Chapter 8 Sound out of place（第2页）

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。