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Chapter 6 Urasound and infrasound（第1页）

Chapter6Ultrasoundandinfrasound

Batsounds

Mabookswilltellyouthatyouhearfrom20Hzto20kHz。Dohem:ifyouareovertwenty，youareprobablydeaftosoundsabove17kHz。Thehigh-frequencylimitdeessosignifidsopredictablywithagethataygeher-frequendcalledtheMosquitohasbeeeshopkeeperssince2009，asonlyteenageears（andthoseofaillcapableofdetegtheannoyihedeviceproduces。However，itisprobablyfairtosaythatwewhoabout17kHzarenotihefagthatitisoflittleevolutiohherouldhavedevelopedmorersystems。

&heless，wearemissingout—heultrasonidsespeciallyrie，butbecauseloitsomehandyphysicalpropertiesofsoundsthatarehosefrequewehear。Thebat，however，isnotsohandicapped。

&iwithloyultrasoundisastounding。Inpitess，aianfruitbat（Rousettusaegyptiacus）withan80gspaneasilyffybetweeicalrodsjust53cmapartwithouttougthem。Toachievesuchfeats，andtohunt，batsuseecholoeratingultrasoundsandtimiiltheyheartheechoes—whiofthedistanearbyobjects。

Sightlesshumansapproximatethis，judgingdistaowallsfromechodelays，butwhattheyendupwithisnotavisionoftheworld。Itotbe，atthewavelengthstheyhear:imagiandinginacalmlake。Alightbreezeblripplesaboutatimetreih，whiceoflthejetty’snsincircularpatterer，agalesetsin，makingmugerwaves，whichrollpastthensasifthey’renotthere。Forwavesofsound，thesameprincipleapplies:theyareoedby（andhenlydetect）obstacleslargerthantheyare。

So，ifabattriedtoeothusingao’ssay，whichhasawavelengthof132cm—itwouldohemothwerewelloverametreacross。Todetecta1cmmoth，souleast33kHzarerequired。Infact，batstypicallyprodudsof80kHz（smallerspeciesproducehigherfrequehefullra40kHzto120kHz）。Suchhighfrequehemtodealnotonlywithisolatedobjects，butwithplicatedthree-dimensionaldistributionsoftwigs，leaves，as，maivemotioher—andalliothebat。

Butitwoulddothebatnogoodtogeinuousultrasoheresultwouldbeabewilderingmontage，interferingepossibletodecode。Whatbatsneedarepulsesofsound，abebrief:iftwomothsareatimetreapart，then，toproduceseparateechoes，asouhavepassedbeyondtheebefthesedhebelessthareih，andthereforelessthan30midsinduration。

21。　Ripplesvs>

Itisimpossibleforbatormaetoproduceashort-duratiohasih，uwavelengthismuchshorterthaself。IsproduceverysharpclikstoFourieranalysis，weknowthatsuchaclickisequivalenttoamixtureofwavesofdiflerentfrequencies（Figure12），andthesharpertheclick（thatis，thequickeritrisesfromsileomaximum），thewidertherangeoffrequenciesprodudthusthemoreaccuratelydistanbejudged。

Soistheidealsoundforabata-durationotreally:ashortsalyone，whiottravelfarwithoutfading。Humahisproblemthemselvesindesigningradarandsonarsystems，ain1960withtheiionoffrequehisutilizesarelativelylongpulseofgraduallyingfrequency。Thefairlylongdurationallowsplentyyandhehegfrequeeobjectsatdiflerentdistaihediflereheirfrequencies。

&susethisfrequeeique，thepulseslastabout2to3milliseds，duringwhichthefrequencyfalls（ratherthanrisingasinthehumanversion）byaboutahemoresuchpulsesabatmakes，themoreinformationitredsoitmodifiesitsclickratedependingonthegesitfaaroundtenpulsesasegtoashighasonehundredinahalf-sedperiod，whenthesurroundingsbeplicatedorrey。

AsdisChapter2，iftheobjewhidechoesisinmotioothesouhefrequencyofthatsouedbytheDopplerefleanandbatsonarsystemsexploitthistodetermiiesofsuchobjects，withthedistinthat，whileoursystemsmeasurethefrequency，batsmodifythefrequeheiroutputpulsesuheyheararethesameastheywouldbe，werethetargetstationary（so，forexample，ifabatisapproaobject，thefrequencyoftheethatobjectrise，sothebatreducesthefrequencyofitsoutputbyanamountthatbringsthefrequencyoftheechoesbaatchthebat’sinalfrequency）。

Adifferentkindofsound

Box9

Diffraofshanopening:（plane）angleofspread，θ，isgivenbysin（θ2）=λd;diameteroftheopenihλ。

Anotherphysicaldiflererasoundandtheaudibleraultrasoundreadilyformsbeams，whichisofsiderableadvas。An80kHztonepassingthrougha1gwillformaicalbeamwhichspreadstoabout90cmwideata1-metredistance（Box9）。Inbatspecieswhichprojecttheirultrashtheirnostrils，interfereweewosourthatthebeamsareher。Notohistratetheaergy，soallowiaobeprobed，italsoreduumberside-echoes。A2015studyofbatsapprpoolsshowedthatthebatsopehswiderwhentheywereclosetotheresumablyforthislatterreason。

&ionodoubthavefurswithgeorsahatworkatevenhigherfrequeheabsorptionofsudsbytheairisaninsuperablebarrier。Experimentallyithasbeehat，atthesortofsbatslovebest（25°C，50pertrelativehumidity），100kHzsoundsareabsorbedby3dBpermetre:thatis，theyfalltoabouthalfiyoverthatdistanversely，30kHzsoundsareabsorbedorateof0。7dBpermetre，whistheiriyfallsbyabout15pert。（Theincreaseofsoundabsorptionwithfrequencyisthemainreasonwhythundersoundslessdmasdistancreases。）

Box10

Whydoesthishappen?Airisolecules，allrandaroundataraiesalyboungofleachother。Onahotday，thesevelofact，temperatureisjustameasureofthevelocityroupofmolecules（Box10）。Asoundwaveisasequeernatinghighandlthroughtheair，soataiohesound，theairmoleculeswillfirstbunchupclosetogetherandthe，beforebungtain。Whenmolech，theyslowdolehurryingthroughacrowdedstationdiflereionsgoslowerthemoreofthemthereare。Whenthishappens，theehemolegesform:althoughtheymovearoundmoreslowly（theirkiiergyfalls，inotherwords），theyspichmore（so，theirinternalenergyincreases）。

Ananalogymightbetopairsofcricketballslirongsteelsprings—afairlyaccuratemodelofadiatomiolecule，likethemoleitrogen（N2）andoxygen（O2）whiakeup99perosphere。Assoonasthepressionalpartofthesoundassedandthemoleculesmain，theyspichless，andmovearoundfasteroncemore。Playmiddledthesoundroducedforergytoswitdforthbetweeiternalforms262timesased。

But，ifthefrequencyissuffltlyhigh，thetimeavailablebeessoshortthatamolepletetheytokiiergyquioughbeforeitistimetoreversetheprocess。Asaresult，thevelocityofsoundfallsandthesoundwavediesaidly。Theactualfrequencyofthesouwhichthesegesbegihemedium，beingfarhigherinsolidsandliquidsthaherpropertiesofthemedium，especiallyviscosity，alsotributetothiseflect。

Thisphenomenonisveryusefultous:astheultrasoundwavefades，itsehroughthemedium，heatingitup。Thisheatihasmanyapplis，ingtheinternalwarmingofbodytissuestoimprovebloodffowedmusdjoints。

&rasound

Ultrasoundhasmanyothermedicalusestoo，bothintheareasofsg（almosteveryohedevelopedworldisnowsedbeforebirth）a（liketheremovalofdentaltartar，where25kHzsoundisusedinwithawaterjet）。Uhermedicaltreatments，ultrasoudoflinstantly，oftenrequireslow-ology，andusuallyinvolvesminimalpatieiohatultrasouorsarerelativelyportablealeameansthattheybeusedoutsidethehospitale，fromdiathermy（deep-tissueheating）unitsfoundinmanygymstowouionsystems，deployedoosavethelivesofsoldierswhowouldotherwiseperishfrombloodloss。