-,

4,

A SURVEY

TECHCAL NOTE2701

OF THEAIRCRAFT-NOBE PROBLEMWITHSPECIAL

REFERENCETO ITS PHYSICALASPECTS

By Harvey H. Hubbard

LangleyAeronautical LalmratoryLangleyField, Va.

Washington

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TECHLIBRARYK.AFB,NM

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suMMARY. . . .

INTRODUCTION.

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EFFECTSOFNOISEAnnoyance. .

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. .Work-OutputandEfficiencyEffectsonCommmication.DeafeningEffects. . . .EffectsofIntenseNoise.

Auditoryeffects. . .Nonauditoryeffects. .

FatigueofStructures. .

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PHYSICALCHARAC!lIERISTICSOFAIRCRKFTNOISEFrequencySpectrums. . . . ... . .

SteadyforcesvaryingindistancePulsi.igflow..” .:..Turbulenceandsetmixing

DirectionalCharacteristicsOver-AllIntensityLevels

Propellers. ; . . .ReciprocatingenginesTurbojets. . . . . .Afterburnerunits. .TurbopropellerunitsRockets. . . . . . .Pulsejets . . . . .Ramjets . . . . . .Aerodynamicnoise. .

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ComparisonofNoisefromVariousPropulsiveUnits

PROTECTIONFROMNOISE. .-.. . . .ExhaustMuffling. . . . . . . .

Reciprocatingengines. . . .Jetengbes. . . . . . . . .

SpatialIsolation. . . . . . .Soundproofing. . . . . . . . .PersonalProtection.. . . . . .

CONCLUDINGREMARK S........

REFERENCES. . . . . . . . . . . .

FIGURES. . . . . . . . . . . . . .

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NATIONALADVISORYCOMMITTEEFORAERONAUTICS

mcmucm NOTE2701

A SURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL

REFERENCETO ITSPHYSICALASPECTS

By ~y H.Hubbard

Thisbriefsurveyisaimedatprovidinga Mckgroundforvariousgeneralphasesoftheairczaft-noiseproblem.MaterialhasbeendrawnfromNationalAdvisoryCommitteeforAeronauticsnoise-researchprogramsattheLangleyLaboratory,fromvisitstovariousorganizationswhichareconcernedin somewaywiththeaircraft-noiseproblem,andfromareviewoftheliterature.Theeffectsofnoisearediscussedbrieflyasa backgroundforthereaderandbriefdiscussionsofthephysicalchar-acteristicsofaircraftnoiseandsoqewaysofprotectionfromnoisearealsoincludedalongwitha limitedbibliographyofresearchworkapplyingtotheaircraft-noiseproblem.

Theseandotherrelatedstudiesindicatethatno easyandinexpen-sivesolutiontotheairc-fi-noiseproblemisavailableatpresent.Reductionsofnoiseatthesourcearepossiblein somecases,as forthepropellerandthereciprocatingengine,butonlyifa possibleperform-ancepenaltyisacceptable.Theproblemofpro@3ingadequateprotec-tionisinmanycasesexpensiveandis complicatedby theintenselow-frequencycontentofthenoisefrommostaircraft-noisesources.

INTRODUCTION

Theproblemofaircraftnoiseanditsreductionhasbeenofinterestformanyyearsbutisbecomingofgreaterconcernbecauseofthehighernoiselevelsbeinggeneratedandtheeverincreasingnumberofpeoplebeingaffected.Developmentsofmorepowertipropulsiomsystemsformilitaryandcommercialusehaveinherentlyresultedinhighernoiselevels.Thusnotonlyarepassengers,crews,andservicepersonnelofairplanesaffectedtoa greaterdegree,butsoarelargergroupsofpeopleworkingandlivingnearairportsandtestfacilities.Becauseofthewidespr@dinterestinthissubjectandbecause.ofitsimportance,theNationalAdvisoryCommitteeforAeronautics,other’governmentalagencies,andaircraftcompaniesarecontinuingtheirbasicresearchrelativetoaircraft-noiseproblemsingeneral,inan attemptto definethepresentandfuturescopeoftheproblemandto explorethepossi-bilitiesoftheirsolution.

2 WA TN 2701

Inthepresentpaperinformationofspecificapplicationto theaircraft-noiseproblemand,inaddition,informationofa generalnaturewhichhasa bearingonthisproblemhasbeencollectedforpurposesofa briefsurvey.Becauseoftheccmrpl@tyandthemanyramificationsoftheaircraft-noiseproblem,onlythemoreimportantresultsonsomeofitsphasesareincluded.Materialforthispaperhasbeendrawnfromnoise-researchprogramsattheLangleyLabomtory,fromvisitstovariousorganizationswhichareconcernedin someway.withtheaircrafi-noiseproblem,andfroma reviewoftheliterature.A.uattempthasbeenmadeto aclmowledgetheavailablesourcesofmaterialwherepossibleandthereferencesofthepresentpaperconstitutea limitedbibliographyofsignificantresearchworkwhichappliestotheaircraft-noiseproblem.Thereadermayobtainmoredetailedinformationfromthereferenceslisted,someofwhicharedigestsofmanyotherreferencesandhencearethemselvessurveysof certainphasesoftheproblem.

Thepaperfirstdealswiththeeffectsofnoiseasa backgroundforthereaderandas a basisfordiscussionoftheothermaterial.

, Nextin ofierofpresentationisa descriptionofthephysicalcharac-teristicsofaircraftnoise,’~ finallysomewaysofprotectionfromnoisearetiscussed.

EJ?FECTSOFNOISE

Theeffectsofnoiseonmanarecomplexsincebothobjectiveandsubjectivereacticmsmaybe experiencedsimultaneously.Ofthese,thelatteraremuchmoredifficultto evaluatesincetheyvarywidelyfrompersontopersonandmaybe affectedbymanyotherfactors.Thereactionofanygivenpersonmayvaryaccordingtotheactivityinwhichhe isengagedandaccomiingtohispersonalorbusinessrelationshipto thenoise-prducingagency.For”~t~ce, ifhewerea neighborhemightbelesstolerantofthenoisethanifhe wereexposedto itinthecourseofhisregularduties.Hisreactionsmightbe stilldifferentifhewerea payingpassengeronanairline. Q

Acceptablenoiselevelswillbe differentforvariousactivitiesandwillvarywiththeindividualandwiththequalityofthenoise.Althoughno attemptismadeinthispaperto establishnoisecriteriaforsubjectivephenomena,somephysiologicalandpsychologimleffectsofthenoisearedescribsd.Sincemanyinconclusiveresultsare.foundintheliteraturein regatitopsychologicalreactions,onlytheresultsofa genemlnaturefroma largenumberofthesestudiesaresummarized(ref.1).

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(Sou~a-pressurelevelsdiscussedinthispaperareexpressedindecibelsand,as is conventional,aremeasuredrelativetoa referencepressurelevelof0.0002dyne/cm2.Theso~d-pressurelevelindecibels

NACAm 2701

isthusequalto 20loglop/O.0002,where p isthesoundpressurein

dynes/cm2.As iswell-lmown,theearrqspondstoa widerangeofpres-surelevels.Forillustration,thepressureamplitudesofnormalcon-

,, versationalspeechmaybea fractionof1 dyne/cm2(X to 70 decibels);whereas,a pressureamplitudeof200dyues/cm2(120decibels)usualIlycausesdiscomfortto theobserver.

3

(Someoftherelationshipsforsound-pressurevariati&sareexpressedaspowerfunctions;theexponentforthepowerfunctionofthesoundener~ istwicethatforthesoundpressure.)

Annoyance

Theannoyancecausedbya steadynoiseisa functionofitsinten-sitylevelandfrequencyspectrum.Annoyanceincreasesastheintensityincreasesandisgenerallygreaterfornoisescontaining.thehigherfre-quenciesthanforthoseofpredominantlylowfrequency.OtherfactorsImownto increasetheannoyanceareunexpectedness,andintermittence.

WorkOutput

Shortexposuresofindividual

andEfficiency

workmentonoise

inapprcrpriateneas,

levelsashighas120decibels‘mdlong-termexposuresto lesserintensitiesdonotseemtoaffectadverselytheirperformanceofmostjobsinvolvingmentalandmuscularwork.A workmanisthusapparentlyableto adapthimselftoanoisyenvironmentforthepurposesofdoingcertainjobs,althoughsub-jectivefeelingsofannoyancearesometimesobserved.Forcertaintaskswhereconcentrationisrequired,thepresenceof somenoisemayevenbebeneficial.Itisrecogn.izeithatadverseauditoryeffectsmaybeobservedby thepersonexposed,unlessadequate‘protectionfortheearisprovided.Asidefromthesepossibleauditoryeffects,someofwhicharediscussedsubsequentlyint~s paper,theremanisaffectalphysiologicallybynoiseletels

Forjobsnecessary,asin-somecases

involvingtalkingorlisteningorinteamwork,noiseisdetrimentalmayconstitutea serioushazard.

is littleevidencethatup to 120decibels.

wherecommunicationsaretoworkingefficiencyand

EffectsonCommunication

Becauseoftheabilityofnoisetomaskspeechfrequencies,itmayinterruptalltypesof communications.Theconsequencesthusmayrangein severityfromannoyanceinthecaseofsocialconversationsto life-and-deathmattersinvitalcommn.icatiops.

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Thequalityofthenoisespectrumissignificantactionisa functionbothofintensityandfrequency.

mu TN2701

sinceitsmaskingAs an example,

themaskingactionofa 400-cpspuretonewith-an~tensi~yof7’O-de~i-belsisillustratedin figure1 as a functionoffrequency(ref.2).Theamountbywhichthehearingthresholdis raisedforeachfrequencyisa measureofthemaskingandis indicatedbythecurvein figure1.A smallamountofmaskingwasprducedat frequenciesbelowthemakingfrequency;whereasa relativelylargeamountoccurredat frequenciesabovethemasl&gfrequency.Themostdetrimentaleffectsoccurredatfrequenciesnearlycoincidentwithandslightlyabovethemaskingfre-guency.Althoughtheseresultsareforoneparticularmaskingfrequency,similsxresultshavebeenobtainedforotherfrequencies.Noisefre-quenciesbelowthespeechrangethuswillhavesomemaskingactiononspeechevenatrelativelylowintensities.,Noisefrequenciesabovethespeechrangewi33havelittleornomaskingeffects.

Ininstanceswheresueechismaskedbyhigh-levelnoise,theuseofearplugswillusuallyimprovetheintelligibility.Thisbenefitarisesbecauseofthenonlinearresponseoftheearto sofidsofvariousinten-sities.b general,high-levelspeechislessintelligiblethanlow-levelspeech.Whenearplugsareused,thesfgnal-t-noiseratioisessentiallyunchangedbutthesignalstrengthmaybe reducedtoa valuewhichmakesitmoreintelligible(ref.1).

DeafeningEffects.

At levelsaboveapproximately85 decibels,themostcommonphysi-ologicaleffectofnoiseonmanistheproductionofhearinglosses.Thenatureandamountofhearinglossproducedisa functionoftheintensityofthenoise,itsfrequency,andthedurationofexposure.Thecompleterelationsofhearinglossto frequency,intensity,anddurationarecomplexandvarysomewhatwiththeindividual.Henceonlya fewofthemoregeneralresultsinreferences1, 3, 4, and5 areincludedherein. .

A givenhearinglossmaybe incurredby exposureto an intensenoiseofshortdurationorbya lessintensenoisefora longerduration.

I Thegreatesthearinglossesaregenerallyproducedatthepredominantnoisefrequencyorina higherbandoffrequencies,orboth.An exposureto a bandoffrequenciesproducesappro~tely thesamehearinglossasa puretoneofthesameintensityandoffrequencycorrespondingtothemiddleoftheband. Figure2 illustratesthetemporaryhearinglossforan observeraftera shortexposuretoa ratherintensejet-enginenoisespectrum(ref.~). Thegreatestlossesinthisinstanceoccurredintherangeoffrequenciesmostusefulforspeechperception(500to 2000cps).

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NACATN 2701 5

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Hearinglossaswellas recoveryofhearimgusuallybeginsrapidlyandthenprogressesmoreandmoreslowlyasa functionoftime.Mostofthesedeafeningeffectsaretempomryandnormalhearingreturnsinapericiioftimerangingframa fewhoursto severaldays.Permnenthearinglossesmayoccuronlyafterrepeatedexposurebeforerecoveryis complete.

EffectsofIntenseNoise

Auditoryeffects.-Inadditionto thewell-knowndeafeningeffectsofnoise,certainothereffectsareprcxlucedintheaudiblefrequencyrange(15to 1~,000cps)at intensitiesabove120decibels.Thislevelisgenerallyre~fiedas theapproximatethresholdoffeelingandmaybe uncomfortabletotheobserver.As thenoiselevelsincreaseabovethisvalue,moreandmorediscomfortisexperienceduntilpainoccursataboutlkOdecibels.Experimentalevidenceexistswhichindicatesthatphysicaldamagetothehearingmechanismsuchaspuncturingoftheear-drummayoccurat levelsofapproximately160decibels(ref.6). Fornoisefrequenciesinthe2000to 15,000cpsrange,themaineffectsatallintensitylevelsareassociatedwiththehearingmechanism.

Nonauditoryeffects.-At frequenciesbelow2000cpsvariousnon-auditoryeffectsalsoappear(refs.5 and6). W&XIa personisexposedtonoiselevelsofapproximately150decibelsinthefrequencyrangeof700to 1500cps,he mayexperienceresonancesoftheheadbonesandcavitiesaswellasblurredvision.At audiblefrequenciesbelow700cps,shilar.sensatimsareexperiencedintheregionofthechestandstomachandvariousmusculargpoupsareaffected.Verylittleinfommtionisavailablefortheloweraudibleandsubaudiblerangesalthoughnoiseatthesefrequenciesmayalsoproducesignificantphysiologicaleffects.

Inlaboratorytests,smallfurredanimalshave%eenkilledby over-heatingduetoabsorbedsoundenergyataudibleandultrasonicfre-quencies(ref.7). Theabilityoftheanimalto absorbthisenerg isindicatedqualitativelyas a functionoffrequencyinfigure3,alongwithsimilardataforman (ref.8). As indicatdinthefigure,asthenoisefrequencyincreases,theanimalis generallyableto absorbtheenergymoreefficiently;whereasthereverseistrueform. !l%esesamegeneraltrendsarebelievedtoapplyintotheul@asonicfrequencyrange.

Resultssuchas indicatedin figure3,togetherwiththeknowledgethattheultrasonicnoisecomponentsinaircraft-noisespectrumsarerelativelylowinintensity,haveledto theconclusionthatthereareatpresentno serioushazatistomanintheultrasonicfrequencyrange.

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WA TN2701

FatigueofStructures

Intheoscillatingpressurefieldsurroundinga noisesourcetheintensitydependsonthedistancefromthesource.At somedistanceawaytheseoscillatingpressuresarerecognizedonlyasnoiseandmayprcducesomeoftheeffectsonmn thathavealreadybeendiscussed.At pointsveryclosetothesource,theypralucemoreintensenoiseandmayalsobe capableofexcitingdestructivevibrationsinnearbypartsoftheaircrafistructure.Studiesoftheseintenseoscillatingpressurefieldsandtheirassociatedvibrationsareofinterestinconnectionwiththefatigueproblem.

Manyfailuresofthesecondarystructureofwingsandfuselageshavebeenobservedinpropellerandjetaircraftduetoacceleratedfatigue.b propellerairplanes(ref.9) thesefailureshaveoccurredgenerallyinthefuselagenearthepropellerplaneofrotationandinthetrailing-edgewingstructureforpusherconfigurations.In jetair-planes,failuresofthetailpipe,fuselage,andwingsecondarystructuresnearthejetetithavebeenobserved.An increaseinclearancebetweenthenoisesourceandthestructureaffectedisusuallybeneficialinalleviatingthiscondition.Forexistingconfigurations,changesintherigidity,mass,anddampingofthestructuremaybe a satisfactorysolution.

PHYSICALCHARACTERISTICSOFAJRCRAI?I’NOISE

Thematerialinthissectionrelativetothephysicalcharacter-isticsofaircraftnoiseisapplicablespecificallytotheconditionofzeroforwardspeedunlessotherwisenoted.Informationforthestaticcase,however,mayalsobe appliedapproximatelyto conditionsoflowforwanispeed,as intake-offandclimb.Low-altitudeandstaticoper-ationsofaircraftarelmowntobringaboutmostofthenoiseproblems.Innormalflight,noisewillgenerallybe a seriousconsiderationonlyforoccupaptsoftheaircraft,andforthisconditiontheeffectsofforwardspeed,especiallyinthehigh-subsonicrangeandabove,arenotlrell-knuwn.

Characteristicsofvariousaircrati-noisesourcessuchaspropel-lers,jets,androcketsareconsidered.Frequencyspectrums,directionalcharacteristics,andintensitylevelsarepresented,andprovisionismadefora comparisonofthenoisefromsomeofthevariousunitsconsidered.

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7

FrequencySpectrums

Figures4, 5,and6 showschematicallythreegenemltypesofsourcesofaircraftnoise,namely,propellers,pulsingflows,andturbulenceandjetmixing.Thefrequencyspectrumsandassociatedwaveformsinthefigureswererecordedphotographicallyfromtheviewingscreensofa PanoramicSonicAnalyzeranda cathode-myoscillograph,respectively.

Steadyforcesvaryimzindistance.-Onebasictypeofnoiseisgeneratedbypropellersandisillustratedin figure4. Therotationalnoise,whichisthemin componentofpropellernoise,is generatedbya constantaer@namicforceonthebladewhich,asa cyclicfunctionoftime,variesindistanceto theobserver.Typicalnoisespectrumsfora propelleroperatingat subsonicandsupersonictipspeeds(ref.10)areshowninthefigurewhereintensityappearsasa functionoffre-quency.Thenoiseconsistsprimarilyofa fewfrequenciesof constantamplitudewhichareharmonicallyrelatedto theblade-passagefrequency.Itis significantthatat supersonictipspeeds(fig.4(b))-someofthehigher-orderharmonicsaremoreintensethanthefundsmentaljwiththeresultthatthecharacteristicwaveformisveryshawlypeaked,asindicatedbythetypicalpressuretime-historyrecordshownat theright-handsideofthefigure.At thesubsonictipspeeds(fig.k(a)),thespectrumgenerallycontainslessharmoniccontentandtheover-au”intensitiesarelower.Ingeneralthemoretitensenoisefrequenciesfromeithertypeofpropellerwillappearinthespectrumbelow1500Cps.

Pulsingflow.-A pulse-jetengine,whichisdesignedto operateina cyclicmanner,isa pulsing-flow-noisesource.Theexhaustgasesareexpelledperiodicallyata frequencywhichisa functionoftheengineconfigurationanditsopemtingtemperature.Noisefroman engineofthistypeisillustratedschematicallyin figure5. Thespectrumassociatedwiththistypeof sourceis shownintheleft-handsideofthefigureandis seento containonlya fewdiscretefrequencies,thefundamentalorfiringfrequencybeingthepredominantone. Thewaveformshownintheright-handsideofthefigireisassociatedwitha spectrumofthistypeandis indicativeofitslow-frequencycontent.

Anothercommonpulsing-flow-noisesourceisthereciprocating-engineetiaust.Exhaustnoisespectrumsindicatethatthemostintensenoisecomponentusuallycorrespondsto thefundamentalfiringfrequencyoftheengineandallothercomponentsareoflesserintensity.Thusthespectrumfromthistypeofpulsing-flowsourcemaybe similartothatoffigure5 exceptforsomepossiblesubharmonicwhicharebelievedtobe causedbydissimilaritiesinthemanifoldsystemandforsomeh.igh-frequencynoiseofanaerodynamicorigin.

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8 WA TN 2701

Eventhoughtheresultsinfigure~ applydirectlyto thepulse-jetengine,similardiscretefrequencyspectrumshavebeenobtainedforramjetsandfora turbojetwithafterburner.Thusthenoisegeneratedbypulsing-flowphenomenaisbelievedtobe,closelyrelatedtothatgener-atedby roughburningortopossibleresonanceswhichmayoccurduringoperationof continuous-typeengines,ortoboth.Verylittleinfor-mationisavailableconcerningthesephenomenasincetheyareavoidedwheneverpossiblebecauseofintensenoiseandvibration.

Turbulenceand.Iet-n g.-Thenetinoisesourcetobe consideredisthemixingregionofa jetissuingfroma chamberintotheatmosphereas shownschematicallyfi figure6,alongwitha typicalspectrumandwaveform.Themechanismofthistypeofnoisegenerationisnotwell-understocdbutexperhentshaveshownthata smoothflowingjetofairissuingintotheatmosphereisan intensenoisesource.Therobingregionofthejetisof interestbecauseitisoneofthemainsourcesofnoisefromtheturbojetengine.Thenoiseisapparentlyassociatedwithturbu-lenceanda typicalspectrumcontainsnearlyallfrequenciesfromthesubaudibleto theultrasonicr-e (ref.Xl.).Sincesomanyfrequenciesarepresentand.inrandomphase,theresultingover-allsignalpresentsa hashypictureas a functionofthe ontheviefigscreenofa cathcde-rayoscillograph.Thisresultisinmrked contrasttothesteadychar-acteristicwaveformofthepropellerin figure4. Thedistributionofenergyinthesoundspectrumofjetshasbeenfoundto be a functionofthejetsize;however,forjetenginesin currentusethepeakintensi-tieswillprobablyoccurat frequenciesnear1500cpsorbelow.

DirectionalChamcteristics

Theradiationpatternsoftheover-allnoisefromsomepropulsivedevicesarehighlydirectionalinnatureandthusmayaffecttheground-handlingofaircraftandeventheirdesignandoperation.An indicationof someofthesedirectionalcharacteristicsisgivenin figure7,whichisa polardiagramshowingtherelativeover-allpressureamplitudeasafunctionofazimuthanglefromthethrustatis(0°in front)fora turbo-jetengine,a propeller(ref.10),anda reciprocating-engineexhaust(ref.12). Propellernoiseisa nmdmumnesrtheplaneofthepropellerwherethedistancevariationbetweentheobserverandthepropellerbladeisgreatest.Jetnoiseisa msxhumtotherearoftheorificenearthejetboundary(seeref.13),andtheazimuthangleatwhichthemaximumoccursisdependentinpartonthesound-propagationvelocityofthejetmedium.As thesound-propagationvelocityincreases,an apparentrefrac.tioneffectcausesthemaxhumvaluetomoveoutwardfromthejetboundary.Thereisno experimentalevidenceto indicatethatthemaximumvalueswilloccuratazimuthangles,asdefinedinfigure7,appreciablylessthan135°.Measurementsoftheover-allnoiseforpulsing-flowsources

..-

NACATN 2701 9

indicatethattheradiationpatternsareonlyslightlyUrectional,asindicatedby thedataforthereciprocatingenginein figure7.

.Variationsinthefrequencycontentofthenoiseaswellas its

intensitylevelmy occuras’afunctionoftheobserversazimuthangle.Forjetsa greaterproporticmoflow-’fiequencynoiseis observednearthejetboundary;whereasatpositionsperpendicularto thejetaxisthehigherfrequenciesbecomerelativelymoreintense.Propeller-noisefrequenciesarea functionoftherotationalspeedoftheprope~erexceptatazimthanglesverycloseto thethrustads wherethenoiseassociatedwithsheddingofvorticesfromthebladesismostintense.Thisvortexnoiseis randominnatureandisusuallyofhigherfrequencythantherotationalcomponent.Forpropellersoperatingat lowtipspeeds,as inreferences14 and15, thevortex-noisecomponentmy be arelativelylargepartofthetotalnoise.Sinceitsintensityincreasesata slowerrateasa functionoftipMachnumberthandoestherota-tionalcomponent,it isa relativelyunhportantpartoftheover-allnoiseathightipMachnumbers.

Propellers.-Foranoisemaybe a functionandthebladegeometry.intensityasa function

Over-AllJntensityLevels

givendiskpowerloadingthebver-allpropellerofthetipMach.number,thenumberofblades,Figure8 shows’therelativeover-allsoundoftipMachnumberfora ,two-anda six-blade

propellerforconstantpowerinput(refs.9,10,andlkto 19). Itcanbe seenthata decreaseinthesoundintensitycanbeachievedbyusingmorepropellerblades;however,thedecreaseobtainableis greateratthelowertipspeedsthanat thehigherone-s.ForbothpropellersthetipMachnumberisa significantpammeterinthesubsonicz%mge;whereas,inthesupersonicrange,thesoundintensityisessentiallyindependentofthetipMachnumber.At subsonictipMachnumbers,largesoundreductionscanbe obtainedby increasingthenumberofbladesandreducingthetipMachnumber.At supersonictipMachnumbersa relativelysmall.benefitwouldbe obtainedfromthistechnique.Somepreliminarytestshaveshownthatbladeplanformwhichapparentlyisnotsignificantat subsonictipMachnumbersmaybea significant-parameterinthesupersonicrange.Thewiderbladesmaycausesomereductiminintensities,particularlyforthehigherharmonics.

Ithasbeendemonstratedthatquietpropellers.aretechnicallyfeasi-bleforsmallpersonal-owner-typeairplanesby thetestsofreferences14and15. It isbelievedthatthesameprinciplesmaybe applklto thequietingofpropellersoftransport-typeairplanesalthoughlargedesignanddevelopmentproblemsandcostswouldprobablybe encountered.Someofthefactorswhichmustbe consideredinthedesignof quietpropellersareoutlinedinreference20.

\.— — -.

10

Reciprocatingengines.- Thewin sourcecatingengineistheexhaust.Theintensity

WA TN 2701

ofnoisefromtherecipro-variesas a functionofthe

typeof&ifold systemusedandin someinstancesmaybe ofthesameorderofmagnitudeasthepropellernoise.Theexhaustnoiseincreasesinamplitudeata slowerrateasa functionofenginerotationalspeedthandoesthepropellernoise(ref.21). Thelatterthenisusuallymostintenseinpresent-dayaircraftfortake-off,climb,andcruisecondi-tions(ref.4),althoughin someinstancesthe.exhaustnoisemaybe moreobjectionable.Whensomeprovisionhasbeenmadeto quietthepropeller,as in references14and15,theexhaustnoisemustalsobe reducedinofierto achieveeffectiveover-allnoisereduction.If stubexhaustsareusedforreascnsofperformance,theexhaustnoiselevelsmaybe somewhathigherthanifthecollector-ringtypeofmanifoldisused.

!l!urbo.iets.-Ikomcurrenttestsithasbeenfoundthatturbojetenginesareverycloselyrelatedto simpleairjetsin regamitotheirnoisegeneration.Itwasalsofoundthatjetnoiseincreasedininten-sityas thejetvelocityandetites densityandturbulenceincreased.Thepredominantpa~ter affectingthenoiseintensityisthejetexit -velocityandis shownin figure9,wheresoundpressureisplottedasafunctionof jetetitgasvelocity.Thesoundpressureisthusseentobe a powerfunctionofthevelocity,withtheexponentofthepowerbeingslightlylargerthan3.0. Thusifthevelocitywasvariedby a factorof 2,thesoundpressureswouldbevariedby a factorofapproximately10.Thisfigureindicatesthata trendtowazd.higherengineoperatingtemper-atureswiththeirhigherassociatedjetvelocitieswillmeancorrespond-inglyhighernoiselevels.

Afterburnerumits.-Figure10,whereover-allnoiselevelsareshownas a functionoftheobserversazimthangle,showsa comparisonofthenoisegene~tedby a turbojetenginewithandwithoutafterburner.Bothsetsofdatawererecordedatthesamedistancefromthesourceandareadjustedto thesamethrustratingforcomparison.Thedirectionalchar-acteristicsareseentobe similarineachcase,butthenoiselevelsassociatedwithafterburneropemtionarehigheratallazimuthangles.Thisincreaseinthenoiselevelisdueinpartto thehigherexitgasvelocitiesoftheafterburnerunit. Theresultsin figure10areforoneparticularengine,andsincethedatamaybe affectedbythedegreeofroughburning)theseresfitsmaYnotbe characteristicofafterburnersin general.

,

TurboPropellerunits.-Noisemeasurementsarenotavailableforturboprupellerunits;h~,ver,someestimatesofthenoiselevelsfortwodiffemt unitshavebeenmadeandtheresultsareshowninfigure11.llrbasitylevelsareplottedas a functionofazimuthangleforcondi-tionsof~00 poundsofthrustanda distanceof300feet.Forthepur-posesofthiscomparisontheassumptionhasbeenmadethatthepropeller

— — .—

NACATN 2701 U

provides90 percentofthethrustandthatthejetprovides10percentofthethrustwhileoperatingat a tailpipetemperatmofapproxi-matelylhOO F abs.

Thesubsonic-turbopropeller(tipMachnumberof0.9) curvehasbeenestimatedfromthetwodashedcurvesshowninthefigurewhichshowthecontributiontotheover-allnoiseofthejete~austandthepro-peller.At thegivenconditions,thepropelleristhemaincontributor,althoughat someazimuthanglesthejet-exhaustnoisemaybe ofthesameorderofmagnitude.Inthecaseofa supersonic-typeturbopropeller,theprop@leris clearlythedominantnoisesourceatallazimthangles.

Rockets.-In general,thenoisegeneratedby rocketenginesappearstobeverysimilartothenoisefromturbojetsinregardto frequencyspectrumsandradiationpatterns.Figure12 showstheover-allnoiselevelsgeneratedbytwotypesofpolid-fuelrocketenginesasa -functionoftheazimuthangle.Alldatahavebeenadjustedtoa distanceof300feetanda thrustratingof5000pounds.Thecurveforsmoothburningrepresentsdatafora thrust-augmentationtypeofrocketenghejwhereasthecurveforroughburning~s obtainedfromtestsofa largerengineusedtopropelmissiles.Thedifferencesinlevelsforthesetwocurvesmayrepresentthedifferencebetweensmoothandroughburning forthistypeofengine.

Pulse.flets.-Thepulse-jetengineisa prolificgeneratoroflow-frequencynoise.Thespectnm,as shownin figure5, consistsprimarilyofa fewdiscretefrequenciesofwhichthefwdamentalorfiringfre.quencyisthemostintense.Measurementsforan engineratedat90poundsofthiustindicatedthattheradiationpatternwasonlyslightlydirec-tionalintheregionto therearoftheengineandtheintensitylevelsata distanceof10 feetweregreaterthan140decibels.

=!!da!l”-Datafora smallsubsonicram-jetunitofthetypeusedtopropelhelicopterbladesindicatdthatthequalityofthenoisewasverysimilartothatshowninfigure5 inthatseveraldiscretefrequencycomponentswerepresentinadditionto thecharacteristic~ndomnoiseofcontinuous-typejets.Measurementsonlargerengtieshavegivens~larresults,andthistypeof spectrumis believedtobe generallyassociatedwithram-jetengines.Testsshowedthenoiseinthefrequency=nge ofO to ~ cpstobe sharplydirectional,withthemaximumnearthejetaxisto therearoftheengine.NoiseinthefrequencyrangeofhOto15,000cpsseemedtobe onlyslightlydirectional,withthemaxhxnalsonearthe3et-s.

Aerodynamicnoise.-Aerc@nami.cnoiseisgeneratedintheboundarylayeroftheairplaneas itmovesthroughtheair. It isassociatedwith

, turbulenceinstancea

andhasa spectrumsimilartothatin figure6. In onenoiselevelofappro-tely 130decibelswasrecordedinthe

.— .—.——— .— — —

12 NACATN 2701

cockpitofa fighterairplaneatan indicatedairspeedof500mph;hencethistypeofnoisemaypredominateinthecockpitandpassengercompart-mentsof somejetaircraft.Thisnoisewillprobablynotbe ofconcezmto observersoutsidetheairplanesinceitisan importantconsiderationonlyathighflightspeeds.

Dataontheaerciiynamicnoiseinthepassengercompartmentofalargegliderareshownin figure13wheresoundpressuresareplottedasa functionofairvelocity(ref.22). Thesoundpressuresareseentobea powerfunctionoftheairvelocitywiththeexponentbeingapproxi-mately2.3. DataobtainedinvarioustypesofaircraftbyRogersandotherinvesti@torsindicatethatthisexponentappliesapproximatelythroughoutthesubsticflightrangefora varietyofaerodynamicshapes.Theintensitylevelmaybehtgherforpooraerodynamicshapesandwillvaryinaccordancewiththesound-transmissioncharacteristicsofthefuselagewall. Itwasalsonotedthattheener~ inthespectrumappar-entlytendsto shiftto thehigherfrequenciesastheairvelocityincreases.ForvelocitiesinexcessofhOO”mphthepeakfrequencies Iwillprobablyoccurinthefrequency~ge of1200to 2400cpsorhigher.

Conventionalmethalsof soundproofingwillprobablybeadequateforprotectionfromthistypeofnoisesincethebulkofthesoundenergy,atthehigherflightspeeds,appearstobe inthefrequencyrangewlieresoundproofingiseffective.It isparticularlyimportanttohaveadequatesealsaroundwhims, doors,canopies,andsoforth,sincefaultsofthistypewhichallowairflowintotheairplanemaymarkedlyincreasethenoiselevel.Pressurizedcabinsareespeciallyeffectiveinminimiz~gaerodynamicnoisesinceallleaksareeffectivelysealed.

ComparisonofNoisefromVariousPropulsiveUnits

It is ofinterestto comparethemaximLuunoiselevelsfromvariouspropulsivedevices.Forthispurposefigure14hasbeenpreparedtoincludethoseunitsforwhichdataareavailable.Allvalueshavebeenadjustedto correspondto a distanceof300feetanda thrustratingof5000pounds.

Thebestestimateobtainableoftheexhaustnoiselevelforareciprocatingengineis considerablylowerthanthepropellernoise;thusa reductionoftheexhaustnoisewithoutalsoreducingpropellernoisewillresultina relativelysmallnoisereduction.If substantialreductionsaretobemadeinthenoisefrompresent-daypropeller-drivenaircraft,boththepropellerandexhaustnoisemustbe reduced.

Itisapparentthatthenoiselevelsassociatedwithsuchhigh-performnceunitsasthesupersonic-t~epropeller,theafterburner,and

.

WA TN 2701.

therocketenginearehigherthanforourConsequentlytheirusewillaggravatethewheretherearelow-fl$ingairplanesand

PROTECTIONFR(2d

13

present-daypropulsivedevices.noiseprobleminneighborhoodsground-testingofengines.

NOISE

Thefactthatsomeaircraftnoiselevelsaresohighindicatesthatsomeformofprotectionshouldbeprovidedforpersonswhoareexposedtothenoiseinthecourseoftheirduties.Protectionisparticularlynecessaryforthosewhoarerequiredtoworkcloseto thenoisesourcewherethelevelsmaybe sufficientlyintensetoaffectthemphysiologically.

Themostdesizablemethodofameliorationisto reducethenoiseitselftoanacceptablelevelat thesource;however,becauseofper-formanceconsiderations,orthehighnoisel$velsinherentinaircraftpropulsionsystems,orboth,thiscryditionisdifficultto realize.Thepresentsectionbrieflydescribesmufflingof jetandreciprocatingenginesanddiscussesothermeansofprotectioninthecaseswherereductionofnoiseat thesourceisnotfeasible.

ExhaustMuffling

Reciprocatingengines.-Foranygivenreciprocatingenginetheexhaustmufflercanbe usedas a meansofreducingtheexhaustnoise.Mufflersareusuallydesignedfora particulartypeofenginesincesuchvariablesas enginefiringfrequency,volumeofgasflow,andthedesiredattenuationChaticteristicsareimportantfactorsinthedesign(refs.12.md23).

Foreffectivemufflingtheuseofa collectorringisgene~llyadvantageoussinceallcylindershavea commonexhaustexit.Thiscon-figurationwillaccomplishsomenoisereductioninitselfandwillall.uwtheuseofonemufflerperengine.Sincetheenginebackpressureshouldbekeptata minimumthemufflerswhichallowa straight-throughpassageoftheekhaustgasesaredesirableforaircraft.Thesemufflersareknownas theresonant-chambertypeandtheiracousticpropertiesaredependentinpartonthechambervolumesinvolved.Ingeneralthistypeofmufflerrequireslargerchambervolumestoattenuatethelowerfre-.quencies;henceitisadvantageousfrcmthestandpointofmufflingfortheenginefiringfrequenciestobe ashighaspossible.

Figure1S,inwhichintensityisplottedasa functionoffrequency,showsthecompositionoftheexhaustspectrumfroma 190-horsepowerreciprocatingenginebothbeforeandaftermuffling.Theverysimple

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14 NK!ATN 2701

mufflerusedforthesetestsisalsoshownschematicallyinthefigureanditseffectivenessisindicatedasthespacebetweenthecurves.Judgingfromthequalityofthespectrum,thetaskofreducingtheover-aunoiseisapparentlyoneofreducingthelow-frequencycomponentssincetheyareof greateststrength.Somediffennceofopinionexistsastowhichfre-quenciesneedthegreatestreductionsincesomeobserversbelievethatthe

.,

higheronesaremoreobjectionable.Thisparticularmufflerdesigniscapableofprovidingsomenoisereductiontluroughoutthespectrumandpro-videsan over-allnoisereductioninthiscaseofapproximately8 decibels.

ThebasicprinciplesofmufflerdesignforsmallreciprocatingenginesarefairlywellunderstoodandthoseWhichapplyspecificallyto smallair-craftenginesareindicatedinreferences12 and23. Suchfactorsashighgasvelocitiesandhighsoundpressuresmaycausethemufflerperformanceto deviatesomewhatfromthatpredictedby thetheorywhichisbasedontheassumptionof smalldisturbances.Forlargerenginesmanyofthebasic”principlesmayapply,butanoptimumdesignwithregardtonoisereduction,weight,andsafetywouldresultonlyfromadditionalresearchanddevelop-mentwork.

Jetendnes.-Sincejetenginesareprolificnoisegene=tors,thereismuchinterestineffectivemufflikgtechniques.Consequently,alargenumberofgovernmentalagenciesandaircraftccmqyudes,workingmoreorlessindependently,havedevisedsatisfactorymufflerswhich,althoughdifferingin constructionandoperation,relyonthesamebasicprinciplestoaccomplishmuffling.Thesemufflersarebeingusedfor~ound-testingofenginesmountedn testcellsandforgroundrun-upsofproductionmdel airplanes<refs.24and25). In contrasttothereciprocating-enginemuffler,thesedesignsareverylargeandmassiveandtodatenoneis’availableto reducejetnoiseinflight.Thelargesizeofthesemufflers,as indicatedin figure16,resultspartlyfromthelargevolumeofexhaustgasesinvolvedandpartlyfromthepresenceofintenselow-frequencynoisecomponentswhichnecessitatetheuseoflargeresonantchambersinadditiontothemoreconventionalsound-absorbingmterials.Theoutsidewallsaremssiveb ordertominimizethetransmissionofnoisegeneratedby thehighvelocitiesandturbulenceinsidethemuffler.

Oneofthebasicrequirementsof jetmufflingisthatthejetexhaustfirstbe cooledandthusreducedinvelocitywithoutbuildingupexcessivebackpressuresontheenginebeingtested.Thee-ust gasesarethusenclosedina compartmentinwhichcoolingprocesses,suchastheadditionof secondaryairor cool+gwatersprays,orboth,maybeapplied.FromthecoolingchambertheexhaustgasesenterthemuffleratvelocitiesoftheorderofhOOft/sec.b geneml,satisfactorymufflingresultshavebeenobtainedwhenthejetvelocitywasreducedtoa valueofappro-tely 200ft/secorlessatthemufflerexit.

Theprinciplesof jet-e~ustmufflingarealsoapplicabletotheairinletsofjetenginesandtowindtunnelswhichhaveheat-exchange

3P WA m 2701 15

towersor openingsto theatmosphere.Intheselattera~”licationstheactualnoisereductiawouldbe accomplishedinthesamemsmneras injet-exhaustmufflingbut,therewouldbe noproblemof coolingtheexhaustgases(ref.26).

SpatialIsolation

S@ce relieffrm noiseis obtainablebymerelyincreasingthedistancebetweentheobserverandthenoisesource,itis’ofinteresttoevaluatethiseffectofdistanceonthenoiseframthevariousair-czafi-noisesources.Noisereductionasa functionofdistance,in quiescentair,is shownin figure17 forfrequenciesof1000,3000,and10,000cps(ref.27). Thesolidlinerepresentsthenoisereductiondueto thenormalspreatigofa soundwaveaccordingtotheinverse-squarelaw.Forfrequenciesoftheorderof1000cpsorbelow,verylittleadditionalnoisereductionduetoatmosphericeffectsoccurs,butathigherfre-quenciestheatmosphericlossesmaybe quitelarge.Sincemostaircraft-noisespectrumshaverelativelylargecomponentsinthefrequencyrangebelowlWO cps,thesolidliqe.infi&re17willessentiallydescribetheintensityasa functionofdistanceexceptforverylargedistances.Differentresultos,particularlyat thelargerdistances,maybe obtainedwhereturbulence,windgmtients,andtemperatureinversionsarepresent.

Thedata offigure17 are ror conditionsofsoundpropa~tionforclearareassuchas overanairportrunwayandfromanairplaneflyingoverhead.Inthecaseofterrainwithobstructionssuchas grass,shrubbery,trees,’andsoforth,theavemgeattenuationis somewhatgreaterthanindicatedinthefi~e.

Soundproofing

In instanceswheregroupsofpeoplearerequiredtobe nearnoisesourcesforlongperialsoftime,soundproofingisgenerallyused.asameansofprotection.Ingeneral,thisinvolvestheuseofa structureto isolatean observerfroma noisesource.TwophysicalphenomenainvolvedM soundproofingarethesound-transmissionandthesound-absorptionqualitiesofthestructure.Londonjin reference28$relatesthesetwophenomenato thereductioninintensityas a noisesignalpassesthroughthewallsofan enclosureby thefollowingexpression:

Noisereduction= 10 loglop +%)

where a istheabsorptioncoefficient,T isthetransmissioncoef-ficient,andthenoisereductionis givenindecibels.(Theabsorptioncoefficientofu materialisdefinedas theratioofthesoundener~

whichisabsortidby thematerialtothetotalener~whichfallsuponit. Similarly,thetmnsmissioncoefficientofa givenpanelistheratioofthesoundenerg transmittedbythepanelto thetotalenergywhichimpingesonit.)

Valuesof a and T mayvarybetweenO and1.0,dependingonthenoise’frequencyandthetypesandamountsofmaterialsusedinthestructure.Twogeneralconclusionsmaybemadefroma studyofLondontsequation:(1)No noisereductioncanbe obtainedunlessthereis someabsorption(a> O)and(2)thevaluesof T mustbe small.in ofiertorealizelargenoisereductions.Dataarepresentedinreferences28and29 whichindicatethatnominalvaluesoftheabsorptioncoefficient(cc= 0.20to O.SO)areeasilyobtainablefora varietyof soundtreat-mentsintherangeof speechfrequenciesat least.Inaccomplishinglargenoisereductionswheresmallvaluesof T smdat leastnominalvaluesof a areneeded,thetrmismissionpropertiesofthestructuremaythereforebe ofprimaryimportance.

Figure18 showsthetheoretical.transmissionlossasa functionoffrequencyforsoundpassingthloughthreehomogeneouspanelsdifferinginsurfacedensity.Thevalueof1 lb/ft2maybe consideredrepresenta-tiveofairplane-fuselageconstruction;whereasthevalueof100lb/ft2ismoreoftheotieroftest-cellconstruction.Althoughsomevariationsinthedataexkt, thetrendsinfigure18havebeenverifiedby experi-ment(ref.30). ~ generalthelargerlossesareseento occuratthehigherfrequenciesandrelativelysmll lossesat thelowerfrequencies.In ofierto increasethetransmissionlossesat thelowerfrequencies,theweightofthestructuremustbe increased.Thevaluesin figure18correspondto cotitionsofperfectabso~tion(a= 1.0)andhenceare~=uy mrgerthanwouldbe obtainedinpractice.

Figure19 showstherelativeamountsofnoisereductionobtainableatvariousfrequenciesby theadditionofabsorbingmaterialsuchasgbss WOO1,trimcloth,=rpeting,andsoforth,toan.airplanefuselageccnnparedwiththatobtainedwiththebarefuselage(ref.31). Fortherangeoffrequenciesof speechfairlylargenoisereductionsareobtaina-bleby theuseofrelativelylightweightabsorbingmaterials.Forthelowerfrequencies,however,thisconventionalmethodofairplanesoundtreatmentprovidesrathersrmllamountsofnoisereduction.

~ geneml, figures18 and19 indicatethatnoisereductionsaremuchmoreeasilyobtainedatthehigherfrequenciesthanat thelowerones.WS findingis significantsinceinfigures4, 5,and6 aircraft-noisespectrumsareshownto containrelativelyintenselow-frequencycomponents.Substantialreductionsat theverylowfrequenciesmayrequiremassivestructuresortheuseof specialtechniques,orboth. Ofspecialinterestinthisregafiistheuseofsomeuniquemethcdsinthesoundtreatmentofa )_argewindtunnel(ref.26).

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mcA m 2701 17

PersonalProtection

Themostwidelyusedtypeofpersonalprotectionforpersonsworkingclosetothenoisesourceistheearplug.This.devicein combi-nationwiththeearmuffprovidedinthestindafiAirForcetypehelmetprovidessubstantialnoisereductionovera wide=ge offrequencies.Theeffectiveness,separatelyandin”combination,oftypicalearplugsandhelmets.underoptimumconditionsisshownin figure20,inwhichthenoiseattenuationisplottedasa functionoffrequency(ref.32). Theattenuationavailablefora combhationnayvaryfromappro~tely30to80decibels,dependingonthenoisefrequency,andforthespeechrangeaveragesappro~tely 50decibels.

Thedifferenceinlevelbetweenairconductionandboneconductionofsoundisapproxhnately50 decibels.Equipmentdesignedforprotectionagainstboneconductionwouldbeverycumbersomeandhenceoflimiteduse. Theprotectionprovidedbya well-fittedearplugandhelmetcombi-nationmaythereforerepresentthep=cticallimitofprotectionfortheear. Thiscombinationtillprovideadequateprotectionforrelativelyshortexposms to levelsuptoappro-tely 145decibelsorforlong-termexposurestonominalintensitylevels.Intensitylevelsofthisofierofmagnitudeareuncomfortableto theobsenerbecauseofeffectsonotherpartsofthebcxtyunlesssomeprotectionisprortded.Sincepersonalprotectionfromtheintenselowfrequencieswhicharefeltbythebodyas a wholewouldalsobe toocumbersome,soundproofingorspatialisolationmaybe thebestsolution.

COI?CLUDINGREMARKS

Thisbriefsurveyhasbeenaimedatprovidinga backgroundfortheunderstandingofinterrelatednoisequestions’.Someoftheeffectsofnoisehavebeenpointedoutbrieflyasbackgroundinformationforthereader,andsomephysicalcharacteristicsofairc=finoiseaswellassomemeansofprotection@om noisehavealsobe’enbrieflydiscussed.

Theseandotherrelatedstudiesindicatethatno easyandinex-pensivesolutiontotheaircraft-noiseproblemisavailableatpresent.-Reductionsofnoiseat thesourcearepossiblein somecases,as for ‘tiepropellerandthereciprocatingengine,butonlyifa possibleperformancepenaltyisacceptable.Theproblemofprovidingadequate

— —...— —- . .—.- —z . —— .————— -———-

protectionisinmanycasesexpensiveandis complicatedbytheintenselow-frequencycontentofthenoisefrommostaircraft-noisesources.

LangleyAeronauticalLaboratoryNationalAdvisoryCommitteeforAeronautics

LangleyField,Vs.,March12,1952

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NACA~ 2701 19

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Theodorsen,Theodore,=d Regier,ArthurA.: TheProblemofNoiseReductionwithReferencetoLightAirplanes.NACATN 11.45,1946.

RegierjArthurA.,andHubbard,HarveyH.: FactorsAffectingtheDesignofQuietpropellers.NACARM L7H05,1947.

Rudmose,H.Wayne,andBeranek,LeoL.: NoiseReductioninAircraft.Jour.Aero.Sci.,vol.14,no.2,Feb.1947,pp.79-96.

Rogers,O.R.: NoiseLevelandItsVariationwithPositionandAirSpeedintheXCG-4GliderNo.29618.MRNO.12Q-M-51/vF2(Addendum13),MaterielCommndjArmyAirForces,June23,1942.

Czarnecki,K.R.,andDavis,DonD.,Jr.: Dynamometer-StandInvesti-gationoftheMufflerUsedintheDemonstrationofLight-AirplaneNoiseReduction.NACATN1688,1948.

GrayjGordon:Quiet,Please!~line, NorthAmericanAviation,Inc.,VO1.9,no.3,Aug.1951,pp.17-19.

Anon.:TheRoarof OurAirPower.TheLockheedStory,LockheedAircraftCorp.,Aug.~, 1950.

—- .—— ————.

rwcAm 2701 21

26.

27.

28.

29.

30.

31.

32.

.

.

Anon.:EvaluationofAcousticalTreatmentfor8 x 6Ft SupersonicWindTunnelattheLewisFlightPropulsionLaboratory,NACA,Clevelsnd(Rep.5). Rep.No.69,Bolt,Beranek,andNewman,Con-sultantsinAcoustics,Aug.1,1951.

Regier,ArthurA.: EffectofDistanceonAirplaneNoise.NACAm1353,1947.

London,Albert:Principles,Fractice,andProgressofNoiseReduc-tioninAirplanes.NACATN 748,1940.

Knudsen,VernO.,and~iS, CyrilM.: AcousticalDeSi@llginArchitecture.JohnWiley&Sons,Inc.,1950.

Nichols,R. H.,Jr.,Sleeper,H.P.,Jr.,WallaCe,R. L.,Jr.,andEricson,H.L.: Acoustical.MaterialsandAcousticalTreatmentsforAircraft.Jour.AcOUS.*C. Am.,vol.19,no.3, May1947.

JaclmanjKennethR.: A ReviewofAdvsncesinSilencingAircraft.preprintofpaperpresentedatA.S.M.E.Semi-AnnualConventionatChicsgo,Ill.jJune16,1947.

Eldredge,IbnaldH.,Jr.,sndParrack,HoraceO.: SoundProblemsh theAirForce.U. S.ArmedForcesMedicalJour.,vol.I,no.4,Apr.1~0, pp.449-461.

—_——_—— ..—— - -...—. —.--.——- —.———

---

“.

300 YitJ 10,OOO

Wmkadfr~anoy,-. .. --—

Figure1.-Threshold Aift aa a function of frequency for a &Q-cpatona of 70 decibek3. (Data obtained frommf. 2.)

I

,

,

MM 3M 1000 lo,ow 30,000

. ~, m

Figure 2.- Temporary hearing 10SE as a function of frequency for anunprotected observer following a 10-minute exposure to ~et-enginenoise of 1~ decibels. (Data obtained from ref. 5.)

“o lax 2000 3otnl moo

F’requsmy,Ops -w

ItLgure3.-Abso@im cmfficients as a function of frequency for afurred animl as.c~rea to those Of mn. (Data obtalmi fromrefS. 7 and8.)

NACATN 2701

Intensity

Intensity

.

III\

(a)Subsonictipspeed.

Frequency,.kc

.Figure

(b)Supersonictip

4.-Noisegenerated

speed.

bypropellers.

—.-.— .—. .— _. ——. ——

Intensity

‘*1 i. o

Frequenoy, kc

IL!J!5.,..

.0

Figure 5.- Noiae generated by a pulsing flow.

II

,

I

~-----”-------- _

---- <--- ---- ----------—— -_ -- -a ----- -- —---- ----, -- ‘----- ----— -

-—----— -- —---- —— -..---- - ,-d~--------- __ -- —__ ,

------ ----- —___ ------- ------

----

1 , ?

I

i

Frequency, kc

Figure6.-l?oisegeneratedby Jetinlxingend~urlulence.

Relative pressure amplitude

o ●2 .4 .6 .8 1.0I/ . 1, 1 1 I

*mrust w

/\

\

/

Reciprocating-engtieexhaust (ref. 10)

/

TPropeller at supersonictip speeds (ref. 12)

/ / I \ \ =!s

ao 7P 90° 10Y 120° 139

#server’s azimuth angle

Figure 7.-Directional characteristicsof thedifferent types of aircraft-noise sources.to equsl maximum values for comparison.)

noise generated by three(Data have &en .ai@stea

180°

L6f.

19°

NACATN 2701 29

.

.

.

●

80

70

60●

TipMachmmher

Figure8.- Effects of tip Machnuniberandnumberof blades on the noiseproducedby single-rotating propellers at constantpowerinput.

— ——.- — —. —>.,,

30 mm m 2701

312

106

100

94

88

82

76400 moo , 2000

Exitvelocity,ft/sec

‘f

.

Figure9.-Turbojet-enginenoiseasa functionoftheexitgasvelocity.(Datacorrespondto a distanceof 300feetatanangleof900rela.tivetothejetaxis.)

— — —. . . —-

5P NACATN2701 31

Uo

2.35

130

HO

105

.

.

/’ ~,/

I \

J \

/ \///

\

//

\

/ \tWithafterburner

/

//

./ /

—“

Withoutafterburner/ \

/~

o 30 60 90 120 Uo MO

Azimuthangle,deg

Figure10.- Comparisonoftheover-allintenaity levelsatvariousazimuthanglesfora turbojetenginewithandwithoutafterburner.(Datacorrespondto conditionsof5000poundsofthrustanda itl.s-tanceof 300feet.)

. . .--—_____ ___ —— __ ..—-—

32 WA TN2701

w

lx ,/ / .—.-. ~&ti&*/

/’ \/ ,\

U!o /’ \/ ‘\,

I,

\

/’ -.\U5 / y \

//\ \

Pro@er-Af/

\A

,/’ $

no Subsonic-t~+ turboprlqmller

a/ \

(Propellerand i\

jetexhauet) \

i /t !\

I \ \g lG

r \\

!

I

I

I

/\/95 / \ \ _ / ‘

//

/’m ,/‘

//

85

T800 30 60 90 120 m U30

Admnthangle,deg

Figure11.- Estimatedintensitylevelsasa functionof azimuthanglefortwoturbopropellerunits.(Datahavebeenadjustedto corre-spondto5000pountiofthrustanda distanceof300feet.)

--— ..— —

WA TN2701 33.

.

.

.Figure3.2.-Intensitylevelasa functionofazimuthanglefortwo

solid-fuelrocketengines.(Datahavebeenadjustedtocorrespondto5000poundsofthrustanda distanceof 300feet.)

.—— .—

34

lf?c

la

lot

20:

96

9C

I) 6

I I I

G3idEu’

100

Velocity,ft/sec

200

Figure13.- Aercdynamicnoise ti a large glider as(Dataobtainai frm ref. 22.)

400

a functionofairspeed.

——— .—.——.— .

.

Reoiprmating engine(25Cn)hp)

Propeller

(TIP Maoh number of O.9)

(sq&%n”&pe)I

I I

TurbojetI I

Rough-burning turbojetuitb afterburner

I I 1Rou@-burnlw

rmket1 I I I 1 A

100 no 120 1.30 Uo 150

MdmmI intensity,decibels

Figure 14.- Comparison of the meximm over-all intensity levels generatedby varioua propulsive units. (Data correapmd approximately to athrust of 5WQ pwnda and a distance of 300 feet.)

5’0

80

50

30 100 300

Requency, opa

Figure 15.- Exhauat noise spectrums for a small&fore and after muffling.

icm

reciprocatingengine

[

.

I,.;t.4/

Cooq chamber

,-\ 1 r Exhaust eXit

:

—-_ —-_1 -’”’—--——_ .._

— . .ID 1

1. J

Figure 16.- Maxhu 6ilencer used by the Lockheed Aircraft Corp. forexhaust-noise reduction in the ground-te6ti~ of Jet aircraft. *(Photogaph is reproduced by courtesy of the Maxim Silencer Co, L-74372ofHartford,Corm.)

3’

I

I,

I

wl-+k

/// // #la

/

/ ,// ,/

/ //”/’,@/

o

I&cm W/“)

///

3000Cpe /’/

/

//

//

/

//

//

/“/ limop

/ .-”

/ ‘/

0

/

/

/’///#/// h-wers~square law

(Resmre x D!.Starme = 00nstant)

3CQ low 30Ca 1o,ooo

Metmoe, ft

Figure 17.- Noise reduction as a function of distance from the sourcefor three different frequencies. (Data obtained from ref. 27.)

%

,

I

I

Hgme 18.- Theoretical transndmslon 10ss as a function of frequencyfor three homogeneous panels varying In surface density. (TkOedata correspond to conditions of perfect absorption.)

,

Frequeaoy bmd8, Opa

Figure19.- Noise reduction obtainable for various frequency banda inan airplane fuBelage by conventional soundproofingtechniques.(Data obtaind from ref. 31.)

I

)

1

~$ w

Figure 20. - Noise attenuation as a function of frequency for V-~lR ear-plugs and the Air Force type helmet under optimm conditions of fit.(Data obtained from ref. 32.)

la) 300 3000., Io,ooo

-r=P