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Chapter6Ultrasoundandinfrasound2
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Asfarashumansareed，theuseroundsouleastto132thefirstseismiadeina.Itwasabrassvesselwitharialfrogsattached，eaetalsphereinitsmouth.Thepresendbearihquakewasihefallofthesphere（s）iion.
Box12
&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.
Prospeg
&ablisheduserounds.Usuallyanexpleissetround（thereisnroutotheloudspeaker），amakesreffectoffltheiweenlayersofdifflerentmaterialsaedbyanarrayofgeophones.
Themeasurementandloofstrairasoundsinsolids，knownasaissioesetoffraallsortsofstructures，fromplanesinffighttotheLohisisknownasstructuralhealthAlertsbetriggeredattheinstantthatevenaninvisiblysmallcrastoforAissionisalsousedtostudytheformationofcragweldiodetesetoflesiocarryhigh-pressureffuidsaheamountofinsidereie.
Undergrous，diflculttoidentifybyesoundswhicharemostlyinfrasoO（prehensivereatyanization）tlymohusinggeophohhydrophohedeepsouederwatersfromthesub-orocks）.ThisworkroundaerdetectorsallowstheCTBTOtopoliatioreaties，ahelosofadotakeplace.
Thischapterhasfothoseareasofsouhear，eventhoughmaswishweChapter8weturntosoundsthathearihoughwe’drathernot.
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