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Chapter 6 Urasound and infrasound2（第1页）

Chapter6Ultrasoundandinfrasound2

Asfarashumansareed，theuseroundsouleastto132thefirstseismiadeina。Itwasabrassvesselwitharialfrogsattached，eaetalsphereinitsmouth。Thepresendbearihquakewasihefallofthesphere（s）iion。

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&ionofsoundingasorliquidmediaisdescribedbyStokes’Lav3;amplitudeαff，dynamicvisu），frequeyρ（rho），velocityofsoundv。

Undergroundsoundhasbeenasourceoffearandwotimes，reasthatevolutionmayhavehardwiredintoourbrains，fromtheexperiencesofourahatsudsapanyavalanches，volis，earthquakes，andnaturaldisasters。Thefactthattheybefeltaswellasheardaddstotheirimpact:tofeelthesolidearthshiftahoisanunnervingexperienderthatthedarkworldoftheundergrardedinmaheabodeofthedead。

&suiroundsoundhaslongbeenusedinwar:duringclassicaltimes，tunnellingeimesdetectedbythesouhroughtheearth，andthereisevenarecordofshieldsbeihegroundandthepreseunnelsbelowbeihesoundsheard。InWorldWarI，detedifflerentpointsalongtrenchwallsallowedtheloytroopsireobeestimated，bytriangulation。InWorldWarII，thePolishresistasignifitperiround，listeningfermansoldiersoverhead—hiletragthembythesamemeans。

Undergroundsouorsarecalledgeophoilthelate2eophohesameprincipleasdynamicmies（Chapter5）:amagionbygroundwavesarialsinawirecoiledaroundit。Today，MEMSareusedinstead:amicroscopicpieceofsiliountedieholderandvibratioroundsouarttomove。Afeedbacksystemhaltsthismotioappliestodosoprovidespreationaboutthesound。However，MEMSdevicesarerelativelyiiveandareusedmainlyfactivelyseismis。Allpracticalgehlydiredareusuallydeployedtorespondtosoundsilyfrombelow。

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&ablisheduserounds。Usuallyanexpleissetround（thereisnroutotheloudspeaker），amakesreffectoffltheiweenlayersofdifflerentmaterialsaedbyanarrayofgeophones。

Themeasurementandloofstrairasoundsinsolids，knownasaissioesetoffraallsortsofstructures，fromplanesinffighttotheLohisisknownasstructuralhealthm。Alertsbetriggeredattheinstantthatevenaninvisiblysmallcrastoform。Aissionisalsousedtostudytheformationofcragweldiodetesetoflesiocarryhigh-pressureffuidsaheamountofinsidereie。

Undergrous，diflculttoidentifybyesoundswhicharemostlyinfrasoO（prehensivereatyanization）tlymohusinggeophohhydrophohedeepsouederwatersfromthesub-orocks）。ThisworkroundaerdetectorsallowstheCTBTOtopoliatioreaties，ahelosofadotakeplace。

Thischapterhasfothoseareasofsouhear，eventhoughmaswishweChapter8weturntosoundsthathearihoughwe’drathernot。