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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