noise.docx
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noise.docx
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noise
Noise
Introduction
AircraftnoiseishardlyanewsubjectasevidencedbythefollowingnotereceivedbyapredecessorofUnitedAirlinesinabout1927.
Althoughinternalnoisewasthemajorpreoccupationofaircraftacousticengineersformanyyearsandstillisimportant,thenoiseproducedbytheaircraftengineandexperiencedonthegroundhasbecomeadominantfactorintheacceptabilityoftheairplane.Withthedevelopmentofhighbypassratioengines,noiseduetoothersourceshasbecomeimportantaswell.
Internalnoiseistreatedbyplacingtheenginestominimizethenoisedirectlyradiatedtothecabin,(e.g.usingthewingasashield)andbyprovidinginsulatingmaterialovertheentiresurfaceoftheflightandpassengercompartments.Iftheenginesaremountedonthefuselage,vibrationisolationisanimportantfeature.Inthelate1980'swhenprop-fanswerebeingdeveloped,internalnoisebecomeanimportantconsiderationagain.Itwas,atonepoint,estimatedthat2000lbsofadditionalacousticinsulationwouldberequiredtoreducecabinnoiselevelstothoseofconventionaljetsifprop-fanswereplacedontheaircraftwings.Thisisonereasonwhymanyprop-fanaircraftweredesignedasaft-mountedpusherconfigurations.
Externalnoiseisaffectedbythelocationofthesourceandobserver,theenginethrust,andanumberoffactorsthatinfluencetheoverallconfigurationdesign.Thesewillbediscussedindetaillaterinthischapter,butfirstwemustunderstandtheoriginsofnoiseanditsmeasurement.
TheNatureofNoise
Asoundwavecarrieswithitacertainenergyinthedirectionofpropagation.Thesoundbecomesaudiblebecauseofenergywhichoriginatesatthesourceofthesoundvibrationsandwhichistransportedbythesoundwaves.Thechangesinairpressurewhichreachtheeardrumsetitvibrating;thegreaterthesechanges,thelouderisthesound.
Theintensityofsound,I,isthequantityofenergytransferredbyasoundwavein1secthroughanareaof1cm.Foraplanesinewave:
I=p2/2ρc
where:
p=theamplitudeofthevaryingacousticexcesspressure
ρ=airdensity
c=speedofsound
Iisusuallyexpressedinergspercm2persec.(mW/m2)
Thehumanearrespondstoafrequencyrangeofabout10octaves.Itrespondstoairvibrationswhoseamplitudeishardlymorethanmolecularsize;italsorespondswithoutdamagetosoundsofintensity1013to1014timesgreaterwithoutdamage.
Theresponseoftheearisnotproportionaltotheintensity,however.Itismorenearlyproportionaltothelogarithmoftheintensity.Ifsoundintensityisincreasedinstepsofwhatseemtobeequalincrementsofloudness,wefindthattheintensitiesformasequenceofthesort1,2,4,8,16,....or1,10,100,1000not1,2,3,4,...or1,10,19,28,....Sincetheearrespondsdifferentlytodifferentfrequencies,thelogarithmicrelationofintensitytoloudnessisnotgenerallyperfect,butitiseasiertohandlethantheenormousnumbersinvolvedintheaudibleintensityrange.Therefore,theintensitylevelofsoundisdefinedindecibelsas10timesthelogarithmoftheratiooftheintensityofasound,I,toareferenceleveldefinedas10-9erg/cm2/sec.
Thus:
Soundintensitylevel(SPL),decibels=10log10I/10-9
Theresponseoftheearisnotexactlyproportionaltothedecibelscale.Inadditiontothephysicalquantities,intensityandfrequency,thepsycho-physiologicalquantitiesofloudnessandpitchmustbeconsidered.Theloudnessofasounddependsbothonintensitylevelandfrequency;pitchdependschieflyonfrequencybuttosomeextentonintensity.ContoursofequalloudnessfortheaveragepersonareplottedinthefollowingfigurefromRef.2.TheactualcontourvaluesarethevaluesofSPLat1kHz.
Contoursofequalloudness,plottedagainstintensityandfrequencyfortheaverageear.
Thedb(A)Scale
Inanattempttodevelopanoisemeasuringscalemoreresponsivetothesecharacteristicsoftheear,the"A"scalewasdefinedtoweightnoiseatfrequenciesabove1000Hzmoreheavily.Noisemeasuredonthisscaleisgiveninunitsofdb(A).
Frequencyresponseweightingforthe"A"scale.(FromPetersonandGross,1967,p.9).
ThePerceivedNoiseLevelScalePNdbandEPNdb
ThescalemostoftenusedforaircraftnoisemeasurementisthePerceivedNoiseLevel(PNL)scale.ThescalerequiresthattheSPLbemeasuredineachofninecontiguousfrequencyrangesandcombinedaccordingtoaspecialprescription,nottoodifferentfromtheA-weightingmethod,toprovideanoiseindicationlevel.TheunitsarePNdb.
Theeffectiveperceivednoiselevel,EPNL,accountsfordurationandpresenceofdiscretefrequencytones.ItinvolvesacorrectionfactorthataddstothePNLwhentherearediscretetonesinthenoisespectrum.ItalsoincludesacorrectionobtainedbyintegratingthePNLovera10secondtimeinterval.(DetailsaregiveninthefulltextofFARPart36.)
Theeffectiveperceivednoiselevelcorrelateswithpeople'sperceivednoisinessasshowninthefiguresbelow.
SubjectiveReactionstoVariousNoiseLevels
Thefactthatpeople'sperceptionofnoisevarieslogarithmicallywithsoundintensityresultsinsomeinterestingrelations.Notethatasintensityisreducedby50%theSPLchangesby10logI1/I2=-3db.Fromtheplotabovethisreductionwouldbeonlybarelyperceptible.Thisiswhynoisereductionisachallenge.TomakesomethingseemabouthalfasnoisyrequiresareductioninSPLbyabout10db.ThisisareductioninIofabout90%!
.
People'sreactionsalsodependonhowoftensuchnoisesoccurandavarietyofmethodsforaveragingnoisinesshavebeenused.Soundexposurelevels(SEL),noiseexposureforecasts(NEF),andDay-Night-Levelsallinvolvesomekindofaveragingofmultiplenoiseevents,usuallywithhigherweightings(e.g.10-20times)fornightflights.Theseareintendedtocapturethecommunityresponseinastatisticalway.(Seefigurebelow.)
CommunityResponsetoDifferentNoiseLevels
Footprints
TheU.S.EnvironmentalProtectionAgency(EPA)usesaDay-NightAverageA-WeightedSoundLevelmetricknownasDNLasamethodforpredictingtheeffectsonapopulationofthelongtermexposuretoenvironmentalnoise.TheDNLmetricislegislatedtobethesinglesystemformeasuringaircraftnoiseimpactandfordetermininglandusecompatibility.
NoisemapstypicallydepicttheDNL65dBcontourasthisisidentifiedbyfederalguidelinesasthethresholdlevelofaviationandcommunitynoisethatis"significant".Ingeneral,mostlandusesareconsideredtobecompatiblewithDNLslessthan65dB.
SampleofEstimatedNoiseFootprintsAtlantaAirportinJan.2000
ContoursofconstantDNLorEPNdBareoftenplottedtodeterminetheareasaffectedatagivenlevels.Differentaircraftmayhaveverydifferentfootprints,thisisespeciallyobviouswhencomparing2vs.4engineaircraft,becauseofdifferentclimbrates.
SourcesofNoise
Aircraftnoiseisgenerallydividedintotwosources:
thatduetotheengines,andthatassociatedwiththeairframeitself.Ashigherbypassratioengineshavebecomemorecommonandaircrafthavebecomelarger,interestinairframe-relatednoisehasgrown,butenginenoisestillaccountsformostoftheaircraftexternalnoise. Therelativeimportanceofvariousnoisesourcesisshowninthefigurebelow.
Propulsion-RelatedNoiseSources
Enginenoiseincludesthatgeneratedatthefaninletandexit,thecombustorcore,theturbine,andthatcausedbyjetmixing.Whilejetnoise,causedbytheturbulentmixingofthehighspeedexhaustwiththeambientair,isabroadbandnoisesource,withmostoftheenergydirectedaftoftheengineata45degreeanglefromtheengineaxis,theturbomachinerynoiseoftenincludesdiscretetonesassociatedwithbladepassagefrequenciesandtheirharmonics.
Jetnoiselevelsvaryasthesixthtoeighthpowerofthejetexhaustvelocityasshowninthefigurebelow.Earlyturbojetengineshadexhaustvelocitiesofnearly2000ft/secandnoisesuppressorswereusedtotrytoobtainbettermixingandlowerthenoiseassociatedwiththestrongshear.Suchsuppressorswereeffectiveinreducingthelowfrequencynoise,butoftennotthehighfrequenciesandaddedweightandcosttothedesign.
Thejetvelocitywasreducedconsiderablyasthebypassratioincreased.Thisisindicatedbythefigurebelowthatappliestoolderengines,butisstillrepresentativeofthetrendobservedforlargermodernengines.
Thenetresultisasubstantialreductioninthenoiseduetojetmixing.Atthesametime,though,thelargerfannoisebecomemoresignificantasseenfromthefigurebelow.
Computationalaerodynamicsisgettingtothepointofpredictingsucheffectsinapracticalway,butitisaverycomplexproblem,involvinginternalunsteadyflowsandpropagationestimates.
WithoutsuchCFDtools,onecanstillestimatetheeffectsofenginethrustlevels,separationdistances,andnumberofenginesbyscalingexperimentalresultsaccordingtothefundamentalphysicsoftheproblemasdescribedinthefollowingsections.
Non-propulsivenoise
Inadditiontotheenginenoise,theshearoftheboundarylayerandunsteadyvortexsheddingfromlandinggear,landinggeardoors,andotherseparatedflowsaswellasflapedgeflowscontributeasignificantpartoftheacousticenergy,especiallyforlargeaircraftonapproach.
Thefigureontherightshowsthatthesenoisesourceswerestillwellbelowtherequirement,butthefigurewasdrawnin1974.Stage3noiseregulationsnowmakeairframenoiseasignificantissue.
NoiseReduction
Withsubstantiallymorestringentnoiseregulationsandadesiretoreducecommunityenvironmentalimpa
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