I'd like to add to a comment Robb N wrote about directional information in=
water.
At 06:44 PM 7/20/2010 +0000, you wrote:
> In practicality it is difficult to use any kind of device to focus
> sounds or accentuate directional information. Something like a parabola=
> or disk will either have too similar of an acoustic impedance as water to=
> be effective, or it will want to float or move around so much that it
> will probably just create noise in your recording. Spacing seems to be
> about the only relatively easy solution to catching a stereo
> sound-field. It is more of a challenge for sure to record in stereo
> underwater because it is much harder to set up the mics and there are so=
> many more unpredictable geologic features to absorb and reflect sound (at=
> least in shallow water).
Direction of acoustic signals in water can be determined and measured quite=
accurately however Robb is correct about the practicality of it. I have
worked in the past on two projects developing solutions to this problem,
the first being a simple hand held diver pinger locator. This was nothing
more than a small parabolic dish about 8" in diameter, a preamp and
earphone electronics with a 9 volt battery in a small waterproof module
with pistol grip the parabolic dish was attached to. Add to this a 2 ft
long cord with a piezioelectric earphone encapsulated in urethane potting
compound that the diver could hold to his ear. The concept that makes this=
work is the 8" plastic parabolic dish is lined inside with a thin layer of=
neoprene closed cell foam which is a near perfect mirror to sound in water=
and focuses the sound waves on a small hydrophone on a short mast in the
center just as parabolic dish mics work in air...
Second directional hydrophone system consisted of three hydrophones spaced=
equidistant apart in a triangular configuration about 6" apart and each
with its own preamplifier/receiver. The signals are fed to a comparetor
and phase shift detector, shortest (closest) two signals were used to
determine angle and direction of sound could be determined to within one
degree accuracy in any direction. This method would be extremely
impractical for the average person and I mention it only to make the point=
that yes, stereo sound recording in water is a viable reality and spacing
can be whatever you wish to make it bearing in mind the speed of sound in
water is a little over 4 times the speed of sound in air...
There is also a concept used to make directional hydrophones by lining a
number of elements up in a straight row almost touching each other and wire=
them in parallel and you get a narrow fan shaped pattern in the direction
the elements are mounted with some gain over a single element. By mounting=
the elements in a ring, the pattern becomes conical or cardioid shaped...
Food for thought but as Robb N puts it, practicality is probably not very
high for NatureSound recordists...
--
Thanks,
Mitch & Shadow...
http://www.4shared.com/dir/UTASxktL/wildlife.html
Shadow's area: http://www.4shared.com/dir/ecfWjyZb/Shadow.html
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