Recommendation for fishery research
Non-linear effects in hydro acoustics are described
in the literature. A brief theoretical explanation of the phenomena
and results of measurements and simulations in the frequency range
38 - 200 kHz are described below.
This article was first
published as a news bulletin in March 2002.
Non-linear effects occur at high sound intensities, and when sufficiently
great, limit the amount of energy that a sound source can radiate
into the water. This is caused by harmonic distortion and cavitation.
Only harmonic distortion is described in this article.
the sound level at a certain range from a transducer will increase
proportional to the power at the source.
As the power at the
source is increased to very high levels, one will find that the sound
level at the 1st harmonic in the field will not result in the same
increase, and finally, no further increase in the received signal
will occur, no matter how much power is radiated by the source.
reason for this effect is that at high intensities, the normally sinusoid
signals, are distorted by the creation of higher order harmonics in
the water, resulting in a sawtoothlike waveform or repeated shock
wave. This is caused by non-linearities in the water.
higher harmonics will suffer higher absorption in the water, the signals
will, at a certain range, return to sinusoid. However, as the higher
harmonics have dissipated into heat, the sound energy is lower.
non-linear effect will also have influence on the transducer beam
pattern. The main lobe will become flatter and thus wider, and the
side lobes will increase compared to the main lobe. The change in
the beam pattern means that target strength (TS) measurements and
integration values will not be correct.
has performed extensive measurements on this effect on the most commonly
used echo sounder frequencies, 38, 70, 120, and 200 kHz, using standard
echo sounder transducers and Simrad EK60 transceivers. The transducers
were mounted 2.5 m below water surface during the measurements.
result from these measurements are:
- The non-linear effect is negligible on 38 and 70 kHz within the
standard maximum limits for the output power, 2000 W and 1000 W respectively.
- For 120 kHz, non-linear effects cause a reduction in signal level
of approximately 1 dB at 10 m range at full power (1000 W).
- For 200 kHz, the reduction in signal level is 4.5 dB at 10 m range
at full power (1000 W).
To avoid the influence from non-linear
effects, we recommend that the output power is reduced to approximately:
- 500 W for 120 kHz and transducer beamwidth 7 degrees
- 100 W for 200 kHz and transducer beamwidth 7 degrees
when performing fishery research investigations.