Chris, Thanks for the info. Kieth Payea suggested a simple test that I can run = to evaluate the adequacy of the speaker magnet that I have. If it does = prove to be too wimpy, I appreciate your suggestions for alternatives. Larry ----- Original Message -----=20 From: Meredith Lamb=20 To: psn-l=20 Sent: Thursday, February 05, 2004 12:08 AM Subject: Re: Another STM-8 question (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.=20 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 clear! ance, 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 an! d add a circular end polepiece, a circular = backing plate and a mild st eel 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=20Chris,Thanks for the info. Kieth Payea suggested = a simple=20 test that I can run to evaluate the adequacy of the speaker magnet that = I=20 have. If it does prove to be too wimpy, I appreciate your = suggestions for=20 alternatives.Larry----- Original Message -----From:=20 Meredith LambTo: psn-lSent: Thursday, February 05, = 2004 12:08=20 AMSubject: Re: Another STM-8 = question
(This is a forwarded email reply from Chris Chapman)I am starting to collect materials for a vertical seismo based on = the=20 STM-8 design. I have a rather small (7") speaker that I am = considering=20 for using for the magnet and feedback coil, but I'm wondering if = the=20 magnet is strong enough. Can anybody give me some insight into = how big a=20 magnet is needed or whether I would have to make adjustments = elsewhere=20 in the design to compensate for a smaller magnet?
Larry=20 Conklin
Liverpool, NY
lconklin@............Hi = Larry,
=20 This 'bit of string' sounds quite a bit too short! But it depends on = how much=20 mass you are planning to use. Check on the values given in Sean's = description?=20 The usual speaker systems use external Alnico cylinder magnets with a = pole=20 piece of 1.0" to 1.5" dia., maybe 4" outside diameter. The = original=20 speakers were 12" to 15" dia or even larger. You can measure the push = well=20 enough with one of the small 100 gm electronic scales, or by lifting a = coin=20 vertically. The cheaper speakers use Ferrite magnets, which were much = more=20 bulky, have lower fields, high stray fields and are much more = temperature=20 dependant.
One of the = problems with=20 speaker magnet systems is that the space for the coil only allows a = few thou=20 radial clearance. Ideally, you would find construction / setup much = easier if=20 you could turn out the pole gap a bit on a lathe to give maybe 15 thou = clear!=20 ance, inside and outside. The older speakers were bolted together, = which made=20 them easy to disassemble. Turned pole faces need a coat of rust = resistant=20 priming paint / electro-plating. The central column and the backplate = of a=20 speaker are usually one item. 16 ohm speakers are preferable, if you = can get=20 them.
Another alternative is = to use=20 four rectangular NdFeB magnets, maybe 5/16" to 1/2" thick, mounted on = two=20 vertical mild steel backing plates held apart with soft iron spacers. = You then=20 wind yourself a rectangular flat coil to fit in the central pole gap. = The=20 movement is parallel to the pole gap. www.Wondermagnet.com sometimes = have=20 magnet pairs already mounted onto a mild steel backing plate, Ref = 0031. They=20 also sell magnet wire. It is easy to visually check the magnet to pole = face=20 clearance in flat systems.
It = is also=20 quite possible to use 1"~1.5" dia NdFeB disk magnets as the centre = column an!=20 d add a circular end polepiece, a circular backing plate and a mild st = eel=20 outer tube. It very much depends on what you can make / have made. I = have used=20 magnet columns about 1" long - two 1/2" thick magnets. I make the = former out=20 of 3.5 thou Al foil with a layer of tissue paper insulation, stuck = onto an=20 1/8" Al end disk with air holes in it. I first machine a wood former, = then dip=20 it in candle wax, turn it down to size and mount up the end plate and = the foil=20 tube with epoxy. You then wind and varnish the coil. You warm the wax = with a=20 hair dryer and slide off the finished=20 coil.
=20 Regards,
Chris Chapman=20