Lets summarize the requirements so far:

Frequency response DC to 25 Hz
Max. acceleration + - 2 g
Noise level 61 ug or better (16 bits)
Environmental Conditions Habitable, Fixed


Comments:
These requirements indicate that the device is an accelerometer. In this case
the freq.
response to DC is a freebie. The fundamental things you need to build an
accelerometer like
this is a mass supported by springs that has a resonant frequency above 25 Hz.
You need to
be able to sense the "position" of the mass. A coil and magnet could be used for
feedback/damping. The position sensor could be a differential capacitor bridge
or a VRDT.
Given this the loudspeaker still looks like a viable approach. You would
probably
need an additional spring to get the resonance up to 25 Hz. The speaker cone and
 spider would
help constrain the mass motion to one axis. The voice coil would be used for
damping/feedback.


From my somewhat limited personal experiences with strong motion quakes the 25
Hz upper response
frequency may be a little excessive.

I sure wish there were an easy way to make the spiral flexures used in
geophones.

Jim Hannon



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