Noise controls for vibrating screen 

mechanisms

M.J. Lowe, D.S. Yantek, H.E. Camargo and L.A. Alcorn

Mechanical engineer, mechanical engineer, research engineer, engineering technician, 

National Institute for Occupational Safety and Health (NIOSH), Pittsburgh, PA

M. Shields

Vice president of engineering, Conn-Weld Industries, Inc., Princeton, WV

Abstract

U.S. National Institute for Occupational Safety and Health (NIOSH) studies show that 43.5% of surveyed coal preparation plant workers had noise exposures exceeding the U.S. Mine Safety and Health 

Administration permissible exposure level of 90 dB(A). Sound levels around vibrating screens in these 

plants often exceed 90 dB(A). NIOSH is currently developing noise controls for horizontal vibrating screens. To characterize noise sources, NIOSH researchers performed sound pressure level (SPL) 

measurements on a vibrating screen at their Office of Mine Safety and Health Research laboratory in 

Pittsburgh. The results show that the entire screen contributes to noise below 1 kHz, and the vibration 

mechanism housings are most significant above 1 kHz. Constrained layer damping (CLD) treatments 

and an enclosure were used to reduce mechanism housing noise in this range from 1-10 kHz. These were 

evaluated using sound power level measurements according to ISO 3744. The CLD treatments reduced 

the A-weighted sound power level by 3.1 dB in the 1 to 10 kHz one-third-octave bands. A panel-on-frame 

vibration mechanism enclosure using various types of panels further reduced the A-weighted sound 

power level from the CLD configuration in the 1 to 10 kHz one-third-octave bands by 3.7, 4.0, and 3.9 

dB for aluminum, steel, and DynalamTM panels, respectively. The combination yielded a 7 dB reduction 

from baseline in A-weighted sound power for the same frequency range.

Key words: Noise control, coal, vibrating screens

Introduction

Hearing loss is one of the most common occupational illnesses in the United States (Franks et al., 1996). However, in 

the mining industry, hearing loss is 2.5-3 times greater than 

what is expected for the average of the population that is not 

exposed to occupational noise. Additionally, the same National 

Institute for Occupational Safety and Health (NIOSH) studies 

have shown that by the age of 50, 90% of coal miners have a 

hearing impairment, versus only 10% of the population that is not 

exposed to occupational noise (Franks, 1996). Noise-induced 

hearing loss is not just a problem in underground mining. In 

fact, a Mine Safety and Health Administration (MSHA) study 

of 60,000 full shift noise surveys showed that based upon federal noise regulations, 26.5% of workers from surface mining 

operations were overexposed to noise, compared to 21.6% of 

workers in underground mines (Seiler et al., 1994). Above 

ground at coal preparation plants, a NIOSH study shows that 

43.5% of employees are overexposed to noise. Furthermore, 

the study found that not only were vibrating screens one of 

the loudest pieces of equipment at the preparation plants, they 

were also the most numerous, thus making vibrating screens a 

key noise source to address (Vipperman et al., 2007).

In that light, a team of NIOSH researchers in partnership 

with Conn-Weld Industries, Inc., have used conventional 

frequency domain beamforming techniques (Mueller, 2002; 

Christensen and Hald, 2004) to locate noise sources on a 

Conn-Weld G-Master 1000 dewatering vibrating screen. From 

the noise sources identified, they developed noise controls to 

mitigate the sound radiated by the mechanism housings. Additional noise controls for the screen body will be the subject 

of future investigations.

A horizontal vibrating screen (Fig. 1) is a large machine 

used to process coal. Vibrating screens can be used for a variety of applications, such as sizing, separating and dewatering 

both coal and refuse (rock) of various sizes. The screen body 

has four sides made of steel plates with a bottom screening 

surface— also known as a screen deck— made of steel wire 

welded to a frame with small gaps between the wires. The body 

of the screen is supported on a steel coil spring suspension. 

For the Conn-Weld screen tested at NIOSH, two vibration 

mechanisms are mounted to a steel beam that spans the width 

of the screen. These vibration mechanisms, which use rotating 

eccentric shafts to generate vibration, are belt-driven using an 

electric motor.

This screen is designed in such a way that it vibrates on 

roughly a 45° angle. In operation, coal flows into the feed end 

of the screen from a delivery chute. As the screen vibrates, the 

material moves along the deck and under a water spray that 

rinses the coal. The liquid and fine coal particles pass through 

the gaps in the screening deck as the material flows toward the 
discharge end of the screen. Finally, the rinsed coal falls off the 

discharge end of the screen to continue with further processing.

a

b

Figure 1 — A horizontal vibrating screen used to process 

coal viewed from (a) feed end and (b) discharge end.

Noise source identification

Noise sources were identified using the beamforming technique. The screen was positioned in the NIOSH Office of Mine 

Safety and Health Research (OMSHR) hemi-anechoic chamber 

in Pittsburgh with the screen directly on the chamber floor with 

wooden wedges driven under the frame rails to prevent rocking. 

The chamber dimensions are approximately 16.7 m long, 10.1 

m wide and 7.0 m high. To collect the acoustic data, a Bruel & 

Kjaer Pulse data acquisition system simultaneously recorded 

the sound pressures from a 42-microphone, 1.9-m-diameter 

wheel array. Based on previous testing, it was known that 99% 

of the overall A-weighted sound level of the vibrating screen 

came from the 100 Hz to 10 kHz one-third-octave bands. Additionally, NIOSH researchers discovered that the screen body 

was the main low-frequency noise source and the mechanism 

housings were the main high-frequency noise source (Yantek 

et al., 2005; Yantek and Camargo, 2009). This gave a starting 

point to determine the frequency ranges that were influenced 

by the mechanism housings and screen body.

The beamforming results showed that above 1 kHz, the 

vibration mechanisms are the most significant noise source 

(Yantek et al., 2008). Figure 2 shows examples of the beamforming results for the 1.6 and 2 kHz one-third-octave bands. 

In the figure, the light colors indicate the locations of high noise 

radiation. The figure clearly shows the vibration mechanism 

housings to be the dominant noise sources at these frequencies 

since the light colors line up with the mechanism housings. 

Figure 2b indicates the belt guard might also be a source of 

noise. Close inspection revealed that during operation the belt 

guard was rattling against the screen structure due to a lack of 

clearance. Since it is easy to eliminate the rattling and it would 

interfere with evaluating other screen noise sources, the belt 

guard was removed for all other tests.

a

b

Figure 2 — Beamforming results viewed from the discharge 

end of the screen for the (a) 1.6 kHz and (b) 2 kHz onethird-octave bands. Light colors indicate areas of high 

noise radiation.

To examine noise sources below 1 kHz, NIOSH contracted 

Acoustical and Vibration Engineering Consultants (AVEC) 

to perform beamforming measurements using their 121-microphone, 3.5-m-diameter array. The vibrating screen was 

positioned in the center of the NIOSH Office of Mine Safety 

and Health Research (OMSHR) hemi-anechoic chamber in 

Pittsburgh, PA. AVEC’s phased array was mounted to a movable truss to position the array for measurements from each 

screen surface. This data was post-processed using AVEC 

beamforming analysis software, which implements a conventional frequency domain beamforming algorithm. To this 

end, scanning grids in the planes of interest are generated 

and potential noise sources are assumed at each grid point. 

Then, the relative phase measured between microphones is 

compared to the relative phase that would be induced between 

the microphones by the assumed sources (Ravetta, 2005). The 

results were obtained in the form of one-third-octave band 

acoustic maps. The results indicated that below the 1 kHz 

one-third-octave band, the screen body is the main source of 

noise radiation (Yantek and Camargo, 2009).

Noise controls

Noise controls tested. To reduce noise from the screen, 

noise controls need to be developed for both noise from the 

vibration mechanism housings and noise from the screen body. 

The work presented here focuses on the noise radiated by the 

vibration mechanism housings. As mentioned in the previous 

section, the vibration mechanism housings are the dominant 
noise source in the one-third-octave bands from 1 to 10 kHz. 

NIOSH researchers developed two separate noise controls to 

reduce the mechanism housing noise in this frequency range. 

The first was a set of constrained layer damping treatments that 

were bonded directly to the outside of the mechanism housings. The second was an acoustic enclosure which surrounded 

both mechanism housings to block noise from reaching plant 

workers.

Constrained layer damping treatments. Constrained 

layer damping applies a flexible damping material— which is 

typically a rubber-like material—to a surface that is vibrating 

and generating noise. The firmness of this rubbery material 

is measured by a property called durometer. The higher the 

durometer, the higher the shear modulus of the material is. 

In other words, a higher durometer value means the material is less pliable. The other side of the damping material 

is constrained by a more rigid material such as steel. As the 

original base layer vibrates, it transmits motion to the damping 

layer. Since the damping layer is constrained on both sides, 

the vibration energy from the base layer is transformed into 

a shear deformation within the damping layer, which reduces 

the noise that is radiated by the system. 

For this application, a thin sheet of 80 durometer, 0.64-mm 

(0.025-in.) thick elastomeric damping material was bonded 

on one side to the flat faces on the front, top and back of each 

housing by using an adhesive. These layers of damping material were then constrained on the opposite side by bonding 

them to 6.4-mm-(1/4-in.)-thick steel plates. Figure 3 shows 

constrained layer damping treatments on the front face and top 

face. To ensure a good bond between the housings, damping 

material and constraining plates, the paint from the housings 

was removed using a grinder and the constraining plates were 

sandblasted prior to applying the treatments.

Figure 3 — Constrained layer damping treatments applied 

to the top and front faces of the mechanism housings.

Acoustic enclosure. An acoustic enclosure was designed 

to enclose both mechanism housings and attach to the same 

H-beam that the mechanism housings attach to. The motor and 

drive belts were not enclosed due to space constraints in coal 

preparation plants, plus they were not found to be significant 

contributors to noise. This first prototype was saltbox-shaped 

(a rectangular prism modified to have an angled face) to 

maximize air space around the mechanism housings (see Fig. 

4). The enclosure walls were comprised of three pieces of 

3.2-mm-(1/8-in.)-thick steel joined together by bolts through 

angle brackets with weld nuts on the back. A hole in the righthand side of the enclosure was cut for the tapered pulley shaft 

to pass through. 

Figure 4 — First enclosure constructed using a three-piece 

design. (a) Model showing internal duct structure and (b) 

enclosure installed on the vibrating screen.

a

b

A shim was installed to move the pulley away from the 

mechanism housing, providing additional clearance between 

the pulley and the right side of the enclosure. The entire 

enclosure was lined with 50.8-mm-(2-in.)-thick Polydamp 

acoustic foam to prevent buildup of reverberant noise inside. 

Ducts were incorporated into the design as shown in Fig. 4a 

to provide both convective cooling and structural support for 

the enclosure.

Test setup. All measurements were taken on a Conn-Weld 

G-Master 1000 horizontal vibrating screen with dual vibration 

mechanisms and a screening deck that was 2.44 m × 4.88 m in 

size. The screen rested on a solid concrete floor and wooden 

wedges were driven under the frame rails in order to prevent 

the screen from rocking during operation. In a preparation 

plant, screens will often be mounted directly to a steel structure 

underneath, where the concrete plant floor has been cut away. 

However, this type of modification was not practical for the 

test chamber and plant configurations vary. The noise reductions from this research in the laboratory will vary in the field 

since each screen and mounting condition will be different.
The screen was running without load (i.e., no 

coal or other material was flowing over the screen 

deck.) This should not significantly affect the results, since earlier field testing of similar vibrating 

screens showed that mechanism and screen body 

noise is a far greater contributor to overall screen 

noise than sound generated by material flow (Yantek 

et al., 2005). The belt guard was removed to avoid 

rattling that was discovered in previous testing. 

The configurations tested were: baseline screen 

with no noise controls, constrained layer damping 

on the mechanism housings, and constrained layer 

damping treatments combined with the enclosure.

The tests were performed in the NIOSH OMSHR 

hemi-anechoic chamber in Pittsburgh, PA. Sound 

pressure levels were measured using 21 B&K Type 

4188 and 4189 microphones set up in a parallelepiped configuration surrounding the screen per ISO 

3744 (ISO 1994). Three 30-second sound samples 

were measured and averaged for each configuration. 

These data were then converted to sound power 

levels following the reference source method listed 

in the ISO 3744 standard.

Experimental results and discussion

Constrained layer damping (CLD) treatments.

Figure 5 shows the A-weighted sound power level 

in one-third-octave bands for the baseline and with 

the constrained layer damping (CLD) treatments on 

the mechanism housings. The figure shows the CLD 

treatments reduced the sound power level in the 250 

Hz through 8 kHz one-third-octave bands. In the 

frequency range that is dominated by mechanism 

housing noise, 1 to 10 kHz, the CLD treatments 

reduced the A-weighted sound power level by 3.1 

dB. In addition, the overall A-weighted sound power 

level was reduced by 1.2 dB.

Figure 5 — A-weighted sound power level in one-third-octave bands 

for baseline and CLD treatment configurations.

First enclosure. The first enclosure prototype 

was tested in conjunction with the CLD treatments. 

Tests could not be performed with only the enclosure, 

due to the failure of several enclosure welds during 

this first test. As can be seen in Fig. 6, the combination of the 

enclosure and the CLD treatments reduced the sound power 

level in the 250 Hz to 2 kHz one-third-octave bands. However, 

the combination was worse for the 100 to 200 Hz one-thirdoctave bands and the 2.5 to 10 kHz one-third-octave bands.

Figure 6 —A-weighted sound power level in one-third-octave bands 

for baseline, with CLD treatments, and with CLD plus enclosure.

The lower frequency degradation is most likely attributable to panel vibration from the large sides of the enclosure. 

An attempt was made to stiffen the panels by welding on 

reinforcement ribs; however, the additional low frequency 

noise was still present. The high frequency noise increase is 

likely caused by the metal-on-metal contact resulting from 

cracked welds on the enclosure which allowed pieces of it to 

slap together as the screen vibrated. This created a noticeable 

jackhammer-like sound.

Second enclosure. While the first enclosure showed 

some promise in the mid-frequency range, there were obvious manufacturing flaws and design issues that needed to be 

corrected. Besides the necessity of better welds to correct the 

high frequency performance degradation, NIOSH and their 

collaborators wanted a stiffer design in order to address the 

low frequency noise and durability issues. Furthermore, they 

desired a design that would easily accommodate preventive 

maintenance and repairs in the field. Removable panels would 

allow easy access to fill ports, drain plugs or entire mechanism 

assemblies without the need to remove the whole enclosure. 

Finally, the design team wanted each component of the enclosure to weigh no more than 222 N (50 lb) to allow the parts to 

be handled and installed more easily.

Based upon the above considerations, NIOSH researchers 

created a modular panel-on-frame design. The steel frame 

provides a relatively stiff structure for the individual panels. 

Using smaller panels further increases panel stiffness, thereby 

reducing the effect of panel modes on low frequency performance. The new enclosure accommodates different numbers 

and spacing of vibration mechanism housings and has no 

pieces that exceed the 222-N (50-lb) weight target. This design 

consists of a series of steel frames that can be bolted together 

to make a larger or smaller enclosure as needed (see Fig. 7).

Bolt-on panels block the noise and can be easily removed to 

reach a fill port, drain plug, or bearing cover. An entire frame 

section with the panels attached can be removed to change a 

mechanism. Further, this design allows the use of interchangeable panels made of various materials with different types of 

sound absorption and/or damping treatments for easier design 

optimization. Cooling ducts bolt onto panels separately and 
can be reconfigured as necessary (Fig. 8).

Figure 7 — Steel frame for the second enclosure showing 

(a) one frame section and (b) the entire frame assembly 

installed on a vibrating screen.

a

b

Figure 8 — Second enclosure installed on vibrating screen.

The frame was composed of steel angle stock and U-channel. 

It was isolated from the H-beam using strips of 57 durometer 

natural rubber. Four sets of panels were made for the second 

enclosure: aluminum, steel, Dynalam damped steel (a CLD 

steel) and Paneltec aluminum honeycomb. All panels were 

3.2 mm (1/8 in.) thick, except the honeycomb panels, which 

were 6.4 mm (1/4 in.) thick (Fig. 9). Panels were lined with 

25.4-mm (1-in.) thick Polydamp acoustic foam. The right panel 

was isolated from the bearing cover plate by a boss made of 

57 durometer natural rubber. The bolts on the bearing cover 

plate were countersunk to increase clearance between the right 

panel and the bearing cover.

Figure 9 — Example of aluminum honeycomb material.

The sound power level was measured with the second enclosure using each of the aforementioned panel materials in 

addition to the CLD treatments on the mechanism housings. 

Figure 10 shows the A-weighted sound power level in onethird-octave bands for the enclosure with aluminum, steel and 

Dynalam panels with the CLD treatments compared to the data 

with only the CLD treatments on the mechanism housings. 

Each type of panel material reduced the A-weighted sound 

power level in the 1 to 10 kHz one-third-octave bands. The 

aluminum, steel and DynalamTM panels reduced the A-weighted 

sound power level in the 1 to 10 kHz one-third-octave bands 

by 3.7, 4.0 and 3.9 dB, respectively. For all practical purposes, 

the results are the same because changes on the order of a 

few tenths of a decibel are insignificant and can be a result of 

test-to-test variation. 

While the honeycomb panels reduced noise from 1-10 kHz, 

the overall A-weighted sound power level increased by 2 dB. 

Due to the construction of the honeycomb panels, the bolts 

that attach the panels to the frame could not be sufficiently 

tightened without crushing the panels. The lack of sufficient 

clamping force allowed the honeycomb panels to rattle against 

the frame, thereby increasing noise. This problem might be 

resolved with press-fit sleeve inserts into the panels for each 

bolt, but this would be cost-prohibitive to manufacture. With 

the aluminum and steel panels, the overall A-weighted sound 

power level was increased by 1.2 and 0.4 dB, respectively. 

Close inspection of Fig. 10 shows the enclosure with either 

aluminum or steel panels increased the sound power level in 

the 160 through 315 Hz one-third-octave bands by several 

decibels. This increase is probably due to the excitation of panel 

modes and the resulting noise radiation. Using the Dynalam 

panels reduced overall A-weighted sound power level by 1.6 

dB. With the Dynalam panels, levels in the 160 through 315 Hz 

one-third-octave bands were approximately the same as those 

with only the CLD treatments on the mechanism housings. For 

the aluminum and steel panels, the low frequency degradation 

from baseline may be avoided by adding a rib pattern to the 

panels for stiffening, or perhaps using thicker panels.

Figure 11 shows a comparison of the A-weighted sound 

power level spectra for the baseline, CLD treatments and CLD 

treatments with the second enclosure using Dynalam panels. 

The figure shows the A-weighted sound power level in the 1 

to 10 kHz frequency range was reduced by 7 dB with the CLD 
treatments and the Dynalam enclosure. Together, the combination reduced the overall A-weighted sound power level by 2.8 

dB, which is nearly a 50% reduction in terms of sound energy. 

Figure 10 — A-weighted sound power level in one-third-octave bands 

for CLD treatments and CLD plus second enclosure.

Figure 11 — A-weighted sound power level in one-thirdoctave bands for baseline, with CLD treatments and with 

CLD plus second enclosure using Dynalam panels.

Conclusions

Noise source identification data show that the main sources 

of noise on the Conn-Weld G-Master 1000 vibrating screen are 

the screen body and the vibration mechanism housings. Below 

1 kHz, the screen body is the dominant noise source, whereas 

noise radiated from the vibration mechanism housings is the 

primary source above 1 kHz. Constrained layer damping (CLD) 

plates and an acoustic enclosure were designed to reduce the 

noise radiated by the vibration mechanism housings.

CLD treatments on the mechanism housings reduced the Aweighted sound power level in the 1 to 10 kHz one-third-octave 

bands by 3.1 dB. In addition, the CLD treatments reduced the 

overall A-weighted sound power level by 1.2 dB. Adding an 

enclosure with Dynalam steel panels in conjunction with the 

CLD treatments reduced the A-weighted sound power level in 

the 1 to 10 kHz frequency range by 7 dB versus the baseline 

values. In addition, this combination reduced the overall Aweighted sound power level by 2.8 dB compared 

to the baseline. We expect these reductions in the 

lab to translate into a reduction of operator noise 

exposure in the field for an estimated production 

cost of less than 15% of the screen price. To 

further reduce noise exposure, controls must be 

developed to reduce noise radiated by the screen 

body. A complete package of noise controls for 

vibrating screens will be the subject of our future 

work. Field testing of this package is planned for 

multiple vibrating screens to evaluate durability as 

well as effectiveness at reducing noise.

Acknowledgments

The authors would like to thank Pat McElhinney, Pete Kovalchik and Arc Weld, Inc. for their 

valuable assistance in the second enclosure design 

process; Bob Michael and Corry Rubber Corporation for their donation of engineering samples and 

expertise for isolation of the enclosure; Conn-Weld 

Industries, Inc. for providing the vibrating screen; John Pack 

for assistance with technical questions on the vibrating screen; 

and Jessie Mechling, Rob Nahay, Tim Matthews, Ben Lewis, 

Shawn Peterson, Alexander Salas, Art Hudson, Adam Smith 

and Kurt Pawlak for additional help with enclosure assembly 

and testing.

Disclaimer

The findings and conclusions in this report are those of the 

author(s) and do not necessarily represent the views of the 

National Institute for Occupational Safety and Health. Reference to specific brand names does not imply endorsement 

by the National Institute for Occupational Safety and Health.

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