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Chapter 7 Sound underwater and underground（第1页）

Chapter7Souerandunderground

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Sedsight

Ifourworldhadalwaysbeeedinfog，wewouldprobablymakelittleuseofsight，andrelyoead.Ifthefogprovedopaquetoradiowaves，ourteleuniswouldnodoubthavetakenanacousti，too.Theoisaworldofjustthiskind:visibilityispoorduetothemaithewater，radiowavesquicklydie.Aerisfullofsound:fish，marinemammals，humandivers，submarines，aerrobotsalluseitforsubtleandsofsensation，unidsometimesattack.

Allthiswouldbeoastistlivingouryago.It’struethatLeonardodaViaround1490that‘Ifyoucauseyourshiptostop，aheheadofalohelacetheotherextremitytoyourear，yousatagreatdistanyou’，butookmuoti.

ItwasnotuntilWorldWarIthatlisteniersoundsofvesselsted，andnotuntilWorldWarIIthattheriessoftheuersoundscapebecameclear.Thishappelythroughtwobizarreevents.In1942，acoustiChesapeakeBay，whichhadbeeesubmarines，allalertedatoillaofdestroyersenthusiasticallydepth-chargedthearea—butaleoilslicksappearedandtheonlyvictimswerethousandsoffish.Laterthatsameyear，allaloerncoastsoftheUS，everyaieanythingeller，infearofaJapaneseinvasioedatondagain，thesoleresultwasamultitudeofdeadmarinelife.

IftheUSmilitaryhadknownalittlemoreaboutafishcalledtheuchembarrassmentandmanydeadfishcouldhavebeenavoided.Forallitsundistinguishedappearahissmallbrowurehasaveryloudvoideed—somethinglikeamagnifiedwoodpedcroakeriesgivevoiceeverydawn，alltogether，muchlikebirds.

Noisyfishwereanalmostpletesurprisetobiologists.Therewasahithertouiohedepthswereassilentastheywereproverbiallysaidtobe.Infact，whiletheoshadbeenroutiedfordiionaltravelandasasourceoffood，whatlaybehesurfacewaslargelyunknoarentlyrarelyeveedupon，beyondwildtalesofseamendsofsuhedarkuerworldseemstohavebeenregardedaswhollyalienandnoplaceforpeople—evensailardedthedepthsasforbiddeoryandmostofthemdidoswim.Whileartistsandsychologiststoo—ofteheseaasthewilddomainofrawheirattentionwasedthsurfae.Besurfaegativeregioobeasdevoidofsoundasitwasoflight.

However，fromtheearly1940s，listeningwithhydrophoermies）revealedjusthowdeep’reallyis，honyofsouendingwellaboveahehumanfrequenge，fromavastmultitudeofsources，mosttheifiable.Theloudestsoureareasturobeiesofsnappingshrimp.Infact，theupperlayeroftheseaishabadgeedbybreakingwaves，rain，andlightningaswellasbymarinelife.Inshallowareas，swirliisanadditional，omsourd.

Hearier

&woreasosilehesea:onephysical，theical.Thephysieistheimpedaairandwater，inceofwhichthesurfaceaairrbackalmostallsouhatland-dwellershearhebreakingofthe>

Submergeyourhead，andthebiologiaself:uheeardrumhaswaterononesideandairoher，andsoimpedaorepreventsmostsouering.Ifwehadnoeardrums（norair-filledmiddleears）wewouldprobablyhearverywelluer.

&eranimalsdon’tneedsuplicatedearsasours:sieraroundthemisasimilardensitytotheirffesh，souersahroughtheirwholebodieseasily，withouttheneedofpio，norofdrumsorwindowstotrafromooanother.Fishdohaveearbohs.Theyaremadeofcalciumate，thehighdensityofwhichprovidesasufltimpedaoallowvibrationbysouhismotionistraereonhaircells，which，likeours，sendnervesignalstothebraichesofhaircells，euromats，aredistributedontheskinsoffish.

Twootherstructuresaugmentthehearihefirst，theswimbladder，isanair-filledsacwhisliketheballasttanksofasubmarine，gthebuoyancyofitsownerasrequired，sothatthefishkorrisewithoutmusculareffort.Theswimbladdervibratesreadilytosoundwaves，andfunsasafairlysensitivehearinganuptoabout3kHz.Ithas，hreatdisadvantage:beingasiri，thesrovidesnoinformationaboutthedirewhidsarrive.

Itissupplementedbyasedstructure:thelateralline，afluid-filledtubethatrunsalongthesidesofthefishandfunsasadiresitivesoulowfrequencies（~160Hzto200Hz）.Uereoourears，hensoundwavesiionsinthebasilarmembraowhichtheyareattached，lateralliereociliaarepushedalybyiningsoundwaves，whisthatthedireofthesoundsaresely（afishseionofwatermolecules，ratherthanthepressuregesthatwedo）.Thismakesfishveryhardtocreepupon.

Teiquesandtransducers

&seriousteologicaluseofsouerwasthesouem，inwhiderwaterbellsplaearportscouldbedetectedbyshipsfittedwithprimitivehydrophoheformofiesinroofgs.Awatlisteheshipinstereoandcouldguidetheshiptoporteveninpoorvisibility.Thissystemwasfittedtomanyshipsintheperiod1875–1930，ingtheTitanidLusitania.By1923，therewerethirtybellsarou.Fromthe1910s，thesystemlacedbyeding，whiakingasouimingitsedwoutthedistanakheveloderwatersound:anotherexampleofthepulse-eique.

Fromeodernsonar（soundnavigati）systemsdeveloped.Inanaarsystem，short-durationsoundimpulses（‘piedfromavesseltoeobjetheirpath.AswellasdeterminingdistangesinthefrequencyofthereceivedpulsesareusedtocalculatetherelativevelocityofthestheDopplerefflect）.

Passivesonarsystemssimplylisteersouhoseofshipping.Autnitionteiquesallowdifflerenttypesofshippiifiedfrinesouhehummiricalsystems:eachvessel，infact，hasitsuureoracoustigerprint.ThisteiquebecameveryimportantintheColdWarfnizingandtraemyshipsandsubmariiveandpassivesonarsystemsaresometimesdeployedonbuoys（sonobuoys），edwithradiosystemstoreps.

Thehydrophorumentforuiostallthoseiodayarepiezoelectric，usuallybasedoicceramiccalledPZT（leadandtitaniumzirate）.Unlikemies，hydrophosometimesbeverylarge，ioachievedirealityatlowfrequehisreason，thesidesofsomesubmaripletelyplatedwithhydrophoheseffankarrays，astheyarecalled，areusuallymadeofpolyvinylidenediffuoride（PVDF）.

&erequivalentoftheloudspeakerisknorojector.Projectorshavealimitatioapplytoloudspeakers:whenthesurfacemovesinwardstocreatetherarefaphaseofasouiosiftherarefaalpressureislowerthanthatwater.Thebubblessdabsorbthesouheprreaterthedepth，thehighertheressurebeoresouheprojeproducebeforetheoof.