Magnetic shielding of acoustic systems. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Speakers

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Most speakers specifically designed for the center channel are shielded from magnetic influence, but when trying to use unshielded speakers or full-range floorstanders for the center, you can easily run into the problem of "color spots" on the TV screen. This effect is also possible if the front speakers are too close to the TV. Magnetic shielding of acoustics will help to get rid of this effect.

The construction of acoustic systems uses sufficiently powerful magnets, and the magnetic field created by them deflects the electron flows in the kinescope of the TV from the correct path. The field created by the speaker coil also has an effect, but it is much less.

The main magnetic field is created in the gap of the speaker's magnetic circuit, but part of it remains scattered in the surrounding space. The field in the gap of the magnetic circuit is necessary for the normal operation of the speaker, and the external scattered field is a side effect of the design. There are three ways to eliminate it. Actually screening, compensation and a combination of these two methods.

Screening

For shielding, a magnetically conductive material (for example, steel) is used in the form of a glass or a cylinder, which is put on the magnetic system. The wall thickness of such a screen is usually in the range of 1-3 mm. In general, if you or your friends work at a factory where there is the necessary material and equipment for their manufacture, you are very lucky. The inner diameter of the glass should be 5-20mm larger than the diameter of the speaker magnet.

Inside, a layer of foamed polyurethane is glued onto the walls of the glass, the thickness of which is selected in such a way that the entire structure is tightly dressed on the magnetic system of the speaker. It remains only to glue the magnetic system of the speaker with glue and put on the "glass". The end part of the glass should be glued to the speaker or 2 mm away from it to avoid possible bounce. It is possible to use not a glass, but a cylinder, for example, a piece of steel pipe of a suitable diameter and wall thickness.

Compensation

To compensate for the magnetic field, you can use ... a magnet. At best, it will be a magnet from another, exactly the same speaker. You can use a magnet close in geometric dimensions, preferably close magnetic induction.

Non-working speakers can be easily purchased inexpensively on radio markets. We cut out the diffuser and the centering washer of such a speaker from the diffuser holder and take it out together with the voice coil. We separate the diffuser holder from the magnetic circuit. To do this, just unscrew 3-4 screws that were previously hidden under the diffuser. Sometimes there is a fastening on rivets, then they need to be drilled. It is not recommended to cut them down, as the magnet can change its magnetic properties from impacts. The magnetic circuit can not be disassembled, but a magnet can be used assembled with it. The magnetic circuit prepared in this way is attached coaxially to the back of the magnetic system of the shielded speaker. Fastening is carried out with the help of glue, through a thin polyurethane gasket.

Compensation with a magnet without a magnetic circuit is carried out in the same way. You can roughly estimate the induction of a magnet by comparing it with the induction of a magnet in dynamics. For example, by measuring the force of separation of the reference sample (a piece of iron) with a spring balance. Don't forget to check the polarity of the magnets. When applied to the attachment point, the shielded speaker and magnet should repel, not attract. If there is a hole to relieve air pressure along the axis of the speaker, do not forget to make the same hole in the gasket.

Combination

After compensation, the resulting system can be additionally shielded. Since the residual external magnetic field is small, a piece of tin rolled into a cylinder or a tin can of a suitable size can be used for the screen (aluminum cans that are not attracted by a magnet are unsuitable for this purpose).

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