Evaluation of stiffeners for reducing noise from horizontal vibrating 

screens 

David S. Yanteka)
 

a) Email address: dyantek@cdc.gov 

National Institute for Occupational Safety and Health
 

Office of Mine Safety and Health Research
 

626 Cochrans Mill Road
 

Pittsburgh, PA 15236
 

Shawn Catlinb)
 

b) Email address: scatlin@mallett.com 

Mallett Technologies, Inc.
 

400 Technology Drive, Suite 200
 

Canonsburg, PA 15317
 

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

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

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

Sound levels around vibrating screens in coal preparation plants often exceed 90 dB(A). 

The National Institute for Occupational Safety and Health (NIOSH) is developing noise 

controls to reduce noise generated by horizontal vibrating screens. NIOSH researchers 

used finite element analysis (FEA) and a NIOSH-written program to estimate the sound 

power level reduction resulting from adding rib stiffeners to key locations on the sides of a 

screen. The researchers evaluated adding rib stiffeners made from steel channel with two 

different cross-sections (C and T) and the effects of orienting the stiffeners horizontally and 

vertically. In addition, the researchers investigated the effects of increasing the modal 

damping from 0.002 to 0.010. The results indicate that for a broadband input, adding 

stiffeners had little impact on the estimated sound power level, but increasing the modal 

damping reduced the predicted A-weighted sound power level by 7.8 dB. 

1 INTRODUCTION 

In 1996, NIOSH published the National Occupational Research Agenda, which identified 

hearing loss as the most common job-related disease in the United States1. Approximately 30 

million workers are exposed to hazardous sound levels alone or to hazardous sound levels in 

conjunction with ototoxic agents2. Despite more than 30 years of noise regulation in the mining 

industry, mine workers develop hearing loss at a significantly higher rate compared to the nonnoise exposed population. According to an analysis of audiograms performed by NIOSH, nearly 

90% of coal miners have a hearing impairment by the age of 50 whereas only 10% of the 

population that is not exposed to occupational noise has a hearing loss by the same age3

. 
To reduce the occurrence of noise-induced hearing loss, the Mine Safety and Health 

Administration (MSHA) modified its rules regarding noise exposure in 19994. MSHA’s new rule 

requires mine operators to use all feasible engineering and/or administrative controls to reduce 

the noise exposures of overexposed miners’. However, for many machines, such as vibrating 

screens, noise controls that reduce the operator’s noise exposure below the MSHA Permissible 

Exposure Level (PEL) are not currently available. 

In 2000, there were 212 preparation plants in operation in the United States5. NIOSH studies 

show that workers who spend a significant portion of their shift working in a coal preparation 

plant can experience noise exposures which exceed the MSHA PEL for noise. NIOSH data 

show that 20 out of 46 coal preparation plant workers had noise exposures that exceeded the 

MSHA PEL noise dose6. MSHA PEL noise doses up to 220% have been recorded for 

preparation plant workers. Workers in preparation plants spend a significant portion of a shift 

working around vibrating screens. The number of screens in a processing plant can range from a 

single screen to more than a dozen. 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 levels7. 

A horizontal vibrating screen (see Fig. 1) is a large machine used to process clean coal that 

has been separated from refuse materials using a water-magnetite mixture. This magnetite is 

recovered so it can be reused in the processing plant and because it lowers the heating value of 

coal. The screen body has two sides made of steel plates with a bottom screening surface made 

of steel wire welded to a frame with small gaps between the wires. Round cross-tubes are used at 

either end of the screen to help stiffen the structure. The feed box, also made from steel plates, is 

where the coal enters the screen. The body of the screen is supported on a steel coil spring 

suspension. One or more vibration mechanisms are mounted to a steel beam that connects the 

sides of the screen. These vibration mechanisms, which use rotating eccentric shafts to generate 

vibration, are belt-driven by an electric motor. The screen is designed such that it vibrates on a 

45 degree angle. Coal flows into the feed end of the screen from a delivery chute. As the screen 

vibrates, the material traverses the deck under a water spray that rinses the magnetite from 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 for further processing. 

Previous NIOSH studies showed the A-weighted sound level around a group of eight 

horizontal vibrating screens used to process clean coal ranged from 94 to 98 dB(A)8

. A series of 

measurements in an operating preparation plant indicated that noise due to vibration was the 

dominant noise source whereas noise from material flow was less significant. Further research 

showed that most of the noise due to vibration is radiated by the screen body and the mechanism 

housings9,10. At frequencies below about 1 kHz, screen body noise is the main noise source 

whereas mechanism housing noise is the primary source above 1 kHz. The sound energy at 

frequencies below 1 kHz accounts for about 80% of the overall A-weighted sound power level. 

Operating deflection shape analysis revealed significant response on the screen sides and feed 

box11 . 

Estimates of the sound power radiated by the screen sides and feedbox using vibration 

measurements showed that each is significant in terms of the total sound power radiated by the 

screen. For vibrating screens, it is undesirable to add weight to the screen body because this 

could adversely affect the performance of the screen. Therefore, it is not possible to make large 

scale changes to the structure of the screen. This work examines the effects of adding stiffeners 

or increasing damping on the sound power level radiated by the screen sides and feed box using 
a finite element model (FEM) of the screen to estimate vibration response and a NIOSH-written 

computer program to estimate the sound power radiated by the screen sides and feedbox. 

2 SCREEN FINITE ELEMENT MODEL DEVELOPMENT 

2.1 Screen Solid Model 

Pro/Engineer was used to develop a surface model of the as-built screen. The solid model 

included nearly all the components of the screen. The drive mechanisms were represented by 

solid blocks with the density adjusted so the mass of the blocks matched the mass of the 

mechanisms. The screen deck was not included in the model because it would significantly 

increase the complexity of the model without significantly changing the results since the screen 

deck does not contribute substantially to either the stiffness or mass of the structure. In addition, 

the frame used to support the screen was not included because previous tests did not indicate it as 

a contributor to noise. The belt guard was also ignored because previous testing showed the belt 

guard was not a significant noise source unless it rattles against the screen. Belt guard rattling 

can be eliminated with a slight increase in clearance. In addition, an assumption was made that 

the belt guard does not have a significant impact on the vibration of the other screen components. 

2.2 Screen Finite Element Models 

The Pro/Engineer solid model was imported into ANSY finite element analysis (FEA) 

software (see Fig. 2a). Some connections between screen components, such as the ribs and 

screen sides, are welded joints. Other components, for example the screen sides and round crosstubes, are connected via bolted joints. For the purposes of the model, all connections between 

individual components were modeled as bonded joints within the software. The screen 

suspension springs were modeled using lumped spring elements at each of the mounting 

locations. Three spring elements were used at each location to model the spring rates in the x 

(for/aft), y (vertical), and z (lateral) directions. Each suspension spring in the model was 

connected to ground with fixed boundary conditions. Steel was used for material properties for 

the entire screen. An example of the mesh for the finite element model of the screen without 

added stiffeners is shown in Fig. 2b. 

After examining previously collected beamforming, ODS, and modal data, it appeared that 

several locations along the screen sides, the sides of the feedbox, and the back of the feedbox 

were dominant contributors to vibration and noise radiation. For ease of reference, the areas 

between the ribs on the screen are called “panels”. Panel 4, panel 7, panel 8, the feedbox side and 

the feedbox back (refer to Fig. 3a) are the areas that exhibited the most significant vibration 

response. These areas were considered as potential locations for adding stiffeners. Initially, 

stiffeners with a T-shaped cross section were considered and one additional model was made 

using stiffeners with a C-shaped cross section (refer to Fig. 3b). For reference, the screen sides 

and feedbox are 8 mm thick. 

Models were made with stiffeners added as shown in Fig. 4. For the first case, a single Tshaped stiffener was added across the feedbox back with a vertical T-shaped stiffener added to 

each side of the feedbox. For the second case, horizontal T-shaped stiffeners were added to 

panels 4, 5, 7, and 8 along with the stiffeners from the first case. Next, for the third case, four 

vertical T-shaped stiffeners were added across the feedbox back along with vertical stiffeners on 

the feedbox sides and panels 4, 7, and 8. Finally, for the fourth case, C-shaped stiffeners were 

used at the same locations with the same orientations as the third case. In every case, the backs of 
the stiffeners were modeled as bonded to the screen sides and feedbox. In addition, the ends of 

the stiffeners considered to be bonded to the existing ribs. 

2.3 Screen Vibration Analysis 

To estimate the effects of adding stiffeners or damping on the noise radiated by the screen 

sides and feedbox, a linear dynamic analysis was performed in ANSY. Forces were applied to 

the screen model at the center of each mechanism perpendicular to the H-beam as shown in Fig. 

5 (note: mechanisms hidden for clarity). Previous work showed that during operation, screen 

natural frequencies are excited due to operation of the mechanisms. It is thought that bearing slap 

within the mechanisms excites the structure through a series of impacts. These impacts provide 

broadband excitation to the screen. For the purposes of evaluating the effects of adding stiffeners 

or damping, each force was assumed to have a flat input spectrum of 4.45 N from 0 to 1 kHz. 

The actual excitation force may differ from this assumption, but the purpose of this effort is to 

examine if adding stiffeners or damping will reduce radiated noise. Therefore, it is not necessary 

to have an exact prediction of noise radiated by the screen; rather the change is what is 

important. 

Within ANSYS, the screen response to these input forces is solved in two steps. First, the 

natural frequencies and mode shapes are determined in the desired frequency range. Next, the 

modal superposition technique is used to estimate the screen response to the input forces. The 

modal superposition technique determines the contribution of each mode of vibration to each 

input force on a frequency-by-frequency basis. The modal responses are then summed to yield 

the response of the screen. To perform the analysis, the modal damping must be defined. A 

constant modal damping ratio of 0.002 was used for all analysis except for the analysis used to 

examine the effects of increasing damping where a constant modal damping of 0.010 was used. 

Previous experimental modal test results indicated the modal damping values were much less 

than 0.010. Table 1 shows the test cases which were simulated. 

The displacement responses at 39 locations were computed and stored for each analysis. 

Figure 6 shows the locations where the responses were computed. Displacement results were 

stored as follows: five locations on panels 4, 6, 7, and 8; four locations on panel 5; five locations 

on the side of the feedbox; and ten locations on the back of the feedbox. These locations were 

used for all analyses. For each analysis, the results were exported as text files that could be read 

into MATLAB. 

3 ESTIMATION OF SOUND POWER RADIATED 

The sound power level radiated by a vibrating object can be estimated by 

LW  10 log 2 

10 v 10log 10 S 10log 10   s,t 146 (1) 

where LW is the sound power level, v 

2 

s,t 

is the surface averaged mean-squared velocity, S is the 

surface area, and  is the radiation efficiency12. Equation 1 was incorporated into a computer 

program that carried out all calculations for each screen location on a frequency-by-frequency 

basis in 1 Hz increments. For each set of displacement results from the FE model (e.g. panel 4, 

panel 5, panel 6, etc.), the velocity response at each location was calculated by multiplying the 

frequency data by 

v  2fd (2) 
Where d is the displacement magnitude as a function of frequency and f is the excitation 

frequency in Hz. The surface averaged mean-squared velocity response of each panel, the 

feedbox sides, and the feedbox back were than calculated by squaring and averaging these 

velocities for each component. 

The radiation efficiencies were estimated from the thickness and perimeter of each 

component using information from Bies and Hansen. Within the program, the radiation 

efficiencies were estimated by curve fitting values from the Bies and Hansen data using a 3rd 

order polynomial. This provided a simple means of estimating the radiation efficiency on a 

frequency-by-frequency basis. 

Estimates for radiated sound power level were performed on each component separately. 

Since we are interested in reducing worker exposure according to the MSHA PEL, which is 

based on A-weighted sound levels, the program was used to apply A-weighting to the estimated 

sound power levels. The sound power levels for each panel on the screen sides were added to 

yield the estimated sound power level radiated by the screen sides. The estimated sound power 

level from the feedbox was estimated by adding the sound power levels from the feedbox sides 

to that of the feedbox back. The estimated A-weighted sound power level was also summed for 

the 0 to 500 Hz and 500 Hz through 1 kHz frequency bands. In addition, the A-weighted sound 

power level was computed in 1/3-octave bands. 

4 RESULTS AND DISCUSSION 

Prior to using the FE model to examine noise radiation, a comparison was made between the 

frequencies of the first few modes of vibration identified by the FE model as compared to those 

obtained via experimental modal analysis (EMA). In addition, a visual comparison was made 

between the animated mode shapes for the first few modes. Figure 7a and shows a comparison 

between the first structural mode of vibration as identified by FEA and EMA. Both results show 

indicate this is a torsional mode of vibration. The FE model predicted the resonant frequency for 

the first mode to be 20.3 Hz where the EMA showed this mode to occur at 18.8 Hz. Figure 7b 

shows the comparison between the FE and EMA results for a mode that involves only the 

feedbox back. The frequency for this mode was 44.0 Hz according to the FE model and 42.0 Hz 

according to the EMA results. For our purposes, these differences were considered to be 

acceptable. 

Figure 8a shows a screen mode at 221 Hz that involves significant contributions from the 

feedbox back and panels 4, 7, and 8. Many of the modes identified from the FE model showed 

significant contributions from these areas. The forced response at 221 Hz is shown in Fig. 8b. As 

expected, the response looks nearly identical to the 221 Hz mode. Significant contributions from 

these same regions were observed on the other modes predicted by the FE model. In addition, the 

previous ODS analysis showed these areas to be dominant contributors to screen vibration. 

The FE models were used to calculate the forced response of the screen as previously 

described. In each case, the exported displacement results at the locations shown in Fig. 6 were 

used to estimate the radiated sound power from the screen sides and feedbox. Table 2 shows the 

resulting estimates of the radiated sound power level for the screen side and feedbox. All values 

in the table are with A-weighting applied because all noise exposure regulations in the mining 

industry are based on A-weighted sound levels. Table 2 shows that none of the cases with added 

stiffeners made a significant improvement on the predicted sound power levels. In fact, the 

estimated sound power levels for the vertical T-shaped stiffeners (Case 4) and C-shaped 

stiffeners (Case 5) were 0.5 and 1.0 dB(A) higher, respectively, than the estimate for the 

baseline, as built model. The only model that showed a significant improvement was the model 
where the modal damping ratio was increased from 0.002 to 0.010. To increase the modal 

damping, constrained layer damping could be applied at various locations on the screen. 

It was expected that adding the stiffeners to the panels would significantly reduce the 

response of the structure and the resulting radiated sound power; however this was not the case. 

Figure 9 shows a comparison of the estimated A-weighted sound power level in narrow bands 

for the as built model (Case 1) compared to the model with vertical T-shaped stiffeners (Case 4) 

narrowband spectra. It appears that some of the modes of the structure simply shifted to a higher 

frequency with little change in response and others did not change much at all. Figure 10 shows a 

comparison of the estimated A-weighted sound power level in narrow bands for the as built 

model with 0.002 modal damping (Case 1) compared to the as built model with 0.010 modal 

damping (Case 6). As expected, the significant increase in damping yielded a substantial 

reduction in the response at each of the peaks which correspond to the modes of the screen. 

Recall that at resonance, the modal damping has the most influence on the response. 

5 CONCLUSIONS 

The effects of adding stiffeners to the sides and feedbox of a horizontal vibrating screen and 

the effects of increasing the damping of a screen were evaluated using the displacement response 

predicted by an FE model. Surface average mean-square velocities calculated from the FE results 

were used with estimated radiation efficiencies to estimate the A-weighted sound power level 

from the screen sides and feedbox. According to the results, adding stiffeners would have little 

impact on the sound power level radiated by these components. However, increasing the modal 

damping for the screen from 0.002 to 0.010 resulted in a predicted reduction of 7.8 dB(A). 

6 ACKNOWLEDGEMENTS 

The authors would like to acknowledge Conn-Weld Industries, Inc. for their support of this 

research. In addition, the authors would like to extend their gratitude to Pat McElhinney, Lynn 

Alcorn, Hugo Camargo, Jenae Lowe, Shawn Peterson, Adam Smith, Pete Kovalchik, and RJ 

Matetic from NIOSH, Office of Mine Safety and Health Research for their assistance with this 

work. 

7 REFERENCES 

1. National Institute	 for Occupational Safety and Health, “National Occupational Research 

Agenda”, DHHS (NIOSH) Publication No. 96-115, (1996). 

2. National Institute for Occupational Safety and Health, “Preventing occupational hearing loss – 

a practical guide”, Edited by J. Franks, M.R. Stephenson, and C.J. Merry. DHHS (NIOSH) 

Publication 96-110, (1996). 

3. J.R.	 Franks, "Analysis of Audiograms for a Large Cohort of Noise-Exposed Miners", 

Unpublished technical report. NIOSH, Cincinnati, OH, (1996). 

4. Department	 of Labor, Mine Safety and Health Administration, “Health Standards for 

Occupational Noise Exposure: Final Rule”, 30 CFR Part 62. Federal Register, (1999). 
5. S. Fiscor and J. Lyles, “Prep Plant Population Reflects Industry”, Coal Age, October 08, 31­

38. (2000). 

6. E.R Bauer, Personal communication. DHHS, CDC, NIOSH, PRL, Pittsburgh, PA, (2004). 

7. M.N. Rubin, A.R. Thompson, R.K. Cleworth and R.F. Olson,.	 Noise Control Techniques for 

the Design of Coal Preparation Plants, Contract J0100018, Roberts & Schaefer Company 

and Bolt Beranek and Newman Inc., BuMines OFR 42-84, (1982). 

8. D.	 Yantek, P. Jurovcik and E.R. Bauer, “Noise and Vibration Reduction of a Vibrating 

Screen”, 2005 SME Annual Meeting, Society for Mining, Metallurgy, and Exploration, Inc., 

Littleton, CO, (2005). 

9. D. Yantek, H. Camargo and RJ Matetic, “Application of a microphone phased array to identify 

noise sources on a horizontal vibrating screen”, NoiseCon08, (2008). 

10. H.E. Camargo, P.A. Ravetta, R.A. Burdisso and D.S. Yantek, “Noise source identification on 

a horizontal vibrating screen”, Mining Engineering, July 2009. 

11. D. Yantek and H. Camargo, “Structural vibration as a noise source on vibrating screens”, 

ASME 2009 International Mechanical Engineering Congress & Exposition, Lake, (2009). 

12. D.A. Bies and C.H. Hansen, Engineering Noise Control Theory and Practice, Third Edition. 

Spon Press, New York, NY, (2003). 

Table 1 - Descriptions of the FE models used to examine the effects of adding stiffeners or 

damping to the screen. 

Case Description Modal 

Damping 

1 As built 0.002 

2 T-shaped stiffeners, 

feedbox back – horizontal, feedbox sides - vertical 0.002 

3 

T-shaped stiffeners, 

feedbox back – horizontal, feedbox sides - vertical 

screen sides, panels 4, 5, 7, & 8 - horizontal 

0.002 

4 T-shaped stiffeners, 

feedbox back and sides – vertical, screen sides, panels 4, 7, & 8 - vertical 0.002 

5 C-shaped stiffeners, 

feedbox back and sides – vertical; screen sides, panels 4, 7, & 8 - vertical 0.002 

6 As built 0.010 
 

 

 

 

 

 

Table 2 - Estimated A-weighted sound power level as predicted using the FE model and 

computer program. 

Case Description 

Estimated Sound Power Level, dB(A) 

0 to 500 Hz 500 to 1000 Hz 0 to 1000 Hz 

Side 

(each) Fbox Total Side 

(each) Fbox Total Side 

(each) Fbox Total Δ 

1 As built, ζ = 0.002 60.3 58.0 64.4 56.7 53.2 60.6 61.9 59.3 65.9 -

2 

T stiffeners, ζ = 0.002 

 fbox back - H 

 fbox sides – V 

59.9 58.4 64.2 57.0 53.0 60.8 61.7 59.5 65.8 -0.1 

3 

T stiffeners, ζ = 0.002 

 fbox back - H 

 fbox sides - V 

 panels 4, 5, 7, & 8 – H 

57.9 58.8 63.0 57.6 53.7 61.5 60.8 60.0 65.3 -0.6 

4 

T stiffeners, ζ = 0.002 

 fbox back and sides - V 

 panels 4, 7, & 8 – V 

60.7 58.9 65.0 57.2 51.7 60.8 62.3 59.7 66.4 +0.5 

5 

C stiffeners, ζ = 0.002 

 fbox back and sides - V 

 panels 4, 7, & 8 – V 

60.8 59.3 65.2 58.3 54.4 62.1 62.8 60.5 66.9 +1.0 

6 As built, ζ = 0.010 52.0 50.1 56.2 50.3 43.1 53.7 54.2 50.9 58.1 -7.8 

Note: Fbox denotes feedbox, ζ denotes the modal damping ratio, H denotes horizontal, and V denotes 

vertical. 

(a) (b) 

Fig. 1 - A horizontal vibrating screen used to process coal viewed from (a) feed end and (b) 

discharge end. 
(a) (b) 

Fig. 2 - (a) Isometric view of the ANSYS model of screen from the feed and (b)isometric view of 

the screen model with FE mesh from as observed from the discharge end. 

(a) (b) 

Fig. 3 - (a) Reference locations on the screen sides and feedbox (note: panels 4,7, and 8 and 

the feedbox exhibited high vibration response) and (b) stiffener cross sections. 
(a) (b) 

(c) (d) 

Fig. 4 - Models of screen with: (a) horizontal T-shaped stiffener on feedbox back and vertical 

T-shaped stiffeners on feedbox sides; (b) horizontal T-shaped stiffener on feedbox back, 

and panels 4,5,7, and 8 with vertical T-shaped stiffeners on feedbox sides; (c) vertical 

T-shaped stiffeners on panels 4, 7, and 8, the feedbox sides, and the feedbox back; and 

(d) vertical C-shaped stiffeners on panels 4, 7, and 8, the feedbox sides, and the 

feedbox back. 
Fig. 5 - Excitation forces applied to the H-beam at the mechanism locations (note: mechanisms 

hidden for clarity). 

(a) (b) 

Fig. 6 - Locations on (a) the screen side and feedbox side and (b) the feedbox back where 

displacement results were stored for each analysis. 
(a) 

(b) 

Fig. 7 - Comparison of the mode shapes for (a) the 20.3 Hz torsional mode predicted by the FE 

model compared to the 18.8 Hz mode from EMA and (b) the 44.0 Hz feedbox back 

mode predicted by the FE model compared to the 42.0 Hz mode from EMA. 
(a) (b) 

Fig. 8 - Comparison of (a) screen mode at 221 Hz with (b) the forced response at 221 Hz as 

predicted by the FE model. 

10 

15 

20 

25 

30 

35 

40 

45 

50 

55 

60 

A-weighted Sound Power Level (dB)

As built, zeta = 0.002 

Vertical Ts, zeta = 0.002 

0 50 100 150 200 250 300 350 400 450 500
 

Frequency (Hz) 

Fig. 9 - Comparison of the A-weighted narrowband sound power level spectra for the as built 

model and model with vertical T-shaped stiffeners. 
10 

15 

20 

25 

30 

35 

40 

45 

50 

55 

60 

A-weighted Sound Power Level (dB)

As built, zeta = 0.002 

As built, zeta = 0.010 

0 50 100 150 200 250 300 350 400 450 500 

Frequency (Hz) 

Fig. 10 - Comparison of the A-weighted narrowband sound power level spectra for the as built 

model with 0.002 and 0.010 modal damping.