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Chapter 6 Urasound and infrasound（第2页）

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

Ultrasoundhasbeehetreatmentoftumoursofmanykinds—ingotherwiseinoperablebraieiquecalledeitherHIFU（high-iyfocussedultrasound），orHITU（high-iherapeuticultrasound）.Aswellasdestroyiissuesbyheatioabout9ionsaboutthesizeofagrairasoniducedbubbleformationintumourshasbeeomakethemmoresusceptibletochemotherapy.Themaiheuseofultrasouslikethese，whieedpredaccuratetargeting，isthatthepathsoftheultrasoundbeamsdepeiesaiesoftheinterveningbodytissues.So，modelsofbodypartsmadefromartificialtissue-mimigmaterialsareusedtocalibrateandprogrammetheequipment.

Amhtforwarduseofultrasoundihotripsy，inwhichhigh-powerpulsessimplypouou，smashiopiecessmallenoughtopassoutwithurine.

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&hebest-krasoundisthesgoffoetuses.Sihediaphragmofaalloudspeakerovequioughtoproducethemegahertzfrequenciesrequired，apiezoelectricprojectorisusedinstead.Gelisappliedtotheabdomen，sothatthereisnoairlayertoreffectorabsorbthesourasoundwavesbouerfacesbetweehdiflerentimpedances，suusnioticffuid.Hence，bymeasuringpreciselyhowlongittakesforechoestoreturerfagatofthedifleringsouiesiheirpositionsbeworkedout.Bymovingthebeamarouhree-dimensionalmapbecalculated，aoareal-timevideoimage.

&loudspeakers，apiezoelesducerisreciprocal—suppliedwithanosgtitprodudwave，and，versely，whenstruckbyasougeesarial.So，ialsertheserheadactsasboththesourceoftheultrasoundsaoroftheirechoes.

Withveryhigh-frequensrelativelysharpimagesbemade:a1MHz（onemillinalimagedetailsatthemillimetrescale，theexactvaluedependiicpropertiesofthetissuebringviewed，andmanysowgoupto15MHz，or50MHzforeyeandskinsers.

&hisispaltryparedtotheeightbilliedbyanaicroscope，whisequeailsassmallas0.03mifortunately，suchhigh-frequendsareabsorbedbyalmostallmediabeforetheyhavetravelledasire，andtheonlyuseableexisliquidhelium—whichwillboilifcolderthan5kelvin（fi268°C）.Theelaborategsystemsrequiredmeanthataicroscopesarehereasonstheyareusedatallarethattheyprobeuhesurfaples，andthatsomematerialswhicharehardtodistinguishfromtheirsurroundingsvisuallyarehighlyreffectivetosound.

Outsidethemedicalfield，oheodiagnosticapplisofultrasoundisiionofffawsandrailwaylines，foriolocatethese，aseriesofsoundburstsissentaloobetested.Inpulse-eode，thetransmitterandreceiverarepositioher:ifaffawispresent，thepulsesarereffedtheirarrivaltimesiheffawposition.Intransmissioedtransmitterareseparatedandahepulsesintransitihepresenogehetestobject.Thisapproaalsobeusedforthedeterminationofmeicalstressesinsolids:becausetheelastiaterialsalterwheheirsouiesalsogelocally.

Thepowerofultrasonics

Thoughthermaleflectsofhigh-powerultrasoundhavemanyapplis，italsodeliveritsehroughmeicalefleedium.Thesnappingshrimpprovidesararenaturalexampleoftheuseoftheshodstressesmadebyultrasouhaudiofrequeokill.Theloudandsuddenclickproduthecreaturesnapsitscludesfrequeo200kHz，phtokillorstunbothpreyandwould-bepredatorsalike.

&ilizedbyshrimpsorhumans，themeicalpowerofultrasoundisusuallydeliveredthrough，whichistheformationacollapseoftiallliquidssuchbubbles，madeeitheroftheiroourorofair.Whenthepressureofaliquidfalls，thesebubblesgrow（whichisthecauseressurizederoffizzydrinkisopened）.Sindwaveisasequenceofloressures，

itcausesbubblestoswellandshrinkrapidly，andathighpowersahebubblespulsatesovioleandimplode，suddeheirvibratiyintheformofheat.Thetemperaturesinvolvedmayexceedthoseonthesurfaceofthesun，andmaketheliquidglowwithlight（aphenomenoninesce，ingshrimpalsoinitiate）.

&heenergyappearsonlywithintihebodyofliquiddoesicularlyhot.Buttheburstsofhighlytratedenergybeusedtoinitiatechemiges（sory），aerilizesubmergedobjectssuchasmedistruments.-basedgismosteflethe20te.Athigherfrequerasoundalsocausesagitationofliquids，whiinatesgefletherange100kHzto1MHz.Irasohsusebotheflects.

High-powerultrasound（without）isroutinelyusedforffuxlesssofpriboards，inwhichtheelectricallyheatedtipofasironisvibratedatultrasostillhigherpowers，finewiresbeweldedtogether，heatedfromwithiiorasonicvibratioageofthisisthat，siiisrestrictedtotheinsonified（sound-filled）wires，theultrasouheatupnearbyaerialsareweldedultrasonically;thefrequendependsohepartstobewelded，from60kHzforthesmallestelementsto1est.

High-powerultrasoundbeusedtolevitatesmallobjects.Althoughtheforvolvedarefeeble，inmicrogravityessuchastheinteriorsofspastheycouldbeusedtoholddelistrumentsinpositiassembly，hlyreactivechemiingintotaything.

&erlimits:phonons

&remelyhighfrequendsbehaveinwaysradicallydifferenttothoseearefamiliar:justlikeeleagicradiation，inwhichthehighestfrequenciesbehavemuchmorelikepartiwaves（heheindividualclicksofaGeigerregisteringthepresenarays），thehighest-frequendsbehavelikeparticlescalledphoencewasdisdirectly:bythelate19thtury，itwasknoecifitofheatwasoraisethetemperatureofasubstanedegree.Thespecificheatherthanthatof，say，oil，whisittakeslooboilakettleofwaterthaleofoil.

Gasesandsolidshavespecificheatstoo，butthereuzzlinganomaly:ihetemperatureofasolidtakesabouttwiceasmuergyasraisiureofthesameamountofthesamesubstasgaseousform.Thismeansthatsolids（aainliquids）musthaveameansheatwhiavailabletogases.Themeismisvibration:amoleasolidoscillatearounditsequilibriumposition，likeapendulum.But，unlikeapendulum，anosgmoleotslowgradually.Thelawsofquantummeithatitmustjumpfromarapidtoaslowos，adoessoittransfersaphononofvibratiytoanothermolecule.Thewaysinwhichsolidsdudelectribeatedforbythebehaviourofphonons.

Infrasound

Asthelowerfrequenanhearingisapproached，theiyofjust-audiblesoundsincreases:apuretoisjustaudibleat20Hzisnearly300milliontimesmorepowerful（allsbeihanajust-audibleto4kHz.Suchpowerfullow-frequendsarerarelyenteredinair，butsolid-borneversionsareorusites，inundergroundstatioorways，andinseismicallyactiveareas，wheretheyeasilybefelt.

Low-poweredairborneinfrasounds，oherhand，surroundusallthetime.Theyareeveedalk:thecyclicairpressuregesatourearsduetoourupanddownheadmovementsstituteaninfrasouaround1Hz.Seawavesgeeinfrasoundsataround0.2Hz.Thelowest-frequenaturalsoundsofallhighintheairanddeepunderground:bothauroraeandvoloesprodufrasoundsataround0.01Hz.

&hekeycharacteristifrasoundisthatittravelfarfurtherthahroughsea，ground，orair;inair，infrasoundsfromthousandsofkilometresaway（made，forexample，byeruptingvoloes）easilybedetected，thoughnotusuallybymies.Justaswefeel，ratherthanhear，infrasound，soweusespecializedbarometerstomeasureit.Infrasoundalsobedetectedbymeauregesitcauses.

Thereisevideinfrasoundhasarasonhumanshertypesofsound，inhaofemotionalresponses:wheofmodernclassicalmusipaniedbyinfrasound，thereeoplewhohatedthemusidfarmorewholovedit，thanataemusioinfrasonient.Otherexperimehatdriversexposedtoinfrasouiredveryquidithaseveedasacauseofsupposedhauntings，partlythroughdirealeffedpartlythroughvibrationoftheeyeball，giviovisualdisturbances.

Infrasoundhasprovedtobeareliablemeaingbolides，whicharemeteoroidsthatexplodeinffight.Siravelsupersonicallythroughtheatmeeinfrasound-risastheyfall.Moreinfrasouedwhenthebolidesexplode.Alltheseairbeesolid-borneversioheyreachtheEarth，andsurfacewavesaremadewhenthebmentscrashtoEarth.Thefrequeiming，andamplitudesofallthesesoundsbiogiveadetailedanalysisofthepath，motiohebolide.

However，infrasoundisusuallyofmorecetouswhehroughtheEarth—andultrasoundreallyesintoitsower.ItistothesemediathatweturninChapter7.