P ers

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Noise source identification on a 


horizontal vibrating screen
 

H.E. Camargo, P.a. ravEtta, 

r.a. Burdisso and d.s. YantEk

H.E. Camargo is research engineer with niosH, Pittsburgh, Pa;
 

P.A. Ravetta is chief research engineer with avEC inc., Blacksburg, va; R.A. Burdisso is president of avEC inc., Blacksburg, 

va; D.S. Yantek is research engineer with niosH, Pittsburgh, Pa. 

Abstract 

In an effort to decrease noise-induced hearing loss (NIHL) 

in coal preparation plant employees, the National Institute 

for Occupational Safety and Health (NIOSH) is conducting research to identify and control dominant noise sources 

in vibrating screen (VS) machines. To this end, acoustic 

measurements of the noise radiated by a dewatering VS 

were conducted at the Pittsburgh Research Laboratory 

(PRL) using microphone phased-array technology.These 

measurements allowed for the identification of the screen 

as the dominant source in the 250 Hz 1/3-octave band.The 

sidewalls are the major contributors in the 315 and 400 Hz 

bands, whereas the eccentric mechanisms and the electric 

motor are the dominant sources in the 800 and 1,000 Hz 

bands, respectively. 

Introduction	 

Since the beginning of the indus-	

trial era, coal has been the primary 	

energy source for electricity generation in the United States (Energy 

Administration Information, 2008). 

Therefore, many efforts have been 

geared to improve the safety and 

health conditions in coal mines. Over 

the past 30 years, most illnesses and 

injuries related to coal mining have 

been significantly reduced, with the 

exception of noise-induced hearing 

loss (NIHL). Overexposure to high noise levels is evident 

along the entire production chain, from its extraction at 

the face to the preparation plants. 

According to a cross-sectional survey conducted by 

the National Institute for Occupational Safety and Health 

(NIOSH), 43.5% of coal preparation plant employees 

are exposed to noise levels that exceed the permissible 

exposure level (PEL) (Bauer, 2008). Furthermore, this 

study identified the vibrating screen (VS) machines used 

to separate coal from refuse and water as the main noise 

contributors in these plants. In this context, NIOSH is 

conducting research to identify dominant noise sources in 

VS machines.This study will allow for the design of noisecontrol devices and improvements to be implemented on 

these machines in the interest of hearing loss prevention. 

Previous measurements conducted at the PRL reverberation chamber show that 84% of the total A-weighted 

sound power is radiated by the VS in the 160 through 

1,000 Hz 1/3-octave bands (Yantek et al., 2008). 

This paper presents the results of 

a study conducted by NIOSH in collaboration with the Acoustical and 

Vibrations Engineering Consultants 

Inc. (AVEC) to determine the most 


dominant noise sources on a VS machine at low frequencies, i.e., 250 to 

1,000 Hz. To this end, phased-array 

measurements of the noise radiated 

by a dewatering VS were conducted 

in the hemi-anechoic chamber at 

PRL. A 121-channel, 3.5-m- (11.5­

ft-) diameter microphone phasedarray built by AVEC was used for the test.To provide the 

most complete data set for noise source identification, the 

array was mounted to a movable truss, so the noise radiated by the VS machine could be measured from multiple 

directions. Though a very large data set was generated, 

this paper presents only the most important results from 

the seven array positions tested. 

Experimental setup 

Acoustic measurements were performed in the hemianechoic chamber at PRL.This chamber is 16.8 m long by 

10.1 m wide by 6.4 m high (55 x 33 x 21 ft). The walls and 

ceiling are treated with Eckel Supersoft Panels (Eckel 

Industries Inc., 2008), resulting in a chamber cut-off frequency of 100 Hz in accordance with ISO 3744. 

The device under test was a dewatering horizontal 

VS machine. The machine consists of a structure that is 

suspended on coil springs. This structure is excited by 

two eccentric mechanisms driven by an electric motor 

through a belt/pulley system. Figure 1 (a) and (b) show 

the VS in the hemi-anechoic chamber. The excitation for 

the noise radiated by the screen body is mainly provided 

by the eccentric mechanisms and the gears used to drive 

them. Each eccentric mechanism uses two rotating unbalanced shafts that are supported on two bearings each. 

One shaft of the first eccentric mechanism is driven by an 

electric motor by a belt. This shaft is directly coupled to 

one of the shafts in the second eccentric mechanism.The 

second shaft in each eccentric mechanism is driven by the 

main shaft through a gear set. 

The microphone phased-array used in these measurements was an AVEC 3.5-m- (11.5-ft-) diameter array. 

The array consisted of 121 microphones arranged in a 

star configuration. This array was used to collect data 

simultaneously and continuously for all 121 channels at a 

sampling frequency of 51.2 kHz.The data were processed 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

using a frequency domain beamforming code (Avec Inc., 

2008). Acoustic maps and integrated spectra in 1/3-octave bands were computed for each array position using 

conventional beamforming with diagonal removal of the 

covariance matrix (Mueller, 2002). 

The sound radiated by the VS was measured from 

seven positions. These positions were chosen to give a 

complete picture of the noise characteristics of the machine. Three of the array positions were above the machine, as illustrated in Fig. 1 (a). These top positions are 

referred to as top/feed, top center and top/discharge. In 

these positions, the array was in a horizontal configuration. In addition, measurements were conducted from 

each side of the machine as shown in Fig. 1 (b).These four 

side positions are denoted as the electric motor, feed, discharge and eccentric side, respectively. In these positions, 

the array was in a vertical configuration. 

Figure 1 

Vibration screen with the array in the (a) top/feed end position and (b) electric motor side position. 

Experimental results 

Results were obtained in the form of acoustic maps 

in 1/3-octave bands. For all array positions, the scanning 

planes were parallel to the plane of the array. Multiple 

scanning planes were processed and they are indicated 

in the pictures to provide a good visualization of their 

locations. For brevity, only the results from three array 

positions are presented.To quantify the contribution from 

various components to the total radiated sound, the integrated sound pressure level (SPL) was computed from the 

acoustic maps. The integration was performed adding the 

levels of all the grid points inside the integration region 

and applying a normalization factor given by integration 

of the point spread function of the array in such region. 

The cutoff for the integration was set to 5 dB from the 

peak level in the integrated region (Mueller, 2002). 

Array on top/discharge end position. Figures 2 and 3 

are illustrative maps obtained with the array in this position.The acoustic maps show noise sources on the eccentric mechanisms, the side walls of the VS and the electric 

motor. At 250 Hz (Fig. 2 (a)), a source is present at the 

screen itself, whereas at 315 Hz (Fig. 2 (b)), two dominant 

sources are seen close to the side walls.These spots show 

larger levels, as the scanning plane is closer to the screen. 

At 800 Hz, the eccentrics are clearly the loudest sources 

(Fig. 3 (a)), while at 1,000 Hz a noise source can be readily identified on the electric motor (Fig. 3 (b)). 

Figure 2 

Acoustic maps at (a) 250 Hz and (b) 315 Hz. Plane at bottom of eccentric mechanisms. 

Array on electric motor side. Figure 4 (a) and (b) show 

sample beamforming maps for the 250 and 315 Hz 1/3-octave bands. From these maps, dominant noise sources can 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

be observed at the wall of the VS, the eccentric mechanisms and the feed end. It should be considered that the 

current array position limits the capability of determining 

the sources positioned along the axis perpendicular to the 

array, i.e., poor depth resolution. 

Figure 3 

(a) Acoustic map at 800 Hz with scanning plane below the eccentric mechanisms shaft; (b) acoustic map at 1,000 Hz with 

scanning plane at electric motor shaft. 

Figure 4 

Acoustic maps at (a) 250 Hz and (b) 315 Hz. Scanning plane at machine side wall. 

The integrated spectra for the four different regions 

shown in Fig. 5 were computed and are presented in Fig. 

6. From Fig. 6, it can be seen that the side wall is the loudest source over most of the frequency range. Given that 

the sides of the machine are parallel to the array, it is expected that plate vibration would radiate directly towards 

the array and generate louder levels for this particular 

array position.Thus, with the array in the horizontal configuration (i.e., above the machine), the noise from this 

component should be less dominant. 

Figure 5 

Integration regions. Array on electric motor side. 

The second highest source in Fig. 6 seems to be the 

eccentric mechanisms. Considering that the sides of the 

machine are “shielding” the propagation path from part 

of the eccentrics to most of the array microphones, the 

spectrum may not be very accurate, but it is still a very 

good estimate of what an operator located at the array 

position would be exposed to. 

Another major source located with the array on the 

electric motor side is at the feed end.These sources could 

also be related to plate vibrations. From other array positions, it was determined that this noise source is not 

related to the suspension springs in the frequency range 

of interest, i.e., 250 to 1,000 Hz. Finally, the integrated 

spectrum over the electric motor indicates that the noise 

radiated from the side of the motor is about 8 to 10 dB 

less than the noise radiated by the side wall (loudest noise 

source) for most of the frequency range. Because these 

values are close to the signal-to-noise ratio (SNR) of the 

array, the integrated levels are due to sidelobes of louder 

sources at the same frequency. This is a clear indication 

that the electric motor does not significantly contribute 

to the side noise radiation below 1,000 Hz. 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

	 	 	 	 	 	 	 	

	 	 	 	 	 	 	 	

 

 

	 	 	 	 	 	 	 	

 

Figure 6 

Integrated spectra. Array on electric motor side. 

Array on discharge side position. This array position 

rendered the most useful results in terms of noise source 

identification. Figures 7 through 10 show the beamforming 

results for different planes and frequencies between 250 

and 1,000 Hz. From Fig. 7 (a) and (b), it can be observed 

that at 250 Hz, the main source is the screen itself. Furthermore, the scanning at the discharge end shows a source on 

the screen and a reflection/source in the floor, suggesting 

that the vibration of the screen is exciting the cavity underneath the VS machine and, thus, radiating more noise 

in the direction of the array. Similar results were observed 

on the feed end. Figure 8 (a) and (b) shows that, at 400 

Hz, the sidewalls of the machine are a major noise source, 

in particular the left wall (electric motor side). At 800 Hz, 

noise from the eccentrics and the electric motor are major 

sources (Fig. 9), whereas, at 1,000 Hz, noise from the electric motor is dominant (Fig. 10).These results are consistent 

with other array positions. 

Figure 7 

Acoustic maps at 250 Hz. Scanning planes at (a) eccentrics shaft and (b) discharge end. 

Conclusions 

Noise source identification on a dewatering VS 

machine was performed by NIOSH at the Pittsburgh 

Research Lab. Measurements were conducted using 

an AVEC 3.5-m- (11.5-ft-) diameter, 121-microphone 

phased-array in a hemi-anechoic chamber. Data were collected from seven 

array positions over and around the 

VS machine. The main goal of these 

experiments was to identify the dominant noise sources at low frequencies, 

i.e., from 250 to 1,000 Hz. The results 

consisted of acoustic maps and integrated spectra over identified noise 

source regions in 1/3-octave bands. 

Sample results for the most representative array positions, frequencies 

and scanning planes were presented. 

Exhaustive analysis was performed to 

determine the dominant noise sources. Whenever possible, the integrated 

spectrum was analyzed in an effort to 

quantify the contribution from different sources. 

The results show that, at 250 Hz, 

the main source is the screen itself, and 

the results show that the vibration of 

the screen is exciting the cavity underneath the VS machine, radiating noise towards the feed and discharge 

ends. It was also found that the sidewalls are the major 

contributors in the 315 and 400 Hz bands; whereas, the eccentric mechanisms and the electric motor are dominant 

sources in the 800 and 1,000 Hz bands, respectively. 

Recommendations 

Based on the results from this work, it is recommended that the following noise controls aimed at reducing the 

sound radiated by the various sources on the VS machine 

should be studied: 

•	 to attenuate the sound radiated from the cavity

below the screen, the cavity could be sealed at the 

edges with acoustic blankets; 

•	 to reduce the radiation from the sidewalls, they

can be treated using constrained layer damping 

or implementing weak radiating cells (Ross and 

Burdisso, 1998); and 

•	 to attenuate the noise radiated from the eccentric

mechanisms,they can be surrounded with an acoustic enclosure and the gear and/or bearing designs 

can be changed. 
 

 

 

 

 

 

 

 

 

 

 

Figure 8 

Acoustic maps at 400 Hz. Scanning planes at (a) eccentrics shaft and (b) discharge end. 

Figure 9 

Acoustic map at 800 Hz. Scanning plane at eccentrics 

shaft. 

Figure 10 

Acoustic map at 1,000 Hz. Scanning plane at electric 

motor shaft. 

In addition, a mechanical filter could be designed to 

transmit the low-frequency force due to the rotation of 

the eccentric shafts at 15 Hz while filtering the higher 

frequency excitations due to bearing and gear forces. 

Acknowledgments 

The authors thank Peter Kovalchik, Lynn Alcorn, Pat 

McElhinney, Adam Smith and Nikolas Macko for their 

help and support during the test. 

References 

Avec Inc., 2008,AVEC Microphone Phased Array Software,Version 1.0: 

www.avec-engineering.com. 

Bauer, E.R., 2008, Personal communication, NIOSH-PRL, Pittsburgh, PA. 

Eckel Industries Inc., 2008, Supersoft panels webpage: http://www1. 

eckel.ca/supersoft.html. 

Energy Administration Information, 2008, U.S. Department of Energy, 

“Net generation by energy source by type of producer,” http://www. 

eia.doe.gov/cneaf/electricity/epa/epat1pl.html. 

Mueller, T. (ed.), 2002, Aeroacoustic Measurements, Springer, ISBN 

3-540-41757-5. 

Ross W.B., and Burdisso R.A., 1998, “Control of low frequency structurally radiated noise with an array of weak radiating cells,” Journal of 

Intelligent Material Systems and Structures, Vol. 9, No. 4, pp. 260-271. 

Yantek, D.S., Camargo, H.E., and Matetic, R.J., 2008, “Application of a 

microphone phased array to identify noise sources on a horizontal 

vibrating screen,” Proceedings of NoiseCon 2008, Dearborn, MI, 

July 28-30.