On Mon, 07 Mar 2005 13:07:25 +1100, jc wrote:

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A lot depends on the range of frequencies, any sensor will have its own
fundamental frequency- multiple sensors may be a good idea. Geophones seem
to be good up to a few hundred Hz, cheap but difficult to get qty one. For
the application a speaker may be the clue (as a second sensor).


Building on this idea - it appears that piezo speakers with a loaded
wafer have done well in this application.
I'm guessing that loading the ceramic to a natural frequency well
below 40Hz would be good. The output is a function of acceleration
of the casing, I imagine.

Brian Whatcott Altus OK