Buy a couple of garmin fish finder transducers (screened kind) chop the ori=
ginal plug off, solder them to two XLR plugs, wrap an elastic band around t=
he pair, sling em in the river / lake / sea - matrix them to form the equiv=
alent of a dipole mic, and adjust mid / side to select the sounds of intere=
st. (In addition, chop the cable near the transducer, add a FET / phantom p=
ower circuit / repot the top, add new or use existing cable.
Fish/depth finde transducers costs peanuts compared to research grade hydro=
phones and are broadband enough for audio, they are resonant at normally 5=
0 or 200 khz, below that quite linear. AIRMAR is the OEM for most well know=
n brands.
-M
--- In "Robb N" <> wrote:
>
> Following Kevin's "Frogs above and below water" thread, he asked about my=
experience with stereo recording in a private message. This seems of gene=
ral interest to the group so I'm going to elaborate and share some of my co=
mments below.
>
> I would love to learn more about the successes and challenges that people=
have had while recoding with multiple hydrophones. Any tips for success? =
Any recordings to share?
> ________
>
> I don't have nearly the level of personal experience with recording as ot=
hers in this group. I have attempted to record frogs above and below water=
on stereo tracks, as was originally asked. I was not happy with the recor=
ding though because the hydrophone needed so much more gain and I didn't ha=
ve the ability to adjust gain independently with my little Zoom H2 that I w=
as using at the time. I didn't even bother to keep the recording, but this=
would be easy to do with the right recorder or preamp.
>
> I think that Mitch's comments are very insightful. He mentioned compensa=
ting for the speed of sound by placing the hydrophones four times further a=
part than what you might place a standard mic. He also talked about surfac=
e reflections and other things. This is all good advise. In practicality =
it is difficult to use any kind of device to focus sounds or accentuate dir=
ectional information. Something like a parabola or disk will either have t=
oo similar of an acoustic impedance as water to be effective, or it will wa=
nt 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 solu=
tion 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 so many more unpredictable geologic features to absorb and r=
eflect sound (at least in shallow water).
>
> I have known of many people that set up two hydrophones on a T-shaped boo=
m. The hydrophones are spaced approximately four times further apart than t=
he width between your ears. Then, if you put a pointer at the top of the b=
oom perpendicular to the plane of the two hydrophones and follow that point=
er with your head while listening in stereo, you can effectively reproduce =
your natural binaural locating abilities underwater. You could be looking =
in precisely the wrong direction with this because this system doesn't prov=
ide front-to-back information. But if the user can narrow in on the source=
, there are, typically, visual clues that can help with that. This techniq=
ue is used as more of a natural underwater direction finder, but it could s=
till help the user set up mic placement for a stereo recording. As noted a=
bove whenever you attach your hydrophone to something underwater you have t=
o be careful about inducing extra mechanical noise into the recording thoug=
h the support structure.
>
> You can also do something similar with a small human-powered boat by clam=
ping the hydrophone cables to each side and and pointing the boat at your s=
ound source. I remember David Kuhn from this group writing about this last=
winter (northern hemisphere), though I remember it from the context of the=
challenges that the currents provided. Calm, still water is always a bles=
sing to clean hydrophone recordings. David, do you have anything to add?
>
> Other than making stereo recordings, there are more tricks that you can d=
o with two hydrophones. You can hang one hydrophone deeper than the other =
for vertical imaging. You can connect the two tightly together and record =
in stereo: With matched gains on each channel, you can sum the two in post=
processing for a theoretical 6dB improvement in self noise; with unmatched=
gains, you can possibly increase your dynamic range with post-processing t=
rickery or at least it will allow you to select for best saturation between=
the two for any given clip. You can even attenuate handling noise to a la=
rge degree by physically connecting the hydrophones, facing in opposite dir=
ections, and summing the two channels--either electrically or in post proce=
ssing.
>
> I'll appreciate anything you guys might add!
>
> Kind regards, Robb
>
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