Cleveland, Ohio

NOISE-CON 2003

2003 June 23-25

Estimated Sound Power Radiated by Surfaces on a

Continuous Miner Tail Section Using Vibration

Measurements

David S. Yantek

Associate Research Fellow

Health Branch

Hearing Loss Prevention Section

National Institute for Occupational Safety and Health (NIOSH)

626 Cochrans Mill Road

P.O.Box18070

Pittsburgh, PA 15236

Phone: (412) 386 – 4498 Fax: (412) 386 – 4865

Email: dcy6@cdc.gov

ABSTRACT

Miners in underground coal mines experience prolonged exposure to high noise levels, often with A-weighted sound

levels in excess of 90 dB. Field tests have shown that the continuous miner conveyor is a dominant noise source.

This paper will identify the most significant noise-radiating surfaces on the continuous miner tail section using

vibration levels and surface areas to estimate the sound power levels of each surface. In addition, using five test

cases, this study will examine the effects of three engineering noise controls on the surface-averaged vibration levels

and average sound levels measured with four microphones located two meters from the tail section. The noiseradiating surfaces of the conveyor’s tail section include the top and bottom decks, the sideboards, and the flex plates.

Emphasis is placed on the changes in the estimated sound power levels in the 500 – 2500 Hz 1/3-octave bands since

this frequency range accounts for 80-85% of the sound energy near the tail section. The results show that the bottom

deck is the most significant noise radiator for the untreated miner. Installing a coated bottom deck reduces the

estimated sound power level radiated from the bottom deck by 11 dB and the average sound level by 2 dB. Using a

coated tail roller results in reductions of the estimated sound power levels for individual components from 1 to 4 dB

and reduces the average sound level by 2 dB. Coating the slide plate has little effect on both the estimated radiated

sound power levels and the average sound level. The test results indicate that modifications that reduce flex plate

vibration and reduce energy transfer from the tail roller into the tail section will be effective in reducing noise when

combined with a coated tail roller and coated return deck.

1. INTRODUCTION

Noise-induced hearing loss is among the most critical health issues affecting miners today. NIOSH studies of

17,260 audiograms for 2871 coal miners show that 80% of coal miners have a moderate to profound high-frequency

hearing loss by age 64.1

 In addition, at age 50, 90% of miners have a hearing impairment by the NIOSH definition

(an average Hearing Threshold Level of 25 dB or greater for the 1000, 2000, 3000, and 4000 Hz frequencies)

compared to only 10% of people without occupational noise exposure.

Mining equipment that is used to drill and cut rock, coal, or other materials radiates noise due to the impacts

associated with the cutting and drilling processes. In addition, conveying mechanisms that move the cut media to

the point where a haulage vehicle carries it away can generate high noise levels. Some conveyors utilize a chain

with metal bars called flight bars to move the material to the loading point. Impacts between the chain and various

components of the conveyor can cause excessive noise levels.
A continuous miner is a machine that uses a rotating cylinder, or cutting drum, with bits attached to its outer surface

to cut coal. The coal is then loaded onto the gathering pan of the conveyor deck by rotating arms. A conveyor chain

with flight bars moves the cut material along the top deck from the front of the machine to the rear of the miner

where a haulage vehicle loads the material and carries it away. The conveyor chain is driven from the front of the

miner via a sprocket. At the rear of the miner, the tail roller guides the conveyor chain back to the front of the

machine via the return deck. The slide plate holds the tail roller in place between the side rails.

The conveyor is one of the dominant noise sources on a continuous miner. Since the operator position is located

near the rear, or tail section, of the miner, reducing the noise radiated by the tail section is necessary to reduce

operator noise exposure. The tail section noise affecting the operator is due to the flight bars scraping the top deck

and impacting the tail roller and other locations on the machine along the return path exciting the structure which

radiates noise.

Huggins and Remington studied the noise generating mechanisms of the conveyor tail section of a loader, which is

similar to a continuous miner without a cutting drum.2

 A fixture was fabricated using the tail section of a loader.

Noise and vibration measurements were conducted in a reverberation chamber to determine the most significant

sources of noise. The dominant noise-generating mechanism was found to be the chain impacting the tail roller

followed by the chain impacting and scraping on the top and the bottom conveyor deck. An isolated tail roller

consisting of a steel sleeve over a rubber coated tail roller and isolation strips for the top and bottom plates were

developed. Application of these treatments was found to reduce the overall A-weighted sound level by 11 dB for

tests conducted on the test fixture. The researchers point out that, although there is a general similarity in the noise

generating mechanisms of conveyors, the relative ranking of the noise-generating mechanisms may vary widely

between machines.

Galaitsis, Madden, and Andersen evaluated the acoustic performance and durability of constrained layer damping

treatments applied to the top and bottom decks and the side rails of continuous miners.3

 Six treated and eleven

untreated continuous miners were tested underground. The results indicated that the average reduction in the Aweighted sound level at the operator’s position was 5 dB with an empty conveyor and no cutting. With a loaded

conveyor, the A-weighted sound level was reduced by 2 dB. After one year of operation, no significant reduction in

the acoustic performance of the treatment was observed. The researchers also evaluated the performance of resilient

wear strips mounted to the conveyor deck. Above ground tests showed the strips to be both durable and effective at

reducing noise. However, underground testing showed excessive wear of the strips. The researchers suggested

modifying the design of the strips to improve their durability.

2. EXPERIMENTAL PROCEDURES / ANALYSIS METHOD

Vibration measurements were used to estimate the A-weighted sound power radiated from the surfaces of the tail

section of the continuous miner using acceleration measurements.4

 The vibration data were measured following

ISO/TR 7849:1987 Acoustics – Estimation of airborne noise emitted by machinery using vibration measurement.

The major noise-radiating surfaces of the tail section that were examined include the left and right side rails, the left

and right flex plates, the top conveyor deck, and the bottom conveyor deck (see Figure 1).

Four microphones were used to measure the sound pressure during testing (see Figure 2). One microphone was

located two meters above a reference box encompassing the tail section and centered across the top deck. Three

additional microphones were positioned at a height of 1.5 m and located two meters from the left, to the rear, and to

the right of the reference box. The microphones to the left and right of the tail section were centered on the length

of the reference box.

Using beeswax, ten to twelve accelerometers were mounted on the left and right side rails, left and right flex plates,

and the return deck. Figures 3 and 4 show the accelerometer locations used on the return deck and left flex plate,

respectively. For the top deck, accelerometers were embedded in the underside of the deck by drilling 6-mm

diameter holes approximately 6 mm deep. Epoxy was then used to mount accelerometers into the holes. Shallow

grooves were made in the underside of the top deck to run the cables to the outside of the machine (see Figure 5).
A water spray of approximately 3.79 l/min (1 gal/min) was applied to the top deck to simulate the wet environment

encountered in mining. The conveyor was operated and the resulting accelerations and sound pressures were

measured at a sample rate of 25 kSamples/sec with 16-bit resolution. The digitized data were subsequently Aweighted. The accelerations were integrated to determine the vibration velocities. Third-octave filtering was then

applied to the A-weighted sound pressures and vibration velocities. For each test, the surface-averaged mean square

velocity on each surface and the four-microphone averaged sound levels were calculated. The surface-averaged

vibration levels were used with the surface areas to estimate the sound power levels of radiation from each test

surface.

Baseline tests were performed with no engineering noise controls applied to the miner. Following the baseline tests,

the effects of several engineering noise controls on the sound power radiated by the test surfaces and the average

sound levels were examined. The additional tests were performed with the following treatments:

1. Coated return deck;

2. Coated tail roller;

3. Coated return deck and coated tail roller;

4. Coated slide plate.

The sound power radiated by a vibrating surface may be determined by

2 P =σρcSv (1)

where σ is the radiation ratio, ρc is the specific acoustic impedance of the fluid, S is the surface area, and 2

v is the

mean square surface-averaged velocity. For the purposes of comparing the radiated sound power from each surface,

the radiation ratio of each surface was assumed to be 1. The estimated sound power level in decibels may then be

found to by

















+













 = 2

0

2

0 w v

v 10 log SS L 10 log (2)

where S0 is the reference surface area taken to be 1 m2

, and 0v is the reference mean square velocity taken to be 50

x 10-9 m/s according to ISO/TR 7849.

3. RESULTS & DISCUSSION

Baseline tests were conducted with no treatments applied to the machine and the data were processed according to

the previously discussed procedures. The four-microphone average sound level for the baseline conditions is shown

in 1/3-octave bands in Figure 6. The average A-weighted sound level for the baseline tests was found to be 99 dB.

The baseline data show that the 500 – 2500 Hz 1/3-octave bands account for 80 – 85 % of the overall A-weighted

sound level. Therefore, the estimated A-weighted sound power levels in the 500 – 2500 Hz 1/3-octave bands from

each surface were examined. Figure 7 shows a bar plot of the estimated A-weighted sound power level attributed to

each test surface in the 500 – 2500 Hz 1/3-octave bands. These data were calculated using equation 2. The sound

power levels due to each surface may not be correct in an absolute sense due to the assumption that the radiation

ratio is unity. However, the data can be used to assess the relative contribution of each surface. As the figure

shows, the return deck is the most significant noise radiator for the baseline conditions followed by the left side

board, the left flex plate, the right side board, the right flex plate, and the top deck. These findings indicate that the

first attempt at controlling noise must focus on reducing the vibration of the return deck.

As the flight bars on the conveyor chain begin their return, the flights have been observed to impact the beginning of

the return deck. This observation correlates with the high vibration levels and estimated sound power for the return

deck under baseline conditions. To address this problem, a urethane coating was applied to the return deck (see

Figure 8) to decrease the energy transmitted to the return deck by the flight bar impacts. Figure 9 shows the

estimated A-weighted sound power levels in the 500 Hz – 2500 Hz 1/3-octave bands for each test surface with the
coated return deck compared to the baseline tests. The figure shows that the sound power level from the return deck

dropped by 11 dB with the coated return deck. Reductions of 1 to 2 dB occurred on the other test surfaces. With the

coated return deck, the figure shows that the most significant noise radiators are the left side board and left flex plate

followed by the right side board and right flex plate. The return deck is shown to be the least significant noise

radiator. The average overall A-weighted sound level dropped to 97 dB, a 2-dB reduction from the baseline sound

level. The results show that the coated return deck is effective at both reducing the average sound level and the

return deck radiated sound power. However, the significantly lower sound power level reductions on the other test

surfaces indicate that other sources of excitation may be controlling the A-weighted sound level.

The flight bars also impact the steel tail roller transmitting energy into the tail section via the tail roller guides, slide

plate, and tension springs along the tail section. A urethane-coated tail roller (see Figure 10) was developed to

reduce the forces transmitted by these paths. Figure 11 shows the estimated radiated sound power level in the 500 –

2500 Hz 1/3-octave bands for each test surface with the coated tail roller compared to the levels for the baseline test

case. The figure shows that the reductions in radiated sound power for individual surfaces ranged from 1 to 4 dB.

The most significant reduction occurred for the left side board followed by the right side board, the top deck, and the

return deck. The reductions in the sound power levels for the left and right flex plates were determined to be 1 dB.

With the coated tail roller, the dominant noise-radiating surface is the return deck followed by the left side board, the

left flex plate, the right flex plate, the right side board, and the top deck. The data show that coating the tail roller

results in higher reductions of overall radiated sound power compared to the coated return deck. However, the

coated return deck reduced the most dominant source better than the coated tail roller. The average overall Aweighted sound level was reduced from 99 dB for the baseline test case to 97 dB with the coated tail roller.

Since both the coated return deck and coated tail roller reduced the estimated radiated sound power levels and the

average overall A-weighted sound level, tests were performed with both treatments in place (see Figure 12). Figure

13 shows a comparison of the estimated sound power levels in the 500 – 2500 Hz 1/3-octave bands with the coated

return deck and coated tail roller compared to the levels for the baseline continuous miner. The sound power level

attributable to the return deck, the most significant source for the baseline machine, was reduced by 16 dB.

Reductions in sound power level for the left side board, top deck, and right side board were 7, 8, and 14 dB,

respectively. The sound power level from the left and right flex plates remained relatively unchanged with

reductions of 2 dB. The average overall A-weighted sound level dropped from 99 dB for the baseline test case to 95

dB with the coated return deck and coated tail roller. With these treatments, the data show that the left and right flex

plates are the dominant noise-radiating surfaces followed by the left side board. The test data indicate that

treatments that reduce the flex plate vibrations, such as constrained layer damping, will be effective in reducing the

noise levels around the tail section when combined with the coated return deck and coated tail roller.

Since the slide plate is an element that transfers the tail roller-chain impact energy into the tail section, a coated slide

plate (see Figure 14) was fabricated. The coated slide plate was installed and the coated return deck and coated tail

roller were replaced with the non-coated components. Figure 15 shows the estimated sound power level for each

test surface in the 500 – 2500 Hz 1/3-octave bands with the coated slide plate compared to the baseline test case.

The figure shows that the return deck was the most significant noise-radiating surface followed by the left side board

and left flex plate. Reductions in sound power levels were less than 1 dB for the left side board and 1dB for the

return deck. The sound power level from the left flex plate increased by 2 dB. This increase is thought to be a result

of the chain being moved towards the left side of the conveyor during installation of the coated slide plate. This

would cause the flights to transmit more energy into the left flex plate. The small change in sound power levels

from the other surfaces shows that the slide plate by itself is not effective at making a significant impact on the

vibration levels. The average overall A-weighted sound level remained 99 dB.

Although the slide plate by itself is ineffective at reducing the surface vibrations, combining the coated slide plate

with other treatments that reduce the force transmitted via the tail roller ends, slide plate, and springs would be

effective. The test data with the coated tail roller prove that reducing the force applied at the tail roller would reduce

the surface vibrations and sound levels. The next step would be to isolate the tail roller and slide plate from the tail

section. To isolate these components effectively, all mechanical connections must be addressed. The existing

design has steel-steel contact at the interface between the ends of the tail roller and the left and right side boards. In

addition, steel compression springs are used on each side of the conveyor deck to apply tension to the chain. These

paths must be addressed in addition to the contact between the slide plate and the tail section to further reduce

energy transmission and radiated noise.
Figure 16 shows a comparison of the estimated radiated sound power for each surface in the 500 – 2500 Hz 1/3-

octave bands in relation to each test case that showed significant reductions in the estimated radiated sound power.

The average overall A-weighted 1/3-octave spectra for each test are plotted on Figure 17. Figure 17 shows that the

coated slide plate had little effect on any of the 1/3-octave band sound levels. Individually, however, both the

coated return deck and coated tail roller were effective at reducing the sound levels in the dominant frequency band.

The data show that the coated tail roller is more effective than the coated return deck in reducing noise above 2500

Hz. Although the higher frequencies are currently insignificant, these reductions may play a more important role if

other controls can reduce the noise levels in the 500 – 2500 Hz 1/3-octave band.

5. CONCLUSIONS

The return deck was the most significant noise-radiating surface for the baseline tests. Installation of a urethanecoated return deck decreased the estimated sound power level of radiation from the return deck by 11 dB in the 500

– 2500 Hz 1/3-octave bands and reduced the average overall sound level by 2 dB. The estimated sound power levels

for the other test surfaces were reduced by 1 to 2 dB. Replacing the existing tail roller with a urethane-coated tail

roller resulted in reductions in the estimated sound power level of radiation from individual surfaces from 1 to 4 dB

and reduced the average overall sound level by 2 dB. Combining the coated tail roller and coated return deck

reduced the estimated sound power level of radiation from the return deck by 16 dB compared to the baseline levels

and reduced the average overall sound level by 4 dB. Using a coated slide plate was found to have minimal effects

on the estimated radiated sound power levels and the average overall sound level.

REFERENCES 1

 John R. Franks, “Analysis of Audiograms for a Large Cohort of Noise-Exposed Miners,” Internal Report,

National Institute for Occupational Safety and Health, Cincinnati, OH, pp. 1-7.

2

 Geoffrey G. Huggins and Paul J. Remington, “Noise Reduction of Conveyors Used in Underground Coal

Mining Machinery,” Contract H0357085, Bolt, Beranek, and Newman, (Bureau of Mines OFR 109-77, 1976).

3

 A. Galaitsis, R. Madden, and D. Andersen, “Noise Reduction of Chain Conveyors Volume II,” Contract

H0155113, Bolt, Beranek, and Newman, (Bureau of Mines OFR 171-83, 1983).

4

Acoustics – Estimation of Airborne Noise Emitted by Machinery Using Vibration Measurement, Technical Report

ISO 7849: 1987 (International Organization for Standardization, Geneva, Switzerland, 1987).
Figure 1. The continuous miner tail section.

Figure 2. Microphone locations two meters from the reference box around the tail section of the continuous miner.
Figure 3. Accelerometer locations on the return deck of the continuous miner.

Figure 4. Accelerometer locations on the left flex plate of the continuous miner.

Figure 5. Accelerometers embedded in the underside of the continuous miner top deck.
Figure 6. Average A-wtd 1/3-octave sound levels for the

baseline continuous miner.

Figure 7. Estimated A-wtd sound power levels for each

test surface in the 500 – 2500 Hz 1/3-octave

bands for the baseline continuous miner.

Figure 8. Coated return deck for the CM. Figure 9. Estimated A-wtd sound power levels for each

test surface in the 500 – 2500 Hz 1/3-octave

bands for baseline versus coated return deck.

Figure 10. Coated tail roller for the CM.

Figure 11. Estimated A-wtd sound power levels for each

test surface in the 500 – 2500 Hz 1/3-octave

bands for baseline versus coated tail roller.
Figure 12. Coated return deck with coated tail roller.

Figure 13. Estimated A-wtd sound power levels for each test

surface in the 500 – 2500 Hz 1/3-octave bands for

baseline versus coated return deck plus coated

tail roller.

Figure 14. Coated slide plate for the continuous miner.

Figure 15. Estimated A-wtd sound power levels for each

test surface in the 500 – 2500 Hz 1/3-octave

bands for baseline versus coated slide plate.

Figure 16. Estimated A-wtd sound power levels for each

test surface in the 500 – 2500 Hz 1/3-octave

bands for baseline versus effective noise

controls.

Figure 17. Average A-wtd 1/3-octave sound levels for the

baseline continuous miner and continuous miner

with attempted noise controls.