Assessment of engineering noise 


controls at a talc processing plant
 

E.R. SPENCER AND E. REEVES 

E.R. Spencer is Mining Engineer with the National Institute 

for Occupational Safety and Health (NIOSH), Pittsburgh, PA; E. 

Reeves is Acoustical Engineer with the U.S. Army Aeromedical 

Research Laboratory, Fort Rucker, AL 

Abstract

National Institute for Occupational Safety and Health 

(NIOSH) researchers conducted an investigation to quantify sound levels and to determine the amount of sound 

reduction provided by engineering noise controls installed 

in a talc processing plant. Baseline sound level and sound 

intensity measurements were performed at the plant, and 

the measurement locations were recorded for comparison 

to post-control measurements. Follow-up measurements 

were then made at the same locations after the initial noise 

controls were installed. The plant subsequently decided to 

implement additional noise controls and the researchers 

returned to conduct measurements for a final analysis of 

all noise controls. The most significant results showed a 

sound level reduction in the main mill area from a range 

of 93 to 104 dB(A) down to a range of 90 to 94 dB(A) and 

a total sound power level reduction of 21 dB(A) for air 

classifying Mill 3. 

Introduction

In 2003, more than 12,000 nonmetal employees worked in preparation or mill plants (NIOSH, 2003).A 

NIOSH study revealed that by age 

50, approximately 49% of metal/nonmetal miners have a material hearing 

impairment (NIOSH, 2007).Accordingly, there is potential for almost 

6,000 nonmetal processing plant 

workers to be hearing impaired by 

age 50. This study’s noise control work will be useful for 

the approximately 150 U.S. nonmetal-processing plants 

(MSHA, 2005) to help them reduce the sound levels of 

their mills. 

With the cooperation of mine officials at a talc processing plant, NIOSH conducted a study to quantify inplant sound levels and to determine the amount of sound 

reduction provided by engineering noise controls installed 

by mine personnel. The long-term goals of the mine officials were to reduce in-plant sound levels and worker 

noise exposure. The noise control evaluation at the talc 

processing plant was performed as part of NIOSH’s effort 

to locate and evaluate state-of-the-art engineering noise 

controls. In addition to locating and assessing existing 

controls, NIOSH is also identifying processes or machines 

in need of noise controls, gaps in technology that impede 

the use of noise controls and barriers to the use of noise 

controls, including collateral hazards (NIOSH, 1996). 

The specific noise controls for this 

study — acoustic curtains and sound 

barrier and sound absorber materials — as well as the theoretical concepts can be applied not only to talc 

plants but also to other comparable 

machinery in all industrial sectors. 

The noise control retrofi t treatments 

for mining machinery can be found 

in the Bureau of Mines handbook 

titled Mining Machinery Noise Control Guidelines (Bartholomae and Parker, 1983).There is 

a high level of consensus about the theory, appropriate 

principles and evaluation methods for engineering noise 

controls (Bies and Hansen, 1987; Lord, 1988; Driscoll, 

1996; Harris, 1998). This study applied the consensus 

noise control approach by identifying and quantifying 

noise sources, developing appropriate engineering and 

administrative controls, and quantifying the extent of 

noise reduction attributable to each control intervention 

alone and in combination. 

To identify noise sources and their relative importance, baseline sound level and sound intensity measurements were performed with equipment turned on or off 

in a pre-selected process.The sound levels and their measurement locations were then entered into SSG-SurferTM 

software1

1 Reference to specific brand names does not imply endorsement by the National Institute for Occupational Safety and 

Health. 

to produce sound level contour mappings of 

the mill floor area.Temporary and fixed acoustic curtains 

and sound absorption material were then used, and postcontrol sound level and sound intensity measurements 

were taken to further identify the noise sources.After additional engineering noise controls were installed, sound 

level and sound intensity measurements were taken 

to quantify the post-control noise levels and the effectiveness of the controls. For this study, the sound level 

measurement was averaged for at least 12 seconds (time 

determined by researcher using B&K and American National Standards Institute [ANSI] recommendations) at 

each location (American National Standards Institute, 

2001). Figure 1 is a top view of the main mill area showing each numbered measurement location. During these 

measurements, the Bruel & Kjaer1

 (B&K) 2260 InvestigatorTM was mounted on a tripod such that the measurement microphone was 1.43 m (56 in.) above the floor 

(International Organization for Standardization, 1987). 
 

 

 

The work patterns and employee locations in this facility fluctuated unpredictably, depending on events that 

occurred during talc processing. In most cases, employees 

would be moving in and out of the noisiest areas, and 

their exposures would probably be very low. However, 

the mine offi cials felt that unusual situations could arise 

where workers would spend prolonged periods of time in 

the noisy areas. These situations were too unpredictable 

to be captured reliably through standard full-shift dosimetry. Instead, the scope of the current study was limited 

to reducing noise sources with the expectation that dose 

reductions could be verified later, if necessary. 

Sound level measurements 

Sound level measurements were conducted at 47 locations, approximately 2 m (79 in.) apart, on the processing 

plant floor under full operating conditions both with and 

without noise controls installed. A spot marking each 

measurement location was painted on the concrete fl oor 

to make the repeated measurements as consistent as possible. At every measurement location, the A-weighted 

equivalent continuous sound pressure level (LAeq) spectrum was measured using a B&K 2260 InvestigatorTM 

running Enhanced Sound Analysis software. The reference used when dealing with sound pressure is 2 x 10-5 

Pascals (Pa), which is the sound pressure that is barely 

audible at 1,000 Hz.When measured, this sound pressure 

would yield a value of 0 dB.The term “level” is commonly 

used to designate a logarithmic ratio of relevant parameters. Therefore, a sound pressure equal to the reference 

pressure of 2 x 10-5 Pa (1 Pascal = 1.45 x 10-4 pounds per 

square inch) produces a sound pressure level (SPL) of 0 

dB. To quantify the change in pressure at any point due 

to a passing sound wave, the root-mean-square (RMS) 

value is used. The SPL for any sound can be calculated 

using 

(1) 

 

where 

PRMS is the root mean square sound pressure in Pascals; 

and 

Pref is the reference sound pressure, 0.00002 Pascals 

(Rossing, 1982). 

Sound intensity measurements 

Because sound pressure level measurements alone 

do not locate the primary noise sources, sound intensity 

measurements were also taken. Once the primary noise 

sources were located, the sound intensity measurements 

were repeated after implementing noise controls to quantify their effectiveness. 

Baseline.To identify noise sources it is recommended 

to turn components on and off while taking measurements (Driscoll, 1996). For the initial visit to the processing plant, sound levels were measured under different 

operating conditions (e.g., fluid energy mill [FEM] fans, 

roller mills or crushers off and/or an air classifying mill 

[ACM] off — Table 1). These measurements were used 

to further identify the noise sources by taking baseline 

measurements with certain machines turned on and/or 

off and then using acoustic curtains and sound absorptive 

material around or next to certain machines, repeating 

the measurements and comparing the results. The baseline sound levels with all machines operating ranged from 

93 to 104 dB(A), with the highest levels being measured 

near ACM 3, Location 20 in Fig. 1. 

FIGURE 1 

Sound level measurement locations (not to scale). 

The long-term goals expressed by the plant’s management were to reduce in-plant sound levels and noise 

exposures of employees. Measuring the baseline sound 

levels with all equipment operating and measuring the resulting reduction of sound levels after the noise controls 

are implemented will quantify these goals for the company and demonstrate potential noise reduction. (Note: 

a second baseline measurement was taken after maintenance and repairs were conducted on ACM 3. This was 

done to evaluate the reduction of the sound levels resulting from only the noise control installation.) 

Initial engineering noise controls. For the initial engineering noise controls, and to quantify the contribution to the sound level from these machines, temporary 

noise controls were recommended to isolate and identify 

noise sources (Driscoll, 1996). For the study, acoustic curtains were installed around the FEM fans (Fig. 2) and 

the Jeffrey crusher (Fig. 3). Both of these controls were 

located on a level above but open to the main mill fl oor. 

The ACM 2 fan room is also above the fl oor area, but it 

is fully enclosed and not a significant noise contributor. 

Additional curtains were added to block sound radiating 

from the lower part of the Jeffrey crusher on the first 

floor.With the FEM fans and crushers operating, a sound 

level reduction was achieved in the main mill area from 

a baseline range of 84 to 91 dB(A) down to an initial 

control range of 83 to 89 dB(A) (Table 1). While this 

reduction of about 2 dB(A) is a small numeric change, a 

reduction of 3 dB(A) would be attributed to reducing the 

sound energy of the noise source by half (Fader, 1981). 

Further, when taken in context with the additional controls to be implemented, this initial step was signifi cant 
because it reduced these noise sources’ sound energy by 

almost 35%, it identified the sound level contributions of 

these noise sources and it allowed for a progression to the 

next noise control effort. 

FIGURE 2 

Curtains around the FEM fans. 

FIGURE 3 

Curtains around the Jeffrey crusher. 

To study the noise contribution of the ACMs to the 

main mill area, temporary welding screens draped with 

acoustical curtains were placed in front of the ACMs. 

The installation of one of these barriers in front of ACM 

2 is shown in Fig. 4. Also, because of the high sound levels measured next to ACM 2, Location 37 in Fig. 1 and 

the 99 dB(A) area in Fig. 11 (c), sound-absorbing material was inserted under the hood of ACM 2. Under 

the same operating conditions, the sound levels without 

noise controls ranged from 90 to 99 dB(A), while the 

sound levels with the noise controls ranged from 88 to 

94 dB(A) (Table 1, Operating Condition 6).

Table 1 

Sound level measurements in the main mill area. 

Sound level measurements, dB(A) 

 Second Initial Final

 Operating condition Baseline baseline1 controls2 test3 

1 FEM fans – On 79-89 78-89 78-88 – 

2 FEM fans and crushers – On 84-91 86-91 83-89 795

-92 

3 FEM fans, crushers, and ACM 2 – On 87-92 – – – 

4 FEM fans, crushers, and ACM 3 – On 87-104 – – – 

5 FEM fans, crushers, all ACMs, and 1.5-m (60-in.) roller mill – On 92-106 91-98 – – 

6 FEM fans, crushers, all ACMs, and 1.7-m (66-in.) roller mill – On – 90-98 88-944 – 

7 FEM fans, crushers, all ACMs and 60- and 66-in. roller mills – On 93-104 91-100 90-99 90-946 

8 All Machines – Off (background measurements) – 70-81 – – 

1 Sound level measurements were taken after maintenance and repairs to ACM 3, with no controls installed. 


2 Initial noise controls; acoustic curtains around FEM fans and Jeffrey crusher, and maintenance and repairs on ACM 3, operating 


conditions 1, 2 and 7 were measured. 

3 Engineering noise controls installed. 

4 For the measurement of operating Condition 6, additional temporary welding screens draped with acoustical curtains were placed in 

front of the ACMs and absorptive noise control material was inserted under the hood of the ACM 2. 

5 The low range is suspect because material stopped fl owing through the Jeffery crusher while measurements were being taken in 

that area. 

6 One FEM was not operational, but it was not considered to be a major noise source contributor per Baseline and Second Operating 

Condition 1. 

The curtains 

reduced the sound levels around ACM 1 and ACM 3 

by about 2 dB(A). Additionally, while using the sound 

absorbing material under its hood and curtains around 

ACM 2, the sound level in front of ACM 2 was reduced 

from 96 to 92 dB(A). This reduction of 4 dB(A) would 

be attributed to reducing the sound energy contribution 

of the noise source to the main mill area by more than 

40% (Fader, 1981). 

Air Classification Mill 3. To further identify the source 
 

of the high sound level of 104 dB(A) measured at Location 20 in Fig. 1, sound intensity measurements were 

taken in front of ACM 3, designated as Surface 1. Figure 

5 shows the measurement grid and the initial sound intensity contour map overlaid on a picture of Surface 1. 

It can be seen in Fig. 5 that the highest intensity level of 

109 dB(A) was measured around the center of the ACM. 

The calculated sound power level of the grid area is 104 

dB(A) (Fig. 6). A frequency analysis of ACM 3 showed 

a high level at 800 Hz. This calculated high sound power 

level was due mainly to this peak and most likely corresponded to the fan blade pass frequency. Once these 

results were discussed with plant management, the ACM 

3’s fan was balanced and the shroud door was sealed. 

Sound intensity measurements after these maintenance 

and repairs were done ranged from 85 to 96 dB(A). The 

reduction of the calculated sound power was 12 dB(A) 

(Fig. 6). 

FIGURE 4 

Acoustic curtain being placed in front of ACM 2. 

FIGURE 5 

Sound intensity measurement grid on Surface 1 of ACM 3. 

FIGURE 6 

Calculated sound power for no noise control treatments, 

maintenance and repairs and noise controls on Surface 

1 of ACM 3. 

Because a signifi cant sound level reduction of about 

3 dB(A), from an average of 95 dB(A) down to about 92 

dB(A), was achieved using the acoustical curtains in front 

of the ACMs, it was decided to engineer noise controls 

for ACM 3. A larger shroud for ACM 3 was built, and 

the interior of the shroud was lined with commercially 

available sound barrier and sound absorber materials — 

Acoustiblok® and Bafl-sorp®, respectively. The sound 

intensity measured after the new acoustical shroud was 

in place ranged from 76 to 86 dB(A), the reduction of the 

calculated sound power level was 9 dB(A) (Fig. 6), and 

the sound level was reduced by about 7 dB(A) in this 

area of the plant. Taken alone, this total reduction would 

be attributed to reducing the sound power contribution 

of Surface 1 by more than 90% (Fader, 1981). 

1.5-m (60-in.) roller mill duct. The area near the 

ductwork of the 1.5-m (60-in.) roller mill (Fig. 7) was 

identified by an analysis of the baseline sound level 

measurements as a noisy area, Fig. 11 (a), > 94 dB(A). 

The baseline sound intensity measurements on the duct, 

shown as a sound intensity contour plot in Fig. 8, ranged 

from 82 to 98 dB(A). The calculated sound power level was 93 dB(A). Using a recommended noise control 

technique (Driscoll, 1996), the duct was wrapped with 

a sound barrier material. Figure 9 shows the duct after 

the treatment was applied.The sound intensity measurements on the treated duct ranged from 82 to 94 dB(A), 
as shown in Fig. 10.The calculated sound power level for 

the measurement surface was 90 dB(A). The reduction 

of the sound power level was 3 dB(A), contributing to 

the overall a sound level reduction of about 2 dB(A) in 

this area of the mill. It can be seen in Fig. 9 that the highest sound intensity levels were measured in the middle 

of the duct. The treatment reduced the sound intensity 

in this area by more than 10 dB(A).The reduction of the 

sound power level by 3 dB(A) would be attributed to 

reducing the sound energy of the noise source by 50% 

(Fader, 1981). 

FIGURE 7	 

Untreated roller mill duct.	 

FIGURE 8	 

Sound intensity measurement results on untreated 1.5-m 

(60-in.) roller mill duct. 

FIGURE 9 

Ductwork treated with a sound barrier material. 

FIGURE 10 

Sound intensity measurement results on treated 1.5-m 

(60-in.) roller mill duct. 

All engineering noise controls installed. The installed 

engineering noise controls consisted of: 

•	 acoustic curtains around the FEM fans and the 

crushers; 

• 	 acoustic curtains in front of ACM 1 and 2 and absorptive noise control material inserted under the 

hood of ACM 2; 

•	 a larger shroud for ACM 3, filled with a noise barrier 

and noise absorptive material; and 

• 	 1.5-m (60-in.) roller mill duct wrapped with noise 

barrier material. 

It must be noted that simple maintenance and repairs 

on ACM 3 reduced the sound level, measured at Location 20 in Fig. 1, by about 10 dB(A). Therefore, a second 

baseline was established (Table 1, operating Condition 

7). This was before the followup test measurements with 

initial noise controls installed were taken. For the fi nal 

test, the main mill area sound levels were again measured 

using the same 47 measurement points that were used 

during the initial visit. The second baseline sound level 

measurements ranged from 91 to 100 dB(A). After the 

implementation of controls the sound levels ranged from 

90 to 94 dB(A) (Table 1). On average, the engineering 

noise controls reduced the sound level on the main mill 

floor by 4 dB(A). 

Figure 11 shows four different operating conditions 
 

 

during the noise control study. The 

following sound level measurements 

were taken in the mill: (a) baseline 

(no noise controls installed), (b) with 

initial acoustic curtains, (c) with additional acoustic curtains in front of 

the ACMs and (d) with fully installed 

noise controls. 

FIGURE 11 

Contour maps for (a) baseline, (b) acoustic curtains around the FEM fans and 

upper and lower levels of the Jeffrey crusher, (c) acoustic curtains around 

the FEM fans, upper and lower levels of the Jeffrey crusher and (d) in front of 

ACMs all engineering controls. 

A long-term goal of the mine officials to reduce in-plant sound levels 

was accomplished and this will subsequently reduce worker noise exposure. 

The remaining levels are still hazardous, so administrative controls and 

hearing protection devices (HPDs) 

are still needed to avoid the risk of 

hearing damage. It is more likely that 

HPDs will provide adequate protection for noise levels of 90 to 94 dB(A) 

than the precontrols levels exceeding 

100 dB(A). Before the controls, worker overexposure to noise would occur 

in the loudest area of the mill in about 

one hour, now over-exposure would 

occur in about 5 hours, under MSHA 

criteria (CFR, 1999). 

Summary 

With the cooperation of mine officials, the National Institute for Occupational Safety and Health (NIOSH) 

conducted a study to quantify sound 

levels and to determine the sound reduction provided by engineering noise 

controls installed in a talc processing 

plant. Sound intensity and sound level 

measurements were performed in the 

plant before and after installing noise 

controls. 

The baseline sound level measurements ranged from 93 to 104 dB(A). 

The initial engineering noise controls 

consisted of acoustic curtains installed 

around the FEM fans and the Jeffery 

crusher. Acoustic curtains were then used in front of the 

other crusher and the ACMs, sound-absorbing material 

was inserted under the hood of ACM 2, and maintenance 

and repairs on ACM 3 were completed. The sound levels 

measured after this work was completed ranged from 88 

to 94 dB(A), a significant reduction of 10 dB(A) from the 

highest baseline sound level. For the loudest ACM, the 

noise controls consisted of building a larger shroud, lining 

the inside of the shroud with a sound barrier material, and 

filling the interior of the shroud with sound absorbing material. Sound intensity measured after the new shroud was 

in place ranged from 76 to 86 dB(A), and a remarkable reduction in the sound power level of 9 dB(A) was achieved 

on the measurement surface. The final control for this 

study was wrapping the duct of a 1.5-m (60-in.) roller mill 

with a sound barrier material.The highest sound intensity 

measurement of the duct was reduced by 6 dB(A), and 

the sound power level on the measurement surface was 

reduced by 3 dB(A). The main mill area second baseline 

sound level measurements taken after maintenance and 

repairs on ACM 3, ranged from 91 to 100 dB(A).The fi nal 

noise control sound levels measurements were reduced to 

a range of 90 to 94 dB(A). 

Using these or similar controls at the other U.S. nonmetal processing plants could reduce the exposure of 

roughly 6,000 workers. Mine management would still 

have to use administrative controls or require the workers 

to wear hearing protection to reduce the risk of hearing 

damage, but the extent of overexposure was decreased 

significantly. MSHA’s preferred method for assessment 

of miners’ exposure and noise controls involve full shift 

dosimetry along with time-motion studies. While this 

method was not accomplished here, an assessment of the 

sound level reduction increases the time a worker can 

spend in the mill without being overexposed to noise by 

about 4 hours. The significant noise reductions that were 

obtained through noise controls and repairs would necessarily reduce exposure, especially in the unusual situation 

where workers needed to remain in the noisiest areas for 

prolonged periods. Capturing these atypical situations 
 

through full-shift dosimetry was beyond the scope of the 

current study. 

Disclaimer 

The findings and conclusions in this report have not 

been formally disseminated by the National Institute for 

Occupational Safety and Health (NIOSH) and should 

not be construed to represent any agency determination 

or policy. 

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