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20.Loudspeaker.
Loudspeakersareintrinsieffltdevices:mostoftheeleergythatpassesintothemappearsasheat，withonlyabout1pergaudible.Hence，amplifiisvital.Transistorsmakeamplifiplemattertoday，themaingebeihateacyisamplifiedtoanappropriateextent，whitheributesofthehearingsystem，meansthatdiflerentdegreesofamplifiustbeappliedtodiflerentfrequehepitvelopeoftheoutputsoundistoremaiisessentialtodeploythemwithcare.Quiteapartfromthedeleteriouseflehearing，loudspeakersbeeasilydamaged，especiallybyartificiallyproduds，whichmayhavearisetime.Also，ifthemiepicksupthesoundoftheloudspeakertowhichitissendingitsoutput，apositivefeedbackloopissetup，resultinginanalltoofamiliarsqueal.
Loudspeakersaretheleasthi-filinkinthemusicprodupleinpriheirdesigsmanypracticalges.Thevoiustretursstartpositiohesignalfallstozero，withnoosbeyondit，abefreetomove.Theustholditsshapewhileitvibrates，beverylight，yetrigidenoughnottosaguy;largeenoughtevolumesofairatlowfrequencies（whereonlypowerfulwavesareaudible），yetsmallenoughsothattheymovebadforthover10，000timesasedathighfrequencies.Also，thegmustanyfrequency.
Iismucheasiertousespeakersingroups，usuallyalli—asmalltweeterforfrequenciesabove2kHz，alargermid-rangespeaker（50Hzto5kHz），andawoofer（30Hzto800Hz）.Forthosewholovelotsofbass，theremayalsobeasubwoofer（20Hzto200Hz）.
Subwoofersareusuallyactiveloudspeakers，whistheytheirolifiers（aheirownpowersupply）.Mostotherspeakersarepassive，drivenbyasignalwhichhasalreadybeenboostedbyahinthehi-fi（orother）syste
Aloudspeakerwithoutacaseisalmostsilent，forthesimplereasonthatthehigh-pressurepulsesgesfrontsimplysliprouofillthelow-pressureareathathasjustformedthere.Hence，loudspeakersmaybemountedinanairtightbox.Iftheboxissmall，however，theairisbeihediaphragmmovesiivesolutionistoplacethediaphragmireofanannuluscalledabafie.Thebafiemustbelargeenoughsuchthat，bythetimethepressurepulsehastravelledroundittothebackofthediaphragm，thelow-pressureareatherehasgooputitahedistahesouravelislohanaquarterofawavelengthfestwaves—thelowestfrequeerest）.
&zresonaoextendthelow-frequenanceofaloudspeaker.Ahole（port）ismadeioftheboxinwhichthespeakerismouhecavitythelowfrequeheresonaheboxliesbelowthatofthespeaker，apressurepulseproduthespeakerdiaphragmmovesback（thebackwave）willmakeitswaywithieattheport，whereitwillbeihaproducedbythefrontofthediaphragThesetulseswillreiher.Thisappliestopulseswavesofafallsbetweentheresonahediaphragmandthatofthebox.
Onedisadvathepuflsofairfromtheportesbeheard，asoundsarelesscrispbecauseeaalisfollowedbyash‘tail’ofresonances.Also，pulsesfsouhfrequeoftheportarecelledoutbythesubsequehediaphragFrequenciesabovebothresoherenhanorreduced.
Ourbrainsaresogoodatfillingingapsinsoundsthatloitthistomakedowithquitebasicloudspeakers.Ioftonesof200Hz，300Hz，400Hz，and500Hzwillalmostalwaysbeharmoones）ofafu100Hz.Becausethebrain’shearire（seeFigure16）‘knows’this，itwilltlydecidethatthe100Hzisactuallythere.But，ifthesetoftonesisallloudspeaker，it’slikelythattherewillnotinfactbea100Hztohisefleownasthemissial，andiswhythelackoflowfrequeheEpidaurustheatre（Chapter1）didnotse:theheariheaudieedtheItalsoexplaiiveclarityoftheearliesttelephones，whiitlowfrequencieswell.
&roduofeflectivemiplifiers，andloudspeakersrevolutioheredustry—aioncouldberapidsihemselvesdidobeged.Sogessooesuddenly，almostanyonecouldaccessmusidselectwhattheylisteoaswell.
AgtosouJonathanSterne，suedia‘toquestiontheverybasisofexperieeheefleerswereprofound:asmusichistoriaPhilippointsout，maatthenumberoferrorstheyheardintheirrecordedperformances.MusiarkKatzsuggeststhatsuersbecametrappedina‘feedbackloop’inwhichtheyattemptedtoproduore‘perfeaobedisappointedallaiesoftheresult.Asaresult，performancebecamelessindividualized，morestandardized，andlostspoheexperieeningtsotherkioo:violinvibrato，forexample，wasinallyaphonographiceflect，butwassooatedbyperformers.
&revolutioniheiionofstereophoniin1933，achievedbyregthetwoelsowowallsofthegroove，at90°toeachotherand45°tothevertical.Theintroduofstereoreeantthatinprihree-dimensionalsouheinalperformancecouldberecreated，giviionsofidealspeakerplatwhichhavefasatedmusicbuflstothisday.Italsogaverisetotheceptof‘fidelity’，sioneasreganinalperformance.Forawhileatleast—andmixingteiques，bythe1970smanypoppieeverperformedassuch;muchoftheirtwasaddedafterthebandhadgonehome.Forclassicalmusichowever，faithfulreghasremaiheless，despitedecadesofibymanymillionsofamateursandprofessionallisteners，recorders，performers，andplayers，‘fidelity’isstillunquantifiable.
Stsounds
&eplifidled，themaihefragilityofrecords.Awholeculturedevelopedghowtohandle，house，d—oraiheTheauto-gercameasagreatrelieftosome，thoughtardedaseitheraspoilsportorareager.Partlybeystiqueandpartlybecauseofthehighqualityofsomeoftheircovers，vinylredespeciallyLPs（latercalledalbums），wereveedasobjeawaythatnmediahaveeverbeen，aillsoldtothisday.
In1964aetapeswereintroducedasarobustandpaativetodisdtobeginwithwereveryphreeswerereadilyavailable，maobuyavinylred（illegally!）traocassettetolistentowhilekeepingtherecordpristine.Radiobroadcastscouldalsoberethiswayandthemusitre，prisingradio，cassetteredrecordplayer，becamepopularsiallowedcassettestoberecordedwithaminimumoffuss.
&womajordisadvaape，however:trackselerequirestime-ingwinding，andhigh-frequencyhissisuterwasreduewhatbythemanyvariantDolbysystemsonofler，allofwhichwaversiorackwiththehighfrequeed，andthehemonplayback.Thisisaeiquepanding（aportmanteauandexpanding）.
Justasstereospawerestsandbehaviours，sodidtape.Mixertapeswereoe，aheSonyWalkman，whichatlastallowedmusicloverstolistentotheirmusicwherevertheyhappeobe，whileonlyaihe
Butwhatwasodoawayehanals，thatis，thoseinwhidisstoredasaupatterherphysiavinylrecord，
iacassettetape）.Inadigitalsystem，therecnaliseringofnumbers，aformitbestored，transmitted，orcopiedwithionofthesignalnorinbaoise.
Atfirstglamayseemthatiuretheintriplidwave（likethoseiheamplitudesofagreatmanypointsofthatwavemustbemeasuredandfact，thesoundneedonlybesampledtwiceasfrequentlyasthehighest-frequenpooopreserve.So，toencodeallfrequenasignalupto8kHz，osampleat16kHz（thisisknowtheorem）.Ifoalowerrate，theeabeedistorted，akno>
Withtheintrodupactdis1982，awholesaleshiftfromaalbegan.OnaCD，digitallyenalsarestoredaspatternsofdarkpitsinashiallayer，whicharesedbyalaser.Thelaserreffeoothareasbutnotfromthepits;theCDplayerihereffesas1sandtheionsas0s，aringsof1saheaudioinformationasasequenarynumbers.inally，CDswereclaimedtohavetheimpressivethoughnotespeciallyusefulcapabilitytoplayevehmarmalade.ThetsionthatthemarmaladehadtobeappliedtotheuppersideoftheCD，asthegisontheunderside.
Nowadaysofusicisrouti，stored，ausingphysicalmedia—audiofilessimplybedowoaputerahroughawideraputdevi，theputerispartofanMP3player（MP3isshPictureExpertsGrPictureExpertsGroup-2AudioLayerIII’）.
ThemagiP3audiofilesishowsmalltheyare:aboutohesizeofequivalentCDfiles，whisaminuteofMP3musibesqueezedihisimpressivereduisachievedpartlybyateiquecalledHuflmanwhichthesymbolsthatappearmostofteheshortestpossibleartlybyorefullythosefrequendswhichpeoplewilliftheyaredisrupted（mainlyspeedprovidingochyversionsofthosefrequenciesoflesstous.
BecauseMP3playerstakeintoausidthelistenerindegwhatinformatiowhenthesongispressed，Sternehascludedthat:
theMP3carrieswithinitpradphilosophiderstandingsofwhatitmeanstoumeanstolistehemind’searworks，andwhatitmeanstomakemusieveryMP3arewholeworldsofpossibleandimpossiblesoundaoriesofsonicpractices....MP3encodersbuildtheirfilesbygamomeiohegtsandthegapsandabsenagiheotherend.TheMP3encoderworkssowellbecauseitguessesthatitsimagiorisaehaions.Itofte.
&usicexploitsthefactthat，overbillionsofyears，systemshaveevolvedtorespondtothesoundswhiostrelevahis，bihlimitatiohenatureofsousourimmediateaccesstotheworldofsoundtoafrequengewhilyathiisactuallyoutthere.ThoseunhearablerealmsarethesubjectofChapter6.
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