Practical Approaches to Practical Approaches to Engineering Noise Engineering Noise 

ControlsControls

Dave YantekDave Yantek

Mining Hearing Loss Prevention WorkshopMining Hearing Loss Prevention WorkshopJune 21June 21--22, 200522, 2005

Pittsburgh Research LaboratoryPittsburgh Research Laboratory

TopicsTopics

•• Basics of soundBasics of sound

•• Measurement of soundMeasurement of sound

•• Measurement practicesMeasurement practices

•• Noise source identificationNoise source identification

•• Noise controlsNoise controls

Basics of SoundBasics of SoundSound Pressure (Sound Pressure (PascalsPascals))•• Pressure fluctuation relative to atmospheric Pressure fluctuation relative to atmospheric 

pressurepressure•• Depends on sound power, distance from source, Depends on sound power, distance from source, 

environmentenvironmentSound Power (watts)Sound Power (watts)•• Sound energy generated by a source per unit timeSound energy generated by a source per unit time•• IndependentIndependent of surroundings, a property of the of surroundings, a property of the 

sound sourcesound source•• Used for rating/comparing sound sources, Used for rating/comparing sound sources, 

calculating sound pressure in a known environmentcalculating sound pressure in a known environment

Sound Intensity (watts/mSound Intensity (watts/m22))•• Measure of the sound power per unit areaMeasure of the sound power per unit area•• Vector quantity (magnitude and direction)Vector quantity (magnitude and direction)

Basics of SoundBasics of SoundThree characteristics of soundThree characteristics of sound1.1.AmplitudeAmplitude –– The amount of deviation of the The amount of deviation of the 

pressure from atmospheric pressure (PSI or pressure from atmospheric pressure (PSI or PascalsPascals))

2.2.WavelengthWavelength –– The distance from maximum to The distance from maximum to maximum or minimum to minimum (feet or maximum or minimum to minimum (feet or meters)meters)

3.3. FrequencyFrequency –– The number of pressure fluctuations The number of pressure fluctuations per second (Hz or 1/s)per second (Hz or 1/s)

Basics of SoundBasics of SoundSound Fields (acoustic environment)Sound Fields (acoustic environment)In typical indoor environments (including In typical indoor environments (including 

mines)mines)•• A region close to the source is dominated by A region close to the source is dominated by 

direct sounddirect sound•• A region far from the source is dominated by A region far from the source is dominated by 

the reverberant soundthe reverberant sound

Note: In general, the sound level measured underground will not be the same as the sound level measured above ground

Measurement of Sound Pressure Level Measurement of Sound Pressure Level (SPL)(SPL)

•• Sound pressure level measurements are made Sound pressure level measurements are made using a sound level meter using a sound level meter 

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AA--Weighting CurveWeighting Curve•• The AThe A--weighted sound level correlates reasonably well with weighted sound level correlates reasonably well with 

hearing loss and is commonlyhearing loss and is commonly--used in noise regulations and used in noise regulations and standardsstandards

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What does A-weighting do to the sound?

Continuous mining machine conveyor noise

Alternating un-weighted for 2.5 sec + A-weighted for 2.5 sec

Measurement of Frequency ContentMeasurement of Frequency Content

•• The frequency range for human hearing is The frequency range for human hearing is 20 Hz to 20 kHz 20 Hz to 20 kHz 

•• The frequency content of sound helps to The frequency content of sound helps to identify noise sourcesidentify noise sources

•• Octave band or 1/3Octave band or 1/3--octave band filters are octave band filters are used to examine frequency contentused to examine frequency content

•• We may think of these filters as a We may think of these filters as a frequency ‘bin’ where the energy in a frequency ‘bin’ where the energy in a small frequency band is countedsmall frequency band is counted

Measurement of Frequency ContentMeasurement of Frequency Content

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Measurement of Frequency ContentMeasurement of Frequency Content

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Adding, Subtracting, &  Adding, Subtracting, &  Averaging Sound LevelsAveraging Sound Levels

•• Sound levels are logarithmic, not linearSound levels are logarithmic, not linear•• Cannot simply add, subtract, or average Cannot simply add, subtract, or average 

levelslevels•• Two sources with equal sound level Two sources with equal sound level 

increase sound level by 3 dBincrease sound level by 3 dB

ExampleExample90 dB + 90 dB ≠ 180 dB90 dB + 90 dB ≠ 180 dB90 dB + 90 dB = 93 dB90 dB + 90 dB = 93 dB

Measurement PracticesMeasurement Practices

•• Several factors may influence Several factors may influence measured sound levelsmeasured sound levels–– Instrumentation Calibration & SetInstrumentation Calibration & Set--upup–– Background noiseBackground noise–– Measurement locations Measurement locations –– Machinery operationMachinery operation–– Measurement environmentMeasurement environment

Measurement PracticesMeasurement PracticesCalibration & Instrumentation SetCalibration & Instrumentation Set--upup

•• Calibrate instrumentation before and after Calibrate instrumentation before and after testingtesting

•• Set the level of the calibrator to 114 dB Set the level of the calibrator to 114 dB when ambient SPL is highwhen ambient SPL is high

•• Make sure that AMake sure that A--weighting is selected weighting is selected prior to measurementsprior to measurements

•• Measure the LEQ with a 15 to 30 second Measure the LEQ with a 15 to 30 second measurement durationmeasurement duration

Measurement PracticesMeasurement PracticesBackground NoiseBackground Noise

•• Measure ambient sound levels (Measure ambient sound levels (akaakabackground noise) before and after background noise) before and after machinery sound level measurementsmachinery sound level measurements

•• Wind (or airflow from ventilation fans) can Wind (or airflow from ventilation fans) can be a source of background noisebe a source of background noise

•• Use a windscreen to reduce the effects of Use a windscreen to reduce the effects of wind noisewind noise

•• Lower BG noise when possible by turning off Lower BG noise when possible by turning off other machinery near the measurement areaother machinery near the measurement area

Measurement PracticesMeasurement PracticesBackground NoiseBackground Noise

• BG SPL must be at least 3 dB below SPL of machinery

• If BG SPL is 10 dB lower than the SPL of machinery, it has little effect on the measured SPL (in practice the effect is considered to be negligible)

• Must correct for BG noise when the BG SPL is 3 to 10 dB lower than the machinery SPL

Measurement PracticesMeasurement PracticesMeasurement LocationsMeasurement Locations

•• Make operator ear SPL measurements as Make operator ear SPL measurements as close to the ear as possibleclose to the ear as possible

•• If we are examining noise radiated to the If we are examining noise radiated to the environmentenvironment–– Measurements should not be made close to the Measurements should not be made close to the 

machine or reflective surfaces, if possiblemachine or reflective surfaces, if possible–– A measurement distance of 3 feet (1 m) is A measurement distance of 3 feet (1 m) is 

commonly usedcommonly used–– Measurements close to the machine or Measurements close to the machine or 

boundaries will be significantly affected, boundaries will be significantly affected, particularly at low frequenciesparticularly at low frequencies

Measurement PracticesMeasurement PracticesEquipment OperationEquipment Operation

•• WarmWarm--up machinery prior to testingup machinery prior to testing•• Operate machinery in a ‘typical’ manner Operate machinery in a ‘typical’ manner 

(RPM, load, equipment functions, etc.) (RPM, load, equipment functions, etc.) when measuring operating sound levelswhen measuring operating sound levels

•• Perform tests with wellPerform tests with well--defined defined parameters to limit testparameters to limit test--toto--test variability test variability when evaluating noise controlswhen evaluating noise controls

Measurement PracticesMeasurement PracticesMeasurement EnvironmentMeasurement Environment

•• Generally, modern test equipment is insensitive Generally, modern test equipment is insensitive to temperature, humidity, and barometric to temperature, humidity, and barometric pressurepressure

•• Clear the area of large reflective surfacesClear the area of large reflective surfaces•• Observers and even the person making the Observers and even the person making the 

measurements can influence the datameasurements can influence the data–– Use a tripod and stand to the side and behind the sound Use a tripod and stand to the side and behind the sound 

level meterlevel meter–– Ask observers to stay away from the measurement area Ask observers to stay away from the measurement area 

until measurements have been completeduntil measurements have been completed

Noise SourcesNoise SourcesSources ofSources of

Mechanical NoiseMechanical NoiseEngine block vibrationEngine block vibrationRoadRoad--tire interactiontire interactionDrilling, cutting, grindingDrilling, cutting, grindingElectric motorsElectric motorsBearingsBearingsGearsGearsConveyor systemsConveyor systems

Sources ofSources ofFlow NoiseFlow Noise

Ventilation systemsVentilation systemsEngine cooling systemsEngine cooling systemsWater spraysWater spraysDust scrubbersDust scrubbersEngine intake & exhaust Engine intake & exhaust 

systemssystems

Noise Source IdentificationNoise Source Identification•• The first step in controlling noise is to The first step in controlling noise is to 

determine the most significant sourcedetermine the most significant source•• In terms of worker exposure, determine In terms of worker exposure, determine 

the machine and/or operation responsible the machine and/or operation responsible for the highest amount of dosefor the highest amount of dose

•• In terms of machinery sound levels, we In terms of machinery sound levels, we must determine the source generating the must determine the source generating the highest sound levelhighest sound level

Why is this important?Why is this important?

We must identify and treat the dominant noise source(s) to get the most sound level reduction for the least cost!

Rank Ordering of Noise SourcesRank Ordering of Noise Sources

Case 2: Reduce 88 dB source to 78 dBOverall Sound Level: 91.4 dB (1.5 dB reduction)

Case 2:Case 2: Reduce Reduce 88 dB88 dB source to 78 dBsource to 78 dBOverall Sound Level: 91.4 dB (1.5 dB reduction)Overall Sound Level: 91.4 dB (1.5 dB reduction)

Case 3: Reduce 90 dB source to 80 dBOverall Sound Level: 90.2 dB (2.7 dB reduction)

Case 3:Case 3: Reduce Reduce 90 dB90 dB source to 80 dBsource to 80 dBOverall Sound Level: 90.2 dB (2.7 dB reduction)Overall Sound Level: 90.2 dB (2.7 dB reduction)

Example: 3 sources of 90 dB, 88 dB & 85 dB Overall Sound Level: 92.9 dBExample:Example: 3 sources of 90 dB, 88 dB & 85 dB 3 sources of 90 dB, 88 dB & 85 dB Overall Sound Level: 92.9 dBOverall Sound Level: 92.9 dB

Case 4: Eliminate 88 dB sourceOverall Sound Level: 91.2 dB (1.7 dB reduction)

Case 4:Case 4: Eliminate Eliminate 88 dB88 dB sourcesourceOverall Sound Level: 91.2 dB (1.7 dB reduction)Overall Sound Level: 91.2 dB (1.7 dB reduction)

Case 1: Reduce 85 dB source to 75 dBOverall Sound Level: 92.2 dB (0.7 dB reduction)

Case 1:Case 1: Reduce Reduce 85 dB85 dB source to 75 dBsource to 75 dBOverall Sound Level: 92.2 dB (0.7 dB reduction)Overall Sound Level: 92.2 dB (0.7 dB reduction)

Multiple Noise SourcesMultiple Noise SourcesProcedure to Rank Order SourcesProcedure to Rank Order Sources

•• List probable noise sources on equipmentList probable noise sources on equipment•• Apply treatments to each source (turn Apply treatments to each source (turn 

source off or disconnect it if possible)source off or disconnect it if possible)–– Treatments do not have to be practical or durableTreatments do not have to be practical or durable–– Goal is to reduce the level of each source by 10 Goal is to reduce the level of each source by 10 

dB or moredB or more

•• Remove treatment from 1Remove treatment from 1stst source, measure source, measure sound levels, reinstall treatmentsound levels, reinstall treatment

•• Remove treatment from 2Remove treatment from 2ndnd source, measure source, measure sound levels, reinstall treatmentsound levels, reinstall treatment

•• Continue for all sourcesContinue for all sources

IntakeIntake silencer 

+ long piping

ExhaustOversized muffler 

+long piping

EngineWrap with

Lead/fiberglass‘blankets’

FanDisconnect belt

or remove fan blade

Drilling Turn Off

Multiple Noise SourcesMultiple Noise SourcesRank Ordering Rank Ordering –– Drill Rig ExampleDrill Rig Example

# 169

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Multiple Noise SourcesMultiple Noise SourcesRank Ordering Rank Ordering –– Vibrating Screen ExampleVibrating Screen Example

Multiple Noise SourcesMultiple Noise SourcesRank Ordering Rank Ordering –– Vibrating Screen ExampleVibrating Screen Example

Multiple Noise SourcesMultiple Noise SourcesRank Ordering Rank Ordering –– Vibrating Screen ExampleVibrating Screen Example

•• The sound level at any location is the The sound level at any location is the result of all surrounding screensresult of all surrounding screens

•• Cannot process coal on the test screen Cannot process coal on the test screen with the other screens offwith the other screens off

•• Even significant changes to the test screen Even significant changes to the test screen will not result in a measurable sound level will not result in a measurable sound level reductionreduction

•• Quilted fiberglassQuilted fiberglass--vinylvinyl--fiberglass barrier fiberglass barrier hung around test screen to reduce hung around test screen to reduce background noise from other equipmentbackground noise from other equipment

Multiple Noise SourcesMultiple Noise SourcesRank Ordering Rank Ordering –– Vibrating Screen ExampleVibrating Screen Example

BARRIER AT DISCHARGE END OF #169BARRIER AT DISCHARGE END OF #169

Multiple Noise SourcesMultiple Noise SourcesRank Ordering Rank Ordering –– Vibrating Screen ExampleVibrating Screen Example

BARRIER BETWEEN #169 and #167BARRIER BETWEEN #169 and #167

Multiple Noise SourcesMultiple Noise SourcesRank Ordering Rank Ordering –– Vibrating Screen ExampleVibrating Screen Example

BARRIER BETWEEN #169 and #167

Remember, treatments do not have to be practical for these tests!

Multiple Noise SourcesMultiple Noise SourcesRank Ordering Rank Ordering –– Vibrating Screen ExampleVibrating Screen Example

Screening noise:Screening noise:Noise generated by the flow of material due to Noise generated by the flow of material due to coalcoal--coal, coalcoal, coal--chute, and coalchute, and coal--screen impactsscreen impacts

Drive noise:Drive noise:Noise radiated by the vibration mechanism Noise radiated by the vibration mechanism housings, screen sides, and the building due to housings, screen sides, and the building due to excitation by the gears, bearings, and eccentric excitation by the gears, bearings, and eccentric weights of the mechanismsweights of the mechanisms

Rinse water spray noise:Rinse water spray noise:Noise due to spraying water onto coalNoise due to spraying water onto coal

Multiple Noise SourcesMultiple Noise SourcesRank Ordering Rank Ordering –– Vibrating Screen ExampleVibrating Screen Example

2.Side 1 and Side 2 processing coal, #169 processing coal (Can calculate level due to #169 by subtracting levels of test 1)

2.2.Side 1 and Side 2 processing Side 1 and Side 2 processing coal, #169 processing coal coal, #169 processing coal (Can (Can calculate level due to #169 by calculate level due to #169 by subtracting levels of test 1)subtracting levels of test 1)

Yields total sound level for test screen (drive noise + screening noise + water spray noise)

SIDE 2 SIDE 1

1.Side 1 and Side 2 processing coal, #169 OFF (BG noise for #169 processing coal)

1.1.Side 1 and Side 2 processing Side 1 and Side 2 processing coal, #169 OFF coal, #169 OFF (BG noise for #169 (BG noise for #169 processing coal)processing coal)

Multiple Noise SourcesMultiple Noise SourcesRank Ordering Rank Ordering –– Vibrating Screen ExampleVibrating Screen Example

4.Side 1 processing coal, Side 2 off, #169 vibe only (Can calculate level of #169 VIBE by subtracting levels of test 3)

4.4.Side 1 processing coal, Side 2 Side 1 processing coal, Side 2 off, #169 vibe only off, #169 vibe only (Can calculate (Can calculate level of #169 VIBE by subtracting levels level of #169 VIBE by subtracting levels of test 3)of test 3)

Yields sound level for drive noise

SIDE 2 SIDE 1

3. Side 1 processing coal, Side 2 off (BG noise for #169 VIBE only & #169 WATER only)

3.3. Side 1 processing coal, Side 2 Side 1 processing coal, Side 2 off off (BG noise for #169 VIBE only & (BG noise for #169 VIBE only & #169 WATER only)#169 WATER only)

Multiple Noise SourcesMultiple Noise SourcesRank Ordering Rank Ordering –– Vibrating Screen ExampleVibrating Screen Example

6.Subtract sound levels due to drive noise and water spray noise from sound level due to all three sources

6.6.Subtract sound levels due to Subtract sound levels due to drive noise and water spray drive noise and water spray noise from sound level due to noise from sound level due to all three sourcesall three sources

5.Side 1 processing coal, Side 2 off, #169 water spray only (Can calculate level of #169 WATER by subtracting levels of test 3)

5.5.Side 1 processing coal, Side 2 Side 1 processing coal, Side 2 off, #169 water spray only off, #169 water spray only (Can (Can calculate level of #169 WATER by calculate level of #169 WATER by subtracting levels of test 3)subtracting levels of test 3)

SIDE 2 SIDE 1

#169Yields sound level due to water spray noise

Yields sound level due to screening noise

Multiple Noise SourcesMultiple Noise SourcesRank Ordering Rank Ordering –– Vibrating Screen ExampleVibrating Screen Example

87 87 dB(AdB(A))Screening noise Screening noise (calculated)(calculated)

80 80 dB(AdB(A))Rinse water spray Rinse water spray (measured)(measured)

91 91 dB(AdB(A))Drive noise Drive noise (measured)(measured)

92 92 dB(AdB(A))Total, processing Total, processing coal (measured)coal (measured)

Overall Sound Overall Sound LevelLevel

Test Condition or Test Condition or Noise SourceNoise Source

Noise ControlsNoise Controls

•• Noise can be reduced by applying Noise can be reduced by applying four basic types of treatmentsfour basic types of treatments(often used together)(often used together)–– AbsorbersAbsorbers–– BarriersBarriers–– Vibration IsolatorsVibration Isolators–– Vibration DampingVibration Damping

Noise Controls Noise Controls -- AbsorbersAbsorbers

•• Applied at a reflective surface to absorb Applied at a reflective surface to absorb energy and reduce reflection of sound energy and reduce reflection of sound 

•• Typically made of porous materials (open Typically made of porous materials (open cell foam, fiberglass, mineral wool)cell foam, fiberglass, mineral wool)

•• The sound absorption coefficient is used to The sound absorption coefficient is used to describe the ability of a material to absorb describe the ability of a material to absorb soundsound

•• Most effective at higher frequenciesMost effective at higher frequencies•• Absorber thickness influences absorptionAbsorber thickness influences absorption

Noise Controls Noise Controls -- AbsorbersAbsorbers

•• Flammability may be a concern with some Flammability may be a concern with some sound absorbing materials or their facingssound absorbing materials or their facings

•• Lining cabs, enclosures, engine Lining cabs, enclosures, engine compartments, overhead guards, and compartments, overhead guards, and ducts may reduce noiseducts may reduce noise

•• Good sound absorbing materials are Good sound absorbing materials are NOTNOTusually good sound barriersusually good sound barriers

Noise Controls Noise Controls -- BarriersBarriers•• Barriers are materials that block Barriers are materials that block 

transmission of soundtransmission of sound•• Good barrier materials are Good barrier materials are densedense, limp , limp 

materialsmaterials•• The transmission loss (TL) is used to The transmission loss (TL) is used to 

describe the performance of barrier describe the performance of barrier materialsmaterials

•• In general, TL increases with frequencyIn general, TL increases with frequency•• Even a small gap will greatly reduce the Even a small gap will greatly reduce the 

overall TLoverall TL

Noise Controls Noise Controls -- BarriersBarriersResulting TL vs. % Open Area for a Material with a TL of 30 dB

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Noise Controls Noise Controls –– Vibration IsolatorsVibration Isolators•• Vibration isolators are flexible components Vibration isolators are flexible components 

typically made from rubber, cork, or even typically made from rubber, cork, or even steel springssteel springs

•• The source of vibration may not be the The source of vibration may not be the dominant source of radiated noisedominant source of radiated noise

•• A large, flat surface set into vibration can A large, flat surface set into vibration can act like a sounding board and radiate act like a sounding board and radiate noisenoise

•• Properly applied isolators reduce Properly applied isolators reduce transmission of vibrationtransmission of vibration

Noise Controls Noise Controls –– Vibration IsolatorsVibration Isolators

•• Selected based on equipment weight, Selected based on equipment weight, required isolation frequency, and operating required isolation frequency, and operating environmentenvironment

•• A rigid mounting location is needed on both A rigid mounting location is needed on both the source and the support structurethe source and the support structure

•• It is critical to make sure that no ‘shunt It is critical to make sure that no ‘shunt paths’ existpaths’ exist

Noise Controls Noise Controls –– Vibration IsolatorsVibration Isolators

Vibration Isolators at corners of two motor-driven pumps

Noise Controls Noise Controls –– Vibration IsolatorsVibration IsolatorsImpact IsolationImpact Isolation

•• Impacts can create noiseImpacts can create noise•• Impacts can cause surfaces to ‘ring like a Impacts can cause surfaces to ‘ring like a 

bell’ or ‘buzz’bell’ or ‘buzz’•• Isolation pads made of rubber, cork, Isolation pads made of rubber, cork, 

urethane and similar can be used to urethane and similar can be used to ‘cushion’ impacts‘cushion’ impacts

Examples:Examples:

Coated flight bars and composite tail rollers for a Coated flight bars and composite tail rollers for a continuous mining machinecontinuous mining machine

Noise Controls Noise Controls –– Vibration DampingVibration Damping

•• Damping material can be applied to Damping material can be applied to vibrating surfaces to convert vibrating surfaces to convert mechanical motion into a small mechanical motion into a small amount of heatamount of heat

•• There are two types of damping There are two types of damping treatmentstreatments–– FreeFree--layer dampinglayer damping–– ConstrainedConstrained--layer dampinglayer damping

Noise Controls Noise Controls –– Vibration DampingVibration DampingFreeFree--layer dampinglayer damping•• Applied to a surface via spray, roller, or brushApplied to a surface via spray, roller, or brush•• Useful for relatively thin structuresUseful for relatively thin structures•• Applied damping material is often thicker than the Applied damping material is often thicker than the 

structure itselfstructure itself

ExampleExampleComposite tail roller for a continuous mining machineComposite tail roller for a continuous mining machine

ConstrainedConstrained--layer dampinglayer damping•• Damping material is bonded to the structureDamping material is bonded to the structure•• Stiff constraining layer is bonded to the damping Stiff constraining layer is bonded to the damping 

materialmaterial•• Must use a stiff adhesiveMust use a stiff adhesive•• Surfaces must be cleanSurfaces must be clean

Noise Controls Noise Controls –– Vibration DampingVibration Damping

No Damping 10% Coverage 50% Coverage

Constrained-layer Damping Demonstration

• Samples are 3-inch wide x 5-inch high aluminum plates

• Samples were suspended and struck with a small metal object

Constrained-layerdamping patch

Noise Controls Noise Controls –– Vibration DampingVibration Damping

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Cabs, Barriers, and EnclosuresCabs, Barriers, and Enclosures

•• Cabs, barriers, and enclosures can be very Cabs, barriers, and enclosures can be very effective in reducing noise exposure if effective in reducing noise exposure if they are applied properlythey are applied properly

•• It is critical to limit gaps to a minimumIt is critical to limit gaps to a minimum–– Pipe penetrations and openings around Pipe penetrations and openings around 

hydraulic lines or electrical wiring should be hydraulic lines or electrical wiring should be sealed as well as possiblesealed as well as possible

–– Doors and windows should use a flexible sealDoors and windows should use a flexible seal

CabsCabs•• Line with sound absorbing material to reduce Line with sound absorbing material to reduce 

build up of reverberant soundbuild up of reverberant sound•• Use barrier material with a closed cell foam Use barrier material with a closed cell foam 

backing on the floor and/or firewall backing on the floor and/or firewall •• Install vibration isolators to reduce structureInstall vibration isolators to reduce structure--born born 

noisenoise•• Hydraulic lines resting on cab surfaces can Hydraulic lines resting on cab surfaces can 

increase noiseincrease noise•• Treat vibrating surfaces with dampingTreat vibrating surfaces with damping•• Use laminated glass for windows because it has Use laminated glass for windows because it has 

higher dampinghigher damping•• Rattling doors and windows can increase noiseRattling doors and windows can increase noise

BarriersBarriers

•• Should be installed either close to the Should be installed either close to the noise source or close to the workernoise source or close to the worker

•• Must be high enough to create an Must be high enough to create an ‘acoustic shadow’‘acoustic shadow’

•• Place sound absorbing material on Place sound absorbing material on reflective surfaces above the barrier to reflective surfaces above the barrier to increase noise reduction increase noise reduction 

Barrier

Sound absorbingmaterial

EnclosuresEnclosures

•• Use materials with high transmission lossUse materials with high transmission loss•• If airflow is required, the worker should not have If airflow is required, the worker should not have 

‘line of site’ to the enclosed noise source‘line of site’ to the enclosed noise source•• Line with sound absorbing material to reduce Line with sound absorbing material to reduce 

buildbuild--up of reverberant noiseup of reverberant noise•• Use barrierUse barrier--absorber materials to increase noise absorber materials to increase noise 

reductionreduction•• Vibration isolate enclosure from structure with Vibration isolate enclosure from structure with 

rubber gasket materialrubber gasket material•• Add damping treatments to the enclosure if Add damping treatments to the enclosure if 

vibration is a concernvibration is a concern

For more informationFor more information

Dave YantekDave Yantek

Phone: 412Phone: 412--386386--4498     Fax: 4124498     Fax: 412--386386--48644864dyantek@cdc.govdyantek@cdc.gov

www.cdc.gov/niosh/miningwww.cdc.gov/niosh/mining

Pittsburgh Research LaboratoryPittsburgh Research Laboratory