Raymond A. Serway Chris Vuille€¦ · • The rate of energy transfer is the power • SI unit:...
Transcript of Raymond A. Serway Chris Vuille€¦ · • The rate of energy transfer is the power • SI unit:...
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RaymondA.SerwayChrisVuille
ChapterFourteenSound
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SoundWaves
• Soundwavesarelongitudinalwaves• Characteris=csofsoundwaveswillhelpyouunderstandhowwehear
Introduc=on
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ProducingaSoundWave
• Anysoundwavehasitssourceinavibra=ngobject• Soundwavesarelongitudinalwavestravelingthroughamedium
• Atuningforkcanbeusedasanexampleofproducingasoundwave
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UsingaTuningForktoProduceaSoundWave
• Atuningforkwillproduceapuremusicalnote
• Asthe=nesvibrate,theydisturbtheairnearthem
• Asthe=neswingstotheright,itforcestheairmoleculesnearitclosertogether
• Thisproducesahighdensityareaintheair– Thisisanareaofcompression
Sec=on14.1
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UsingaTuningFork,cont.
• Asthe=nemovestowardtheleJ,theairmoleculestotherightofthe=nespreadout
• Thisproducesanareaoflowdensity– Thisareaiscalledararefac&on
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UsingaTuningFork,final
• Asthetuningforkcon=nuestovibrate,asuccessionofcompressionsandrarefac=onsspreadoutfromthefork
• Asinusoidalcurvecanbeusedtorepresentthelongitudinalwave– Crestscorrespondtocompressionsandtroughstorarefac=ons
Sec=on14.1
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CategoriesofSoundWaves
• Audiblewaves– Laywithinthenormalrangeofhearingofthehumanear– Normallybetween20Hzto20000Hz
• Infrasonicwaves– Frequenciesarebelowtheaudiblerange– Earthquakesareanexample
• Ultrasonicwaves– Frequenciesareabovetheaudiblerange– Dogwhistlesareanexample
Sec=on14.2
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Applica=onsofUltrasound
• Canbeusedtoproduceimagesofsmallobjects• Widelyusedasadiagnos=candtreatmenttoolinmedicine– Ultrasonicflowmetertomeasurebloodflow– Mayusepiezoelectricdevicesthattransformelectricalenergyinto
mechanicalenergy• Reversible:mechanicaltoelectrical
– Ultrasoundstoobservebabiesinthewomb– CavitronUltrasonicSurgicalAspirator(CUSA)usedtosurgically
removebraintumors– High-intensityFocusedUltrasound(HIFU)isalsousedforbrainsurgery
• Ultrasonicrangingunitforcameras
Sec=on14.2
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SpeedofSoundinaLiquid
• Inafluid,thespeeddependsonthefluid’scompressibilityandiner=a
– BistheBulkModulusoftheliquid– ρisthedensityoftheliquid– Compareswiththeequa=onforatransversewaveonastring
Sec=on14.3
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SpeedofSound,General
• Thespeedofsoundishigherinsolidsthaningases– Themoleculesinasolidinteractmorestrongly
• Thespeedisslowerinliquidsthaninsolids– Liquidsaremorecompressible
Sec=on14.3
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SpeedofSoundinaSolidRod
• Thespeeddependsontherod’scompressibilityandiner=alproper=es
– YistheYoung’sModulusofthematerial– ρisthedensityofthematerial
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SpeedofSoundinAir
• 331m/sisthespeedofsoundat0°C• Tistheabsolutetemperature
Sec=on14.3
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IntensityofSoundWaves• TheaverageintensityIofawaveonagivensurfaceisdefined
astherateatwhichtheenergyflowsthroughthesurface,ΔE/Δtdividedbythesurfacearea,A
• Thedirec=onofenergyflowisperpendiculartothesurfaceateverypoint
• Therateofenergytransferisthepower• SIunit:W/m2
Sec=on14.4
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VariousIntensi=esofSound
• Thresholdofhearing– Faintestsoundmosthumanscanhear– About1x10-12W/m2
• Thresholdofpain– Loudestsoundmosthumanscantolerate– About1W/m2
• Theearisaverysensi=vedetectorofsoundwaves– Itcandetectpressurefluctua=onsassmallasabout3partsin1010
Sec=on14.4
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IntensityLevelofSoundWaves
• Thesensa=onofloudnessislogarithmicinthehumanear
• βistheintensitylevelorthedecibellevelofthesound
• Ioisthethresholdofhearing
Sec=on14.4
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Intensityvs.IntensityLevel
• Intensityisaphysicalquan=ty• Intensitylevelisaconvenientmathema=caltransforma=onofintensitytoalogarithmicscale
Sec=on14.4
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VariousIntensityLevels
• Thresholdofhearingis0dB• Thresholdofpainis120dB• Jetairplanesareabout150dB• Table14.2listsintensitylevelsofvarioussounds– Mul=plyingagivenintensityby10adds10dBtotheintensitylevel
Sec=on14.4
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SphericalWaves
• Asphericalwavepropagatesradiallyoutwardfromtheoscilla=ngsphere
• Theenergypropagatesequallyinalldirec=ons
• Theintensityis
Sec=on14.5
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IntensityofaPointSource
• Sincetheintensityvariesas1/r2,thisisaninversesquarerela&onship
• Theaveragepoweristhesamethroughanysphericalsurfacecenteredonthesource
• Tocompareintensi=esattwoloca=ons,theinversesquarerela=onshipcanbeused
Sec=on14.5
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Representa=onsofWaves
• Wavefrontsaretheconcentricarcs– Thedistancebetweensuccessivewavefrontsisthewavelength
• Raysaretheradiallinespoin=ngoutfromthesourceandperpendiculartothewavefronts
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PlaneWave
• Farawayfromthesource,thewavefrontsarenearlyparallelplanes
• Theraysarenearlyparallellines
• Asmallsegmentofthewavefrontisapproximatelyaplanewave
Sec=on14.5
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PlaneWaves,cont
• Anysmallpor=onofasphericalwavethatisfarfromthesourcecanbeconsideredaplanewave
• Thisshowsaplanewavemovingintheposi=vexdirec=on– Thewavefrontsareparalleltotheplanecontainingthey-andz-axes
Sec=on14.5
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DopplerEffect
• ADopplereffectisexperiencedwheneverthereisrela=vemo=onbetweenasourceofwavesandanobserver.– Whenthesourceandtheobserveraremovingtowardeachother,theobserverhearsahigherfrequency
– Whenthesourceandtheobserveraremovingawayfromeachother,theobserverhearsalowerfrequency
Sec=on14.6
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DopplerEffect,cont.
• AlthoughtheDopplerEffectiscommonlyexperiencedwithsoundwaves,itisaphenomenacommontoallwaves
• Assump=ons:– Theairissta=onary– Allspeedmeasurementsaremaderela=vetothesta=onarymedium
Sec=on14.6
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DopplerEffect,Case1(ObserverTowardSource)
• Anobserverismovingtowardasta=onarysource
• Duetohismovement,theobserverdetectsanaddi=onalnumberofwavefronts
• Thefrequencyheardisincreased
Sec=on14.6
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DopplerEffect,Case1(ObserverAwayfromSource)
• Anobserverismovingawayfromasta=onarysource
• Theobserverdetectsfewerwavefrontspersecond
• Thefrequencyappearslower
Sec=on14.6
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DopplerEffect,Case1–Equa=on
• Whenmovingtowardthesta=onarysource,theobservedfrequencyis
• Whenmovingawayfromthesta=onarysource,subs=tute–voforvointheaboveequa=on
Sec=on14.6
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DopplerEffect,Case2(SourceinMo=on)
• Asthesourcemovestowardtheobserver(A),thewavelengthappearsshorterandthefrequencyincreases
• Asthesourcemovesawayfromtheobserver(B),thewavelengthappearslongerandthefrequencyappearstobelower
Sec=on14.6
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DopplerEffect,SourceMoving–Equa=on
• Usethe–vswhenthesourceismovingtowardtheobserverand+vswhenthesourceismovingawayfromtheobserver
Sec=on14.6
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DopplerEffect,GeneralCase
• Boththesourceandtheobservercouldbemoving
• Useposi=vevaluesofvoandvsifthemo=onistoward– Frequencyappearshigher
• Usenega=vevaluesofvoandvsifthemo=onisaway– Frequencyappearslower
Sec=on14.6
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DopplerEffect,FinalNotes
• TheDopplerEffectdoesnotdependondistance– Asyougetcloser,theintensitywillincrease– Theapparentfrequencywillnotchange
Sec=on14.6
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ShockWaves
• Ashockwaveresultswhenthesourcevelocityexceedsthespeedofthewaveitself
• Thecirclesrepresentthewavefrontsemijedbythesource
Sec=on14.6
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ShockWaves,cont
• TangentlinesaredrawnfromSntothewavefrontcenteredonSo
• Theanglebetweenoneofthesetangentlinesandthedirec=onoftravelisgivenbysinθ=v/vs
• Thera=ovs/viscalledtheMachNumber• Theconicalwavefrontistheshockwave
Sec=on14.6
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ShockWaves,final
• Shockwavescarryenergyconcentratedonthesurfaceofthecone,withcorrespondinglygreatpressurevaria=ons
• Ajetproducesashockwaveseenasafogofwatervapor
Sec=on14.6
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InterferenceofSoundWaves
• Soundwavesinterfere– Construc=veinterferenceoccurswhenthepathdifferencebetweentwowaves’mo=oniszeroorsomeintegermul=pleofwavelengths• Pathdifference=nλ(n=0,1,2,…)
– Destruc=veinterferenceoccurswhenthepathdifferencebetweentwowaves’mo=onisanoddhalfwavelength• Pathdifference=(n+½)λ(n=0,1,2,…)
Sec=on14.7
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StandingWaves
• Whenatravelingwavereflectsbackonitself,itcreatestravelingwavesinbothdirec=ons
• Thewaveanditsreflec=oninterfereaccordingtothesuperposi=onprinciple
• Withexactlytherightfrequency,thewavewillappeartostands=ll– Thisiscalledastandingwave
Sec=on14.8
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StandingWaves,cont
• Anodeoccurswherethetwotravelingwaveshavethesamemagnitudeofdisplacement,butthedisplacementsareinoppositedirec=ons– Netdisplacementiszeroatthatpoint– Thedistancebetweentwonodesis½λ
• Anan&nodeoccurswherethestandingwavevibratesatmaximumamplitude
Sec=on14.8
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StandingWavesonaString
• Nodesmustoccurattheendsofthestringbecausethesepointsarefixed
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StandingWaves,cont.
• Thepinkarrowsindicatethedirec=onofmo=onofthepartsofthestring
• Allpointsonthestringoscillatetogetherver=callywiththesamefrequency,butdifferentpointshavedifferentamplitudesofmo=on
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StandingWavesonaString,final
• Thelowestfrequencyofvibra=on(b)iscalledthefundamentalfrequency(ƒ1)
• Higherharmonicsareposi=veintegermul=plesofthefundamental
Sec=on14.8
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StandingWavesonaString–Frequencies
• ƒ1,ƒ2,ƒ3formaharmonicseries– ƒ1isthefundamentalandalsothefirstharmonic– ƒ2isthesecondharmonicorthefirstovertone
• Wavesinthestringthatarenotintheharmonicseriesarequicklydampedout– Ineffect,whenthestringisdisturbed,it“selects”thestandingwavefrequencies
Sec=on14.8
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ForcedVibra=ons
• Asystemwithadrivingforcewillcauseavibra=onatthefrequencyofthedrivingforce
• Whenthefrequencyofthedrivingforceequalsthenaturalfrequencyofthesystem,thesystemissaidtobeinresonance
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AnExampleofResonance
• PendulumAissetinmo=on
• Theothersbegintovibrateduetothevibra=onsintheflexiblebeam
• PendulumCoscillatesatthegreatestamplitudesinceitslength,andthereforeitsnaturalfrequency,matchesthatofA
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OtherExamplesofResonance
• Childbeingpushedonaswing• Shajeringglasses• TacomaNarrowsBridgecollapseduetooscilla=onscausedbythewind
• UpperdeckoftheNimitzFreewaycollapseduetotheLomaPrietaearthquake
Sec=on14.9
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StandingWavesinAirColumns
• Ifoneendoftheaircolumnisclosed,anodemustexistatthisendsincethemovementoftheairisrestricted
• Iftheendisopen,theelementsoftheairhavecompletefreedomofmovementandanan=nodeexists
Sec=on14.10
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TubeOpenatBothEnds
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ResonanceinAirColumnOpenatBothEnds
• Inapipeopenatbothends,thenaturalfrequencyofvibra=onformsaserieswhoseharmonicsareequaltointegralmul=plesofthefundamentalfrequency
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TubeClosedatOneEnd
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ResonanceinanAirColumnClosedatOneEnd
• Theclosedendmustbeanode• Theopenendisanan=node
• Therearenoevenmul=plesofthefundamentalharmonic
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Beats
• Beatsarealterna=onsinloudness,duetointerference
• Waveshaveslightlydifferentfrequenciesandthe=mebetweenconstruc=veanddestruc=veinterferencealternates
• Thebeatfrequencyequalsthedifferenceinfrequencybetweenthetwosources:
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Beats,cont.
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QualityofSound–TuningFork
• Tuningforkproducesonlythefundamentalfrequency
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QualityofSound–Flute
• Thesamenoteplayedonaflutesoundsdifferently
• Thesecondharmonicisverystrong
• Thefourthharmoniciscloseinstrengthtothefirst
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QualityofSound–Clarinet
• ThefiJhharmonicisverystrong
• Thefirstandfourthharmonicsareverysimilar,withthethirdbeingclosetothem
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Timbre
• Inmusic,thecharacteris=csoundofanyinstrumentisreferredtoasthequalityofsound,orthe&mbre,ofthesound
• Thequalitydependsonthemixtureofharmonicsinthesound
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Pitch
• Pitchisrelatedmainly,althoughnotcompletely,tothefrequencyofthesound
• Pitchisnotaphysicalpropertyofthesound• Frequencyisthes=mulusandpitchistheresponse– Itisapsychologicalreac=onthatallowshumanstoplacethesoundonascale
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TheEar
• Theouterearconsistsoftheearcanalthatterminatesattheeardrum
• Justbehindtheeardrumisthemiddleear
• Thebonesinthemiddleeartransmitsoundstotheinnerear
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FrequencyResponseCurves• Bojomcurveisthe
thresholdofhearing– Thresholdofhearingis
stronglydependentonfrequency
– Easiestfrequencytohearisabout3300Hz
• Whenthesoundisloud(topcurve,thresholdofpain)allfrequenciescanbeheardequallywell
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