Shielded How?

Sansui

Many speakers are magnetically shielded. How are they shielded? Are the speakers lead lined or something like that? Just curious...

McLoki

It is something wrapped around the speaker magnet. Not something against the speaker cabinet walls. I am not sure what it is though...

McLoki

Once again - google is your friend....

I have speakers near my TV (or computer monitor) that are causing distortion (jitter) to the image. How can I shield these speakers?

Conventional speakers incorporate both a permanent magnet and an AC magnetic field to produce sound. The field from the permanent magnet is present whether the speaker is active or not. The AC magnetic field is only present when the speaker is activated, and varies in frequency and strength with the pitch and volume of the sound produced. The magnetic field from the two sources can deflect the electron beam in a cathode ray tube monitor (TV) causing distortion of the image, sometimes called jitter (and possible damage to the equipment).

You will have to use magnetic shielding alloys to shield these magnetic fields and you have a choice of several methods. Keep in mind that with magnetic fields, you can either shield the source of the offending field, or shield the thing(s) that you wish to protect.
Note:Unlike the bucking magnet method, these shielding methods do not alter the sound quality of the speaker.

1] Method for maximum aesthetics

To achieve maximum aesthetics you will need to be able to open the speaker cabinet and get access to the back of each speaker. There, you will find a donut shaped magnet, proportional in size to the size of the speaker, over which you will place a cup shaped shield.

Because you will be placing the shielding material in close proximity to this strong magnetic field, you will have to take saturation into account. This means using at least 2 layers of shielding.

For the layers closest to the magnet, choose a high saturation material such as MagnetShield. This material has the ability to "absorb" the initial blast of the field without saturating and becoming useless, but it will only give a limited attenuation. It is very low cost, so 2 or 3 layers are practical.

The outermost layer should be a high permeability material such as Joint-Shield. This outer layer will "absorb" much of the field which has evaded the first layer and yield a very high degree of attenuation. Note that attenuation will be greatest close to the speaker magnet, where the field is strongest (most interfering) anyway.

MagnetShield and Joint-Shield are both offered in a convenient 4 inch wide strip. The material is thick enough to provide good shielding, but still can be cut with a scissors and shaped by hand. For especially strong magnets, you may need more than one layer of each material.

Here is how you do it:

Wrap the MagnetShield around the speaker magnet (notice that it is attracted to the magnet) in a cylinder shape. Cut it so that you have about 1" of overlap at the seam. Use duct tape to tape the seam securely. Cut the material which extends backward at several locations so you can bend these "tabs" inward to form the "bottom of the cup" shape. Leave this layer in place.

Joint-Shield is provided with a peel-and-stick adhesive on one side. Before removing the adhesive backing, cut and shape the material just like the first layer, but on top of the first layer. Remove the adhesive backing and press the second layer onto the first layer. You are done!

Just be careful not to disturb or allow the shield to touch the electrical contacts on the speaker.

2] The Quick and Easy Way (and Maximum Field Reduction!)

If you need maximum field reduction, or cannot open the speaker cabinet, or you simply want to take the easy route, you can simply place flat magnetic shielding alloy between the speaker and the TV.

The magnetic fields at the side of the speaker magnet have different characteristics compared to at the back of the magnet, and different shielding materials are required. Take this into account when considering where your speakers will be positioned relative to the TV.

Shielding the side of the speaker:
Most times you can get away with one or two layers of Magnetic Shielding Foil. On each speaker, place the foil flat against the side of the speaker cabinet which faces the TV, or place it against the side of the TV. A good, inexpensive way to check for shield performance and the best position for the shield is to use a Pocket Magnetometer. You can get additional attenuation by using multiple layers of foil, especially if you use a spacer (such as cardboard) in between the layers. This may offer some aesthetic challenges, but you don't need much technical expertise.

Shielding the back of the speaker:
A high saturation material of significant dimensions such as 36"x15" MagnetShield Plate is required here. If your speakers cabinets are small, you can cut the 36"x15" sheet in half to get two 18"x15" pieces. Note: There will be a position somewhere between the back of the speaker magnet and the front of the TV which will yield "near perfect" shielding. Move either towards the speaker or towards the TV from this point and you will loose shielding effectiveness. Therefore be sure to check the position with a Pocket Magnetometer or digital DC Gaussmeter. Naturally, some situations may require shielding of both the back and the sides of the speaker cabinet.

3] The Third Alternative

You can always place the TV or monitor inside a shielded enclosure. This will protect the monitor from external fields produced by the speakers and any other sources.

Jed Leland

Hello,
From my understanding there's two types of shielding, "hard" and "soft" shielding. For soft shielding a reverse polarity magnet is attached to the back of the usual magnet. This reduces the amount of stray magnetism by cancellation. For hard shielding the reverse polarity magnet and the normal magnet are covered by a shielding cup. This reduces the amount of stray magnetic fields even more.
Jed Leland

ben62670

Jed Leland wrote: »

Hello,
From my understanding there's two types of shielding, "hard" and "soft" shielding. For soft shielding a reverse polarity magnet is attached to the back of the usual magnet. This reduces the amount of stray magnetism by cancellation. For hard shielding the reverse polarity magnet and the normal magnet are covered by a shielding cup. This reduces the amount of stray magnetic fields even more.
Jed Leland

Bingo!
You can get "bucking magnets" from Parts Express on the cheap if you need to shield old drivers.