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Chapter 7 Sound underwater and underground（第1页）

Chapter7Souerandunderground

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。

Fh-polis，suchasgeophysicalsurveyingforoilaiuoproduderwatersound。Inthese，asmallcavityischargedwithpressedairandarelayreleasesthepressuresuddenly，rapidlyfalargeairbubbleandanapanyingburstofsound。Thepulsefrequencyisaround20Hzto200Hzaudeisveryhigh（probablythe‘loudest’man-madesourtheoexglargeexplosioravelsthroughtheseabedasfromtheiweentherocklayersbel（upto10km）arraysofhydrophoowedhesurfaagetherefled，asareprocessedbyputerstoprovideathree-dimensionalmap。