Simrad EK15 - Page 280

SimradEK15
Inordertoknowhowmuchpoweryoucanuseyoumustknowwhatkindoftransducer
youareusing.ProvidedthattheechosounderhasbeeninstalledwithaSimrad
transducer,andyouknowwhattypeitis,thisisnoproblem.Allnecessaryparameters
aboutthetransducerarethenknownbytheechosounder,andthesoftwareinthesounder
willensurethatyoudonotoutputtoomuchpower.Ifyouuseathirdpartytransducer
youmustmanuallycheckthattheoutputpowerfromtheSimradEK15doesnotexceed
thepowerrating.
Note
Ifyousendtoomuchpowerintothetransduceritwill–justlikealoudspeaker–be
damagedbeyondrepair.
Ifthetransducerreceivestoomuchpowerfromtheechosounder,itwillalsocavitate.
Thisisaphysicalphenomenoncausingtheappearanceofgasbubblesimmediately
belowthetransducerface.Whenthishappenshardlyanyenergyissentintothewater,
andthetransducerfaceissubjecttophysicaldamage.Thecavitationdependsonthe
powerapplied,thephysicalsizeofthetransducerface,howdeepthetransduceris
mounted,andtheamountofcontamination(airandparticles)underthetransducerface.
Transducerswithalargefacecanacceptmorepower.
Nearsealevel,minutebubblesofmicronorsubmicronsizearealwayspresentin
theocean.Whentherarefactiontensionphaseofanacousticwaveisgreatenough,
themediumrupturesor"cavitates".Forsoundsourcesneartheseasurface,the
ever-presentcavitationnucleipermitrupturetooccuratpressureswingsofthe
orderof1atm(0.1MPa),dependingonthefrequency,duration,andrepetition
rateofthesoundpulse.CavitationbubblesmayalsobeproducedbyBernoulli
pressuredropsassociatedwiththetipsofhigh-speedunderwaterpropellers.Natural
cavitationiscreatedbyphotosynthesis.
Severalextraordinaryphysicalphenomenaareassociatedwithacousticcavitation.
Chemicalreactionscanbeinitiatedorincreasedinactivity;livingcellsand
macromoleculescanberuptured;violentlyoscillatingbubblesclosetoasolid
surfacecanerodethetoughestofmetalsorplastics;lightmaybeproduced
bycavitation(sonoluminescence).Thehighpressuresandhightemperatures
(calculatedtobe30,000°Kelvin)attheinteiorduringthecollapsingphaseof
cavitatingsinglebubblescancauseemissionofareproduciblepulseoflightof
durationlessthan50picoseconds.
Ofdirectimportancetotheuseofsoundsourcesatseaisthefactthat,asthesound
pressureamplitudeincreases,ambientbubblesbegintooscillatenonlinearly,and
harmonicsaregenerated.Atsealevel,theamplitudeofthesecondharmonicis
lessthan1percentofthefundamentalaslongasthepressureamplitudeofthe
fundamentalofaCWwaveislessthanabout0.01atmrms(lkPa)(Rusby1970).
Thisincreasestoabout5percentharmonicdistortionwhenthesignalisabout10
kPa.
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