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) 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 Pressure Level (dB) 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 Pressure Level (dB) 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