(Zpeqp + Zseqs), (3.20) where Zpe is an L x M matrix of acoustic transfer impedances from the primary sources to the error sensors, Zse is an L x N matrix of acoustic transfer impedance from the secondary sources to the error sensors, and the superscript H denotes the Hermitian transpose. The strengths of the secondary-source array are adjusted to minimize the cost function. I f L = N , the matrix of optimal secondary-source strengths is [36]: q^ o Zse Zpe(\p (3.21) The sound pressure at any position in the space after control becomes P ^p^p ~ZsZse Zpe{\p. (3.22) 40 The acoustic power outputs of the primary sources and the secondary sources are given by: ^ = i k R e ( Z w ) q / , + q ; R e ( Z ^ ) q s ] , (3.23) , Ws = i[q 1 f f R e ( Z„)q I + q f R e ( Z ^ ) q p ] . (3.24) where Zpp is the M x M transfer-impedance matrix between the primary sources, Zss is the NxNtransfer impedance matrix between the control sources, and Zps is the Mx TV transfer impedance matrix between the primary and secondary sources. The total radiated acoustic power of the system, which is the summation of the power outputs of the primary sources and the secondary sources, can be written as WT=±[qHpRe(Zpp)qp + q ; R e ( Z ^ ) q s + q f R e f Z J q , + q f " R e ( Z ^ ) q p (3.25) The distances between the control speakers must be within an optimal range as determined as follows [30]: r =< j s - m a x — 2 V NX 2 V N-INA N = 2,4, 6, N = 3, 5, 7, 1.4A, (In 20 (N + \)X v 2 J 20(7V-2) (1.25 In + 0.5)ln| f r 1 X 2 (r.„ n X • + -X 2 V + 0.7) In (3.26) W = 2, 4, 6,---, + • X 4 ( A f - l ) + | , W = 3, 5, 7 , - , (3.27) A n r M distance greater than rss.max w i l l result in a control-source separation which is too large to produce a quiet zone. A n rss distance lower than rss.min w i l l produce a quiet zone, but at the expense of significant sound pressure increases in areas outside the quiet zone. 41 3.4 Active Noise-Barrier Theory In Chapter 1 we presented the Maekawa method for estimating the insertion loss o f a barrier. It was seen from this expression that the insertion loss o f a barrier is inversely related to the wavelength o f the sound; that is, the higher the frequency o f the sound, the greater the insertion loss o f the barrier. The area of attenuation behind the barrier is called the 'dark' area. The multi-channel active-noise-control system presented in Section 3.3 was used to improve the insertion loss o f the barrier. The optimal spacings between the control sources and the error microphones were the same, as expressed by Equations (3.26) and (3.27). The A N C system was located between the noise source and the barrier. The error-microphone array was located on top of the barrier, and the control source array was located between the primary source and the error-microphone array (see Figure 3.5). Before estimating the extra insertion loss provided by an active noise-control system, we must first develop the expressions for a radiating point source and the y Figure 3.5. Active Noise Barrier •J 42 resulting 'dark area' created by the barrier alone. The sound-pressure field created by a point source in open space is expressed as [37,38] P = —e ikr 0 kr ' (3.28) and the diffracted field arriving at the receiver position in the 'dark area' can be approximately expressed as [39] P. =-nkR, -Ae -in/4 ikR sgn(7t + a - ())) F[jk(Rx-R) (3.29) + sgn(7t - a - <>) F\\k{Rx-R)\ y«(/c , +R ) for kR/»l, where k is the wavenumber of the sound, A=-iZoq with q the strength of the source and Zo=a> 2polA-KCo, and F(ix)=£e*2d1 (3.30) is the Fresnel integral. R and R' are the direct distances from the receiver to the source and to the source mirror image in the barrier, respectively. R/=r+r0 is the shortest distance from the source to the receiver around the barrier top and a and § are the angles of the incident and diffracted sound waves to the barrier, as shown in Figure 3.6. When the reflections from the ground on both sides o f the barrier are negligible, the sound field in the 'dark area' comes directly from the noise source through diffraction source mirror image point source noise barrier Figure 3.6. Active Noise Barrier parameters. 43 over the barrier top, and the total diffracted sound pressure at the receiver is [30] 1=1 where Ppd is the diffraction caused by the primary noise source only - this also represents the diffracted sound while the active control system is off - and P$ is the diffraction caused by the z'th control source. Both Ppd and P$ are predicted using Equation (3.29). The additional insertion loss created by the multi-channel active-noise-control system can then be described as 3.4.1 Effect of Rigid Ground on Control Efficiency A s discussed in a previous section, the existence of a rigid surface below an A N C system w i l l affect its performance. It was found that a reflective ground (half-space condition) generally reduces the size of the quiet zone created in open space (free-field condition). The same holds true in the case of an active noise barrier. When the ground on both sides of the barrier is reflective, there are four propagation paths from each noise source to the receiver over the barrier top, as shown in Figure 3.7. They are: (1) direct from the source to the receiver; (2) via the reflective ground on the source side, which is equivalent to the path from the source image to the receiver; (3) via the reflective ground on the receiver side, which is equivalent to the path from the source to the receiver image; and (4) via the reflective ground on both sides, which is equivalent to the path from the source image to the receiver image. A t low frequencies, such as in the case of the first three harmonics of the Beechcraft 1900D run-up noise, the ground can be assumed to be a perfect reflector. The total diffracted sound pressure at the receiver position, expressed by Equation (3.31), becomes (3.32) 4 4 N (3.33) 44 error sensor control source primary source receiver reflective ground primary source image control source image -J receiver image Figure 3.7. Four propagation paths of sound diffraction over the barrier on reflective ground where j represents they'th pathway. The additional insertion loss of the barrier can then be calculated from where AZ = 201og I 0( |^/ji>, |) , 7=1 (3.34) (3.35) is the diffracted sound from the primary source only. Both Ppa and PSd are expressed in the form of Equation (3.29). 3.5 Mul t i -po le Source Generation In Chapter 2 we explained how noise radiates from propellers as monopoles, dipoles, quadrupoles, or a combination of these. Monopoles are omni-directional point sources that radiate sound-pressure wavefronts as dictated by the inverse-square law, which was presented in Chapter 1. Dipoles are modeled by placing two monopoles of opposite phase at a distance less than one wavelength from each other. The result of the opposite phases is 45 cancellation of sound pressure in the directions perpendicular to the line connecting the ' two monopole sources (see Figure 3.8). A quadrupole source is modeled in the same fashion, but using two dipole sources, distanced less than one wavelength from each other. Adjacent sources must be of opposite phase. A quadrupole is shown in Figure 3.9. Simulations of A N C systems using the expressions and models developed here w i l l be presented in the next chapter. < > d< A Figure 3.8. Directivity pattern of a dipole sound source. Figure 3.9. Directivity pattern of a quadrupole sound source. 46 - CHAPTER 4 -ACTIVE NOISE CONTROL COMPUTER SIMULATIONS 4.1 Introduction Computer simulations were performed to estimate the amount of run-up noise attenuation provided by an Active Noise Control ( A N C ) system to be located at the Vancouver International Airport. In addition, the sizes of the quiet zones were predicted for different arrays of control sources. The computer models used for the simulations are based on the expressions and algorithms developed in Chapter 3. These computer models assume that the meteorological conditions are neutral; i.e. that no wind is present, and that temperature and humidity are constant. In the following sections, sound-attenuation predictions using single- and multi-channel A N C systems w i l l be discussed, for different source-radiation patterns. In addition, simulations of A N C systems working in conjunction with acoustical barriers, forming active-noise barriers, w i l l be presented. 47 4.2 A N C System Design In Chapter 1, we introduced A N C systems, and how the type o f control attained is dependent on the proximity o f the control source to the primary source. We w i l l now discuss how an A N C system can be designed to attenuate run-up noise. When designing an A N C system, we must first choose a frequency in the primary noise spectrum that we want to target. Naturally, the most dominant frequency in the A -weighted spectrum would be the first choice; however i f no one frequency dominates, the 'target' frequency may be anywhere within the frequency range that is to be attenuated. The selection o f this frequency, and the effects on other frequencies in the range under consideration, w i l l be covered in section 4.3.5. The type o f control - global or local -depends strongly on the wavelength o f the target frequency. A s discussed in Chapter 1, for global control to be achieved the control source(s) must be positioned within X/4 (one-quarter o f a wavelength) o f the primary source [16,30]. I f the two sources are separated by more than X/4, then the system becomes a local-control system. Since lower frequencies have longer wavelengths, they are generally easier to be controlled by A N C . The spectra in Chapter 2 o f the noise measured from the Beechcraft 1900D have their lowest dominant frequency at 112 Hz , which is the blade-passing frequency (BPF) . The wavelength at 112 H z is 3.07 m meaning that, for global control the control sources would have to be positioned within 3.07/4 = 0.77 m o f the primary source. This would entail positioning a loudspeaker within 0.77 m o f a spinning propeller, which is not feasible, for safety reasons. Furthermore, a reference microphone must be positioned within this distance in order to capture a reference signal, further reducing the space in which to work. Having a microphone so close to a propeller would also result in inaccurate noise measurements, since the strong air flows and turbulence would distort the signal. A global-control system thus is unfeasible, and we must adopt a local-control strategy. Simulations o f local-control systems using single and multiple control sources, as well as different primary-source radiation patterns, were performed. A single monopole primary source is used in the next three sections (4.3 to 4.5), enabling us to draw general conclusions as to the effects o f A N C systems on the redistribution o f energy in the sound 48 field, as wel l as the effects on run-up noise spectra. Section 4.6 w i l l present simulations o f A N C systems using primary-source radiation patterns closer to actual propeller-noise patterns. 4.3 Single-Channel Local Control In this section, the discussion o f a single-channel A N C system involving one control source and one error microphone w i l l be presented. For simplicity, the primary and control sources are assumed to be single monopole (or 'point') sources, which radiate spherically as defined by the inverse-square law. A s was explained in Chapter 3, two separate sound-field conditions w i l l be considered: free-field and half-space. The quiet zone is defined as the area where the primary sound pressure is attenuated by 10 dB or more [32,33]. In local control this attenuation occurs at the expense o f an increase o f sound pressure in other areas. The size o f the quiet zone is dependent on the relative positions o f the primary source, the secondary source and the error microphone. The wavefronts o f the primary and secondary sources must be equal in amplitude and opposite in phase at the location o f the error microphone for cancellation to occur. This is the wavefront-matching concept, which was introduced in Chapter 1 (see Figure 1.10). A s was seen in Figure 1.10, a triangular quiet zone manifests itself as longer arc lengths as the primary-source wavefronts coincide with the wavefronts o f the control source. In a 3-dimensional free-field, the quiet zone is conical, with the zenith at the control source. A s was demonstrated in Chapter 1, the width o f this quiet zone can be adjusted by varying the distance between the primary and control sources. 49 4.3.1 Single-Channel Control Simulations 4.3.1.1 Effect o f increasing primary-to-control-source distance (rm) Simulations o f a single-channel system were performed in free-field and half-space, for the 112 H z B P F o f the Beechcraft 1900D. Various distances between the primary and secondary sources (rps) were chosen to study their effect on the size o f the quiet zone. For these simulations, the distance between the secondary source and the error microphone (rse) was varied accordingly, so that rse= rps. The heights o f the single monopole primary source, the control source and the error microphone were equal to 2 m, the height o f the propellers o f the Beechcraft 1900D. Results for control systems in free-field and half-space for rse= rps=5, 10, 20 and 40 m are shown in Figures 4.1 to 4.4, respectively. The results are plotted as contour maps that show the change ALp in sound-pressure level with position in a horizontal plane 2 m high behind the primary source. The colorbars to the right o f the graphs display the ALp values associated with the colors in the contour maps. Attenuation zones appear as the areas with ALp <0 dB , and quiet zones as the areas with ALp <-10 dB. A value above zero indicates an increase in sound level. The x andy axes are the distances in meters, with the primary source located at x -y = 0 m . The y axis extends as far as 8 km, which exceeds the furthest location o f run-up complaints, i f we take.y = 0 m at the Vancouver Airport. In Figures 4.1 to 4.4 we can see that the maximum attenuation provided by a single-channel system along thej^-axis at JC = 0 m ranges from 6 dB with rse- rps~5 m in the free-field condition, to 8 dB with rse= rps^ 40 m for both conditions. The half-space condition for rse= rps=5 m is clearly the worst situation, for which no attenuation is present. The variation o f the shape o f the attenuation areas with rse= rps is clearly seen i f we look at, say, the - 6 dB attenuation line for a l l cases; the widest area is generated with rse= fps~5 m and the narrowest with rse- rps= 40 m. In addition, more and more secondary, smaller attenuation areas, appear on either side o f the primary attenuation area (centered along y=0 m) as rse and rps are increased. It can also be seen how the energy is redistributed to other areas, called 'hot zones', where levels are increased by 2 to 4 dB on 50 either side o f the quiet zones in most cases. The only case to exhibit a larger increase is, again, the half-space condition for rse= rps=5 m (Figure 4.1 (bottom)), where increases o f up to 9 dB are observed. In addition, the sound energy in the areas from}' = 0 to y = 2000 m display increasingly complex behaviour as rse= rps increases. 51 Figure 4.1. Single-channel control with r3e= rps= 5 m,for free-field (top) and half-space (bottom) conditions. Figure 4.2. Single-channel control with rse= rps= 10 m,for free-field (top) and half-space (bottom) conditions. 5 3 Figure 4.3. Single-channel control with rse= rps= 20 m.for free-field (top) and half-space (bottom) conditions. 5 4 4.3.1.2 Effect o f increasing control-source-to-error-microphone distance Ow) Figures 4.1 to 4.4 show the attenuation areas that are generated when both r„ and rps are increased, but are equal in magnitude. We w i l l now look at the effect o f increasing rse, keeping rps constant. Figures 4.6 to 4.8, respectively, show this effect on the attenuation areas, using rps= 20 m, and rse= 5, 40 and 100 m (the rse= rps= 20 m case is shown in Figure 4.3). From these Figures, we can see that increasing rse while keeping rps constant increases the size o f the ALp < 0 dB attenuation area. Quiet zones (ALp < -10 dB) appear with r s e = 40 m and rse= 100 m only. We can therefore deduce that quiet zones are generated when the r^jr^ ratio is 2.0 or larger, for a single-source, single-channel A N C system. These figures also reveal that the free-field and half-space simulations become increasingly similar as rse increases when rse/rps is 2.0 or larger. We can observe that the size o f the quiet zone increases only slightly from the rse= 40 m case to the rse= 100 m case. Quantifying the size o f the quiet zone is best defined by the quiet-zone angle, 6qz, which is taken as the angle between the two -10 dB attenuation lines (see Figure 4.5). For instance, 6qz for the rse= 40 m case (Figure 4.7) in the free-field is 9qz = 2 x tan'(900/8000) = 12.8°. Since 0q- for the rse= 40 m and the rse= 100 m cases remain roughly the same, we can deduce that it is Tps that governs 0qz\ rse determines the position o f maximum attenuation (at the error microphone). Figure 4.9 offers a closer look at the area in the vicinity o f the A N C system for the rse= 40 m configuration, showing how the majority o f the sound increase is localized at the location o f the control sources. Figure 4.9 also demonstrates how the maximum attenuation occurs around the error microphone. quiet zone Figure 4.5 Definition of quiet zone and quiet-zone angle 0qz. 56 Figure 4.6. Single-channel control with rps= 20 m, r « - 5 m for free-field (top) and half-space (bottom) conditions. 5 7 Figure 4.8. Single-channel control with rps= 20 m, rse= 100 m for free-field (top) and half-space (bottom) conditions. 5 9 100 80 60 40 20 0 -20 -40 -60 -80 -100 J i i lis -100 -50 0 y(m) 50 100 Figure 4.9. Single-channel control with primary source aty= 0 m, control source aty=20 m and error microphone aty=60 m, showing sound level increase localized around control source, and maximum attenuation around the error microphone. 4.3.1.3 Effect o f increasing control-source and error-microphone height We w i l l now investigate the effect o f varying the heights o f the control source and the error microphone in relation to the primary source, which was fixed at the height o f the propellers (hp = 2 m). In the previous section, we discovered that quiet zones only started appearing in the rps - 20 m and rse - 40 m case, and with higher values o f rse. We w i l l therefore again use this configuration for these simulations. Let us first look at the result o f placing the control source and error microphone on the ground (hc=he=0). The results are shown in Figure 4.10. When comparing Figures 4.10 and 4.7, we see that they are very similar; the configuration with hc=he=2 m appears to be slightly better, judging by the lower limit shown in the colorbar. 6 0 Figure 4.10. Single-channel control with rps= 20 m, rse= 40 m, hp= 2 m, hc=he= 0 m for free-field (top) and half-space (bottom) conditions. 61 Guo [30] showed that i f the control source is at a different height from the primary source, then the primary source, control source and error microphone should be aligned (see Figure 4.11). For instance, i f the primary-source height is 2 m and the control-source height is 4 m at a distance o f 20 m, then the error microphone positioned 40 m from the control source should be at a height o f 8 m. The results for this configuration are shown in Figure 4.12. This configuration is clearly non-optimal, since increases o f up to 4 dB and 25 dB occur in the free-field and half-space conditions, respectively. From the results o f the simulations in this section, we are able to conclude the following, for a single-source, single-channel A N C system: - the sizes o f the attenuation areas increase as the primary-to-control source distance (rps) decreases; - quiet zones are generated when the rse/rps ratio is 2.0 or larger; - the free-field and half-space conditions are almost identical when the r^jr^ ratio is 2.0 or larger; - the optimal heights o f the control source and error microphone are equal to the primary source height. When larger quiet zones are required, more channels must be introduced to the A N C system. Multiple-control-channel A N C systems provide more attenuation over a wider area. error microphone Figure 4.11. Alignment of an ANC system. 6 2 Figure 4.12. Single-channel control with rps= 20 m, r„= 40 m, hp=2 m, hc= 4 m and he= 8 m,for free-field (top) and half-space (bottom) conditions. 63 4.4 Multiple Control Sources A control system with more than one control channel w i l l increase the amount o f attenuation, as well as the size o f the quiet zone [31]. Each control channel requires its own error microphone to ensure proper performance o f the system. A multiple-control system in a linear configuration was shown in Figure 3.6. 4.4.1 Multiple-Channel Local-Control Simulations Simulations o f multiple-control systems with 3, 9,21 and 61 channels were performed with rps - 20 m, rse = 40 m, and hp= hc= he- 2 m in the free-field, based on the optimal arrangements discussed in the previous sections. The results are shown in Figs. 4.13 to 4.17, respectively. In Chapter 3, we discussed the optimal range o f the distances between the control sources (rss), which were dictated by Equations 3.26 and 3.27. Figure 4.13 shows the difference in the results when we use rss values close to the upper and lower limits o f the optimal range, for a 3-channel system in the free-field. It can be seen that a value closer to the lower limit, rss.mi„, yields a larger quiet zone, as well as smaller increases in the hot zones than does a value close to rss.max. We w i l l therefore use rss values closer to the lower limit for these simulations; the exact values are given in the figure captions. The results in Figures 4.13 to 4.16 show that the quiet zone does, in fact, increase as more channels are added to the A N C system. In addition, the sound levels in the hot zones decrease, as more channels are introduced. 64 Figure 4.13. 3-channel control with rps=20 m, rse= 40 m, hp- hc= he= 2 m and rss values close to rss.max = 9.17m (top) and rss.min = 6.17m (bottom) for free-field conditions. 6 5 0 2000 4000 6000 8000 y(m) Figure 4.14. 9-channel control with rps=20 m, rse= 40 m, hp= hc= he= 2 m and rss= 2. Figure 4.15. 21-channel control with rps=20 m, rse= 40 m, hp= hc= he= 2 m and rss= 2 0 2000 4000 6000 8000 Figure 4.16. 61-channel control with rps=20 m, rse= 40 m, hp= hc= he= 2 m and r„= 1.7 m. 4.4.2 Effect of Primary-to-ControI-Source Distance (rp!) on Quiet-Zone Size In section 4.2, we determined that the area o f maximum attenuation becomes wider as the control source is positioned closer to the primary source. However, for all single-channel cases, a quiet zone for which at least 10 dB o f attenuation occurs was not created. We have seen in section 4.3 that quiet zones are created with as few as 3 channels. Let us therefore study the relationship between the quiet zone angle 9qz and the rps distance for a 21-channel system. The rse distance was varied accordingly, maintaining a r^jr^ ratio o f 2.0. rps distances o f 5, 10, 20 and 40 m were used. These results were plotted, to see i f a trend emerged. This is shown in Figure 4.17, along with the best-fit regression curve. The best-fit curve follows a power-law trend (y = 258.Ix" 0 5 6 ) . Note when using this curve that the control system becomes global when rps < A/4; therefore it is only valid for rps>0.11 m, for the 112-Hz frequency. The maximum quiet-zone angle that can be achieved with this 21-channel local-control system is 169°. This curve is very useful when a particular quiet-zone angle is sought, and the primary-source to secondary-6 7 source distance (rps) is to be determined. 4.4.3 Effect of Number of Control Channels on Quiet-Zone Size A s was seen in Figures 4.13 to 4.16, the size o f the quiet zone can be increased by adding more control channels to the A N C system. The quiet-zone angles achieved with the different numbers o f control channels, with rps = 20 m and rse - 40 m, were plotted, to see i f a consistent pattern emerged. This is shown in Figure 4.18. Again, the best-fit curve follows a power-law trend (y = 8. l x ° 5 6 ) . This curve is very useful in deciding the number o f control sources to use to achieve a desired quiet-zone angle. 68 s £ 120 110 100 90 80 70 60 50 40 30 20 10 0 11 16 21 26 31 36 41 46 number of control sources 51 56 61 66 Figure 4.18. Quiet-zone angle vs. number of control sources. 4.4.4 Effect on Harmonics So far we have only looked at systems which target the 112-Hz BPF, and the results for that frequency only. However run-up noise also contains harmonics of the BPF and our goal is to rninimize the total A-weighted sound pressure in a certain area. Thus, we also have to consider which frequency to target and the effect of a system designed to target one frequency on the attenuation at the other frequencies. In Chapter 3, Equations 3.26 and 3.27 defined the optimal range of the distances between the control sources rss in terms of the frequency that is targeted for optimization. Table 4.1 shows the optimal range for the BPF (f0) and its first 4 harmonics (/}, /}, f$, ft), using a 9-channel system configured with rps= 20 m and rse= 40 m. As seen in Table 4.1, if a system is designed for the 112 Hz BPF, the upper limit (rssmca) for higher frequencies tends to fall below the optimum r^ range for 112 Hz. Conversely, if a system is 69 Frequency fss-max fss-min (Hz) (m) (m) f0=112 4.40 2.86 ft = 224 3.02 1.95 f2 = 336 2.43 1.55 f3 = 448 2.10 1.31 f4 = 560 1.87 1.14 Table 4.1. Optimal ranges for a 9-channel system, rps- 20 m and r„= 40 m. designed for, say, the 3 r d harmonic (448 Hz) , then the optimal range for 112 H z tends to lie above this range. To study the effects on the higher harmonics, simulations using rss = 2.9 m, close to the lower limit for f0, were first performed. Since this value is also in the optimal range for / / , we would expect to see favourable results for this frequency as well . Figures 4.19 to 4.23 display the quiet zones for rss = 2.9 m for fQ, ft, f2, fi and ft, respectively. Figure 4.19. 9-channel attenuation of fo with rps= 20 m, rse= 40 m and rss = 2.9 m. 7 0 Figure 4.20. 9-channel attenuation of fi with rps= 20 m, rse= 40 m and rss = 2.9 m. Figure 4.21. 9-channel attenuation of f2 with rps= 20 m, rse= 40 m and rss = 2.9 m. 71 Figure 4.22. 9-channel attenuation of f3 with rps= 20 m, rse= 40 m and rss = 2.9 m. Figure 4.23. 9-channel attenuation of f4 with rps= 20 m, r i e = 40 m and rss = 2.9 m. 7 2 It can be seen from Figures 4.19 to 4.23 that, using this configuration, -10 dB quiet zones are present only for fo and / ; , since the rss value chosen is within both o f their optimal ranges. The other harmonics (/j, fi and ft) experience some attenuation, but in irregular shapes and patterns, in 'strips' on either side o f the x=0 m centerline. Increases outside o f the quiet zones can be seen, by as much as 15 dB for the _/} and f4 cases. Another approach was, therefore, taken, this time using rss= 2.0 m, a value close to the maximum for / j . Results o f simulations done using this value are shown in Figures 4.24 to 4.27. This time, we would expect better results for / / , 72 and fi and worse results for fo, since the value chosen is within the optimal range o f / ; , 72 and f. Observing Figures 4.24 to 4.27, we see that this configuration significantly improves the situation in terms o f quiet-zone generation, providing much larger quiet zones for fo,fi,f2, and fi. The larger quiet zones are generated at the expense o f substantial increases o f up to 20 dB in the hot zones for fo, as expected. Figure 4.24. 9-channel attenuation of fo with rps= 20 m, rse= 40 m andrss = 2.0 m. 73 Figure 4.25. 9-channel attenuation of fi with rps= 20 m, rse= 40 m and rss = 2.0 m. Figure 4.26. 9-channel attenuation of f2 with rps= 20 m, rse= 40 m and rss = 2.0 m. Figure 4.27. 9-channel attenuation of fi with rps= 20 m, rse= 40 m andrss = 2.0 m. Figure 4.28. 9-channel attenuation of f4 with rps= 20 m, rse= 40 m andrss = 2.0 m. 75 In Chapter 2 we presented the spectra o f the run-up noise as measured in the Deering Island community, located approximately 3 k m from Vancouver Airport (see Appendix C ) . The attenuations o f the main spectral peaks simulated at a distance 3 k m from the primary source, using the A N C system configuration with rss = 2.0 m, are shown in Table 4.2 for both free-field and half-space conditions. In Table 4.2, attenuations can be seen for the fundamental and harmonics up to the 4th harmonic (/*), for which there is an increase for the half-space condition. This problematic increase can be avoided by filtering the reference signal through a low-pass filter (LPF) - with cut-off frequency say at 500 H z - so that signals at frequencies higher than 500 H z are not reproduced by the control source. This prevents increases o f sound pressure at frequencies above 500 H z . A more extensive discussion o f the effect o f low-pass filters on A N C system performance w i l l be presented in Chapter 5. Figure 4.29 shows how the attenuations from Table 4 affect the harmonics o f the Beechcraft 1900D spectrum measured in the community, for the 015° heading. A low-pass filter at 500 H z would have to be used to eliminate the possibility o f an increase o f sound pressure for harmonics higher than /}. We can see that a total attenuation o f 15 d B A is provided by this system. Frequency (Hz) ALn (dB) free-space half-space fo=H2 -14.9 -14.8 f i=224 -15.0 -14.7 f 2=336 -15.0 -14.4 f3=448 -11.1 -20.1 J4=560 -6.5 2.5 Table 4.2. Attenuations provided with an ANC systems optimizedfor f\ (rss = 2.0 m). 76 80 7 112 224 336 448 560 Total Frequency (Hz) Figure 4.29. ALp from a 9-channel system using rss = 2.0 m and a LPF at 500 Hz, for • • 1900D WKM free-field W half-space 4.5 Active Noise Barrier Simulations Another application o f Active Noise Control is in conjunction with a barrier; this is termed an Active Noise Barrier ( A N B ) . It was explained in Chapter 1 how acoustical barriers are only effective at high and mid frequencies, and that the attenuation provided by a barrier is termed Insertion Loss (IL). A t Vancouver Airport, the blast fence located near the Canadian Airlines hangar (see Figure 4.30) acts as an acoustical barrier, in addition to shielding an adjacent parking lot from wind blast from aircraft engines. The measured insertion loss o f this barrier was discussed in Chapter 2, and the results are shown in Appendix E . These graphs demonstrate that the blast fence is quite effective at attenuating the mid and high frequencies (ALP = 8 to 14 dB), but not so effective at low frequencies (<200 Hz) (ALP = 0 to 8 dB). In the following sections, simulations o f the additional IL provided by an A N B w i l l be presented. The height o f the barrier was assumed to be 4 m, the same as the height o f the blast fence, and the barrier was assumed to be infinitely long, meaning that 77 Figure 4.30. Existing Blast Fence at Vancouver International Airport diffraction around the ends o f the barrier is negligible. Furthermore, the simulations assume that the barrier has no gaps, which is clearly not the case. A N B simulations for 3, 9, and 21 control sources using rps=20 m and rse= 40 m are shown in Figures 4.31 to 4.33, for a single monopole source in the free-field. The figures display the extra attenuation provided by an active-noise-control system, above and beyond the insertion loss o f the barrier. The barrier lies along the y= 0 m axis. The results o f the A N B simulations are nearly identical to those for the A N C simulations without barriers (Figures 4.13 - 4.15). A s in the A N C case without barriers, the size o f the quiet zone increases and the sizes o f the hot zones decreases as more channels are introduced into the A N B system. Active Noise Barriers are very useful when sound attenuation over the entire spectrum is needed. Communities located at large distances from the airport benefit from atmospheric absorption which attenuates high frequencies, as was seen from the Beech-1900D spectra measured on Deering Island. Communities which are closer to the airport, however, do not benefit as much from this phenomenon, making Active Noise Barriers a very interesting option, since the barriers attenuate the high and mid frequencies while the A N C system attenuates the low frequencies. 7 8 Figure 4.31. 3-channelANB with rps=20 m, rse=40 m andrss=6.2 m. Figure 4.32. 9-channel ANB with rps=20 m, rse=40 m and rss=2.9 m. 7 9 4.6 Mul t i -po le Source A N C Simulations The simulations presented in the previous sections assumed a single monopole point source, radiating spherical wavefronts, as the primary noise source. It assumes that all points o f the sphere are in phase; therefore the radiation is essentially the same in al l directions (omnidirectional). In Chapter 2 we discussed the radiation pattern generated by aircraft propellers, and how it varies with rotational speed. Since run-up propeller speeds vary from one aircraft to another, we cannot generalize as to which mechanism w i l l govern the run-up noise generated. Therefore, it would be best to perform simulations for al l forms o f radiation from the two propellers: monopole, dipole and quadrupole. In Chapter 3, we explained how these patterns can be modeled. The corresponding radiation patterns were shown in Figures 3.9 and 3.10, for a single source. Note that none o f these resembles the apparent measured directivity pattern o f the Beechcraft 1900D (see Figures 2.4 to 2.7) -in particular with respect to symmetry. The reflections from the ground and/or blast fence 8 0 could be responsible for the asymmetry seen in those noise directivity patterns; in any case this suggests that further information on propeller noise radiation, and how to model it, is required. Since the Beechcraft 1900D is a twin-propeller aircraft, we w i l l use two sources in the simulations, spaced 5.23 m apart, the same spacing as that o f the Beechcraft 1900D propellers. It is quite possible that the two propellers do not spin in perfect synchrony; the resulting noise from the two sources might then be out-of-phase with each other. Let us therefore look at two scenarios for each type o f radiation - in phase and out-of-phase - to simulate the two extreme cases. The resulting directivity patterns are shown in Figure 4.34. Simulations o f a 9-channel system, again with rps=20 m, r, e=40 m and rss=2.0 m, in the freerfield, are shown in Figure 4.35 for monopole sources, Figure 4.36 for dipole sources and Figure 4.37 for quadrupole sources. Observing Figures 4.35 to 4.37, it can be seen that the quiet zones achieved using the different radiation patterns are similar to those using one monopole source. The main differences are in the significant increases in the hot zones outside o f the quiet zones; these reach as high as 50 dB in some cases. Sharp localized peaks are present for all pattern types, mostly on the edges o f the quiet zone. When we studied single monopole sources, we noticed that levels in the hot zones decrease significantly with 61-channel (or more) systems, for which the sizes o f both the quiet zones and the hot zones are optimized. These simulations allow us to observe how the efficiency o f an A N C system deteriorates as the radiation patterns become more complex. This is the result o f poorer wavefront matching between the primary and control sources. A s we move from monopole to dipole to quadrupole sources, more lobes are present with increasingly smaller radii o f curvature; these do not match properly the larger radii o f curvature o f the control-source wavefronts. This is particularly obvious in the case o f the quadrupole simulations, for which a 'ridge' o f sound-level increase occurs along the centerline o f the sound field; again, this could be improved i f more channels were introduced into the A N C system. 81 Figure 4.34. Sound radiation patterns: a) two monopoles in phase (left) and out of phase(right); b) two dipoles in phase(left) and out of phase(right); and c) two quadrupoles in phase(left) and out of phase(right). 82 Figure 4.35. 9-channel attenuation of two monopoles in phase (top), and out-of-phase (bottom). 83 Figure 4.36. 9-channel attenuation of two dipoles in phase (top), and out-of-phase (bottom). 8 4 8000 70 -|60 50 40 30 -120 10 0 -10 Figure 4.37. 9-channel attenuation of two quadrupoles in phase (top), and out-of-phase (bottom). 8 5 - CHAPTER 5 -ACTIVE NOISE CONTROL EXPERIMENTS 5.1 A N C Exper iment Setup Experiments were conducted to demonstrate and study the ability o f an A N C system to attenuate pure tones and run-up noise. A further objective was to compare the results with those from the A N C simulations, in order to validate the simulation model. A n A N C system involving 1 and 3 channels was set up in an anechoic chamber, simulating free-field conditions. The pure-tone results allow us to draw general conclusions as to the amount of attenuation, and the size of quiet zones provided by an A N C system. The run-up noise results determine the number of harmonics of the B P F of the Beech- 1900D that can be attenuated, as well as the amount of attenuation o f each harmonic provided by single- and 3-channel systems. Note that a 3-channel system was the largest that fit in the anechoic chamber. The experimental setup for the 3-channel system is shown in Figure 5.1. Experiments were performed in the fully-anechoic chamber in the Department o f Mechanical Engineering at The University of British Columbia. The chamber measured 4.7.x 4.3 x 2.3 m 3 . Previous tests [40] showed the chamber to be highly anechoic at frequencies above about 200 Hz , with anechoicity decreasing at lower frequencies, and near the chamber surfaces. The primary and control sources were 8-inch A M X low-frequency loudspeakers (or 'woofers'), enclosed in boxes to give monopole radiation. This type of loudspeaker 86 was chosen since, as discussed in previous chapters, A N C is mostly a low-frequency attenuation system and, therefore, adequate low-frequency signals are required. The loudspeaker was connected to a Stanford Research Systems Model SR770 F F T Network Analyzer which generated tones for the pure-tone tests, and a Tascam D A - 3 0 D A T player playing recorded aircraft noise for the run-up tests. The error microphones were Panasonic P9931-ND electret condensers, chosen for their manageability and low-frequency accuracy, connected to custom-made preamplifiers. The primary-source loudspeaker was positioned at one end of the anechoic chamber. Due to the limited space in the anechoic chamber, the reference signal was taken directly from the D A T player; otherwise, the close proximity of the control sources to the primary source would have caused 'bleeding' (the capturing of an unwanted signal) of the control signals into the reference signal. The E Z - A N C controller was connected to a computer running associated software which allowed the control parameters to be set. For the single-channel experiments, the control speaker was positioned 1.4 away from the primary source and the error microphone was positioned 1.1m from the secondary (control) source. These distances were chosen to allow sufficient spacing between the primary, control and error-microphone lines, while leaving enough room behind the error microphones to measure attenuation areas. For the 3-channel experiments, the same values were used for the control-source and error-microphone lines. The three control sources, as wel l as the error microphones, were positioned 0.85 m apart. For both cases, the primary source, control sources and error microphones were positioned at a height o f 1.3 m above the wire-grid floor of the anechoic chamber. A receiver microphone connected to a Larson-Davis model 2800 Real-Time Analyzer was used to measure the resulting sound field at different points in the chamber. A photograph of the 3-channel system is shown in Figure 5.2. A horizontal 0.5 m x 0.5 m grid was set up to define the measurement positions in the horizontal plane; this yielded 10 positions along the main axis of the system and 9 positions along the perpendicular direction, for a total of 90 horizontal-plane measurements. These were all recorded at the height of the system, which was 1.3 m. The same grid spacing was used for the vertical-plane measurements, which yielded 6 x 1 0 = 60 measurements (see Figure 5.3). 87 h + + H 1.4 m 1.1m 1.65 m Figure 5.1. 3-channel ANC setup in anechoic chamber Figure 5.2. 3-channel ANC setup in anechoic chamber. From back to front: primary source, control sources, error microphones and receiver microphone. 8 8 2.0 0.5 m -1.0 -2.0 I 0.5 m| 0 1.0 2.0 3.0 4.0 y(m) 2.0 4 4 • v 0.5 m <= => 0.5 m 0 1.0 2.0 3.0 4.0 y(m) Figure 5.3. Horizontal (top) and vertical (bottom) grids set up for receiver measurements. 5.2 Pure-Tone Attenuation Measurements Before performing the A N C experiments with the run-up noise, the system was tested with 100- and 400-Hz pure tones, for both single- and 3-channel systems. These frequencies were chosen since they are close to the B P F and 3 r d harmonic o f the B P F o f the Beech- 1900D run-up noise. A s was explained in previous chapters, A N C is mainly effective at low frequencies, with the amount of attenuation and the size of the quiet zone 89 increasing as frequency decreases. The purpose of the 100- and 400-Hz tests was to verify the simulation results, as well as to quantify the amount of attenuation provided in both cases. A s we observed in Chapter 4, we would expect to see larger quiet zones for the 100-Hz tone than for the 400-Hz tone. The results in the following sections are presented as contour plots, showing areas of attenuation. The colourbars in the figures refer to the sound-pressure level attenuations, ALp in dB, provided by the A N C system. The experimental results are plotted alongside theoretical results for an identical system simulated in free-field, for comparison. 5.2.1 100-Hz Tone Attenuation Measurements The horizontal- and vertical-plane attenuations of a 100-Hz tone provided by the single-channel system are shown in Figure 5.4. For both planes, the experiment results show a large ALp <-10 dB quiet zone around the error microphone, similar to the theoretical results. Sound level increases of up to 2 dB occur at the y = 0 m to y = 1 m end of the chamber for the experimental results; for the theoretical results the levels are higher and more localized at the primary and secondary source locations. Triangular-shaped quiet zones are not seen for the 100-Hz results, since the control sources are located close to the primary source in relation to the wavelength at 100 Hz . However, this does not constitute global control, since the rps<X/4 condition for global control is not met. The results from the 3-channel system are shown in Figures 5.5. This system provides larger quiet zones, as expected. The horizontal-plane experimental results show increases occurring in the corners of the receiver area; this is most likely due to the anechoicity of the chamber not being fully effective at 100 Hz . A n average increase in sound level of 2 dB again occurs in the vicinity of the primary source for the theoretical results, as opposed to being localized at the primary and secondary sources, as seen in the theoretical results. 90 91 92 5.2.2 400-Hz Tone Attenuation Measurements The horizontal- and vertical-plane attenuations for the single-channel system of a 400-Hz pure tone are shown in Figure 5.6. In comparison with the 100-Hz tone results, the quiet zone for the 400-Hz tone results is smaller, localized around the error microphone. In the experimental results, we are now seeing level increases at the location of the control source for the horizontal plane, similar to the theoretical results. For the vertical-plane experimental results, the increases occur above and below the control source, as opposed to in front of the control source as seen in the theoretical results. The emergence o f a triangular-shaped quiet zone is more apparent for the vertical plane. The attenuation provided by the 3-channel system is shown in Figure 5.7. In the horizontal-plane experimental results we see three separate small quiet zones, as opposed to the one large quiet zone seen in the theoretical results. The optimal rss range for this system at 400 H z is from 0.48 to 0.90 m; the spacing of the control sources in these experiments was closer to the maximum (0.85 m). The results might suggest that a spacing of 0.85 m might be too large, and that Equation 3.26 might require revisions. However, an inaccurate measurement of the spacing, resulting in a value closer to, or over 0.90 m, is a more likely explanation. The sound-level increases outside of the quiet zone are now higher, reaching almost 10 dB near the location of the center control source, as predicted by the theoretical results, although these are still more localized and reach up to 15 dB. These results enable us to see how the quiet zones increase in size as the pure-tone frequency is lowered, and when more control channels are present in the A N C system. It was also seen how the sound level increases outside the quiet zone for higher frequencies, and with more channels. The emergence of triangular-shaped quiet zones seen in the A N C computer simulations was apparent in the 400-Hz pure-tone experiments. These experimental results appear to validate the theory, with the exception of the maximum control-source spacing discussion which, as with any experiment, could be attributed to human error. A smaller receiver-measurement-grid spacing would offer more detail into the results. 93 94 9 5 5.3 R u n - U p Noise Attenuation Measurements Single- and 3-channel A N C experiments were performed using recorded run-up noise as the primary-source signals. The run-up noise was recorded on D A T tapes as part of the run-up noise analysis. The recorded run-up noise from microphone position 1 (73 m away from the aircraft), when the aircraft was at a heading of 075°, was used. The benefit o f performing these experiments in an anechoic chamber was that no additional reflections were added to the primary-source run-up noise, other than those from the ground and blast fence already present in the recordings. A L P F at 500 H z was used, to filter out high frequencies that could not be attenuated by the A N C system and, thus, were possibly subject to increases in sound level. 5.3.1 Single-Channel A N C Experiments The sound-pressure level spectrum at a position 0.15 m behind the error-microphone measured in the single-channel experiment is shown in Figure 5.8. The attenuations provided by the A N C system were: 21 dB atf0, 11 dB at_/} and 7 dB at f2. N o attenuation was obtained at/} (448 Hz) , and harmonics higher than/} were not attenuated, since a L P F at 500 H z was present. A n 8 dB attenuation also occured from 112 H z to 150 H z and from 210 H z to 224 H z ; however a significant increase of 10 to 30 dB was also found below 112 Hz . The horizontal- and vertical-plane contour plots for the single-channel experiments are shown in Figures 5.9(a) to 5.11(a), for the fundamental and first two harmonics of the B P F (f0,fi and f2), respectively. We would expect the results for f0 (112 Hz) to be similar to the theoretical results for the 100-Hz tone, since the two frequencies are very close. Likewise, we would expect the results for f2 (336 Hz) to be similar to the theoretical results for the 400-Hz tone, and the results for// to lie somewhere between the two. ALp < 0 dB attenuation areas were present for all three harmonics. These were mostly located in front of the control source; however the location of these zones became more randomly distributed in the horizontal plane as the frequency increased. A significant 96 o - ^ f c n o ' a - i ^ T - ^ o o - i - ^ c o i - t o c o c M i n c o c M i n l0^,r!P>l^-«00)T-C0<000OC0inN.O<M^-|>-01 n ID " h - a> ^ Frequency (Hz) Figure 5.8. Spectra from single-channel ANC experiment, 0.15 m behind error microphone before control, after control. quiet zone was only present for f0; for// andf2 they were much smaller, localized around the error microphone. In the case of/o, the emergence of a triangular shape was apparent in the horizontal plane. Sound-level increases occurred mostly in the areas near the primary and secondary sources for fo and/}; for f2 they were located throughout the horizontal and vertical planes. 5.3.2 3-Channel A N C Experiments In this section, results from a 3-channel A N C system using run-up noise are presented. In the A N C computer-simulations chapter, the effects on higher and lower harmonics o f the B P F were studied, with the spacing of the control speakers (rss) optimized for a particular frequency. In an attempt to validate these findings, we studied the effect of 3-channel A N C experiments on the fundamental and first two harmonics (f0, fufi) o f the run-up noise. The r „ = 0 . 8 5 m spacing falls in the optimum range for/} and f2, but not for f0 (see Table 5.1). 97 98 99 100 Frequency (Hz) rss-max (m) rss-min (m) fo = H2 2.14 0.92 fi = 224 1.30 0.61 f2 = 336 1.00 0.51 Table 5.1 Optimum range of rssfor a 3-channel system with rps=1.4 m, rse=l.l m. Two receiver positions, at 0.15 m and 1.15 m behind the center error microphone, were chosen to display the spectra resulting from a 3-channel system. The spectrum resulting from the 3-channel experiment at the error-microphone location is shown in Figure 5.12. The attenuations provided by the system are: 7 dB at fQ, 9 dB at// and 0 dB at f2. Again, no attenuation is present for f (448 Hz), and harmonics higher than f were not affected, due to the L P F present at 500 Hz . In addition, the sound level in the frequency range between fo and f was attenuated by about 5dB, whereas the range, from f2 to 500 H z was increased by about 5 dB. Less attenuation was provided than in the single-channel case. This is because the quiet zone tends to be projected further from the error microphones as more control channels are added to the A N C system, as was shown by the A N C computer simulations (Chapter 4). The resulting spectrum at a position 1.15 m behind the center-error microphone is shown in Figure 5.13. For this case, 25 dB of attenuation was obtained at fo and 10 dB at f; higher harmonics were not attenuated. Again, the sound level in the frequency range between fo and / / was attenuated by about 5dB; in the range from f2 to 500 H z the level was increased by about 5 dB. The horizontal- and vertical-plane contour plots are shown in Figures 5.9(b) to 5.11(b). In the horizontal results for fo, three small quiet zones are seen directly behind the two outer error microphones, and a little further behind the center error microphone. Fo r / / , the quiet zones are randomly located; for f2 they are nonexistent. Increases in sound level are present in the vicinity of the control sources and the primary source for 101 iiiiiiiimiiiiiiuiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiui.iiiiiiiiiiiiiiiiii^ o - ' t c n o - ' t i ^ i - ' ^ i - o o i - T r o o ' v - m o o c M m o o c M m t O m t o C s j ' ^ r c o a j T - c o c D o o o c o m r ^ o c M ^ f r ^ o ) CO CO *J t-~ a> ^ Frequency (Hz) Figure 5.12. Spectra from 3-channel ANC experiment, 0.15 m behind error microphone before control, after control. !lllllllllllll!I.IIIIIIINIIIIII!llllllll!Nllldllllllllllllllllllllllllllll^  O ' * 0 ) O ' * I ^ T - ^ t C O ' r - ' « t 0 0 T - l D 0 0 C N j m 0 0 C M m i n ^ f o t N ' < t ( D 0 > T - c 0 C D 0 0 O C 0 m i ^ O C M - > d - h - 0 5 CO CO T - T - T - T - C N C N C N C N C O C O C O C O - ^ - T f ' ^ - ' ^ - T j -Frequency (Hz) Figure 5.13. Spectra from 3-channel ANC experiment, 1.15 m behind error microphone before control, after control. and// ; for f2 they are present throughout the horizontal plane. Triangular quiet zones do not appear for any of the three frequencies. A s previously discussed, the control-source spacing of 0.85 m is in the optimal range for/} and f2; therefore we would expect larger quiet zones at these two frequencies. The area of ALp < 0 dB f o r f is larger than at 102 fo, suggesting that the optimal-range theory is valid, although we would expect larger quiet zones for// and f2, i f we compare with the theoretical results o f the previous section. This, again, leads us to wonder i f the maximum value of rss is a little large, or that run-up noise that contains both tonal and broadband characteristics is more difficult to control than as predicted for pure tones. 5.4 Summary These experimental results enable us to see how an actual A N C system attenuates sound radiated from a loudspeaker. The pure-tone tests displayed how larger quiet zones are present at lower frequencies, and when more control channels are used. Significant attenuations were found for both the 100-Hz and 400-Hz frequencies, at the error-microphone location. The emergence of triangular quiet zones were seen, for the 400-Hz tests. The results of the run-up noise experiments showed that the B P F and first harmonic (fo and fi) could be significantly attenuated - the second harmonic to a lesser extent - in the vicinity of the center error microphone. It was also shown how the control-source spacing of 0.85 m is more optimal for the first harmonic than for the B P F , since it yielded a larger overall ALp < 0 dB quiet zone. 103 - CHAPTER 6 -IMPLEMENTATION OF ACTIVE NOISE CONTROL AT THE VANCOUVER INTERNATIONAL AIRPORT 6.1 Introduction In Chapters 4 and 5, we discussed the application o f A N C to run-up noise, by studying the results from simulations and experiments. When applying A N C to a real^ world noise problem, we must consider several design considerations, such as the A N C system location, equipment and compatibility with different types o f aircraft. This chapter presents a preliminary discussion o f these issues. 6.2 Possible Location of A N C Systems Locating an A N C system at the Vancouver Airport w i l l involve careful planning and design. The Vancouver Airport has strict zoning restrictions that prevent the erection o f structures or the installation o f equipment in certain areas to avoid interfering with navigational and communications equipment (see Appendix F). A brief description o f these are as follows: - avoid electromagnetic interference ( E M N ) with the Instrument Landing System (ILS) (ILS); ensure that E M N radiated by power lines, substations and I S M apparatus do not interfere with the proper reception o f guidance signals in the aircraft approach path. N o objects higher than 1.2 m can be present or no objects protruding above a vertical angle o f 0.8° from the center o f radius o f the ILS can be installed; 104 - limit the height o f structures to less than 5 m in the vicinity o f ramp radar located within a 1000-m radius o f the radar site; - avoid the placement o f structures within a 300-m radius o f the Distance Measuring Equipment ( D M E ) , and structures that extend more than 0.5° vertically from a 600-m radius o f the D M E ; - locate V H F / U H F radio-communication systems as far as possible from the airport radio antennae - in no circumstance should any source o f electrical noise (engine ignitions, electrical switching gear, high-tension lines, etc.) be within 500 m o f the airport radio antennae; - restrict the height o f structures in the vicinity o f the takeoff/approach surfaces - no object o f height above 0.95 ° within 3000 m o f the surface. Most o f these restrictions limit the height o f structures or equipment, either as fixed heights, or as maximum vertical angles from the center o f the restricted location. The most stringent o f the restrictions is that o f the D M E ; no objects can be placed within a 300-m radius o f the D M E , unless by special permission o f Transport Canada and "only where calculations show that the proposed obstructions has no impact on the operation o f the navigational aid". In our simulations, the A N C systems were positioned at a height o f 2 m from the ground, the height o f the propellers o f the Beechcraft 1900D aircraft. Should this height violate certain restrictions, we could position the control sources and error microphones on the ground, since doing so made little difference in the results o f the simulations, relative to positioning them at a height o f 2 m. When selecting a location for an A N C system, we must first address the areas which generate the most run-up noise complaints from residents. In Chapter 1 we mentioned that there are specific locations on the airfield designated for run-ups. Figure 6.1 shows the designated run-up locations at the Vancouver International Airport. Propeller aircraft such as the Beechcraft 1900 Series and deHavilland D H C - 8 normally perform their run-ups at the Button 08 (# 1 and #2) and A i r B C hanger (# 6) locations. We w i l l therefore focus on attenuating noise generated from these two locations. 105 Button Runway OIL 'Lima 1 Buton Runway OS. Alpha' Tax< W/'H2' Intemactton TaxfC' VUda Ana Taxi W w a n Asa AtrBCHangar<AfrcnV) Air Canada Htrga- (Ajjron V) Canadian Airtnas Hangar (Apion VTI) Hsrboir A*r Ap»n VANCOUVER INTERNATIONAL AIRPORT AUTHO RITY D e s i g n a t e d A b o v a ^ d t o R u n - u p L o c a t i o n s ( as o f D e c e m b e r 1 9 9 0 ) Figure 6.1. Run-up locations at the Vancouver International Airport. Based on the distribution o f run-up complaints reported in the 1992 Engine Run-U p Noise Study report [3], and on the results from Chapter 4, we w i l l look at three different scenarios: 1) protecting south Vancouver and north Richmond from run-up noise generated by run-ups at Button 08; 2) protecting north and west Richmond from run-up noise generated by run-ups at Button 08; 3) protecting north and west Richmond from run-up noise generated by run-ups at the A i r B C hangar. Scenario 1 In this scenario we envisage a system which would protect south Vancouver and north Richmond from run-up noise generated by run-ups at Button 08. The required 106 quiet-zone angle 0qz can be found by drawing a line from the westernmost complaint location in the Musqueam lands, to the Button 08 area, and then from there to the southernmost complaint location in the south Vancouver area. This angle is approximately 80°, the quiet-zone angle obtained using a 61-channel A N C system with rps= 20 m, rse = 40 m and r,s = 1.7 m, as seen in Chapter 4. Figure 6.2 shows the expected result o f the implementation o f this system at the Button 08 location. With this configuration, most o f the hot zones are directed over the Pacific Ocean to the west o f the airport, thus avoiding increases in sound energy propagating into other communities. Figure 6.2. Implementation of a 61-channel ANC system at Button 08. Expected change in noise level: CD <-10 dB, LZD -10 to 0 dB, CZ3 0 to +4 dB. 107 Scenario 2 This scenario involves implementing a system to attenuate run-up noise in north and west Richmond generated by run-ups at Button 08. Using the same procedure to find the required quiet-zone angle as for in Scenario 1, we find &qz= 63°. There are several ways o f accomplishing this: we could use a 61-channel system again, but increase rps to reduce 0qz from 80° to 63°; however this would not be the most economic solution. The next approach would be to use the curve plotted in Figure 4.18 (or the associated formula), to determine the number o f channels required i f we have rps= 20 m and rse = 40 m. The required angle corresponds to 35 channels for &qz= 63°. The resulting expected attenuations are shown in Figure 6.3. For this configuration, hot-zone 'strips' alternate with smaller quiet-zone 'strips' (as was seen in Chapter 4); these are denoted by the red areas in Figure 6.3. We can see that the complaint locations in Richmond have been attenuated, but the residents o f south Vancouver might experience a level increase o f as much as 6 dB. Scenario 3 This scenario involves implementing a system at the A i r B C hangar, to attenuate run-up noise propagating into west and north Richmond. This run-up location has the benefit o f being to the south o f several airport buildings, which provide some shielding against noise propagating to south Vancouver. Due to its closer proximity to the complaint locations than the Button 08 run-up location, a greater 0qzof approximately 162° is required, i f we want to protect the Burkeville community immediately east o f the airport. We have seen in Chapter 4 that the maximum quiet-zone angle achieved by a 21-channel system is 169°. We could then use a 21-channel system, distancing it from the primary source to achieve the required angle. However, this would require positioning the control sources 1 m from spinning propeller blades (see Figure 4.17). Therefore, it would be safer (though less economical) to use, say, a 61-channel system, which can be located further from the primary source to achieve the same angle. In order to do this, we 108 Figure 6.3. Implementation of a 35-channel ANC system at Button 08. Expected change in noise level: L Z D <-10 dB, L"Z3 -10 to 0 dB, C Z H -10 to+6dB. can adjust the curve in Figure 4.17 to reflect the results for a 61-channel system. We found that a 61-channel system with rps= 20 m and rse = 40 m yielded a 0q:of 80°. B y shifting the point on the curve at rps= 20 m upwards so that it corresponds to 80° on the y-axis, we get the approximate curve for a 61-channel system (Figure 6.4). Observing this figure, we see that an rps o f 4 m would be required to achieve an angle o f 163°. Figure 6.5 shows the attenuation results for this configuration. Again, the system would create hot-109 zones strips alternating with quiet-zone strips in the red area. Since this system would be relatively close to the residential areas and, therefore, would not benefit from atmospheric absorption, this would be a good situation in which to implement an active noise barrier, which would attenuate the entire noise spectrum. We see that, in scenarios 2 and 3, increases o f up to 6 dB occur in the hot zones. Adding another A N C system to attenuate sound levels in these areas would not be recommended; this would cause hot zones from two systems to overlap, possibly doubling the noise-level increase in certain areas. This problematic increase would have to be looked at carefully; environmental conditions such as wind speed and direction, temperature, relative humidity and cloud coverage would have to be taken into account and a careful evaluation be made as to which residential area would most likely suffer the worst effects o f a particular aircraft run-up at a particular location. A s seen in Chapter 1, the environmental conditions could be studied on a daily basis and used in our favour to avoid sound-level increases in residential areas; for example, the hot zones could be 110 Figure 6.5. Implementation of a 35-channel ANC system at the AirBC hangar. Expected change in noise level: (ZD <-10 dB, LZD -10 to 0 dB, UZ2 -10 to +4 dB. directed upwind (see Figure 1.3) in order to create shadow zones, possibly ' l if t ing' the hot zones to higher altitudes, away from residential areas. This would also mean that the quiet zone would be downwind, possibly projecting it further. Solutions such as these, in conjunction with the Vancouver Airport Authority's noise-management strategies, such as ensuring optimal aircraft orientation during run-ups [3], should result in less complaints from residents about excessive run-up noise. I l l 6.3 A N C Equipment In this section, we w i l l briefly discuss the selection o f A N C equipment for implementation at Vancouver Airport. This equipment includes the controller, microphones, loudspeakers and cables. A N C Controller A s was explained in Chapter 1, the A N C controller is the heart o f an A N C system. It consists o f a powerful digital-signal processor that detects an mcoming signal, performs a phase inversion on selected frequencies, and sends the resulting signals to the control loudspeakers. Currently, 6-chanriel controllers are available for R & D purposes; they can be connected in parallel to obtain more channels. It is not known whether controllers which incorporate significantly more channels or more are available or possible, but with digital-signal-processing technology advancing at an exponential rate, they may be available in the near future. A computer may be required, to run software with which the control parameters are set. A 386SX computer is adequate for a 6-channel control system; however a system with more channels might require a computer with a faster processing speed. Both the controller and computer should be located indoors, or should at least be properly protected against the elements. However, many A N C controllers used in practical applications come as 'stand-alone' systems which do not require an external computer. Microphones The microphones o f an A N C system have two functions: to provide the reference signal and the error signals. The reference microphone does not have to be o f the same make and model as the error microphones; however the error microphones should be identical. The reference microphone should be rugged enough to withstand the elements, as well as strong wind blasts from aircraft engines. Protection can be achieved by the use o f a proper windscreen, and by ensuring that the aircraft orientation which produces the strongest wind blast is not pointed directly at the microphone. The reference microphone should also be able to reproduce low-frequency signals accurately. 112 Electret condenser microphones were used as the error microphones for the A N C experiments described in Chapter 5. Though accurate and manageable, these are not microphones that are built for outdoor use; i f they were to be used outdoors, proper shielding from the elements would be required. More rugged outdoor microphones can be used, provided that they are all o f the same make and model and that they have sufficient low-frequency accuracy. Stands would be required i f these were to be placed at the height o f an aircraft engine, although the simulation results have shown that they can also be placed on the ground, along with the control sources. Loudspeakers The loudspeakers used as the control sources should also be able to withstand outdoor conditions. These should be low-frequency loudspeakers ('woofers'), although full-frequency-range loudspeakers could also be used. The loudspeakers do not have to be excessively powerful; a power output o f 40 watts per loudspeaker may be sufficient. Higher power outputs might be required i f the control sources are positioned very close to the primary source. Adequate amplifiers, able to produce 40 W (or more i f necessary) per channel, would also be required. These should be properly protected against the elements as well , perhaps along with the computer and the controller. Stands would be required i f the control loudspeakers were to be placed above ground although, as with the error microphones, the control loudspeakers could be placed on the ground. Cabling Proper cabling for the microphones and speakers should be provided, and steps should be taken to ensure that they do not obstruct access routes for aircraft, vehicles and airport personnel. Running the cables underground would be the best solution. Wireless systems are available on the market, but these would not be recommended, as they most likely would violate the restrictions with respect to electromagnetic ( E M F ) compatibility outlined at the beginning o f this chapter. 113 6.3 Implementation of A N C for Different Aircraft This thesis presented a particular example o f applying A N C technology to the reduction o f run-up noise from the Beechcraft 1900D aircraft. The coherence analysis o f Chapter 2 indicated that noise from this aircraft is sufficiently coherent to be controlled by A N C . The general findings o f this thesis can be applied to any propeller aircraft, such as the deHavilland D H C - 8 aircraft. The B P F w i l l vary from one aircraft to another; for example, it is around 70 H z for the D H C - 8 . However, the spacings o f the A N C components would have to be reviewed accordingly. The B P F o f propeller aircraft in use at Vancouver Airport would have to be measured, and an A N C system would have to be designed to accommodate al l o f these, or at least the ones which perform the most run-ups and generate the most complaints from nearby residents. Since the algorithm used by the controller is adaptive, the A N C system is able to adjust itself to attenuate a wide range o f B P F ' s , provided that the control-source spacing (rss) is within the optimal range of the B P F ' s for the aircraft in question. It can also adapt to the changes in noise radiation that occur as the propellers accelerate up to, and decelerate down from, full-power run-up speed. One important issue that would need to be addressed is the repeatability o f the positioning o f aircraft during run-ups. It has been shown that i f the primary source shifts away from the central axis o f the system, the control system can still create a quiet zone i f the primary source location shift Ax does not exceed the half width o f the control sources array (|zlx| <(N- l)ra / 2 ). A s Ax increases, the optimal spacing range (r M ) becomes narrower, and the quiet zone becomes smaller, shifting to the opposite direction with respect to the source location shift [16]. The application o f A N C to broadband jet-engine noise is also possible and, in fact, is currently being researched for use as a global control strategy in engine-nacelle designs by N A S A [4]. Loca l control o f jet-engine noise using the systems discussed in this thesis is also possible by making adjustments to the control parameters, but A N C generally does not provide as much attenuation for broadband noise as it does for tonal noise, and might generate greater increases in the hot zones. The local control o f jet-engine noise is clearly an area that requires further research and development. 114 CHAPTER 7 -CONCLUSION 7.1 Accomplishments The purpose of this research was to study the feasibility of using active noise control to reduce run-up noise generated at Vancouver Airport, and affecting noise-sensitive communities. We began by studying annual noise-management reports produced by the Vancouver Airport Authority, and a study of run-up noise at Vancouver Airport performed by an acoustical consultant in 1992. This literature led us to target propeller-aircraft noise, partly because of its tonal nature, but mostly because the deHavilland D H C - 8 aircraft has remained at, or near the top of, the list o f complaints by aircraft type over the past few years. A review o f available literature on the topic of active noise control, focussing on published work on local control vs. global control, applications to tonal noise, and applications to aircraft noise, was done. A n important discovery was that, in order for global control to be achieved, it is necessary to place control sources within X/4 o f the primary noise source, which is not recommended for propeller noise for safety reasons. Our literature review objective was, therefore, narrowed down to finding published work on local control. It was discovered that most research and applications of active noise control used global control, and that local-control applications were limited to a few experiments, only one of which involved aircraft noise [12]. Applications to aircraft noise - more specifically to propeller-aircraft noise - are limited to reducing interior cabin noise 115 using global control. There was, therefore, a void to be filled with respect to local control of aircraft noise, underlining the importance of this research. The next step was to perform noise measurements on a propeller aircraft, in order to study its spectral and radiation characteristics and their suitability for active control. A Beechcraft 1900D twin-engined propeller aircraft was provided by Central Mountain A i r for the measurements. Four microphone positions were used. Two recorded the near-field noise from the aircraft, allowing noise directivity patterns and a coherence analysis to be performed, another recorded the run-up noise heard in a community located 3 km from the airport, and a fourth microphone measured the apparent insertion loss of an existing blast fence near the run-up area. The resulting spectra from these recordings showed that the Beechcraft 1900D has its blade-passing frequency at 112 H z and that this frequency, as wel l as the first two harmonics, represented dominant levels in the spectra measured in the community. Noise directivity patterns were generated using these spectra; it was found that the noise radiated in irregular, asymmetrical directional patterns. A coherence analysis was performed; it suggested that the run-up noise would be coherent enough to be controlled by an A N C system. A N C simulation models were developed using algorithms based on the filtered-X least-mean-squared feedforward theory [16,31] commonly used in many A N C applications. Both free-field and half-space conditions were considered, simulating the two extremes of ground reflectivity. The application of A N C to a barrier in order to supplement its insertion loss at low frequencies, thus creating an active noise barrier, was also modeled. Modeling of different source-radiation patterns was also discussed. Computer simulations were performed to predict the attenuation provided by single- and multi-channel A N C systems for possible implementation at Vancouver Airport. It was found that 10 dB or more of sound-level attenuation can be provided over areas (called 'quiet zones') large enough to cover the complaint receiver locations. The quiet zones, however, are created at the expense of sound-level increases in areas outside the quiet zones, called 'hot zones'. It was found that a properly designed system can limit the majority of the sound-level increases to localized areas at the locations of the control sources. 116 When using a multi-channel system, the spacing of the control sources must be within an optimal range; a value close to the range minimum yielded the best simulation results. The sizes of, and levels in, the hot zones decrease as more control channels are added to the A N C system, which also has the beneficial result of increasing the size of the quiet zone. It was shown that as many as four peaks (the fundamental and first three harmonics) of the run-up noise can be attenuated for a given configuration, resulting in a total attenuation of 15 d B A in a community 3 km away from the airport. Act ive noise barriers ( A N B ) , using the same configuration as an A N C system without barriers, yielded very similar sizes of quiet zones; the advantage of A N B ' s over regular A N C systems is that A N B ' s also provide attenuation at mid and high frequencies. Multi-pole sources, simulating radiation patterns more similar to those of actual propellers, were found to create more complex quiet zones, in which 'ridges' and peaks of sound-level increases occur. A N C experiments using single- and 3-channel systems were performed, in an attempt to validate the A N C simulations. It was found that experiments using pure tones matched theoretical simulations of the same system quite accurately. Experiments using recorded run-up noise demonstrated that the first two peaks (fundamental and first harmonic) could be attenuated by 25 dB and 10 dB, respectively. Implementation of A N C at Vancouver Airport would involve a number of practical considerations: identifying the run-up site and aircraft from which most complaints are generated, selecting the number of channels for the system, as wel l as the proximity o f the control sources to the primary source required to achieve a required quiet-zone angle, and selecting equipment that would withstand adverse weather conditions. Making use of weather data could help in the orientation of a quiet zone towards a particular area. 7.2 Future Work Further research is required before a final installation of an active noise system is realized at Vancouver Airport. A greater understanding of aircraft noise radiation would enable more accurate radiation models to be produced, and their effect on A N C system design and performance to be studied. The incorporation of environmental conditions into 117 the simulation models would help in predicting its effect on outdoor sound propagation and the effectiveness of A N C systems. This would, of course, require modeling of the effect o f environmental conditions on sound propagation with phase. More detailed experiments, in anechoic and outdoor environments, would help in generating and validating these models. O f course, A N C experiments at the airport, at first using a loudspeaker to replace the aircraft and with receiver microphones in different communities, would be best, simulating an actual case. The greatest improvement to the local-control scenarios would be to further minimize the hot zones, above and beyond the methods proposed in this thesis. Wi th digital-signal-processing technology advancing at an exponential rate, we can expect significant improvements to controller design in the future; these may involve multiple A N C systems to be used without overlapping the hot zones. Perhaps more directionally radiating loudspeakers (as opposed to radiating omnidirectionally) to limit sound projection into unwanted areas, could also remedy the hot-zone problem. Finally, more research into developing A N C systems that would attenuate noise from both propeller-and jet-engine aircraft while limiting hot zones locally at the run-up site would be ideal, since this would benefit not only Vancouver Airport, but all airports worldwide. 118 B I B L I O G R A P H Y 1. " Y V R Aeronautical Noise Management - 1997 Annual Report", Vancouver International Airport Authority. 2. Hubbard, H . H . , "Aeroacoustics o f Flight Vehicles- Theory and Practice", V o l 1, 1995. 3. 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Principles of optics, Pergamon Press, London. 39. Bowman J.J., Senior T . B . A . , and Uslenghi P . L . E . (1969). Electromagnetic and acoustic scattering by simple shapes, North-Holland, Amsterdam. 40. Busch, T. (1997). Scale-Model Investigation of Highway Traffic Noise Barriers, M . A . S c . Dissertation, University of British Columbia. 121 A P P E N D I X A Spectra Measured at Position 1 1/3 Octave Band Spectrum •1- z -t : : _ _ : { : : - : - - -r T -r -r T i [ ft - - - - - - - - - - - - - - - - - - - - - - - ~ I ~ \~ -- -- 1 - -! : - - : --1 - 1 --~ ~ F t --m t o o i n t n o o c o o o u j o o o m o o o o o o o o o o o o o o o o o o t ' S i p j r - c M C ^ ^ T j - m c o c o o r M c n o u ^ ^ - o o c o o o m o o o L O O o o o o o o o > ~* V- M ' - ' - • ^ C N C N C T ) - » t u n c D C 0 O C S i C D O i r ) T - O O C 0 O O i r ) O O J - 0 T- T- 1- CM Frequency (Hz) Linear jA-weighted | 0 - 1250 Hz FFTSpectrum 45 -, Frequency (Hz) Position: 1 Heading: Ambient Noise 123 1/3 Octave Band Spectrum 100 T 90 80 Frequency (Hz) | a Linear m A-weighted | 0-1250 Hz FFT Spectrum 70 Frequency (Hz) Position: 1 Heading: 075° Engine Noise at Idling 124 120 1/3 Octave Band Spectrum 100 r- eg Frequency (Hz) | B Linear jA-weighted | 0 - 1250 Hz FFT Spectrum 100 90 20 10 c o r t c o c M t o i - m o - ^ - c o — • o c o r - O T r r - O T j - h - r i n t o r i n a j N i f l o o j i n o i N i D o i n m u ) i O ( D ( D C 0 h - h - h - C 0 C 0 0 0 0 ) a ) O ) O CO O Tf CJ) CO CO CO O CO CD O CO O t- ^ N N Frequency (Hz) Position: 1 Heading: 075° 125 1/3 Octave Band Spectrum 100 • co S 60 Ibi rt-n CM N n if) (£> 00 O OJ CD T - - T - ^ c N C N r O T f u n t D c o i- i- i- CM Frequency (Hz) [ H Linear b A-weighted | 0-1250 Hz FFT Spectrum 120 100 w - W - w - W - W - w w \ < ffi 2. 60 o o c 3 c o c s j t D ^ m o j T r c o c o r ^ T - ( o o T f a ) c o < j o c N ( O T - i f ) o ) T t ( 0 ( n t ^ T - < o - c o c o r ^ O T j - r ^ o ^ r ^ ^ ^ - o o ^ m o o ^ m c D C M U i o j c s i u i a j c g t D O T c o t o Frequency (Hz) Position: 1 Heading: 105° 126 1/3 Octave Band Spectrum 100 80 IT) tO CO Ul <0 00 < Frequency (Hz) [^Linear aA-weightedj 0 - 1250 Hz FFT Spectrum CO CO CO ° 00 CM tO CO r- O -<---- CM CM CM CO S O) (O CO U) (D LO to to to Frequency (Hz) N N CO CO Ol CM CD Ol CO 0 ) 0 ) 0 ) 0 CD CO CO Position: 1 Heading: 135° 127 120 1/3 Octave Band Spectrum 100 80 40 20 ? 5 in co oo < Frequency (Hz) p Linear m A-weighted | 0-1250 Hz FFT Spectrum 190 . 20 *f CO CO CO CM CD CO CO f- T-5 o i. 1. to co CM CM CM CO c D o ^ o c o c o c M i O T - m m - ^ - c o c o r - i - i D k n i D U ) c D ( O c D r - r » r ^ > c o c o c o o ) 0 > c n o o Frequency (Hz) Position:! Heading: 165° 128 120 1/3 Octave Band Spectrum 10(1 80 40 20 I O ( D ( O O C M C D O W T -c N t N c O T j - i n i D e o o c j c o o i n T - o o c o o o i n o o ^ i -i - T - T - c s | C M M ^ m i O C O O C M ( D O < * > Frequency (Hz) I Ei Linear f|A-weighted 0-1250 Hz FFT Spectrum 100 - i Position: 1 Heading: 195° 129 1/3 Octave Band Spectrum 120 - i : 100 Frequency (Hz) | g Linear m A-weighted | 0 - 1250 Hz FFT Spectrum 100 Frequency (Hz) Position: 1 Heading: 225° 130 120 1/3 Octave Band Spectrum 100 80 40 20 in co co o o o o o o o o o o o o o o o o o o - e m o o c o o o c n o o o i o o o o o o o o o ? * * ^ • m c o c o o c N c D o i n ^ o o c o o o m o o > o T - ^ - ^ - t N C M C O ^ l O C O C O O O J C O O ^ t — t- r- t- CM Frequency (Hz) H Linear • A-weighted 0 - 1250 Hz FFT Spectrum 120 . ::::: I vyyv/v 40 Position: 1 Heading: 255° CM CM CM CO C D O ^ C D C O C O C M C O i - m c o T - m c o t N L O l o w i n c o c o c o r ^ h -Frequency (Hz) T - t n c ^ T r c o c o r - ~ T -c n c M i n m c M c o c n c o N ( D ( D O D C f l O ) 0 ) 0 Tf O) P ) (D CO CO O CO r r W N 131 120 1/3 Octave Band Spectrum 100 80 40 20 o o i n o o o o o o o o o o o o o o o o o o - g m o i n * - o o < o o o m o o o i D o o o o o o o o ? » 5 c M o i r t T t w t o c o o r j i D o m T - o o p j o o m o o J - o . - T - T - O J C S J C O - ^ - W C O C O O C M U J O * 1 ! — T~ T- C M Frequency (Hz) Id Linear •A-weighted 0 - 1250 Hz FFT Spectrum 100 a. in m co oo 30 c g c \ i ( D T - i n c n ^ r o o o o K T - c o o 5 c n r t o o c M i D ^ i o c n ^ a 3 c o h - ^ < o o ^ c D c o o o e o r ^ o ^ i ^ o * 5 r ^ T - ^ - o o ^ u l o o ^ i n o o c s i i o o » c j t n c n c g c o c n c o t o o e O ( D O e o r r ( N N N n n n v t ^ B i n m O I D f O N S N O O ) 0 ( D ( l l D 1 0 0 r r r N P I Frequency (Hz) Position: 1 Heading: 285° 132 120 1/3 Octave Band Spectrum 100 80 40 20 10 Frequency (Hz) Position: 1 Heading: 315° 133 120 1/3 Octave Band Spectrum 20 O O CO o i f in to co CM CM CO CM CM CO lO CO CO Frequency (Hz) ra Linear • A-weighted 0-1250 Hz FFT Spectrum 100 T r ' o o c o o o c M < O T - i i > m ^ - o o ( O t ^ ^ t o o ^ O ) c o c o c N « ) ^ i n c n ^ - c o c o r ^ ^ - ( O o ^ r c D < 0 ( 0 ^ ^ o r t f ^ o ^ r ^ o ^ t ^ T - T r { D * - i n o O T - i n c o c j u i o i r g i n o j c \ i ( o c n c o ( O o e o t D O < o Frequency (Hz) Position: 1 Heading: 015° 134 120 1/3 Octave Band Spectrum 100 . 80 40 20 10 CO CO CD CM CO j • " s P7 CO CM CM CM CO CO LO c n c o c o c M c o * - m a > ^ - c o c o r ~ ! - c o — L o c o c g c o c n c M L O c n c M c o m c o c o CO LO CO CO CO CO Frequency (Hz) CO CO CO O Oi o> r t- r P| N Position: 1 Heading: 045° 135 A P P E N D I X B Spectra Measured at Position 2 136 90 1/3 Octave Band Spectrum 70 . _ 50 . m a. <n 40 20 CM CM CO m (o co 1 I CM CM CO m co oo Frequency (Hz) | • Linear m A-weighted | 0 - 1250 Hz FFT Spectrum Tf ID CO CD CM CM CM CO U 3 0 T f m c o c o c M c o ^ L n c n T i - c o r o N - ' « - c D O T f ^ i n c o ^ u i c o c g i n c n c M i n o i c M t o o i c o c o o c o W U l i n c D C D < D r ^ h - h - C O C O C O C n O ) 0 1 0 0 ' « - ' « -Frequency (Hz) Position: 2 Heading: Ambient Noise 137 1/3 Octave Band Spectrum 90 80 Frequency (Hz) | @ Linear m A-weighted | 0 - 1250 Hz FFT Spectrum 70 , 60 10 0 I _ Frequency (Hz) Position: 2 Heading: Engine Noise at 075° 138 120 1/3 Octave Band Spectrum 100 80 40 20 < CO <0 40 30 10 K ^ T - T - c M o j c v j t o r t c O ' d ' T r - v i o i o i n t o t D t D t ^ r ^ r - . o o c o a i o i c D c n o o Frequency (Hz) Position: 2 Heading: 075° 139 120 1/3 Octave Band Spectrum 20 m to co ui o o o m CM to o m <r- CM CM CO tn to co < Frequency (Hz) ^Linear m A-weighted ] 0 - 1250 Hz FFT Spectrum 100 a > c o e o c M i O T - L O O i - M - o o < o r ^ T - ( D c - T - L O a 3 C N i n c n c g L O c o C M t D a ) < o c o c - . L O L O i n c o c o c o r ^ i ^ - r ^ c o o o c o c n c n c n o o ^ T -oi co oo Frequency (Hz) Position: 2 Heading: 105° 140 120 1/3 Octave Band Spectrum 100 80 40 20 30 c o c N ( O T - i o o i ^ r c x ) < o i ^ ^ ( O o ^ - C D c o o o c g t D ^ i n c n ^ ' O o c 3 r » - - c o o - * T - T - t s i c M C M M c o r t ^ ^ ^ r i n i n i n c D c o c o r ^ r ^ K c o o o c o c D c n c D o o * - ^ Frequency (Hz) Position: 2 Heading: 135° 141 120 1/3 Octave Band Spectrum 80 m S 60 Q. in 40 20 0 [ «•, HI t If I, I'M, (.'•, Bl , E l II l-l | PI | M | I'l t II 11 11 11 11 11 11 II 11 11 11 11 11 11 II h i 11 11 11 11 11 H li m i D o m m o o c o o o i n o o o i n o o o o o o o o o o o o o o o o o o - C m ^ ^ O J O J - ^ i O U J C O O C N C D O U l T - O O M O O i n O O O i n O O O O O O O O > * ' lj — ^ ^ ^ C M C M C O ^ W C O C O O C M C O O U ^ ^ O O C O O O I O O O J - O " T - ^ - T - C N C N C O T r i O C O C O O C M C O O * * - ! -n - CM Frequency (Hz) |~E3 Linear a A-weighted~j 0-1250 Hz FFT Spectrum 100 , Frequency (Hz) Position: 2 Heading: 165° 142 1/3 Octave Band Spectrum 120 -. 20 10 o i ., ,. " , " " „ : : : : , : : T : : : , . : r : ~ , : , ~ ~ ~ : ^ m o n ^ 0 ^ ^ ° ^ ^ ^ ^ ° O ^ U 1 0 0 T - U 1 0 0 C N U ) 0 1 C ^ U > C n C « J C D C f ) C O t D O C O t O O < 0 Frequency (Hz) Position: 2 Heading: 195° 143 120 1/3 Octave Band Spectrum 100 IE i I o o m CM CM co in co co Frequency (Hz) 3 Linear • A-weighted 0 - 1250 Hz FFT Spectrum 100 5 » ¥ V-CO CD CM CM CM CO O C O C O C M t D T - m o ^ t C O C O r ^ ' r -T - m c O C M l O O l C M i n O C M C O O T C O ~ ^ h « . C O C O C O O ) 0 1 0 1 0 CD S CO T- CM CM Frequency (Hz) Position: 2 Heading: 225° 144 120 1/3 Octave Band Spectrum 100 80 40 20 m to co o o o o o o o t ; " m o o o o o o o >•*• ! o < o o o m o o > o Frequency (Hz) | m Linear m A-weighted | 0 - 1250 Hz FFT Spectrum < D c o o o c M c o * - m c o ' » r a c o i ^ * - ( D 0 3 O T M » c j ( O T - m c n * t » r o r ^ T - ( o o - * c > o o c o « o c o r - o ^ r ^ 0 5 T ^ T - T r c o * - i n o o * - i n o o c > i i n c n c N j i n m c \ i c o O T < o i o o r t t o o « o S r r r W N P i n r t P ) t t * « l M l « ) U l ( 0 ( D S S S O ) ( 0 ( 0 0 1 0 ) ( D O O i - r p N N Frequency (Hz) Position: 2 Heading: 255° 145 120 1/3 Octave Band Spectrum 100 1 H-1L 80 40 20 O TJ- CO CO CO CM CO T- eg CM o i co C D O T t C O C O C O C M C D ^ m O J T j - C O C O h - T -T - W C O ' - i n c O C M l O m C M W C D C M C D c n M m i n L n c o c o c D t ^ K i ^ c o o o c o c n c n c n o Frequency (Hz) Position: 2 Heading: 285° 146 120 1/3 Octave Band Spectrum : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : i : i : 100 ; ) _ 80 40 20 20 10 O O C O O O N C D ^ L O C f t T f ( D C O r ^ T - C D O ^ m c O C O C N l ( D T - u ^ O > ^ C O C O r ^ ^ C D 0 ^ 0 1 C O O O m O C O f ^ O T T r ^ 0 ^ r - ^ ^ 0 3 ^ l 0 0 3 r - L O C O C \ J U 1 0 ) C \ l l O O T C > J O O C n C O t D O C O t O O r O Frequency (Hz) Position: 2 Heading: 315° 147 1/3 Octave Band Spectrum 120 - i 100 80 m 40 20 i n c o o i o u ) o o c o o o m o o o i n o o o o o o o o o o o o o o o o o o " e " m ^ T - c s i r j - - * i - o t o c o o c N i o o i n T - o o c o o o i o o o o i n o o o o o o o o S - w V- en ^ T - T - C N C M C O T f W C D O O O C N I t D O i n T - O O C O O O l O O O j - O w ^ - i - ' - C N C M C O T j - L O C D O O O C N C O O ^ r — T" T- T- OJ Frequency (Hz) [ E Linear a A-weighted~| 0 - 1250 Hz FFT Spectrum 90 Frequency (Hz) Position: 2 Heading: 015° 148 120 1/3 Octave Band Spectrum 100 80 m 5 60 a. 40 20 0 | « , H ,!>•,! I l ^ l l l l l j l l l l l l l j l l l l l l l l l l l l l l l l i r i l l l l l l l i r l l l l l l l t l H m c o o u i i f ) o o c o o o m o o o m o o o o o o o o o o o o o o o o o o " C " m ^ T - C M N - T t W t D C O O C N C D O L O ' i - O O C O O O l O O O O i n o O O O O O O o S i a l j rr\ ^ ^ ^ C N C \ J C 0 ^ U ) ( D C 0 O 0 J C D O i n T - O O C 0 O O i n o O - < . 0 * ' ^ - i - ^ - C M C N C O T r i n t O C O O C M C D O * 1 ! — T~ T- T~ CM Frequency (Hz) m Linear a A-weighted 0 - 1250 Hz FFT Spectrum 100 90 Frequency (Hz) Position: 2 Heading: 045° 149 A P P E N D I X C Spectra Measured at Position 3 90 1/3 Octave Band Spectrum 80 60 _ 50 CO 30 20 10 Frequency (Hz) | a Linear m A-weighted [ 0-1250 Hz FFT Spectrum 40 15 5 CO CO CM CO CM CM CM CO S ' - i o o t o i n c o i N i D ' - i n o i t c o n s ' - c D T t c o T - i o c o ' » - i n c o c M i n o ) C M i n c n c M ( D a ) c o c o T r ^ f m i O L O c o c o c o r - r ^ r ^ c o c o c o o o a i o o TJ- rj) CO CD CO CO O CO T- T- CM CM Frequency (Hz) Position: 3 Heading: Ambient Noise 151 1/3 Octave Band Spectrum 80 40 30 . raLinear •A-weighted 0-1250 Hz FFT Spectrum 35 Frequency (Hz) Position: 3 Heading: 315° 152 90 1/3 Octave Band Spectrum 80 I 70 i » . : " : : = ^ r ^ J | £ j f i j 1 - " fl" so - n _ _ _ ; _ , _ > _ ; 1-1- - - - - -fk-rt • • | L | - - - - - J ' l i l l - - r • " l i l l l I t FL-lr 0- t| : : P | B Linear a A-weighted | 0-1250 Hz FFT Spectrum 60 -i 50 o4 o ^ c o c o c o c N C D T - L o m ^ c o c o r ^ T - c o o ^ m c o c o c M t o ^ u i a > ^ c o c o ^ ^ ( o o ^ O ) r o c o Frequency (Hz) Position: 3 Heading: 015° 153 1/3 Octave Band Spectrum 80 70 50 40 30 20 nritii • Linear • A-weighted 0-1250 Hz FFT Spectrum Frequency (Hz) Position: 3 Heading: 045° 154 A P P E N D I X D Spectra Measured at Position 4 155 1/3 Octave Band Spectrum 90 80 Frequency (Hz) 10] Linear m A-weighted~| 0-1250 Hz FFT Spectrum 40 10 ^ m O W r ^ O ^ I ^ O ^ f ^ i - T t O O ^ L O O O ^ L O C O C N I U I C O C N i m O T C J t D O J C O t O O C O t O O t O Frequency (Hz) Position: 4 Heading: Ambient Noise 156 100 1/3 Octave Band Spectrum 90 80 70 m 2- 50-30 20 10 O O L O O O O O O O O O O O O O O O O O O O - g m o w * - o o c o o o m o o o i n o o o o o o o o ? • » O J C M C O - t f W l D r a O C M t D O m T - O O C O O O L O O O i - O T - T - T - C M C M r O ^ - L O C D C O O C M C D O ^ f — T- T- T- C\t Frequency (Hz) S Linear • A-weighted 0-1250 Hz FFT Spectrum 70 c o c o c o t s i t o ^ u i m ^ c o e o K ^ t o o ^ m c o c o c j c o T - t o c n ^ - c o c o f ^ T - t o o ^ - C D Position: 4 Heading: Engine Noise at 075° Frequency (Hz) 157 1/3 Octave Band Spectrum 80 40 ii -ftH tn to co < H Frequency (Hz) B Linear • A-weighted 0 - 1250 Hz FFT Spectrum CO CO CO CM to CM CM CM CO O i n O N I D ' - l O O l t l O O S ._ > - l f l ( D ( N i r ) 0 ) M L O O l N t 0 0 1 i n i n i n t o c o t o r - - h - h - e o c o c o o ) c n c n f- to o cn co co co to o co to o co O O T- CM CM Frequency (Hz) Position: 4 Heading: 075° 158 120 1/3 Octave Band Spectrum 100 80 40 20 i n c o c o o c M t D o i o * -D O O O O O O O O O O O O O O O O - g r a — O C O O O L O O O O L O O O O O O O O O ? * ' * - C \ I C M C O ^ L O t O C O O C v | C D O U l * - O O C O O O C O O O J - 0 ^ T - T - C M C M C O ^ I - i O C O C O O C M t O O * ^ ! — Frequency (Hz) a Linear •A-weighted 0 - 1250 Hz FFT Spectrum 10 0 U - - , - — . . . u . ^ . . . ^ ^ o ^ c o c o o o c N t D T - u i o i T j - c o c o r ^ T - o o o ^ c n c o c o c M c o r - L o c n ^ c o c o ^ T - t o o ^ c n r o c o J g r ^ r N N N n n n T f l T U i n i n i D I D I O S N N O O O f f l O l O I O O ' - ' - r N N Frequency (Hz) Position: 4 Heading: 105° 159 1/3 Octave Band Spectrum 120 It -fl-i t "r Itt 20 ? 1 < .2 Frequency (Hz) | m Linear a A-weighted | 0 - 1250 Hz FFT Spectrum 80 < CO S 40 20 CO CO CO CM CO 0) CO CO CN CO CO CO I*. T- CO T- T- CM CM CM CO LO LO LO C O C M L O C n C M L O O l C M C O C n C O C O O) O) O) O O T~ Frequency (Hz) Position: 4 Heading: 135° 160 1/3 Octave Band Spectrum 120 100 -It r f b f I t ' IT) CO CO T- CM CM CO O CO O O IT) in I D co r - CM CM CO U) CO CO O CM CD I 3 < h-Frequency (Hz) • Linear • A-weighted 0 - 1250 Hz FFT Spectrum 60 < 50 CO <0 40 10 CO CO CM CD CM CM CM CO t o o ^ c n c o c o c M C D I T - i n c n T f c o c o r - - ^ - t D O , * ' O J c o c o T - u i t o i - i n « c M u i o ) t N i n o ) C M < D a ) c o t o o ( O i D o c o i o i n u i t D t D t o r ^ r ^ r ^ c o c o c o o ) 0 ) c n o o T - T - ' » - c M C M Frequency (Hz) Position: 4 Heading: 165° 161 1/3 Octave Band Spectrum 100 CM CO o o o O O LO CM CM CO LO CO CO T- T- T- CM Frequency (Hz) [jJLinear m A-weighted] 0 - 1250 Hz FFT Spectrum Frequency (Hz) Position: 4 Heading: 195° 162 100 1/3 Octave Band Spectrum CO O O LO < I-Frequency (Hz) | rg Linear a A-weighted | 0-1250 Hz FFT Spectrum T f r C O C O C O C N l C O T - l f l O ) $ « > - r - , - , - C N J C N C N 4 C O T - CD O ^ O) CO CO co T- m co ^- m co TJ- m m in co co co ( O T - i n o j ^ c o c o r ^ T - c D w c n c M w c n c M c o a > c o c o N - h - c o c o c o c n o o o o cn co co co to O CO r r N IN Frequency (Hz) Position: 4 Heading: 225° 163 1/3 Octave Band Spectrum 100 70 60 ST B 5 0 a. in 40 30 CM Frequency (Hz) rj Linear • A-weighted 0 - 1250 Hz FFT Spectrum 60 < m 20 CO CO CM to in CD •* co co CM CM CM CO < 0 0 * * O C O C O C M C Q T - W C n T t - C O C O J ^ * - < O O T f * - i n c o * - m c o c M m c n ( M i o o > C M C D C o c o c o © c o m i n i n c D t D t o r - r ^ - i ^ c o c o c o c n c D c n o o T - T -cn co co Frequency (Hz) Position: 4 Heading: 255° 164 120 1/3 Octave Band Spectrum 100 M m Si? 40 I o c o o o m o o o L O O o o o o o o o o o m t o a s o c M t o o m t - o o c o o o i n o o o u i T - T - T - C N C M C O t m C D C O O C M t D O m ' r -r r N N (I Frequency (Hz) JO] Linear a A-weighted | 0-1250 Hz FFT Spectrum LO CD CO 1 I 10 O T r o 3 c o < D C N i u 3 ^ i n O T ^ c o c o t ^ ^ t D o ^ o i p o c o c M t O T - i n m ^ c o c o r ^ . ^ t o o ^ c D c o c o rtST'1"T'NrJwn"n^*flinwini0(D10NSS00no<n0J<,)OOT"1"T"NN Frequency (Hz) Position: 4 Heading: 285° 165 120 1/3 Octave Band Spectrum 100 n FU — - " -r - - - - - - - - - - - - -i \ - -- -- - - --- - --- - --- ---- -----12.5 D C - C 3 U si C t M 31.5 o -* o LO CO CO o CO o m O CM o 0 o o CM o LO CM 0 o o o s •o 800 1 000 L o LO N 0091. O o o CM 2500 " o 0 CO 4000 o o o o o CO ID 0009 10000 CD O LO \ o o o D 20000 " A-wt " Total " Frequency (Hz) '•Linear » A-weighted | 0 - 1250 Hz FFT Spectrum 80 Frequency (Hz) Position: 4 Heading: 315° 166 120 1/3 Octave Band Spectrum 100 80 40 20 in co co C M C N C O ^ l ' m c D C O O C M C D o m i - O O C O O O W O O - i -T - T - T - c M C M c o ^ r m c D c o o c N j c o o * * ' i- T- CM Frequency (Hz) ©Linear • A-weighted 0 - 1250 Hz FFT Spectrum 90 80 Frequency (Hz) Position: 4 Heading: 345° 167 1/3 Octave Band Spectrum 100 , 90 Frequency (Hz) [ra Linear m A-weighted | 0-1250 Hz FFT Spectrum 90 Frequency (Hz) Position: 4 Heading: 015° 168 120 1/3 Octave Band Spectrum 80 co 40 20 0 | ™! W, 11111J1 J . l J ,1,11, W J1111! 11 J j l t i ' l , I'l, 11J1,1,1, i I l l l l l l l l h l 111! i l h l II ! • I 1 J 1 I 1 II II i n t D o i o m o o c o o o m o o o m o o o o o o o o o o o o o o o o o o t i m ^ T - O J C M - T f i f t ( D C O O C N C D O l O ^ O O C O O O U 1 0 0 0 U ) O O O O O O O o S - W V- m ^ ^ ^ C N I C N C O ^ i O t D C O O C S l t D O W ^ O O C O O O i n O o J - P w v - i - T - C N C N C O ^ t m i D C O O C N J C O O * * - ! -T" T~ T- CN Frequency (Hz) I g| Linear g A-weighted | 0 - 1250 Hz FFT Spectrum 90 Frequency (Hz) Position: 4 Heading: 045° 169 A P P E N D I X E Blast Fence Insertion Loss 171 173 o o o o o o o CM O CO CO CM (ap) ids 181 A P P E N D I X F Current Sea Island Zoning & Geographic Coordinates 182 

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