Simrad ES80 - Page 462

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Alargeschoolofshreectssoundwavessimilarly.Thistypeofspreadingisreferred
toassquare-lawor20logTVG(TimeVaryingGain)spreading.IntheES80'sEchogram
dialogbox20logTVGspreadingisreferredtoasSchoolGainandBottomGain,while
40logTVGspreadingisreferredtoasFishGain.
Thesituationisslightlydifferentwhenobservingtheechoesfromindividualsh.
Thetransmittedwaveundergoessquare-lawspreadingwhentravellingfromthesurface
anddowntothesh.Theswimbladderoftheshscattersasmallfractionofthe
arrivingenergyinalldirections.Travellingfromtheshandbacktowardsthesurface
thescatteredwaveundergoesanothersquare-lawspreading.
Thecombinedeffectisreferredtoasquad-lawor40logTVG(TimeVaryingGain).
Propagationlossesduetoabsorptionaremuchhigherinseawaterthaninfreshwater.
Absorptionalsoincreaseswithfrequency.At38kHztheabsorptionis0.5dB/kmin
freshwaterand10dB/kminseawater.At200kHztheabsorptionis10dB/kminfresh
waterand50dB/kminsaltwater.TheES80mustknowwhichwatertypeispresentin
ordertocompensatefortheselossescorrectly.
ThedB(decibel)unithaslongtraditionsinunderwateracousticsandothereldsin
physics.Itisalogarithmicmeasurefortheratiobetweentwoquantities.
Relatedtopics
AdjustingtheTVG(TimeVariableGain)setting,page113
TVGfunction,page414
Echogram:Echogrampage,page424
Environmentdialogbox,page340
WaterColumnpage,page340
SimradES80ReferenceManual