From: Winfield Hill
Subject: Re: Any info about depth sounders?
Date: 7 Dec 2002 08:31:45 -0800
Organization: Rowland Institute
X-Newsreader: Direct Read News 4.20
> Can anybody give any advice, or preferably a schematic for a depth
> sounder for a boat?
Visit a marine supply store and ask to see the manuals of the depth
sounders/fish finders, which often include schematics at the back.
Most use 200kHz transducers. The attenuation in sea water at this
frequency is about 60dB/kyd so a 500 yard range is practical if the
beam is narrow enough. High frequencies like 200kHz are preferred
in part because one can easily make small directional transducers.
Because transducer mount design is a big deal for a boat, you can
usually purchase just the transducer at marine supply stores. Don't
try to adapt an air transducer for underwater use, because they have
very different acoustic impedances.
Deep Sea (8-mile range) depth-sounding transducers are usually at
low frequencies like 10-12kHz, where the attenuation is 0.5 dB/kyd,
or about 100x less. However they are large, expensive beasts.
> All those old films of people sweating in submarines while the
> destroyer above tries to sound them out, used an audible 'ping'.
> Is that a more trasmissive frequency to use in sea water, or was
> it just that ultrasonics wasn't really available during the last
> world war?
Submarine sonars are usually flexible, with a frequency settable to
take into account the intended use and range.
> When I find the best frequency to use, how many cycles should in
> build into the ping?
Usually 3 to 20 cycles are used, to avoid smearing the echo and to
maintain high resolution. It's also a matter of receiver bandwidth.
Fairly high power is used during the ping, e.g., 200 watts minimum.
Because piezo-ceramic transducer acoustic impedances are different
than water, power-coupling is limited and the transducer looks like
a capacitance to the driver. This is why resonant drive circuits
are usually used.
> An echo from a hard bottom probably isn't too difficult, but if the
> bottom is soft mud?
Sandy bottoms scatter nearly as well as rocky bottoms, if the angle
of incidence is 90 degrees. Silty bottoms are down about 6dB and
saturated clay bottoms are down about 12dB more, which means that
a rather strong echo can still be observed.
If you are serious about in learning about these things, buy a copy
of Robert J. Urick's book "Principles of Underwater Sound."