Simrad SU90 - Ambient Noise; Fishing Gear Noise

Preparations
Cavitationusuallyoccursmorewillinglyinairlledwaterandtheoccurrenceis
dependentonthehydrostaticpressure.Cavitationisaseveresourceofnoise.Thenoise
ismadewhenthevoidsimplode.Cavitationnoiseoftenoccursatthepropellerand
nearextrudingobjectsathigherspeeds.
Contributionstotheownoisemayalsobecausedbyairbubbleshittingthetransducer
face,orbythesplashcausedbythewavessetupbythevessel.Insomecasesaresonant
phenomenonissetupinaholenearthehull.Thissoundwillhaveadiscretefrequency,
whileallotherownoisewillhaveawidefrequencyspectrum.
Rattlenoises
Rattlenoisemaybecausedbylooseobjectsinthevicinityofthetransducer,likexing
bolts.Therattlemayalsocomefromlooseobjectsinsidethehull.
Interference
Interferencefromotherhydroacousticequipmentonboardthesamevesselmaybean
annoyingsourceofdisturbance.Unlessthesamefrequencyisusedformorethanone
pieceofequipmentonlythetransmittedpulsewillcontributetotheinterference.
Ambientnoise
Theambientnoisemaybesplitupasfollows:
1Seanoise:Airbubbles,Seismicdisturbances,Waves,Boundaryturbulence,etc.
2Biologicalnoise:Fish,Mammals,etc.
3Manmadenoise:Othervessels,Interference
4Precipitationnoise
Theambientnoiseisusuallynotalimitingfactortotheperformanceofthesonarand
echosounders.Insomeareas,however,wheremanyshingvesselsareoperating
togethertheengineandpropellernoisefromothervesselsmightbedisturbing.
Interferencefromhydroacousticinstrumentslocatedinothervesselsmayalsobea
limitingfactor.Theseanoiseisascanbeexpecteddependentontheweatherconditions
andinbadweathertheseanoisecanbequitehigh.
Fishinggearnoise
Wheneveryourshinggearisinuse,itwillcreatenoise.Abottomtrawl,forinstance,
isaconsiderablenoisemaker.Still,thisnoisewillseldombealimitingfactorforhull
mountedsonarsorechosounders.However,foroperationofacatchmonitoringsystem
oratrawlsonar,withthetransducer(s)placedonthetrawl,thegearnoiseisoneof
themaincontributorstothenoiselevel.
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