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Chapter6Ultrasoundandinfrasound
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Batsounds
Mabookswilltellyouthatyouhearfrom20Hzto20kHz.Dohem:ifyouareovertwenty，youareprobablydeaftosoundsabove17kHz.Thehigh-frequencylimitdeessosignifidsopredictablywithagethataygeher-frequendcalledtheMosquitohasbeeeshopkeeperssince2009，asonlyteenageears（andthoseofaillcapableofdetegtheannoyihedeviceproduces.However，itisprobablyfairtosaythatwewhoabout17kHzarenotihefagthatitisoflittleevolutiohherouldhavedevelopedmorersystems.
&heless，wearemissingout—heultrasonidsespeciallyrie，butbecauseloitsomehandyphysicalpropertiesofsoundsthatarehosefrequewehear.Thebat，however，isnotsohandicapped.
&iwithloyultrasoundisastounding.Inpitess，aianfruitbat（Rousettusaegyptiacus）withan80gspaneasilyffybetweeicalrodsjust53cmapartwithouttougtheToachievesuchfeats，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
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