DEPARTMENT OF HEALTH AND HUMAN SERVICES

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health

Noise Control

in Underground Metal Mining

IC 9518

INFORMATION CIRCULAR/2009

IC 9518  Information Circular

Noise Control in 

Underground Metal Mining

by Efrem R. Reeves1, Robert F. Randolph2, David S. Yantek3, and 

 J. Shawn Peterson4

DEPARTMENT OF HEALTH AND HUMAN SERVICES

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health

1 Acoustical Engineer, U.S. Army Aeromedical Research Laboratory, Fort Rucker, AL

2 Manager, Hearing Interventions Team, Pittsburgh Research Laboratory, National Institute for 

Occupational Safety and Health, Pittsburgh, PA

3 Research Engineer (Mechanical Engineer), Pittsburgh Research Laboratory, National Institute 

for Occupational Safety and Health, Pittsburgh, PA

4 Research Engineer (General Engineer), Pittsburgh Research Laboratory, National Institute for 

Occupational Safety and Health, Pittsburgh, PA
ii | Noise Control in Underground Metal

This document is in the public domain and may be freely copied or reprinted.

Disclaimer

Mention of any company or product does not constitute endorsement by the 

National Institute for Occupational Safety and Health (NIOSH). In addition, citations to Web sites external to NIOSH do not constitute NIOSH endorsement of the 

sponsoring organizations or their programs or products. Furthermore, NIOSH is 

not responsible for the content of these Web sites.

Ordering Informa

To receive documents or other information about occupational safety and health 

topics, contact NIOSH at

Telephone: 1–800–CDC–INFO (1–800–232–4636)

TTY: 1–888–232–6348

E-mail: cdcinfo@cdc.gov

or visit the NIOSH Web site at www.cdc.gov/niosh.

For a monthly update on news at NIOSH, subscribe to NIOSH eNews by visiting 

www.cdc.gov/niosh/eNews.

DHHS (NIOSH) Publication No. 2010–111

December 2009
Noise Control in Underground Metal Minin

Conten

Acronyms and Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Noise-Induced Hearing Loss. 

Three Variables of Noise Exposure. 

Noise Problem Analysis. 

Noise Dosimeters. 

Sound Levels. 

The Role of Engineering Noise Controls in Reducing NIHL. 

2 Noise Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Hierarchy of Noise Control. 

Barriers and Sound-Absorbing Materials. 

Field Studies. 

3 Evaluating Noise Controls for Haul Trucks . . . . . . . . . . . . . . . . . . 11

Absorptive Material. .

Partial Engine Enclosures. .

Discussion. .

4 Evaluating Noise Controls for Load-Haul-Dumps . . . . . . . . . . . . 21

Engine Enclosure 1. .

Engine Enclosure 2. .

Enclosed Operator Cab. .

Discussion. .

5 Evaluating Noise Controls for Jumbo Drills and Bolters . . . . . . 33

Covers for Electric-Motor-Powered Hydraulic Pumps. .

Absorptive Material in Canopy. .

Absorptive Material on Sides of the Cab Around Operator Area. .

Absorptive Material in Lower Front of Cab. .

Windshields. .

Discussion. .

6 Summary and Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Haul Trucks. .
iv | Noise Control in Underground Metal

Load-Haul-Dumps (LHDs). .

Jumbo Drills and Bolters. .

Lessons Learned. .

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Appendix A: What is Noise? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

The Physics of Sound. .

Decibels. .

Average Sound Levels. .

Appendix B: Noise Exposure as a Health Hazard . . . . . . . . . . . . . . 55

Effects of Noise. .

Dose Accumulation. .

Appendix C: Sound Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

Sound Level Meters. .

Frequency Bands. .

Decibel Addition. .
Noise Control in Underground Metal Min

Acronyms and Abbrevia

ANSI American National Standards Institute

CFR Code of Federal Regulations

dB decibels

dB(A) decibels, A-weighted

f frequency

Hz Hertz

L

eq equivalent continuous sound level

L

avg average sound level

LHD load-haul-dump

m meters

MSHA Mine Safety and Health Administration 

NIOSH National Institute for Occupational Safety and Health

NIHL noise-induced hearing loss

Pa Pascals

PEL permissible exposure level

psi pounds per square inch

REL recommended exposure limit

rms root mean square

SLM sound level meter

TWA8 time-weighted average (8-hour)

λ wavelength
vi | Noise Control in Underground Metal

Acknowledgeme

The authors are indebted to the following colleagues who participated in data 

collection, analysis, and reviews of early versions of this report: Daniel R. Babich, 

Michael DiMartino, Thomas M. Durr, Gerald L. Finfinger, Patrick J. Hintz, Peter 

G. Kovalchik, R.J. Matetic, Timothy J. Matty, Alex D. Prokop, and Adam K. Smith.
Noise Control in Underground Metal Min

1 Introduc

1.1 Noise-Induced Hear

Noise-induced hearing loss (NIHL) is the most common occupational illness 

in the United States, with 30 million workers exposed to excessive noise levels 

[NIOSH 1996] every day. Of particular concern is the mining industry; which has 

the highest prevalence of hazardous noise exposure of any major industry sector 

[Tak et al. 2009] and is second only to the railroad industry in prevalence of workers reporting hearing difficulty [Tak and Calvert 2008].

This document is for operators, safety personnel, and mechanics in the mining 

industry who are not specialists in noise control engineering or acoustics. Evaluations of successful and unsuccessful attempts at controlling noise on several large, 

underground metal mining machines are detailed to illustrate the basic principles 

of noise control. Once personnel understand the guidelines and principles of noise 

control, they will be able to

■ evaluate the extent of a noise problem;

■ determine the best approach to the problem; and 

■ apply the most appropriate solution.

Because of the insidious nature of NIHL, it can go unnoticed until a considerable 

loss of hearing has occurred. In some cases, diagnosis is delayed because an exposed individual claims to have become accustomed to the noise. In reality, that 

person may have already suffered irreversible hearing loss. 

Humans can hear sounds in the frequency range from about 20 to 20,000 Hertz 

(Hz). Within this range, NIHL usually begins in the frequency region around 

4,000–6,000 Hz, the upper levels of the speech region. The first noticeable symptoms include difficulty understanding higher pitched voices, such as the voices 

of females and children, and difficulty understanding certain consonant sounds, 

which are primarily high frequency in nature. 

The extent of NIHL varies depending on the level and duration of noise exposure 

and on an individual’s susceptibility; despite having similar noise exposure, individuals can experience differing degrees of hearing loss, or none at all. 

NIHL is almost always preventable. To reduce or eliminate the possibility of NIHL, 

an individual’s noise environment must be analyzed and appropriate action taken 

to reduce noise exposure.
2 | Noise Control in Underground Metal

Chapter 1 | Introduction

1.2 Three Variables of Noise

The three elemental components to consider when devising an engineering noise 

control are source, path, and receiver, which interact with each other to produce 

a unique situation for a given environment; the same source can yield different 

sound levels when the path or the location of the receiver is changed. Engineering 

noise controls can be implemented to reduce the amount of sound energy generated by the noise source and to divert the flow of sound energy from the propagation path, all with the aim of protecting the receiver (worker) from being exposed 

to high levels of sound energy. 

1.3 Noise Problem An

The Mine Safety and Health Administration (MSHA) established noise exposure 

limits for mine workers, and these are given in 30 CFR Part 62 (the noise rule). 

MSHA regulations state that an individual’s occupational noise exposure should 

not exceed an 8-hour time-weighted average (TWA8) of 90 dB(A). This level is defined as the permissible exposure level (PEL). Higher noise exposures are permitted for shorter periods of time as shown in Table 2. The noise rule applies to the 

sound received by a worker, not to sound emitted by a machine or process.

Table 1. Permissible time allowed for a given noise exposure

Exposure duration 

(hours) Sound level (dB[A])

8 90

6 92

4 95

3 97

2 100

1 105

0.5 110

0.25 115

Source: 30 CFR Part 62

The noise rule stipulates that noise exposures resulting in the MSHA action level 

(AL) TWA8

 of 85 dB(A) or more require that hearing protection is provided to 

the exposed miner. Exposures resulting in a TWA8 of 105 dB(A) or more require 

dual hearing protection. In practice, this usually means that an earmuff-type hearing protector must be worn over an earplug-type hearing protector. Dual hearing 
Chapter 1 | Introduction

Noise Control in Underground Metal Min

protection is required in addition to the actions required for enforcing the PEL. 

Further, the noise rule states that no miner can be exposed to sound levels exceeding 115 dB(A) for any amount of time.

Under the MSHA noise rule for the PEL, sound levels below 90 dB(A) do not 

contribute to the calculation of partial noise doses (Appendix B). In other words, 

a miner could be exposed to a sound level of 89 dB(A) for a full shift and—from a 

regulatory point of view—receive zero noise dose. This does not mean, however, 

that this individual has zero risk of receiving hearing damage. The NIOSH recommended exposure limit (REL) is a TWA8

 of 85 dB(A). Noise levels exceeding this 

REL are considered hazardous by NIOSH.

1.4 Noise Dosimeter

To determine the amount of noise workers are exposed to during the course of 

their day, workers can wear noise dosimeters. A dosimeter is designed to be worn 

on a person during all or part of a work shift, and it measures and stores sound 

levels and computes total noise exposure. Dosimeters are especially useful in 

environments where the noise levels are variable or intermittent or when workers 

move to and from different areas of a plant or mine during the course of a work 

shift. If sound levels are constant and the worker does not move, a sound level 

meter (SLM) can also be used to assess exposure. The procedure for using SLMs to 

measure noise and assess exposure is detailed in Appendix C. 

Dosimeters and SLMs incorporate filters—or weighting networks—that can be 

applied to affect the meter’s sensitivity to desired sound frequencies. The weighting 

is performed according to accepted standards. The A-weighting network approximates human perception of the loudness of low level sounds (around 40 dB). It 

is the most widely used weighting network because it is a reasonable estimator of 

the risk of NIHL. Without weighting in place, the SLM would indicate the same 

sound pressure level for sound waves having the same amount of physical energy 

regardless of the sound’s frequency. In reality, very low and high frequency sounds 

are less damaging than mid frequency sounds. so A-weighting de-emphasizes the 

extreme frequencies. In the test examples in the following sections, the A-weighted 

scale is used, resulting in A-weighted decibels symbolized by units of dB(A).*

A dosimeter must be calibrated before and after each measurement period with 

a calibrator that fits the specific type of microphone for the meter. The pre-measurement calibration is necessary to ensure the instrument is functioning properly 

prior to making measurements. The post-measurement calibration is especially 

important in a mining environment because the instrument is likely to be subjected to jolting and jarring during a work shift and because temperature or humidity 

extremes could affect the accuracy of the meter. 

* See Appendix C for more details about weighting networks.
4 | Noise Control in Underground Metal

Chapter 1 | Introduction

The microphone, the most fragile part of the instrument, is especially susceptible 

to damage. The documentation provided by the instrument manufacturer should 

give the valid operating ranges for humidity, pressure, and temperature.The SLM 

should be calibrated by a qualified laboratory at the interval recommended by the 

manufacturer, typically every 1-2 years. Their calibration should be traceable to the 

National Institute of Standards and Technology.

Proper placement of the dosimeter microphone is important. ANSI S12.19-1996, 

Measurement of Occupational Noise Exposure, specifies that the microphone 

should be located on the mid-top of the wearer’s most noise-exposed shoulder. The 

microphone should be set approximately parallel to the plane of wearer’s shoulder, 

and the cable should be routed and fastened such that it does not interfere with 

job performance or create a safety hazard. For miners, the best place to attach the 

dosimeter case is usually the miner’s belt. 

1.5 Sound Levels

When a weighting filter is applied in determining the level of a sound, the word 

pressure is dropped from the term sound pressure level. For context, a list of the 

A-weighted average sound levels of some common sounds are shown in Table 2. 

Table 2. Typical sound levels of common sources

Sound source Level (dB(A))

Threshold of pain 120

Rock concert 110

Subway train 100

Heavy truck or bus (15 meters away) 90

Urban traffic 80

Passing cars (15 meters away) 70

Normal conversation 60

Classroom 50

Suburban neighborhood at night 40

Bedroom at night 30

Broadcast studio 20

Rustling leaves 10

Threshold of hearing 0

Source: Cowan 1994
Chapter 1 | Introduction

Noise Control in Underground Metal Min

1.6 The Role of Engineering Noise Controls i

Reducing NIHL

The mining industry recognizes how important engineering noise controls are 

in reducing noise exposure during underground operations. But, because of the 

relatively small market for mining equipment, manufacturers have limited incentives to develop less noisy machinery or more innovative noise controls. Also, the 

specialized equipment designs imposed by the sometimes-hostile mining environment has limited the transfer of noise control technologies from other industries.

Despite this lack of proven control technologies, mine operators work with what’s 

available to try to create noise control solutions at the mine level. However, many 

operators install noise treatments without knowing how much noise reduction to 

expect or how much noise reduction is actually achieved after installation. In some 

cases, because of improper material selection, placement, or installation, the noise 

treatment reduces sound little—if any. In other cases, noise treatments are applied 

when the source sound level does not warrant treatment, thus wasting effort and 

resources. Unsuccessful noise controls cost the industry time and money, and they 

do nothing to decrease workers’ risk of NIHL—though they give the false impression that the problem, if there is one, has been addressed.

Noise Control in Underground Metal Min

2 Noise Control

2.1 Hierarchy of Noise Contro

Three methods to reduce worker noise exposure are:

1. Implementing engineering noise controls to reduce noise at the source or at 

the worker

2. Using administrative controls to limit the amount of time workers spend in 

noisy environments

3. Wearing personal protective equipment, such as hearing protectors, to 

reduce the sound level entering the ears.

Using engineering noise controls is the most desirable option because they address 

noise sources directly. Administrative controls and hearing protectors are indirect 

interventions and are less easily monitored and therefore more readily circumvented.

2.2 Barriers and Sound-Absorbing Material

A barrier is a solid obstacle that is at least somewhat impervious to sound and 

interrupts the direct path from the sound source to the receiver. The sound transmission loss (TL) of a material is a measure of its ability to block sound. To block 

sound most effectively, the barrier should be

■ placed as close as possible to either the source or receiver;

■ assembled to be as tall and wide as practical so it extends well beyond the 

direct source-receiver path; and

■ constructed of a material that is solid and airtight.

Low frequency sounds are difficult to block with barriers because low frequency 

sounds pass directly through and bend around obstacles relatively easily. This is 

why the bass tones from a passing car stereo are audible even inside buildings. Mid 

to high frequency sounds, which often dominate a worker’s noise exposure, cannot pass through or bend around barriers as easily as low frequency sounds. In 

general, adding mass to a barrier improves its ability to block noise. Another way 

to improve the TL of a barrier is to use multiple layers of material with each layer 

separated from the others using a compliant material such as foam. This method 

decouples the vibration of each layer from the other layers and, therefore, increases 

the TL.
8 | Noise Control in Underground Metal

Chapter 2 | Noise Control

Sound-absorbing treatments are usually made of porous materials that absorb 

incident sound energy and reduce the reverberation due to sound reflected from 

surfaces. Fiberglass and open-cell foam are often used for sound absorbers. A 

material’s degree of sound absorption depends on its flow resistance and thickness, 

the way it is mounted, and the frequency of the incident sound. Thicker sound 

absorbing materials are needed to absorb low frequency sounds. For frequencies 

above about 1 kHz, 1-inch-thick sound absorbing material has sufficient sound 

absorption. Two-inch-thick sound-absorbing material has good absorption for 

frequencies above about 500 Hz. Protective facings on sound absorbing foam tend 

to improve the sound absorbing capabilities of the material at lower frequencies.

To improve the sound absorption of an installed material, the material can be 

mounted with an air space between it and the surface behind it. To achieve the 

best results, the material should be spaced one-quarter wavelength from the 

surface behind it. In this case, the wavelength is based on the lowest frequency of 

interest. In addition, the optimal absorption of a material occurs when the thickness is equal to one-quarter wavelength for the frequency of interest. Or,

 t = λ /4 (1.1)

where t = material thickness, inches

 λ = wavelength of sound for lowest frequency of 

 interest, inches

Equation 1.1 shows that in some instances it can be impractical to install material with the optimal thickness or spacing to absorb low frequency sounds. For 

example, the optimal material thickness for noise at 1 kHz is roughly 3.5 inches 

and the optimal material thickness for noise at 500 Hz is about 7 inches. Knowing 

the frequency content of a noise problem enables one to select a sound-absorbing 

material that has sufficient absorption at the frequencies where the noise energy is 

greatest.

2.3 Field Studie

In its advice concerning controlling noise, MSHA emphasizes a reduction in daily 

noise exposure or dose. To accommodate this, noise controls should be evaluated in the environments in which they will be used. Initially, a sound level meter 

should be used to assess a noise control by measuring the sound level without and 

with the noise control. If the application of a noise control does not reduce sound 

levels, then this control is ineffective at reducing noise, assuming all other factors have remained constant. Once a control is applied that reduces sound levels, 

changes in noise exposure can be assessed through dosimetry.
Chapter 2 | Noise Control

Niose Control in Underground Metal Min

Engineering noise controls can be difficult and expensive to implement. For complex 

noise problems, it is best to consult with a professional noise control engineer before 

implementing a solution. However, some simple measures to reduce sound levels, 

including proper maintenance of mining equipment, are easy to implement. This 

document details results of testing of barriers and sound-absorbing materials used 

to control sound levels on and around machinery at several mine sites. To show the 

detail of the measurements, the sound levels are reported to the nearest 0.1 dB. However, for general use, rounding to the nearest whole number is permissible.

Noise Control in Underground Metal Mini

Haul trucks at metal mines operate both underground and on the surface. Haul 

truck noise is a good example of the challenging problem of reducing noise in both 

reverberant and non-reverberant environments.

3.1 Absorptive Materi

Several of the tested haul trucks had 0.75-inch-thick, vinyl-covered material installed 

in the area in front of the operator. It is not known if this material was a sound absorbing material or if it was only padding. Figure 1 shows an example of these haul trucks. 

The material was attached with Velcro for easy removal (Figure 2).

NIOSH researchers measured sound levels at the operator position above ground 

at low and high idle, with and without the vinyl-covered material in place. Table 3 

gives the results. The data for haul truck 1 confirm that the vinyl-covered material 

had little effect on sound levels at the operator’s position. The results for haul truck 

2 show that the sound level at low idle was higher with the vinyl-covered material 

than without it, possibly due to fluctuations in engine output between the tests. At 

high idle, the vinyl-covered material had no measurable effect on the sound level.

Evaluating Noise Controls for 3 Haul Trucks
12 | Noise Control in Underground Metal

Chapter 3 | Evaluating Noise Controls for Haul Trucks

Figure 1. Haul truck with vinyl-covered material installed in the area i

front of the operator.

Figure 2. Haul truck without vinyl-covered materia
Chapter 3 | Evaluating Noise Controls for Haul Trucks

Noise Control in Underground Metal Mini

Table 3. Sound level at the haul truck operator’s position, surface

measurement

Haul 

truck 1 

without 

material 

(dB[A])

Haul 

truck 

1 with 

material 

(dB[A])

Haul 

truck 1 

noise 

reduction 

(dB[A])

Haul 

truck 2 

without 

material 

(dB[A])

Haul 

truck 

2 with 

material 

(dB[A])

Haul 

truck 2 

noise 

reduction 

(dB[A])

Low idle 81.9 81.7 0.2 85.3 86.5 -1.2

High idle 101.3 100.6 0.7 101.3 101.3 0.0

The third haul truck had the same 0.75-inch-thick, vinyl-covered material in 

the area in front of the operator as the previous two haul trucks. However, haul 

truck 3 also had vinyl-covered material attached to the underside of the canopy 

(see Figures 3 and 4). NIOSH researchers compared sound levels measured at 

the surface with those measured underground under the same conditions: at the 

operator position at low and high idle, with and without the vinyl-covered material 

in place.

Figure 3. Haul truck with vinyl-covered material installed in canop

above the operator.
14 | Noise Control in Underground Metal

Chapter 3 | Evaluating Noise Controls for Haul Trucks

Figure 4. Haul truck with vinyl-covered material remove

Table 4 shows the effect of having vinyl-covered material in the canopy by comparing the measurement with all material installed to the measurement taken without 

material in the canopy or in front of the operator. Table 4 also shows the effect of 

using all the material in the underground environment. At high idle, using the 

vinyl-covered material only on the underside of the canopy reduced the sound 

level by 0.6 dB(A) whereas installing all the material reduced the sound level by 

1.0 dB(A). 

Table 4. Sound level at the haul truck operator’s position, underground 

measurement

Without 

vinyl-covered material (dB[A])

With vinylcovered 

material 

in canopy 

only (dB[A])

With all 

vinyl-covered material (dB[A])

Noise reduction with 

vinyl-covered 

material in 

canopy (dB[A])

Noise reduction with all 

vinyl-covered 

material 

(dB[A])

Low idle 83.2 82.4 82.6 0.8 0.6

High idle 100.6 100.0 99.6 0.6 1.0
Chapter 3 | Evaluating Noise Controls for Haul Trucks

Noise Control in Underground Metal Mini

Table 5 shows the sound levels for haul truck 3 measured above ground at the operator’s position. Comparing the data from Table 5 with that of Table 4, the underground environment increased the sound level at the operator’s ear by 2.4 dB(A) at 

low idle and 1.5 dB(A) at high idle.

Table 5. Sound level for haul truck 3 at the 

operator’s position, surface measurement

Full padding (dB[A])

Low idle 80.2

High idle 98.1

3.2 Partial Engine Enclosure

Engine enclosures are used to contain engine noise. Sound-absorbing material can 

be used to line engine enclosures to absorb noise within the enclosure. This can 

reduce the sound level emitted from the enclosure. The actual amount of noise reduction achieved depends on many factors. To contain engine noise, the enclosure 

must be made from a material with a high TL. Adequate space is needed between 

the engine and its enclosure to allow proper flow of cooling air. If the space between the engine and enclosure is insufficient, the cooling fan will not be able to 

efficiently move air and the noise due to the fan may increase substantially. Gaps in 

an enclosure greatly reduce its ability to contain noise.

To test how effective partial engine enclosures are at reducing sound levels on 

haul trucks, sound levels were measured underground at the haul truck operator’s 

position. Data were collected on eight different haul trucks at high idle. Each haul 

truck had a partial engine enclosure similar to the one shown in Figure 5, fashioned from a piece of 0.5-inch-thick rubber that NIOSH researchers believed to be 

used conveyer belt material. Figure 6 shows the engine without the partial enclosure. Measurements were made with and without the barrier in place. The results 

showed that the barrier reduced the sound level reaching the operator by about 1 

dB(A).
16 | Noise Control in Underground Metal

Chapter 3 | Evaluating Noise Controls for Haul Trucks

3.3 Discussio

3.3.1 Absorptive Materia

The results of this testing demonstrate that using absorptive materials is not a 

cure-all fix for haul truck noise. Often, the low cost and relative ease of application 

of some absorptive materials leads inexperienced personnel to apply them as a 

quick fix. However, before installation the situation must be analyzed to determine 

Figure 5. Partial engine enclosur

Figure 6. Open engine compartmen
Chapter 3 | Evaluating Noise Controls for Haul Trucks

Noise Control in Underground Metal Mini

whether they will be effective. Without this initial assessment, the time and expense of applying the materials may be completely wasted.

For example, Figure 7 shows a haul truck with an open cab. The arrows represent 

sound waves and the blue area in the canopy of the haul truck represents soundabsorbing material. For the material to be effective, the majority of the sound 

reaching the operator must be due to reflections from the underside of the canopy. 

In this example, the operator would be in the direct path of the sound waves from 

the engine compartment. In addition, sound waves reflected from the rib have a 

direct path to the operator. Only a fraction of the sound waves reaching the operator are reflected from the underside of the canopy. So, in this case, the material 

would have little to no effect.

The test results showed that the underground environment increased sound levels 

at both low and high idle. NIOSH researchers attribute this increase to the reverberation of sound that occurs in enclosed spaces. The amount of increase also 

depended on whether the machine was running at low idle or high idle. This is 

due to the different frequency content associated with the noise emitted at high 

and low idle and how each of these is affected by the mine environment. Since the 

environment and operating conditions can have a significant impact on equipment 

noise, controls should be assessed in the environment where they are used under 

all operating conditions.

Figure 7. Haul truck with sound-absorbing material installed i

canopy and depiction of how sound may enter the operator area, 

reaching operator before padding.
18 | Noise Control in Underground Metal

Chapter 3 | Evaluating Noise Controls for Haul Trucks

3.3.2 Partial Enclosur

When use of a full enclosure to control noise is impractical, testing has shown that 

a properly designed partial enclosure or barrier can provide some level of sound 

reduction. When a partial enclosure is used, it is critical to block the line of sight 

to the engine as much as possible. Adjacent sections of material should be overlapped to prevent sound waves from escaping the engine compartment. 

Engine enclosures cannot be sealed completely due to requirements for cooling air. 

Airflow paths should be designed such that the air must follow a maze-like path. 

The surfaces of the airflow path should be lined with sound-absorbing material. As 

noise travels down the flow path, the sound-absorbing material will absorb some 

of the sound energy.

3.3.3 Sealing Ga

An often overlooked noise control measure is sealing gaps. A hole or gap in an 

enclosure, even if small, can greatly compromise noise reduction. Gaps provide a 

direct path for sound to travel from the engine to the haul truck operator. Sealing 

gaps reduces the noise exposure of the operator. Figure 8 shows a large gap around 

the perimeter of the instrument panel, which is part of the engine enclosure. Sealing the gaps around the instrument panel, as shown in Figure 9, can significantly 

reduce the operator’s noise exposure. Sound-absorbing foam should not be used 

to seal gaps. Due to its open cell nature, sound-absorbing foam is not very good at 

blocking noise. When sealing gaps, closed cell foam should be used instead.
Chapter 3 | Evaluating Noise Controls for Haul Trucks

Noise Control in Underground Metal Mini

Figure 8. Direct openings to the engine compartment aroun

instrument panel.

Figure 9. Gaps around instrument panel sealed with foa

Noise Control in Underground Metal Mini

Evaluating Noise Contr

4 Load-Haul-Du

Load-haul-dump vehicles (LHD) are widely used in underground metal mines for 

moving material. NIOSH researchers assessed several noise controls applied in the 

engine compartment and the operator’s cab.

4.1 Engine Enclosure 

LHD1 is shown in Figure 10. The engineering noise controls on the machine 

consisted of a partial engine enclosure and sound-absorbing material in the 

engine compartment and in the cab. The engine enclosure on the left (operator) 

side of the machine was composed of 0.125-inch-thick steel panels insulated with 

1.5-inch-thick fiberglass sound-absorbing material. Engine areas not covered by 

steel were covered with 0.25-inch-thick rubber that NIOSH researchers believed to 

be used conveyer belt material. The left side of the engine was completely enclosed 

as seen in Figure 11.
22 | Noise Control in Underground Metal

Chapter 4 | Evaluating Noise Controls for Load-Haul-Dumps

Figure 12 shows the front and back of the steel panels that covered the left side of 

the engine. The right side of the engine enclosure, shown in Figure 13, consisted 

of 0.25-inch-thick rubber that did not completely cover the engine compartment 

opening.

Figure 10. LHD

Figure 11. Left side of the engine enclosur
Chapter 4 | Evaluating Noise Controls for Load-Haul-Dumps

Noise Control in Underground Metal Mini

Sound levels at the operator’s position were measured both on the surface and underground with the engine at high idle. Measurements were made with both sides 

of the engine enclosure on or off, and then with the right side off and the left side 

either on or off. Table 6 gives the results. The measurements on the surface show 

Figure 12. Front and back of the steel panels insulated wit

1.5-inch-thick fiberglass.

Figure 13. LHD1 partial engine enclosure, right sid
24 | Noise Control in Underground Metal

Chapter 4 | Evaluating Noise Controls for Load-Haul-Dumps

that the full enclosure reduced the sound level by about 1 dB(A). Because these 

LHDs are primarily used underground, the underground results are more important. Comparing the surface and underground measurements, the underground 

environment adds about 3 to 4 dB(A) to the sound level at the operator’s ear. The 

table shows the application of all controls in the underground environment resulted in an attenuation of 1.5 dB(A) at the operator position.

Table 6. Sound level for LHD1 at the operator’s position, high idle

Surface measurement Underground measurement

Without 

enclosure 

(dB[A])

With 

enclosure 

(dB[A])

Noise reduction 

(dB[A])

Without 

enclosure 

(dB[A])

With enclosure 

(dB[A])

Noise 

reduction 

(dB[A])

Both sides 

on or off

91.9 90.8 1.1 95.8 94.3 1.5

Right side 

off, Left side 

on or off

91.6 90.8 0.8 95.1 94.3 0.8

4.2 Engine Enclosure 

LHD2 is shown in Figure 14. The noise controls on the machine consisted of a 

partial engine enclosure, absorptive material in the engine compartment, and 

professionally installed noise control material in the open cab. The top portion 

of the engine enclosure was composed of 0.375-inch-thick steel panels lined with 

quilted fiberglass sound-absorbing material. The left side of the engine was almost 

completely enclosed, as shown in Figure 15, by 0.125-inch-thick steel panels with 

quilted sound absorption. However, the picture shows several gaps around the 

perimeter of the steel panels. Figure 16 shows the inside of the steel panels covered with absorptive material. The right side of the engine compartment was not 

enclosed as shown in Figure 17.

Sound levels were measured at the operator’s position with the engine at low and 

high idle with the noise controls described above. At low idle, the sound levels 

were about 75 dB(A) on the surface and slightly less than 80 dB(A) underground. 

With the engine at low idle, the sound level was measured underground with 

the machine in reverse and the back-up alarm sounding. In this case, the sound 

level was 95.4 dB(A). Table 7 shows the A-weighted sound levels measured at the 
Chapter 4 | Evaluating Noise Controls for Load-Haul-Dumps

Noise Control in Underground Metal Mini

Figure 14. LHD

Figure 15. Left side partial engine enclosur
26 | Noise Control in Underground Metal

Chapter 4 | Evaluating Noise Controls for Load-Haul-Dumps

Figure 17. Right side and top of engine compartmen

Figure 16. Quilted absorber inside the engine compartmen
Chapter 4 | Evaluating Noise Controls for Load-Haul-Dumps

Noise Control in Underground Metal Mini

operator position at high idle on the surface and underground. On the surface, the 

engine enclosure offers 1.5 dB(A) of noise reduction. However, examination of the 

underground results shows a reduction of only 0.1 dB(A). This is most likely due 

to noise from the right side of the engine reflecting from the rib and back to the 

operator station. Figure 17 shows that the right side of the engine is completely 

exposed.

 Table 7. Sound level for LHD2 at the operator’s position, high idle

Sound level 

without enclosure 

(dB[A])

Sound level 

with enclosure 

(dB[A])

Noise reduction 

(dB[A])

Surface 89.4 87.9 1.5

Underground 92.2 92.1 0.1

Figure 18 shows the one-third-octave-band sound pressure level spectrum measured underground at the operator’s position with the engine at high idle without 

the partial enclosure. The sound level was the same as that of LHD2, 92.2 dB(A), 

as shown in the column labeled “Without enclosure/Underground” of Table 7. The 

figure shows that most of the sound energy is below 500 Hz. To effectively absorb 

Figure 18. In-cab one-third-octave band spectrum for LHD2 at high idle wit

engine compartment open.

16 31.50 63 125 250 500 1000 2000 4000 8000 A L

30

40

50

60

70

80

90

100

110

120

Frequency (Hz)

Sound Pressure Level (dB)

Overall sound level = 92.2 dB(A)

Greatest amount of 

sound energy below 500 Hz
28 | Noise Control in Underground Metal

Chapter 4 | Evaluating Noise Controls for Load-Haul-Dumps

low frequency sounds, a thick material (4–7 inches) or a thin material backed by 

an airspace is required [Moulder 1998]. Unfortunately, either solution would be 

difficult to implement in the limited space of the engine compartment.

It is interesting to note that the 80 Hz one-third-octave band is the highest. Noise 

at the engine firing frequency is contained in this band. Perhaps a larger muffler 

would have reduced the sound level of the machine. The presence of significant 

low frequency energy points to the cooling fan as a likely noise source. Neither 

exhaust noise at the engine firing frequency nor cooling fan noise would be significantly affected by noise controls applied to the engine enclosure.

4.3 Enclosed Operator Ca

Figure 19 shows a 6-yard LHD similar to LHD3. (The machine in the figure is not 

the actual machine evaluated.) The noise control installed on the evaluated machine consisted of a glass-enclosed cab lined with vinyl-faced, 1-inch-thick open 

cell foam with an attached barrier material. The material used in the cab is shown 

in Figure 20.

To enclose the cab, shown in Figure 21, window-frame and upper door modifications were required for glass panes to be installed within the frames. All of the 

modifications were cleared with the manufacturer to ensure that the integrity of 

the falling object protective structure was not adversely affected.

Figure 19. LHD

Figure 20. Open cell foam used for in-ca

sound absorption.

Barrier material

Vinyl facing
Chapter 4 | Evaluating Noise Controls for Load-Haul-Dumps

Noise Control in Underground Metal Mini

With the machine underground, NIOSH researchers simultaneously measured 

sound levels inside and outside of the cab with the engine at high idle. In addition, 

sound levels were measured above, in front, to the right, and to the rear of the cab 

with and without the glass panels installed. 

Table 8 shows the measurement results. The column labeled “Interior” is the 

average of four measurements taken inside the cab. The “Exterior” column is the 

average of the four exterior measurements and can be used as a reference to judge 

the noise reduction provided by the cab. The exterior measurements were also 

used to ensure that the sound level generated by the LHD did not vary much during the course of the measurement period. The results indicate that the completely 

enclosed cab produced greater than 20 dB(A) of noise reduction. Even without the 

front window installed (row 4), more than 10 dB(A) of reduction was achieved.

Table 8. Sound level for LHD3 at the operator’s position, high idle, underground measurement

Exterior (dB[A]) Interior (dB[A])

All windows open 99.9 96.9

Left window closed 98.2 93.8

Back and left windows closed 98.4 92.9

Back, left, and right windows closed 99.9 89.1

All windows closed 100.3 77.7

Figure 21. Enclosed cab with glass in plac
30 | Noise Control in Underground Metal

Chapter 4 | Evaluating Noise Controls for Load-Haul-Dumps

4.4 Discussion

Partial engine enclosures with openings of any size greatly compromise the noise 

reduction capability of the enclosure. This is especially true underground, where 

sound initially directed away from an operator can strike the walls and reflect back 

to the operator. To be effective at reducing the sound levels reaching the operator, 

enclosures must be designed to minimize holes and gaps, especially those with line 

of sight between the noise source and the operator.

The most effective noise-reducing enclosures are airtight. However, an airtight 

enclosure for a source that requires ventilation, such as an engine, is impractical 

because it could lead to overheating and engine damage. The only openings in the 

engine compartment should be those to allow cooling air into and out of the cooling package. For an LHD, if solid panels cannot be used for the engine enclosure, a 

partial enclosure that incorporates overlapping materials or baffles, similar to that 

suggested for haul trucks, should be used. Using a partial engine enclosure will 

decrease the sound levels compared to an open engine compartment. However, an 

engine compartment with solid panels is the best approach. 

As a rule of thumb, enclosures should be lined with sound-absorbing material 

to reduce build up of reverberant noise within the enclosure. Full coverage of all 

surfaces is not completely necessary as the effect of adding sound-absorbing material decreases as more and more of the surfaces are covered. The best approach to 

develop an enclosure is to first eliminate any gaps or leaks and then to add soundabsorbing material inside.

For LHD2, the lined partial engine enclosure reduced the sound level by 1.5 dB(A) 

above ground. However, in the underground environment, the sound levels were 

not affected (refer to Table 7). This is probably due to an increase in the contribution of cooling fan and exhaust noise to the sound level at the operator’s ear in the 

underground environment. Underground, fan noise and exhaust noise can reflect 

from the rib to the operator. In addition, the underground environment may have 

amplified the exhaust tones corresponding to the engine firing rate.

A fully enclosed environmental cab can provide 20 dB(A) or more of noise reduction. Besides providing protection and comfort for the operator, environmental 

cabs are designed to reduce exposures to occupational hazards such as dust and 

noise. When installing a retrofit cab, it is wise to contact the original equipment 

manufacturer to ensure that the integrity of the falling object protective structure 

is not compromised. In addition, once the cab is enclosed, a climate control system 

should be installed to ensure the safety and comfort of the equipment operator.
Chapter 4 | Evaluating Noise Controls for Load-Haul-Dumps

Noise Control in Underground Metal Mini

The results of testing done with LHD3 indicate that the completely enclosed cab 

reduced noise by more than 20 dB(A). Sound levels were reduced more than 10 

dB(A) even with the back window removed. This shows that if completely enclosing an operator’s cab is not feasible, a properly designed 3/4-cab enclosure can 

provide a substantial noise reduction. The resulting noise reduction will depend 

on the location of noise sources on the machine relative to the open area of the 

3/4-cab. If there are no significant noise sources near the open area, the partial cab 

can still provide a substantial noise reduction. However, if a noise source has line 

of sight with the operator due to the exclusion of a side, the partial cab will not be 

effective.

Noise Control in Underground Metal Mini

Evaluating Noise Controls f

Drills and Bolt

Noise controls for jumbo drills and bolters consisted of several motor covers or 

barriers, treatments applied inside the cabs, and different cab windshield designs.

5.1 Covers for Electric-Motor-Powered Hydraulic Pump

Ten machines were tested: five roof bolters and five jumbo face drills. All of the 

tested face drills and bolters were equipped with at least one electric motor used to 

drive hydraulic pumps. The dual-boom face drills were equipped with two electric 

motors used to drive hydraulic pumps. The motors were directly behind the operator area as shown in Figure 

22. Five of the tested machines—two roof bolters and 

three jumbo drills—had noise 

controls installed around the 

electric motor and hydraulic 

pumps. All of the reported 

measurements were made 

underground at the operator’s ear position with only the 

electric motors operating.

5

Figure 22. Dual-boom jumbo face dril
34 | Noise Control in Underground Metal

Chapter 5 | Evaluating Noise Controls for Jumbo Drills and Bolters

Table 9 shows the sound levels without and with noise controls applied to the 

electric-motor-powered hydraulic pumps. Several of the tested controls are shown 

in Figures 23–25. It should be noted that the sound levels generated with only the 

electric motors on were less than 85 dB(A). Sound levels during drilling and bolting can exceed 100 dB(A). Therefore, the noise due to the electric-motor-powered 

hydraulic pumps is insignificant in terms of the operator’s dose. The data show 

the motor enclosures built from barrier-type materials reduced the sound level by 

about 2 dB(A). However, the enclosures built from absorptive material reduced the 

sound level less than one-half dB(A). Sound-absorbing materials do not usually 

provide much TL.

Table 9. Sound level for jumbo drills and bolters at the operator’s position, 

underground

Motors Noise control

Without 

control 

(dB[A])

With control 

(dB[A])

Noise reduction (dB[A])

Bolter 1 0.25-inch-thick heavy 

conveyor belt 84.9 83.2 1.7

Bolter 2 1.5-inch-thick fiberglass 

blanket 77.3 76.9 0.4

Face drill 1 0.5-inch-thick heavy 

conveyor belt 79.4 77.2 2.2

Face drill 2 1.5-inch-thick quilted 

fiberglass absorptive 

material

79.9 79.5 0.4

Face drill 3 0.25-inch-thick Plexiglas 84.3 81.9 2.4

Figure 23. Heavy conveyor belt barrieFigure 24. Fiberglass blanket barrieFigure 25. Plexiglas moto

cover.
Chapter 5 | Evaluating Noise Controls for Jumbo Drills and Bolters

Noise Control in Underground Metal Mini

5.2 Absorptive Material in Canop

Most of the tested machines had sound-absorbing material under the canopy. 

However, only three of the machines had the material installed in such a way that 

it could be easily removed and replaced so that NIOSH researchers could directly 

measure its effectiveness at reducing noise. In all of the cases, the absorptive material was a 1-inch-thick quilted fiberglass blanket. Figure 26 shows the material 

being removed for testing.

Figure 26. One-inch-thic

quilted fiberglass blanket being 

removed for testing.

Table 10 shows the sound levels without and with the material and the resulting 

noise reduction achieved by applying it to the canopy. The face drill measurements 

were taken underground during the drilling cycle, and the bolter results were 

measured above ground with the percussive hammer operating. Face drill 2 had 

a removable windshield, so the effect of the absorptive material in the canopy was 

measured with and without the windshield. The data show the sound-absorbing 

material did not significantly change the sound levels at the operator’s position in 

this case.

Table 10. Sound level of jumbo drills and bolters at the operator’s position

Motors

Without absorptive material 

(dB[A])

With absorptive 

material (dB[A])

Noise reduction 

(dB[A])

Bolter 2 97.4 97.3 0.1

Face drill 1 99.1 99.3 -0.2

Face drill 2

(with windshield) 99.6 99.6 0.0

Face drill 2

(without windshield)

100.3 100.1 0.2
36 | Noise Control in Underground Metal

Chapter 5 | Evaluating Noise Controls for Jumbo Drills and Bolters

5.3 Absorptive Material on Sides of the Cab Aroun

Operator Area

One bolter and one face drill had a removable 1-inch-thick quilted fiberglass blanket around the operator’s area. For bolter 1, measurements were performed underground with the windshield in place while drilling and bolting. The operator’s area 

of bolter 1 is shown in Figure 27. For face drill 2, the sound levels were measured 

while only the electric-motor-powered hydraulic pumps were operating. 

Figure 27. Quilted fiberglass material in the operator’s area of bolter 

Table 11 shows the sound levels with and without the absorptive material placed 

around the operator and the resulting noise reduction. The data indicate that the 

absorption around the operator has essentially no effect on the sound level during the drilling process. During the bolting process, the measured sound level at 

the operator’s ear was 0.3 dB(A) higher with the material in place. However, this 

difference is most likely due to changes in the noise produced at the drill steel, not 

due to the installation of the sound-absorbing material.

Table 11. Sound level of jumbo drills and bolters at the operator’s position, 

absorptive material around operator

Without quilted 

material around 

operator area 

(dB[A])

With quilted 

material around 

operator area 

(dB[A])

Noise reduction 

(dB[A])

Bolter 1 (drilling) 97.5 97.6 -0.1

Bolter 1 (bolting) 98.4 98.7 -0.3

Face drill 2 (motor) 78.1 77.2 0.9
Chapter 5 | Evaluating Noise Controls for Jumbo Drills and Bolters

Noise Control in Underground Metal Mini

5.4 Absorptive Material in Lower Front of Ca

Figure 28 shows a 1-inch-thick quilted fiberglass blanket applied to the lower front 

of the operator area on bolter 2. This material is located where the operator’s knees 

would be positioned while operating the drill boom.

Figure 28. Quilted fiberglass materia

in the lower front of the operator’s 

area of bolter 2.

Table 12 shows the effect that the absorptive material placed in this position had 

on sound levels measured at the operator’s ear during drilling and bolting. The 

table shows the levels were virtually unchanged in each case. This is not surprising. 

Most of the drilling and bolting noise probably reaches the operator by bending 

around the windshield or by first reflecting off the rib. The noise reflected from the 

front lower area to the operator is most likely minimal.

Table 12. Sound level of bolter 2 at the operator’s position, absorptive 

material in lower front of cab

Without absorption 

(dB[A])

With absorption 

(dB[A])

Noise reduction 

(dB[A])

Bolter 2 (drilling) 98.1 97.9 0.2

Bolter 2 (bolting) 99.9 99.9 0.0

5.5 Windshield

The most common noise control installed on the tested face drills and bolters was 

a windshield. The amount of noise reduction achieved varied greatly depending on 

how the windshield was designed. Several examples of windshields are shown in 

Figures 29–31.
38 | Noise Control in Underground Metal

Chapter 5 | Evaluating Noise Controls for Jumbo Drills and Bolters

Figure 30. Windshiel

installed on bolter 3.

Figure 29. Windshiel

installed on bolter 2.

Figure 31. Windshiel

installed on bolter 5.
Chapter 5 | Evaluating Noise Controls for Jumbo Drills and Bolters

Noise Control in Underground Metal Mini

Most of the windshields were designed to be flipped up into the canopy. This feature allowed the operator an unobstructed view while operating and tramming the 

machine. The windshield on bolter 2 had gaps between sections that were arranged 

vertically and did not wrap around the operator station (see Figure 29). The windshield of bolter 3 had no gaps between sections of windshield, and the windshield 

wrapped around the operator (see Figure 30). Bolter 5’s windshield was continuous, but it did not wrap around the operator station. Strips of belting material had 

been installed on the sides of the operator station on bolter 5 in an effort to block 

noise (see Figure 31). Face drills 1 and 4 had wrap-around windshields without 

gaps between sections.

Table 13 shows the effect the windshields had on sound levels reaching the operator’s ear. The greatest noise reductions were achieved for bolter 3, face drill 1, and 

face drill 4, all having wrap-around windshields with no gaps. The only difference 

between the windshields of bolters 2 and 5, was that bolter 2’s windshield had gaps 

between panes and bolter 5’s windshield was continuous. Bolter 5 experienced a 

1-dB(A) greater noise reduction than bolter 2 when drilling.

Table 13. Sound level of bolters 2, 3, 4, and 5 at the operator’s position

Without windshield 

(dB[A])

With windshield 

(dB[A])

Noise reduction 

(dB[A])

Bolter 2 while drilling 98.5 97.9 0.6

Bolter 2 while bolting 101.2 99.9 1.3

Bolter 5 while drilling 100.6 99.0 1.6

Bolter 3 while drilling 99.2 96.0 3.2

Bolter 3 while bolting 105.7 102.5 3.2

Face drill 1 101.7 99.3 2.4

Face drill 2 100.1 99.6 0.5

Face drill 3 97.1 95.3 1.8

Face drill 4 with single 

boom 94.0 91.9 2.1

Face drill 4 with double 

boom 98.9 95.6 3.3

Face drill 5 101.9 100.6 1.3
40 | Noise Control in Underground Metal

Chapter 5 | Evaluating Noise Controls for Jumbo Drills and Bolters

5.6 Discussio

Covers for electric-motor-powered hydraulic pumps constructed of a heavy barrier material, such as conveyor belting, as opposed to an absorptive material such as 

fiberglass, produced the most substantial sound level reductions. However, on the 

tested machines the A-weighted sound levels created by the untreated motors were 

below 85 dB(A). Section 3.3.1 advised having the environment analyzed for noise 

levels prior to incurring the expense of noise treatments. If multiple noise sources 

generate sound levels of 85 dB(A) individually, it may be necessary to treat each of 

these sources to reduce the operator’s noise exposure. For example, four 85-dB(A) 

noise sources operating together would result in a sound level of 91 dB(A). However, on a case-by-case basis, the contribution of each noise source to the operator’s noise exposure should be determined before installing noise controls. With 

bolters and jumbo drills, the sound level due to drilling and bolting often reaches

100 dB(A), whereas the pumps generate a sound level less than 85 dB(A). Therefore, the noise exposure due to the electric-powered-hydraulic pumps is insignificant and, in this case, noise controls should not be applied to the pumps. 

The application of fiberglass absorptive material to the canopy, seat area, and lower 

portion of the open cab had little to no effect on the sound level at the operator’s 

ear during drilling and bolting. To be effective at reducing the sound level reaching the operator, sound-absorbing materials must be placed on surfaces that reflect 

sound toward the operator’s hearing zone. Furthermore, a significant portion of 

the noise at the operator’s ear must be due to noise reflected from these surfaces. If 

the majority of the noise at the operator’s station arrives directly from the face or 

from reflections from the rib, treating the surfaces around the operator will have 

virtually no effect on the sound level at the operator’s ear. For machines with open 

cabs, such as those installed on the face drills and roof bolters tested, absorptive 

materials will be of limited benefit.

For face drill 2, a reduction of nearly 1 dB(A) was achieved with absorptive material in the operator area with only the electric-motor-powered hydraulic pumps 

in operation. This reduction probably occurred because the noise from the pumps 

must reach the operator by an indirect path. Line of sight with the pumps is 

obscured by body of the machine. As this noise reflects off surfaces around the 

operator, the material around the operator reduces the noise. However, when the 

operator began drilling, the primary noise source became drilling noise. Since 

drilling noise reaches the operator mainly via a direct path, or by bending around 

the windshield, the absorbing material around the operator would have no effect.

In general, well-designed windshields were the most effective noise controls 

implemented on the drills and bolters because they block drilling and bolting 

noise from reaching the operator. Also, the noise generated by drilling and bolting 
Chapter 5 | Evaluating Noise Controls for Jumbo Drills and Bolters

Noise Control in Underground Metal Mini

is predominantly high frequency in nature. High frequency sounds are easier to 

block and absorb because of their shorter wavelengths. The windshields that had a 

gap between an upper and a lower pane of glass were the least effective at reducing 

sound levels because the gaps allow drilling and bolting noise to pass through.

The conveyor belt strips serving as a makeshift enclosure on bolter 5 were installed 

in an attempt to supplement the noise reduction due to the windshield. Because 

no sound level measurements were taken without the strips in place, the noise 

reduction they offer is unknown. NIOSH researchers assume they have little, if 

any, effect on sound levels reaching the operator’s ear because of gaps between the 

strips. The strips should be overlapped a few inches to improve the noise reduction 

due to their use.

Noise Control in Underground Metal Mini

Summary and Guidel

Engineering noise controls are the preferred solution to a noise problem because 

they address noise sources directly. Administrative controls and personal protective equipment should be explored as secondary solutions.

Basic noise controls include barriers and sound-absorbing materials. A barrier is a 

solid obstacle that is somewhat impervious to sound and that interrupts the direct 

path from the sound source to the receiver. For the best reduction in sound level, 

the barrier should be

■ placed as close as possible to either the source or receiver;

■ assembled to be as tall and wide as practical so it extends well beyond the 

direct source-receiver path; and

■ constructed of a material that is solid and airtight.

Sound-absorbing treatments reduce reflections and the resulting echoes and 

reverberation. Usually, these materials are porous. Compared to high frequency 

sounds, low frequency sounds are more difficult to absorb with materials and to 

block with barriers. Therefore, it is important to know the frequency content for a 

particular noise problem.

The effectiveness of barriers and absorptive materials as noise controls on mining 

equipment was tested during field studies. Following are some of the key findings.

6.1 Haul Truck

The use of absorptive materials in the operator’s area of tested haul trucks had very 

little effect on sound levels underground. Sound level reductions were on the order 

of 1 dB(A). Most of the sound reaches the operator via the direct path from the 

noise source to the operator. In addition, noise reflects from the walls to the operator station. Open cabs allow the direct and reflected sound to enter the operator 

station. Therefore, a large reduction in sound levels from installing sound-absorbing material at the operator station is not expected. 

6.2 Load-Haul-Dumps (LHDs

A fully enclosed environmental cab can provide 20 dB(A) or more of noise reduction. If a fully enclosed cab is impractical, a partial cab can provide useful protection as long as the openings face away from the primary noise sources. A partial 

6
44 | Noise Control in Underground Metal

Chapter 6 | Summary and Guidelines

cab with three sides and a top was found to provide more than 10 dB(A) of noise 

reduction. Both full and partial cabs should have similar results on other underground equipment. When installing a retrofit cab, it is wise to contact the original 

equipment manufacturer to ensure that the integrity of the falling object protective 

structure (FOPS) is not compromised.

6.3 Jumbo Drills and Bolter

When applying noise control treatments, care should be taken to use the right 

product for the job. The 0.5-inch-thick rubber conveyor belt mats used to cover 

the electric-motor-powered hydraulic pumps on the jumbo drills and bolters were 

effective at reducing noise because the heavy rubber is a barrier material, which is 

the correct choice for the application. Rubber is usually not the best material to use 

for a barrier, but in this case it was effective. On bolter 2, the electric motor and 

hydraulic pumps were covered with sound-absorbing material. In this instance, the 

treatment had almost no effect on the noise from the electric motor and hydraulic 

pumps because sound-absorbing material makes a poor barrier. Sound-absorbing 

material is most effective when it is used at a reflective surface. The cover should 

have been constructed using a barrier lined with sound-absorbing material to surround the electric motor and hydraulic pumps.

Prior to developing noise controls for a source, the significance of this source 

should be considered relative to other noise sources on a machine. In this case, the 

sound level with the electric motor and hydraulics operating was 85 dB(A) whereas noise due to drilling and bolting is about 100 dB(A). In this case, the noise due 

to the electric motor and hydraulic pumps is insignificant. 

Windshields on jumbo drills and bolters reduced the sound level at the operator’s station during the drilling/bolting cycle up to 3 dB(A). The noise generated 

from drilling and bolting is relatively high frequency in nature. Therefore, the 

windshield provides an effective barrier. Gaps between and around sections of the 

windshield should be sealed for the most effective noise control. In addition, wrapping a windshield around the operator station improves the noise reduction by 

forcing drilling/bolting noise to travel further around the windshield to get to the 

operator. In addition, this helps block noise reflected from the rib from reaching 

the operator.
Chapter 6 | Summary and Guidelines

Noise Control in Underground Metal Mini

6.4 Lessons Learne

Through evaluating different noise controls on underground machinery, NIOSH 

researchers discovered several findings. Both the effective and ineffective treatments rendered valuable information.

1. Although it is tempting to use sound-absorbing materials for noise controls 

because they are inexpensive and simple to attach to existing surfaces, 

sound barriers were always more effective in the examples NIOSH studied 

for this report.

2. Windshields and environmental cabs can be highly effective noise controls, 

especially for high frequency noise.

3. Plugging gaps in machine panels and windshields with a material that 

creates an airtight seal can greatly enhance the noise reduction benefits of 

existing barriers.

4. Gaps in barriers compromise noise control effectiveness.

5. When openings in enclosures are necessary, a partial enclosure can provide 

some benefit. Enclosures should be lined with an absorptive material thick 

enough to absorb the dominant sound frequencies. Openings to let air in 

and out of the enclosure should have lined ducts with multiple bends to 

absorb sound and to force it to follow a circuitous pathway before exiting 

the enclosure.

In order to reduce noise-induced hearing loss from work-related noise exposure, 

mine workers, union representatives, mine managers, equipment manufacturers, NIOSH, and MSHA must work in partnership to successfully construct and 

implement new and better noise controls. To ensure the success of a noise control 

program, appropriate materials must be applied and the noise sources treated must 

be significant in terms of the worker’s daily noise exposure.

Noise Control in Underground Metal Mini

References

ANSI [2001]. American national standard: measurement of occupational noise 

exposure. New York: American National Standards Institute, Inc., S12.19–1996.

ANSI [2007]. American National Standard Specification for Personal Noise Dosimeters. New York: American National Standards Institute, Inc., S1.25–1991 

(R2007).

CFR. Code of Federal Regulations. Washington, DC: U.S. Government Printing 

Office, Office of the Federal Register.

Cowan JP [1994]. Handbook of environmental acoustics. New York: Van Nostrand 

Reinhold, p. 37.

Kryter KD [1994]. The handbook of hearing and the effects of noise. San Diego, 

CA: Academic Press.

Moulder R [1998]. Sound absorptive materials. In: Harris CM, ed. Handbook 

of acoustical measurements and noise control. 3rd ed. New York: McGraw-Hill, 

Chapter 30.

NIOSH [1979]. Industrial noise control manual. Cincinnati, OH: U.S. Department 

of Health, Education, and Welfare, Center for Disease Control, National Institute 

for Occupational Safety and Health, SHEW (NIOSH) Publication No. 79–117.

NIOSH [1996]. National occupational research agenda. Cincinnati, OH: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) 

Publication No. 96–115.

Tak S, Calvert GM [2008]. Hearing difficulty attributable to employment by industry 

and occupation: An analysis of the National Health Interview Survey—United States, 

1997 to 2003. Journal of Occupational and Environmental Medicine 50:46–56.

Tak S, Davis RD, Calvert GM [2009]. Exposure to hazardous workplace noise and 

use of hearing protection devices among US workers—NHANES, 1999–2004. 

American Journal of Industrial Medicine 52:358–371. 

Noise Control in Underground Metal Mini

Appendix A 

Noise is generally defined as sound that is loud, unpleasant, or otherwise undesirable. As a type of sound, it can be characterized according to its physical properties.

A.1 The Physics of Soun

A basic understanding of the physics of sound is crucial for measuring and controlling it. Sound results from a physical occurrence created by a pressure disturbance that travels, or propagates, through an elastic medium via longitudinal 

wave motion. When this disturbance reaches the ear, it is perceived by the brain 

as sound. The propagation of sound can be compared with the motion that takes 

place along a stretched spring. If one end of the spring is repeatedly compressed 

and released, a traveling wave is produced. As the wave propagates, sections of the 

spring compress and then expand. This compression and expansion of the spring is 

analogous to the compression and expansion of air particles caused by the propagation of a sound wave. The compression and expansion of the air particles cause 

a fluctuation of the air pressure above and below the atmospheric pressure. This is 

shown graphically in Figure A.1.

What is Noise?

Figure A.1. An illustration of wave motion (sine wav

One cycle

PRESSURE

Amplitude

TIME

Expansion

Compression
50 | Noise Control in Underground Metal

Appendix A | What is Noise?

The rate at which the air particles expand and contract is referred to as the frequency of the wave. Frequency is measured in cycles per second and is commonly 

expressed in Hertz (Hz). Figure A.1 depicts a pure tone that has only one frequency. For a pure tone, the frequency can be determined by counting the number of 

full cycles that occur in a fixed time and then dividing the number by the time. For 

example, Figure A.1 shows two full cycles. Assuming the two cycles are completed 

in 0.01 seconds, the frequency would be 200 cycles per second, or 200 Hz. Young 

listeners with normal hearing can usually hear sounds from a low frequency of 

about 20 Hz to a high frequency of roughly 20,000 Hz.

In general, real sounds are made up of many frequencies. Knowing the frequency 

content of a sound is useful in determining how to design noise controls to attenuate offensive sounds, or noise. The frequency content of a particular noise can be 

used to determine which materials to use to control it. In addition, the frequency 

content of a noise can be used to determine its source.

The time it takes for the wave to go through one complete cycle, from trough-totrough (minimum-to-minimum) or crest-to-crest (maximum-to-maximum),as 

shown in Figure A.1, is the period of the wave. The period (T) is inversely related 

to frequency (f) by:

T = 1/f (A.1)

The distance a sound wave travels during one complete cycle is its wavelength. 

Wavelength is a distance commonly expressed in meters or feet. The wavelength of 

a sound, λ, can be found using the frequency, f, and the speed of sound, c by:

λ = c/f (A.2) 

The speed that a sound wave travels in any medium depends only on the properties of the medium through which it propagates. The speed of sound in dry air at a 

temperature of 20 °C, or 68 °F, is about 343 meters per second or about 1,125 feet 

per second. Using equation A.2, the wavelength for a 100 Hz tone in air would be 

3.43 meters, or 11.25 feet.

Assuming the speed of sound is held constant, Equation A.2 shows that as frequency increases, wavelength must decrease, and vice versa. For high frequency 

sounds, the distance between each area of compression or expansion along the 

wave will be small. For low frequency sounds, the distance between each area of 

compression or expansion will be relatively large. Stated another way, low frequency sounds have long wavelengths and high frequency sounds have short wavelengths.
Appendix A | What is Noise?

Noise Control in Underground Metal Mini

In air, sound travels as a pressure wave. As a pressure disturbance passes a point in 

space, minute changes occur in the atmospheric pressure at that point. The sound 

pressure alternates between positive and negative values so it adds to and subtracts 

from the atmospheric pressure. This sound pressure is analogous to an AC voltage 

on top of a DC voltage. Here, atmospheric pressure is analogous to the DC voltage. 

Sound pressure is typically measured in Pascal (Pa). One Pascal is equal to 1.45 × 

10-4 pounds per square inch (PSI). Sound pressure causes the eardrum to vibrate. 

These minute changes in pressure can be measured with a microphone. The magnitude of the pressure change corresponds to the amplitude of the wave as shown 

in Figure A.1. The perceived loudness of a sound is directly related to the amplitude of the disturbance. Several other factors, such as frequency and fluctuations 

in the amplitude of a sound, also influence the perceived loudness of a sound.

To quantify the change in the pressure at any point due to a sound wave, the rootmean-square (RMS) pressure is used. The RMS is the square root of the average of 

the instantaneous pressure squared. If a simple average of the changes in pressure 

were used to quantify the pressure wave, the result would be equal to zero because 

the positive pressure fluctuations would be cancelled by the negative pressure 

fluctuations.

The human ear responds to a wide range of pressures from 20 × 10-6 to 20 Pa. 

The threshold of hearing for an undamaged ear corresponds to an RMS pressure of 20 x 10-6 Pa at 1,000 Hz. An RMS sound pressure near the high end of 

the range would be painful and could damage the auditory system, even with 

brief exposures. Working with values of pressure that cover such a large range 

can be unwieldy. Therefore, to simplify the numbers and reduce the range of 

values to a manageable size, the value of the RMS sound pressure is converted 

to decibels (dB).

A.2 Decibel

Decibels are logarithmic values formed by taking the ratio of a power to a reference power. In terms of time-varying signals, such as AC voltage or sound pressure, the power is related to the RMS value squared. The reference used when 

dealing with sound pressure is 20 × 10-6 Pa, which, as mentioned previously, is the 

RMS pressure that is barely audible at 1,000 Hz. When measured, this pressure 

would yield a value of 0 dB. The term level is commonly used to designate a logarithmic ratio of relevant parameters. The sound pressure level for any sound can be 

calculated using the following equation:
52 | Noise Control in Underground Metal

Appendix A | What is Noise?

 (A.3)

where SPL is the sound pressure level in dB;

PRMS = RMS sound pressure in Pascals; and

Pref = Reference RMS sound pressure, 20 × 10-6 Pa

The RMS sound pressure commonly referred to as the threshold of pain is 20 Pa. 

Using this value in Equation A.3 yields a sound pressure level of 120 dB.

A.3 Average Sound Levels

Often, one is more interested in determining the average sound pressure level over 

a period of time rather than the sound level at a particular time. Two commonly 

used averages are the equivalent continuous sound level, denoted Leq, and the average sound level, denoted Lavg.

The L

eq is the constant sound level that has the same energy as a fluctuating sound 

over the measurement time. For example, if the fluctuating sound in the cab of a 

haul truck produced an Leq of 80 dB over a 1-hour period, the sound would have 

the same sound energy as a continuous sound of 80 dB over the same 1-hour 

period. A 3-dB increase in the Leq corresponds to a doubling of the sound energy 

over the measurement time.

The L

avg is commonly used in the assessment of worker noise exposure. In some 

texts L

avg is called TWA or TWA(x) where the ‘x’ denotes the measurement time 

in hours. The L

avg

 is similar to the L

eq except the value of the Lavg depends on the 

exchange rate, threshold level, and criterion level used to measure a worker’s noise 

dose. For a given criterion level and allowable exposure time, the exchange rate is 

the change in sound level corresponding to a doubling or halving of the allowable 

exposure time. On many dosimeters, the exchange rate can be set to 3, 4, 5, or 6 

dB. The current MSHA exchange rate is 5 dB.

The 8-hour time-weighted average sound level (TWA8

) is used in occupational 

noise measurements. It is the constant A-weighted sound level for an 8-hour 

time period that would expose a person to the same noise dose as did the actual 

time-varying sound level over the time used to perform the measurement. If the 

measurement time is less than 8 hours, the TWA8 will always be less than Lavg. If 

the measurement period is greater than 8 hours, the TWA8 will always be greater 

than L

avg.

D = C1 +

C2 + C3 + . . . +

 Cn

T1 T2 T3 Tn

= 8

 20.2(LA-90)

SPL = 10log P2

 = 20log P

 P2

 P

RMS

ref ref

RMS
Appendix A | What is Noise?

Noise Control in Underground Metal Mini

Table A.1 lists various measurement times and their associated TWA8 values for an 

L

avg of 90 dB(A) using a 5-dB exchange rate. For a measurement time of 4 hours, 

the corresponding TWA8 would be 85 dB(A). Further, a measurement time of 12 

hours would result in a TWA8

 of about 93 dB and a 16-hour measurement time 

would yield a TWA8 of 95 dB.

Assessing an operator’s TWA8

 is a very good method to compare noise exposures 

before and after installing controls. A reduction in TWA8 that reaches the intended 

target after implementation of engineering noise controls would indicate that the 

effort was successful and achieved the desired result. MSHA PIB 08-12 considers a 

noise control to be technologically achievable if it reduces a worker’s noise exposure by 3 dB(A). TWA8

 measurements can also be used to assess compliance with 

MSHA regulatory requirements.

Time (hours) TWA8 (dB[A])

 1 75

 2 80

 4 85

 8 90

10 92

12 93

14 94

16 95

Table A.1. TWA8 as a function of Measurement Time 

for an L

avg of 90 dB(A)

Noise Control in Underground Metal Mini

Appendi

Noise is “audible acoustic energy (or sound) that is unwanted because it has adverse auditory and nonauditory physiological or psychological effects on people” 

[Kryter 1994]. Generally, noise is unwanted sound. 

B.1 Effects of Nois

As stated above, noise can have both psychological and physiological effects. The 

psychological effects of noise include annoyance, speech interference, sleep interference, and decreased work performance [Kryter 1994]. Sudden noises can trigger the physiological response of muscular reflex as the body prepares for defensive action against the source of the noise. Sometimes, this spontaneous reaction 

can interfere with tasks or cause accidents. However, the physiological effect most 

commonly associated with noise is temporary or permanent hearing loss.

It is widely known that exposure to high sound levels can cause damage to the 

mechanisms of the inner ear and reduce hearing sensitivity. A noise-exposed individual may experience a temporary threshold shift, which is a temporary reduction in sensitivity to sound. Normal sensitivity may return after a period without 

noise exposure. However, continued exposure to high sound levels can eventually 

result in a permanent threshold shift—or noise-induced hearing loss (NIHL). 

Because NIHL generally occurs gradually over time, it can go unnoticed until a 

considerable hearing loss has occurred. In some cases, an exposed individual will 

claim to have become accustomed to the noise. However, in reality, it is impossible 

for the ears to become resistant to noise. 

B.2 Dose Accumulatio

In many mining environments, sound levels vary throughout the day. Changes in 

geology, the amount of material mined, or mining method can all affect the generated sound levels. Also, many miners move throughout the mine or operate their 

machines in various modes, which can expose the miner to different sound levels. 

When sound levels are not constant, the total dose for a worker can be calculated 

using the partial dose for each task or sound level. With MSHA PEL criteria, exposure to any sound level at or above 90 dB(A) results in the accumulation of a noise 

dose. The total, or daily, noise dose can be calculated from the partial dose due to 

each task using the following equation:

Noise Exposure 

Health Haza
56 | Noise Control in Underground Metal

Appendix B | Noise Exposure as Health Hazard

 (B.1)

D = C1 +

C2 + C3 + . . . +

 Cn

T1 T2 T3 Tn

= 8

 20.2(LA-90)

SPL = 10log P2

 = 20log P

 P2 P

RMS

ref ref

RMS

 where D = total noise dose

 Cn = the actual exposure time for task or sound level 

 n, in hours

 Tn = the allowable exposure time for task or sound 

 level n, in hours (see Table A.1) 

If the allowable exposure time is something other than the values listed in Table 

A.1, the value of Tn

 based on an 8-hour workday can be calculated from the following equation (assuming MSHA PEL parameters):

Allowable Exposure Time (hours) (B.2)

 where LA = the A-weighted sound level at the worker’s 

 position for a given task 

To calculate the allowable exposure time for a criterion level other than 90 dB(A), 

the desired sound level in dB(A) would replace 90 in the denominator of Equation 

B.2.

D = C1 +

C2 + C3 + . . . +

 Cn

T1 T2 T3 Tn

= 8

 20.2(LA-90)

SPL = 10log P2

 = 20log P

 P2

 P

RMS

ref ref

RMS
Noise Control in Underground Metal Mini

Appendi

In a mining environment, sound levels are usually measured for either compliance 

or diagnostic purposes. Compliance measurements are made in accordance with 

Mine Safety and Health Administration (MSHA) regulations to determine if exposures exceed the Permissible Exposure Level (PEL). Diagnostic measurements, on 

the other hand, are used to help locate and quantify noise sources. Another important difference is that compliance measurements focus on workers in the vicinity 

of one or more noise-generating machines, whereas diagnostic measurements primarily focus on the machines themselves. This appendix summarizes basic techniques of diagnostic noise measurement as the first step in planning a successful 

noise control strategy. The appendix also summarizes diagnostic usage of the two 

most basic and essential instruments available to measure and characterize sound: 

sound level meters and dosimeters.

C.1 Sound Level Meter

A sound level meter (SLM) is the instrument used to measure the sound level of 

a noise source. MSHA regulations allow the use of a Type 1 (precision) or Type 2 

(general purpose) SLM as designated by the American National Standards Institute (ANSI S1.4). A Type 2 meter has broader performance tolerances and is usually less expensive. According to ANSI S1.4, the expected total error for an SLM 

measuring steady broadband noise in a reverberant field is approximately +/- 1.5 

dB for a Type 1 SLM and +/- 2.3 dB for a Type 2 SLM.

When using an SLM, it is important to set the frequency weighting and the time 

weighting to the appropriate settings. For measurements related to assessment 

of noise exposure, the SLM should be set to “A-weighting” and “slow response”. 

A single sine wave (refer to Figure A.1) produces a sound at only one frequency. 

This type of sound is called a pure tone. Most real-world sounds consist of many 

frequencies. Therefore, when an overall sound level measurement is performed, 

a single decibel value is obtained that accounts for the sound energy over a broad 

range of frequencies. However, the human ear does not respond to all frequencies in the same way. At low sound levels, the ear is most sensitive to sounds with 

frequencies from 500 to 5,000 Hz. Below 500 Hz, the sensitivity of the ear falls off 

quite rapidly as frequency decreases. As the sound level increases, the response of 

the ear “flattens” somewhat with respect to frequency. For high sound levels, the 

sensitivity of the ear to sounds below 500 Hz does not decrease as rapidly as it does 

for low level sounds. 

Sound Measure
58 | Noise Control in Underground Metal

Appendix C | Noise Control in Underground Metal Mining

Figure C.1. Figure C.1. Frequency response for weighting networ

20 50 100 200 500 1,000 2,000 5,000 10,000 20,000

FREQUENCY IN CYCLES PER SECOND

RELATIVE RESPONSE-DECIBELS

+5

0

-5

-10

-20

-30

-35

-25

-15

-40

-45

-50

A

B

C

Key

Frequency responses

for SLM weighting characteristics

Sound measurements are usually conducted to assess human reaction to sound or 

to determine the potential risk for hearing damage. Since the sensitivity of the ear 

is a function of both frequency and level, several weighting networks, or filters, 

were developed to approximate the sensitivity of the ear. These filters simulate 

the response of the ear under certain conditions. A typical SLM has two different frequency weighting networks, identified as the A-weighting and C-weighting 

networks. Some meters also have a B-weighting network, which falls between the 

A and the C networks. 

In terms of judging loudness, A-weighting is most appropriate for low level sounds 

(40 dB), B-weighting is most appropriate for mid level sounds (70 dB), and Cweighting is most appropriate for high level sounds (100 dB). When assessing the 

potential risk for causing hearing loss, the A-weighing network is used because 

it is a reasonable estimator of the risk of hearing damage. Figure C.1 shows the 

frequency response characteristics of the A, B, and C networks. When a weighting 

filter is applied in determining the level of a sound, the word pressure is dropped 

from the term sound pressure level. If an RMS pressure of 20 Pa is measured using 

the A-weighted network, sound level would be written as 120 dB(A).
Appendix C | Noise Control in Underground Metal Mining

Noise Control in Underground Metal Mini

Time weighting, or response, adjusts the response time of the meter. Typically, 

it can be set to Fast, Slow, Impulse, or Peak. Fast time weighting can be used to 

closely track rapid fluctuations in sound levels. Slow time weighting is useful when 

estimating the average level of a rapidly fluctuating sound. For an integrating-averaging SLM, a time weighting of Fast or Slow should not affect the measurement of 

the equivalent continuous sound level, or Leq. However, for compliance measurements, MSHA requires the use of Slow for the time weighting and A-weighting for 

the frequency weighting.

Impulse time weighting is used for measuring the sound level of impulsive events 

such as engine backfires or explosions. This setting is usually not used for measurements related to hearing conservation. Peak time weighting indicates the level 

of the absolute peak sound level when the level reached its greatest magnitude. 

This setting is sometimes used to determine the peak level of impulse sounds.

An SLM must be calibrated before and after each measurement period with a 

calibrator that fits the size of the microphone on the meter. The pre-measurement 

calibration is necessary to ensure the SLM is functioning properly prior to making measurements. The post-measurement calibration is important in a mining 

environment because the instrument is likely to be subjected to jolting and jarring 

during a work shift and because temperature or humidity extremes could affect the 

accuracy of the meter.

The microphone, the most fragile part of the instrument, is especially susceptible 

to damage. The documentation provided by the instrument manufacturer should 

list the operating ranges for humidity, pressure, and temperature. The SLM should 

be calibrated by a qualified laboratory for a calibration traceable to the National 

Institute of Standards and Technology at the interval recommended by the manufacturer, typically every 1-2 years.

When performing measurements with an SLM, care should be taken to avoid 

contaminating the readings with reflected sound. To avoid doing this, the microphone should generally be a minimum of 3 feet, or 1 meter, away from all reflecting surfaces. This includes walls, ribs, ceilings, the ground, and the body of the 

person doing the measurements. If conditions allow, the SLM should be mounted 

on a tripod. This will yield a consistent microphone height and further reduce the 

chances of the measurement being influenced by reflections from the body of the 

person performing the reading.

After calibrating the SLM, but before measuring the sound of interest, the background sound level, or ambient sound level, should be measured. The background 

level is the sound level that is measured when the piece of equipment or the device 

to be measured is shut down. It is desirable to have at least a 10-dB difference be-
60 | Noise Control in Underground Metal

Appendix C | Noise Control in Underground Metal Mining

tween the background level and the sound level measured when the object under 

test is operating. This is usually not a problem in the mining industry. However, 

measurements near ventilation fans may have high background sound levels. If a 

10-dB or greater difference is not achieved, correction factors must be used to account for the background noise. For example, a measurement taken with a motor 

operating yielded a result of 87 dB(A) and the level with the motor shut down produced a level of 83 dB(A); the difference is 4 dB(A), which is less than 10 dB(A), 

and Table C.1 must be consulted. The correction associated with the difference is 2 

dB(A). Subtracting this value from the combined sound level of 87 dB(A) yields a 

result of 85 dB(A).

Air passing over the grid of the microphone, which can happen when using a SLM 

in a windy environment, is picked up by the microphone element as noise that typically has predominantly low frequency components. This wind noise is a spurious 

contribution to the actual measurement. Using a windscreen can minimize these 

contributions. A windscreen is a ball of open-cell foam that is placed over a microphone to minimize wind noise while having minimal effect on the directional and 

frequency response characteristics of the microphone. The manufacturer of the 

SLM should be consulted for information regarding proper windscreen selection.

When measuring sound levels, it is important to remember that sound pressure 

varies with distance from the source. Therefore, for consistent results, it is crucial 

to record the distance from the noise source to the measurement location. Furthermore, when measuring the sound level of a piece of machinery, the side of the 

machine from which the measurements are taken should be noted. This is necessary because the sound level around the machine may vary.

 Table C.1. Correction for background noise*

Noise level minus background 

noise level (dB[A])

Correction to subtract 

from noise level (dB[A])

8 to 10 0.5

6 to 8 1.0

4.5 to 6 1.5

4 to 4.5 2.0

3.5 2.5

3 3.0

*

Correction to obtain the sound level of the source if the background

 noise were not present
Appendix C | Noise Control in Underground Metal Mining

Noise Control in Underground Metal Mini

C.2 Frequency Band

The overall level of a sound is an important metric for determining the extent of 

a noise problem in an occupational setting. However, when a noise problem has 

been determined to exist, more information is needed to determine how to best 

approach the problem. An important measure of a noise is its frequency distribution, or spectrum. In some cases, knowing the frequency distribution of a noise 

will help to identify its source. The spectrum may also help in the selection of the 

most appropriate noise control materials for a problem.

Octave band analysis is one type of spectrum analysis. An octave is a frequency 

interval between two sounds with a frequency ratio of 2, such as from 125–250 Hz 

or 2,000–4,000 Hz. In octave band analysis, the frequency spectrum is divided into 

bands on a logarithmic basis. Each band is identified by a center frequency, which 

is the geometric center of the band. The octave band center frequencies commonly 

used are 63; 125; 250; 500; 1,000; 2,000; 4,000; and 8,000 Hz.

When more detailed information is required, one-third-octave bands are used for 

spectral analysis. This type of analysis divides each octave band into three logarithmic bands. One-third-octave band analysis is useful for locating the frequency 

region of tones that may become obscured when using octave band analysis.

C.3 Decibel Additio

To fully evaluate a noise problem, it is usually necessary to sum the sound levels 

due to multiple noise sources. To calculate the sum of the sources, the sources are 

assumed to be random and unrelated to each other—or incoherent—as is usual 

when dealing with machinery or equipment noise. 

Since sound levels are formed by taking a logarithmic ratio, simple addition will 

not yield the correct total sound level. This means that 80 dB + 80 dB ≠ 160 dB. 

To obtain a correct result of adding two sound levels, first, the difference between 

them is determined. Table C.2 shows the number of decibels to add to the higher 

of two sound levels to be added. The corresponding value from Table C.2 is added 

to the larger of the two original sound levels. For example, in Equation C.1 the 

two values are the same, so the difference is zero. In this case, 3 dB is added to the 

higher of the original measurement (in this case they are the same): 

80 dB + 80 dB = 83 dB (C.1)

If the levels to be summed are 80 dB and 82 dB, Table C.2 indicates that 2.1 dB 

should be added to the higher level, which is 82 dB:

80 dB + 82 dB = 84.1 dB (C.2)
62 | Noise Control in Underground Metal

Appendix C | Noise Control in Underground Metal Mining

When dealing with decibels, it is generally unnecessary to obtain decimal accuracy. The result of Equation C.2 should be recorded as 84 dB. When adding several 

decibel levels, it is best to begin by combining the two lower levels and then adding 

their sum to the next highest level. This trend should be continued until all of the 

levels are summed and a final result is found.

Table C.2. Chart for calculating the sum of two decibel levels

Difference between two 

levels being added (dB) Decibels added to higher level

 0 3.0

 1 2.6

 2 2.1

 3 1.8

 4 1.4

 5 1.2

 6 1.0

 7 0.8

 8 0.6

 9 0.5

 10 0.4

Source: NIOSH 1979

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