Noise and vibration reduction 

of a vibrating screen 

D.S. Yantek, P. Jurovcik and E.R. Bauer 

Mechanical Engineer, general engineer and mining engineer, respectively, 

NIOSH, Pittsburgh, Pennsylvania 

Abstract 

Workers in coal-preparation plants, where vibrating screens are significant noise sources, are often exposed 

to sound levels exceeding 90 dB(A). The National Institute for Occupational Safety and Health, a major coal 

company and a vibrating-screen manufacturer conducted a cooperative study of vibrating-screen noise. 

Rubber isolators reduced sound levels by 1 dB. Urethane screen panels did not reduce sound levels in this case. 

Drive noise was determined to be the dominant noise source on the screen. Damped side plates reduced 

vibration levels on the screen sides by 1 to 7 dB(A). Detailed results of the study are presented. 

Introduction 

In 2000, there were 212 preparation plants in operation in the 

United States and 129 of these plants were located in three 

states: Kentucky, Pennsylvania and West Virginia (Fiscor and 

Lyles, 2000). Studies by The National Institute for Occupational Safety and Health (NIOSH) have shown that workers 

who spend a significant portion of their shift working in a coal 

preparation plant can experience noise exposures that exceed 

the Mine Safety and Health Administration (MSHA) Permissible Exposure Level. Noise doses up to 220% have been 

recorded for preparation-plant workers in jobs with titles such 

as stationary equipment operator, froth cell operator, plant 

operator, plant controls man, third floor operator, wet plant 

attendant, sump floor operator, plant backup and plant mechanic (Bauer, 2004). These job classifications require the 

worker to spend a considerable portion of a shift in the plant 

while working around slurry pumps, dryers, centrifuges and 

vibrating screens. Vibrating screens generate sound levels 

ranging from 90 to 95 dB(A) during clean bituminous coal 

processing and 95 to 100 dB(A) during refuse and anthracite 

processing (Ungar et al., 1974). 

Because they are used to size, separate and dewater both 

coal and refuse (rock) of various sizes, screens may be located 

on many floors within a preparation plant. The number of 

screens in a processing plant can range from a single screen to 

more than a dozen. Consequently, preparation-plant workers 

can be exposed to high sound levels generated by the screens 

multiple times during a shift as they move and work throughout 

the floors within the plant. Vibrating screens are a major noise 

problem in most coal preparation plants because screens are 

used extensively in the plants, are usually located in high traffic 

areas and can generate high noise levels (Rubin et al., 1982). 

Due to the revised Health Standard for Occupational Noise 

Exposure (30 CFR Part 62) passed in 1999, the Mine Safety 

and Health Administration (MSHA) no longer gives credit for 

wearing hearing protection in determining a worker’s noise 

dose. This new MSHA regulation has reemphasized the use of 

engineering noise controls to reduce worker noise exposure. 

To address the problem of worker noise exposure in coal 

preparation plants due to vibrating screens, NIOSH, a major 

coal company and a vibrating screen manufacturer at a coal 

preparation plant conducted a cooperative study of vibrating 

screen noise. 

The consortium selected a coal preparation plant for the 

study based on its proximity to all of the involved researchers. 

In the selected plant, preliminary measurements were made in 

the area surrounding a group of eight horizontal vibrating 

screens used to process clean coal. These measurements 

indicated that the sound levels ranged from 94 to 98 dB(A) 

with the plant processing coal (see Fig. 1) and from 89 to 97 

dB(A) without the plant processing coal and the screens 

operating with vibration only. The sound levels decrease 

significantly with increasing distance from the screens, indicating that the screens dominate the overall A-weighted sound 

level in this area of the preparation plant. To reduce the 

potential for overexposing preparation-plant workers to noise, 

the collaborators agreed to test several commonly used engineering noise controls for vibrating screens. In addition, the 

collaborators initiated a research effort to rank the noise 

sources on one of the horizontal vibrating screens and to 

develop engineering noise controls to address the dominant 

noise sources. 
Figure 1 — Contour plot of the A-weighted sound levels around a group of eight 

clean coal screens. 

Experimental procedures 

After the plant operation, test options and available screens 

were discussed, a single 2.4- x 4.9-m (8- x 16-ft) horizontal 

vibrating screen used to drain and rinse a 1 x 10-mesh 

cyclone clean coal product was selected as the test screen. 

The test screen, Screen 169, operates within a group of eight 

similar screens as shown in Fig. 2. 

Figure 2 — Overhead view of test area. 

The plant is designed such 

that the eight screens are split into two groups of four screens, 

referred to as Side 1 and Side 2. The sound levels at any 

location within the test area are influenced by each of the 

eight screens. Ideally, the sound level around the test screen 

could be measured with only the test screen processing coal. 

However, the seven other screens cannot be turned off while 

processing coal on only the test screen. 

All of the screens from either Side 1 or 

Side 2 can be turned off while processing 

coal. In addition, one screen from Side 1 

or Side 2 can be turned off while processing coal with the remaining screens. Therefore, without some means of reducing the 

contribution of the other screens to the 

sound level around the test screen, evaluating the sound levels due to the test 

screen and the noise reductions due to 

installing engineering controls would be 

impossible. 

To reduce the background sound levels 

around the test screen due to the nearby 

plant machinery, quilted barrier-absorbers consisting of two fiberglass layers 

with a loaded vinyl septum and covered in 

a nylon material were hung from the plant 

support beams around the test screen (see 

Fig. 3). The barriers were attached to the 

support beams of the above floor with 

bolts through grommets. The barriers were 

sized such that they touched the floor 

when installed. The seams between individual barrier strips were sealed with 

Velcro, and gaps around pipe penetrations 

were kept to a minimum. The above steps 

were necessary to achieve the maximal 

reduction in the background noise near 

the test screen. 

Six microphones were positioned at a 

height of 1.5 m (5 ft) and a distance of 0.6 

m (2 ft) from a reference box surrounding 

the test screen (see Fig. 4). Four microphones were positioned along the left side 

of the test screen with two microphones 

behind the feed chute. Due to the proximity of the barrier to the right side of the test 

screen, it was not possible to position 

microphones along the right side of the 

screen. Microphones could not be positioned at the discharge end of the screen 

between the test screen and Screen 170 

due to the screen design. The data-acquisition equipment was positioned in the corner of the building as shown in Fig. 5. 

A series of sound-level measurements 

was performed to assess the impact of the 

barrier-absorbers on the background noise 

near the test screen. Sound-level measurements were performed with the plant processing coal with the test screen off. Next, measurements 

were performed with the screens in Side 1 processing coal and 

Side 2 off. These conditions represent the background sound 

levels that would be observed during testing. The barriers 

were then installed and the measurements were repeated. The 

measurements indicated that the background sound level was 

reduced by more than 11 dB(A) with the plant processing coal 

with the test screen off and by more than 7 dB(A) with the 

screens in Side 1 processing coal and the screens in Side 2 off. 

These results show that the barrier was effective in reducing 

the background noise and would enable the researchers to 

evaluate noise controls and to study the noise generated by the 

test screen. 
Figure 3 — Barrier-absorber curtain between Screens 

169 and 167 showing tight seal around piping. 

Figure 4 — Overhead view of microphone positions around test Screen 169 showing barrierabsorber curtain location. 

Figure 5 — Data acquisition equipment location and 

microphones along left side of Screen 169. 

Trial testing of commonly used engineering 

noise controls 

In the beginning of the project, the coal company wanted to 

test several engineering noise controls that are commonly 

applied to vibrating screens: rubber isolation mounts, urethane screen panels and a urethane sieve. Prior to all testing, 

a new set of steel coil springs was installed to ensure the 

baseline sound levels would not be affected by broken or worn 

springs. Next, rubber isolation mounts replaced the new steel 

coil springs. After the tests with the rubber mounts were 

performed, the standard steel screen panels were replaced 

with urethane screen panels. After examining the test data, it 

was determined that the coal/water flow was a secondary 

noise source and the urethane siev would, therefore, not have 

an impact on sound levels for this screen. 

Steel coil springs vs. rubber isolators. After the barrierabsorber curtain was in place, sound-level measurements 

were performed with new steel coil springs installed. To 

determine the effect of switching to rubber isolators on building vibration, accelerometers were attached to the screen at 

the steel coil spring locations on both the screen side and 

building side of the springs to measure accelerations in both 

the fore/aft (x) and vertical (z) directions as shown in Fig. 6. 

Figure 7 shows the accelerometers at one mounting location 

with the steel coil springs (upper picture) and rubber isolators 

(lower picture). 

First, the sound level due to the test screen processing coal 

was determined. With the barrier in place and the new steel 

coil springs installed, the sound levels around the screen and 

the vibration at the screen mounts were measured with the test 
screen off and all other screens processing coal. The test 

screen was then turned on and the measurements were repeated with all screens processing coal. The sound level 

produced due to the test screen processing coal could then be 

determined by 

 L = 10LOG ( 100.1L P,2 − 1001 . LP ,1

P ,/ w caol ) (1)

where 

LP,w/coal is the background-noise-corrected sound level 

due to the test screen processing coal, 

LP,1 is the sound level measured with the test screen off and 

the seven other screens processing coal and 

LP,2 is the sound level measured with all of the screens 

processing coal. 

It should be noted that all of the equations presented in this 

report were used to calculate both overall and 1/3-octave band 

sound levels. 

Next, sound-level measurements were performed with 

Side 1 processing coal and Side 2 off. The test screen, Screen 

169, was then operated with vibration only with the Side 1 

screens processing coal and the sound level measurements 

were repeated. The background-noise-corrected sound levels 

due to the test screen for vibration only operation were then 

calculated by 

 L = 10LOG (100.1L P,4 − 100 1. LP,3 

P v, ib ) (2)

where 

LP,vib is the background noise corrected sound level due to 

operating the test screen with vibration only; 

LP,3 is the sound level measured with the Side 1 screens 

processing coal and the Side 2 screens off; and 

LP,4 is the sound level measured with the Side 1 screens 

processing coal, the test screen operating with vibration 

only and the other screens in Side 2 off. 

Figure 6 — Accelerometer locations used during steel coil 

spring and rubber isolator testing. 

Figure 7 — Accelerometers affixed to screen and base at 

isolator mount locations. 

Similar procedures were followed throughout the study 

when sound level measurements had to be taken with the nontest screens operating. 

Figures 8 and 9 show the background-noise-corrected Aweighted 1/3-octave band sound levels due to the test screen 

for full operation and vibration only, respectively. The figures 

show that using rubber isolators reduced the overall sound 

level by 1 dB(A) for full operation and 2 dB(A) for vibration 

only operation. The figures show that small reductions occur 

throughout the 100 to 10 kHz frequency range. 

When altering the suspension of a vibrating screen, the 

effects of the modifications on both the sound levels and the 

building vibration are important considerations. Vibration 

measurements at the mounting locations with the steel coil 

springs and rubber isolators were compared to examine the 

vibration associated with noise radiation and building vibration. Vibrations at frequencies above 20 Hz are significant in 

terms of noise radiation. In addition, spikes in the vibration 

time history are indicative of impacts that would generate 

significant noise levels. With regard to building vibration, 

vibrations at the eccentric mechanism rotation frequency, 15 

Hz in this case, are most important. 

To examine the vibration associated with noise radiation, 

the time waveform of the accelerometer signal on the building 

side of the mounting location highlighted in Fig. 10 was 

compared for the steel coil springs and rubber isolators. 

Figure 11 shows the time waveform with the steel coil 

springs and with the rubber isolators. The figure shows that 15 

impacts per second occurred with the steel coil springs. This 

impact rate corresponds with the rotation frequency of the 

vibration mechanisms. The impacts cause peak-to-peak highfrequency vibrations on the order of 75 g at this location. 

Similar results were observed at the other mounting locations. 
Figure 8 — A-weighted 1/3-octave band sound levels for steel coil springs vs. 

rubber isolators, full operation. 

Figure 9 — A-weighted 1/3-octave band sound levels for steel coil springs vs. 

rubber isolators, vibration only. 

The impacts could be due to contact between an inner and outer coil spring, a coil 

spring and its positioning disk, a friction 

check and the screen, or the screen and a 

fixed object such as a chute, pipe, or support structure. These impacts cause the 

screen and its support structure to vibrate 

and radiate noise. Therefore, these impacts must be eliminated to reduce screen 

noise in the most efficient manner. 

The accelerations in the 16 Hz 1/3­

octave band were compared for the steel 

coil springs and the rubber isolators, because this frequency band contains the 

mechanism rotation frequency. These low 

frequency vibrations can be a concern with 

respect to building vibration. 

Figure 12 shows the percentage change 

in the accelerations in the 16 Hz 1/3­

octave band for the fore/aft (x) and vertical 

(z) directions on both the building (base) 

and screen side of each mount. The figure 

shows that the building vibration at the left 

mount on the discharge end and the right 

mount on the feed end of the screen increased substantially. Subjectively, an increase in the building vibration could be 

felt in the corner of the building where the 

acquisition equipment was located. The 

low-frequency vibration with the rubber 

isolators was significantly higher than with 

the steel coil springs, even though the 

high- frequency vibrations were lower with 

the rubber isolators. 

The increase in building vibration is 

probably due to a difference between the 

system spring rates with the rubber isolators and the steel coil springs. With the 

steel coil springs, two sets of inner and 

outer springs are used at each mount at the 

discharge end whereas one inner and outer 

spring and a third spring are used at each 

mount on the feed end. However, with 

this arrangement, the product of the feed 

end spring rate with the distance from the 

feed end mounting location to the screen 

center of gravity is equal to the product of 

the discharge end screen rate with the 

distance from the discharge end mounting location to the screen center of gravity. In this case, the translational and 

rotational motions of the screen are 

decoupled so the screen motion is primarily translational in the direction of the force applied by the 

eccentric mechanisms. 

When the rubber isolators were installed, two isolators 

were used at each mounting location at the discharge end and 

a single isolator was installed at each mounting location at the 

feed end. The spring rates of all of the rubber isolators were 

equal. With this arrangement, the product of the total spring 

rate at each screen end and the distance from the screen center 

of gravity to the mounting locations is not equal. This creates 

coupling between translational and rotational screen motions, 

causing the screen to translate in the direction of the force 

applied by the eccentric mechanisms and to rotate with a 

pitching motion about the screen center of gravity. This 

coupling tends to increase the vibration transmitted to the 

building. If the spring rates at the discharge and feed ends of 

the screens with rubber isolators could be matched to those 

with the steel coil springs, the steel-on-steel impacts observed 

with the steel coil springs could be eliminated without increasing the building vibration. 

Standard steel screen panels vs. urethane screen panels. 

Following the tests with the rubber isolators, the steel screen 

panels were replaced with urethane screen panels. Sound level 

measurements were then performed with the screens process-
ing coal. Figure 13 shows the background-noise-corrected 

A-weighted 1/3-octave band sound levels with the standard 

and urethane screen panels. A slight increase in sound level 

was observed. However, the increase was only a few tenths 

of a decibel, which is within measurement error. The figure 

indicates that the urethane screen panels did not reduce the 

sound level around the test screen in this case. These results 

are consistent with the findings of a 1977 report to the U.S. 

Bureau of Mines, which states that particle size must ordinarily be greater than 12-mm diameter before coal impact 

noise exceeds 90 dB(A) near a screen (Rubin, 1997). Generally, coal flow noise is dominant in screens handling coarse 

coal, while noise due to the drive mechanism, which may be 

radiated by the screen body or mechanism housings, is the 

most significant noise source in screens handling finer coal 

(Ungar et al., 1976). 

Figure 10 — Location used to compare the acceleration 

time history with steel coil springs and rubber isolators. 

Figure 11 — Acceleration in Z-direction at Mount 2 with steel coil springs and 

rubber isolators. 

Ranking of screen noise sources 

The first phase of the study involved a trial-and-error approach to reducing the noise generated by the test screen. In 

some cases, this approach will result in finding a successful 

noise control. However, in many cases, the attempted controls 

will not reduce sound levels because the controls do not 

directly address the primary noise sources on a piece of 

equipment. Noise controls that address the dominant noise 

sources on machinery are the most effective means of reducing noise. Therefore, the next phase of the project consisted of 

rank ordering the noise sources on the screen. 

Contributions of major noise sources. Figure 14 shows the 

A-weighted 1/3-octave band sound levels for the test screen 

with the steel coil springs and standard steel return deck while 

processing coal. The figure shows that the sound level in the 

16 Hz 1/3-octave band, which contains the mechanism rotation frequency, is unimportant in terms of the overall Aweighted sound level. The 100 Hz through 10 kHz 1/3-octave 

bands, however, account for 99% of the overall A-weighted 

sound level under full operation. Therefore, emphasis was 

placed on this frequency range in determining the contributions of each noise source on the screen. 

A vibrating screen is a complex piece of machinery. 

However, the noise radiated by a screen is primarily due to 

two noise sources: screening noise and drive noise (Hennings, 

1980). Screening noise consists of the noise generated by the 

flow of the coal/water mixture down the chute and across the 

top of the screen deck. As the mixture flows out of the chute 

and across the screen, impacts between individual pieces of 

coal and between the mixture and the 

chutes and screen generate noise. Drive 

noise refers to the noise radiated due to 

vibration of the mechanism housings, 

screen sides and the building, resulting 

from excitation by the gears, bearings and 

eccentric weights of the mechanisms. Generally, screening noise is more significant 

than drive noise for coarse coal screens, 

while drive mechanism noise is dominant 

for fine coal screens (Ungar et al., 1976). 

For drain and rinse screens, such as the test 

screen in this study, the spray of rinse 

water onto the processed coal is another 

potential noise source. 

To determine the relative contribution 

of these sources, several measurements 

were performed. First, the sound levels 

around the screen were measured with the 

test screen processing coal. In this instance, 

all of the aforementioned noise sources 

make a contribution to the measured sound 

levels. Next, the sound levels were measured with the screen vibrating without 

processing coal. This yields the sound level 
due to drive noise. Finally, the sound levels 

were measured with the rinse water spray 

on and the screen turned off. The above 

measurements may be used to calculate the 

sound level due to screening noise. In these 

calculations, the assumption was made that 

the damping and/or mass-loading effects 

of the flow of the coal/water mixture across 

the screen has little impact on the noise 

generated by drive noise. 

The total sound level produced by the 

screen may be calculated by 

LP t, ot = (3)

10 LOG ( 100.1L P ,scr + 10 01 . L . L Pdr + 10 0 1 p r, in )

where 

Lp,tot is the total sound level,

Lp,scris the sound level due to screening

noise, 

Lp,dr is the sound level due to drive 

noise and 

Lp,rin is the sound level due to rinse 

water spray. 

Because the first set of measurements 

consists of the total screen noise and the 

second and third measurements consist of 

the drive noise and the rinse spray noise, 

the contribution of screening noise to the 

total sound level can be calculated by rearranging Eq. (3) to yield 

LP s, cr = (4) 

10LOG (100.1L P t, ot + 10 0 1. L 0 1. L Pdr + 10 p r, in ) 

Drive noise was found to be the most 

significant contributor to the total screen 

noise, yielding a sound level of 91 dB(A). 

The sound level resulting from the rinsewater spray was found to be only 80 

dB(A), which is insignificant in relation 

to the total sound level around the screen. 

Using Eq. (4), the sound level due to 

screening noise was calculated to be 87 

dB(A). Table 1 shows a summary of the 

A-weighted sound levels due to each individual noise source as well as the total 

sound level. 

The A-weighted 1/3-octave band sound 

levels for the total screen noise, drive noise, 

rinse-water spray noise and screening noise 

are shown in Fig. 15. The figure shows that 

drive noise is dominant in the 100 through 1,600 Hz 1/3-octave 

bands, whereas screening noise is dominant above the 1,600 

Hz 1/3-octave band. Therefore, the most effective approach to 

reduce the sound level around the screen would be to first 

address the drive noise in the 100 through 1,600 Hz 1/3-octave 

bands. Once the sound level due to drive noise is reduced to 87 

dB(A), drive noise and screening noise would be equal contributors to the sound level around the screen during full 

operation. At this time, a decision would have to be made 

regarding which of these two noise sources would be most 

easily addressed to further reduce sound levels, if necessary. 

Figure 12 — Percentage change in the acceleration in the 16 Hz 1/3-octave 

band due to replacing steel coil springs with rubber isolators. 

Figure 13 — A-weighted 1/3-octave band sound levels while processing coal 

with standard steel screen panels or urethane screen panels and rubber 

isolators coal. 

Contributors to drive noise. Because drive noise was determined to be the dominant noise source on the screen, further 

testing was performed to examine the drive noise. Drive noise 

can be separated into the noise radiated by the screen sides and 

the noise radiated from the housings of the eccentric mechanisms. To identify noise controls that will best reduce drive 

noise, the relative contributions of the screen sides and mechanism housings to drive noise must be determined. The total 

sound level due to drive noise is 

 LP d, rive = 10LOG (100.1L P ,side + 100 1. L p ,mech ) (5)
where 

Lp,side and Lp,mech are the vibration-radiated sound levels due to screen side 

vibration and mechanism housing vibration, respectively. 

A series of sound-level measurements 

was performed to determine the sound 

levels due to screen side and mechanism 

housing vibration. Because the goal was to 

determine contributors to drive noise, these 

measurements were performed with vibration only on a day when the plant was not 

operating. 

For these tests, the barrier-absorber curtains used to block airborne background 

noise from the nearby screens at the beginning of the study were taken down. For the 

first test, the eccentric mechanism housings were wrapped with quilted lead-fiberglass barrier-absorbers. The goal of wrapping the mechanisms was to reduce the 

contribution of the noise radiated by the 

mechanism housings by at least 10 dB, 

thereby enabling the measured levels to be 

treated as the sound levels due to screen 

side vibration only. Next, the barrier-absorber wrap was removed to measure the 

contributions of both screen side and housing vibration to the drive noise. The sound 

level due to mechanism housing vibration 

was then calculated by rearranging Eq. (5) 

to yield 

L = 10LOG ( 100.1L P ,drive 0 1. L p s, ide 

P , mech + 10 ) (6)

Figure 16 shows the A-weighted 1/3­

octave band sound levels due to screen 

body and mechanism housing vibration. 

The sound levels due to screen body and 

mechanism housing vibration were found 

to be 87 and 84 dB(A), respectively. Because a 3-dB difference in levels indicates 

a doubling of energy, screen side vibration can be said to have twice the significance of mechanism housing vibration in 

terms of drive noise. The spectra indicate 

that the noise radiated by the screen body 

is dominant in the 100 through 500 Hz 1/ 

3-octave bands. The screen sides account 

for 83% of the sound energy for this band. 

In the 630 through 1,600 Hz 1/3-octave 

bands, the figure shows that the screen 

sides and mechanism housings generate similar sound levels 

with each contributing approximately 50% to the drive noise 

in this frequency range. In the 2 through 10 kHz 1/3-octave 

bands, the sound levels due to mechanism housing vibration 

are higher than those associated with screen side vibration. 

Mechanism housing vibration accounts for 58% of the drive 

noise sound level in this frequency range. 

Figure 14 — A-weighted 1/3-octave band sound levels for test screen while 

processing coal. 

Figure 15 — A-weighted 1/3-octave band sound levels for the total screen noise, 

drive noise, rinse water spray noise, and screening noise. 

Table 1 — Summary of noise sources and their sound 

levels with steel coil springs and steel deck. 

Overall A-weighted 

sound level 

Test condition/source dB re 20 μPa 

Total, processing coal (measured) 92 

Drive noise (measured) 91 

Rinse water spray (measured) 80 

Screening noise (calculated) 87 

Reducing vibration-radiated noise 

To reduce the noise radiated by the vibrating screens in the 

most efficient manner, the dominant noise source must be 

addressed first. The noise source ranking tests showed that 
drive noise was the most significant noise 

source. Therefore, the noise radiated with 

the screen operating without coal and 

with vibration on should be reduced prior 

to attempting to reduce screening noise. 

Figure 16 — A-weighted 1/3-octave band sound levels for noise radiated by 

screen body and mechanism housings. 

Rebuilt mechanisms. Because drive 

noise was determined to be the dominant 

noise source on the screen, several methods of reducing drive noise were considered. The first idea that was discussed 

was replacing the existing set of mechanisms, which were approximately 5 years 

old, with rebuilt mechanisms. It was 

thought that wear of the internal gears and 

bearings over time could have increased 

the vibration generated by the mechanisms at audible frequencies (above 20 

Hz). These vibrations are then transmitted to the screen sides via the H-beam, 

causing the screen sides to radiate noise. 

Therefore, replacing the mechanisms 

could potentially reduce the drive noise 

radiated by both the mechanism housings 

and screen sides. 

Baseline sound levels were measured 

with the existing set of mechanisms, the 

rubber isolators and the urethane screen 

panels installed on the test screen with 

the plant shut down and the test screen 

operating with vibration only. The

mechanisms were then replaced with 

rebuilt mechanisms and the measurements were repeated. Figure 17 shows 

the 1/3-octave band sound levels with 

the original and rebuilt mechanisms. 

Figure 17 — A-weighted 1/3-octave band sound levels for vibration only operation 

with original and rebuilt mechanisms. 

The 

figure indicates that installing the rebuilt 

mechanisms reduced the overall Aweighted sound level by 1 dB. According to the screen manufacturer and the 

coal company, installing rebuilt mechanisms costs approximately $10,000. The 

results suggest that the cost associated 

with installing a set of rebuilt mechanisms is high relative to the observed 

sound level reduction. 

Due to the increased building vibration with the rubber isolators, the mounts 

were switched back to the steel coil 

springs. In addition, the urethane screen 

panels were removed and standard steel 

screen panels were installed, because the 

urethane screen panels did not reduce the 

sound levels and were prone to plugging. 

After approximately one year, a new set of measurements 

was performed to establish the baseline sound levels for the 

screen operating with vibration only with the steel coil 

springs and steel deck. The measurements were performed 

on a maintenance day with the plant shut down. At this time, 

an unexpected result was observed. The vibration only sound 

levels for the test screen with the steel coil springs and rebuilt 

mechanism decreased to 87 dB(A), a 4-dB reduction compared to the original measurements from Feb. 25, 2003 

Figure 18 shows a comparison of the vibration only sound 

levels with steel coil springs, steel screen panels and the 

original mechanisms; rubber isolators, urethane screen panels 

and the original mechanisms; rubber isolators, urethane screen 

panels and the rebuilt mechanisms; and steel coil springs, steel 

screen panels and the rebuilt mechanisms. The test dates for 

these data were Feb. 25, 2003, July 12, 2003, July 18, 2003, 

and July 23, 2004, respectively. 

The figure shows that the average sound level measured on 

Feb. 25, 2003, with new coil springs, the steel return deck and 

the original mechanisms was 91 dB(A). When testing was 

conducted within a one-week period (July 12, 2003, and July 

18, 2003) with the original versus rebuilt mechanisms with 
rubber isolators and urethane screen panels, the sound levels were 91 and 90 

dB(A), respectively. However, after the 

steel coil springs replaced the rubber isolators and the steel screen panels replaced 

the urethane screen panels and one year 

had passed since the installation of the 

rebuilt mechanisms, the sound level for 

vibration only operation was 87 dB(A). 

On the reinstallation of the steel coil 

springs, vibration measurements were 

taken at the spring mounting locations to 

determine the cause of the steel-on-steel 

impacts observed in the original data 

(refer to Fig. 11). The steel-on-steel impacts were not observed at this time. It is 

possible that the original test data were 

influenced by an installation problem 

with the springs, which would increase 

the sound level around the screen. It is 

also possible that during the year that had 

passed, any high spots on the gears in the 

rebuilt mechanisms had worn away resulting in smoother tooth engagement. In 

addition, with the rubber isolators, more 

vibration was transmitted to the building, which could have increased the drive 

noise. Proper installation of the steel coil 

springs may have eliminated the steelon-steel impacts and reduced the vibration transmitted to the building, thus decreasing the noise radiated by the screen 

and the building. 

Figure 18 — Comparison of sound levels with original or rebuilt mechanisms, 

steel coil springs or rubber isolators, and steel or urethane screen panels for 

several test dates. 

Figure 19 — Approximate accelerometer locations used for vibration 

measurements on the left side of the screen. 

Figure 20 — A-weighted surface-averaged velocity levels. 

Damped side plates and mechanism 

cover. Because screen side vibration was 

determined to be the most significant 

source of drive noise, constrained layer 

damping was applied to the screen sides. 

Constrained layer damping involves 

bonding a viscoelastic material to the 

vibrating object using a stiff adhesive and 

then bonding a stiff constraining layer to 

the viscoelastic material. When subjected 

to vibration, the constraining layer places 

the viscoelastic material in shear. Shearing the viscoelastic material converts the 

vibration energy into a small amount of 

heat, thereby reducing vibration and, 

hence, radiated noise. 

Prior to constructing the damped side 

plates, a series of vibration measurements 

was performed on the left side of the 

screen to determine the relative vibration 

levels along the side of the screen. The 

goal was to determine if constrained layer 

damping was necessary on the entire side 

plate or if some areas, which had insignificant vibration levels, could be left untreated. The left side was divided into six 

areas and a group of accelerometers was 

attached to each area as shown in Fig. 19. 

The test screen was operated with vibration and without coal. Because noise 

radiation is directly related to the surface-
averaged velocity, the resulting 1/3-octave band accelerations were integrated to 

yield vibratory velocities, and the surface-averaged A-weighted velocity levels 

for the six areas were calculated (see Fig. 

20). A-weighting was applied to the vibratory velocities to deemphasize low frequency vibration, which is not a significant contributor to the A-weighted sound 

level in this case. The figure shows that 

the velocity levels are highest in the 100 

through 400 Hz 1/3-octave bands for the 

Group 2 and Group 5 accelerometers. 

However, above the 500 Hz 1/3-octave 

band, the variation in vibration levels from 

group to group is generally less than 10 

dB. Therefore, each area is a significant 

contributor to the drive noise radiated by 

the screen sides and should be treated. 

Constrained layer damped side plates 

were constructed by bonding a layer of 

viscoelastic damping material to new side 

plates using epoxy. A 3.2-mm- (1/8-in.-) 

thick layer of steel was then attached to 

the viscoelastic material with epoxy to 

complete the construction. Mechanism 

covers were fabricated from a barrierabsorber material similar to the material 

used to exclude background noise from 

the test area during the initial tests. Prior 

to installing the damped side plates, 

baseline sound level measurements were 

taken during miner’s vacation with the 

plant shut down and the test screen operating with vibration only. In addition, 

acceleration measurements were taken 

on the sides of the screen at the locations 

shown in Fig. 21. 

Figure 21 — Accelerometer locations used to evaluate the effects of installing 

constrained layer damped side plates. 

The damped side plates were installed 

during the first week of September 2004, 

and sound-level measurements were collected on Sept. 14, 2004, with the mechanism covers and damped screen sides 

installed with the plant shut down and the 

test screen operating with vibration only. 

The mechanism covers were then removed 

and the measurements were repeated. In 

addition, acceleration measurements were 

performed at the locations shown in Fig. 

21 for comparison with the baseline vibration measurements collected in July. 

Figure 22 shows the A-weighted 1/3-octave band sound 

levels for baseline conditions and with the engineering noise 

controls installed.

Figure 22 — A-weighted 1/3-octave band sound levels for vibration only operation 

for mechanism cover and damped side plate tests. 

 As the figure shows, neither the damped 

side plates alone nor the damped side plates and mechanism 

covers reduced the overall A-weighted sound level. It was 

expected that the sound level would be reduced by 3 dB or 

more. The figure shows that the sound levels in the 200 

through 630 Hz 1/3-octave bands were reduced by 1 to 2 dB 

with the damped side plates. With the damped sides and 

mechanism covers, the reductions in this band were again 1 to 

2 dB. In the 800 through 10 kHz 1/3-octave bands, the sound 

levels measured with the engineering controls installed increased from 83 to 84 dB(A). Because it is unlikely that 

installing these noise controls would increase the sound levels 

in these bands, something must have changed with the vibrating screen during the period between baseline testing in July 

2004 and the testing with the noise controls in September 

2004. 

Further investigation of the screen revealed a possible 

cause for the increase in sound levels. One of the inner springs 

on the discharge end of the screen was broken when the 

damped side plates were installed. As a result, only three 

springs were installed on the right side of the discharge end 

when the damped side plates were installed. With three 

springs at this location, it is likely that the screen would 

undergo a pitching/rolling motion that could create steel-onsteel contact between the support springs or with the screen 

and the base. Either case would increase noise radiation. 
To evaluate the effects of the damping treatment on the 

screen side vibration, the average A-weighted vibration velocity levels for the left and right sides of the screen were 

calculated from the measured accelerations at the locations 

shown in Fig. 21. Although the aforementioned problems with 

the screen installation would tend to increase the radiated 

noise, it was thought that the damping treatment would still 

reduce the screen side vibration. This is because both of the 

installation problems would tend to cause additional noise to 

be radiated from the building and from local areas more than 

from the screen sides. Figures 23 and 24 show the A-weighted 

1/3-octave band vibration levels on the left and right sides of 

the screen, respectively. Figure 23 shows 

reductions in the A-weighted velocity levels on the left screen side ranging from 2 to 

7 dB in the 200 through 5 kHz 1/3-octave 

bands with the damped sides.

Figure 23 — A-weighted average velocity level on left side of the screen without 

and with damped sides. 

 For the right 

screen side, Figure 24 indicates reductions 

of 1 to 6 dB in the 200 Hz through 5 kHz 1/ 

3-octave bands.

Figure 24 — A-weighted average velocity level on right side of the screen without 

and with damped sides. 

 In the 200 through 630 Hz 

1/3-octave bands, the A-weighted vibration levels were reduced by 3 dB on the left 

side and 4 dB on the right side. The Aweighted vibration levels in the 800 Hz 

through 5 kHz 1/3-octave bands were reduced by 6 dB on the left side and 3 dB on 

the right side. With these substantial reductions, the A-weighted sound levels should 

have been reduced by 3 dB or more. More 

research should be performed to evaluate 

the effects of mechanism covers and the 

damped sides on the radiated sound levels. 

Future work 

Due to problems with unintentional 

changes to the test screen in the plant, 

evaluating the barrier-absorber mechanism 

covers and damped side plates in the plant 

proved to be difficult. The reductions in 

vibration levels due to the damping treatments should have produced a corresponding reduction in the drive noise, but instead the overall A-weighted sound level 

was not reduced. In addition, the sound 

levels in the higher frequencies increased 

significantly despite the significant reduction in vibration velocity levels at these 

frequencies. The variation of test conditions in the preparation plant clearly presents a problem when trying to evaluate 

engineering noise controls. To properly 

evaluate these engineering controls, the 

sound and vibration measurements should 

be performed on a similar screen at the 

manufacturer’s location in a well-controlled environment. 

Summary 

Two commonly applied engineering noise 

controls, rubber isolators and urethane 

screen panels, were tested on a 2.4- x 4.9­

m (8- x 16-ft) horizontal vibrating screen. 

Replacing the steel coil springs with rubber isolators reduced the sound level when 

processing coal by 1 dB(A) and the vibration-only sound level by 2 dB(A). However, switching to the 

rubber isolators resulted in a 157% increase in the building 

vibration at the left discharge end mounting location and a 

245% increase in the building vibration at the right feed end 

mounting location. These increases are probably due to coupling of translational and rotational vibration due to a spring 

rate imbalance with the rubber isolators. Steel-on-steel impacts, which can substantially increase noise, were observed at 

the screen mounting locations with the steel coil springs during 

the initial testing. After switching from the steel coil springs to 

the rubber isolators and back to the steel coil springs, the steelon-steel impacts that were observed during the initial testing 
were not observed. It is important to note that without the steelon-steel impacts, the sound levels with the steel coil springs 

and rebuilt mechanisms were significantly lower than the 

sound levels with the rebuilt mechanisms and rubber isolators. 

Because one year had passed between these tests, it is possible 

that the mechanisms had gone through a break-in period, 

reducing drive noise. The differences between the sound levels 

with correctly installed steel coil springs and a properly designed suspension using rubber isolators would probably be 

insignificant. However, this must be evaluated in a wellcontrolled environment. The urethane screen panels were 

ineffective at reducing the sound level in this instance. 

A series of sound level measurements was performed to 

rank order the screen noise sources. Drive noise was found to 

be the most significant contributor. Screening noise was 

found to be the second most important contributor to the noise 

generated by the test screen. Noise resulting from the rinsewater spray was found to be insignificant. Screen side vibration was found to be the primary contributor to drive noise, 

while mechanism-housing vibration was determined to be the 

secondary contributor. 

Several engineering noise controls to reduce drive noise 

were tested. Replacing the original mechanisms with rebuilt 

mechanisms was initially found to reduce the drive noise by 

only 1 dB(A). However, subsequent tests showed that installing the rebuilt mechanisms reduced the drive noise by 4 dB(A) 

compared to the initial measurements. It is possible that some 

type of installation problem that increased the noise radiated 

due to steel-on-steel impacts elevated the baseline sound 

levels. A series of tests should be conducted at the 

manufacturer’s site in a well-controlled environment to compare the sound levels on a similar screen with used and rebuilt 

mechanisms. Installing a barrier-absorber wrap over the mechanism housings and applying constrained layer damping to the 

screen sides did not reduce the drive noise. However, the 

damped side plates did reduce the vibration velocity levels on 

the screen sides between 1 and 7 dB(A). The reduced vibration 

levels on the screen sides should have produced a sound level 

reduction. It is thought that a missing steel coil spring on the 

discharge end of the screen influenced the results with the 

mechanism cover and damped screen sides. The missing 

spring may have increased vibration transmitted to the building or may have caused steel-on-steel impacts between the 

other inner and outer coil springs. Either effect could increase 

radiated noise beyond the baseline levels. Further investigations with the mechanism covers and damped side plates 

should be performed at the manufacturer’s site in a wellcontrolled environment. 

References 

Buer, E.R. 2004, personal communication, CDC, NIOSH, PRL, Pittsburgh, 999, 

“Health Standards for Occupational Noise Exposure, Final Rule,” Department of Labor, Mine Safety and Health Administration, 30 CFR Part 62, Vol. 

64, No. 176, September 13, pp. 49630-49634. 

Fiscor, S., and Lyles, J., 2000, “Prep plant population reflects industry,” Coal 

Age, October, pp. 31-38. 

Hennings, K., 1980, “Noise Abatement of Vibrating Screens, Using NonMetallic Decks and Vibration Treatments,” Contract H0387018, AllisChalmers Corporation, U.S. Bureau of Mines OFR 120-82, 61 pp. 

Rubin, M.N., 1977, “Demonstrating the Noise Control of a Coal Preparation 

Plant - Volume I. Initial Installation and Treatment Evaluation,” Contract 

H0155155, Bolt Beranek and Newman Inc., U.S. Bureau of Mines OFR 

104-79, 179 pp. 

Rubin, M.N., Thompson, A.R., Cleworth, R.K., and Olson, R.F., 1982, “Noise 

Control Techniques for the Design of Coal Preparation Plants,” Contract 

J0100018, Roberts & Schaefer Company and Bolt Beranek and Newman 

Inc., U.S. Bureau of Mines OFR 42-84, 125 pp. 

Ungar, E.E., Fax, G.E., Patterson, W.N., and Fox, H.L., 1974, “Coal Cleaning 

Plant Noise and Its Control,” Contract H0133027, Bolt Beranek and 

Newman Inc., U.S. Bureau of Mines OFR 44-74, 99 pp. 

Ungar, E.E., Dym, C.L., and Rubin, M.H., 1976, “Practical Reduction of Noise 

from Chutes and Screens in Coal Cleaning Plants,” Contract H0144079, 

Bolt Beranek and Newman Inc., U.S. Bureau of Mines OFR 59-77, 65 pp.