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Chapter 8 Sound out of place（第2页）

Fightingback

Withtheexeneighbournoiseandsnals，noonewouldbelikelytoplaiobeelimihisbedone?

Iunatelyrare）whioiseishighlyregular，sistsofoneorfewfrequendisbothersomeonlyinasmallandwell-deflion，aoise（ANbehighlyefieCreliesohatsouofpressionsfollowedbyrarefaapressureplotsuchasFigure1，thisisthefactthattheliaboveahemidpoint.Ifasedwaveweresuperimposedosuchthatateverypointatwhichthereisapressionintheiherewasanequallyiioninthesilencewouldresult，theaergybeiirelytoheat.However，ihislybeachievedoververylimitedareas，suecockpits，thedriver’sheadareaofsomed—perhapsmostusefully—thespasideeardefenders（Figure23）.

Manyhighlyefiectiveteologicalsolutionstonoisehavebeeheproblemisthatallofthemhavebeenappliedalready.Today’svehiclesareextremelyquietparedtotheirmeicalpowers，farquieterthantheirmuchfeebleraorseverwere，butthisfaedbythevastlygreaterhemioday.Butitisstillworthlookingathowtoday’smaesarekeptsoquiet，sihesameprincipleswillalmostlybeapplifuture，especiallyaserialsbeeavailable.

23.Aoise.

&prinoisetrolistoidentifythesourdremoveit.Forperhapsthesiimportaoday，theinternalbustiohisisimpossible:thenoiseismadebytheexplosionswithin，whitrinsioisy.HolainedinChapter3，systemsdohetyofasouuorespondmlytofrequend4kHz，anditisusuallypossibletoshifttheoutputfrequeobelowthis.Themaiheadjustmeimieoffueliomodifythebustionofthefuelsothatsuchhigh-frequenposarereduced.

Haviasfaraspossiblewiththenoisesourextstepistoit.Again，ICEsaregsiobeectedtotheairarouodrawinoxygegases，andairpathsarees.Toreduissioheanswer.（Asilehesamething，butismoreoftenso-calledwheoquietenflrearms.）

&wobasicmufierprindmostICEsuseboth.Absorptivesilengisachievedsimplybylihabsorbersothatadoraightdowedtoheat.Thosewhichdotravelstraightdownarealsoreducetheiredgesareretardedbythedug.Butthiseffelyslightatlowfrequencies，whicharebetterdealtwithbyreactivemufiers.TheseHelmholtzresonatorsturoublesomefrequendsatthesefrequehusenhansidethemufier，allowiobeabsorbedthere.AfewmufiersalsouseANC.

&hefewdecades，itseemslikelythatICEswilllargelygivewaytoelectris.Thereisnoteiwhytheseshouldsilent，butavehicleroatlycouldbedeadly，soallwillbeprovidedwithspeeratedrunningandwarningsounds（whichmightalsobeofvalueforbicycles—especiallythosewithhereisnoreasoobeu，however—evenabealarming.Toattration，‘shushing’soundsejobwithlessannoyance.

&eheotherhand，areprobablyheretostayfortheforeseeablefuture.Intheircase，thesourceoftheheburhemselves，buteheyemergefromtheehighspeedandmixwiththerelativelyslow-movingairoutsidetheplaurbulehiscausesisthesourostallthenoise.Eshavebeehthemixing，mainlybyihecirceofthejetbygitandbygtheexhaustjetinairofiespeed.Nomoreeiionswithoutanexcessivelossofthrust，butotherapproacheshavebeensidered.Ohasmetwithmodestsucouhewings.Arategige，suchastheuseofairshipsforthetranspoodswheretraanissue.

Allbuttheslowestormostantiquatedmovingvehiowstreamlined，whichavoidstheturbulewouldproduoiseaswellasingeffcy.Streamliningisachievedpartlybydesignihasuitableshapesothattheairstreamslipspastitwithoutthesuddengeofdireorspeedthatwouldresultinturbulendpartlybysm.heless，asfarascardriversareed，theaerodynamioisewhichstreamliniodealwithisstillthemostsignifltatgandhigherspeeds.TheprincipalsourcesareusuallythewingmirrorsandtheApillars（theoneswhichholdthesidesofthewindsplace）.

Forthoseoheloudestnoiseisfrequeyres，especreteroads.Low-noisesurfadoagreatdealtoreducethis.Thebestavailablearelayersofasphaltwithmanyair-fllledpores（about25pere），butsuchsurfacesared.Alessefiective，butmoredurableaerheuseofthin（~2cm）asphaltlayers.

Timewaswhenthedesignofmaesdidnotsiderquietall，andnoisemaliedonlyasastigplasterafterwardsorasunreliablerulesofthumb（likestretgafewstrihallfflotheatrestageswithvases）.Now，maesareusuallywelldesignedandcarefullyeobeatleastfairlyquiet—thularmaintenandustrialmadstallatioicesareessential.

Whennoiseeitheravoidednoted，theokeepitssourceswellseparatedfrompotentialsufierers.Oneapproach，usedforthousandsofyears，iszoiionofnoisyactivitiestoparticularareas，sudustrialzones，whicharedistaialdistricts.Theflrstrepledatesfromarouinsmiths，blacksmiths，ters，potters，aerswerebahetrereektheAegeancoast.

Wherezoionbydistanceisimpracticabledueteographisiderations，souhemainsolution:abarrierthatjustcutsofithesightofanoisesourcewillreduoiselevelbyabout5dB，aiorewillprovideaboutara1.5dBredu.Also，thebarriermustbemassiveenoughtopreventmudpassingstraightthroughit;about10kgundereachsquaremetreisusuallysuffcebarrierslargelyrefflectratherthanabsorb，reffledsneedsideration，butotherwisedesignandstruple，resultsarepredidcostsarerelativelylow.

Temporaryfflexibleversionsofnoisebarriers，knownasacoustig，bepladstrusitesand，ifcarefullypositionedaoavoidleavinggaps，beveryefiective，reduoiselevelsbyupto30dBinsomelikepermanoisebarriers，acoustigaoisereduarilybyabsorption.

Thebigproblemwithnoisebarriersistheirunattraesstotheeye.Thisbeofisetbyintrodugasofvegetatioweenbarrierandobserver.Ifspacepermitsatleast1etatioswillhelpreduewhattoo.Atlowfrequencies（around25Hz），thisismostlyduetroundabsorptioncausedbyfallehighfrequencies（above~1kHz）itiscausedbyfoliage.Itisbestiftheleavesarehalfaslongasthesoundwaves，so，forboththesereasons，somedeciduoustisessential，（amixoftreesandbushes）reagashighaspossible（uptoabout10metres，afterwhichrefflelargebraheefiewindydays，thesoundofsuchvegetationisanotherbonus，asadistrathenoise.

&ionofnaturalsoundsourdsg）isespeciallyefieurbanparks.Insuchspaooimproveasoundscapeisbyaddingfountains，butbirdsong（realorrecorded）workwelltoo.

&eninghomes

Ofallspaces，themostpree，becausethere，aisaensionofourpersonalspagis，therefhpriority:thebestwaytodothisisthroughgoodacousti，ingselehtmaterials，and，crucially，theirproperinstallationahisismuchcheaper，moreaestheticallypleasing，andmoresuccessfulthatretroflttiionsiingdwelling.

&roflttedornot，thebasicapproaesouionaresimple:stoperiroywhatdoesgetin，anddon’taddmoretoityourself.Therearethreewaysforsouer:viaopenings;bystruevibration;andthroughs，doors，gs，andfflasdiaphragms.Inallthreecases，themainpointtobearinmindisthatanacousticshellisonlyasgoodasitsart:justasevenasmallholeiertightship’shullreuseless，sodoesasingleopenwindowinadouble-glazedhouse.Infact，thesituationwithnoiseismuchworsethanwithwaterduetothelogarithmiseofourears:ifwesealowoidenticalholesinaboatwewillhalvetheinfflow.Ifweeoftwoidentidowsintoahouse，theinfflowwillagainhalf—butthat50pertreduatensityisonlyabouta2pertreduloudness.

Thesedisdoubleglazing，single-glazedwindowsmakeextdiaphragms.Struesoundisamuchgreaterge，thoughifthesourceiswithiheroomwhichit（or（s）he）occupies，betreated.Butthiswillnotbedwillusuallyrequiretreatmentofthegaswellasallwalls，theffloor，andthedoor.

Oneiable，aivesolution—thoughmayberactive—isthehangingofheavyvelourdrapes，withasmanyfoldsaspossible.Ifsomethiicisrequired，itisvitaltoinvolvea:whileanobvioussolutionistothiwalls，it’simportanttobearinmindthatdoublingthiessredusmissionlossbyonly6dB（asouionofaboutthree-quarters，butaloudionofonlyabout40pert）.Thismeansthatsolidwallshicktoworkwell.

Afarbetterapproachistheuseofporousabsorbersandofmulti-layerstruaporousabslassflbre，higher-frequendwavesarelhmultiplereffleanyinternalsurfagstud-mouhairgapsbetweehewallwilldealwithlowerfrequehroughreffleoflowerfrequeheairsolidianotherexampleofanimpedach）.Awell-flttedacoustisulateddoorisalsovital.

Thefioorshoulded:eveniftherearenoroomsbeh，hardffloorsareextbothatgeingnoisewhenwalkedonandintransmittingthathroughoutthebuildiandunderlayarehighlyefiectiveathighfrequearealmostuselessatlowerones，soifyouhaveffloor-mountedloudspeakersoraohehome，somethingspecialwillberequired，andagainthereiseri.

Thoughrarelyaproblemihemostsignifltissueinthedesignofpublicspacesisreverberatily，thoughithasbeenabugbearforarchiteers，andaudiencesalikeformillennia，nowaytoquantifyiteduntil1898，wheSabihereverberationtimeofaroomastheperiodnecessaryfortheiyofthesouooh（fi60dB）ofitsinitiallevel（Box13）.Evenmoreusefully，Sabinederivedanempiricallybasedequationthatallowedthistimetobecalculatedfromthesizeandshapeofaroom—soarchitectscouldpredictitatlast.

Box13

Sabiion:T=0.161（VA），reverberationtimei，roomvolumeV，totalabsorptiohisequationisstillusedtodayexceptfhlyabsorbentrooms.

Howevermuchey，aiseisspentonanindividualnoiseproblem，theresultswillbeiiveuherootdthetextofthatproblemarei:itmaybepoiorepladarddoorswithaesiftheaisthattheyareroutiopetlepthespeedlimitonatrunkroadiftheefiectistosendtraffresides.Suchholisticapproaoisereduextendbeyondacoustits:thebesofflttingspabebalatheirvisualimpatroduofartifldstodistractparkusersfromtraffayahanitpleases，andmovingpeoplefromopen-plaoquieterayreduceteamw.Similarly，whilewell-builthousesoiseout，thisnecessarilyiheinhabitantsin，gofithesoundsofoo，aoalienation.AsDavidHendyputsit:‘ithdrawiesoundscapes...wemakestraher’.

Thewiderview

&hegreatimportaoussystems，theirperformausuallyamatteroflifeahisisnottheeuerspecies.Theprimeimportananymarineanimals，biheeffcywithwhidstraveluer，meansthatsomemariuresaredevastatedbytheefieoise，inparticularhins.Thescaleoftheefieclearbutsomearesubtle:sincewhalesuheircalvesbysoundsigherkeasilybebrokeraneoussoundsandthecalfseparated.Inothercases，thestartleefieiliarloudsoundsonhiremethatrapidsurfagoccurs，leadihfromtheformatiasbubblesiheraiondiversdescribeas‘thebends’）.

Therearemanytributorstoueriohesoundsofshipping，soing，andsignallingalladdtheirload.Fortuhereisnnitiosonanimals;moreinfadivers，whoroutinelyriskseverehearihough，astheefiectsareoftehoseesihescaleoftheproblemishardtoestablish）.

&erdisaryandpubliptstobattlehonlandaer，makecollaborationatalllevelsvital，andotsucceedwithouttheactivesupportofiianizationssuchastheWorldHealthanization，natios，localauthorities，specialistanizatioheUKemehepublieactivitywhichshalltheseplayerstogetherisionandawarenessweeks.

Asteologieshaveevolved，citieshavegrowravelhasinoisesourceshaveproliferatedandnoiseisnowafactoflifeformahermadedeliberatelyandselflshlyvialoudspeakersorunavoidablyahrougheherhardware.Althoughthereismuchthatsdotohelp，itmustbeapahedudlegislatiohatnoiseisuniversallyregardedasthepollutantthatitis.

Ultimately，noise-makingmustbeeuable，ratherthanjustannoyihismayseemimpractical，exactlysugehastakeheUKregardingsmoking—andthistookonlyafewyears.Thegecameabhabinationofnelemehroughbothnationalandloent，andaloudandclearmessagethatsmokingcausesseriousillhesameapproaoise，justasradigemightbeaplished.

Ofalldises，soundhasbyfarthewidestvarietyofiies.Uaheworkofactors，advertisers，aerospaeers，automotiveehropologists，architects，artists，broadcasters，builders，unigineers，ps，educators，elegineers，ealhealthoffcers，fllm-makers，historians，marinebiologists，musis，physis，physicists，politis，prospectors，psychologists，seismologists，sociologists，townplanners，zoologists，andmanyothers.

&o，manyofthesedomainshavebeeoverthepastferoacheslikethoseofsoundsg，souhedesignofevermoreintimateandpersoemshavestartedtetherexpertisefromacrosstheseflelds.Suchcollaboratioehenarrofieredtomostpraers，butmeaheyaremakingpeoplemoreawareofthevastrangeahesubjeultitudeofprademotiosaknowledgeofsouo，andofthemanynelidinsightsthebriherofitsmanybranchesmayyield.Suehebeofeveryone.Forallofus，thefuturesoundsgood.