(This is a forwarded email reply from Chris Chapman)
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@............
Hi Larry,
This 'bit of string' sounds quite a bit too short! But it depends on how much mass you are planning to use. Check on the values given in Sean's description? The usual speaker systems use external Alnico cylinder magnets with a pole piece of 1.0" to 1.5" dia., maybe 4" outside diameter. The original speakers were 12" to 15" dia or even larger. You can measure the push well enough with one of the small 100 gm electronic scales, or by lifting a coin vertically. The cheaper speakers use Ferrite magnets, which were much more bulky, have lower fields, high stray fields and are much more temperature dependant.
One of the problems with speaker magnet systems is that the space for the coil only allows a few thou radial clearance. Ideally, you would find construction / setup much easier if you could turn out the pole gap a bit on a lathe to give maybe 15 thou clearance, inside and outside. The older speakers were bolted together, which made them easy to disassemble. Turned pole faces need a coat of rust resistant priming paint / electro-plating. The central column and the backplate of a speaker are usually one item. 16 ohm speakers are preferable, if you can get them.
Another alternative is to use four rectangular NdFeB magnets, maybe 5/16" to 1/2" thick, mounted on two vertical mild steel backing plates held apart with soft iron spacers. You then wind yourself a rectangular flat coil to fit in the central pole gap. The movement is parallel to the pole gap. www.Wondermagnet.com sometimes have magnet pairs already mounted onto a mild steel backing plate, Ref 0031. They also sell magnet wire. It is easy to visually check the magnet to pole face clearance in flat systems.
It is also quite possible to use 1"~1.5" dia NdFeB disk magnets as the centre column and add a circular end polepiece, a circular backing plate and a mild steel outer tube. It very much depends on what you can make / have made. I have used magnet columns about 1" long - two 1/2" thick magnets. I make the former out of 3.5 thou Al foil with a layer of tissue paper insulation, stuck onto an 1/8" Al end disk with air holes in it. I first machine a wood former, then dip it in candle wax, turn it down to size and mount up the end plate and the foil tube with epoxy. You then wind and varnish the coil. You warm the wax with a hair dryer and slide off the finished coil.
Regards,
(This is a forwarded email reply from Chris Chapman)
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@............
Hi Larry,
This 'bit of string' sounds quite a bit too short! But it depends on how much mass you are planning to use. Check on the values given in Sean's description? The usual speaker systems use external Alnico cylinder magnets with a pole piece of 1.0" to 1.5" dia., maybe 4" outside diameter. The original speakers were 12" to 15" dia or even larger. You can measure the push well enough with one of the small 100 gm electronic scales, or by lifting a coin vertically. The cheaper speakers use Ferrite magnets, which were much more bulky, have lower fields, high stray fields and are much more temperature dependant.
One of the problems with speaker magnet systems is that the space for the coil only allows a few thou radial clearance. Ideally, you would find construction / setup much easier if you could turn out the pole gap a bit on a lathe to give maybe 15 thou clearance, inside and outside. The olde
r speakers were bolted together, which made them easy to disassemble. Turned pole faces need a coat of rust resistant priming paint / electro-plating. The central column and the backplate of a speaker are usually one item. 16 ohm speakers are preferable, if you can get them.
Another alternative is to use four rectangular NdFeB magnets, maybe 5/16" to 1/2" thick, mounted on two vertical mild steel backing plates held apart with soft iron spacers. You then wind yourself a rectangular flat coil to fit in the central pole gap. The movement is parallel to the pole gap. www.Wondermagnet.com sometimes have magnet pairs already mounted onto a mild steel backing plate, Ref 0031. They also sell magnet wire. It is easy to visually check the magnet to pole face clearance in flat systems.
It is also quite possible to use 1"~1.5" dia NdFeB disk magnets as the centre column and add a circular end polepiece, a circular backing plate and a mild s
teel outer tube. It very much depends on what you can make / have made. I have used magnet columns about 1" long - two 1/2" thick magnets. I make the former out of 3.5 thou Al foil with a layer of tissue paper insulation, stuck onto an 1/8" Al end disk with air holes in it. I first machine a wood former, then dip it in candle wax, turn it down to size and mount up the end plate and the foil tube with epoxy. You then wind and varnish the coil. You warm the wax with a hair dryer and slide off the finished coil.
Regards,
Chris Chapman