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Study of the effect of underground drilling environments on the noise produced by percussive rock drills Higginson, John Francis 1973-12-31

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cf  A STUDY OF THE EFFECT OF UNDERGROUND DRILLING ENVIRONMENTS ON THE NOISE PRODUCED BY PERCUSSIVE ROCK DRILLS  by J. F. Higginson B.A.Sc., University of British Columbia, 1971  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE  in the Department of Mineral Engineering  We accept this thesis as conforming to the required standard.  THE UNIVERSITY OF BRITISH COLUMBIA June, 1973  In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make i t freely available for reference and study.  I further agree  that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representative. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission.  (Signed)  ii  ABSTRACT  The aim of the investigation is to identify and interrelate particular parameters that influence the magnitude of noise levels to which rock drillers  are subjected.  excessively noisy machine.  The percussive rock d r i l l is known to be an  Currently, exhaust mufflers and other silencing  devices are being developed but as yet acceptable noise levels have not been established.  The definition of acceptable sound power levels for d r i l l s must  recognize that the sound levels to which the d r i l l operator is exposed are modified by the acoustic properties of the working environment. For the i n i t i a l phase of the investigation a representative rock d r i l l was selected as a noise source.  Comparative sound levels generated by  this machine were measured in a free field environment and in typical underground working places.  Increases in the sound pressure levels in each octave  band from 63 to 16,000 hertz were observed when the d r i l l was operated in both stopes and drifts. For the subsequent phase of the investigation, studies were conducted on an assortment of commercially available rock d r i l l s .  The changes in  measured sound levels have been related to: the acoustic properties of the working place, the d r i l l position relative to the walls, the length of d r i l l steel exposed from the hole, and the d r i l l a i r supply pressure. Based on the measurements taken throughout the investigation, sound pressure level correction factors are proposed.  By applying these factors to  sound levels generated under free field conditions, predictions of rock d r i l l sound pressure levels present in underground working places can be made. In addition, when studies of rock d r i l l noise levels in various operating configurations are being conducted, use of the factors permits reduction of observed sound level measurements to a common datum.  iii  TABLE OF CONTENTS  Page Introduction  <  1  Measurement of Sound Pressure Levels  3  Sound Radiation Patterns  3  Sound Power Level  5  Semi-Reverberant Fields  7  Underground Drilling Conditions  7  Sound Levels in Stopes  10  Sound Levels in Drifts  12  Rating the Sound Levels of Rock Drills  13  Derivation of the Sound Pressure Level Correction Factors  14  In-Situ Comparison of Rock Drills  15  Silencing the Rock Drill  15  Conclusions  17  Acknowledgements  17  Bibliography  18  Appendix I  The Rated Sound Levels of Some Percussive Rock Drills  Appendix II The Effect of Drift Size on Sound Pressure Levels Appendix III The Effect of Operating Pressure on the Sound Pressure Levels.  LIST OF ILLUSTRATIONS  1.  Standard Measurement Positions for Airleg Drills  2.  The Measurement of Sound Pressure Levels in  2'  Free Fields  2  3.  Plan Views of Sound Radiation Patterns Underground  4  4.  Plan Views of Sound Radiation Patterns in Free Field  4  5. The Sound Power Level of the Test D r i l l  6  6.  Reflection of Sound Waves in Stopes with High Backs  8  7.  Reflection of Sound Waves in Stopes with Low Backs  8  8.  Reflection of Sound Waves in Open Ended Drifts  8  9.  Reflection of Sound Waves Near the Face of a Drift  8  10. Sound Pressure Level Correction Factors for Stopes  11  11.  Sound Pressure Level Correction Factors for Drifts  11  12.  Comparison of In-Situ Measurements  15  1.  INTRODUCTION One of the more d i f f i c u l t problems in noise control in the mining industry is the abatement of percussive rock d r i l l noise.  Legislation in  the 1960's resulted in the development of efficient exhaust mufflers (1,2,3,4) and compulsory hearing conservation programs (5).  Further legislation is con-  templated but at this'stage more detailed knowledge regarding the characteristics of the d r i l l as a noise source and the effect of the d r i l l i n g environment on sound fields is required before meaningful  limits can be specified.  Reduction in the noise levels by use of exhaust mufflers has increased the relative importance of other noise sources in the rock d r i l l .  The major  noise sources have been identified by Hoi do (6) as the exhaust and mechanical noises, comprising 87.5 and 12.5 percent respectively.  Beiers (7) extended  the identification to eleven separate sources and showed the effectiveness of. some muffling devices.  This paper will present ratings for the sound levels of  various d r i l l s and an indication of the relative importance of the sound sources in each. Rock d r i l l s are most commonly operated in relatively confined spaces in underground mines.  Fischer (8) and Botsford (9) have both drawn attention  to the increase in sound pressure levels (SPL's) that have been encountered underground and present a general indication of the increases to be expected. In order to formulate regulations governing the use of rock d r i l l s in underground mines i t is necessary to have a quantitative description of these sound pressure level increases. Such a description must be related to identifiable acoustic characteristics of the environment. This paper develops a procedure to interpret the sound pressure levels measured underground.  The object is to reduce these working environment levels  to their equivalent in a standard test environment.  This information can then  be applied to an in-situ determination of the effectiveness of silencing devices and the prediction of average exposure levels for the d r i l l operators.  It is  hoped that this investigation will aid in the establishment of target sound pressure levels for percussive rock d r i l l s and the identification of guidelines within which silencing of these machines can be accomplished.  Fig. 1 Standard measurement positions for airleg d r i l l s as specified by the CAGI-PNEUROP Test Code.  2 cubic meters of diorite rock  Fig. 2 The measurement of sound power levels was carried out in a quarry to obtain free f i e l d conditions.  3  MEASUREMENT OF SOUND PRESSURE LEVELS The d r i l l s employed in a l l tests were the commercially available models of three manufacturers, in this paper designated as manufacturer A, B, and C.  Four airleg d r i l l s and two stoper d r i l l s were tested utilizing  6 foot long, 7/8 inch diameter hexagonal steels and four winged tungsten carbide bits as standard ancillary equipment. Noise levels were measured using Bruel & Kjaer instruments; a sound level meter, type 2209, an octave band f i l t e r , type 1613, and a one inch condenser microphone, type 4145.  The microphone, covered with a foam wind-  shield protector and mounted on a tripod, was  connected to the sound  level meter by a ten meter long extension cable. Output from the sound level meter was recorded at 7.5 ips speed setting on Ampex low noise tapes by a Uher 4200 Report tape recorder. Calibration levels were obtained from a Bruel & Kjaer pistonphone, type 4220. The tapes were analyzed in the laboratory by replay through the sound level meter and i t s associated octave band f i l t e r set. As the percussive rock d r i l l is a variable and directive noise source, the measurement positions and techniques must be standardized to obtain reproducible results.  The Compressed Air and Gas Institute (CAGI) and  the European Committee of Manufacturers of Compressed Air Equipment (PNEUROP) have prepared a test code (10) for the free field measurement of sound from pneumatic machinery including airleg and stoper rock d r i l l s .  The CAGI-PNEUROP  Test Code has been adopted as a standard procedure for this work. The standard measurement positions for airleg d r i l l s as shown in Figure 1 are referenced by position number in the remainder of this paper. An idealized sketch of the free f i e l d test environment, as described in Section 7.2 of the CAGI-PNEUROP code, appears in Figure 2. SOUND RADIATION PATTERNS The sound fields around a type B airleg d r i l l have been mapped in the free field test environment and in an underground d r i f t .  Figure 4 is a plan  view of the so-called free f i e l d sound pattern, contoured at one decibel intervals on two horizontal planes. These contours illustrate the directivity of the d r i l l noise and indicate that the major noise sources are the exhaust air and the d r i l l steel.  The relationship between the SPL's measured at the standard  measuring points 1 through 4 and the overall sound f i e l d , indicates that these  Fig. 3(a)  Fig. 3(b)  SPL contours one meter above the d r i l l  SPL contours through the plane of the d r i l l  Fig. 3 The plan views of the sound radiation patterns around the d r i l l in a 10' x 10' d r i f t i l l u s t r a t e how the sound levels are increased by reflections from the d r i l l i n g face.  Fig. 4(a)  Fig. 4(b)  SPL contours one meter above the d r i l l  SPL contours through the plane of the d r i l l  Fig. 4 The plan views of the sound radiation patterns around the d r i l l in a free f i e l d indicate the d i r e c t i v i t y of this noise source.  5.  points can be used to obtain representative measurements of the noise produced by the d r i l l . The SPL's measured at the positions recommended by the CAGI^PNEUROP Test Code represent "near-field" levels.  These measurements are not indica-  tive of the noise levels that will be produced elsewhere in the working place but only of the levels in the immediate vicinity of the d r i l l .  If further in-  formation on the sound radiation pattern is desired, readings should be taken at points seven meters from the d r i l l , well into the "far-field". The sound field mapped 1n a 10' x 10' underground d r i f t is contoured in Figure 3.  The levels measured vary from six to eight decibels higher than at  the corresponding points in Figure 4, the equivalent free field condition, and illustrate what is known as "reverberancy".  The measured SPL is composed of  the direct sound from the d r i l l plus sound reflected from the floor and the walls of the d r i f t .  Beranek (11) calls fields like those of Figure 3 semi-  reverberant fields and proposes that the SPL in such a field can be computed from the sound power level of the noise source and the acoustic properties of the enclosure. SOUND POWER LEVEL The sound power level of the test d r i l l was determined by measuring the SPL's at eight points on the surface of a hemisphere 40 feet in diameter. The technique used was described by Zaveri (12) and gives the sound power level via equations 1, 2, and 3.  1. SPL  2.  3. where r = radius of the hemisphere = 20" P  Q  = reference SPL = .0002 ybars  SPL = averaged sound pressure level in decibels PWL = sound power level in decibels  6.  -  Sound Octave  Measured Sound Pressure Levels (dB)  Power  Microphone Positions  Level  Band Mid-Frequency  1  2  3  4  5  6  7  8  63  88  84  87  87  87  83  84  84  120  125  85  83  82  84  86  86  83  84  118.5  250  84  84  86  81  81  87  88  85  119  500  85  85  86  84  82  90  86  86  120  1,000  83  84  84  82  78  87  83  86  118  2,000  83  86  86  85  75  82  85  87  118.5  4,000  82  86  87  86  74  83  87  87  119.5  8,000  78  82  84  82  70  83  82  83  116  Linear  93  94  94  93  91  96  94  93  128  A-scale  90  92  92  91  83  90  92  92  125.5  (dB)  Fig. 5 The Sound Power Level of the test d r i l l was found to be 128 decibels overall.  7.  The PWL for the test d r i l l was measured  in each of eight octave bands and the  results so obtained are shown in Figure 5. SEMI-REVERBERANT FIELDS The sound pressure level in semi-reverberant fields can be computed by equation 4. SPL = PWL where  4.  Q = directivity factor with values 1,2,4, and 8, when the source is in mid a i r , on a hard floor, at an edge between walls, or at a corner of three hard walls, respectively. r = radius of test sphere in feet (3.28 feet for CAGI positions) a = sound absorption in sabines  Beranek (11) calls f(a) a room constant R which he defines by equation 5. R= where  Aa  5.  a = average sound absorption coefficient of the room A = area of the bounding surfaces  Young (13) lets f(a) = a and notes that "a" can be derived from measurements of the reverberation time of the room.  where  V = room volume in f t •T = reverberation time in seconds  The above considerations mean that before the SPL's can be computed in accord with equation 4, the acoustic characteristics of the environment must be identified. UNDERGROUND DRILLING CONDITIONS In this investigation the standards for a l l d r i l l s are defined in terms of free f i e l d ratings.  As shown by Figure 2, these ratings were obtained  with the d r i l l located over a hard surface floor, d r i l l i n g into a boulder the nominal surface area of which was less than 0.5 square meters.  Fig. 6 - In stopes with a height of more than 3 meters the back i s not an important reflecting surface.  Fig. 8 - When sidewall cut i s i n i t i a t e d at a point more than 3 meters from the d r i f t face the sound levels are Increased by reflections from four surfaces.  /  Fig. 7 - In stopes with a height of less than 3 meters the f l o o r , the back, and the face being d r i l l e d reflect the sound at the d r i l l operator.  /  Fig. 9 - D r i l l i n g into the face of a d r i f t places the d r i l l and i t s operator Into the most reverberant situation underground with three walls, the f l o o r , and the back reflecting the sound waves.  9.  In normal underground application the drilled medium is a solid rock wall, the surface layer of which is composed of interlocked rock fragments, and wherein the depth and extent of rock fragmentation are variable quantities.  In addition, in such situations the d r i l l is often located  within a few meters of additional enclosing walls.  Thus, in the free field  condition only one significant sound reflecting surface, the floor, exists; whereas in actual underground situations additional reflecting surfaces of variable reflectivity occur.  Figure 6, representing a high backed stope  situation, shows the presence of one more reflecting surface additional to the floor surface.  Figure 7, representing a low backed stope situation,  shows the presence of two more reflecting surfaces additional to the floor surface. One of two situations usually applies when a rock d r i l l is operated in a d r i f t .  When a d r i f t side slash is being d r i l l e d , as shown in Figure 8,  the acoustic environment consists of four reflecting surfaces, whereas the d r i l l i n g of a d r i f t face, as shown in Figure 9, yields a total of five surfaces capable of reflecting sound back to the d r i l l  operator.  The rock walls encountered underground are not perfectly reflective but are typically very rough with r e l i e f that can be measured in feet.  The  absorption coefficients of these walls vary with rock type and with the sound frequency. 0.05  Values of "a", the absorption coefficient, range from less than  for a smoothly blasted quartz wall to more than 0.3 for a wall treated  with shotcrete.  Diehl (14) quoted some values of absorption coefficients that  can be applied to the d r i l l i n g environment. With respect to floor conditions in underground working places, since this reflecting surface is rarely hard and smooth i t s influence on working place acoustic properties is important.  Normally the floor will be a flattened  broken rock pile composed of rock fragments ranging from two feet in diameter down to slime size, and often the floor is partially covered by water.  The  range of conditions is sufficiently wide that a careful correlation was  not  attempted in this investigation but the influences are considered in the statement of correction factor tolerance limits.  The correction for an area with  an abnormally absorptive floor will fall near the low end of the range of a particular factor, whereas the correction for an area displaying a relatively  hard floor surface will be near the high end of the range.  The correlations  presented in this paper are for average floor conditions such as those found in drifts where the broken rock is reasonably coarse (like gravel), packed by moderate t r a f f i c loads, and wet, but with less than 10% of the area covered with standing water. When a rock d r i l l is operated as shown in Figure 6, the geometric centre of the d r i l l must be kept at least 1.0 meter from the wall to satisfy the CAGI requirements.  In addition to this restriction, i t must be recog-  nized that the d r i l l is a complex noise generator with sources of various sound power levels distributed over a length of from one to three meters in normal operation. These considerations mean that the directivity "Q" as employed in equation 4 i s not well defined but has a value somewhere between 1 and 2. Under the simplifying assumptions that the d r i l l is a point source located at a distance "d" from a perfectly reflecting wall, the levels at the points 3 and 4 of Figure 1 can be computed from geometric considerations. The effect of wall distance shown in Figure 6 is to increase the SPL by one decibel when "d" equals one meter, and increasing to three decibels as "d" approaches zero.  —•  SOUND LEVELS IN STOPES Stopes are dynamic production areas displaying continuously changing geometric configurations.  From an acoustic point of view stopes are reasonably  large chambers with low absorption coefficients except over broken rock covered areas.  These rock piles can be substantial in both volume and surface area.  Usually, when rock d r i l l s are operated in stopes they are located against one wall distant from the other walls. The roof (or back) of the stope is from two to ten meters above the floor so that often i t may be considered to be far from the d r i l l and have no effect on the sound levels.  Figures 6 and 7 con-  veniently idealize the typical stoping situations. The SPL's at points 3 and 4 may be expected to increase by about four decibels for a d r i l l operating under a very low back.  These increases should  f a l l to less than one decibel when the back is more than two meters above the  SOUND PRESSURE LEVEL CORRECTION FACTORS FOR DRIFTS  SOUND PRESSURE LEVEL CORRECTION FACTORS FOR STOPES  FOR AIRLEG DRILLS OF TYPE B  FOR AIRLEG DRILLS OF TYPE B Octave Octave Band Mid-frequency  Correction Factors 1n dB re .0002 ubars  Mid-frequency  Back > 3 meters high  Back < 3 meters high  Mean (C)  Mean (C)  Deviation  Deviation  63  4  2  6  3  125  3  2  5  2  250  3  2  5  2  500  2  1  4  2  1,000  2  1  4  1  2,000  1  1  3  1  4,000  1  1  2  1  8,000 .  0  1  1  1  0  1  0  1  15,000  Fig.  10 The sound pressure levels 1n a stope may be predicted by adding the above correction factors to the rated levels f o r the d r i l l 1n question. A l l levels must be measured at the positions defined as 3 and 4 in F i g . 1.  Correction Factors i n dB re .0002 ubar Cross Section Area ( f t ) 60 70 80 90 100 2  40  50  63  10  8  7  125  11.5  11  10  8  7  120  140  160  6  6  5  4  4  3  3  9.5  9  8.5  7.5  6  5  4  6  5  4  3.5  2  1  .5  9.5 7.5  6.5 5.5  5  4  3.5  3  2  1.5  1,000  12  8.5  7  5.5  4.5  3.5  3  2.5  2  2,000  6.5 5.5  4.5  3.5  3  2.5  2.5  2  1.5  1  250 500  9  10  4,000  5  4  3  2  1.5  1  1  .5  0  0  8,000  7  5  4  3  2  1.5  1  .5  0  0  2.5  2  1.5  1  1  1  .5  0  0  0  16,000  Fig.  11 The sound pressure levels 1n d r i f t may be predicted by adding the above correction factors to the rated l e v e l s f o r the d r i l l . ATI the levels must be measured at the posi t i o n s defined as 3 and 4 i n Figure 1.  drill.  Measurements made underground have indicated that the actual increases  are always affected by the floor conditions. A set of correction factors is presented in Figure 10.  These factors  represent the increases over the free field levels of a type B airleg d r i l l as measured in eight stopes.  For comparative purposes, the SPL's as measured in  stopes may be reduced to the levels the d r i l l would produce in a free f i e l d environment by subtracting the appropriate correction factors from the levels observed in-situ. SOUND LEVELS IN DRIFTS As the acoustic conditions of drifts are reproducible, the detailed investigations in this paper were carried out in such areas.  In comparison  to stopes, during the d r i l l i n g cycle drifts will usually exhibit a higher degree of symmetry, have more smoothly blasted walls, and contain less broken rock material piled on the floor. The equations 4, 5 and 6 show that the acoustic characteristics of the d r i l l i n g environment are described by the volume, bounding surface area, and the average absorption coefficient.  The volume and surface area cannot  be calculated because drifts are open ended and thus the length is not defined. There are some cases in which an effective length corresponding to a few sound wavelengths may be applicable, but a definition of this situation is not available.  The question is left open by describing the drifts by their cross-  section area.  The volume and surface area are simply related to the cross-  section area of square drifts when an effective length of d r i f t is chosen. The SPL's of the test d r i l l have been measured in a large number of drifts (see Appendix II) and the increases over the d r i l l rating levels are tabulated in Figure 11.  These correction factors are given as average values  and possess a tolerance limit of two decibels for a l l octave bands above 125 hertz. i.e.  Errors will be incurred in measurements taken in very small d r i f t s ,  cross-section less than 40 sq. f t . , where the CAGI positions can no  longer be maintained with respect to the walls. Definite reverberancy and standing waves have been observed in a l l drifts.  These effects are most marked in the 63 and 125 hertz octave bands  with SPL variations of from 4 to 10 dB observed over a distance of less than 0.5 m. Smaller variations of the SPL have been observed in a l l octave bands up to the 8,000 hz. range.  These variations are also caused by direct re-  flection of sound due to the slabby nature of the walls, large rock fragments lying on the floor, and hardware lying about the working place. Operation of a d r i l l at the face of a drift as shown in Figure 9 caused more reverberancy than the open drift especially when the crosssection area is less than 60 sq. f t . The increases were mainly in the lower octabe bands and no significant changes in the average levels could be shown above the 500 hz. band.  Insufficient data have been obtained to predict con-  fidently the increases in the 63 and 16,000 hz. octave bands. RATING THE SOUND LEVELS OF ROCK DRILLS Testing f a c i l i t i e s were arranged at the Britannia Mine where d r i l l s were tested underground in a 10' x 10' drift located in chlorite schist and in a free f i e l d environment d r i l l i n g into the same rock type.  The latter  testing station was an area located near the portal of a mine access drift where the sound levels were not influenced by surrounding walls. This location is judged to satisfy the CAGI designated requirements for SPL measurements of airleg type d r i l l s , but for the operation of stoper type d r i l l s the CAGI requirements cannot be met fully at this location.  Since the stoper is a  directive sound source, the SPL measurements at fixed points around i t are markedly increased when there is a rock face close to the d r i l l .  This problem  was recognized at the Britannia station by the poor reproducibility of SPL's produced by the stopers. The SPL's measured in the 10' x 10' drift and in the free field area are defined as the rated levels for the rock d r i l l s .  Tables 1 to 6 of Appen-  dix I present ratings for some airlegs and stopers. All the operating conditions during testing were maintained as close to CAGI specifications as possible.  In the course of these tests a variable not identified in the CAGI  specifications was discerned. The CAGI Test Code specifies 0.5 to 2 m. of steel exposed from a d r i l l hole during testing provided that the d r i l l is more than 1 m. from the face at a l l times.  The graphs of Appendix III represent data gathered in the  test d r i f t described earlier.  These graphs show that the SPL's produced  when 0.5 m. of steel is exposed are significantly less than those produced when 1.0 m. is exposed.  In order to make ratings reproducible the length of  steel exposed was maintained at 0.75 to 1.0 m. despite penetration rates of up to a meter per minute.  For the short time intervals in which such a  reading was possible, taped records were found to be somewhat superior to SPL observations written in the f i e l d . DERIVATION OF THE SOUND PRESSURE LEVEL CORRECTION FACTORS The "scatter" of data plotted in the graphs of Appendix II warrants comment. These data points represent d r i l l s working in rocks ranging from chlorite schists to diorites and under operating air pressures of between 80 and 90 psi. If the noise produced by the d r i l l s could be shown to have a marked dependence on a i r pressure or on the rock type, some of the scatter could be explained. The graphs of Appendix III present the results of the studies into the effect of compressed air supply pressure on the sound levels of some airleg drills.  As the dynamic pressure increased from 80 to 90 psi the SPL's i n -  creased by about one decibel in each octave band. This change is too small to account for the scatter mentioned above. An airleg d r i l l of type B was tested in drifts in a chlorite schist rock (9' x 10'), in a mineralized dolomite (8' x 11'), and in a quartz diorite rock (8'6" x 10').  A l l the data points are plotted on the lower graphs of the  Appendix II and illustrate no clear dependence on the hardness of the rock or the size of the d r i f t .  It was concluded that the rock type was not a s i g n i f i -  cant variable in the tests. A number of transformations were carried out on the decibel and crosssection scales of the graphs of Appendix II in an attempt to find an analytical function to predict the SPL's.  Inverse and logarithmic transformations,  functions of the form of equation 4, and simple polynomials were tried but none of these gave a suitable f i t , consequently the points were plotted linearly as shown and graphical curve fitting was employed.  •  15.  The SPL Correction Factors were obtained by subtracting the free f i e l d ratings from the levels predicted by the curves for a given crosssection area.  Since the errors associated with the ratings are at least ±1 dB  and the errors on the curves as drawn are also ±1 dB, the correction factors possess a total error of ±2 dB.  This error has been used as the estimate of  the tolerance limits quoted earlier.  The factors quoted are for an a i r  pressure of nominally 85 psi dynamic and 0.5 dB per 5 psi are added or subtracted when the corrections are applied to the observed levels. IN-SITU COMPARISON OF ROCK DRILLS The sound levels produced by rock d r i l l s are most readily measured when the d r i l l is operating in the working place. This situation means that the data will indicate the SPL's under a special set of conditions and as such should not be used as a direct measurement of the efficiency of the d r i l l ' s silencing devices.  The correction factors presented in this paper can be  selected to f i t the conditions in the working place and subtracted from the measured SPL's, thereby reducing the levels to those the d r i l l would produce in a free f i e l d .  When the reduced SPL characteristics of the d r i l l are plotted  c r i t i c a l comparisons can be made with the levels produced by other d r i l l s of the same type and with the rated levels presented in Appendix I. In Figure 12 the SPL's of three d r i l l s of type B producing from I H to 118 dB were reduced to a common datum.  It was found that a l l three d r i l l s  were in good condition with efficient mufflers and would have produced about 111.5 dB had they been operating in a free f i e l d .  The SPL in the 7' x 8'  drift used in this example was 78% higher than the SPL in free f i e l d , in the 8' x 8' drift i t was 65% higher and in the 9' x 11' drift the SPL was i n creased by 44% overall. SILENCING THE ROCK DRILL The major sources of noise from the percussive rock d r i l l have been identified (2) as impact, exhaust and resonance.  PARAMETER  AIRLEG DRILL 1 SPL measured in 7' x 8* drift  Rock Type Air psi  AIRLEG DRILL 2  SPL predicted for free field operation  Argillite  SPL measured in 8' x 8' dri ft  SPL predicted for free f i e l d operation  Dolomite  AIRLEG DRILL 3 SPL measured in 9' x 11' drift Chlorite Schist  95  80  90  140  140  80  Measurement Position  3  3  3  Overall SPL  118  111.4  116  110  103.5  103  125  106  94.5  102.5  250  110  101.5  107  500  106  98.5  1000  105  2000  Water psi  SPL predicted for free field operation  111.3  114  111.3  98  100  98  92.5  101.5  94.5  100  103.5  100.5  109  102.5  103.5  100.5  95  106  98  101  98  109  103  108  103.5  105  103  4000  110  105.5  109.5  106.5  108.5  108  8000  no  105  107  104.5  104.5  104  SPL's by Octave Band 63 hertz  Figure 12  The sound levels measured in a variety of working environments have been reduced to a common datum (free field) by application of the appropriate SPL Correction Factors.  16.  Frequency  Source  40-100 hertz  Impact of piston and d r i l l steel Impact of d r i l l steel and rock  100 - 2000  Exhausting of a i r from the exhaust ports  2000 up  Resonance of parts of d r i l l Resonance of d r i l l steel  The graphs of Appendix III show that the three airleg d r i l l s , A, B and C are efficiently muffled.  At an operating pressure of 90 psi the contri-  bution to the overall SPL by the frequency components above and below the 2000 hertz octave band are: Source of Noise  Drill A  Drill B  Drill C  Exhaust  110.5dB  109.5dB  106.5dB  Steel  112.5dB  113.5dB  111.5dB  These numbers indicate that the steel noise forms 62%, 70% and 76% of the total noise produced by d r i l l s A, B and C respectively. Efficient muffling has decreased the exhaust noises to such an extent that further silencing can best be achieved by solving the steel noise problem. Simon (15) has shown that the d r i l l i n g efficiency of a percussive rock d r i l l is an inverse function of the mechanical impedance of the steel. As the amplitude of the stress waves in the steel is increased, the rate at which work can be done on the rock increases. An increase in the number of blows per minute will also increase the rate of work provided the thrust on the d r i l l is increased proportionately.  A l l of these steps call for an increase in  the strength of the steel being used so that the mechanical impendance need not be increased.  Silencing of the steel usually involves a sizable increase in  its cross-section area and a decrease in d r i l l i n g efficiency.  Visnapuu and  Jensen (16) were able to drop the steel noise by 6 dB but found that the penetration rate dropped by 28%. Percussion d r i l l designers have increased the penetration rate by i n creasing the frequency of d r i l l steel blows, however this condition has accentuated the SPL of the 4,000 hz. octave band to the point where the maximum amplitude of the rock d r i l l noise occurs in this band.  There appears  to be some correlation between the rate of penetration and the level observed  1;  in this band.  If this SPL is reduced by 2 or 3 dB through manipulation of  the steel i t s e l f the d r i l l i n g rate is decreased very noticeably.  Since  these losses are not acceptable i t is suggested that special earmuffs might be designed for d r i l l i n g operations.  These muffs would be similar to those  worn by aircraft ground crew and would provide extra attenuation for frequencies between 2,000 and 10,000 hz. the  Such muffs would reduce effectively  noise hazard of percussion drilling provided complete operator acceptance  could be obtained. CONCLUSIONS The CAGI-PNEUR0P Test Code can be used as a guide for sound pressure level measurements in underground environments.  The correlation of free  f i e l d SPL's with underground SPL's has been achieved satisfactorily for a number of percussion rock d r i l l s .  Sound levels in the working place can be  predicted from a knowledge of the free f i e l d levels of these d r i l l s . the  Likewise  sound levels measured in the working place can be related to expected free  field levels.  ACKNOWLEDGEMENTS The author wishes to acknowledge the financial assistance of the Department of Energy, Mines and Resources and the B. C. Department of Mines and  Petroleum Resources, and to express his appreciation to Anaconda American  Brass Ltd., Joy  Gardner-Denver Co. (Canada) Ltd.,  Atlas Copco Canada Ltd., and  Mfg. Co. (Canada) Ltd. for providing equipment and supplies.  The author  also extends his thanks to those individuals associated with the B. C. mining industry, the B. C. Department of Mines, and the University of B. C , who have contributed to this investigation.  18.  BIBLIOGRAPHY  1.  WEBER, B. H., "Noise Suppression on Rock D r i l l s " , Canadian Mining Journal, September, 1972.  2.  Report of Investigations 6165, U.S. Bureau of Mines.  3.  Report of Investigations 6345, U.S. Bureau of Mines.  4.  Report of Investigations 6450, U.S. Bureau of Mines.  5.  Province of B. C. Mines Regulation Act, Chapter 25, Rule 94.  6.  H0LD0, J.,  "Energy Consumed by Rock Drill Noise", Atlas Copoo Publication  7.  3  1958.  BEIERS, J. L., "A Study of Noise Sources in Pneumatic Rock D r i l l s " , Transactions  8.  50l3  FISCHER, H.C.,  of the IMM, July, 1965.  "Noise caused by rock drilling  depends on working  conditions", Compressed Air Comments 4,  9.  B0TSF0RD, J.H., "Noise Abatement", Canadian Mining  1958.  Journal,  October, 1960. 10.  The CAGI-PNEUROP Test Code for the Measurement of Sound from Pneumatic Machinery, The Compressed Air and  Gas  Institute, New York, 1969. 11.  BERANEK, L.L., Acoustics,  McGraw Hill Book Co. Inc., New York,  1954, pp. 314 - 317. 12.  ZAVERI, K., "Sound Power", B & K Technical Review #3, 1971.  13.  YOUNG, R.W.,  "Sabine Reverberation Equation and Sound Power Calculations", Journal of the Acoustical  Society  of  America, July, 1959. 14.  DIEHL, G.M.,  "Think Quiet", a reprint from Compressed Air,  1971,  p. 15. 15.  SIMON, R., "Transfer of the Stress Wave Energy in the Drill Steel of a Percussive Drill to the Rock", Int. J. Rock Mech. Mining Sci. (1), 1964, pp. 397-411.  16.  VISNAPUU, A., and JENSEN, J.W.,  "Noise Control for the Pneumatic  Rock D r i l l " , AIME Preprint #73-AU-62.  APPENDIX I  THE RATED SOUND LEVELS OF SOME PERCUSSIVE ROCK DRILLS  TABLE 1  RATED LEVELS FOR AIRLEG A  A i r Pressure = 95 p s i .  Penetration = = 35 1pm.  Water Pressure = 85 p s i .  Integral  Position 4  Position 3  Position 1  Octave Band  1inear  Muffler:  Free Field  Drift  107  112  Free Field  Drift  Drift  Free Field  111.5  115  111.5  115  111  113 104  dBA  107  109  111  113  63  95  100  95  102  96  125  97  105  97  105  96  250  94  102  91  102  93  104 •  103  500  98  105  102  106  101  106  1,000  95  100  95  101  96  101  2,000  101  102  102  104  104  105  4,000  101  106  105  108  106  110  8,000  95  102  103  106  104  105  16,000  90  94  93  95  96  96  TABLE 2  RATED LEVELS FOR AIRLEG B  A i r Pressure = 90 p s i .  Penetration = =30 fpm.  Water Pressure = 60 p s i .  Muffler:  Position 1  Octave Band  Integral  Position 3 Free Field  Position 4  Free Field  Drift  linear  108  113  113  114  112  114  dBA  107  112  112  113  111  113  63  90  102  97  104  97  103  125  94  104  96  105  95  104  250  95  103  101  102  100  105  500  97  105  100  105  100  104  1,000  95  103  98  103  96  101  2,000  100  104  104  105  102  105  Drift  Free Field  Drift  4,000  101  105  108  108  107  109  8,000  100  103  107  104  105  105  16,000  88  95  98  98  98  98  TABLE 3  RATED LEVELS FOR AIRLEG C Penetration = = 30 1pm.  Air Pressure = 90 psi.  Muffler:  Water Pressure = 80 psi.  Position 3  Position 1  Octave Band  Integral  Free Field  Drift  Free Field  Position 4  Drift  Free Field  Drift  106.5  112  111.5  113  111  112.5  dBA  106  no  111  112.5  111  112  63  86  106  96  102  96  102  125  81  98  90  96  88  98  250  88  98  90  99  88  99  500  89  96  91  98  90  98  1,000  94  98  95  99  94  99  2,000  96  103  102  103  102  103  4,000  104  106  107  108  108  . 108  8,000  100  103  105  106  106.  107  16,000  88  92  93  94  94  94  1inear  TABLE 4  RATED LEVELS FOR AIRLEG D  Air Pressure = 95 psi.  Penetration = = 35 ipm.  Water Pressure = 80 psi.  Octave Band  linear  Muffler:  Position 1  Position 3  Integral  Position 4 Free Field  Free Field  Drift  Free Field  Drift  108  113  111.5  115  111  114.5  Drift  dBA  107  no  111  113.5  no  113  63  95  95  102  102  101  106  125  100  103  97  104  96  104  250  94  105  100  107  101  106  500  98  101  99  103  101  102  1,000  95  103  100  102  98  102  2,000  101  104  107  106  104  107  4,000  101  106  110  103  106  108  8,000  95  104  107  106  104  16,000  90  98  95  98  96  -  106 97  TABLE 5  RATED LEVELS FOR STOPER 8  Pressure = 85 psi. Air 1 Water iPressure = 60 psi.  Free Field  dBA 63 125  Muffler: Integral  Position 4  Position 3  Position 1  Octave Band  linear  Penetration = = 30 ipm.  Drift  Free Field  Drift  Free Field  Drift  110.5  113.5  109.5  113  110  113  107  109  108  110  108  110  105  109  104  109  104  107.5  101  101  101  101.5  103  97  102  98  102  97  101  97  101 99  .103.5  250  99  100  500  100  101  .  1,000  98  101  95  98  95  2,000  101  103  100  104  100  103  4,000  100.5  105  103  105  103  105  8,000  99  100  101  102  101.5  101  16,000  94  94  94  94  TABLE 6  RATED LEVELS FOR STOPER C  Air Pressure = 90 psi.  Penetration = = 30 ipm.  Water Pressure = 80 psi.  Octave Band  linear  Muffler:  Position 1 Free Field  95  95  Integral  Position 3  Drift  Free Field  Position 4  Drift  Free Field  Drift  111.5  113.5  112  113.5  112  113.5  dBA  110  112  110  112  no  112  63  103  103  103  108  104  106  125  97  101  102  101  102  103  250  97  104  103  104.5  103  105  500  97  102  96  101  1,000  102  103  100  102  100  102  2,000  103  105  101  103.5  101.5  103.5 108.5 106  96.5  4,000  107  109  106.5  109  106  8,000  105  104  103.5  105  104  16,000  96  96  97  97  96  •  99  97.5  APPENDIX II  THE EFFECT OF DRIFT SIZE ON SOUND PRESSURE LEVELS  1.  The data points represent readings taken at the positions designated 3 and 4 by the CAGI-PNEUROP Test Code.  2.  Open circles o represent single data points. Closed circles • represent two or more coincident data points.  3.  The observations were made in drifts with a width to height ratio between 0.6 and 1.4.  Sound Pressure Level ( decibels )  Sound Pressure Level  ( decibels )  Sound Pressure Level ( decibels )  9Z  Sound Pressure Level ,( decibels )  Sound Pressure Level ( decibels )  9Z  Sound Pressure Level ,{ decibels )  Sound Pressure Level (decibels)  Sound Pressure Level  .(decibels)  APPENDIX III  THE EFFECT OF OPERATING PRESSURE ON THE SOUND PRESSURE LEVELS  1. A l l observations were made tn the 10' x 10' drtft at Anaconda Britannia Mines. 2. The data points are© 70 pst dynamic afr pressure °80 psi • 90 psi 3. The overall SPL's for the graphs which follow are: Dynamic Air Pressure (psi)  Graph A  70  114  112  103  80  114.5  113.5  110.5  90  114.5  114  112  70  113.5  113  110.5  80  114  114  111  90  115  115  112.5  Graph B  Graph C  AIRLEG  A  CAGI Position:  3  Steel Exposed:  1.0 nri.  105-  250  8K  500  Full Octave  Band  AIRLEG  I6K  Mid-frequencies-  A  CAGI' Position:  4  S t e e l Exposed-.  1.0 mi,  I6K Full  Octave  Band  Mid-frequencies  115  AIRLEG B CAGI Position:  4  Steel Exposed: 0.5 m  8K Full Octave  Bond  I6K  Mid-frequencies-  I6K Full  Octave  Band  Mid-frequencies  AIRLEG C CAGI Position*  4  Steel Exposed: 0.5 m  Full Octave  Band  Mid-frequencies-  AIRLEG  C  CAGh Position: Steel Exposed:  Full  Octave  Band  4 1.0 m  Mid-frequencies  


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