Shielding a DIY center

Discussion in 'Archived Threads 2001-2004' started by MarkBurdette, Mar 19, 2003.

  1. MarkBurdette

    MarkBurdette Auditioning

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    I've been able to round up all the older drivers and the crossover I need to build a center channel speaker to match my older Polk Monitor series... they're all designed to work together so all I have to do is build a box. The unit will be MTM.

    The problem is shielding. Neither the tweeter nor the 6.5" midwoofer is shielded. I have bucking magnets I can put on them, but from what I've read - while it helps - that won't allow me to put the speaker on top of the TV. Worst case, I'll have to build a shelf or in some other way mount the center in position far enough away from the TV.

    So I have two questions.. first, what is the rule of thumb (rules of thumb?) regarding driver or magnet size and distances and magnetic interference? Of course I can just put the driver close to the TV and see what happens, but I'm wondering what is considered "usual" or minimum distance? There is a VCR shelf under the TV.. I considered building the center into that slot.

    Second.. is there a way to add some shielding beyond bucking magnets? Would it help to line the interior of the cabinet (or maybe just the bottom?) with metal?

    Any insight, ideas or recommendations are appreciated.

    Thanks.
     
  2. Brian-K-Owens

    Brian-K-Owens Stunt Coordinator

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    At one time, I had (2) 15" subs under a 36" TV. . .What a nightmare!!!

    I ended up having my dad build metal enclosures that the wood enclosure would slide into. Worked GREAT!!! Maybe you could build a metal enclosure within the wood enclosure if you have not built the wood enclosure yet.

    Maybe not the easiest way, but that is what I did. . .

    Brian
     
  3. Jonathan M

    Jonathan M Second Unit

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    What sort of TV do you have - one presumes it is a direct view CRT. If you have a rear projection, then the guns are down the bottom, so putting the centre on top will probably be OK.

    Magnetic fields usually decay between r^2 and r^3 but because of the shape of the field about a speaker, it is very hard to predict the effect.

    Tweeters are not normally too much of a problem, it's the woofer that will be the problem.

    Your best plan of attack is to see if you have an issue. Hold the woofers in the position they are going to end up with the bucking magnets on, and see what sort of interference you get. It shouldn't be too strong, so shouldn't do any damage to the TV. It would pay to sneak up on it though, to be sure. As soon as you get inference, you know you're too close.

    Additional shielding usually consists of a steel cup over the magnet/bucking magnet configuration - make sure it's isolated from the magnet assembly.

    As for metal lining enclosures etc. this may work, but make sure the metal is ferrous based so that it effects the magnetic field.

    I've found that a 6.5" woofer can be as close as 10cm from my CRT, but it very much depends on orientation, as the magnetic field is not uniform. Generally 30cm or so is enough for all scenarios in my experience.

    Hope this helps!
     
  4. MarkBurdette

    MarkBurdette Auditioning

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    "Additional shielding usually consists of a steel cup over the magnet/bucking magnet configuration - make sure it's isolated from the magnet assembly."

    I thought about trying to add a metal cup... you say isolated from the magnet assembly - so obviously the cup shouldn't touch the magnets themselves.. what about the steel of the frame? **IF** I tried to add my own cup, can it attach to the frame directly or does it need to be isolated from all metal on the driver?

    I really don't think it would be that difficult to do, I just am unsure of the specifics on where to attach the cup in order to not defeat the purpose... I thought of lining the inside of the cup with insulation or foam or something in order to get a tight, secure fit over the magnet structure, yet ensure it doesn't touch the magnets.

    The TV is a 26" direct view. I've got an inexpensive Advent center sitting right on top of it right now with no problems.
    Thanks.
     

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