For something vaguely like this, I've done two things -- One was
to epoxy (it's a rigid glue) a cheap lavalier condenser to the
floor, and the other was to epoxy a 1" piezoelectric disc to the
platform. The lav had a bit more subtlety in frequency response,
although most of the sound is hugely bass. The piezo produce a
very loud signal and also didn't require a battery. (Just don't
plug the piezo into a sensitive mic circuit and then bang on
it -- you could generate several volts, which could damage some
mic inputs.)
By playing with different glues, you could attenuate some of the
bass. And of course flip on the LP filter on the recorder.
But note that in both cases, I'm miking the resonant flooring
surface, not the sand. Sand doesn't transmit sound more than a
few inches without huge attenuation. So ultimately you're
recording the floor...
HTH,
ac
Allen Cobb
http://timbreproductions.com
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