Moving a bit off topic,
And a question for Mitch,
Im wondering what the best method is to pressure test a hydrophone?
Without actually dunking on to 20 - 50 meters?
I have experience using the Teledyne Benthos, AQ2000 - one failed so I took=
a peep inside it. Rubber outer coat, 2-3mm wall of epoxy, long piezo cylin=
der and I was quite surprised to find the center was just an air space. Max=
depth of the AQ2000 as its name suggests is 2000 meters. (or 1 1/4 miles i=
n old money)
-Mike.
--- In Mitch Hill <> wrote:
>
> I'd like to add to a comment Robb N wrote about directional information i=
n
> 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 parabol=
a
> > 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 s=
o
> > 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 qui=
te
> 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 th=
is
> work is the 8" plastic parabolic dish is lined inside with a thin layer o=
f
> neoprene closed cell foam which is a near perfect mirror to sound in wate=
r
> 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 space=
d
> 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 poin=
t
> 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 wi=
re
> 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 mounti=
ng
> 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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