Preparations
Cavitationusuallyoccursmorewillinglyinairîš¿lledwaterandtheoccurrenceis
dependentonthehydrostaticpressure.Cavitationisaseveresourceofnoise.Thenoise
ismadewhenthevoidsimplode.Cavitationnoiseoftenoccursatthepropellerand
nearextrudingobjectsathigherspeeds.
Contributionstotheownoisemayalsobecausedbyairbubbleshittingthetransducer
face,orbythesplashcausedbythewavessetupbythevessel.Insomecasesaresonant
phenomenonissetupinaholenearthehull.Thissoundwillhaveadiscretefrequency,
whileallotherownoisewillhaveawidefrequencyspectrum.
Rattlenoises
Rattlenoisemaybecausedbylooseobjectsinthevicinityofthetransducer,likeîš¿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,wheremanyîš¿shingvesselsareoperating
togethertheengineandpropellernoisefromothervesselsmightbedisturbing.
Interferencefromhydroacousticinstrumentslocatedinothervesselsmayalsobea
limitingfactor.Theseanoiseisascanbeexpecteddependentontheweatherconditions
andinbadweathertheseanoisecanbequitehigh.
Fishinggearnoise
Wheneveryourîš¿shinggearisinuse,itwillcreatenoise.Abottomtrawl,forinstance,
isaconsiderablenoisemaker.Still,thisnoisewillseldombealimitingfactorforhull
mountedsonarsorechosounders.However,foroperationofacatchmonitoringsystem
oratrawlsonar,withthetransducer(s)placedonthetrawl,thegearnoiseisoneof
themaincontributorstothenoiselevel.
381293/A
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