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Chapter 2 The nature of sound（第1页）

Chapter2Thenatureofsound

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。

&fallsonaseriesofparallelliripes，esasihapart（orthereabouts），itisdiffradsierwaveleedthrles，suchdifiragratingssplitwhitelightintoitspocolours—thebaakesrainbowsfromsunbeamsinjustthisway。Siularseriesof‘stripes’ofincreasedpressure，italsoactasadiffragrating，sglightwithawavelengtharoundthatofthedistaripes（thatdistanghalfthewavelengthofthesound）。Usuallythemediumhereisaesolid，suchasfusedquartz。Thisacousto-opticeffect，wheresouerlight，isusedbothuerandinairasaurbiandimagingtool（seeFigure7）。