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Old 20th July 2001, 08:23 PM   #1
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I want to build a center speaker for my home-theater setup. I would like to use the same drivers I used for my surrounds. However, these drivers are not magnetically shielded. Is there any way I can shield them or the enclosure myself, without screwing up the TS-parameters?

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Old 20th July 2001, 09:24 PM   #2
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Default one option...

Bucking magnets, perhaps?
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Old 21st July 2001, 01:35 AM   #3
grataku is offline grataku  United States
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aren't you the guy that wanted to "buck" a transformer? Are you some kind of cowboy?
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Old 22nd July 2001, 12:07 AM   #4
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Default Re: one option...

Originally posted by Rory
Bucking magnets, perhaps?
Yeah, but won't that screw up the TS parameters, as mentioned before? Because that's what I've heard from a DIY speaker fanatic.
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Old 22nd July 2001, 07:06 AM   #5
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I assume that you want to use the same drivers for the center channel to keep the sound of the speakers the same (similar timbre). If the driver manufacturer has a shielded version, the T/S parameters are usually different, but the sound should be close enough to work.

The only possibilities for shielding that I know of are: Bucking magnets, Shielding the magnet and Shielding the entire enclosure.

The first two are the easiest to do (and often used together), but are likely to change the T/S parameters. I wouldn't do these, unless you have a way to check the effectiveness of the shielding, and a way to check the T/S parameters both before and after the modifications. If the enclosure and crossover are properly adjusted for this, I don't think there will be a significant effect on the timbre of the speakers.

Shielding the entire enclosure is a difficult task. It would require a single piece of heavy-gauge steel, formed to fit the box. Welding multiple smaller sheets (1 per side) together won't work as well, as the gap and weld will interrupt the magnetic flux.

Another idea is to place a sheet of steel (as large as the top of the TV) under the speaker (I assume the speaker's above the TV). This should work, unless you have some BIG drivers in the speaker. It might be useful to also put another sheet on the inside of the speaker, on the side towards the TV (usually the bottom). If you tried the bucking magnet route, and got too much change to the T/S parameters, a smaller bucking magnet, along with these methods might also work.

Good luck.
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Old 22nd July 2001, 10:54 AM   #6
hifi is offline hifi  Sweden
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an alternative is going at it from a different direction.....its more room to place a shield in a tv....

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Old 23rd July 2001, 12:01 AM   #7
grataku is offline grataku  United States
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Or buy a projector TV.
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Old 17th July 2004, 04:15 PM   #8
valnar is offline valnar  United States
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Sorry to resurrect an old thread.

I have a "shielded" bookshelf speaker that I purchased, which I believe just means a bucking magnet attached, that doesn't appear to be very shielded after all. I want to place it on top of my 36" TV, but it creates the rainbow halo effects. No go.

What is the exact material (and from where) that I can buy which would negate this affect? I want to place it between the TV and my speaker, if possible.

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Old 17th July 2004, 05:22 PM   #9
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Pure iron is a good material, and so is low-carbon steel. The better stuff is expensive.
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Old 17th July 2004, 05:48 PM   #10
Tobbe_L is offline Tobbe_L  Sweden
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Iron works quite well (normal steel is actually much worse) and is cheap so that is what you should try first.

There are special alloys that are used for this in "real" applications, there are many types but the most common one is probably permalloy which is relatively cheap. You should be able to buy this form several companies.

The really good stuff is very expensive and needs to be handled with care. once it is shaped into the ocrrect form (or welded) it needs to be heat-treated (depending on the material 700-1000 degress C) in a hydrogen-argon mixture so basically this is not an option for a DIY project (I have done it at work, but I have access to real equipment).

With all of these (including iron) you need to be carefull, avoid high magnetic fields.
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