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Study of the effect of underground drilling environments on the noise produced by percussive rock drills 1973

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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) i i ABSTRACT The aim of the investigation is to identify and interrelate par- ticular parameters that influence the magnitude of noise levels to which rock drillers are subjected. The percussive rock dr i l l is known to be an excessively noisy machine. 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 drills must recognize that the sound levels to which the drill operator is exposed are modified by the acoustic properties of the working environment. For the initial phase of the investigation a representative rock dr 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 under- ground working places. Increases in the sound pressure levels in each octave band from 63 to 16,000 hertz were observed when the dri 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 dri 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 dr i l l steel exposed from the hole, and the dr i l l air 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 dr i l l sound pressure levels present in underground working places can be made. In addition, when studies of rock dr i l l noise levels in various operating con- figurations are being conducted, use of the factors permits reduction of observed sound level measurements to a common datum. i i i 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 ' 2. The Measurement of Sound Pressure Levels in 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 Drill 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 difficult problems in noise control in the mining industry is the abatement of percussive rock drill 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 character- istics of the d r i l l as a noise source and the effect of the drilling environ- ment 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 drills and an indication of the relative importance of the sound sources in each. Rock drills 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 drills in under- ground 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 drills and the identification of guidelines within which silencing of these machines can be accomplished. Fig. 1 Standard measurement positions for airleg drills 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 field conditions. 3 MEASUREMENT OF SOUND PRESSURE LEVELS The drills employed in all tests were the commercially available models of three manufacturers, in this paper designated as manufacturer A, B, and C. Four airleg drills and two stoper drills 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 its 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 drills as shown in Figure 1 are referenced by position number in the remainder of this paper. An idealized sketch of the free field 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 drift. Figure 4 is a plan view of the so-called free field sound pattern, contoured at one decibel inter- vals 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 dr i l l steel. The relationship between the SPL's measured at the standard measuring points 1 through 4 and the overall sound field, indicates that these Fig. 3(a) SPL contours one meter above the d r i l l Fig. 3(b) 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 illustrate how the sound levels are increased by reflections from the d r i l l i n g face. Fig. 4(a) SPL contours one meter above the d r i l l Fig. 4(b) 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 directivity 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 CAGÎ 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 drift 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 drift. 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" PQ = 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 Band Microphone Positions Level Mid-Frequency 1 2 3 4 5 6 7 8 (dB) 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 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. where Q = directivity factor with values 1,2,4, and 8, when the source is in mid air, 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. where 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 ft •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 all drills are defined in terms of free field ratings. As shown by Figure 2, these ratings were obtained with the drill located over a hard surface floor, drilling into a boulder the nominal surface area of which was less than 0.5 square meters. SPL = PWL 4. R = A a 5. Fig. 6 - In stopes with a height of more than 3 meters the back is not an important reflecting surface. Fig. 7 - In stopes with a height of less than 3 meters the floor, 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. 8 - When sidewall cut is initiated 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. 9 - Drilling 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 floor, 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 frag- ments, 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 drift. When a drift side slash is being drilled, as shown in Figure 8, the acoustic environment consists of four reflecting surfaces, whereas the drilling of a drift face, as shown in Figure 9, yields a total of five sur- faces capable of reflecting sound back to the dr i l l operator. The rock walls encountered underground are not perfectly reflective but are typically very rough with relief that can be measured in feet. The absorption coefficients of these walls vary with rock type and with the sound frequency. Values of "a", the absorption coefficient, range from less than 0.05 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 drilling environment. With respect to floor conditions in underground working places, since this reflecting surface is rarely hard and smooth its 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 state- ment 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 traffic loads, and wet, but with less than 10% of the area covered with standing water. When a rock dr i l l is operated as shown in Figure 6, the geometric centre of the dr 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 is not well defined but has a value somewhere between 1 and 2. Under the simplifying assumptions that the dr 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 drills 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 dr 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 dr i l l operating under a very low back. These increases should fall to less than one decibel when the back is more than two meters above the SOUND PRESSURE LEVEL CORRECTION FACTORS FOR STOPES FOR AIRLEG DRILLS OF TYPE B Octave Band Mid-frequency Correction Factors 1n dB re .0002 ubars Back > 3 meters high Mean (C) Deviation Back < 3 meters high Mean (C) 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 15,000 0 1 0 1 Fig. 10 The sound pressure levels 1n a stope may be predicted by adding the above correction factors to the rated levels for the d r i l l 1n question. A l l levels must be measured at the positions defined as 3 and 4 in Fig. 1. SOUND PRESSURE LEVEL CORRECTION FACTORS FOR DRIFTS FOR AIRLEG DRILLS OF TYPE B Octave Correction Factors in dB re .0002 ubar Mid-frequency 40 50 Cross Section 60 70 80 Area 90 ( f t 2 ) 100 120 140 160 63 10 8 7 6 6 5 4 4 3 3 125 11.5 11 10 9.5 9 8.5 7.5 6 5 4 250 9 8 7 6 5 4 3.5 2 1 .5 500 9.5 7.5 6.5 5.5 5 4 3.5 3 2 1.5 1,000 12 10 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 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 16,000 2.5 2 1.5 1 1 1 .5 0 0 0 Fig. 11 The sound pressure levels 1n d r i f t may be predicted by adding the above correction factors to the rated levels for the d r i l l . ATI the levels must be measured at the pos- itions defined as 3 and 4 in Figure 1. d r i l l . 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 field 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 drilling 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 drilling 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 avail- able. 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 drift 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 all octave bands above 125 hertz. Errors will be incurred in measurements taken in very small drifts, i.e. 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 all 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 all 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 dr i l l at the face of a drift as shown in Figure 9 caused more reverberancy than the open drift especially when the cross- section area is less than 60 sq. ft. 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 facilities were arranged at the Britannia Mine where drills were tested underground in a 10' x 10' drift located in chlorite schist and in a free field environment drilling 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 drills the CAGI re- quirements 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 dri l l s . Tables 1 to 6 of Appen- dix I present ratings for some airlegs and stopers. All the operating condi- tions 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 dril l hole during testing provided that the dr i l l is more than 1 m. from the face at all times. The graphs of Appendix III represent data gathered in the test drift 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 field. 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 drills 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 drills could be shown to have a marked dependence on air 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 d r i l l s . As the dynamic pressure increased from 80 to 90 psi the SPL's in- 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'). All 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 drift. It was concluded that the rock type was not a signifi- cant variable in the tests. A number of transformations were carried out on the decibel and cross- section 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 field ratings from the levels predicted by the curves for a given cross- section 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 air pressure of nominally 85 psi dynamic and 0.5 dB per 5 psi are added or sub- tracted when the corrections are applied to the observed levels. IN-SITU COMPARISON OF ROCK DRILLS The sound levels produced by rock drills are most readily measured when the dri 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 drill'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 field. When the reduced SPL characteristics of the dr i l l are plotted critical comparisons can be made with the levels produced by other drills of the same type and with the rated levels presented in Appendix I. In Figure 12 the SPL's of three drills of type B producing from I H to 118 dB were reduced to a common datum. It was found that all three drills were in good condition with efficient mufflers and would have produced about 111.5 dB had they been operating in a free field. The SPL in the 7' x 8' drift used in this example was 78% higher than the SPL in free field, in the 8' x 8' drift i t was 65% higher and in the 9' x 11' drift the SPL was in- 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 AIRLEG DRILL 2 AIRLEG DRILL 3 Rock Type Air psi Water psi Measurement Position Overall SPL SPL's by Octave Band 63 hertz 125 250 500 1000 2000 4000 8000 SPL measured in 7' x 8* drift Argillite 95 140 3 118 110 106 110 106 105 109 110 n o SPL predicted for free field operation 111.4 103.5 94.5 101.5 98.5 95 103 105.5 105 SPL measured in 8' x 8' dri ft Dolomite 80 140 3 116 103 102.5 107 109 106 108 109.5 107 SPL predicted for free field operation 111.3 98 92.5 100 102.5 98 103.5 106.5 104.5 SPL measured in 9' x 11' drift Chlorite Schist 90 80 3 114 100 101.5 103.5 103.5 101 105 108.5 104.5 SPL predicted for free field operation 111.3 98 94.5 100.5 100.5 98 103 108 104 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 dr i l l steel and rock 100 - 2000 Exhausting of air from the exhaust ports 2000 up Resonance of parts of d r i l l Resonance of dr 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 drills 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 drilling efficiency of a percussive rock dr 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 dr i l l is increased proportionately. All 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 drilling efficiency. Visnapuu and Jensen (16) were able to drop the steel noise by 6 dB but found that the pene- tration rate dropped by 28%. Percussion dr i l l designers have increased the penetration rate by in- 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 itself the drilling rate is decreased very noticeably. Since these losses are not acceptable i t is suggested that special earmuffs might be designed for drilling operations. These muffs would be similar to those worn by aircraft ground crew and would provide extra attenuation for fre- quencies between 2,000 and 10,000 hz. Such muffs would reduce effectively the 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 field 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 field levels of these d r i l l s . Likewise the 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., Gardner-Denver Co. (Canada) Ltd., Atlas Copco Canada Ltd., and Joy 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 Drills", 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 50l33 1958. 7. BEIERS, J. L., "A Study of Noise Sources in Pneumatic Rock Drills", Transactions of the IMM, July, 1965. 8. FISCHER, H.C., "Noise caused by rock drilling depends on working conditions", Compressed Air Comments 4, 1958. 9. B0TSF0RD, J.H., "Noise Abatement", Canadian Mining 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 Cal- culations", 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 Air Pressure = 95 p s i . Penetration = = 35 1pm. Water Pressure = 85 ps i . Muffler: Integral Octave Position 1 Position 3 Position 4 Band Free Field Drift Free Field Drift Free Field Drift 1inear 107 112 111.5 115 111.5 115 dBA 107 109 111 113 111 113 63 95 100 95 102 96 104 125 97 105 97 105 96 104 250 94 102 91 102 93 • 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 Air Pressure = 90 p s i . Penetration = =30 fpm. Water Pressure = 60 ps i . Muffler: Integral Octave Band Position 1 Position 3 Position 4 Free Field Drift Free Field Drift Free Field Drift l inear 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 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 Air Pressure = 90 psi. Water Pressure = 80 psi. Penetration = = 30 1pm. Muffler: Integral Octave Band Position 1 Position 3 Position 4 Free Field Drift Free Field Drift Free Field Drift 1inear 106.5 112 111.5 113 111 112.5 dBA 106 n o 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 TABLE 4 RATED LEVELS FOR AIRLEG D Air Pressure = 95 psi. Water Pressure = 80 psi. Penetration = = 35 ipm. Muffler: Integral Octave Position 1 Position 3 Position 4 Band Free Field Drift Free Field Drift Free Field Drift linear 108 113 111.5 115 111 114.5 dBA 107 n o 111 113.5 n o 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 - 106 16,000 90 98 95 98 96 97 TABLE 5 RATED LEVELS FOR STOPER 8 Air 1 Pressure = 85 psi. Penetration = = 30 ipm. Water i Pressure = 60 psi. Muffler: Integral Octave Position 1 Position 3 Position 4 Band Free Field Drift Free Field Drift Free Field Drift linear 110.5 113.5 109.5 113 110 113 dBA 107 109 108 110 108 110 63 105 109 104 109 104 107.5 125 .103.5 101 101 101 101.5 103 250 99 100 97 102 98 102 500 100 101 . 97 101 97 101 1,000 98 101 95 98 95 99 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 95 95 TABLE 6 RATED LEVELS FOR STOPER C Air Pressure = 90 psi. Penetration = = 30 ipm. Water Pressure = 80 psi. Muffler: Integral Octave Position 1 Position 3 Position 4 Band Free Field Drift Free Field Drift Free Field Drift linear 111.5 113.5 112 113.5 112 113.5 dBA 110 112 110 112 n o 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 96.5 99 1,000 102 103 100 102 100 102 2,000 103 105 101 103.5 101.5 103.5 4,000 107 • 109 106.5 109 106 108.5 8,000 105 104 103.5 105 104 106 16,000 96 96 97 97 96 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 ) Sound Pressure Level ,( decibels ) 9Z Sound Pressure Level ( decibels ) Sound Pressure Level ,{ decibels ) 9Z Sound Pressure Level (decibels) Sound Pressure Level .(decibels) APPENDIX III THE EFFECT OF OPERATING PRESSURE ON THE SOUND PRESSURE LEVELS 1. All 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 Graph A Graph B Graph C (psi) 70 114 112 103 80 114.5 113.5 110.5 90 114.5 114 112 70 80 90 113.5 114 115 113 114 115 110.5 111 112.5 105- AIRLEG A CAGI Position: 3 Steel Exposed: 1.0 nri. 250 500 8K I6K Full Octave Band Mid-frequencies- AIRLEG A CAGI' Position: 4 Steel Exposed-. 1.0 mi, I6K Full Octave Band Mid-frequencies 115 AIRLEG B CAGI Position: 4 Steel Exposed: 0.5 m 8K I6K Full Octave Bond 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: 4 Steel Exposed: 1.0 m Full Octave Band Mid-frequencies


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