NOISE SOURCE IDENTIFICATION ON A CONTINUOUS MINING MACHINE 

Hugo E. Camargo, Adam K. Smith, Peter G. Kovalchik, Rudy J. Matetic 

National Institute for Occupational Safety and Health – PRL 

626 Cochrans Mill Road, Pittsburgh, PA 15236 

ABSTRACT 

Noise Induced Hearing Loss is the most common occupational 

disease in the U.S. and of paramount importance in the mining 

industry. According to data for 2006 from the Mine Safety and 

Health Administration (MSHA), Continuous Miner operators 

accounted for 30.2% of underground mining equipment 

operators with noise doses exceeding the Permissible Exposure 

Limit (PEL). This figure becomes more significant considering 

that 49% of the 2006 national underground coal production was 

extracted using continuous mining methods. Thus, there is a 

clear need to reduce the sound radiated by Continuous Mining 

Machines. The first step towards efficient noise control of a 

Continuous Mining Machine requires identification of the 

various noise sources under controlled operating conditions. To 

this end, a 42-microphone phased array was used in 

conjunction with 4 reference microphones to sample the 

acoustic field of a machine in the Hemi-anechoic chamber of 

the Pittsburgh Research Laboratory. These data were processed 

using a frequency-domain beamforming algorithm to obtain 

acoustic maps of 5 sides of the machine. The focus of the test 

was on the conveyor noise since previous studies showed that 

operation of the conveyor is the most important contributor to 

the sound radiated by the machine. From the acoustic maps, the 

following potential areas for noise control were identified, and 

included: chain-tail-roller interaction, chain flight tip-side 

board interaction, and chain-upper deck interaction. 

INTRODUCTION 

In today’s industrialized world, noise constitutes one of the 

most common environmental and occupational hazards. 

Consequently, occupational Noise Induced Hearing Loss 

(NIHL) has been recognized by the National Occupational 

Research Agenda (NORA) as the most common job related 

diseases in the U.S1

. Particularly in the mining industry, noise 

has been found to have the most devastating consequences. 

According to a study conducted by the National Institute for 

Occupational Safety and Health (NIOSH), 90% of coal miners 

and 50% of metal/non-metal miners had hearing loss by age 50, 

in contrast to 10% of those who were not exposed to 

occupational noise2

. 

One of the most common extraction methods in the mining 

industry is the continuous mining. In this method, a self 

propelled machine known as the Continuous Miner (CM), 

controlled by an operator and its helper, rips coal from the face 

and loads it into conveyors or shuttle cars in a continuous 

operation. In 2006, 49% of the total coal production in the U.S. 

was extracted using this method3

. 

Analysis of noise samples collected by the Mine Safety and 

Health Administration (MSHA) in 2006 revealed that of the 

various mining equipment operators, 30.2% of CM operators 

were exposed to noise levels exceeding the Permissible 

Exposure Level (PEL) stipulated in the Title 30 of the Code of 

Federal Regulations4 (see Appendix A). This constitutes the 

largest group of overexposed mining equipment operators that 

according to the Bureau of Labor Statistics employs 

approximately 9660 people5

. Other mining equipment with 

overexposed operators were Roof Bolters (20.1%), Bulldozers 

(10.0%), Shuttle Cars (3.6%), and Longwall Shearers (2.6%). 

In this context, NOISH is currently conducting research aimed 

at reducing the noise radiated by the CM. Efficient noise 

control of a CM requires Noise Source Identification (NSI). 

This process involves the determination of the spatial location, 

as well as the frequency content of the dominant sources. 

Traditionally, NSI has been performed by conducting acoustic 

intensity measurements over a grid enclosing the device under 

test. However, this approach is very time intensive as it requires 
 

 

 

the acquisition of data at every grid point for a single test 

configuration. 

The use of phased array technology for noise source 

identification is a relatively new technique that is very time 

efficient. This technique consists of collecting acoustic pressure 

data using an array of microphones and processing the data 

with a beamforming algorithm. 

The present work describes the use of phased array technology 

for noise source identification on a CM. The three main 

components that radiate noise from a CM are the cutting head, 

the conveyor and the dust collector fan, which are shown in 

Figure 1. Of these three components it was determined that 

conveyor is the most significant contributor to the total radiated 

sound6

. 

Tail Section 

Conveyor Path 

Cutting Head 

Dust Collector Fan 

Tail Roller 

Foot Shaft 

Figure 1. Noise generating components on a CM. 

Previous studies involved measurements of Sound Pressure 

Level (SPL) over a grid enclosing the entire machine on a quasi 

free field environment6

. Results from these measurements 

showed that most of the sound was being generated at the tail 

section of the CM. However, it was not clear what mechanisms 

or sources were generating these high noise levels in this area 

of the CM. Consequently, conveyor noise has been the subject 

of previous research6,7,8. From these studies, three noise 

controls were proposed: 1) A jacketed Tail Roller (TR), i.e. a 

resilient material between an inner and an outer steel shell, 2) a 

urethane-coated Tail Roller, and 3) urethane-coated chain 

flights. Figure 2 shows these noise controls. 

(a) (b) (c) 

Figure 2. Controls for tail section noise: a) Jacketed TR, b) 

urethane-coated TR, and c) urethane-coated chain flights. 

Use of a urethane-coated TR resulted in a noise reduction of 2 

dB(A) in Sound Power Level. However, the main drawback of 

this control was its durability when tested underground it only 

lasted 10 days of operation before the urethane was torn from 

the roller. A standard TR is not replaced until the CM is rebuilt. 

The jacketed TR yielded a modest reduction of approximately 1 

dB(A) in Sound Power Level. The most promising control was 

the chain with urethane-coated flights. Implementation of this 

control attenuated the Sound Power Level by 10 dB(A)6

. 

NOMENCLATURE 

yn(t) Acoustic pressure recorded by microphone n. 

Y(f) Fourier Transform of y(t).
 

p(xb,t) Acoustic pressure induced by a source at xb. 

P(f) Fourier Transform of p(t).
 

g Free-field Green’s function. 

Δt Propagation time between source and observer. 

b(xb) Beamform output. 

R Resolution in terms of the spotsize [m]. 

λ Acoustic wavelength [m]. 

L Distance between array and potential source [m]. 

D Diameter of the array [m]. 

psf Point Spread Function.
 

EXPERIMENTAL SETUP 

Acoustic measurements were conducted in the Hemi-Anechoic 

chamber of Pittsburgh Research Laboratory (PRL). This is a 

17.7-meter long by 10.4-meter wide by 7.0-meter high chamber 

that meets the requirements of ISO 3744 down to 

approximately 100 Hz. The device under test was a new JOY 

14CM-15 Continuous Miner provided with a 38-inch wide 54-

flight conveyor chain. The chain was set under tension by 

means of a hydraulically controlled take up system. Figure 3 

shows the CM in the Hemi-Anechoic chamber. 

Phased Array 

Figure 3. Continuous Miner in the Hemi-Anechoic chamber. 
 

Data were collected using a 1.92-meter 42-element Bruel & 

Kjaer microphone phased array. The array has a spoke-wheel 

configuration with a total of seven arms, each holding 6 

microphones. Due to the large size of the CM with respect to 

the array, various measurements along the sides, front, back, 

and overhead of the CM were performed. Figure 4 shows the 

various positions of the array with respect to the CM at which 

data was collected. For the side, front and back measurements, 

the array was at a distance of 2 meters from the CM, whereas 

for the overhead measurements the array was at 1.75 meters. In 

addition to the phased array measurements, the Sound Power 

radiated by the CM was measured in PRL’s reverberation 

chamber9

, according to the ISO3743-2 standard. 

Area 

covered 

Array 

position 

Figure 4. Phased array positions with respect to the CM in the 

Hemi-Anechoic chamber. 

During normal operation of the conveyor, i.e. moving coal from 

the front to the back of the CM, coal has a damping effect on 

the noise radiated by the conveyor. Previous work at PRL 

showed that a continuous water flow supplied to the middle of 

the conveyor has the same effect as coal. Therefore, in order to 

simulate the noise damping effect of coal, a water flow rate of 4 

gal/min was used in the present test. 

BEAMFORMING 

The underlying principle on which beamforming theory was 

developed is that a noise source is sensed by the microphones 

in the array with a time delay, i.e. phase shift, and amplitude 

differences as shown in Figure 5. It is these differences that are 

used to identify the location and the strength of the source. The 

array is electronically steered to each potential source point of 

interest by adjusting the phase of the microphone data. The 

approach to make this phase adjustment is to assume a noise 

source exists at each potential point of interest, xb. The simplest 

and most commonly used source model is that of a monopole 

given by: 

s(t − r / c)

p(x,t) = , (1) 4πr 

where p is the acoustic pressure perceived at time t by the 

observer located at x; s is the signal emitted by the monopole 

located at xs, r is the distance between the source position and 

the observer position, and c is the speed on sound. 

1

r

nr

Wavefronts Potential source

Nr

z

location, xb

1

r

nr Nr

z

location, xb 

x Phased Array
 

Microphones
 

(1 y t) y ( n )t y ( N )t 

Figure 5. Phased array measurements of a point source. 

The frequency-domain model of equation (1) is given by: 

P(x, f ) = S( f )g(x, f ) , (2) 

where f is the frequency in Hz, S(f) is the Fourier Transform of 

s(t) and g(x,f) is the free-field Green’s function: 

−i2πfΔt e 

g(x, f ) = (3) 4πr 

Δt is the time required for the sound to travel from the source at 

xb to an observer located at x, and thus it is also known as the 

propagation time: 

Δt = r / c (4) 

For an array of N-microphones, at each potential source point 

of interest xb, the following expression is evaluated: 

g *

(x ) g(x ) b * b b(xb ) = Y( f )Y ( f ) (5)

g(xb ) g(xb ) 

where Y(f) is a vector containing the Fourier Transforms of the 

time series of the acoustic pressure recorded by each 

microphones of the array: 

⎡Y1( f ) ⎤ ⎢ ⎥ Y2 ( f ) Y( f ) = ⎢ ⎥, (6) ⎢ M ⎥ ⎢ ⎥ Y f ⎣ N ( )⎦
and g(xb) is a vector containing the acoustic pressure induced at 

the array microphones by the assumed source at xb, given by 

the free-field Green’s function: 

⎡ g1 ( f )⎤ ⎢ ⎥ g2 ( f ) 

g(x =
 ⎢ ⎥

b )
 (7) ⎢ M
 ⎥ ⎢ ⎥ ⎣gN ( f )⎦

The normalized vectors g(x)/║g(x)║, are also known as the 

steering vectors and are usually denoted as w(x). The beamform 

expression b(xb) represents an estimate of the acoustic pressuresquared at the array location caused by the source at xb. 

The product Y(f)Y*

(f) is the cross-spectral matrix of the 

microphone data. The diagonal elements of this matrix are the 

auto-powers which do not carry phase information but usually 

contain microphone self noise. Therefore, in order to improve 

the beamform output, these diagonal elements are usually 

removed from the cross-spectral matrix and are substituted by 

zeros. This process is also known as Diagonal Removal (DR). 

In order to evaluate the performance of a microphone phased 

array at any frequency of interest, a simple point source is 

simulated at some desired distance from the array center. Then, 

using a beamforming algorithm, an acoustic map of this 

simulated source is computed. This particular acoustic map is 

also known as the array beam pattern or the array Point Spread 

Function (psf)

10, and is given by: 

psf (x , x ) = w 

2 * 

x b S ( b )w ( x S ) (8)

The Point Spread Function of a phased array contains a mainlobe and various side-lobes. The main-lobe is due to the 

simulated source. However, the side lobes are purely 

mathematical, i.e. do not represent any simulated source, and 

are a consequence of the finite number of microphones in the 

array. Therefore, the psf yields information regarding: 1) the 

resolution of the acoustic maps also known as the spotsize 

which is defined as the width of the main-lobe 3 dB down from 

its peak value, and 2) the dynamic range, that is the difference 

between the main-lobe peak value and the maximum side-lobe 

peak value. The variables that determine the resolution R of the 

acoustic maps are the size of the array D, the distance between 

the array and the source L, and the frequency of interest 

represented by the wavelength λ. The following expression can 

be used as a rule of thumb11: 

R = 1.22λL /
 D (9)

Equation (9) was used to compute the resolution of the array at 

three different source-array distances used in the present test. 

Figure 6 shows these values as a function of frequency. 

From Figure 6 it can be observed that for this particular array 

and test setup, a moderate resolution is obtained for frequencies 

between 400 Hz and 1000 Hz. However, for frequencies above 

1000 Hz, the spotsize compared to the dimensions of the 

machine imparts a good resolution. 

175 

At 1 50 meters 

150 At 1.75 meters 

At 2 00 meters

125

Spotsize (cm) 100 

75 

50 

25 

0 

0 250 500 750 1000 1250 1500 1750 2000 

Frequency (Hz) 

Figure 6. Resolution of the array used in the present test for 

source located at three different distances. 

RESULTS 

Phased array data were processed using a frequency-domain 

beamforming algorithm that removes the diagonal elements of 

the cross-spectral matrix. Beamform output was obtained in the 

form of 1/3-octave band acoustic maps. The frequency range of 

interest was 400 Hz to 3150 Hz. Sound power measurements in

a reverberation chamber revealed that around 91% of the

acoustic power radiated by a CM is emitted in this frequency

range. Figure 7 shows the A-weighted sound power measured

in a reverberation chamber.

50 

80 

125 

200 

315 

500 

800 

1250 

2000 

3150 

5000 

8000 

120 

91 % 

110 

Sound Power Level (dBA) 100 

90 

80 

70

60

1/3-Octave Band Frequency (equency (equency (Hz)

Figure 7. A-Weighted Sound Power Level emitted by the CM

with the conveyor operating in a reverberation chamber. 
Overhead measurements revealed dominant noise sources 

located at the tail section of the CM; more specifically in the 

vicinity of the tail roller. Examination of the acoustic maps 

suggests three different noise mechanisms: 1) Chain-tail roller 

interaction, 2) flight tip-side board interaction, and 3) flightupper deck interaction. Figure 8 shows these sources at four 

different frequencies. 

 

 

(a) 

Tail Roller 

(dB) 

80 81 82 83 84 85 83 84 85 86 87 88 

(b) (c) 

(dB) 

73 74 75 76 77 78 74 75 76 77 78 79 

(d) (e) 

 

 

Figure 8. Dominant sources. a) Covered area by the acoustic 

maps at b) 500 Hz, c) 1000 Hz, d) 1600 Hz, and e) 2500 Hz. 

The acoustic maps shown in Figure 8 unveil three different 

types of noise sources that were previously suspected but not 

 

confirmed. It can be seen that not only the impacts from the 

chain links onto the tail roller generate the noise at the tail 

section. The impacts from the flight tips onto the side boards, 

and the impacts from the chain flights onto the upper deck are 

also significant noise radiators; this explains why the coated 

flight chain, which attenuates these impacts, yielded the most 

promising results. Therefore, a combination of both the coated 

TR and the chain with urethane-coated flights is required to 

effectively reduce the strength of the sources at the tail section. 

Secondary sources were identified at the front end of the left 

flex-board guide at 1600 Hz and 2000 Hz as shown in Figure 9. 

It is suspected that impacts from the chain flight tips cause the 

flex-boards to vibrate which in turn rattle against the guides. 

Since there is a small clearance of approximately 1 cm between 

the flexi-boards and the flight tips, a small transverse 

displacement of the chain will cause a shift of the impacts from 

the right to the left flex-boards and vice versa. A similar 

phenomenon was observed in previous studies when the 

vibration of the left flex-plate was larger than that of the right 

flex-plate8

. 

 

(a) 

Flexiboards Guides 

(dB) 

73 74 75 76 77 78 72 73 74 75 76 77 

(b) (c) 

Figure 9. Secondary sources. a) Covered area by the acoustic 

maps at b) 1600 Hz, and c) 2000 Hz. 

Measurements with the array at the rear of the CM confirmed 

the three noise mechanisms suggested by the overhead 

measurements. Figure 10 shows the acoustic maps obtained 
 

 

 

 

 

with the array at the back for the same frequencies shown in 

Figure 8. 

75 

75 

(a) 

87 

86	 

84	 

85	 

(dB) 

83	 

82	 

88	 

87 

86 

(dB) 

85 

84 

83 

77 

76	 

(dB) 75

74 

73 

72 

77 

76 

(dB) 75

74 

73	 

72 

(b) 

(c) 

(d) 

(e) 

Figure 10. Dominant sources. a) Covered area by the acoustic 

maps at b) 500 Hz, c) 1000 Hz, d) 1600 Hz, and e) 2500 Hz. 

Acoustic maps from the left side of the CM show two dominant 

sources: 1) at an opening of the lower deck where the tail 

section pivots right and left, and 2) at the place where the chain 

enters the lower deck upon leaving contact with the tail roller. 

Figure 11 shows these two noise source locations. 

Acoustic maps from the right side of the CM reveal a source at 

the place where the flexi-board interacts with the guide, as 

shown in Figure 12 on pg 7. Note that overhead results in 

Figure 8 show a noise source at the left flexi-board-guide 

interaction place and none on the right side. However, it should 

be kept in mind that these measurements were taken at different 

times and thus the triggering mechanism, i.e. the impacts from 

the flight tips to the flexi-boards, probably shifted from left to 

right. 

Measurements were also conducted with the array in front of 

the CM. Since the focus of this work was on conveyor noise, 

the front part of the conveyor. Acoustic maps from these 

measurements show a dominant source where the chain 

the cutting head of the CM was raised, leaving a direct view to

engages the upper deck. Figure 13 shows this source at 1600 

Hz and 2000 Hz. However, considering the operators location, 

shown in Figure 13a, these sources are not as significant as 

those in the tail section. Furthermore, use of a coated chain will 

also reduce the strength of this source.

(a) 

Source 1 Source 2 

(dB)

73 74 75 76 77 78 

(b) 

Figure 11. Sources on the left side of the CM. a) Covered area 

by the acoustic maps at b) 1250 Hz.
-

(a) 

Flexi board-guide 

interaction noise 

(dB)

67 68 69 70 71 72 

(b) 

Operator 

Location 

(a) 

(b) 

83 

82 

81 

(dB) 

80 

79 

78 

80 

79 

78 

(dB) 

77 

76 

75 

(c)

 

 

 

 

Figure 12. Sources on the right side of the CM. a) Covered 

area by the acoustic maps at b) 1600 Hz. 

Figure 13. Sources on the front of the CM. a) Covered area by 

the acoustic maps at b) 1600 Hz, and c) 2000 Hz. 

CONCLUSIONS 

Noise source identification on a CM was performed using 

phased array measurements and a frequency domain 

beamforming algorithm with Diagonal Removal. The focus of 

this test was on conveyor noise. Dominant noise sources were 

identified at the tail section of the CM. Acoustic maps obtained 

with the array overhead and in the back of the CM reveal that 

not only the impacts from the chain onto the tail roller generate 

noise at the tail section, but also the impacts from the chain 

flights onto the side boards and onto the upper deck are 

significant noise radiators. Therefore, in order to effectively 

reduce the noise radiated by the tail section of the CM, a 

combination of both a coated tail roller and a chain with 

urethane-coated flights will be required. Even tough the coated 

TR exhibited durability issues, the advent of new engineering 

materials such as carbon fiber and Kevlar composites opens the 

possibility for further research on this control. 

A noise source was identified at the front of the CM, in the 

place where the chain engages the upper deck. However, since 

this source is located away from the operator’s position, it is 

not considered as significant as the noise sources at the tail 

section. Secondary noise sources were also identified at the 

guides of the flexi-boards. It is suspected that the impact from 

the flight tips onto the flexi-boards triggers these sources. Most 

of these noise sources can be attenuated by implementation of a 

chain with urethane-coated flights. 

Based on these results, future work will involve testing tail 

rollers coated with urethane of different hardness. Also, the 

acoustic performance of flexi-boards with constrained layer 

damping will be investigated. 

ACKNOWLEDGMENTS 

The authors would like to thank JOY Mining 

Manufacturing Co. for providing the CM. Thanks are also due 

to Lynn Alcorn and Patrick McElhinney for their help and 

support during the test. 

REFERENCES 

1

 Smith, A.K., Spencer, E.R., and Alcorn, L.A., “Underground 

Evaluation of Coated Flight Bars for a Continuous Mining 

Machine”, Proceedings of Inter-Noise 2006, Honolulu, Hawaii, 

USA, Dec. 3-6, 2006. 

2 Franks, J.R., “Analysis of Audiograms for a Large Cohort of 

Noise-Exposed Miners”, National Institute for Occupational 

Safety and Health, Internal report, Cincinnati, OH,1996. 

3

 Energy Administration Information website, Underground 

Coal Production by State and Mining Method, Data for 2006, 

http://www..eia.doe.gov/cneaf/coal/page/acr/table3 html. 
 

4

 MSHA [2008]. MSHA Standardized Information System, 

Number of Noise Samples @90PELDose(computed)>=100 by 

Machine Code, 2006. Denver, CO: U.S. Department of Labor, 

Mine Safety and Health Administration, Office of Injury and 

Employment Information. Date accessed: March 20, 2008. 

5

 Bureau of Labor Statistics website, Occupational Employment 

and Wages, May 2006, 47-5041 Continuous Mining Machine 

Operators, http://www.bls.gov/oes/current/oes475041.htm. 

6

 Kovalchik, p., Johnson, M., Burdisso, R., Duda, F., and Durr. 

M., “Noise Control for Continuous Miners”, Proceedings from 

the 10th International Meeting on Low Frequency Noise and 

Vibration and its Control, York, England, Sept. 11-13, 2002, 

PP. 299-306. 

7 Smith, A.K., Spencer, E.R., Alcorn, L.A., and Kovalchik, P.G., 

“Underground Evaluation of Coated Flight Bars for Continuous 

Mining Machines”, Inter-Noise 2006 Proceedings, Dec. 3-6, 

2006, Honolulu, Hawaii, USA. 

8

 Smith, A.K., Yantek, D.S., and Peterson, J.S., “Development 

and Evaluation of a Urethane Jacketed Tail Roller for 

Continuous Mining Machines”, Proceedings from

IMECE2007, IMECE2007-41821, Seattle, Washington, Nov. 

11-15, 2007. 

9

 Peterson, J.S., and Bartholomae, R.C., “Design and 

Instrumentation of a Large Reverberation Chamber”, 

Proceedings of Noise-Con 2003, Cleveland, Ohio, June 23-25, 

2003. 

10 Muller, T.J. (Editor), Aeroacoustic Measurements, Springer, 

2002. 

11 Christensen, J.J., and Hald, J., B&K, “Beamforming”, Bruel 

& Kjaer Technical Report No 1-2004. 

 

 

 

 

APPENDIX A 

Permissible Exposure Level 

According to the Title 30 Code of Federal Regulations, Section 

62.101, the Permissible Exposure Level is a Time-Weighted 

Average for an 8-hour shift (TWA8) of 90 dB(A). This value is 

equivalent to a dose of 100% of that permitted by the standard, 

given by: 

⎛ C C C ⎞ 1 2 n D =100⎜

⎜ + +L+ ⎟

⎟ (A.1) 

⎝ T1 T2 Tn ⎠

where Cn is the time a miner is exposed to a specific sound 

level and Tn is the reference duration exposure for that specific 

sound level, given by the following expression: 

8 T = (A.2) n L −90) / 5 n 2( 

where Ln is the measured A-weighted, slow-response sound 

pressure level. 

Once the dose percentage D is computed using equations (A.1) 

and (A.2), the equivalent TWA8 can be obtained from: 

TWA8 =16.61⋅ log10 (D /100) + 90 (A.3) 

Equation (A.3) yields the TWA8 which represents the sound 

level that, if constant throughout the 8-hour shift, would result 

in the same measured dose D.