Application of Phased Array Technology for Identification 

of Low Frequency Noise Sources 

Hugo E. Camargo, NIOSH, USA., E-mail: HCamargo@cdc.gov

Patricio A. Ravetta, AVEC Inc., USA, pravetta@avec-engineering.com

Ricardo A. Burdisso, AVEC Inc., USA, rburdisso@avec-engineering.com

Adam K. Smith, NIOSH, USA, ASmith9@cdc.gov

The findings and conclusions in this report have not been formally disseminated by the National Institute for 

Occupational Safety and Health and should not be construed to represent any agency determination or policy. 

Reference to specific brand names does not imply endorsement by NIOSH. 

Summary 

Noise Induced Hearing Loss (NIHL) is the most common occupational disease in the 

U.S. with devastating consequences particularly in the mining industry. A study 

conducted by the National Institute for Occupational Safety and Health (NIOSH) 

revealed that 90% of coal miners have hearing impairment by age 50, compared to 

only 10% of those not exposed to occupational noise. According to the Mine Safety 

and Health Administration (MSHA), Continuous Mining Machine (CM) operators 

account for 30% of workers exposed to noise doses exceeding the Permissible 

Exposure Level (PEL). This number becomes more dramatic considering that 49% of 

the total national underground coal production is extracted using these machines. 

In this context, NIOSH is conducting research to identify and control dominant noise 

sources in CMs. Previous noise source identification was performed using a Brüel 

&Kjær (B&K) 1.92-m diameter, 42-microphone phased array. These measurements 

revealed that the impacts from the conveyor chain onto the tail roller, and the impacts 

from the conveyor chain onto the upper deck are the dominant noise sources at the 

tail-section of the CM. The objectives of the work presented in this paper were: 1) To 

rank the noise radiated by the different sections of the conveyor, and 2) to determine 

the effect of a urethane-coated tail roller on the noise radiated by the tail-section. This 

test was conducted using an Acoustical and Vibrations Engineering Consultants 

(AVEC) 3.5-m diameter, 121-microphone phased array. The results from this new test 

show that a urethane-coated tail roller yields reductions in the tail-section of 2 to 8 dB 

in Sound Pressure Level in the frequency range of 1 kHz to 5 kHz. However, 

integration of the acoustic maps shows that the front-section and mid-section of the 

conveyor also contain dominant noise sources. Therefore, a urethane-coated tail 

roller in combination with a chain with urethane-coated flights that reduces the noise 

sources in the front and mid sections of the conveyor is required to yield a significant 

noise reduction on the CM operator’s overall exposure. These results show the 

applicability of phased array technology for low frequency noise source identification. 
1. Introduction

Coal is one of the main energy sources used in the United States of America. During

the last decade, coal-generated electricity accounted for 48.98% to 52.83%1 of the 

total electricity generated nationwide. As such, coal mining is a well-regulated 

industry. However, in spite of over 30 years of regulation, NIHL continues to be one of 

the most common occupational diseases among miners. A study conducted by the 

National Institute for Occupational Safety and Health revealed that 90% of coal 

miners have hearing impairment by age 50, compared to only 10% of those not 

exposed to occupational noise2

.

Half of the underground coal production is extracted using the room and pillar 

method. In this mining method, a self-propelled machine known as the CM is used to 

rip the coal from the face and load it into shuttle cars in a continuous operation. A CM 

has three main noise-radiating components: the cutting head, the chain conveyor, 

and the scrubber system. The cutting head is used to rip the coal from the face and 

consists of a rotating steel drum equipped with cutting bits. The chain conveyor is 

used to transport the ripped coal from the front to the back of the machine; it is driven 

by a sprocket wheel located at the front, and it goes around an idler tail roller at the 

back of the machine. The scrubber system is used to collect the dust generated 

during the cutting process by means of a vane axial fan that pulls air through the dust 

collector. The three noise radiating components, as well as the tail roller are shown in 

Figure 1. 

Operator Location 

Figure 1. Noise-radiating components on a CM. 

The use of a CM during the extraction process and the proximity of the operator and 

helper to this machine in a diffuse field environment, are definitely among the 

predominant factors leading to NIHL. According to MSHA, 30.2% of CM operators in 

2006 were exposed to noise levels higher than the Permissible Exposure Level 

(PEL)3

. Within this framework, and in an effort to reduce NIHL in CM operators and 

their helpers, NIOSH is currently conducting research to attenuate the sound radiated 

by a CM.

Previous tests involved using a 1.92-m diameter, 42-microphone B&K phased array 

to identify noise sources on a Joy 14CM9 CM. Measurements along the sides, front, 

back, as well as from overhead of the CM were performed. The acoustic maps 

obtained from these measurements showed that the impacts from the conveyor chain 

onto the tail roller, upper deck and the side walls of the conveyor are the dominant 

noise sources at the tail section in the 1.0 kHz to 1.6 kHz frequency range4

. To 

reduce the impacts from the conveyor chain onto the tail roller, a prototype coated 

roller was built. This prototype consisted of a standard steel roller coated with an 84-

shore A durometer urethane. The use of this prototype yielded a reduction in the total 

radiated sound power level of 3.6 dB(A)5

.
The objectives of the work presented in this paper are: 1) To rank the noise radiated 

by the different sections of the conveyor, and 2) to determine the effect of a urethanecoated tail roller on the noise radiated by the tail-section. To this end, a 40-shore A 

durometer coated tail roller was built and tested using a microphone phased array. 

2. Experimental Setup 

Acoustic measurements of a JOY 

14CM9 CM were conducted in a 

Hemi-anechoic chamber at the 

Pittsburgh Research Laboratory 

(PRL), as shown in Figure 2. This 

chamber is 18.7-meter long, by 

10.1-meter wide, by 7.0-meter high 

and meets the requirements of ISO 

3744 down to 100 Hz. Figure 2 also 

shows the microphone phased array 

used to collect the data in the 

overhead configuration. 

 Figure 2. CM in the Hemi-anechoic chamber.

2.1 Phased Array Measurements 

Measurements were performed using an AVEC beamforming system that consists of 

a 3.5-meter diameter, 121-microphone phased array arranged in a star configuration, 

instrumented with a 128-channel data acquisition system. This system was used to 

collect data simultaneously and continuously for all the microphones at a sampling 

frequency of 51.2 kHz. Sixteen seconds of data were collected, providing sufficient 

data for 100 averages of 8192 samples. 

Since the focus of the test was on the noise radiated by the conveyor of the CM, the 

phased array was set up at two different overhead positions: one position close to the 

middle of the conveyor and one position above the end of the tail section, as shown in 

Figure 3a and in Figure 3b, respectively. 

(a) (b)

Figure 3. Microphone phased array positions with respect to the CM: 

 a) Array position 1, and b) array position 2. 
2.2 Test Configurations 

Two different configurations were tested: 1) The CM provided with a standard steel 

tail roller, and 2) the CM with a prototype urethane-coated tail roller. Figure 4 shows 

the two tested tail rollers.

 (a) Standard steel tail roller. (b) Urethane coated tail roller. 

Figure 4. Tested tail rollers. 

To simulate the damping effect of coal on the noise radiated by the conveyor during 

normal operation, a water flow at a constant rate of 4 gal/min was supplied at the midsection of the conveyor. 

3. Results 

Based on the different noise sources identified along the conveyor path, the conveyor 

can be divided in three parts: Front, mid, and tail sections, shown in Figure 5.

Tail-section

Mid-section

Tail Pivot Front-section

Figure 5. Conveyor sections as seen from overhead. 

3.1 Tail Roller Noise 

The acoustic maps obtained from the CM with the standard steel tail roller are shown 

in Figure 6. These maps confirm that the conveyor chain/tail roller interaction is one 

of the dominant noise sources in the back of the CM. Other noise sources can be 

seen in the mid-section of the conveyor, in particular near the tail pivot, and further 

upstream in the conveyor. For all frequencies, multiple sources or a distributed

source in the very end of the tail were identified.

Figure 7 shows the acoustic maps obtained from the CM with the urethane-coated tail 

roller. On the one hand, this figure shows that the urethane-coated tail roller reduces 

the noise in the tail-section, i.e., there are no noise sources on the tail roller except at

1 kHz. On the other hand, this figure shows that the noise sources on the mid-section 
and front-section of the conveyor remain unchanged, i.e., the urethane-coated tail 

roller has no effect on the noise radiated by these sections.

a) Standard tail roller at 1000 Hz. 

b) Standard tail roller at 1250 Hz. 

c) Standard tail roller at 1600 Hz. 

d) Standard tail roller at 2000 Hz. 

Figure 6. Standard steel tail roller.

a) Coated tail roller at 1000 Hz. 

b) Coated tail roller at 1250 Hz. 

c) Coated tail roller at 1600 Hz. 

d) Coated tail roller at 2000 Hz. 

 Figure 7. Urethane-coated tail roller. 

A quantification of the noise reduction achieved with the urethane-coated tail roller 

was obtained by analyzing the integrated spectra over the tail-section, as shown in 

Figure 8. The integration was performed by adding the levels of all the grid points

inside the integration region and applying a normalization factor given by integration 

of the point spread function of the array in the region6

. The cutoff for the integration 

was set to 5 dB from the peak level in the integrated region. 
Figure 8 shows a significant reduction in Sound Pressure Level in the 1 kHz to 5 kHz 

frequency range. Such reduction varies from 2 to 8 dB. Between 250 Hz and 1 kHz, 

the reduction is not significant. It should be noted that below 630 Hz the roller is not 

the main noise source, and thus, the integrated spectra is most likely reflecting levels 

due to sidelobes of louder sources at such frequencies. 

60

65

70

75

80

85

90

250 315 400 500 630 800 1000 1250 1600 2000 2500 3150 4000 5000

Frequency[Hz]

SPL[dB]

BaselineRoller

UrethaneRoller

Overall Levels

Baseline: 93.4 dB, 93.1 dB(A) 

Urethane: 91.6 dB, 90.9 dB(A) 

Figure 8. Integrated spectra over an area enclosing the tail roller. 

3.2 Conveyor Noise 

This section “combines” the results obtained with the two array positions to identify

and perform a relative comparison of noise sources in the mid-section and frontsection of the conveyor. To better compare the noise from different conveyor 

sections, the results obtained with the urethane-coated tail roller were used, 

assuming that the tail roller noise can be reduced with the urethane treatment. 

Figure 9 shows sample acoustic maps obtained with the array close to the center of 

the CM (Figure 3a). These maps show, on one hand, that the front-section of the 

conveyor contains dominant noise sources over the entire frequency range. 

Furthermore, no other sources are visible above 2 kHz in the mid-section, i.e., the 

conveyor front-section is at least 5 dB louder than any other source at such 

frequencies. On the other hand, the tail roller noise is not as significant as seen with 

the array above the tail-section (Figure 3b) due to the urethane-coated tail roller and 

the relative array position. Therefore, a direct comparison cannot be made between 

the noise levels on the front part of the conveyor and the noise levels on the tailsection. However, this comparison can be made from different array positions and 

different tail roller configurations using the integrated spectra. 
Acoustic maps obtained with the array above the tail-section (Figure 3b), show that 

the noise levels on the mid-section of the conveyor are slightly higher at some 

frequencies. Since both array positions cover one half of the mid-section, as shown in 

Figure 3, such louder levels corresponding to position 2 were used for the integrated 

spectra analysis. Thus, the noise from the front-section was obtained with the array 

closer to the center of the conveyor (Figure 3a) while the spectra for the mid-section 

and tail-section were obtained with the array closer to the end of the machine (Figure 

3b).

 (a) 250 Hz. (b) 500 Hz. 

 (c) 800 Hz. (d) 1600 Hz. 

 (e) 2000 Hz. (f) 3150 Hz. 

Figure 9. Noise sources identified with the array close to the center of the CM (array

position 1). Conveyor with urethane-coated tail roller. 

The integrated spectra results are shown in Figure 10. From this figure it can be seen 

that the front-section and mid-section contain the loudest sources. The levels for such 

regions are similar below 2.5 kHz. Above 2.5 kHz the front-section of the conveyor is 

the loudest source. Note also that the mid-section is louder than the tail-section at all 

frequencies except for the band with center frequency at 3.15 kHz. In the case of the 

urethane-coated tail roller, the treatment renders a reduction in noise levels that 

positions the tail-section as the quietest source on the conveyor. As explained before, 

this results in the noise from the tail-section being at least 5 dB quieter than the other 

sections for most frequencies and hence not “visible” in the previous maps where a 

cutoff of 5 dB from the peak levels was used. 
From Figure 10 it can be seen that a treated tail roller per se will not yield a significant 

overall noise reduction. The sources in the front-section and in the mid-section of the 

conveyor need to be attenuated to have a positive impact on the CM operator’s noise 

exposure. Based on previous research conducted by NIOSH using a conveyor chain 

with urethane-coated flights7, , 8 9, it is expected that a combination of the 40-shore A 

urethane-coated tail roller, and a conveyor chain with 84-shore A urethane-coated 

flights will yield a significant noise reduction in the overall Sound Power Level of at 

least 9 dB(A). To this end, future tests will be conducted to confirm this hypothesis. 

60

65

70

75

80

85

90

250 315 400 500 630 800 1000 1250 1600 2000 2500 3150 4000 5000

Frequency[Hz]

SPL[dB]

Frontsection

Midsection

Tailsection

Tailsection(urethane)

Overall Levels

Front-section: 95.3 dB, 94.7 dB(A) 

Mid-section: 95.1 dB, 94.5 dB(A) 

Tail-section: 93.4 dB, 93.1 dB(A) 

Tail-sect. (uret.): 91.6 dB, 90.9 dB(A) 

Figure 10. Integrated spectra over conveyor sections.

4. Conclusions 

hased array measurements were conducted at the Pittsburgh Research Laboratory. 

he results indicate that the urethane treatment on the roller is effective in reducing 

P

Measurements were performed using an AVEC 121-microphone, 3.5 meter diameter 

phased array in a hemi-anechoic chamber. Data was collected from two array 

positions over the CM. The main goals of these experiments were to determine the 

effect of a urethane-coated tail roller on the noise radiated by the tail-section, and to 

rank the noise radiated by the different sections of the conveyor. The urethanecoated tail roller was compared to an original steel tail roller. The results consisted of 

acoustic maps and integrated spectra over identified noise source regions in 1/3rd

octave bands. 

T

the noise in the tail section between 1 kHz and 5 kHz (about 2-8 dB depending on the 

frequency band). With such treatment, the tail/roller noise is at least 5 dB below the 

maximum level on the acoustic maps. Unfortunately, the integrated spectra over the 

three conveyor sections show that the front-section and the mid-section contain the 

loudest sources. Above 2.5 kHz the dominant noise sources are in the front-section; 

below this frequency, the mid-section and the front-section radiate similar noise 

levels. Therefore, a combination of a urethane-coated tail roller and a chain with 

urethane-coated flights is required to attenuate the noise sources in the front, mid and 

tail sections of the conveyor in order to have a significant impact on the CM’s 

operator overall exposure. 
Acknowledgements 

The authors would like to thank Lynn Alcorn, Pat McElhinney, and Nikolas Macko for 

their help and support during the test. 

References 

 

1

 Energy Information Administration website, Net Generation by Energy Source by Type of Producer: 

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 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. 

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@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: 

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9

 Smith, A.K., Peterson, J.S., and Kovalchik, P,G., “Continuous Mining Machine Conveyor System 

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