NoiseNoise ControlControl EngineeringEngineering BasicsBasics
DaveDave YantekYantek NIOSHNIOSH -- PittsburghPittsburgh ResearchResearch LaboratoryLaboratory HearingHearing LossLoss PreventionPrevention BranchBranch TopicsTopics
•• BasicsBasics ofof soundsound •• MeasurementMeasurement ofof soundsound •• MeasurementMeasurement practicespractices •• NoiseNoise sourcesource identificationidentification •• NoiseNoise controlscontrols BasicsBasics ofof SoundSound PhysicalPhysical ParametersParameters Sound Power (watts) • Sound energy generated by a source per unit time • Independent of surroundings, property of a source • Used for comparing sound sources, calculating sound pressures Sound Intensity (watts/m2) • Measure of the sound power per unit area • Vector quantity (magnitude and direction) Sound Pressure (Pascal) • Pressure fluctuation from atmospheric pressure • Depends on sound power of source, distance from source, environment source, environment Note: 1 PSI = 6,900 Pascal BasicsBasics ofof SoundSound CharacteristicsCharacteristics ofof Sound:Sound: AmplitudeAmplitude Deviation of the pressure from atmospheric pressure atmospheric pressure amplitude
TIME PRESSURE
The higher the amplitude, the higher the sound pressure level BasicsBasics ofof SoundSound CharacteristicsCharacteristics ofof Sound:Sound: FrequencyFrequency The number of pressure fluctuations per second Period, T
TIME PRESSURE
Frequency is related to the period: f = 1/T Frequency is measured in Hertz (Hz) 1 Hz = 1 cycle per second BasicsBasics ofof SoundSound CharacteristicsCharacteristics ofof Sound:Sound: WavelengthWavelength The distance required for the wave to repeat itself wavelength
DISTANCE PRESSURE
wavelength Wavelength is related to frequency by the speed of sound: λ = c/f low frequency – long wavelength high frequency – short wavelength BasicsBasics ofof SoundSound SoundSound FieldsFields (acoustic(acoustic environments)environments) InIn typicaltypical indoorindoor environmentsenvironments (including(including mines)mines) • A region close to the source is dominated by direct sound • A region far from the source is dominated by reverberant sound
• For a given source, the sound level measured indoors (underground) will usually be higher than the sound level measured outdoors (above ground) MeasurementMeasurement ofof SoundSound A sound level meter (SLM) or a microphone & data acquisition system is used to measure sound pressure levels
microphone preamplifier MeasurementMeasurement ofof SoundSound
90 Sound level w/ fast time constant (125 ms) Sound level w/ slow time constant (1000 ms) 89 Equivalent continuous sound level
88
87 Sound Level, dB(A)Sound Level,
86 The equivalent continuous sound level (LEQ) is often used for machinery measurements
85 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Time [seconds] MeasurementMeasurement ofof SoundSound Examples of Sound Pressures and SPLs SPL Sound Source Sound Pressure (dB) Military jet takeoff with Military jet takeoff with 89 Pascal afterburner from aircraft 130 (0.013 PSI) carrier at 50 feet (0.013 PSI) 0.51 Pascal Leaf blower at 25 feet 85 (0.000074 PSI) 0.020 Pascal Conversation at 3 feet 60 (0.0000041 PSI)
NOTE: Atmospheric pressure is 101,325 Pascal (14.7 PSI)
Source: Noise and Vibration Control, Edited by Leo L. Beranek, Revised Edition. 1998. Published by the Institute of Noise Control Engineering, Washington, DC. MeasurementMeasurement ofof SoundSound Examples of Sound Pressures and SPLs % of % of SPL Sound Source Atmospheric (dB) Pressure (dB) Military jet takeoff with afterburner from aircraft 0.088% 130 carrier at 50 feet Leaf blower at 25 feet 0.00050% 85
Conversation at 3 feet 0.000028% 60
NOTE: Atmospheric pressure is 101,325 Pascal (14.7 PSI) MeasurementMeasurement ofof SoundSound AA--weightingweighting 10
0
-10 The A-weighted sound level -20 What doescorrelates A-weighting reasonably do to the wellsound? Example:with Air-rotary hearing drill loss rig and sound is clip -30 Alternating un-weighted for 2 seconds commonly used in noise
Attenuation, dB Attenuation, + -40 regulationsA-weighted for and 2 seconds standards A-weighting significantly -50 attenuates the level of sounds below 1000 Hz 10000 20 30 40 50 100 300 400 500 200 2000 1000 3000 4000 5000
Frequency (Hz) MeasurementMeasurement ofof SoundSound MathematicsMathematics ofof DecibelsDecibels
•• DecibelsDecibels areare logarithmic,logarithmic, notnot linearlinear •• CannotCannot simplysimply add,add, subtract,subtract, oror averageaverage soundsound levelslevels •• TwoTwo sourcessources withwith equalequal soundsound levelslevels increaseincrease soundsound levellevel byby 33 dBdB
ExampleExample 9090 dBdB ++ 9090 dBdB ≠≠ 180180 dBdB 9090 dBdB ++ 9090 dBdB == 9393 dBdB MeasurementMeasurement ofof SoundSound FrequencyFrequency ContentContent
•• TheThe frequencyfrequency rangerange forfor humanhuman hearinghearing isis 2020 HzHz toto 2020 kHzkHz •• TheThe frequencyfrequency contentcontent ofof soundsound helpshelps toto identifyidentify noisenoise sourcessources •• OctaveOctave--bandband oror 1/31/3--octaveoctave--bandband filtersfilters areare usedused toto examineexamine frequencyfrequency contentcontent •• WeWe maymay thinkthink ofof thesethese filtersfilters asas aa frequencyfrequency ‘‘binbin’’ wherewhere thethe energyenergy inin aa smallsmall frequencyfrequency bandband isis countedcounted Example:Example: 1/31/3--OctaveOctave--BandBand SpectrumSpectrum Vibrating Screen Noise – Vibration Only, Without Coal 95
90 ‘broadband source’
85
80
75
70
65
60
55
A-wtd Sound Level (dB) Pressure 50 8000 1250 2500 5000 1000 1600 2000 3150 4000 6300 125 160 250 315 500 630 45 100 200 400 800
1/3-Octave Band Center Frequency (Hz) Example:Example: 1/31/3--OctaveOctave--BandBand SpectrumSpectrum Air-rotary Drill Rig - Drilling Noise 95
90 Source dominated by high frequencies 85
80
75
70
65
60
55
A-wtd Sound Level (dB) Pressure 50 1250 2500 5000 1000 1600 2000 3150 4000 6300 8000 125 160 250 315 500 630 45 100 200 400 800
1/3-Octave Band Center Frequency (Hz) MeasurementMeasurement PracticesPractices
•• SeveralSeveral factorsfactors maymay influenceinfluence measuredmeasured soundsound levelslevels ––InstrumentationInstrumentation calibrationcalibration && setset--upup ––BackgroundBackground noisenoise ––MeasurementMeasurement locationslocations ––MachineryMachinery operationoperation ––MeasurementMeasurement environmentenvironment MeasurementMeasurement PracticesPractices CalibrationCalibration && SetSet--upup
•• CalibrateCalibrate beforebefore andand afterafter testingtesting •• SetSet thethe levellevel ofof thethe calibratorcalibrator toto 114114 dBdB whenwhen ambientambient SPLSPL isis highhigh •• SelectSelect thethe desireddesired weightingweighting priorprior toto testingtesting •• MeasureMeasure thethe LEQLEQ withwith aa 1515 toto 3030 secondsecond measurementmeasurement durationduration MeasurementMeasurement PracticesPractices BackgroundBackground NoiseNoise
•• MeasureMeasure ambientambient soundsound levelslevels (aka(aka backgroundbackground noise)noise) beforebefore andand afterafter machinerymachinery soundsound levellevel measurementsmeasurements •• WindWind (or(or airflowairflow fromfrom ventilationventilation fans)fans) cancan bebe aa sourcesource ofof backgroundbackground noisenoise •• UseUse aa windscreenwindscreen toto reducereduce windwind noisenoise •• TurnTurn offoff otherother machinerymachinery nearnear thethe measurementmeasurement areaarea toto reducereduce BGBG noisenoise MeasurementMeasurement PracticesPractices BackgroundBackground NoiseNoise
• 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 MeasurementMeasurement PracticesPractices MeasurementMeasurement LocationsLocations
•• MakeMake operatoroperator earear SPLSPL measurementsmeasurements asas closeclose toto thethe earear asas possiblepossible •• IfIf wewe areare examiningexamining noisenoise radiatedradiated toto thethe environmentenvironment – Measurements should not be made close to the machine or reflective surfaces, if possible – A measurement distance of 3 feet (1 m) is commonly used – Measurements close to the machine or boundaries will be significantly affected, particularly at low frequencies MeasurementMeasurement PracticesPractices EquipmentEquipment OperationOperation
•• WarmWarm--upup machinerymachinery priorprior toto testingtesting •• OperateOperate machinerymachinery inin aa ‘‘typicaltypical’’ mannermanner (RPM,(RPM, load,load, equipmentequipment functions,functions, etc.)etc.) whenwhen measuringmeasuring operatingoperating soundsound levelslevels •• PerformPerform teststests withwith wellwell--defineddefined parametersparameters toto limitlimit testtest--toto--testtest variabilityvariability whenwhen evaluatingevaluating noisenoise controlscontrols MeasurementMeasurement PracticesPractices MeasurementMeasurement EnvironmentEnvironment
•• ModernModern testtest equipmentequipment isis relativelyrelatively insensitiveinsensitive toto atmosphericatmospheric conditionsconditions •• ClearClear thethe areaarea ofof largelarge reflectivereflective surfacessurfaces •• ObserversObservers andand thethe personperson makingmaking thethe measurementsmeasurements cancan influenceinfluence thethe datadata – Use a tripod and stand to the side and behind the sound level meter – If a tripod is not used, hold the SLM away from the body – Ask observers to stay away from the measurement area NoiseNoise SourceSource IdentificationIdentification ExamplesExamples ofof NoiseNoise SourcesSources
MechanicalMechanical NoiseNoise FlowFlow NoiseNoise Engine block vibration Ventilation systems Road-tire interaction Engine cooling systems Drilling, cutting, grinding Water sprays Electric motors Dust scrubbers Bearings Engine intake & exhaust systems Gears Conveyor systems NoiseNoise SourceSource IdentificationIdentification
•• TheThe firstfirst stepstep inin controllingcontrolling noisenoise isis toto determinedetermine thethe mostmost dominantdominant sourcesource •• InIn termsterms ofof workerworker exposure,exposure, determinedetermine thethe machinemachine and/orand/or operationoperation responsibleresponsible forfor thethe highesthighest percentpercent dosedose •• InIn termsterms ofof machinerymachinery soundsound levels,levels, wewe mustmust determinedetermine thethe sourcesource generatinggenerating thethe highesthighest soundsound levellevel
WhyWhy isis thisthis important?important? NoiseNoise SourceSource IdentificationIdentification MultipleMultiple NoiseNoise SourceSource ExampleExample
ThreeThree NoiseNoise Sources:Sources: 9090 dB,dB, 8888 dB,dB, && 8585 dBdB OverallOverall SoundSound Level:Level: 92.992.9 dBdB CaseCase 1:1: ReduceReduce 8585 dBdB sourcesource toto 7575 dBdB OverallOverall SoundSound Level:Level: 92.292.2 dBdB (0.7(0.7 dBdB reduction)reduction) CaseCase 2:2: ReduceReduce 8888 dBdB sourcesource toto 7878 dBdB OverallOverall SoundSound Level:Level: 91.491.4 dBdB (1.5(1.5 dBdB reduction)reduction) CaseCase 3:3: ReduceReduce 9090 dBdB sourcesource toto 8080 dBdB OverallOverall SoundSound Level:Level: 90.290.2 dBdB (2.7(2.7 dBdB reduction)reduction) CaseCase 4:4: EliminateEliminate 8888 dBdB sourcesource We must identify and treat the dominant WeOverallOverall must SoundSound identify Level:Level: and 91.291.2 treat dBdB (1.7(1.7 the dBdB reduction)dominantreduction) noisenoise source(ssource(s)) toto getget thethe mostmost soundsound levellevel reductionreduction forfor thethe leastleast cost!cost! MultipleMultiple NoiseNoise SourcesSources ProcedureProcedure toto RankRank OrderOrder SourcesSources •• ListList noisenoise sourcessources onon equipmentequipment •• ApplyApply treatmentstreatments toto allall sourcessources (turn(turn offoff oror disconnectdisconnect whenwhen possible)possible) – Treatments do not have to be practical or durable – Goal is to reduce the level of each individual source by 10 dB or more •• RemoveRemove treatmenttreatment fromfrom 11st source,source, measuremeasure soundsound levels,levels, reinstallreinstall treatmenttreatment •• RemoveRemove treatmenttreatment fromfrom 22nd source,source, measuremeasure soundsound levels,levels, reinstallreinstall treatmenttreatment •• ContinueContinue forfor allall sourcessources MultipleMultiple NoiseNoise SourcesSources RankRank OrderingOrdering –– VibratingVibrating ScreenScreen ExampleExample Primary W.O. 18 Cyclones (12) Electrical Room 15
12 Machine Well Circuit 2 Cycl 9 Refuse Scr
Meters Stairs
6 Circuit 1 Cycl Refuse Scr # 169 3
0 0 3 6 9 12 15 18 21 24 Meters Side 2 Side 1 MultipleMultiple NoiseNoise SourcesSources RankRank OrderingOrdering –– VibratingVibrating ScreenScreen ExampleExample MultipleMultiple NoiseNoise SourcesSources RankRank OrderingOrdering –– VibratingVibrating ScreenScreen ExampleExample
• The sound level at any location is the result of all surrounding screens • Cannot process coal on the test screen with all of the other screens off • Large changes to the sound level from the test screen would result in only a small (insignificant) change in the measured sound level • Quilted fiberglass-vinyl-fiberglass barrier hung around test screen to reduce background noise from other equipment MultipleMultiple NoiseNoise SourcesSources RankRank OrderingOrdering –– VibratingVibrating ScreenScreen ExampleExample
VibratingVibrating ScreenScreen NoiseNoise Sources:Sources: Screening noise: Noise generated by the flow of material due to coal-coal, coal-chute, and coal-screen impacts Drive noise: Noise radiated by the vibration mechanism housings, screen sides, and the building due to excitation by the gears, bearings, and eccentric weights of the mechanisms Rinse water spray noise: Noise due to spraying water onto coal MultipleMultiple NoiseNoise SourcesSources RankRank OrderingOrdering –– VibratingVibrating ScreenScreen ExampleExample BARRIER AT DISCHARGE END OF #169
Remember, treatments do not have to be practical for these tests! MultipleMultiple NoiseNoise SourcesSources RankRank OrderingOrdering –– VibratingVibrating ScreenScreen ExampleExample
BARRIERBARRIER BETWEENBETWEEN #169#169 andand #167#167 MultipleMultiple NoiseNoise SourcesSources RankRank OrderingOrdering –– VibratingVibrating ScreenScreen ExampleExample 1.Side 1 and Side 2 processing coal, #169 OFF (BG noise for #169 overall level)
SIDE 2 SIDE 1 2.Side 1 and Side 2 processing coal, #169 processing coal (Can calculate level due to #169 by subtracting levels of test 1) Yields total sound level for test screen (drive noise + screening noise + water spray noise) MultipleMultiple NoiseNoise SourcesSources RankRank OrderingOrdering –– VibratingVibrating ScreenScreen ExampleExample 3. Side 1 processing coal, Side 2 off (BG noise for #169 drive noise & #169 water spray noise)
SIDE 2 SIDE 1 4.Side 1 processing coal, Side 2 off, #169 vibe only (Can calculate level of #169 drive noise by subtracting levels of test 3)
Yields sound level for drive noise MultipleMultiple NoiseNoise SourcesSources RankRank OrderingOrdering –– VibratingVibrating ScreenScreen ExampleExample 5.Side 1 processing coal, Side 2 off, #169 water spray only (Can calculate level of #169 water spray noise by subtracting levels of test 3)
Yields sound level due to water #169 spray noise
SIDE 2 SIDE 1 6.Subtract sound levels due to drive noise and water spray noise from total sound level due to all three sources Yields sound level due to screening noise MultipleMultiple NoiseNoise SourcesSources RankRank OrderingOrdering –– VibratingVibrating ScreenScreen ExampleExample
Test Condition or Overall Sound Noise Source Level Total, processing 92 dB(A) coal (measured) Drive noise 91 dB(A) (measured) Screening noise 87 dB(A) (calculated) Rinse water spray 80 dB(A) (measured) NoiseNoise ControlsControls •• FourFour basicbasic typestypes ofof treatmentstreatments (often(often usedused together)together) ––AbsorbersAbsorbers ––BarriersBarriers ––VibrationVibration IsolatorsIsolators ––VibrationVibration DampingDamping NoiseNoise ControlsControls AbsorbersAbsorbers •• AppliedApplied atat aa reflectivereflective surfacesurface toto absorbabsorb energyenergy andand reducereduce reflectionreflection ofof soundsound •• TypicallyTypically mademade ofof porousporous materialsmaterials (open(open cellcell foam,foam, fiberglass,fiberglass, mineralmineral wool)wool) •• TheThe soundsound absorptionabsorption coefficientcoefficient isis usedused toto describedescribe thethe abilityability ofof aa materialmaterial toto absorbabsorb soundsound •• MostMost effectiveeffective atat higherhigher frequenciesfrequencies •• AbsorberAbsorber thicknessthickness influencesinfluences absorptionabsorption NoiseNoise ControlsControls AbsorbersAbsorbers •• FlammabilityFlammability maymay bebe aa concernconcern withwith somesome soundsound absorbingabsorbing materialsmaterials oror theirtheir facingsfacings •• LiningLining cabs,cabs, enclosures,enclosures, engineengine compartments,compartments, overheadoverhead guards,guards, andand ductsducts maymay reducereduce noisenoise •• GoodGood soundsound absorbingabsorbing materialsmaterials areare NOTNOT usuallyusually goodgood soundsound barriersbarriers NoiseNoise ControlsControls BarriersBarriers •• BarriersBarriers areare materialsmaterials thatthat blockblock transmissiontransmission ofof soundsound •• GoodGood barrierbarrier materialsmaterials areare densedense,, limplimp materialsmaterials •• TheThe transmissiontransmission lossloss (TL)(TL) isis usedused toto describedescribe thethe performanceperformance ofof barrierbarrier materialsmaterials •• InIn general,general, TLTL increasesincreases withwith frequencyfrequency •• EvenEven aa smallsmall openingopening willwill greatlygreatly reducereduce thethe overalloverall TLTL NoiseNoise ControlsControls BarriersBarriers Resulting TL vs. % Open Area for a Material with a TL of 30 dB 30
25
20 Opening
15
10 Transmission Loss, dB Transmission Loss,
5
0 0% 1% 2% 3% 4% 5% 6% 7% 8% 9% 10% % Open Area NoiseNoise ControlsControls VibrationVibration IsolatorsIsolators
•• VibrationVibration isolatorsisolators areare flexibleflexible componentscomponents usedused toto reducereduce transmittedtransmitted vibrationvibration •• TheThe sourcesource ofof vibrationvibration energyenergy maymay notnot bebe thethe dominantdominant sourcesource ofof radiatedradiated noisenoise •• AA large,large, flatflat surfacesurface setset intointo vibrationvibration cancan actact likelike aa soundingsounding boardboard andand radiateradiate noisenoise NoiseNoise ControlsControls VibrationVibration IsolatorsIsolators
•• SelectSelect basedbased onon equipmentequipment weight,weight, operatingoperating speed,speed, andand environmentenvironment •• AA rigidrigid mountingmounting locationlocation isis neededneeded onon bothboth thethe sourcesource andand thethe supportsupport structurestructure •• ItIt isis criticalcritical toto makemake suresure thatthat nono ‘‘shuntshunt pathspaths’’ existexist NoiseNoise ControlsControls VibrationVibration IsolatorsIsolators
Vibration isolators at corners of two motor-driven pumps NoiseNoise ControlsControls VibrationVibration IsolatorsIsolators -- ImpactImpact IsolationIsolation
•• ImpactsImpacts cancan createcreate noisenoise •• ImpactsImpacts cancan causecause surfacessurfaces toto ‘‘ringring’’ oror ‘‘buzzbuzz’’ •• IsolationIsolation padspads mademade ofof rubber,rubber, cork,cork, urethaneurethane andand similarsimilar cancan bebe usedused toto ‘‘cushioncushion’’ impactsimpacts Example: Coated flight bars for a continuous mining machine NoiseNoise ControlsControls VibrationVibration DampingDamping
•• DampingDamping materialmaterial cancan bebe appliedapplied toto vibratingvibrating surfacessurfaces toto convertconvert mechanicalmechanical motionmotion intointo aa smallsmall amountamount ofof heatheat •• TwoTwo typestypes ofof commonlycommonly--usedused dampingdamping treatmentstreatments ◦◦ FreeFree--layerlayer dampingdamping ◦◦ ConstrainedConstrained--layerlayer dampingdamping NoiseNoise ControlsControls VibrationVibration DampingDamping
Free-layer damping • Applied to a surface via spray, roller, or brush • Useful for relatively thin structures • Applied damping material is thicker than the structure itself Constrained-layer damping • Damping material is bonded to the structure • Stiff constraining layer is bonded to the damping material • Must use a stiff adhesive • Surfaces must be clean NoiseNoise ControlsControls ConstrainedConstrained--layerlayer DampingDamping DemonstrationDemonstration Constrained-layer damping patch
No Damping 10% Coverage 50% Coverage • 3-inch wide x 5-inch high aluminum plates • Samples were suspended and struck with a small metal object NoiseNoise ControlsControls ConstrainedConstrained--layerlayer DampingDamping DemonstrationDemonstration No Damping 10% Coverage 50% Coverage 4 4 4
2 2 2
0 0 0 Pressure [Pa] Pressure [Pa] Pressure -2 Pressure [Pa] -2 -2
-4 -4 -4 1.0 1.2 1.4 1.6 1.8 2.0 1.0 1.2 1.4 1.6 1.8 2.0 1.0 1.2 1.4 1.6 1.8 2.0 Time [s] Time [s] Time [s] 80 80 80 80 80 80 75 75 LP = 77.4 dB 75 75 LP 70.7 dB 75 75 LP = 68.5 dB 70 70 70 70 70 70 65 65 65 65 65 65 60 60 60 60 60 60 55 55 55 Pressure dB/2e-005 [Pa] Pressure dB/2e-005 [Pa] 55 Pressure dB/2e-005 [Pa] 55 55 SPL [dB] SPL [dB] SPL [dB] 50 50 50 50 50 50 45 45 45 45 45 45 10000 1000 2000 5000 10000 10000 100 200 500 1000 2000 5000 1000 2000 5000 100 500 200 200 500 40 40 100 40 40100 200 500 1000 2000 5000 10000 40100 200 500 1000 2000 5000 10000 40100 200 500 1000 2000 5000 10000 Frequency [Hz] Frequency [Hz] Frequency [Hz]
Frequency [Hz] Frequency [Hz] Frequency [Hz] NoiseNoise ControlsControls Cabs,Cabs, Barriers,Barriers, andand EnclosuresEnclosures
•• Cabs,Cabs, barriers,barriers, andand enclosuresenclosures cancan bebe veryvery effectiveeffective atat reducingreducing noisenoise •• ItIt isis criticalcritical toto limitlimit gapsgaps toto aa minimumminimum – Pipe penetrations and openings around hydraulic lines or electrical wiring should be sealed as well as possible – Doors and windows should use a flexible seal NoiseNoise ControlsControls CabsCabs • Hydraulic lines resting on cab surfaces can increase noise • Rattling doors and windows can increase noise • Use laminated glass for windows because it has higher damping (higher TL) • Use barrier material with a closed cell foam backing on the floor and/or firewall • Install vibration isolators to reduce structure-born noise (watch out for shunt paths!) • Line with sound absorbing material to reduce build up of reverberant sound • Treat vibrating surfaces with damping material NoiseNoise ControlsControls BarriersBarriers •• ShouldShould bebe installedinstalled eithereither closeclose toto thethe noisenoise sourcesource oror closeclose toto thethe workerworker •• MustMust bebe highhigh enoughenough toto createcreate anan ‘‘acousticacoustic shadowshadow’’ •• PlacePlace soundsound absorbingabsorbing materialmaterial onon reflectivereflective surfacessurfaces aboveabove thethe barrierbarrier toto increaseincrease noisenoise reductionreduction
Sound absorbing material
Barrier NoiseNoise ControlsControls EnclosuresEnclosures • Use materials with high transmission loss • If airflow is required, the worker should not have ‘line of site’ to the enclosed noise source • Line with sound absorbing material to reduce build-up of reverberant noise • Use barrier-absorber materials to increase noise reduction • Vibration isolate enclosure from structure with compliant materials • Add damping treatments to the enclosure to reduce vibration of the enclosure ForFor moremore informationinformation
DaveDave YantekYantek
Phone:Phone: 412412--386386--44984498 Fax:Fax: 412412--386386--48654865 [email protected]@cdc.gov www.cdc.gov/niosh/miningwww.cdc.gov/niosh/mining