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Chapter 7 Sound underwater and underground（第2页）

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

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.