fund092 - unt digital library/67531/metadc55864/m... · nationaladvisorycommitteeforaeronautics...
TRANSCRIPT
![Page 1: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/1.jpg)
-,
4,
A SURVEY
TECHCAL NOTE2701
OF THEAIRCRAFT-NOBE PROBLEMWITHSPECIAL
REFERENCETO ITS PHYSICALASPECTS
By Harvey H. Hubbard
LangleyAeronautical LalmratoryLangleyField, Va.
Washington
-,.-...—. ..— — ._. ,. . . . .-, - .-. ..=- . ..- . ... ..-—. - . ..—--
![Page 2: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/2.jpg)
1P
TECHLIBRARYK.AFB,NM
Illlillllolllmuulflulll10L5751
Page
r,
suMMARY. . . .
INTRODUCTION.
.
.
EFFECTSOFNOISEAnnoyance. .
●
✎
✎
✎
.
.
●
✎
.
.
.
.
.
.
.
.
. .
. .
. .
. .Work-OutputandEfficiencyEffectsonCommmication.DeafeningEffects. . . .EffectsofIntenseNoise.
Auditoryeffects. . .Nonauditoryeffects. .
FatigueofStructures. .
.
.
.
.
.
.
.
.
.
.
.
.
.
.●
●
✎
✎
✎
✎
✎
✎
.
.
.
.●
●
✎
✎
✎
✎
✎
.
.
●
✎
✎
✎
✎
✎
✎
✎
✎
●
✎
✎
✎
✎
✎
✎
●
✎
✎
✎
.
.
.
.
.
.
.
.
.●
●
.
.
.
.
.
.
.
.●
✎
✎
PHYSICALCHARAC!lIERISTICSOFAIRCRKFTNOISEFrequencySpectrums. . . . ... . .
SteadyforcesvaryingindistancePulsi.igflow..” .:..Turbulenceandsetmixing
DirectionalCharacteristicsOver-AllIntensityLevels
Propellers. ; . . .ReciprocatingenginesTurbojets. . . . . .Afterburnerunits. .TurbopropellerunitsRockets. . . . . . .Pulsejets . . . . .Ramjets . . . . . .Aerodynamicnoise. .
.
.
.
.
.●
✎
●
✎
.
.
.
.
.
.
.
.
.
.
.’.
...●
✎
✎
●
✎
✎
✎
✎
✎
●
✎
●
●
✎
✎
✎
✎
✎
✎
✎
✎
●
.
.
.●
✎
✎
✎
✎
✎
✎
✎
●
✎
●
●
✎
✎
✎
✎
●
✎
✎
✎
✎
✎
✎
.
.
.
.●
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
.●
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.●
✎
✎
✎
✎
✎
.
●
✎
●
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.●
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
ComparisonofNoisefromVariousPropulsiveUnits
PROTECTIONFROMNOISE. .-.. . . .ExhaustMuffling. . . . . . . .
Reciprocatingengines. . . .Jetengbes. . . . . . . . .
SpatialIsolation. . . . . . .Soundproofing. . . . . . . . .PersonalProtection.. . . . . .
CONCLUDINGREMARK S........
REFERENCES. . . . . . . . . . . .
FIGURES. . . . . . . . . . . . . .
. . . . . . . . .
. . . . . . ● ✎☛
✎ ✎ ✎ ✎ ✎ ✎ ✎ ✎ ✎
✎ ✎ ✎ ✎ ✎ ✎ ✎ ✎ ✎
✎ ✎ ✎ ✎ ✎ ✎ ✎ ✎ ✎
✎ ✎ ✎ ✎ ✎ ✎ ✎ ✎ ✎
. . . . . . . . .
● O.... . . .
. ..0. ..*O
. . ...0 ..0
;L
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.●
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.●
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.●
✎
✎
✎
✎
✎
✎
✎
✎
●
✎
✎
✎
✎
✎
✎
✎
✎
✎
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
......
.
.
.
●
✎
✎
✎
●
✎
✎
●
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
●
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
.
●
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
.
.
.
.
.
.
.●
✎
✎
✎
✎
●
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
✎
1
1
;33455-56
677
i899
10101010ILI-1111112
13131314151517
17 “
19
22
.- —...—. .— —— .-— —. __— .—.
![Page 3: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/3.jpg)
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.
![Page 4: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/4.jpg)
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).
—.— —.
(Sou~a-pressurelevelsdiscussedinthispaperareexpressedindecibelsand,as is conventional,aremeasuredrelativetoa referencepressurelevelof0.0002dyne/cm2.Theso~d-pressurelevelindecibels
![Page 5: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/5.jpg)
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.
.
-—.-—— ———. ——.
![Page 6: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/6.jpg)
4
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).
. . ..-
.
—
![Page 7: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/7.jpg)
NACATN 2701 5
.
.
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.
— ..—. ~—
![Page 8: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/8.jpg)
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.
‘..
![Page 9: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/9.jpg)
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.
.—— .- — --—— .— —.
![Page 10: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/10.jpg)
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
..-
![Page 11: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/11.jpg)
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.
\.— — -.
![Page 12: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/12.jpg)
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
— — .—
![Page 13: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/13.jpg)
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
.— .—.——— .— — —
![Page 14: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/14.jpg)
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
![Page 15: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/15.jpg)
.
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
——. — —-- — --——
![Page 16: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/16.jpg)
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
![Page 17: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/17.jpg)
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~
![Page 18: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/18.jpg)
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).
—.
![Page 19: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/19.jpg)
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 . —— .————— -———-
![Page 20: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/20.jpg)
protectionisinmanycasesexpensiveandis complicatedbytheintenselow-frequencycontentofthenoisefrommostaircraft-noisesources.
LangleyAeronauticalLaboratoryNationalAdvisoryCommitteeforAeronautics
LangleyField,Vs.,March12,1952
——
![Page 21: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/21.jpg)
NACA~ 2701 19
REFERENCES
.
.
1.
2.
3*
4.
5.
6.
7.
a.
9*
10.
11.
12.
Kryter,KarlD.: TheEffectsofNoiseonMan. Jour.of SpeechandHearingDisorders,MonogaphSUPP.1, ~Pt. 1950.
Davis A. H.: ModernAcoustics.G. BellandSonsLtd.(London),1934p.246. ~.
Anon.: FinslReportonTemporsryDeafnessFollowingExposuretoLoudTonesandNoise.ACTAOTO-IARINGOLCCICA,Supp._lI~f3t~~lm, 1950. (ReprintofOSRDCont&ctOE2Blcmr-194,Sept.30,
.
McFarland,RossA.: HumanFactorsinAirTmnsportDesign.McGraw-Hillmok Co.,fiC.,1946.
Parrack,HomceO., Eldredge,DonaldH., adologicalEffectsofbtenseSound.MR No.MaterielComand,U. S.Air Force,May24,
Koster,HenryF.: Physi-McRExo-69~m,Air1948.
Parrack,Ho=ce O.: AircraftNoiseandNoiseSuppressionFacilities-AnEvaluation.Paperpresentedto theJointConferenceofAirMaterielCommnd- AircraftIndustriesAssoc., LosAngeles,Cal-if.,Apr.11-12,1950.
VonGierke,HenningE.,Psrrack,HoraceO.,andEldredge,DonsldH.:HeatingofAnimslsbyAbsorbedSoundEnergy.AF TRNo.G240,AirMaterielComand,U. S.AirForce,Oct.1950.
VonGierke,HendngE.: MeasurementoftheAcoustichpedanceandtheAcousticAbsorptionCoefficientof theSurfaceoftheHumanBody. AFTRNo.601o,AirMaterielCommandjU. S.AirForce,Mar.1950.
Hubbard,HarveyH.,andRegier,ArthurA.: Free-SpaceOscillatingPressuresneartheTipsofRotatingPropellers.NACARep.996,1950. (SupersedesNACATN1870.)
Hubbard,HarveyH.,andLassiter,LeslieW.: Soundfroma Twp-BladePropellerat SupersonicTipSpeeds.NACAIUdL51C27,1951.
El&edge,DonaldH.,Jr.,andParrack,HoraceO.: JetEngineSoundSpectra.AF TRNo. X27, ATI57639,AirMaterielCcmmnd,U. S.AirForce,June1949.
Davis,.DonD.,Jr.,andCzarnecki,K. R.: Dynamometer-StandInvesti-gationofa GroupofMufflers.MCA ~ 1838,1949.
-— _.— -——. ——
![Page 22: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/22.jpg)
20
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
~A ~ 2701
Ernsthausen,Wilhelm:SoundandVibratiainAircraft.Vol. IIofGermanAviationMetictieWorldWarII,pt.VII,ch.VII-A,Dept.oftheAirForce,pp.651-685. -
Vogeley,A.W.: Sound-LevelMeasUrementaofa LightAirplanetoReduceNoiseReachingtheGround.NACARep.926,1949.sedesNACATN 1647.)
Modified 4(Super-
Bermek,LeoL., Elwell, FredS.,Roberts,JohnP.,andTaylor,C. Fayette:ExperimentsinExternalNoiseReductionofLightAirplanes.NACATN 2079,1950.
Hicks,ChesterW.,andHubbard,HarveyH.: Comparisonof SoundEmissionfromTwo-Blade,Four-BladejsndSeven-BladePropellers.NACA!D?1354,1947.
Hubbard,HarveyH.: SoundfromDual.-RotatingandMultipleSingle-RotatingPropeU.ers.NACATN 1654,1948.
Hubbard,HarveyH.,andRegier,ArthurA.: Propeller-LmxinessChsrtsforLightAirplanes.NACATN1358,1947.
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.
—- .—— ————.
![Page 23: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/23.jpg)
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.: [email protected]&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.
—_——_—— ..—— - -...—. —.--.——- —.———
![Page 24: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/24.jpg)
---
“.
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
,
![Page 25: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/25.jpg)
,
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.)
![Page 26: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/26.jpg)
“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.)
![Page 27: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/27.jpg)
NACATN 2701
Intensity
Intensity
.
III\
(a)Subsonictipspeed.
Frequency,.kc
.Figure
(b)Supersonictip
4.-Noisegenerated
speed.
bypropellers.
—.-.— .—. .— _. ——. ——
![Page 28: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/28.jpg)
Intensity
‘*1 i. o
Frequenoy, kc
IL!J!5.,..
.0
Figure 5.- Noiae generated by a pulsing flow.
II
![Page 29: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/29.jpg)
,
I
~-----”-------- _
---- <--- ---- ----------—— -_ -- -a ----- -- —---- ----, -- ‘----- ----— -
-—----— -- —---- —— -..---- - ,-d~--------- __ -- —__ ,
------ ----- —___ ------- ------
----
1 , ?
I
i
Frequency, kc
Figure6.-l?oisegeneratedby Jetinlxingend~urlulence.
![Page 30: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/30.jpg)
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°
![Page 31: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/31.jpg)
NACATN 2701 29
.
.
.
●
80
70
60●
TipMachmmher
Figure8.- Effects of tip Machnuniberandnumberof blades on the noiseproducedby single-rotating propellers at constantpowerinput.
— ——.- — —. —>.,,
![Page 32: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/32.jpg)
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.)
— — —. . . —-
![Page 33: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/33.jpg)
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.)
. . .--—_____ ___ —— __ ..—-—
![Page 34: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/34.jpg)
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.)
--— ..— —
![Page 35: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/35.jpg)
WA TN2701 33.
.
.
.Figure3.2.-Intensitylevelasa functionofazimuthanglefortwo
solid-fuelrocketengines.(Datahavebeenadjustedtocorrespondto5000poundsofthrustanda distanceof 300feet.)
.—— .—
![Page 36: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/36.jpg)
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.
——— .—.——.— .
![Page 37: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/37.jpg)
.
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.)
![Page 38: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/38.jpg)
5’0
80
50
30 100 300
Requency, opa
Figure 15.- Exhauat noise spectrums for a small&fore and after muffling.
icm
reciprocatingengine
![Page 39: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/39.jpg)
[
.
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’
![Page 40: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/40.jpg)
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.)
%
![Page 41: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/41.jpg)
,
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.)
,
![Page 42: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/42.jpg)
Frequeaoy bmd8, Opa
Figure19.- Noise reduction obtainable for various frequency banda inan airplane fuBelage by conventional soundproofingtechniques.(Data obtaind from ref. 31.)
![Page 43: fund092 - UNT Digital Library/67531/metadc55864/m... · NATIONALADVISORYCOMMITTEEFORAERONAUTICS mcmucm NOTE2701 ASURVEYOFTEEAIRCRAFT-NOISEPROBLEMWITHSPECIAL REFERENCETOITSPHYSICALASPECTS](https://reader034.vdocument.in/reader034/viewer/2022050222/5f67731ee7f7cf79bf3ca3e1/html5/thumbnails/43.jpg)
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