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Chapter5Eleid
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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）.AcapacitorisapairofparallelmetalplateswithathiherairorsomeothermaterialwhiotductelectriowritheAttaeplatetotheivepoleofabatterychargesitwitheleetalsfreeeledthoseiearerepelledbytheelectricalfieldarisingfrenumberniiveoherepelledelethatplate，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，everytypeisnaturallyfairlyomionalinrespthatthewavelengthsofthesouimpiarelargerthanthediaphragIomakeamieseosoundfromimmediatelyinfrontorbehiional），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.
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