As Chris suggests, the gap is the part that is hardest to work around.  This
varies a lot from woofer to woofer.  My experience has been that a high
power but very cheap unit will have the biggest gap.  Expensive woofers have
small gaps for better performance, at high power they do this with exotic
voice coil designs.  I got lucky with a woofer given to me by a friend
because it had a rip in the cone.  It turned out to have a gap big enough to
work with.  I did spend some time with a Dremel tool opening the entry area
to the gap.  In the seismometer, the coil moves through a slight arc, so a
little extra clearance is needed there.
 
The electrical performance of woofers doesn't vary that much, and the "gain"
or generator constant is easily measured and compensated.  Much easier than
trying to get bits of metal filings out of the magnet gap!
 
I have some pictures of my version at:
http://www.bryantlabs.net/VertSeis.html
 
Good Luck!
 
    Keith
 
Keith Payea
Bryant Labs
kpayea@..............
www.bryantlabs.net  
(707) 566-8935
 
  _____  

From: psn-l-request@.............. [mailto:psn-l-request@............... On
Behalf Of ChrisAtUpw@.......
Sent: Wednesday, February 28, 2007 5:11 AM
To: psn-l@..............
Subject: Re: STM-8


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 





As Chris=20
suggests, the gap is the part that is hardest to work =
around. =20
This varies a lot from woofer to woofer.  My experience has been =
that a=20
high power but very cheap unit will have the biggest gap.  =
Expensive=20
woofers have small gaps for better performance, at high power they do =
this with=20
exotic voice coil designs.  I got lucky with a woofer given to =
me by a=20
friend because it had a rip in the cone.  It turned out to =
have a gap=20
big enough to work with.  I did spend some time with a Dremel=20
tool opening the entry area to the gap.  In the =
seismometer,=20
the coil moves through a slight arc, so a little extra clearance is =
needed=20
there.
 
The electrical=20
performance of woofers doesn't vary that much, and the =
"gain" or=20
generator constant is easily measured and compensated.  Much =
easier=20
than trying to get bits of metal filings out of the magnet=20
gap!
 
I =
have some=20
pictures of my version at: http://www.bryantlabs.ne=
t/VertSeis.html
 
Good=20
Luck!
 

    Keith
 
Keith=20
Payea
Bryant Labs
kpayea@..............
www.bryantlabs.net
(707)=20
566-8935

 



From: psn-l-request@.................
[mailto:psn-l-request@............... On Behalf Of=20
ChrisAtUpw@.......
Sent: Wednesday, February 28, 2007 5:11 =

AM
To: psn-l@..............
Subject: Re:=20
STM-8


In =
a message=20
dated 2007/02/28, mckimzey@........... writes:


2) I have been reading about the STM-8 vertical at the =
site and in=20
  the 
aggregate emails at PSN.  How specific does one have to =
be with=20
  the voice 
coil?  I can't find the Radio Shack models.  =
Would any=20
  subwoofer work, or 
would there be significant electronic =
modifications=20
  needed if one used 
another subwoofer?

Hi=20
Mike,

       I suspect that many =
subwoofers=20
could be made to work, but do note that Sean turned out the =
polepieces to=20
increase the pole gap and wound his own feedback coil, which had =
more turns.=20
The pole gap spacing likely needs to be increased for seismometer use. =
The=20
magnet systems chosen were demountable / bolt together. Many =
current=20
speaker systems use ferrite magnets firmly glued in place and have very =
narrow=20
pole gaps, which could make setup / alignment=20
difficult.
       An alternative is to =
use a=20
flat rectangular coil with quad NdFeB magnets on mild steel backing =
plates. You=20
might find this construction easier? See drawings at=20
http://jclahr.com/science/psn/chapman/lehman/index.html The more =
powerful NdFeB=20
magnets can give larger magnetic fields in a wider=20
gap.
       You need to be able to =
measure the=20
force / current characteristic and the resistance to calculate the =
feedback=20
required in a triple loop =
system.
       Look=20
also how Bob McClure has used NdFeB magnet / coil systems at=20
http://jclahr.com/science/psn/mcclure/index.html
   &nb=
sp;  =20

      =20
Regards,

       Chris =
Chapman=20