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What Is A "Shielded" Speaker Anyway, And When Do I Need One?

by Mithat Konar

Originally published in Audio Basics: A Monthly Newsletter of Audio Information, May 1993, published by Audio By Van Alstine, Inc.


M
any readers of Audio Basics have no doubt noticed in other publications a near proliferation of so called "magnetically shielded" speaker systems. Not everyone, however, may know exactly what this "shielding" is and when it might be required. For those, I offer the following discussion.

Every magnet has a "field" around it--a region of space where the magnetís influence is non-negligible. If youíve ever seen the demonstration with a magnet under a piece of paper with iron filings on top of the paper, you have observed the influence of the magnetís field on the filings. Almost all loudspeaker systems use magnets as a component in the systems that generate the forces that move the diaphragms that move the air, etc. Basically, when a magnetic field cuts across a speakerís voice coil that is carrying a current, a force is generated that forces the coil and anything attached to it to move either outward or inward, depending on the polarity of the magnet and the applied current.

With conventional loudspeaker magnet systems, the field is not limited only to the region immediately around the voice coil (which is the only place that the field is of any use); there exists a significant amount of "stray" field. And unfortunately, iron filings are not the only things that can be effected by a magnetís field, stray or otherwise. Electrons hurling through space, the kind you find inside your television or computer display tube, can be wildly affected by magnetic fields, as can the physical elements that store information on computer floppy discs and hard drives.

In the case of the television or computer display tube, the effect of an external magnetic field is to distort the displayed image. This effect can range from mild warping to total havoc depending on the intensity and location of the field and the design of the particular tube. In almost all cases, the magnetic fields surrounding a typical high fidelity speaker system are more than enough to distort images if the speakers are within a foot or so of the television. Luckily, the distortion is usually temporary. If you move the speakers away from the screen, the image will go back to normal in a matter of seconds or sometimes days if youíve applied a particularly strong field. (However, DO NOT try this experiment at home unless you are willing to risk the consequences. There is a small chance you will permanently damage your picture tube.)

In the case of floppy disks and hard drives, the situation is somewhat worse. If you subject a floppy disk or hard drive to a strong enough magnetic field, for even a split second, the information on it will disappear. Forever. And the kinds of fields most speakers generate are enough to erase even the best of disks.

The recent crop of "shielded" speakers are designed primarily to circumvent the first of these problems and manage to take care of the second in the process. The term "shielded" is actually something of a misnomer. A more correct term would be "negligible stray field" speaker, but most people, particularly on this side of the Atlantic, have an aversion to identifiers exceeding two words and to words exceeding two syllables. Such people would prefer to be inaccurate for the sake of linguistic convenience. A pity...

At any rate, as the term suggests, a "negligible stray field" speaker (a.k.a "shielded" speaker) is a speaker that is designed to have a negligible stray field about it. This means that you can place it as close as you want to a television/computer screen or computer media without fear of distorting the image on the screen or destroying the data on the disks. In reality, it is impossible to remove all the stray field (hence the somewhat vague qualifier, "negligible"). This means that there is a practical limit to how close you can get to such a speaker, and some are better than others.

There are two popular methods currently used to reduce the stray magnetic fields around a speaker. The first involves placing a driverís magnet system in a magnetically conductive assembly that effectively limits the magnetic field to within the assembly. This type of structure can properly be referred to as a "shielded" structure. The second and equally popular method uses an additional magnet on the back of the woofer or tweeter to bend the existing stray field back in on itself. This arrangement is sometimes called a "bucking magnet" configuration; it involves no shielding whatsoever. Occasionally, both methods are incorporated into a single speaker's design. No matter the method, the effect, as far as our discussion here is concerned, is the same: a significant reduction in the intensity of a speakerís stray magnetic field.

When are these systems useful?

  • If you would like to incorporate high quality audio into a video system and you want to place the speakers closer than a couple of feet to the screen, then you need a "negligible stray field" system.
  • If you want to incorporate high quality audio into a computer system, or if you want to put high quality audio anywhere close to a computer, then you most emphatically need a "negligible stray field" system. Even if you locate the speakers well away from your computer, it will only be a matter of time before someone finds that floppy with your most highly prized data on it, and seeing a convenient place to put it, lands it right atop the speaker. At 3 a.m., after a long night of computing, that person may even be you.

Finally, you should note that there are some speaker technologies that do not use magnets and are thus without further modification free of stray magnetic fields. The most common of these technologies is the electrostatic loudspeaker. If a system is a full range electrostatic design--like the QUAD ESL-63--it will be entirely free of stray magnetic fields. However, many electrostatic systems are actually hybrids using an electrostatic element for the mids and highs and a conventional dynamic speaker for the low frequencies. These are not free of magnetic fields unless the designer has taken steps to eliminate the stray field from the wooferís magnet--as in the (stock) Acoustat Spectra 11. Full range ribbon systems, like those from Magnepan, should not be confused with full range electrostatic speakers. These ribbon systems use magnets--though in a different configuration than the dynamic speaker--and so possess a stray magnetic field.

copyright © 1993 Mithat Konar--all rights reserved

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