Following Kevin's "Frogs above and below water" thread, he asked about my e=
xperience with stereo recording in a private message. This seems of genera=
l interest to the group so I'm going to elaborate and share some of my comm=
ents below.
I would love to learn more about the successes and challenges that people h=
ave had while recoding with multiple hydrophones. Any tips for success? A=
ny recordings to share?
I don't have nearly the level of personal experience with recording as othe=
rs in this group. I have attempted to record frogs above and below water o=
n stereo tracks, as was originally asked. I was not happy with the recordi=
ng though because the hydrophone needed so much more gain and I didn't have=
the ability to adjust gain independently with my little Zoom H2 that I was=
using at the time. I didn't even bother to keep the recording, but this w=
ould be easy to do with the right recorder or preamp.
I think that Mitch's comments are very insightful. He mentioned compensati=
ng for the speed of sound by placing the hydrophones four times further apa=
rt than what you might place a standard mic. He also talked about surface =
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 direc=
tional information. Something like a parabola or disk will either have too=
similar of an acoustic impedance as water to be effective, or it will want=
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 soluti=
on 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 a=
nd there are so many more unpredictable geologic features to absorb and ref=
lect sound (at least in shallow water).
I have known of many people that set up two hydrophones on a T-shaped boom.=
The hydrophones are spaced approximately four times further apart than the=
width between your ears. Then, if you put a pointer at the top of the boo=
m perpendicular to the plane of the two hydrophones and follow that pointer=
with your head while listening in stereo, you can effectively reproduce yo=
ur natural binaural locating abilities underwater. You could be looking in=
precisely the wrong direction with this because this system doesn't provid=
e 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 technique=
is used as more of a natural underwater direction finder, but it could sti=
ll help the user set up mic placement for a stereo recording. As noted abo=
ve whenever you attach your hydrophone to something underwater you have to =
be careful about inducing extra mechanical noise into the recording though =
the support structure.
You can also do something similar with a small human-powered boat by clampi=
ng the hydrophone cables to each side and and pointing the boat at your sou=
nd source. I remember David Kuhn from this group writing about this last w=
inter (northern hemisphere), though I remember it from the context of the c=
hallenges that the currents provided. Calm, still water is always a blessi=
ng to clean hydrophone recordings. David, do you have anything to add?
Other than making stereo recordings, there are more tricks that you can do =
with two hydrophones. You can hang one hydrophone deeper than the other fo=
r 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 p=
rocessing for a theoretical 6dB improvement in self noise; with unmatched g=
ains, you can possibly increase your dynamic range with post-processing tri=
ckery or at least it will allow you to select for best saturation between t=
he two for any given clip. You can even attenuate handling noise to a larg=
e degree by physically connecting the hydrophones, facing in opposite direc=
tions, and summing the two channels--either electrically or in post process=
ing.
I'll appreciate anything you guys might add!
Kind regards, Robb
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