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Chapter 4 Hearing sound（第2页）

ensinthebrain’shearingandlares（seeFigure16）isirelyclear，buttheflrststageinprogistoextratfeaturesfromthestreamofinputdatafromtheauditoryuresaretheinuouslyupdate，amend，aalmodelofthethieune，aspokenphrase，ihemodel’saccuracyistestedbypredigwhatthesoundwilldothebrainistryingtodoisestablishthedegreetooofasoundtributestothemeaning，aprocesscalledhierarg.Ourincredibleprowessinperfthisfunonstratedbyourabilityto，forexample，reizethatsometohum‘SomewhereOvertheRaiheygetmost.Thespeedwithwhichthemammalianbraihesoundsthatreachtheearsisalmostincredible—say‘squirrel’toadogord，andtheresponseisalmostinstantaneous.

Homingin

Folloieusicisaratherunusualandnewfangled（iiforabraintodo:usuallythefoterestisaheexternalworld，alyanimportantfunoftheheariolocatethatobject.Fhfrequehebrailyohatasoundtheleftwillarriveflrstattheleftear，andpartlyontheblogeffectofthehead，whisthatthelevelattherightearwillbelower.Forasouhaaheears，thebraiheglevelsofsoundattheears:ifalongwavearrivesfromtheleft，eachofitsantinodeswillreachtheleftearflrst，sothepressureatthatearwillinitiallybehighest.Astherogresses，theleftearpressurefallswhiletherightrisesui，whenitwillbegintofallagaihawavemorethanab，thereisverylittlelevelovertheiasdireotbejudged.

&ermihesoundsourceisaboveorbelowtheears，thebraiheeffectsoftheshapesoftheheadahelevelofthesouionflndingisnottheonlyadvantageofhaviheauditoryimesflreevenwhennosouthebraisualsiftheyoneside.

Thebrain’sprogsystemhasevolvedtomakereasoionsaboutthesoundsitreceives，leadingtosuenaasthepreceeffect（alsoknowheflrstwavefrontortheHaaseffect）.Thebrain’sassumptionisthatasoundthatarrivesifraillisedicatesthedireofthesoundsource.So，subsequentsardedasedireastheflrst.Thisallowsustolocateasoundsourdarkspaceswithoutbeingfusedbyechfrommaions.Suchassumptionsmislead，espesituationsthatwouldnotoaturally:forinstanelistenstoasoundfromaloudspeakeraboutametreawayandat45°totheleft，andthissoundisgraduallyreplaidentialoudspeakerat45°tht，thesoundwillstillseemtotheleft.

Averyusefulfeatureforpetounicrowdsisthecocktailpartyeffewhichparticularphrases（sue）standoutfromthehubbub.Thisworksfornon-vodstoo:ductorsareoftenhighlyattuistrumentsormusicalphrases.Thiseffectworksbecausethebrainistlymodel-buildiherweareactivelylisteningornot，apreferecheswithsoundsthatithasclassifledashavingasignifliso>

Theroleisaveryriplexone—aslinguistRolasout，soundsaindsinthreeways:as‘ihealarmingsoundofanexplosion），‘sigeralmeaningofaword），and‘signiflers’（unsciousassostriggeredbyawordlike‘end’）.AndhearingisalsoverymuchasocialactivityttoLabelle:

theridulationsofauditorymaterialdomuflxdeliioheprivateandthepublidoperatesbyflinks，groupingsandsthatatuateindividualidentityasarelatio...[and]weaveanindividualintersocialfabric...tributingtothemeaningofsharedspaces.

&sout，too，thatwhetherwelikeitornot，susintoiwithbabies，noisyneighbfelloorters.Asheputsit:‘Souiohatismostofteioious，ffluid.’

Hearingbones

Theeardrumsarewoiiweactuallyhearfairlywellwithoutthem，sindwavesalsoreaerearbytravellingthroughthebohehead，speciflcallythemastoidboheear.Submergihebathlargelyswitchesofftheairborhesoundsthatremainarrivemainlybyboion.Thissystemisratheriivehair，wehearsoundsabout40dBlesspowerfulthaectablethroughourmastoids.Oherhaionallowsustodetedswithfrequeo30kHz，whichiswellbeyondtheupperfrequencylimitoftheairbor，presumablybecausesudsareoflittlevaluetous，theyareallehesameway，sotheygiverisetothesamepitsationas20kHzsounds.

Bottlenosedolphins（Tursiopstruncatus）takebone-basedhearingmuchfurther:theirjawsbearteeththatarespacedatregularintervalsahesameangle，areallofaverysimilarshape，ahatdependupoion.Thisaddsuptoafogarray，inwhidarticularwavelengthsaresignifltlyamplifled—butonlyifthesourceisdirectlyahead.Thus，thedolphinshearveryquietsounds，aheirdireplybymovingtheirheadsuntiltheloudnessismaximized.

Deafness

Thehearingsystemisadelie，aotheeardrumsorossiooccurswemustrelyinsteadoionandartiflcialaids:Edisoeethtotrahesoundsfromhisphonographtohismastoid，asthemarksoilltestify.

Thisisductivehearingloss.Ifdamagetotheinnerearorauditorynerveoccurs，theresultissensorineuralor‘nerve’hearingloss.Itmostlyaffectshigherfrequendquietersounds;inmildfivesrisetoacalledret，inwhichthereisasuddenjumpiy’ofsounds.Apersfromretaoasoundofgraduallyinglevelatflrstdetegandthensuddehesound，whichseemsparticularlyloud.Hehere’soshout’protestihosewhoraisetheirvoicesjustalittletomakethemselvesheardoempt.

Sensorineuralhearinglossistheosocauseisphysicaldamageihehaircells.Withveryhighlevelsofsound，theeardrumsberuptured（andtheyagedbyblowstotheheadorbyiion）.Remarkablyhowever，bursteardrumslyheal，theyusuallythenperformalmostaswellasbefore.About360millionpeopleworldwide（over5pertoftheglobalpopulation）have‘disabling’hearingloss—thatis，hearinglossgreaterthan40dBier-hearingearinadultsandahearinglossgreaterthan30dBier-hearingearin（about10pertofthetotal）.Aboutohreepeeofsixty-flvesufferfromsugloss.

Intermsofalleviatievariesgreatly.Forductivehearingloss，ifsfunains，hearingaidstailor-madetofltthearticularpatternhlyeffective.Theymayalsobeequippedwithnoise-gfundofteionalmies，sothattheuserfosourcesthattheyarelookingat.Theyalsoiiimulatethemastaidsarelesssuctreatingsensorineuralhearingloss，bealsaredistortedbyret.

Withpletedeafhegeismuchgreater，butifewyearstheintroduoffairlyreliableplantshasgivehefuturel:in2012，byengstemcellstogrowintohaircells，deafenedgerbilshadtheirheariypicallyby45pert，butby90perafewcases.Thisapproaedaybeapplicabletothosepeople（arou）whlossiscausedbydamagetonervescalledspiralgaherearealsosomeanimals，ingowls，inwhichloststereociliasimplygrowbaditmaybethatthiscapabilitycouldbegeiduhumans（tiherpeculiarfeatureof，whichisthatitonlyworksatitsbestinspring，presumablybecauseowlschextrapreyfortheirchi）.

Thereareagreatmanyhearingproblemswhiplycharacterizedbylossoffun.Theoinnitus:ringiscausesarelargelymysterious，aype，anddurationofthesreatlyfrompersoisoftenassociatedwithpastiis（especiallytibiotia，alyapanieshearingloss.

Structuresforspeech

Aroundamillio，thehearingsystemsofouraorsulegestoflerally—todetectspeeowlittleabouttheevolutionofourvocalsystems;althoughtheabilitytomakesoundsinatrolledwayisostanimalspecies，speechisimmeasurablymoreplex.So，uionof，say，theleg，weotlookbackatalongofaralformsandwatchthesystemadapttothegrequirementsofthoseusersaingdemandsoftheires.

17.Vocalapparatus.

Initsmostbasiakingofsoundsissimple:Figure17showsthestruvolved.Airexitsfromthelungsthroughatube—thewirachea—whichisequipped-likevocalfolds（vocalcords）thatrestricttheairfflowaheyaretensedbymusclesatta.Ihistensiohevibratiothelengthofthefoldssetsitslowerlimit，resultingialfrequend125Hzformen，200Hzforwomen，and300Hzorabovefor.Inboys，thereisasuddehduriy，whichcausesthe‘breaking’ofthevoice.

Afteremergingfromtheerachea，thislow-frequendehebackareaofthevocaltract，roofedbythesoftpalate.Infrontofthisisthehardpalate.Togetherthesestruawhisresonas.Thecharacteristigthsofvowelsaresethere，andvariedbyraisiogethevolumeofthetractortodivideitintotwoliies.

tsinvolvemorepartsofthevocalapparatusthanvowels，areusuallyshorterinduration，and，inmanycases，gewhiletheyarebeiherearefourmaintypes，deflheirmai.

Plosivesaremadebythesuddenstoppingofairfflow（heerops）.Fridliquidsrequirepartialstopping，withorwithoutturbuleively.heairstreamtothenasalcavity.Glides（semivowels）involvearapidtransitionfromooaheunusualhelpfulhisnamingsystemissharedbythesubdivisionofthetsagtotheirplaceofarti，asTable4shows.Alsoshowherornotthetisvoiced—thatis，whetherthevocalcordsareinvolvedinmakiriloquistsattemptingtoproducelabialorlabiodentalsouymiedbytheheirlipsseparatedandmotionless.Skilledpropohearttthisbyspeakiisuts.）

&hewavelengthsoftheresohevocaltradonlyonitsstructure，gthevelocityofsouhefrequehosewavelehe‘DonaldDuck’voiceproducedbybreathinghelium（14pertofthedensityofair），andthemorerarelyheardgravellyvoiceproducedafterbreathingxenon，whichis4.6timesdehanair.

Table4.Englishts

Hearingsound

Lifewouldbeadullthing，however，ifallwedidwithourvocalapparatuseak.Singingis，physiologiodifferentfromspeakieveryaspectofthesoundmadeismoreflrolled，andpitchisofteoaernallydeflliihevocalcords:itrequirestheproduofturbulehelips，whisfersehevocalcavity，whichactsasaHelmholtzresonator.Shoutingsimplyrequiresgreaterairforthelungs.Ihevocalapparatusworksasitdoeswhenprodualspeech，exceptthatthevocalfoldsareednorfullyrelaxed，sothatassesthroughthemitproducesturbulehisiscalledaddu）.Sincemuchmoreairpassbetweenthefoldswithoutexgsoundwaves，whisperingisnecessarilyrelativelyquiet.

systemsarefarmoresophistiourmostadvanes，aosuitusadmirably.Butwhatnaturehasgivenusislimitedinrange.Inallbutthetigroupsofpeople，uni—whichweprizesohighly—mustspreadfarbeyondthereachofvoice，anditwastoaflrstelectridtheronicswerepressedioaugmentandtoreplaceourffleshandourheydothisisthesubjectofChapter5.