noise control opencast mines, underground mines and mineral processing plants report.pdf

Department of Mining Engineering

Mines and Mineral Processing Plants.

1.1 Sources of Noise ........................................................................................................................... 9
1.2 Effect of Noise on Hearing Mechanism ...................................................................................... 10
1.3 Noise Abatement Methods .......................................................................................................... 10
1.4 Noise-Induced Hearing Loss ....................................................................................................... 10
1.5 Three Variables of Noise Exposure ............................................................................................ 11
1.6 Noise Dosimeters ........................................................................................................................ 11
1.7 The Role of Engineering Noise Controls in Reducing NIHL ..................................................... 12
2 Noise Control in Opencast Mines ....................................................................................................... 13
2.1 Introduction ................................................................................................................................. 13
2.4 Industrial Area ............................................................................................................................ 13
2.6 Dragline Operations .................................................................................................................... 14
2.7 Coal Transport ............................................................................................................................ 14
2.10 Crushing and Screening Station .................................................................................................. 15
2.11 Bins ............................................................................................................................................. 15
2.15 Evaluating Noise Controls for Haul Trucks (Case Study) .......................................................... 19
2.15.1 Absorptive Material ............................................................................................................ 19
2.15.3 Sealing Gaps ....................................................................................................................... 23
2.15.4 Evaluating Noise Controls for Jumbo Drills and Bolters .................................................... 23
2.15.5 Absorptive Material in Canopy ........................................................................................... 24
2.15.6 Absorptive Material on Sides of the Cab Around Operator Area ....................................... 25
2.15.7 Absorptive Material in Lower Front of Cab ....................................................................... 26

3.1 Hierarchy of Noise Control ......................................................................................................... 31
3.2 Barriers and Sound-Absorbing Materials.................................................................................... 31
3.3.1 Noise Exposure Reduction .................................................................................................. 33
3.3.2 Dose from Multiple Noise Sources ..................................................................................... 34
3.3.3 Acoustical Materials ........................................................................................................... 35
3.3.4 Installation Methods ............................................................................................................ 36
3.3.5 Compliance Assistance ....................................................................................................... 36
3.4.2 Retrofit Noise Controls ....................................................................................................... 38
3.4.3 Alternative Technology ....................................................................................................... 40
3.5.4 Administrative Controls ...................................................................................................... 43
3.6 Conveyors – Chain ...................................................................................................................... 43
3.6.4 Administrative Controls ...................................................................................................... 45
3.8 Hand-Held Pneumatic and Electro-Pneumatic Drills ................................................................. 45
3.8.1 Original Equipment Manufacturers (OEM) ........................................................................ 46
3.8.3 Alternate Technology .......................................................................................................... 46
3.10 Loaders – Face ............................................................................................................................ 48
3.11 Locomotives - Diesel .................................................................................................................. 49
3.12 Longwalls - Shear and Plow ....................................................................................................... 50
3.12.1 Original Equipment Manufacturers (OEMs) ...................................................................... 50
3.14 Roof Bolters ................................................................................................................................ 53
3.15 Roof Scalers ................................................................................................................................ 55
3.16 Shuttle Cars - Diesel ................................................................................................................... 56
3.16.1 Original Equipment Manufacturers (OEM) ........................................................................ 56
3.17 Case Study: Field Studies by MSHA .......................................................................................... 57
3.18 Evaluating Noise Controls for 4 Load-Haul-Dumps: Field Studies by MSHA .......................... 57
3.18.1 Engine Enclosure ................................................................................................................ 57
3.18.2 Engine Enclosure 2 ............................................................................................................. 59
3.18.3 Enclosed Operator Cab ....................................................................................................... 62
3.19 Adapting Active Noise Control Headsets for the Mining Industry ............................................. 64
4 Noise Control in Mineral Processing Plants ....................................................................................... 66
4.1 Introduction ................................................................................................................................. 66
4.3 Mobile Operations ...................................................................................................................... 66

4.7 Engineering Controls .................................................................................................................. 67

4.17 Screens – Classifying .................................................................................................................. 86
4.17.2 Retrofit Noise Controls (Screens with built-in noise controls) ........................................... 86
4.17.3 Retrofit Noise Controls (Screens without noise controls) ................................................... 87
Figure 1: Haul Truck ................................................................................................................................. 19
Figure 2: Haul truck with vinyl-covered material installed in the area in front of the operator. ............... 20
Figure 3: Haul truck with vinyl-covered material installed in canopy above the operator. ....................... 21
Figure 4: Partial engine enclosure. ............................................................................................................. 22
Figure 5: Haul truck with sound-absorbing material installed in canopy and depiction of how sound may
enter the operator area, reaching operator before padding. ......................................................................... 22
Figure 6: Heavy conveyor belt barrier. ...................................................................................................... 24
Figure 7: Fiberglass blanket barrier. .......................................................................................................... 24
Figure 8: Plexiglas motor cover. ................................................................................................................ 24
Figure 9: One-inch-thick quilted fiberglass blanket being removed for testing. ....................................... 25
Figure 10: Quilted fiberglass material in the operator’s area. .................................................................... 26
Figure 11: Quilted fiberglass material in the lower front of the operator’s area of bolter 2. ..................... 26
Figure 12: Wind Shields for Protection ..................................................................................................... 27
Figure 13: Wind Shields for protection ...................................................................................................... 28
Figure 14: Continuous Miners - Auger Type ............................................................................................. 38
Figure 15: Areas Where Retrofit Noise Controls can be Applied to an Auger-Type Continuous Miner .. 38
Figure 16: Installation of Wear Strips on Pan Line.................................................................................... 39
Figure 17: Full Coverage Treatment to Both Upper and Lower Pan Lines ............................................... 39
Figure 18: Sand-Filled Auger Cutting Head .............................................................................................. 40
Figure 19: Areas Where Retrofit Noise Controls May be Applied on a Continuous Miner – Drum Type 41
Figure 20: Constrained-Layer Damping of Conveyor Pan Using Individual Strip .................................... 41
Figure 21: Example of Layering Applied to Motor Covers ....................................................................... 42
Figure 22: Constrained-Layer Damping of Conveyor Pan Using Individual Strips .................................. 44
Figure 23: Constrained-Layer Damping of Conveyor Pan Using Full Coverage ...................................... 44
Figure 24: Areas Where Retrofit Noise Controls Should be Installed ....................................................... 48
Figure 25: Absorption Material Used to Insulate Inner Surfaces of Cabs or Passenger Compartment ..... 52
Figure 26: Left side of the engine enclosure .............................................................................................. 58
Figure 27: Front and back of the steel panels insulated with 1.5-inch-thick fiberglass. ............................ 58
Figure 28: LHD partial engine enclosure, right side. ................................................................................. 59
Figure 29: Left side partial engine enclosure. ............................................................................................ 60
Figure 30: Quilted absorber inside the engine compartment. .................................................................... 60
Figure 31: Right side and top of engine compartment. .............................................................................. 61
Figure 32: In-cab one-third-octave band spectrum for LHD2 at high idle with engine compartment open.
.................................................................................................................................................................... 61
Figure 33: Open cell foam used for in-cab sound absorption. ................................................................... 62
Figure 34: Enclosed cab with glass in place .............................................................................................. 62
Figure 35: Barriers ..................................................................................................................................... 68
Figure 36: Noise Damping Material Applied at a Conveyor Transfer Point ............................................. 72
Figure 37: Noise Damping Material Applied to the Base of a Chute ........................................................ 72
Figure 38: Example of a 90-degree Elbow ................................................................................................ 75
Figure 39: Installation of a Resilient Crusher Feed Plate .......................................................................... 77

List of Tables
Table 1: Sound level at the haul truck operator’s position, surface measurement ..................................... 20
Table 2: Sound level at the haul truck operator’s position, underground measurement ............................ 21
Table 3: Sound level for haul truck at the operator’s position, surface measurement ............................... 21
Table 4: Sound level for jumbo drills and bolters at the operator’s position, underground ....................... 23
Table 5: Sound level of jumbo drills and bolters at the operator’s position .............................................. 25
Table 6: Sound level of bolters 2, 3, 4, and 5 at the operator’s position .................................................... 29
Table 7: Data for Example Calculations Involving Multiple Sound Sources ............................................ 34
Table 8: Sound level for LHD1 at the operator’s position, high idle ......................................................... 59
Table 9: Sound level for LHD2 at the operator’s position, high idle ......................................................... 61

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1 Introduction One of the highest prevalent chronic diseases in the mining industry, as per National Institute for
Occupational Safety and Health (NIOSH), is Noise Induced Hearing Loss (NIHL). Some of the other
industries where high risk of NIHL exists are wood product manufacturing, building construction, and real
estate. It has been concluded after a lot of examination on the audiograms on workers that there is a need
for better noise controls and hearing conservation strategies in mining, manufacturing and construction
sectors.
One of the most common and preventable causes of NIHL is noise exposure. Though it is entirely
preventable, but once acquired, it is totally irreversible. Pressure generated by high noise caused over 96%
of workers’s compensation claims for hearing loss in the early 2000’s. Ageing, which is a non-work factor
also causes hearing loss. But all man-made high noises act as additives to further increase the hearing loss
of a worker at workplace. Audiometric results are the correct methodology to access the effects of NIHL
vis-à-vis work related and non-work related hearing losses.
People working in mines (underground or open cast mines) have the highest incident of noise-induced
hearing loss among all occupations. Nearly 80% of miners are exposed to noise levels that exceed 85 dBA.
About 25% of these miners are exposed to noise levels higher than the 90 dBA Permissible Exposure Limit
(PEL). Ninety per cent of all coal miners above the age of 50 have a hearing impairment. By the time coal
miners retire, they are nearly guaranteed a moderate hearing loss.
The use of heavy equipment, the drilling of rock, and the confined work environment all contribute to high
levels of noise exposure in mining. As a result, as many as 70% of all miners have NIHL significant enough
to be considered a disability. In one study, it was found that almost half of the workforce of these miners
never used hearing protectors. One NIOSH study found that by age 50, about 90% of coal miners and 49%
of metal/nonmetal miners had a hearing impairment (as compared with 10% of the non-occupational noise-
exposed population). Simply stated, most miners have a hearing loss by retirement.
Many programs are underway to help reduce work-related noise induced hearing loss in the mining sector.
Jurisdictions are undertaking or planning a significant amount of work in inspections and auditing, targeting
highest risk sectors. Noise induced hearing loss is irreversible; therefore it is important to prevent exposure
at the earliest possible opportunities. Many awareness programs have encouraged more companies to
introduce a noise policy, and a noise control and hearing loss prevention program.
One of the major focus is to make the companies aware of the possibilities of using the higher levels
(elimination, substitution and engineering control), and encouraging them to think about opportunities. The
feasibility of engineering or administrative noise controls is related to total cost of application and overall
effectiveness of the control in reducing noise. Retrofit engineering controls have been shown to be largely
ineffective, and the development of quieter mining equipment has been slow. To date, these approaches
have not led to an acceptable reduction of NELs.
As part of these awareness campaigns, opportunities lie in training in the use of hearing protectors (eg, how to
fit properly), choosing appropriately and in customising the fitting (eg, by the use of custom-moulded devices).
Education and training have a significant role to play in preventing work related hearing loss. With rate of hearing
loss greatest in the first 10 years of exposure, it is important to prevent exposure at the earliest possible
opportunities.
Noise exposure and noise-induced hearing loss are still prevalent in the mining industry. Most of the risk

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materials can be used to minimise this. Some degree of residual hearing protection may well be required,
but this should be part of a well-designed hearing protection programme.
Noise arising out of prospecting mining beneficiations or metallurgical operations shall be abated or
controlled by the holder of prospecting licence or a mining lease at the source so as to keep it within the
permissible limit.
Noise level standards:
-Operational/working zone-not to exceed 85 dB (A) Leq for 8 hours exposure
-The ambient air quality standards in respect of noise as notified under the Environmental (protection)
Rules, 1986, shall be followed at the boundary line of the coal washery.
Works carried out in order to expand productivity in the mining industry have pointed out the necessity to
utilize larger machinery in parallel with improvements in technology. An increase in mechanisation also
has resulted in an increase in noise levels, leading underground and open pit mines and mineral processing
plants to generate enormous levels of noise. Occupational noise in underground mines has reached
unbearable levels due to the reverberant nature of the narrower spaces. Therefore, it is hard to find a
relatively low-noise environment for workers. Although the equipment employed in open pits are
comparatively larger in size than the ones encountered underground, they may be said to be less significant
as the noise emitted from them easily spreads hemi-spherically in the free sound field. In reality, the noise
occurring during extraction works (i.e. drilling-blasting, excavation, loading and transporting) that take
place in both open and underground pits is noteworthy when considering labour health and job performance
as the highest disease and illness rates in mining continue to be mine worker’s permanent or temporary
hearing loss
Additionally, it appears that noise can account for quickened pulse rates, increased blood pressure and a
narrowing of the blood vessels. Workers exposed to noise sometimes complain of nervousness,
sleeplessness and fatigue. Therefore, it is of foremost importance to conduct research on this matter to give
suggestions to mine management with respect to the health of workers and maximizing the competence in
productiveness. In comparison with the levels of noise exposure in various industries (airport, forest
machinery, cement industry, foundry, textile industry, printing, metal plate workshop, ship engine room,
riveting workshop), noise levels encountered in the open cast mining industry are second only to that
encountered near jet engines at airports. Noise-induced hearing loss usually occurs initially at high
frequencies (3k, 4k, or 6k Hz), and then spreads to the low frequencies (0.5k, 1k, or 2k Hz).
1.1 Sources of Noise
Noise, defined as undesirable sound, is a by-product in many industries. This is particularly true for mining.
Many miners are exposed not only to loud but sustained noise levels. Most of the large excavation
equipment utilized at open pits are not said to be responsible for the excessive noise levels as they are
mostly equipped with noise-protected operator cabs. However, excavators with lower capacity and mobile
diesel-powered machines have been accepted as the primary noise sources in surface mining activities. On
the other hand, equipment such as continuous miners, stage loaders, shearers, compressors, fans and
pneumatic drilling machines may be counted as the main contributors to excessive noise levels in
underground mining. Additionally, equipment like vibrating screens, rotating breakers and mills which are
commonly in use in most of the mineral processing plants may be defined as the important sources of noise.
The length of period during which workers are exposed to excessive noise is rather important as it takes a

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The parameters which are effective for hearing loss due to noise are exposure period, noise level, age of
workers and physical condition of workers (existence of other illness etc.). For most effects of noise, there
is no cure. However, prevention of excessive noise exposure is the only way to avoid health damage.
1.2 Effect of Noise on Hearing Mechanism
Upon receiving an acoustic signal, pressure changes occurring in the auditory canal move the drum
membrane. The bones called hammer, anvil and stirrup, which are located behind the eardrum are connected
in a chain between the tympanic membrane and the round window of the cochlea. In the case of these bones
being exposed to noise, they start to vibrate. Therefore, the sound energy caused by this vibration is
converted into mechanical energy and then into hydraulic energy in the cochlea. The motion in the cochlea
will affect the small hair-like cells in the cochlea depending on the electrical signal frequency. When a cell
is stimulated it sends an electrical signal to the brain. The loss of hearing in the inner ear, apart from natural
diseases, may be faced in the case of small hair-like cells becoming damaged or weakened due to excessive
noise levels for a long period. Noise-induced hearing loss is 100% preventable but once acquired, hearing
loss is permanent and unfortunately irreversible. Miners have to put up with a variety of noise sources
during their daily working environment. Contrary to popular thought, hearing loss arising from instant high
levels of noise rarely happen; however, the main cause is prolonged levels of sound.
1.3 Noise Abatement Methods
Efforts made to reduce excessive noises from any source to tolerable levels by changing acoustic features
and decreasing the period of exposure may be covered as the principles of noise control. It should be
noted that noise controls and administrative actions should be the first line of defence. These methods
may be classified into three groups:
a) Equipment practice: This practice relates directly to the selection and utilization of mining machinery
to obtain reduced noise levels.
b) Operational and administrative practice: This practice is also related to the design and execution of the
mining operation to obtain reduced noise exposure.
c) Engineering noise controls: Removing hazardous noise from the workplace by means of engineering
controls is the most effective way to prevent noise-induced hearing loss. For this purpose, equipment
hardware changes are implemented, especially to reduce machine noise emission levels.
1.4 Noise-Induced Hearing Loss
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;

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 ac tion taken to
reduce noise exposure.
1.5 Three Variables of Noise Exposure
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.
1.6 Noise Dosimeters
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. The microphone, the most fragile part of the instrument, is especially susceptible to damage. The

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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.
1.7 The Role of Engineering Noise Controls in 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.

2 Noise Control in Opencast Mines 2.1 Introduction
Bridges et al (1998) identified certain areas specifically as being likely sources of noise emission. These
included:
• Train loading
2.2 Equipment Specifications
To minimize noise generation, all potential vendors were required to guarantee noise levels for their
equipment, and the values provided by the vendors were a major factor in the choice of equipment
purchased. In several cases the company purchased equipment which was not the lowest price, but had the
lowest noise emissions.
This applied to mobile as well as fixed plant. Where equipment suppliers could not guarantee acceptable
noise levels from standard designs, the company engaged in negotiations with the equipment manufacturers,
in order to modify their equipment to meet the noise requirements. For mobile equipment, this involved the
use of acoustic panels, shrouds and louvres. One major approach adopted was to shut the dragline down
between the hours of 1 pm on Saturdays and 7 am on Mondays during the initial few months when the
dragline was working in unshielded areas. This represented 42 hours out of a week of 168 hours, or 25% of
the total available time, which was a significant financial and productivity sacrifice on the part of the
company.
• Construction of large earth embankments to sh ield nearby properties
• Locating the truck dump station in a pit
• Incorporation of an overland conveyor for raw coal transport instead of traditional truck haulage
• Enclosure of many conveyors
• Complete enclosure of the coal preparation plant and trans fer stations
• Sealing of floors within the coal preparation plant wherever possible.
2.4 Industrial Area
The main industrial area, including raw and clean coal stockpiles, is enclosed on the southern and eastern

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directions, but it has also provided a visual barrier, so that the visual amenity of the area from the south and
east is that of a rolling hill, with only the very tops of one or two structures visible.
2.5 Truck Dump Station
The truck dump station is located at the eastern end of the property closest to the pit, minimising noisy truck
movements across the site. The dump hopper has been constructed in a pit, so that much of the structure,
hopper, and unenclosed equipment is below ground level. The station, and trucks moving in the area around
the station have also been shielded by an embankment. The truck dump area is enclosed on three sides by
a double-sheeted structure, and is only open to the west (the mine side). Thus the noise from dumping is
constrained within the dump station and to the west. This enclosure has the additional advantage of
minimizing any dust generated during the dumping operation. The design of the structure was also selected
to blend into the surrounding landscape.
2.6 Dragline Operations
The dragline operation commenced at the eastern end of the lease, so that the dragline would quickly be
“hidden” behind the spoil piles acting as noise barriers. Contouring and seeding of the spoil piles will
quickly give them a more natural appearance.
2.7 Coal Transport
It is common practice to use truck haulage to deliver coal to the processing plant. However, to constrain
truck noise within the pit as much as possible, at Bengalla the coal is transported to the nearby truck dump
station for primary crushing, and then is carried by a 4½ km long overland conveyor to the stockpile and
processing area.
2.8 Conveyors
All elevated conveyors are fully enclosed outside the coal preparation plant. The overland raw coal
conveyor is sheeted on the southern/eastern side, and roofed. The stockpile area conveyors, which are inside
the main industrial area embankment, cannot be enclosed due to the operations of the stackers and
reclaimers. The fully enclosed elevated gantries include a concrete floor and corrugated steel sheeting walls
and roof. All conveyors have been constructed with specially machined idlers, while the overland conveyor
has been constructed with idlers which are also specially balanced. The drive stations for all conveyors are
located either in fully enclosed transfer stations or at ground level, inside the main embankment at the
stockpile area.
2.9 Coal Preparation Plant and Transfer Station Cladding
It has been standard practice to enclose coal preparation plants in the region down to first floor level, leaving
the ground floor level open for maintenance access. In this case, the plant has been fully enclosed to ground
level on the sensitive eastern and southern sides, and partially on the western and northern sides. This has
been achieved without compromising maintenance access. In addition, noise modelling had shown that the
translucent sheeting often provided to improve the lighting inside plants was a source of increased noise
transmission to the exterior, so the plant is entirely clad in steel sheeting. No natural ventilation could be
permitted, as the vents would have allowed noise emission, so ventilation fans are included in the upper
level of the preparation plant. Transfer stations have been similarly treated, with minimal openings for
access and maintenance. Inside the preparation plant, the floors are constructed of concrete wherever
possible, with minimal penetrations, to prevent noise transmission between floors and to the ground floor

2.10 Crushing and Screening Station
At the raw coal sizing and crushing station, it would have been normal practice to use an inclined vibrating
screen to size the coal before crushing. As these are known to be very noisy, a screen type not previously
used in Australia was adopted instead. This is the roller screen, using rotating rolls to separate the oversize
from the undersize. Adoption of this type of screen has actually allowed a reduction in height of this
building, in addition to the expected benefits from reduced noise and vibration.
2.11 Bins
Other than the truck dump hopper, which has already been discussed, the bins designed for the site included
• Plant feed surge bin
• Plant rejects bin
• Train loadout bin
Of these, the train loadout bin is located outside the main embankment, to the south-east of the industrial
area, while the others are inside the main embankment. The train loadout conveyor head end is not fully
enclosed, however, the drive station is located back inside the embankment in the industrial area, reducing
noise emissions and also reducing the size of the bin support structure. The train loading system, which is
hydraulically controlled, includes a 700t capacity bin above a 100t capacity weigh flask to accurately load
each wagon. There is the potential for noise generation from two sources, coal filling the weigh flask, and
coal from the weigh flask loading the wagon. The hydraulic pump which powers the system is fully enclosed
below the bin. The bin level control system operates such that the bin itself is never empty, so coal entering
the bin falls on coal, which minimises noise generation. The wagon loading takes place in an acoustically
lined tunnel, which constrains noise emissions to the north-east and south-west. The ventilation provided
to this tunnel was carefully designed to permit noise emissions only to the north-west, the direction of the
mine industrial area. (Colin, 2000)
The plant feed surge bin and the plant rejects bin both have fully enclosed feed conveyors, as mentioned
earlier. In both cases, the conveyor drives are located at ground level, within the embankment. The bins are
not acoustically lined, and the operating philosophy has been to endeavour to maintain a bed of material in
these bins to minimise noise generation.
2.12 Stockpile Equipment
Stockpile machines, i.e. stackers and reclaimers, were identified as potential noise sources. The
manufacturer was required to take particular steps to minimise noise generation from the machines,
including:
• Fully enclosed tripper discharge chute
• Low height tripper transfer discharge
• Fully enclosed boom conveyor load skirts

(ii) Reclaimers
• Vibration absorbing rubber plates attached to chain guide liners
• Large diameter chain sprockets and guide rollers and tumblers
• Low noise motors on harr ow sled drive
• Fully enclosed impact loading table at discharge to yard conveyor.
2.13 Construction Activities
The noise generated during construction was to be limited to the same constraints as the development
consent. With activities including heavy earth moving machines, cranes, and other typically “noisy”
equipment, and the fact that initially the embankments themselves had to be constructed, this period was
seen to be potentially more difficult than for the actual operations. Construction of the embankments was
anticipated to be a significant noise generating activity, with construction contracts placing the
responsibility for noise management and control onto the contractor. The contracts specified daily noise
monitoring in the closest occupied rural areas, with noise control options available to the contractor
including relocation of noisy plant, replacement of noisy plant with quieter machines, or simply stopping
work at the contractor’s expense until weather conditions can be more favourable.
2.14 Operational Results
2.14.1 Mobile Equipment
2.14.1.1 Dragline
Prior to placing into service, the dragline was found to exceed its required static sound power of 107 dB(A),
due to relatively minor defects around the ventilation fans on the roof of the machine. With these defects
rectified, the machine has continued to meet the required static sound power level and fan noise is rarely
heard in nearby residential areas. The dragline’s bucket and rigging components were recognised early in
the mine’s design phase as a potential source of intermittent noise, particularly while the machine is
dumping as the bucket is high above the spoil piles. A detailed investigation was conducted into methods
of reducing impact noise as the bucket works, including design modifications to prevent or minimise
impacts and coating materials which reduced noise from remaining unavoidable impacts.
Recommendations arising from the investigation were to coat some components with a resilient material,
particularly the spreader bar and lift chains as these are not subject to regular contact with spoil. An archless
bucket was chosen to minimise the chances of the spreader bar and drag chains coming into contact with
the bucket. Resilient pads were mounted around the rim of the bucket and on the sides near the lifting
points, preventing direct metal contact with the lift chains. These modifications resulted in impact noise
associated with the spreader bar and lift chains being reduced from a typical sound power of 132 dB(A) to
around 116 dB(A), while impacts on the bucket arch were totally eliminated. Remaining metal impacts with
a sound power over 130 dB(A) are still possible, mainly associated with the drag chains and rope sockets
which cannot be coated due to their service conditions. Minimising remaining impact noise has been left to
the machine’s operator, with ongoing training sessions being conducted to ensure each operator has the

These 2 loading units were supplied with significant noise control modifications, including radiator louvres
or plenum chambers, engine enclosures, acoustically treated cooling air inlet chambers and more effective
exhaust silencers. Both machines have been operating satisfactorily with these modifications, with regular
cleaning required to the acoustic material to prevent dust buildup and maintain its performance. These units
produce a sound power typically around 111 dB(A) and no greater than 113 dB(A) while working,
compared to over 120 dB(A) for a standard dual-engine excavator and 117 dB(A) for a standard loader of
the same model. With their working location usually in a pit, noise from these machines is rarely audible at
any property.
2.14.1.3 Drills
Diesel powered drill rigs, with the engine exposed on standard machines, were supplied with enclosed
engines, louvred radiators, acoustically treated cooling air outlets and more effective exhaust silencers.
Minor treatment was also required to the dust extractor, and rubber skirts around the drill head minimised
compressed air noise as the bit contacted the ground. Some initial cooling problems were experienced with
these machines, with airflow restricted to the radiator. After relatively minor modifications to the fans and
louvres, these machines have been working satisfactorily with regular cleaning of the acoustic material.
Modifications to the machines have reduced their sound power from 117 dB(A) to below 112 dB(A) while
working.
2.14.1.4 Track Dozers
Standard track dozers typically produce a sound power around 116 dB(A) from the engine, exhaust and
radiator, with track noise in reverse producing up to 128 dB(A). Machines supplied to this mine produced
a sound power below 112 dB(A) due to engine enclosures, radiator louvres and better exhaust silencers,
with track noise slightly reduced to 126 dB(A) due to third gear not being available in reverse. Subsequent
modifications to the tracks, involving a block of resilient material being imbedded in each plate to reduce
vibration, resulted in a 2 to 3 decibel reduction in track noise. A further investigation aimed at achieving a
maximum sound power of 113 dB(A) from track noise is being conducted jointly by the mine and the
machine’s manufacturer, with no clear results to date.
2.14.1.5 Wheel Dozer
A significant noise control initiative adopted by the mine was to use a wheel dozer wherever possible on
exposed dump areas, particularly at night, to eliminate excessive track noise from a track dozer. A wheel
dozer was therefore supplied with a maximum sound power of 111 dB(A), with modifications similar to
the loader described above. Due to a wheel dozer’s limited traction, a track dozer is still regularly required
on dump areas. This work is carried out during the day wherever possible, with remaining work at night
carried out at low speed in first gear to eliminate track noise.
2.14.1.6 Dump Trucks
Trucks are used on the mine to haul prestrip overburden to dump areas, to haul reject material from the
processing plant to dump areas, and to haul coal from pits to the truck dump station. The first two of these
tasks requires the trucks to travel to exposed elevated dumps, making these machines among the most
critical for site noise control. Standard trucks typically produced a sound power over 120 dB(A), although
early noise controlled machines were specified in 1996 by another coal mine in the Hunter Valley which
resulted in their sound power being reduced to 116 dB(A). Machines at Bengalla typically produce a sound
power of 110 dB(A) while operating, with up to 113 dB(A) with the retard (braking) system engaged. This

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This low sound power was achieved generally in the same way as the other machines described above, with
engine enclosures, radiator louvres and treated cooling air outlets under the truck’s chassis.
Some ongoing problems have been experienced with the machines, mainly with the unit regularly used to
haul reject material. This material has a high water content and occasionally spills a small amount onto the
radiator louvre, covering the sound absorbent material and encouraging dust to settle on the louvres. Regular
cleaning has proved to be the most practical solution, however careful cleaning is required in a number of
barely accessible areas and results in unwanted downtime and a slight loss of production.
2.14.1.7 Water Carts
Water carts of 90 tonne capacity are used to minimise dust from haul roads in the mine while producing a
sound power no greater than 112 dB(A). Engine, exhaust and radiator noise has been treated in these
machines in a similar manner to the dump trucks, but with a hydraulically cooled braking system water
carts produce less noise than dump trucks while travelling downhill. As for the other machines, regular
cleaning of all noise control components is required to maintain satisfactory performance.
2.14.2 Fixed Plant
2.14.2.1 Processing Area Conveyors
It became apparent at an early stage that the noise generated by several of the conveyors was significantly
greater than expected. In particular the stockyard conveyors, not being enclosed to allow access for the
stackers and reclaimers, were perceived as very noisy. Measurements conducted at representative locations
near a number of these noisy conveyors showed up to 17 decibels over the guaranteed noise levels for many
affected conveyors. Earlier tests on a nearby mine site had indicated that careful machining of idlers
contributed to a significant reduction in noise from conveyors, so these conveyors were inspected to find
the cause of excessive noise. It was discovered that the idlers had an uneven surface, which was found to
be the rust preventative coating which had been applied by the manufacturer after machining. The idlers
had been stacked for shipment before this coating had properly dried and consequently their surface was
uneven. An intensive programme of scraping, grit-blasting and polishing was undertaken, and noise levels
were significantly reduced. It was further found that many idlers were out of balance and did not conform
to the specification. This caused noise generation to an unacceptable level. A program of idler replacement
was begun, and noise reductions of up to 14dB(A) have been realized through the installation of conforming
idlers. Some conveyors were found to have ripples in the belt covers. These caused severe difficulties with
belt scraping, and also created a drumming noise as the ripples passed over the idlers. Following
negotiations with the belting vendor, affected belts were replaced. Examples showing the results of the
various improvements discussed above are given below in Table 3. The overland conveyor did not have the
same problems as the other site conveyors. The idlers were larger diameter, they were balanced as well as
machined, the rust inhibiting coating was very thin and even and did not affect the idler’s mass or balance,
and the belt did not contain significant defects. This conveyor met the guaranteed noise levels from the
outset, and continues to meet these levels.
2.14.2.2 Bins
Noise from the raw coal dump hopper has been found to be somewhat higher than expected, and this appears
to be related to the proportion of harder waste material present in the coal seam being mined. Noise from
the station is occasionally audible at the closest properties, although it is rarely the subject of noise
complaints. The main noise reduction strategy adopted for this and other bins was to ensure some material
always remains in the bin to dampen noise from falling material. Noise from the plant feed surge bin

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for processing. At other times, noise from this bin is insignificant. Similarly, noise levels generated at the
train loadout bin have been acceptable, although it is audible at times at a few close properties. The train
loading process has also been demonstrated to produce acceptable environmental noise levels, and the
acoustic tunnel shielding noise from the filling of the rail wagons is effective in reducing noise levels,
especially to the south-east and east where the town and several properties are located. The plant rejects bin
itself is satisfactory from a noise perspective, but the process of placing rejects in the trucks for disposal
has proved to be noisy. Remedies for this are still under investigation at the time of writing this paper, and
may include additional shrouding at the bin/truck interface, or modifications to the truck body.
2.14.2.3 Stockpile Machines
The stackers produce an average of 2 decibels over the expected levels, although stacker noise is strongly
dependent on the condition of idlers which therefore must be carefully maintained. The reclaimers have
also produced 2 to 3 decibels more than expected, with dominant sources of noise including the main bucket
chains and sprockets and the interface chute between the reclaimer and the conveyor. Of these sources, only
the chains and sprockets have the potential to be audible at a residential property due to their intermittent
noise character. These machines have been the subject of a few noise complaints, however, these have
occurred during strongly noise-enhancing atmospheric conditions. The machines are currently being
reviewed by the mine.
2.15 Evaluating Noise Controls for Haul Trucks (Case Study)
Haul truck noise is a good example of the challenging problem of reducing noise in both reverberant and
non-reverberant environments.
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 The
material was attached with Velcro for easy removal. 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.
The vinyl-covered material had little effect on sound levels at the operator’s positio n. The results for haul

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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.
Figure 2: Haul truck with vinyl-covered material installed in the area in front of the operator.
Table 1: Sound level at the haul truck operator’s position, surface measurement
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

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Figure 3: Haul truck with vinyl-covered material installed in canopy above the operator.
Table 2: Sound level at the haul truck operator’s position, underground measurement
Table 3: Sound level for haul tr uck at the operator’s position, surface measurement
2.15.2 Partial Engine Enclosures
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

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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. 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).
Figure 4: Partial engine enclosure.
Figure 5: Haul truck with sound-absorbing material installed in canopy and depiction of how sound may enter the operator area, reaching operator before padding.
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.
2.15.3 Sealing Gaps
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.
2.15.4 Evaluating Noise Controls for Jumbo Drills and Bolters
2.15.4.1 Covers for Electric-Motor-Powered Hydraulic Pumps
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.
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 bu ilt 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.

Figure 7: Fiberglass blanket barrier.
Figure 8: Plexiglas motor cover.
2.15.5 Absorptive Material in Canopy
Most of the tested machines had sound-absorbing material under the canopy. However, only three of the

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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.
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.
Figure 9: One-inch-thick quilted fiberglass blanket being removed for testing.
Table 5: Sound level of jumbo drills and bolters at the operator’s position
2.15.6 Absorptive Material on Sides of the Cab Around 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.
Figure 10: Quilted fiberglass material in the operator’s area.
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 oper ator’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.
Sound level of jumbo drills and bolter s at the operator’s position, absorptive material around operator
2.15.7 Absorptive Material in Lower Front of Cab
Figure 11: Quilted fiberglass material in the lower front of the operator’s area of bolter 2.
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

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Sound level of bolter 2 at the operator’s position, absorptive material in lower front of cab
2.15.8 Windshields
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.
Figure 12: Wind Shields for Protection

Figure 13: Wind Shields for protection
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. 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.

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Table 6: Sound level of bolters 2, 3, 4, and 5 at the operator’s position
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). 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 thos e 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

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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 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.

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3 Noise Control in Underground Mines 3.1 Hierarchy of Noise Control
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.
3.2 Barriers and Sound-Absorbing Materials
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 bey ond 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.
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

Or,
It 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.
3.3 Noise Control Resource Guide – MSHA
The Mine Safety and Health Administration’s (MSHA) Noise Control Resource Guide series is a
compendium of resource information and guidance for reducing miners’ noise exposures at coal and metal
and nonmetal surface mines, underground mines, and mills and preparation plants. The Noise Control
Resource Guides represent the Agency’s continuing efforts to assist mine operators in lowering noise
exposure, preventing miner hearing loss, and achieving compliance with the Occupational Noise Exposure
Standard.
-market suppliers of noise controls;
and in some cases, provide information on engineering controls that can be designed, fabricated, and
installed at the mine site.
for machinery suppliers and suppliers of sound and vibration controls and materials.
Technical experts and practitioners in the field of noise in the mining industry, as well as manufacturers of
noise control equipment, provided information contained in this noise control resource guide. The material
found in this guide should be considered a resource and not be construed to be a mandatory requirement.
This guide should be used in conjunction with MSHA Program Information Bulletin (PIB) P11-45
Technologically Achievable, Administratively Achievable and Promising Noise Controls.
Due to the variability of the mining environment, it would be difficult to compile a document that would
present controls that are feasible in each and every situation. The individual noise controls or series of
controls found herein can reduce the exposure of most miners; however, they must be designed, tailored,
and implemented according to the specific situation. Questions regarding technical applicability and
feasibility of the controls to a specific mining situation should be referred to the local MSHA office.
MSHA promulgated Health Standards for Occupational Noise Exposure for the metal, nonmetal, and coal
mining industry (30 CFR Part 62) in an effort to reduce the number of miners who will experience a material

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Weighted Average over eight hours (TWA8) of 90 dBA (100% Dose) and establishes an Action Level (AL)
at a TWA8 of 85 dBA (50% Dose). The operator is required to enroll affected miners in a Hearing
Conservation Program if the AL is met or exceeded. If the PEL is exceeded, the mine operator is required
to use all feasible engineering and/or administrative controls to reduce miner’s exposure to the PEL. The
Noise Control Resource Guides deal with noise controls that are available on types of mining equipment
typically used in different mining environments. The first guide covers surface mining; the second,
underground mining; and the third, mills and preparation plants. These guides will reference the type of
mining equipment and noise controls that are available from the manufacturer of the equipment or as a
retrofit for the equipment. The guides do not address generic administrative controls that are universally
accepted as being effective, i.e. rotation of workers, time limitations, distance, etc. However, if specific
administrative controls have been shown to provide significant noise reduction, these administrative
controls will be discussed with the equipment or the process. The guides also contain appendices that list
equipment manufacturers, noise control products, aftermarket manufacturers, reference sources, and
contact information; however, these lists are not all inclusive
3.3.1 Noise Exposure Reduction
In general, the amount of noise reduction achievable by, and the technologically achievability of a given
noise control or a group of noise controls is widely variable and must be considered on a case-by-case
basis. The amount of noise reduction that can be obtained from an individual noise control or suite of
controls is dependent on a large number of factors:
For these reasons, each of the machine and noise controls shown in this guide do not have specified noise
reductions. Such figures are only obtainable after a complete acoustical investigation is conducted on each
individual machine. Each noise control case study has a set of conditions that are unique to it. Since the
noise standards treat engineering controls equally with administrative controls, one may use either
engineering or administrative controls or a combination of both to reduce miner’s exposures . Each noise
control guide is a valuable source of information for mine operators to use when deciding what type of
mitigative action is best suited for the conditions encountered at their operation. In addition to the
applicability of the control, the operator will need to consider the specific materials used when installing an
engineering control. It is important to remember that the effectiveness of any engineering control used to

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maintained acoustical material. As with most everything used in the mining industry, if an effective
maintenance program is not put in place, the noise control will not last. Sometimes noise controls are
expensive. It is in the o perator’s best interest to maintain the controls so as to reap the benefits of their
investment.
3.3.2 Dose from Multiple Noise Sources
Special considerations should be afforded to multiple noise sources, a situation common in the mining
industry. Multiple noise sources present unique challenges in their measurement and control. The
effectiveness of noise controls on multiple noise sources needs to be systematically evaluated in light of
their contribution to a miner’s exposure. To further illustrate this, consid er the following: When it is
determined that there are multiple noise sources that contribute to a miner’s noise exposure, and that these
sources expose the miner to high levels of noise in a serial fashion, general noise control practices would
direct you to lower the sound level of the highest noise source. However, noise exposure (dose) is a function
of the sound level AND the amount of time the miner is exposed to the noise. Therefore, in planning which
noise source(s) to treat, it is important to look at the sound level and the exposure time. Table 1 illustrates
the roles of sound level and exposure time. A particular miner’s exposure is comprised of four levels and
intervals: S1, a source of 90 dBA for 4 hours; S2, a source of 95 dBA for 2 hours; S3, a source of 100 dBA
for 1 hour; and S4, a source of 88 dBA for 1 hour.
Table 7: Data for Example Calculations Involving Multiple Sound Sources
The miner’s exposure [S1 + S2 + S3 + S4], computed in terms of percent dose compared to the permissible
exposure level (PEL), with a 90 dBA threshold for 8 hours, is 150% [50 + 50 + 50 +0]. By treating only the
highest sound level source (S3) by application of an engineering noise control and reducing it from 100
dBA to 97 dBA (S3 mod), the miner’s exposure [S1 + S2 + S3 mod + S4] would be 133% [50 + 50 + 33 +
0]. However, if the source to which the miner is exposed for most of the time (S1) is modified to obtain a
3 dBA reduction from 90 to 87 dBA [S1 mod], the impact is to reduce the miner’s exposure [S1 mod +S2
+ S3 + S4] to 100% [0 + 50 + 50 + 0]. Actually, a noise control yielding only a 1 dBA reduction applied to
(S1) would achieve the same result. If sources (S1) and (S2) are treated by 3 dBA each and reductions from
90 dBA to 87 dBA and from 95 dBA to 92 dBA obtained, the miner’s resultant exposure [S1 mod + S2
mod + S3 + S4] would be 83% [0 + 33 + 50 + 0]. It is very important when conducting noise control work
to examine the makeup of the miner’s full shift noise exposure. The exposur e may not be based solely on
the highest sound level or the longest exposure time. It is the total noise dose, not just the individual sound
levels or exposure times.
3.3.3 Acoustical Materials
Acoustical materials can reduce noise either by absorbing or blocking sound waves, or damping
vibrations. These materials are generally referred to as absorption, barrier, composite, and damping
materials, and they can substantially increase the effectiveness of other acoustical devices. Selection of
appropriate acoustical materials must be made based on a firm noise control engineering basis and
commensurate to the task, properly installed, used, and maintained. Acoustical devices include, but are
not limited to, mufflers, silencers and enclosures. Absorption, barrier, composite, and damping/isolation
materials are defined as follows:
A material designed to absorb sound waves. It is not intended to be used for blocking sound waves. Some
examples of absorption materials are foam and fiberglass. It may be used inside a cab or enclosure to
prevent the reverberation of sound waves.
A material designed to block sound waves. It does not absorb sound waves. A typical use of barrier
materials would be on the firewall of a bulldozer to block low frequency engine noise. Some examples of
sound barriers are massloaded vinyl curtains, lead, plywood, glass, steel, and concrete.
A material designed to both absorb and block sound. It may be used to provide additional barrier qualities
to an enclosure or operator cab as well as absorption of radiating sound waves. Some examples are
combinations of foam, vinyl, fiberglass, and lead.

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