Echosoundertheory
Whenthepeakpressureamplitudeissomewhatgreaterthan1atm,theabsolute
pressureforasoundsourceatsealevelwillbelessthanzeroduringtherarefaction
partofthecycle.InusingCWbelow10kHz,thisnegativepressure,ortension,is
thetriggerforasharplyincreasedlevelofharmonicdistortionandtheissuanceof
broadbandnoise.Anyattempttoincreasethesoundpressureamplitudeappreciably
beyondtheambientpressurewillcausenotonlytotaldistortionbutalsothe
generationofalargecloudofbubbleswhichwillactuallydecreasethefar-eld
acousticpressure.
Thedetailedbubbleactivitiesduringcavitationhavebeenstudiedinseveral
laboratories.Acousticianshaveidentiedgaseouscavitationresultinginstreamers
ofhissingbubblesthatjetawayfromregionsofhighacousticpressureswings,and
vaporouscavitation,whichradiatesshockwavesofbroadbandnoise.
—HermanMedwin&ClarenceS.Clay(1998)
[2]
InordertocontrolthepoweroutputontheSimradEK15,usetheNormalOperation
functionontheOperationmenu.
Relatedtopics
•Choosingpassivemodeofoperationonpage78
•Adjustingthepulseduration(length)onpage78
•NormalOperationdialogboxonpage154
Pulseduration
Theechosounder'spulsedurationisameasurementforhowlong
theacousticpulselasts.
Thepulsedurationcanbeadjustedaccordingtothecurrentdepthandwhatkindofsh
youarelookingfor.Thedeeperyouwishtosee,thelongerpulsedurationshouldbe
used.RememberthatinEK15echosounder,thepulsedurationandthebandwidth
ismutuallydependant.
•Longpulse-lotsofacousticenergy-narrowbandwidth-lesssensitivefornoise
fromownvesselandenvironment
•Shortpulse-lessacousticenergy-widebandwidth-moresensitivefornoisefrom
ownvesselandenvironment
2.from“FundamentalsofAcousticalOceanography”,AcademicPress,SanDiego,1998
351718/C
279
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