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Chapter4Hearingsound
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Therange
Beiohearisunremarkable:powerfulsouhebodyaedevenbysingle-s.Butbeiohearaswellaswedoislittleshortofmiraculous:wequiteeasilydetedwhichdeliversapowerof10fi15wattstotheeardrums，despitethefactthatitmovesthemonlyafraofthewidthenato
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，thesethatcyclehasfallenhalfwaytoitsmiminimumahwhenitishalftomaximuInthisway，suchaquartetofcell-groupscouldrespondtoatoimeshigherihantheirindividualmaximumflringrates.
Thehaircellsofthebasilarmembranealsetherinaverydifferentway:thoseiorsoouterrowsrespondtoiningsoundwavesbygtheirlengthsiheThismotiohevibratioereotheinnerrowofhaircells（theosendsignalstothebrain）andhenceprovidessignifltamplifl，allowingustohearsoundswhichare40dB（ohousandth）lesspowerfulthaherwisecould.Thisactivitygesownfaintsouoaissions.Theseemissireatvalueihefunalityofthehearingsystemininfantswhoaretooyouherorwhattheyhear.Also，theyoftehereisaheiheyareausefulcheckfists（hearis）too.Theyare（fortuooquiettobeheard，sosensitivein-earmiesareusedtomeasurethe
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.
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