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Chapter 5 Electronic sound（第2页）

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）system.

Aloudspeakerwithoutacaseisalmostsilent，forthesimplereasonthatthehigh-pressurepulsesgesfrontsimplysliprouofillthelow-pressureareathathasjustformedthere.Hence，loudspeakersmaybemountedinanairtightbox.Iftheboxissmall，however，theairisbeihediaphragmmovesiivesolutionistoplacethediaphragmireofanannuluscalledabafie.Thebafiemustbelargeenoughsuchthat，bythetimethepressurepulsehastravelledroundittothebackofthediaphragm，thelow-pressureareatherehasgooputitahedistahesouravelislohanaquarterofawavelengthfestwaves—thelowestfrequeerest）.

&zresonaoextendthelow-frequenanceofaloudspeaker.Ahole（port）ismadeioftheboxinwhichthespeakerismouhecavitythelowfrequeheresonaheboxliesbelowthatofthespeaker，apressurepulseproduthespeakerdiaphragmmovesback（thebackwave）willmakeitswaywithieattheport，whereitwillbeihaproducedbythefrontofthediaphragm.Thesetulseswillreiher.Thisappliestopulseswavesofafallsbetweentheresonahediaphragmandthatofthebox.

Onedisadvathepuflsofairfromtheportesbeheard，asoundsarelesscrispbecauseeaalisfollowedbyash‘tail’ofresonances.Also，pulsesfsouhfrequeoftheportarecelledoutbythesubsequehediaphragm.Frequenciesabovebothresoherenhanorreduced.

Ourbrainsaresogoodatfillingingapsinsoundsthatloitthistomakedowithquitebasicloudspeakers.Ioftonesof200Hz，300Hz，400Hz，and500Hzwillalmostalwaysbeharmoones）ofafu100Hz.Becausethebrain’shearire（seeFigure16）‘knows’this，itwilltlydecidethatthe100Hzisactuallythere.But，ifthesetoftonesisallloudspeaker，it’slikelythattherewillnotinfactbea100Hztohisefleownasthemissial，andiswhythelackoflowfrequeheEpidaurustheatre（Chapter1）didnotse:theheariheaudieedthem.Italsoexplaiiveclarityoftheearliesttelephones，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—oraihem.Theauto-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，whileonlyaihem.

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.