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Mu metal


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In one of the articles discussing shielding of speaker magnets, it was suggested to use metal like a coffee can to shield it. I tried that and it still leaked thru. It was also suggested that a person could use Mu metal to shield the magnet. The speakers I have are 15 inch woofers made by Acoustron in Houston in about 1967. Acoustron has since gone out of business an may have been purchased by another company or at least the technology. I'm not sure, but what is Mu metal and where would I find it? and would I still use a similar box to cover magnet as suggested in using a coffee can? or would I line the inside of the speaker enclosure to shield it from the T.V. I installed the speakers in the bottom of an entertainment center and one of the woofers is about 8 to 10 inches from from the circuit board in the bottom of the case for the T.V. Thanks for any suggestions. Gary

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Dear Gary,

mu-metal is constructed of a sandwich of three layers of metal. A layer of copper sandwiched between two layers of soft steel. One possible source of mu-metal is located at: http://www.russellind.com/Russell/index.html. Unfortunately, while mu-metal is quite effective in preventing small magnetic fields from affecting magnetic pickups, it is not as effective in containing HUGE magnetic fields like those created by large Woofer magnets. mu-metal is usually used to shield transformers or tape heads. The BEST method is to move the offending magnetic source away from CRT's. Another thing you *might* try is to use a small flat plastic magnetic sheet to counteract the magnetic field from the Woofer magnet. This will require a LOT of experimentation to determine the best position and location of the "bucking" magnet. I have never had much luck using this method, but your experience may be better. One more thing, mu-metal is VERY expensive! The UTC Corporation (United Transformer Company) used to make mu-metal shields for their small transformers. They cost almost as much as the transformers themselves, in many cases. A panel of mu-metal the height and width of a typical television would cost hundreds of dollars. Perhaps more. Distance is the BEST choice.

Sincerely,

Bill

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Dear Gary,

"mu-metal" is a substance constructed of three layers of metal sandwiched together. The outer layers are usually steel and the inner layer is copper. It is generally quite heavy and is quite pricey. Shields of mu-metal were manufactured by UTC (United Transformer Corporation) these were formed into a "can" that the UTC transformers fit inside. Shields of mu-metal were also used to shield magnetic tape-heads from external hum fields. Mu-metal is expensive and quite hard to find. Two less expensive options come to mind. *Occasionally* you can get a small magent, such as a flat flexible refrigerator magnet to counteract the magnetic interference from the large voice-coil magnet. You will have to experiment to find the correct orientation to cause the interfering magnetic field to cancel sufficiently. I have to admit that I have had little success with this technique, personally. The BEST solution is to provide distance from the woofer magnet and the CRT. Sometimes a few inches is all that is necessary. It is one of the serious problems in using a non-shielded speaker with a CRT. Mu-metal is expensive and I am not sure where you might find it these days. I hope this helps a little.

Sincerely,

William

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  • 4 weeks later...

Mu Metal is available from Fair Rite Magnetic Products near Chicago. They are quite fair on an expensive item. One solution is to cold cut a piece of thick steel pipe large enough to surround the magnet and extend an inch or two beyond. The other solution is a bucking magnet. In any case you are just pointing the field in another direction.

Tom Maguire

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