Hi Tony!
Great idea and project. I think a few experimental tows in a swimming pool=
before committing to the whole swim would yield lots of helpful info - and=
I, for one, would love to know what you find out. Hydrophone purchases is=
a little out of my department, and I might try Klaus's idea of a cheap
Radio-Shack lavellier mic sealed in a (dry) condom, with most of the air
removed.
I would NOT try for the radio-signal approach, but for a one-time project
would place a minidisc machine in about three freezer bags, all loaded with=
battery and blank disc. Then as I closed each bag, starting with the
innermost, I would silicon-seal each bag shut around the mic cable,
allowing the goo to cure for several hours before sealing the NEXT outer
bag the same way.
To record, you would just press the proper machine buttons through the bags=
.
Towing a hydrophone, one alone or in an array, has lots of military
history. Listening for subs, you know. The idea was to get the cable long=
enough so you were listening for bad-guys, and not only to the racket of
your own ship. Streamlining so as to produce the least water noise around=
the hydrophone is a critical idea, I believe, but your swimming velocity
(with all due respect) will be modest as compared to a battleship or
destroyer. Your own swimming noises may require you to tow, rather than to=
attach to, your hydrophone. I am guessing here.
Historically, in order to get underwater sounds without the noise of the
ship and the towing array, the sonabuoy was invented. Each is deployed
from airplane, and when it hits the water, flies open, rights
itself, drops a weighted hydrophone on a l-o-o-o-n-g bungie cord, erects
an antenna, and adds ocean water to its silver silver-oxide battery - which=
is the very electrolyte necessary to produce a few hours of FM modulated
audio radio signal on a crystal controlled specific frequency. The battery=
current also begins to electro-corrode a scuttle plug at a
resistor-controlled rate, so after a specified time the salt water pours in=
and the buoy sinks to the ocean bottom, leaving no easy-to-find trace of
where the military power has been interested in listening.
I mention all this because there are several billion on the sea floor, and=
one can occasionally pick up a new one surplus. I would perform immediate=
surgery on it to change the crystal frequency (a local ham radio expert
might be helpful, here, if this is getting too adventurous for your own
electronic expertise . . ) to a nearby NON-military frequency, such as I
once did with about 50 Sonabuoys for Roger Payne's Argentinean right whale=
research. You do not want to be transmitting on a military frequency
accidentally at anytime these days.
If nothing else, a Sonabouy is a source of a cheap hydrophone, and myriad
other fascinating working parts. But watch out when it deploys, it tends
to puncture nearby things like yourself. ;^)
my best regards,
Marty Michener
MIST Software Associates PO Box 269, Hollis, NH 03049
EnjoyBirds.com - Software that migrates with you. http://www.EnjoyBirds=
.com
At 10:41 PM 7/23/2003 -0700, you wrote:
>Hello,
>
>This might be slightly off the course of normal conversation for this grou=
p,
>but I need to harness the collective brainpower for some help with a
>hydrophone conundrum.
>
>First let me explain how I plan to use it. In October, I=B9ll be
>participating in a group swim from Alcatraz to San Francisco. While
>training for the swim, I couldn=B9t help but wonder what 250 swimmers
>high-tailing it through the 62 degree water of San Francisco Bay would sou=
nd
>like, from just below the surface. Getting a hydrophone and attaching it =
to
>my wetsuit is the easy part, but I=B9m having trouble figuring out how to
>record. I=B9d like to use my consumer minidisc recorder, which can easily=
be
>kept dry in a pelican case, but the problem is how to get the cable from t=
he
>hydrophone to the recorder. So my questions are:
>
>1. Is there such a thing as a wireless hydrophone that would work with a
>consumer MD recorder mounted nearby?
>
>2. If not, any ideas as to how I might connect the hydrophone=B9s cable t=
o
>the MD recorder while keeping the recorder dry? (bearing in mind that bot=
h
>items would probably be strapped or taped around my torso, and thus expose=
d
>to the water for the whole hour or so I=B9ll be swimming)
>
>3. Since I=B9ve never used a hydrophone before, is this even a worthwhile
>thing to try? Or would the drag created by my swimming just make a lot of
>noise and not capture anything else? I think I can rig it in such a way
>that it won=B9t tangle in my feet and so forth, but I=B9m not sure how eff=
ective
>the hydrophone is if it=B9s constantly moving.
>
>4. Any recommendations on a fairly cheap hydrophone?
>
>Any other suggestions would be greatly appreciated. The recording might e=
nd
>up being just a bunch of swimmers breathing and arms slapping the surface,
>but I thought it might be worth a shot anyway...
>
>Thanks very much in advance,
>
>Tony Cross
>
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