Magnetic Shielding
The old center channel does have one thing going for it that I will be hard pressed to build as well on my own; magnetic shielding. In order to convert the electrical signals produced by the amplifier into acoustic energy (aka sound), a speaker employs a combination of permanent and electromagnets to vibrate the diaphragm. Check out this site for a great overview of how speakers work. There's nothing inherently bad about speakers being magnetic, except when you place them by a device sensitive to stray magnetic flux, such as a television. TVs use cathode ray tubes to produce a picture. For those who know who CRTs work, or those who have read this equality great site about how TV's work, you might begin to see the problem.
The speaker's magnets and the TV's magnets do not like to play nicely
together, and it’s always the speaker who is the bully. A decent
sized speaker, such as the woofer that I am using, produces a tremendous
amount of magnetic flux. Just look the side view photo of the speaker
and you can see the size of the magnet. When a speaker is placed close
to a TV, it grabs hold of the electron beam from the TV's electromagnets
and pulls and pushes it away from its desired path. This produces a very
cool, but often very permanent effect on the display. If you put the magnet
close enough to the TV, overtime (like 5 seconds) it magnetizes the casing
of the CRT, and your TV is now permanently psychedelic. This effect is
called guassing. Computer monitors are equipped with a degaussing button
that can flush the tube of stray flux. Televisions are not that well equipped,
so for the purposes of testing, I volunteered my brother's Dell monitor.
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My Brother's unsuspecting CRT. I tried to minimalize
the impact of the scan line in the photos. Also, notice how his
computer speakers have no effect on the monitor. |
Notice the distortion of the colors. The screen
is supposed to be solid white, but the magnet bends the RGB beams
away from their intended path. |
How
Since I am building a center channel speaker, it’s going to be placed on top of the TV, so I have to eliminate any flux that will leak out of the speaker. There is no such thing as a magnetic insulator; the only way to eliminate flux from leaking is to divert it away from the TV. Magnetic flux travels through magnetically permeable materials. Air is not a particularly permeable material, but ferromagnetic materials, such as iron and steel are. Magnetic flux is lazy, so it takes the path of least resistance back to its source. We use this to our advantage by putting the speaker in a metal container. When the flux leaves the speaker, it travels through the air to the metal enclosure. Rather than travel through the metal, it prefers to travel inside it until it arrives on the other side. By enclosing the speaker, I can keep all the flux inside the cabinet, and prevent it from ruining the TV.
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| Notice the absence of distortion as the coffee
can redirects the flux lines away from the monitor. |
Monitors can degauss themselves by overpowering
the magnetism aquired by the tube. TV's cannot. |
For this project I anticipate using a double layer of sheet metal to line the interior of the cabinet. One layer, such as with the coffee can eliminate most of the flux, but there is still some left over. An aditional layer would remove (hopefully) all the flux that manages to pass through the first one.
Some Good Links
