In a message dated 2007/02/28, mckimzey@........... writes:

> 2) I have been reading about the STM-8 vertical at the site and in the 
> aggregate emails at PSN.  How specific does one have to be with the voice 
> coil?  I can't find the Radio Shack models.  Would any subwoofer work, or 
> would there be significant electronic modifications needed if one used 
> another subwoofer?

Hi Mike,

       I suspect that many subwoofers could be made to work, but do note that 
Sean turned out the polepieces to increase the pole gap and wound his own 
feedback coil, which had more turns. The pole gap spacing likely needs to be 
increased for seismometer use. The magnet systems chosen were demountable / bolt 
together. Many current speaker systems use ferrite magnets firmly glued in 
place and have very narrow pole gaps, which could make setup / alignment 
difficult.
       An alternative is to use a flat rectangular coil with quad NdFeB 
magnets on mild steel backing plates. You might find this construction easier? See 
drawings at http://jclahr.com/science/psn/chapman/lehman/index.html The more 
powerful NdFeB magnets can give larger magnetic fields in a wider gap.
       You need to be able to measure the force / current characteristic and 
the resistance to calculate the feedback required in a triple loop system.
       Look also how Bob McClure has used NdFeB magnet / coil systems at 
http://jclahr.com/science/psn/mcclure/index.html
       
       Regards,

       Chris Chapman   
In a me=
ssage dated 2007/02/28, mckimzey@........... writes:



2) I have been reading about th=
e STM-8 vertical at the site and in the 

aggregate emails at PSN.  How specific does one have to be with the voi=
ce 

coil?  I can't find the Radio Shack models.  Would any subwoofer w=
ork, or 

would there be significant electronic modifications needed if one used 

another subwoofer?




Hi Mike,



       I suspect that many subwoofers could be=
 made to work, but do note that Sean turned out the polepieces to increas=
e the pole gap and wound his own feedback coil, which had more turns. Th=
e pole gap spacing likely needs to be increased for seismometer use. The mag=
net systems chosen were demountable / bolt together. Many current spe=
aker systems use ferrite magnets firmly glued in place and have very narrow=20=
pole gaps, which could make setup / alignment difficult.

       An alternative is to use a flat rectang=
ular coil with quad NdFeB magnets on mild steel backing plates. You might fi=
nd this construction easier? See drawings at http://jclahr.com/science/psn/c=
hapman/lehman/index.html The more powerful NdFeB magnets can give larger mag=
netic fields in a wider gap.

       You need to be able to measure the forc=
e / current characteristic and the resistance to calculate the feedback requ=
ired in a triple loop system.

       Look also how Bob McClure has used NdFe=
B magnet / coil systems at http://jclahr.com/science/psn/mcclure/index.html<=
BR>
       

       Regards,



       Chris Chapman  =20