Installation manual

SimradES70
Figure5Wavepropagation
fromaatbottom
(CD010217C)
(CD010217G)
TheES70transmitshighenergysoundwavepulses
intothesea.Aatbottomreectsthetransmitted
waveasifitwasamirror.Thepropagatingenergy
isspreadoveralargerandlargerareaasittravels
downtothebottomandupagain.Theenergyis
spreadoverafourtimeslargerareaeverytimethe
traveldistancedoubles.
Alargeschoolofshreectssoundwavessimilarly.
Thistypeofspreadingisreferredtoassquare-law
or20logTVG(TimeV aryingGain)spreading.
Thesituationisslightlydifferentwhenobserving
theechoesfromindividualsh.Thetransmitted
waveundergoessquare-lawspreadingwhen
travellingfromthesurfaceanddowntothesh.Theswimbladderoftheshscattersa
smallfractionofthearrivingenergyinalldirections.Travellingfromtheshandback
towardsthesurfacethescatteredwaveundergoesanothersquare-lawspreading.The
combinedeffectisreferredtoasquad-lawor40logTVGspreading.
Figure6Wavepropagation
fromash
(CD010217H)
Intheechosounder’sEchogramdialog20logTVG
spreadingisreferredtoasSchoolGainandBottom
Gain,while40logTVGspreadingisreferredto
asFishGain.
Propagationlossesduetoabsorptionaremuch
higherinseawaterthaninfreshwater.Absorption
alsoincreaseswithfrequency.At38kHzthe
absorptionis0.5dB/kminfreshwaterand10dB/km
inseawater.At200kHztheabsorptionis10dB/km
infreshwaterand50dB/kminsaltwater.Theecho
soundermustknowwhichwatertypeispresentin
ordertocompensatefortheselossescorrectly.
ThedB(decibel)unithaslongtraditionsinunderwateracousticsandothereldsin
physics.Itisalogarithmicmeasurefortheratiobetweentwoquantities.
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