Klas,
I also have no experience but purely speculating:
There may be an interface problem as Walter correctly points out.
But you should get a good reflection from a metal surface (ie Sonar
detects subs well.)
The speed of sound is much higher in water (1500 m/s vs 330 m/s).
So the wavelength will be about 4 times longer for the same frequency.
Therefore the diameter of the parabola must be significantly larger
to capture the same range of frequencies.
Good Luck, and let us know what you find out.
Drew
At 01:13 PM 9/26/2002 -0400, you wrote:
>Klas Strandberg wrote:
> > I have a question from a client about parabols and underwater recording=
. I
> > have no experience or understanding of it. Is there any clue out there?
>
>I looked into this a while back, and it seems there is little value with
>parabolas made out of most plastic materials. The problem being twofold,
>often the sound transmission characteristic of the parabola material is
>not much different from water and it fails to reflect but the sound goes
>right through it. The second has to do with the relative
>uncompressability of water, with several places seeming to state that
>the parabola will provide no gain or directionality. While others seem
>to be trying it:
>
>I did run across this, which is about a talk on the subject, maybe you
>can track it down.
>http://www.auditory.org/asamtgs/asa92nwo/4aEA/4aEA13.html
>
>Even so, I've been meaning to give it a try.
>
>Sound in water reflects very well off a air/water interface, so one
>parabola which should definitely reflect is to take two reflectors and
>seal a thin airspace between them. You would have to do pressure
>compensation if going very deep.
>
>Be aware that the surface of the water is a big reflector, and causes
>phase effects in shallow water. I've had occasion, using my hydrophone
>in very shallow ponds where I knew there were frogs making sound but I
>could not pick them up from a little ways away.
>
>The speed of sound in water is different than air, which will change the
>size of reflector needed.
>
>Let me know if you find out anything.
>
>Walt
>
>
>
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