On 02/18/2013 10:23 PM, Fons Adriaensen wrote:
On Tue, Feb 19, 2013 at 05:49:37AM +1100, David Worrall wrote:
Does anone know of such a beast?
(I thought of putting the TetraMic in a plastic bag, but I'm not sure Len would
approve :-)
Don't know what Len will think of it, but putting a Tetramic
(or any such mic) in a plastic bag isn't likely to produce
anything usable. Basic problem is that the acoustic impedance
of water is around 3400 times higher than that of air, so the
water/air interface will reflect almost all energy. You need
a transducer that is more or less matched to the acoustic
impedance.
Or how one might go about building one...
Failing which, does anyone have a recommendation on a cost-effective
single-channel unit?
Have a look at <http://www.aquarianaudio.com/hydrophones.html>
Prices start at around $130.
I know of one experimental underwater AMB mic using these -
but can't tell much more.
Most hydrophones (and certainly any low-cost ones) will be
omnis, so the A/B processing will be very different from
what is required for a normal tetra mic using directional
capsules. If they are not omnis then calibration becomes
a real practical problem, it requires underwater impulse
response measurements which in turn require a calibrated
underwater source - an expensive item.
like fons says, your usual hydrophone will be an omni.
the speed of sound is quite high under water, so you cannot easily
design an acoustic delay element to obtain a useful directional response.
if you don't want to use spaced omnis, check out he sonar surround stuff:
http://www.ambient.de/en/products/ambient-recording/underwater/directivity-sphere.html
not ambisonic, but at least you get some surround information, which is
quite an engineering feat. the guy who makes them is really kind of
cool, i've talked to him at tonmeistertagung 2010 in leipzig...
--
Jörn Nettingsmeier
Lortzingstr. 11, 45128 Essen, Tel. +49 177 7937487
Meister für Veranstaltungstechnik (Bühne/Studio)
Tonmeister VDT
http://stackingdwarves.net
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