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Chapter 2 The nature of sound（第1页）

Chapter2Thenatureofsound

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Twofacesofsound

Ifatreefallsihoutaohearit，doesitmakeasound?Thedualmeaningof‘sound’，asphysieion，providesaswer:yesaiohephysidsesofsoundareplex，inthatmanyoftheimpressionssoundmakesoedtoitsphysicalparametersbutothem.So:high-frequendssouched—usually.Andmorepowerfulsoundssouhewhole.Furthermore，manysounds，fromsirenstoskirlsandfromlullabiestolions’roars，makeemotionalimpauswhilythevaguestrelationshipstotheirphysicalparameters.

Thephysicalaspectsofsoueruhaioiswithphysicsthatweshouldbegin.

Pressure>

Soundsareusuallymadebysomethingmovinginahediaphragmofalinandout，thegapbetweenthevocalfandwidening，uitarstringvibratingbadforth.Itisthetransmissioionstmedium（solid，liquid，as），andtheirprogressihthatmedium，thatstitutesound.Ihemotiohemediumitself，suchastheairintheneckofabottlewhenoneblowsacrossit.Non-movingsourcludesuddeenergy，suchasbyexplosionsorsparks，andrapidlyosgheatsources.

&ionisthatofaloudspeaker’sdiaphragm，thecauseisavaryirialwiththesamepatternasthesouhediaphragmwillproduce.Eachtimethediaphragmmovesout，itsqueezestheairmoleculesimmediatelyinfrontofitclether，fionofhighpressure.Thesemoleculespresshbthemcletheriurn，andsoapulseofolecules（ahthemedium，folloressurearea（rarefa），whichisproducedasthediaphragmmovesin>

Thediaphragmtheagain，makingasedpulse.Howoftenthediaphragmmovesinand1sedgivesthefrequencyofthesouz，abbreviatedHz）.Thesimplestsounduretoneasmade，forexample，byatuningfork;asnapshotofthevariationihdistansuchaforkwouldbeasinewave，asshowninFigure1.

&aeaks（hs）ofasouhewavelength（λ）.Thesoundwilltravelthroughtheairatavelocityv，whichwillbearouresperseperature.Thefrequency（f）isgiveionf=vλ.Aplotofthevariationofpressureovertimeatasiinspaceisalsoasinewave，sowecouldlabeltheabovex-axisofFigure1as‘time’ifwewished.

1.Soundressureplot.

2.Molecularviewofsound>

ImageslikeFigure1aresooiseasytoimagiheyprovidesomekindofapictureofasoundwave，andmanybooksusetheminthisway.Infact，however，thereisnoupanddowioninasouhereisin，say，aheonlymotionisofmoleculesshuttlielyawayfromandtowardsthesource，liketheballsofaon’scradle.Suchwavesarereferredtoaslongitudinal，andifwecouldseeairmoleculestheywouldlooksomethinglikeFigure2.

Ifauoussinatesfromapoispreadsiions，asanexpahedeteareaissmall（likeamiediaphragmoreardrum）aresfromthesource，thecurvatureofthesligible，inwhichcasethesouheformofplanewaves.Evenifthesourceofthesoundhasaparticulardiretoit（likemostloudspeakers），thesoundwillstillspreadspherically，solongasthediaphragmislargerthahofthesound.Fths，thesousiosomeextent，andatsufitlyhighfrequencieswillformabeam（seeChapter6）.

gsound

&yofsounddependsoiddensityofthemedium（Chapter1）.Inair，souyihinghumidityahiemperature，butonlybecauseofthedensitythatthesefactorsplesaregiveninTable1.

Table1.Souiesimediaands

&hevelocityofsoundinairiemperature，oheairmaresupishotterthaheground，souravelfasterthehighertheyare.Theeffectofthisvelocreaseistobehemdownwardsfromthewarmerair，returhsomedistanceaway，asshowniimessoundsbeheardmreatdistaashortohiseffect.Refraalsoexplainswhyitishardtaihewihegroundslowsthesouly（paredtotheground）buttheairafewmetresupisfaster，sothesoundwavesareslowedalittlemorethere.Thesousfromthelow-veliontothehigher-veloe，andhencecurvesupawayfromthegroundandfromyourears（seeFigure4）.

3.Sationwhenairisearthegroundthanhighaboveit.

4.BarbarahearAlan，butnotClive.

ohadiaphragm，opushshtheairthatsurroundsitanyfaster.Mliespressurepulsesthatareclether，whiarriveatapoint—youreardrum，say—morefrequently.Thatistosay，thesoundfrequencywouldrise.Iforiespushingtheairharderbymovingthediaphragmfurtherinandout，theofpression（aion）inthepulsesincreases，leadingtoahighersoundpressure（heardasaloudersound）.Ifohediaphragmtomovefasterthayofsoundinthemedium，apulsehasomoveawayfromthediaphragmbeforethepulseforms.Heheypileupintoasiremehigh-pressurepulseknownasashockwave（thecauseofsonisand>

Movingadiaphragmmorerapidlyisnottheonlywaytoihefrequencyofasound:iftheloudspeaker（orothersource）israpidlyapproagyouoryouarerapidlyapproagit，thepressurepulsesarriveatyourearmorefrequently，becauseeachstartsclosertoyouthantheohefrequencyofthesouhesourcehaspassedby，thepulseshityourearwithloervalsiween，becauseeachhasalittlefarthertotravelthantheohefrequencyfalls.Thisisthewell-know，enswheneveroneispassedbyaspeedingmotorbikeorthesirenofthepolicedit（Box2）.

&，sous，andaswithrefleirreofthesoundsouredifasurfaceissmoothandhard，soifyouaresomewherebetweensourdsurface，youroximatelythesamesoundfromeachside（butquieterfromthereflectorside）.‘Smooth’isarelativetermthough，meaning‘withbumpssmallerthah’.Sindwavesareaboutonemilliontimeslht（paring3kHztoyellow），evenquiteroughsurfaceslikeakegoodairrors.irrorsfocusthesoundsthattheyrefleWorldWarIjustsuirrorswerebuiltalohcoastofEnglandtofocusthesoundplaheearsoflisteningsolders.Wheweentwoormorecurvedreflectors，theresultbeawhisperinggallery，liketheoneinLondon’sSaintPaul’sCathedral.

Box2

Soufromtheiweenaherairaerandair，ordifferentrocklayersih.Howmuchofthesoueddependsonthediffereicesofthetwomedia，andtheimpedaurhedensityofthemediumayofsoundinit.Apedance（Box3）issimilartoelectricalresistaitmeasuresthedificultywithwhidtravelthroughamedium.Itiskeytomasandapplisofsound.Forirubberysurfacewillabsorbsoundaoheat，sirubberhasaremelyhighapedaingsonsubmarinesarebasedonthisfafortunessofrubberisverytemperature-depe，so，whenColdWarsubmarineswereredeployedfromtheNorthAtlantictotheGulffromthelate1980s，thehighereraturesrobbedthemoftheirstealthaoffaflurryofreseardresurfag.

Soundbefocussedbypassingitthroughanacoustiadeofacrylicplastisesworkbecauseawaveisrefraitpassesfromoasitstrikestheiweeaheahroughwhichthewaveisrefradsoioofitsvelothetwomedia（Snell’slaw，Box4）.

&whichisusuallyfarmorenoticeableforsoundthanfhtisitsabilitytobendrounddoverreadthroughanopening，aphenomenonknownasdiffraure5）.

Box3

Characteristipedanedium:Z0=ρ0v0;densityρ0，velocityofsouistheRayl，andthe0sihesearethevaluesofthemediumwhenitis‘uis，whennosouinit.

Box4

Snell’slaw:sinθ1sinθ2=v1v2;θ（theta）istheahedireofthesoundaaheinterface，vissouy.

5.Difira.

&hewaveleerthebending.Soifahighwallisiweenasoundsourd，say—ahebasssoundsdiffraitstopbatoearthbutthehigh-pitesarelost（Figure6）.Thismuffliisausefulcluethathelpsusgaugethedistaniliarsoudoors.