There is an interesting webpage at http://www.hexmite.com/he123txrx.htm
which seems to offer underwater transducers in the 23.5 KHz range for a
"low cost."   Don't know exactly what they mean by "low cost," since it is
relative.  What is low cost to Bill Gates may be extremely high cost to a
hobbyist like me. :-)

Anyone ever heard of this company (Hexmite)???   The only dealer seems to
be in Shanghai, China, but it appears to be a U.S. email address.  And they
say they only take payment through PayPal.  Which is OK, but which kind of
implies they are a small outfit.

They even have a schematic of their transducer interfaced to a PIC!!  How
about that? :-)

John

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