Hi Patrick.
I also saw your request several days ago from the phonography list, but
have just been busy. Sorry for the delay.
Your hydrophone doesn't need to be filled with oil. This is just
something that is easy to get that has an acoustic impedance fairly
close to that of water. If you have a big air cavity in front of your
transducer, it will have its own resonance which will color the sound.
Maybe this is good or maybe it's bad; it depends on your ear, I
suppose. It may resonate above that of the piezo device and the sonic
difference is negligible. It is good practice to avoid air cavities
outside your transducer assembly if you can. Besides, air floats pretty
well in water, which isn't usually what we're looking for while trying
to record beneath the surface. Of course, you will need some air or
other compliant gas behind the piezo disc if you're using it in a
bending mode as these devices are design to be. (I am assuming that
your "piezo" is a brass disc, which is common for DIY projects.)
Most synthetic rubbers have an impedance pretty close to that of water.
There is no need to have a void between the outer "shell" and your
transducer. Just put the rubber right up against your piezo. You could
probably get creative with a bicycle inner tube and some rubber cement
from a patch kit and make an encapsulent that is not bad at all. You
might also find rubber caps made of SBR at a hardware store to put over
your hydrophone. People use Plastisols (Plasti-Dip is a retail brand)
with some degree of success, but vinyls (plastisols included) are not
very close to water acoustically--nor are silicones. Urethane rubbers
can be very good, but most that are used with hydrophones are 2-part
mixes that are not easy in which to buy small quantities. You can buy
urethane marine sealants in caulking tubes at most boating stores. This
might work well as an encapsulent. Epoxy is not bad acoustically, but
it is too hard to use as an encapsulent for a bending element. However,
you can use it as a thin layer over a piezo bender. Back in the early
days, I used to saturate a nylon mesh with epoxy and lay it over piezo
benders to get a controlled thickness of epoxy--kind of like you'd work
with fiberglass.
I don't mean to discourage these DIY projects at all, because they're
fun and affordable. And you can get great results. But you're not
likely to get great performance out of a simple brass piezo bender
without getting pretty tricky. They're cheap. So I wouldn't
over-design it. Just keep making several iterations of these and use
the one you like the best.
If you have a few specific questions, feel free to contact me off-list.
Kind regards, Robb
. m u r m e r . wrote:
>
>
> and while i'm out of lurk mode...
>
> i posted this question to the phonography list, but got no response, so
> i thought i'd try here. any thoughts about this?
>
> i've been fiddling with building a simple hydrophone for awhile now.
> i'd made several attempts with hollow cavities filled with vaseline,
> poster tack, oil, or nothing. i'd read that a hydrophone ought to be
> filled with oil, something about it having a similar density to water
> and therefore letting the soundwaves travel through the cavity to the
> piezo inside. working with oil is messy, however, and it's very
> difficult to reliably seal the cavity without any air bubbles inside. i
> managed one though, but i also had an identical one which i'd sealed
> with an empty cavity. i compared them, and both work great, and sound
> remarkably similar, if not identical. so what i wonder is: why do i
> think the cavity needs to be full of oil? what does that provide that
> my empty one does not have? anyone have any insight?
>
> best,
> patrick
>
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