DIY Magnetic Shielding

Flippit

Member
2001-07-20 9:20 pm
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?

Tnx
 
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.
 
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.

Thanks,
Robert
 

Tobbe_L

Member
2003-12-08 11:09 pm
Gbg
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.
 
I found this stuff on the web. Whadaya think? I was thinking some of it between my speaker and top of the TV would work?

It's the right stuff, but 4 thou' is pretty thin compared with what I've seen for this sort of screening job.
You might need several thicknesses, depending on the flux of your speaker, the physical arrangement, and the 'sensitivity' of your TV (they seem to vary considerably).
 

dhenryp

Member
2003-08-03 12:52 am
Mass.
It should not affect speaker properties as all it is doing is taking stray magnetic fields, that are lost to the speakers magnetic circuit anyway, and diverting them through the mu-metal shield.

Regarding the advice to not bend it, as the FAQ says you SHOULD bend it. It works better if bent into a curved surface around the madnetic source. In addition to wrapping the magnet you could also put a layer inside the cabinet on the the side that is closest to the TV, if the one layer is not enough.

FYI you can also by mu-metal from McMaster-Carr. It think it's a little cheaper.
 
dhenryp said:
It should not affect speaker properties as all it is doing is taking stray magnetic fields, that are lost to the speakers magnetic circuit anyway, and diverting them through the mu-metal shield.

Regarding the advice to not bend it, as the FAQ says you SHOULD bend it. It works better if bent into a curved surface around the madnetic source. In addition to wrapping the magnet you could also put a layer inside the cabinet on the the side that is closest to the TV, if the one layer is not enough.

FYI you can also by mu-metal from McMaster-Carr. It think it's a little cheaper.
If you read the whole FAQ it says the best way to shield is to use flat sheets of MU metal between the entire speaker and the TV. Bending mu metal has an effect on its ability to aborb the magnetic fields, that is way speaker manufacturers cast their shielding cups rather than just bend and weld them. Mu metal is a crystaline structure that is damaged when bent. I've done a lot of custom work using it and trust me, it makes a difference.
 
So does anyone have any direct experience with MuMetal shielding a speaker when used outside of the speaker?

If I bought.... say... a 12" by 14" sheet which would easily cover the bottom of my speaker, and put it between my TV and the speaker, would that work?

Anything less expensive that works? Like a ¼" sheet of solid steel, iron or copper?

-Robert
 

dhenryp

Member
2003-08-03 12:52 am
Mass.
markp said:
If you read the whole FAQ it says the best way to shield is to use flat sheets of MU metal between the entire speaker and the TV. Bending mu metal has an effect on its ability to aborb the magnetic fields, that is way speaker manufacturers cast their shielding cups rather than just bend and weld them. Mu metal is a crystaline structure that is damaged when bent. I've done a lot of custom work using it and trust me, it makes a difference.

I 've read the entire FAQ plus I've also read through the description of the materials they are talking about. A few points:

1. I don't think the magnet shield plates the FAQ talks about using between the speaker and the TV are even mu-metal. They don't call it mu-metal and it's maximum permeability is 80 times less than the magnetic shielding foil they DO call mu metal (re: http://www.lessemf.com/276.html).

2. The FAQ says nothing about not bending the magnet shield plate.

3. The shileding cups used by speaker manufacturers are almost certainly NOT made of mu-metal. Look at the prices for .004" foil and figure out what shield at ten or twenty times that thickness would cost. The cups I've seen aren't cast but are stamped steel. Steel works well to reduce magnetic fields so long as the fileds are relatively weak.

4. Look at the descriptions the site uses for the mu-metal magnetic shielding materials. All of them talk about making rounded bends. They do say you should not heat them nor bend it a sharp angles. I bought a sheet of mu-metal from McMaster Carr (8912K32) and it CAME rolled into a 2-3 inch tube.