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Old 2nd January 2007, 07:59 AM   #11
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I'm pretty sure i've seen numerous references to use of faraday cage to block magnetic feilds..
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Old 2nd January 2007, 08:16 AM   #12
batee is offline batee  United States
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Faraday cages shield from exterior electric fields. Gaussian shields shield from exterior magnetic fields. If a Faraday cage is made of a material such as Iron (or Liquid Oxygen or ...) with positive magnetic susceptibility (in other words, a permeability > 1), it will provide some magnetic shielding. Soft Iron shields well because of its permeability. MuMetal has an ultrahigh permeability, and therefore shields very well.

How much steel? I tried sticking a 2" dia x 1/4" thick (along the axis of the poles) NeFeB N42 (1.33 Tesla) magnet to a 1/4" steel plate, and it feels 99% shielded on the back side of the steel. I tried the same magnet with some 16ga steel, and I felt almost no shielding happening on the back side of the steel.

Note: When you shield magnetically, you also change the strength (by changing the effective length) of the magnet you're shielding. On the magnet side of my magnet/steel contraption, the magnetic field appears to be close to as strong as it would be if I had a magnet 2x as long (along the axis of flux) as the one I started with. What does this mean? That it's also in your interest to move the shielding as far away from the magnet you're shielding as possible to minimize the changes you make to the magnet.

If you're interested, there are some awesome Electricity and Magnetism lectures online from the MIT OpenCourseware site. Excellent demos to go along with the in-depth mathematics behind all of this (including the LOX sticking to a magnet that I mentioned above).

http://ocw.mit.edu/OcwWeb/Physics/8-...VideoLectures/

Bryan A. Thompson
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Old 2nd January 2007, 08:21 AM   #13
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Thanks for the info, i'll have to do some reading (dial-up bites) I guess
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Old 2nd January 2007, 08:35 AM   #14
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What would it take the shield a typical 12" subwoofer such as AE speakers AV12 and similar?

I had a look at the PE bucking magnets. The biggest is around 100mm x 45mm x 18mm and they mention the size should be around 1/2 - 2/3 the size. So lets say the magnet in question is 140mm diam and 35mm deep, what size should I use? 75mm diam x 12mm (2 stacked?)

Or just the biggest size a single?

Where do you place the bucking magnet if not on the back of the driver magnet?
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Old 3rd January 2007, 12:54 PM   #15
Volenti is offline Volenti  Australia
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Quote:
Originally posted by paulspencer
What would it take the shield a typical 12" subwoofer such as AE speakers AV12 and similar?

I had a look at the PE bucking magnets. The biggest is around 100mm x 45mm x 18mm and they mention the size should be around 1/2 - 2/3 the size. So lets say the magnet in question is 140mm diam and 35mm deep, what size should I use? 75mm diam x 12mm (2 stacked?)

Or just the biggest size a single?

Where do you place the bucking magnet if not on the back of the driver magnet?
Magnet area is the primary determing factor in magnet strength (all else being equal), not so much the thickness, the only reason manufactures stack magnets is because magnets are cheaper than the high grade steel they'd have to use otherwise.

So in choosing a suitable bucking magnet I would use one the same diameter as on the driver, or failing that the largest diameter one I could get my hands on.

My peerless tweeters have a bucking magnet that's exactly the same diameter as on the tweeter motor only half the thickness.
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Old 3rd January 2007, 08:26 PM   #16
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Sadly enough I gave up on this project myself... I just decided it'd be better to move the sub away from the TV!

Its good to see this thread may be of use to someone else
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