Chris,

Regarding your questions about the speaker used for the feedback
coil/magnet for the VBB vertical leaf spring seis:

I haven't been in a Radio Shack store for some time, so I have
no idea what speakers are available. What is not available in
their stores is available from "Tech America" and others by mail.   
The selection is not at all critical. I chose the 10-inch, DUAL COIL 
sub-woofer speaker because I needed the higher coil resistance of 
8 ohms total. The larger coil diameter means more clearance to work 
with in the seismometer, since the coil moves in an arc at the boom end.
I would prefer a coil resistance closer to 50 ohms for stability
of the VBB transfer function, but the 8-ohm total does work. None of
the other parameters are much of a concern. The force constant of
the coil-magnet needs to be measured, and can range anywhere from 10
to 40 Newtons/ampere and work nicely.

For my current research, I am making my own magnet and coil assemblies.
The coil has an ID on only 5/8". With the present "bought" hardware
(that I described for the PSN on December 6), I have a constant of
10.6 N/A.  The rare-earth magnet costs $28, and the other hardware
is less than $10, so the total is less than the large speaker. If there is
any interest in it, I will finish the description of winding the formless 
multilayer coil next week. It has about 1200 turns in 12 layers of
#36 wire, with a resistance of 90 ohms. Some variations, like using
two magnets, increase the constant by as much as 20%. A properly
machined soft-iron frame might double the output over that of the
"bought" hardware, but the bought hardware is within everyone's reach.


I discussed your other concerns (pressure compensation, capacitive
transducers, etc) for the benefit of the PSN on December 8.

Regards,
Sean-Thomas

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