Chapter 6 Urasound and infrasound（第2页）

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。