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Chapter 5 Electronic sound（第1页）

Chapter5Eleid

Souy:mies

AsiedbyCharlesWheatstohemieurelyacousticaldevietalplatesclampedtotheearsbyaspringyrodahofdressmaker’sribbo，onepressedone’sheadagainstthesouonehelpfullysuggestedaboilile）which，withabitofluck，couldbeheardmoreclearly。Tonoone’ssurprisebutWheatstodidn’t。

&oroftoday’sdevices—whivertsouyratherthansimplydiregthemtotheearasWheatstone’sversiondid—wastheie，iedbyDavidHughesinthe1870s。Inthis，athie（thediaphragm）pressesavesselfilledwithgrahroughwhichatispassed，andthepressiveforcealterstheelectricalresistahoughtheirperformanceoor，ieswereusedintelephonesfordecades。

Althoughtherehavebeenmanymiedesigheyears，andseveralspecialisttypesareavailabletoday，onlythreemajoronesarenowinohedynamiovingie，theie，aricmie。

18。Dynamicmie。

Inadynamicmie（Figure18）thediaphragmisattachedtoacoilofwirethatsurroundsastati。Asmallvoltageisiheitmoves，whigivesrisetoaweakt。Thelowvoltagemeansthatthequalityoftherespooopoorformeasurementwork，sodynamiainlytobefoundisandregstudios。

Inaie（Figure19），thediaphragmformsoeofaerlycalledaser，hehename）。Acapacitorisapairofparallelmetalplateswithathiherairorsomeothermaterialwhiotductelectriowrithem。Attaeplatetotheivepoleofabatterychargesitwitheleetalsfreeeledthoseiearerepelledbytheelectricalfieldarisingfrenumberniiveoherepelledelethatplate，toleaveitpositivelycharged。Theacitor，therefore，nowhasavoltage（calledapvoltage）acrossit，andthemieisreadyforuse:thediaphragmplatemovesinandoutwiththepressionsaionsofthesouimpi。Alargeelectricalresistaherapidlyesg，soihesouraernsofvoltageripples。ieshaveafrequenseandareusedasmeasurementmiesinlaboratoriesandinSLMs。Theyalsorespondmorerapidlytosuddensoundsthandodynamicmies。

19。ie。

Crystalmiesandceramicmiesexploitthepiezoelectriceflewhichquartzorsomeotherematerialproducesavoltagewhenslightlypressed。Mostlaelephohese，asdotreheadsets。

&ypesofmie，ofhistoricalorspeterest，ihefollowing:

&miadeofpermalychargedmaterial（theelectricalequivalentofamagioninasimilarwaytoies。

MEMS（mieicalsystems）miesareiesetcheddiretosilichips。

Justafewsquaremillimetresiheiressandrobustheyareusedinmobilephones，amongmanyotherapplis。

Opticalmiesuseashinysilibraneasadiaphragm，whichreffectsthelightfromalight-emittingdiode（LED）。Aphotodeteeasuresthelightwhenthemembraneisvibratedbysoundwaves，aroniccirvertsthesetoarial。Suchmiesarepadrobustaedbylocaleleagheyareused，forexample，toallowunibetweeaflduringMRIss。

Pressurezonemis）aredesignedforusenearahardreffectivesurface（suplatdireastageffoorisrequired）。Withaie，reffethesurfacewouldihediredstothemie，butaPZMoveresthisbyhavingitsdiaphragmsoclosetothesurfacethatmostwavelengthsoverlap。

Ribbonmiesuseastrip（‘ribboherthanadiaphragm，aheeardrumandmostotherkindsofmie，theyrespondtothepressurediflereheribbon’ssides，whithatvelocitieswithwhichairmolethesoundwavemovearedetected，ratherthansouterns。Suchmiesareoftenusedbyentatorsinns，becauseasoundwhialldireswillimpactbothsidesoftheribbonsimultaneously，sothatitwillnotrespond。Forthisappliieshaveaprojethatisheldtotheupperlip，whichhelpsguidetheentator’svoicesothatitimpingesoheribbononly。Aivefspeeoisyehelavaliermie，whichisasmalleleicmie。Lavaliershavetheadvaheyreadilybederg。

Asouyprobesistsofapairoffaieswhichmeasurethepressuresofdiflerentpartsofthesamesouhesethemolecularvelodhehesouyiscalculated。

Animportantinieisdireality:anidealomionalmieisequallyseosoundfromaion，aocaptureafullsoundsple。Auionalmie，whichpioiononly，isidealfupspeeginnoisyes。

&heribbonmieandsouyprobe，everytypeisnaturallyfairlyomionalinrespthatthewavelengthsofthesouimpiarelargerthanthediaphragm。Iomakeamieseosoundfromimmediatelyinfrontorbehiional），allthatisoallowthebackaswellasthefrootheair。Thediaphragmwillnotmoveverymurespoosoundwavesarrivingfromallarouheriseandfallofpressureatthefrontwouldbeverysimilartothatatthebaywavesihefrontorbaiewillbepickedupreadily。

Asimplewaytoachievedirealityistomountamieatthefocusofareffectorwhosecrossseisaparabola。Thisshapereffetsouhediaphragm，aslongastheyaresmallerthaor。Alternatively，amiebemouheendofatubewithslitsalongitssides，tomakeashotgun（boom）mie。Soundassingstraightdowraveluhemie，butthosefromotherdireterthroughtheslits。Eachsudwilleipleslits，somanyversionsofitwillbeformed，eachwithadiflereheversionswilltherefelyceleachotherhdestruterferegunmiesarewidelyusedfor，oftenapaheyare，hhlyfreque。

&ytosound:loudspeakers

Loudspeakersaremiesinreverse:ifadynamiser，icmieissuppliedwithavaryirodudwaves（suchmiesarethereforereferredtoasreciprosducers）。Mostactualloudspeakersaredynamicmiesinreverse，andarecalledmovingcoillure20）。Asthenameimplies，arialisfedtoacoil（thevoicecoil）attachedtmwhichisusuallyintheformofae。Theagheeleagupinthecoilbythesig，andthediaphragm，tomove。

20。Loudspeaker。