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Simrad EK15
In order to know how much power you can use you must know what kind of transducer
you are using. Provided that the echo sounder has been installed with a Simrad
transducer, and you know what type it is, this is no problem. All necessary parameters
about the transducer are then known by the echo sounder, and the software in the sounder
will ensure that you do not output too much power. If you use a third party transducer
you must manually check that the output power from the Simrad EK15 does not exceed
the power rating.
Note
If you send too much power into the transducer it will – just like a loudspeaker – be
damaged beyond repair.
If the transducer receives too much power from the echo sounder, it will also cavitate.
This is a physical phenomenon causing the appearance of gas bubbles immediately
below the transducer face. When this happens hardly any energy is sent into the water,
and the transducer face is subject to physical damage. The cavitation depends on the
power applied, the physical size of the transducer face, how deep the transducer is
mounted, and the amount of contamination (air and particles) under the transducer face.
Transducers with a large face can accept more power.
Near sea level, minute bubbles of micron or submicron size are always present in
the ocean. When the rarefaction tension phase of an acoustic wave is great enough,
the medium ruptures or "cavitates". For sound sources near the sea surface, the
ever-present cavitation nuclei permit rupture to occur at pressure swings of the
order of 1 atm (0.1 MPa), depending on the frequency, duration, and repetition
rate of the sound pulse. Cavitation bubbles may also be produced by Bernoulli
pressure drops associated with the tips of high-speed underwater propellers. Natural
cavitation is created by photosynthesis.
Several extraordinary physical phenomena are associated with acoustic cavitation.
Chemical reactions can be initiated or increased in activity; living cells and
macromolecules can be ruptured; violently oscillating bubbles close to a solid
surface can erode the toughest of metals or plastics; light may be produced
by cavitation (sonoluminescence). The high pressures and high temperatures
(calculated to be 30,000° Kelvin) at the inteior during the collapsing phase of
cavitating single bubbles can cause emission of a reproducible pulse of light of
duration less than 50 picoseconds.
Of direct importance to the use of sound sources at sea is the fact that, as the sound
pressure amplitude increases, ambient bubbles begin to oscillate nonlinearly, and
harmonics are generated. At sea level, the amplitude of the second harmonic is
less than 1 percent of the fundamental as long as the pressure amplitude of the
fundamental of a CW wave is less than about 0.01 atm rms (l kPa) (Rusby 1970).
This increases to about 5 percent harmonic distortion when the signal is about 10
kPa.
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351718/C
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