Chapter 4 Hearing sound（第2页）

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。