Chapter 7 Sound underwater and underground（第2页）

Thoughprojectorsareinprihydrophoheirphysiisofte。OwidelyusedtypesistheTonpilz（Germanfmushroom’）transduwhichseveralpiezoelectricPZTdisdwichedbetweeomakeastackwhichisterminatedbyaicalordricalheadmass，thebusiheprojepilztransdugeefrequehe2kHzte。

Formaerapplis，ingsignallinganddistang，direalsoundsarerequired。Justasinair，asoundsourcewillbenaturallydirealifthesouproduceshavesmallerwavelengthstharansducerface。Butbecausethevelocityofsoundisaboutfivetimesgreaterihaninair，thewavelengthdingtoaparticularfrequencyisalsoaboutfivetimesgreaterthanitsairbor，sodirealityishardertoeby。

&waytomakeadirealsouheparametricarray。Iftwoseewaveswhichdifflerjustalittleihehatdifflerencywillbeprodugwithotherswhosefrequehoseofthesources。Thewavelengthofthedifflerencewavebeaslongasrequired，butitmaiionalityofitsparent>

Parametricarraysexploitthefactthatsouydepehighsouhepressureintheesverylarge，ingdensitysignifidthereforebrieffyspeedingupthesouhereversehappeioofthesevelogesistodistortthewavefromitsusualsinusoidalform。

Thisisaoinhigh-powerultrasound，too。AsAugusteFouriershowed，anon-sinusoidalwaveisequivalenttoasumofposinusoids。Iheparametricarray，theseposiheiherwiththesumanddifflerehedifflereheo。Parametricarraysalsobeusedinair，tomakeaudio-frequenoredireal。

Althoughthereislittlethateleagicradiationdoesabovewaterthatsoundotdobelowit，soundhasoneunavoidabledisadvasvelowaterismuchlowerthanthaticradiationinair，whisthatsgtakesfarlonger。Also，whenwavesareusedtosenddata，therateofthatdatatransmissiolyproportionaltothewavefrequendaudiosoundwavesarearound1，000timeslowerihanradiowaves。Forthisreasonultrasouead，sisfrequechthoseofradiowaves。Aageisthatitiseasiertoproducediresatultrasoosendthesignaliionyouwant。However，adisadvaabsorptiohsouheraed。

Soundsworldwide

&ancesoverwhidtraveluerareamazing。Itisbelievedthatbeforetheproliferationofengine-poweredvessels，AntarcticwhalescouldbeheardbytheirArctis。Suchvastrangesarepossiblepartlybedwavesareabsorbedfarlessihaninair。At1kHz，absorptionisabout5dBkminair（at30perthumidity）butonly0。06dBkmier。Also，uersoundwavesaremuchmoreed;anoisemadeinmid-airspreadsiions，butihebedandthesurfacelimitverticalspreading。

Box11

Souyihemostacs1）equationistheLeroyRobinsonformula:c=1402。5+5T-0。0544T2+0。00021T3+1。33S-0。0123ST+0。000087ST2+0。0156Z+0。000000255Z2-0。0000000000073Z3+0。0000012（Φ-45）-0。00000000000095TZ3+0。0000003T2Z+0。0000143SZ;Ttemperature（°ity（pert），Zdepth（metres），Φlatitude（degrees）。

Therangeofsouiesuerisfarlargerthaninair，becauseoftheenormousvariatioy，whichisaffectedbytemperature，pressure，andsalinity（Box11）。Levelsatwhidvelogesrapidlywithtemperaturearecalledthermodtheyfolloatterninmostseas:wheheriscalm，theuppermostlayeroftheseaischaracterizedbyarapidfalliure，andhendvelocity，withdepth。Becauseditionsaremoreer，thisisknownastheseasonalthermoe。Belowthisisthemainthermoe，wherethetemperatureandsouyuetofallwithdepth，iofseason。Atthebaseofthemainthermoe（thedepthofwhichvariesgreatlywithlatitude），asteadytemperatureofaround4°Cisreadhardlygesatdeeperlevels。Ihermallayer，pressurebeesthedominantfadeterminingsouy—whisequehdepth，asshowninFigure22。

22。Uersouyprofile。

Sosomewhereunderallosthereisalayeratwhidvelocityislow，saweenregionsinwhichitishigher。Byrefra，soundwavesfrombothaboveandbelowaredivertedtowardstheregionofminimumsouy，arappedthere。Thisisthedeepsouhinsphericalshellextendingthroughtheworld’sos。

Sindwavesinthedeepsoundelmoveonlyhorizontally，theiriyfallsinproportionoaravel，ratherthantothesquareofthedistaheywouldinairoriasiure（inotherwords，theyspreadoutispheres）。Soundabsorptioninthedeepsoundelisverylow（itisstronglyfreque，butaround0。2dBperkmfor4kHzwaves），andsouhedeepelreadilyavigatetheEarth。

ThedeepsoundelloitedtosetuptheSOFAR（soundfixingandranging）system，whiitiatedin1960bytheAustralia-BermudaSoundTransmissio，inwhichexplosioofflnearHeardIslandintheIhecoastofAustralia。TheyweredeteBermuda，atadistan。AedsoundwasdiscoveredbytheSOFARreseardlateridehecallsoffinteraphysalus），wholongagodiscoveredtheexistendpropertiesofthedeepsounddregularlyvisitittosigantkin。

SOFARopeotheATOC（AcoustietryofOClimate）system，whichcalculatesglobalseatemperaturesbymeasuringaveragesouyes，thushelpingtoquantifyclimateges。

gweathercausesgseas，whichleadteoftemporaryacousticstatesoftheogshadowzonesfromwhichmostsoundisexdtemporarysoundels，whichallowlong-distaion。Soundswhichtraveldower，likethosewhichtravelaroundthedeepshlydistortedeothemanygesofvelotheirjoururntovariatioureandsalinity）。Inthe1990s，variesoundsofunknin（theBloopisperhapsthebestkeofimagierpretations，ineliterallymonstrousohemostlikelysourceisthegreatlymodifiedsoundsofdistanticebergsg。

&hsounds

Souravelreadilyinsolids（seeBox12），asdopressurewavesofotherkinds。Butnotallseismicwavesaresound（forprimary）wavesarelongitudinal:theyaresequenpressionsaioyisdetermiyayoftheground，andthereforearesound>

However，S（sedary）waves，beingtrasouhPandSwavesarebodywaves—theytravelthroughtheEarth，andtheirrefrabythelayersundergrouhinformationaboutourplaructure。Therearealsoavarietyofsurfaceseismicwaves，butnonearesound>

Manylargeanimalsmakeandhearlow-frequends。ThesoundsmakebyAfrielephantsarelowsimplybecausetheirvocalceaivelyslow-moving—somueoftheircallsareinfrasonic。Thisisofsiderableadvahemsinfrasoundtravelsfarwithlittleattenuation（inair，veryroughly，a10Hzsignaltravelsoimesfurtherthana100Hzone，and10，000timesfurthertha1，000Hz）。

Undergrouionishighlyvariable，butusuallymuchlowerthaninair:femaleelephantsuseinfrasoundstoattractmales（over3kmawaybyairorover1round）andtotacttheiryousalsouseinfrasouhuorms（ausefulwatersource）over500kmaway。In2004，SriLasffedthecoast，probablybecausetheydetectedtheinfrasoundsoftheoningtsusgeheirsigherbymakingrumblingsoundsorbystampi，whichalsodeted-bornesoundusingvibratioorscalledPacorpuscles。