The one clue we can get from STM's web page is buried in one of the
documents:

"...3. The feedback transducer (which was the moving coil velocity sensor of
the basic seismometer) has a low DC resistance but a very high force
constant (Newtons/Ampere), of the order of 10x the mass (in kg). ..."

I think that in one of his e-mails, Sean-Thomas mentioned a simple way to
measure this with your candidate speaker.  It went something like this:

1.  Set the speaker on the bench with the magnet down.
2.  Measure the position of the center dome.
3.  Set a known mass on the center dome.
4.  Put a current into the voice coil, and record how much current it takes
to return the center dome
of the speaker to it's original position.

I think I used a 9V battery in series with a 10K pot, a meter, and the voice
coil.

Since you just need to return the cone to it's original position, no
expensive measurement tools are needed.  Just rig up some kind of pointer.
The only important things are a known mass and a decent ammeter.

His original speaker was an 8" woofer from Radio Shack.  I'm sure it was not
particularly hefty.  A modern 6" woofer would probably have a higher force
constant.

Good Luck,

        Keith

Keith Payea
Bryant Labs
kpayea@..............
www.bryantlabs.net
(707) 566-8935
----- Original Message ----- 
From: "Larry Conklin" 
To: "PSN List" 
Sent: Friday, January 30, 2004 8:12 AM
Subject: Anther STM-8 question


I am starting to collect materials for a vertical seismo based on the STM-8
design.  I have a rather small (7") speaker that I am considering for using
for the magnet and feedback coil, but I'm wondering if the magnet is strong
enough.  Can anybody give me some insight into how big a magnet is needed or
whether I would have to make adjustments elsewhere in the design to
compensate for a smaller magnet?

Larry Conklin
Liverpool, NY
lconklin@............


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