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Discussion in 'Home Theater Projects' started by David Giesbrecht, Jun 20, 2001.
Is it possible to magnticly sheild some of your own speakers.
Unfortunately, not without changing the parameters of the drivers (speakers), which would change the crossover points, the dynamics of the speakers and other traits of the speaker system.
I got two turntables and a microphone
How would you go about to shield a speaker ?
Even if it alters the speakers characteristics I'm interested in how it can be done (for future needs, if I'll get around to build a good center speaker).
Subsonic - can't beat the feeling!
I can only give you a basic response to how shielding is accomplished. Perhaps someone else can go into the details. The manufacturer uses what is called a "bucking magnet" in addition to the primary magnet around the voice coil. The bucking magnet evidently results in a tightly-contained magnetic field around the speaker.
Two ways I know of: 1. "bucking" magnets epoxied on the back of the driver's magnet - the counteractive magentic field turns the field flux lines back in toward the magnets so the flux field doesn't go very far beyond the driver. I don't believe they completely change the T-S parameters of a driver, but I'll let the pros step in here and correct me.
2. A "mu-metal" cup installed to completely cover the exposed driver magnet. Mu-metal is a special (expensive) alloy. Exciting stuff, huh?
Yes, a "bucking magnet" is used to contain the magnetic field of the speaker. (the most basic method, not the best method) The bucking magnet is a magnet which is reversed in magnetic polarity, which reduces the magnetic flux of the main magnet. Which is the primary reason parameters change, and changes the quickness and damping of the speaker, among other things.
Manufacturers of high-end speakers use a "bucking magnet" with the least amount of reverse polarity, and use a shielding cup, a metal cup which covers the magnet system, to magnetically shield the speaker.
I got two turntables and a microphone
Ahhh... shielding, this topic comes up every so often.
There are three (typical) ways to provide shielding of a speaker drivers magnetic field (or this is the extent of my knowledge).
1. Bucking Magnets: These are magnets that are applied to the drivers magnet (or in close proximity) in a "reverse" orientation... i.e. poles are opposite. The magnetic fields cancel each other out. This usually effects the Thiele/Small parameters of the driver.
2. Magnetic Cups: These are metal cups placed around the magnet of the driver (sometimes in conjunction with Bucking Magnets). The magnetic field flows through the metal cup easier then through the air, so the emitted field is reduced. This all deals with permeability... science type guys can explain better. Very little effect on driver T/S parameters.
3. Shielded Enclosures: Same idea as the cups, but you line the speaker cabinet with metal to keep the magnetic field within the enclosure. No measurable effect on driver T/S params. You can also shield the CRT tube instead of the drivers.
The best magnetic field shielding metal is MU metal. Very expensive. Low carbon steel (Heating and A/C duct stuff) can be used but "MAY" over time become magnetized. Line the inside of the cabinet with the metal, covering every panel with holes for wires and the driver itself.
Do a search on Yahoo for "Magnetic shielding". You will find a number of firms that provide materials and designs for commercial applications. Read their pages and become educated.
This is the extent of my knowledge, those that know of errors in my post, please feel free to correct me!
My Small Collection
Thanks, I was already thinking about the sheilded box idea so I will probably go with that.
FWIW, CRS (cold rolled steel) is dirt cheap and an excellent shielding material. 14ga should be sufficient for all but the strongest motors. As an example, I can lay a sheet of it on top of the TV, with it overhanging the CRT, and sit a 22lb motored pro 15" on it with nary a bobble in the picture.
Shielding by lining the speaker is a way to 'kill two birds with one stone', as mounting the sheets with non-hardening mastic damps the cab via constrained layer construction.
Loud is beautiful, if it's clean
Looks guys, this is the reply I got from one of the manufacturers.
"A ceramic bucking magnet plus a steel cap."
Could a person use a non ferrous material such as lead that is used on roof vents? or aluminum which resists magnetism?
darn it all.. i know i should have held on to the sheets of 3/4" mdf lined with metal that they were using to line the MRI room at the hospital i worked at... 200 bux/sheet i think... just gave it away.. made a shop table out of it.....