格格党

手机浏览器扫描二维码访问

Chapter 6 Urasound and infrasound（第1页）

Chapter6Ultrasoundandinfrasound

banner"

>

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