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Chapter 4 Hearing sound（第1页）

Chapter4Hearingsound

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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.