Stereo Towing Hydrophone
Single
element hydrophones are broadly omni-directional, but a simple two channel two
element system can provide some directional information by exploiting the stereo
effect. If the piezos are one behind the other and separated by 750mm
or more and we wear headphones our ears can distinguish which received the
sound first and form a rough idea of how far ahead or behind the boat’s beam it
came from. If the sound persists, changing course by 90 degrees can then reveal
which side it was originally on, and perhaps an approximate direction of the
source. Multi-element arrays in conjunction with computers can do this much more
accurately, and with software that identifies characteristic echo clicks allow
researchers or whale watchers to home in on particular species that are not even
on the surface.
The photo above shows the same nose and tail cones built for my original
towing hydrophone, but fitted to a meter long PVC tube, as a stereo towing hydrophone.
The photo above shows one of two DIY piezo bender hydrophones,
with its associated pre-amplifier, secured to a length of white rigid plastic
with a couple of stretched rubber O-rings. The plastic separated the two
hydrophones by the desired amount for a stereo system, but probably also
acoustically links them which is not ideal. I did not in the end deploy
this system, deciding instead to replace the DIY components with a pair of SesorTech SQ12 bender style hydrophones and a pair of SensorTech
SA02 pre-amplifiers.
The photo above show the
associated stereo amplifier project box containing a pair of "canakit"
amplifiers,www.canakit.com
which use an
LM380 chip. The two individual potentiometers in the kit were replaced with a
double gang variable potentiometer. Their "audio in" connections were fed with
the outputs from towed hydrophone, and the output was to be fed to the line-in
connection of my laptop. This combination produced a very sensitive system with
good volume for a speaker, but unfortunately during its first field trials (on Volo in New Caledonia in October 2002) the amplification proved unstable. There
was nothing I could do with the resources on board to fix this, so in retrospect
I should have stuck with my DIY gear, or at least brought it along for the trip
just in case!
Squelch Circuit?
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