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Chapter 4 Hearing sound（第1页）

Chapter4Hearingsound

Therange

Beiohearisunremarkable:powerfulsouhebodyaedevenbysingle-s。Butbeiohearaswellaswedoislittleshortofmiraculous:wequiteeasilydetedwhichdeliversapowerof10fi15wattstotheeardrums，despitethefactthatitmovesthemonlyafraofthewidthenatom。

Almostasimpressiveistherangeofsoundpowerswehear。Thegapbetweeaudiblesouhreshold，0dB）tothethresholdofpain（around130dB）ishuge:130dBis1013，whichisthenumberofpenahundredbillionpounds。

Wealsohearafairlywiderangeoffrequetenoctaves，aorethanapianokeyboard。Wedetect，thoughnottrulyhear，frequencieswellbelowaoo，aswillbeexplainedinChapter6。Andourfrequencydisatio:mostofusdetectdifferencesofaboutaquarterofasemitohpradiions，adiffereohofasemitodistinguishable。emey，bytrast，ismedioinfavourableswelydetermiionofasound’ssourabout10°horizontallyor20°vertiyotheranimalsuchbetter。

14。Theear（themiddleandinnerearsaregreatlyenlarged）。

&hemostimpressiveabilitiesisthatwederstandwordswhoselevelsarelessthaofthatofbaoiselevel（ifthatbadisabroadspreadoffrequehisfarsurpassesanymae。

Ourearshavetwofuns，hearingandbalandbalaakeirelybythesemicirals（seeFigure14）。Therestoftheearhasthusbeeoevolvethebestpossiblesystemforoursoourvocalapparatus;asarelativelateeriionithadtofltitselfincheekbyjstructurespreviouslyearmarkedfaing，lidsug，kissingandflghtiraiororaplishedsihespeechsystemfunsjustasperfectlyasaStradivariusviolin。

15。Frequendence。

Whatdoearshear?

AsFigure15shsystemshaveomeasurethephysicalpowersofsounds:apiple，hasamaximumpowerofabout0。08watt，whileatromboneageabout6watts，yetitsouerihismeansthattromboworkalotharderthanpiccoloplayers（whoinfaeverplayatfullblastinanyordinarypieusic）。Butallialpowersarelow。Aloudatfullstretchmightmas;ifitcouldkeepitupfortwomiwouldbeenoughtoboilatablespoonof>

&hispowerspreadsfromthesourydires，so，ify10metresfromsuorchestra，lessthawouldreachyoureardrums。Andwhattheyactuallydeteorethanaseriesofrapidprodsofair，whioinformatiohanhowhardtheyhitandhoidlytheyarrive。Thaterienceawholeworldofsedwithemotionahmeanihepreciseationofhighlyevolvedanatomical，eleieurolsystems。

&erear:gthesounds

&hevisiblepartoftheear，alyasafuocolleds;itsfront-to-backasymmetryalsoprovidessomeinformationabouttheirdireoveablepisomemammalspossessgreatlyassistiionflnding;thoughsomeofusalsohavethisabilitytoo，itisofbeyoaivalue。

&heauditoryalisaderaround30mmlong，ithasaresoawavelehis，dingtoafrequend3kHz。Thegaihisfrequesinalossatothers，sothealactsasapartialband-passfllter。

Attheendofthealistheeardrum（tympanie），aroughlycirculardiscofstretabout1cmacross。Itisiaheizeitsareaandsocaptureasmuchforceaspossible。Itisalsoslightlyical，whichallowsittotrahanitcouldifitwerefflat。Thedrumhasnoresonances—forfrequenciesabove~3kHz，itssurfacemovesinachaotidforlowerfrequeallmovesasohewavesarelargerthanitis。Asaresultittrapeoffrequehminimalflltering。Thisfrequenseisapartthroughitsasymmetridinpartthroughaninternalscaffoldofflbres。

Itisimportantthatthedrumistautbutnotstiff，andthisismadepossiblebytheequalizationofinterernalpressurebytheEustatube（alethatitneveris，exceptforanihereisasignifitpressurege—makingadistinctivegsouoperates）。

Themiddleear:sharpeningtheblow

Theeardrumisectedtoaseriesofthreeti，infaescalledossicles，whichoccupytheair-fllledmiddleear。Theirmainjobistocovertthewideshallowmotiooahigher-pressure‘tap’onase-likemembraheroundhichisthegatewaytotheinnerear。Bywaslevers，theossicreasetheforcebyabout1。5times。However，themainwayinwhichtheforhancedissimplythroughtheratiooftheareasoftheeardrumandtheroundhicreasesthepressureabouttwentytimesbytratingtheforallerarea。Theossiclesalsoprovidesomeprotefortheihroughtheacousticrefflex（seeChapter8）。

Theinnerear:souy

Theinnerearisfullofliquid，and，justastheeardrumvertsairboroboheroundwihelattertoaffluid-borhispassesalongthecochlea，acoiledtubeabout2g。Atitsextremityisahole（thehelia），andtheassesthroughthisaravelsbagasedtubewhiedalohefirst。Whentheleteditsdoubled-baey，itmustbeelimiherwiseitwouldrefflectbackupthedihnewlyarrivedwaves。So，thesedtubetermiaheovalwindow。Thisbulgesoutavesreachit，dissipati。

Ruhetubesliketheflllingihebasilarmembrane，whivertsthewavestonerveimpulses。Theside-by-sidetubeswouldbeabout5g，weretheynotcoileduplikeasnail（cochleaisLatinforshelengthofthecochleaisrelatedtothewavelengthofsouissimilarinallmammals:onlyabout50pergerihanaperson，sothegisprobablysimplyaspagmeasure，ratherthanhavinganyacoustiidothertinymammalsodateafull-sizecochlea。Theirsareabout1dtlytheylyhearaboutthreeorfouroparedtotheeighttotenewhidmeanimalsdetebraedtheanofwhichgrowaroundninerowsofshorthairscalledstereocilia（arouheserowsruhemembrane，andhavenerveflbresatta。Theseflbresbuoformtheauditory（coerve，whismitsimpulsestothebrain。

Thebasilarmembranemovesihesouimpi。Itisstifferasrootthanatitstip，whisthatlowerfrequendscauseossclosertothelatter。Theseosscausethestereoove，andthehaircellstowhichthestereociliaareattaserochemicalimpulsestothebraihebrainknowswhichcellsarewhere，itdetermihefrequendsbythismeans（thisisknolacetheory）。However，whehsou1kHz，thewholemembraes。Inthiscase，adifferentmeismbeportant，

inwhichthehaircellsflreihepulsesthatmakeupthesoundwave—flringoimesaserespooa100Hztoance。

&hecellsareincapablemorerapidlythanabout500timesased。Torespondtohigherfrequehereforefaceasimilarproblemtothatofasquadofsoldiersarmedwithfflintlockswhichtake，say，amioreload:howthesquadproduedguervalsof10seds?Theaodividethesquadintosixgroups。Theflrstgroupflresandstartstoreload。Teer，thesedgroupflres，andsooerthesixthgrouphasflred，theflrstgroupleteditsreloadingandwillflreagain。Haircellswroupsinjustthisway:theflrstgroupmightsignalthebrainwhehesouspeak，thesethatcyclehasfallenhalfwaytoitsmiminimumahwhenitishalftomaximum。Inthisway，suchaquartetofcell-groupscouldrespondtoatoimeshigherihantheirindividualmaximumflringrates。

Thehaircellsofthebasilarmembranealsetherinaverydifferentway:thoseiorsoouterrowsrespondtoiningsoundwavesbygtheirlengthsihem。Thismotiohevibratioereotheinnerrowofhaircells（theosendsignalstothebrain）andhenceprovidessignifltamplifl，allowingustohearsoundswhichare40dB（ohousandth）lesspowerfulthaherwisecould。Thisactivitygesownfaintsouoaissions。Theseemissireatvalueihefunalityofthehearingsystemininfantswhoaretooyouherorwhattheyhear。Also，theyoftehereisaheiheyareausefulcheckfists（hearis）too。Theyare（fortuooquiettobeheard，sosensitivein-earmiesareusedtomeasurethem。

Nervesandbraiosubjective

Thenervesignalsthatemergefromthebasilarmembramimidwaves，butessageswhithreepieation:（a）howmanynerveflbresaresignallingatonce，（b）hthebasilarmembrahoseflbresare，and（gtheintervalisbetweenburstsnals。Thebraisloudnessinformationfromabinationof（a）and（dpitationfrom（b）and（c）。

16。Losuage，aivitiesiheprimaryauditnizessounds。Broca’sareabothanalysesaits。Wernicke’sareadealswiththesequencesofspeeds（heard，made，ahesupramarginalgyrusmaydealwitharti，andtheangulargyrusmayassistrog。

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