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What is speaker shielding?

 

Curtis
Posted on   
I know the simple answer - it's what protects the video image on your tv from getting discolored/distorted when you plop that center channel on top of the set.

But - my question is this: Is the shielding just a sheet of lead or something lining the cabinet? Or is it something around the magnet itself?

Reason I ask is in case I buy a non-shielded speaker - how tough would it be to add shielding myself?
 

Anonymous
Posted on   
Did you ever find out?? I've been seaching for
the answer too and can't seem to find one ,
unless I take my sheilded PC speakers apart.
Dave
 

Pat
Posted on   
I'm also wanting to find a workable retrofit solution to the shielding issue. Due to room size constraints it's necessary to put my existing Bose 201 speakers so close to the TV that I'm afraid of permanent magnetic field damage to the CRT. Any suggestions would be appreciated.
 

Porcdoc
Posted on   
Modern tvs have built in Degaussers for demagnitization. As it is there is so little
metal in a tv these days , it leaves nothing
to become magnitized. You are probably safe
testing how close you can get with your speakers
as you'll definitely see the field effects as
you get too close. If for some reason your set
retains the halo distorsion, just cycle it off/on.
I've done this to mine, a 31" , placing standard unshielded speakers on top without resisdual
effects. Hope this helps.
 

D.Jackson
Posted on   
Ive read that adding magnets to the back of the drivers, making sure the new magnets are about 2/3 the size of the original magnets, helps to cancel out the magnetic field. Used in this way they are called bucking magnets. I've also heard that lining the speaker cabinets with 1/8 inch thick iron or steel sheeting will help as well. There is also a magnetic shielding "fabric", but it is incredibly expensive. Another option I've read about is to adhere steel cans that have been cut to size over the driver magnets. I don't know anything, but I've read a few things! Hope this helps, some!
 

alan
Posted on   
All screening is is containing the speaker in an aluminium box. There's a lot of theory behind it, but the idea is that the magnetic field caused small "eddy" currents to be induced, like a transformer in the screen. This requires energy, and so reduces the effective power of the magnetic fields. Repeating this makes it better and better (putting screen boxes inside screen boxes with insulation between).

The army have done huge amount of research on this to protect their senistive equipment from interference from a nuclear bomb blast, so ask them howe they do it
 

Derek
Posted on   
There are two ways to sheild speakers;

1. Mu-metal. Look inside a cassette. There is a little piece of metal behind where the read/write head goes (in the middle). Most television have this kind of shield over the speaker magnets. I have no idea what it is or where to get it though.

2. Bucking Magnet. This is a smaller but more powerful magnet inverted on the back of a speaker [driver]. It is strong enough to cancel out stray magnetic fields but not so strong as to prevent the speakers from working.

Try going to www.partsexpress.com for some examples of both.
 

p w Van Smith
Posted on   
What are the options in magnetically shielding of center channel speaker???
 

timn8ter
Posted on   
Like Derek says, bucking magnets. Try this web site.

http://www.partsexpress.com/webpage.cfm?webpage_id=3&SO=2&&DID=7&CATID=45&ObjectGroup_ID=571&filter=ferrite
 

Deepak Rao
Posted on   
I am planning to place tower speakers which are not shielded about 10 cm from my TV. Will these too cause a problem or is it only with centre speakers which sit on top of the TV? Are you guys sure there is no permanent damage to the TV? How do I know if there is interference?
 

timn8ter
Posted on   
I don't think it's a good idea to place a non-shielded speaker close enough to a TV to cause the "rainbow" effect. An earlier comment stated that all you have to do is cycle the power on the TV and it goes away. What's actually occurring is degaussing or realigning the magnetically charged particles inside the picture tube which newer TVs do each time you turn them on. This will work for a while but eventually you'll cause permanent damage to the picture tube. I've even had "shielded" speakers from JBL cause this problem on a 32" JVC TV. These weren't "old". Both were purchased in the last 3 years. BTW, both have been replaced by better items.
 

Kev Wallbridge
Posted on   
I have also had problems with a new tv and stereo system set-up, I called in the engineer for the T.V and he said this was due to the speakers being to close. This had made the rainbow effect on my TV and I think it is permanent. Both the TV and speakers were brand new. I am currently checking the instruction manuls for both to see if I can get a new TV.
 

Anonymous
Posted on   
If you open up a pc speaker, you will notice that the magnet itself is encased in metal. I do not know what metal is used. The cabinet has no lining.

Robert
 

John Spencer
Posted on   
As this is a really common and extremely frustrating problem then why isn't the problem tackled at source - why isn't the TV screen itself magnetically shielded? Surely this would make much more sense? Does anyone know if any TV manufacturers have actually done it?
 

Derek
Posted on   
It is being handled at the source - the speaker. It's also MUCH cheaper to shield a speaker than a TV. Besides, how would you shield the front of the TV where all the light comes out?Derek
 

Anonymous
Posted on   
I too am worried about non-shielded subwoofer placement in my HT setup. There are several HT type large cone subwoofers in the $800 and (way) up range that say they are shielded, and a lot of the small cone subwoofers (like the new velodyne VX10) with shielding.

I was looking into the DIY subwoofer arena, but none of those sources have addressed this problem
(typically a quality Shiva 12" driver, 250 - 300 watt internal amp, and a 3qf ported box) for $400 to $600.

I have found some solutions, but essentially to buy and build them would put one in the same ballpark as buying a ready-made shielded subwoofer like the M&K V1250THX for $800:

Bucking magnets: Parts Express has them including one whooping 20 oz. ring magnet. Actually using them sounds like voodoo magic. Cost: about $4.

Mu-Metal: There are actually quite a few sources, but the stuff is expensive. However, you can usually get "Engineering Samples" for discount. For example, AdvanceMag.com offers a kit for $200 which includes (1 roll of 5'x15" High Perm., 1 roll of 5'x4" High Perm., and 1 roll of 5'x15" Low Perm foil). Again, for the average person the actual usage of the stuff is one-step above voodo magic.

Shielded Driver: I have found one 12" subwoofer driver source that indicates the driver is shielded (the magnet is in a mu-metal case). This is the Madisound-Sonicraft SC12NRT. X-max peak is 15mm. Performance looks to be slightly less than the Shiva driver ($125). Cost: $230

So build your own: $230 for the SC12NRT driver, $300 for a KG-5230 300 watt internal amp, $150 or so for a decent MDF box (like the Parts Express Titanic subwoofer box kit) = $680+.

I like to hear from anyone who has built a shielded subwoofer for themselves or has experience with any of the under $900 12" shielded subwoofers. Thanks.
 

Derek
Posted on   
I've never built a shielded sub but I would imagine two woofers on opposite sides of the box might reduce the strength of the stray maganetic field.

Also, try http://www.partsexpress.com/pe/showdetl.cfm?&User_ID=14088186&St=6126&St2=-55154136&St3=54986648&DS_ID=3&Product_ID=15827&DID=7
or
http://www.partsexpress.com/pe/showdetl.cfm?&User_ID=14088186&St=6126&St2=-55154136&St3=54986648&DS_ID=3&Product_ID=7824&DID=7

...but you're right. Almost all shielded drivers are 6 1/2 inches or less in size.

Just order one of these http://www.hsustore.com/vtf2.html. It's shielded. I used to own one and let me tell you $500 for this bone crushing thing - it's amazing.

I must say that keeping the sub about 3 feet away from the television is usually enough. Most televisions these day do a degauss on power up. You television will adjust to the magnetic field if it's weak enough to a point where you wont notice it. Try flipping the woofer after a week to see if you realy have a problem.

Hope this helps.
 

Derek Johnson
Posted on   
I would think lining the box with 1/8 inch thick aluminium would do it if not completely fixing it. Look at the coils that are incased in aluminium they dont allow a stray magnetic field. I would also rap the magnet in aluminium some how maybe even a few layers of aluminium foil but I have heard that you cant have the aluminium directly touching the magnet. Why would this be ? Derek(not the same derek as earlier posts :) )
 

chris somersett
Posted on   
so what do magnets go in to like tv s and speakers
. so if any one can tell me what elts thay go in to it will be a big help. its for a schoole prodgect
 

Derek
Posted on   
There aren't that many magnets in TVs. There's a coil like the one in cars that raises the 110 volt voltage high enough to jump from the back of the picture tube to the front (through a vacuum). That coil probably produces a very week field.

There are some other coils on the PC board used as adjustable chokes. There's a degaussing coil around the picture tube and the magnets on the TVs speakers (usually shielded).

Oh yeah. There are at least four coils at the back of the picture tube to guide the electron gun.

Hope that helps.
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