Simrad ST90 - Ambient Noise

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Airbubblesabsorbandreectthesoundenergy,andtheymayinworstcasesblockthe
soundtransmissionaltogether.
Theboundarylayerincreasesinthicknesswhenitbecomesturbulent.Theboundary
layeristhinintheforwardpartofthevesselhull,andincreasesasitmovesaft.The
thicknessdependsonshipsspeedandontheroughnessofthehull.Allobjectssticking
outfromthehull,ordentsinthehull,willdisturbtheowandwillincreasethethickness
oftheboundarylayer.Whentheowspeedishigh,theturbulencecanbeviolentenough
todestroytheintegrityofthewater.Smallvoidsorcavitiesinthewaterwilloccurand
thisiscalledcavitation.
Rattlenoise
Rattlenoisemaybecausedbylooseobjectsinthevicinityofthetransducer,likexing
bolts.Therattlemayalsocomefromlooseobjectsinsidethehull.
Interference
Interferencefromotherhydroacousticequipmentonboardthesamevesselmaybean
annoyingsourceofdisturbance.Unlessthesamefrequencyisusedformorethanone
pieceofequipmentonlythetransmittedpulsewillcontributetotheinterference.
Inphysics,interferenceisthephenomenoninwhichtwowavessuperposeeach
othertoformaresultantwaveofgreaterorloweramplitude.Interferenceusually
referstotheinteractionofwavesthatarecorrelatedorcoherentwitheachother,
eitherbecausetheycomefromthesamesourceorbecausetheyhavethesameor
nearlythesamefrequency.Interferenceeffectscanbeobservedwithalltypesof
waves,forexample,light,radio,acoustic,surfacewaterwavesormatterwaves.
https://en.wikipedia.org/wiki/Interference_(wave_propagation),April2016
Relatedtopics
Acousticnoise,page76
Ambientnoise
Ambientnoiseisusuallynotalimitingfactortotheperformanceofsonarsandecho
sounders.
Theambientnoisemaybesplitupasfollows:
•Seanoise:Airbubbles,seismicdisturbances,waves,boundaryturbulence,etc.
•Biologicalnoise:Fish,mammals
•Manmadenoise:Othervessels,interference
•Precipitationnoise:Heavyrainorhail
Insomeareas,wheremanyvesselsoperatetogether,theengineandpropellernoisefrom
othervesselsmaybedisturbing.Interferencefromhydroacousticinstrumentslocated
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