How to shield cabinet using unshielded drivers?

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I am looking to use some unshielded drivers (mid-basses & tweeters) in a cabinet that will end up as a center channel near monitor. Are there any lessons learned or recommendations to shield the cabinet for this application? Will lining the inside of cabinet with mu-metal or copper work? I welcome any experience on this subject.


Joined 2002
Will lining the inside of cabinet with mu-metal or copper work?

Sorry feke67, copper has very little effect on magnetic fields, and mu-metal is very expensive, it would be cheaper just to buy proper shielded drivers!

The commercial magnetically shielded drivers that I have seen have a simple 2mm thick iron cup shaped pressing glued over the the magnet assembly.

Correct Eric, that is the case, but as the shielding and magnet can interfere with a driver's T/S parameters, unless very well designed, (unlikely in a tv), modified drivers may not work as advertised, and the shielding may be less effective than on a commercial shielded driver.

But feke, why not try it anyway if you have access to the right materials, and please post a follow up to the forum if it works.
ok, sheilding drivers hmm...

...there are two methods to sheilding drivers, the first of which is to use a bucking magnet and the second of which is sheilding.

A bucking magnet is a magnet with the same shaped field as the driver only having its opposing pole facing the driver - this does not stop the magnet field it merely creates an area where the net field is zero by the two fields cancelling each other out.

So-called mu-metals are metals that have a certain structure as to allow magnetic fields to permiate them easily - using mu-metal shields creates a path of short magnetic resistance so the magnetic field lines pass along the planes of the metal rather than through the air.

I hope this helped?

A-level physics does come in handy sometimes I guess!

Also shielded magnets rarely sound better than a comparable unsheilded magnet as they consist of two interacting fields - this inherantly loses the exponential flux density of a single magnet.
Joined 2001
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The little cans on the magnets of shielded speakers are there to act as a magnetic shunt over the magnet+the bucking magnet glued to it.

One can add a bucking magnet (or build a bi-pole with the two drivers magnet-to-magnet). This tends to lower the Qts because more of the field is concentrated in the gap.

I don't know really how well mu-metal works in this situation (be easy enuff for me to do some experiments -- i have a whack of it for sale It has been suggested that shielding the TV would be more effective than shielding the speaker.

I'll do some experiments and get back.

Joined 2001
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feke67 said:
Will lining the inside of cabinet with mu-metal or copper work?
Copper will only shield a *changing* magnetic field e.g. one radiated from a crummy transformer. The copper gets a current induced into it, and this current has it's own magnetic field which opposes the one that made it. OTOH, any ferrous metal just provides a much easier path for flux than what air does, so it follows the metal once it meets it instead of continuing on it's original path.

I am familiar with magnetic circuits, flux density and saturation. At least in theory....
Seing as I doubt I will be able to find transformer laminations to shield the box (even if I do how many will it take). I can do the calculations, but I was more or less wondering if anybody has ever tried it. The magnets strength is given but what will the coils field strength be? In another forum I read that most drivers are sheilded with a 2mm cap, and this made he hopeful. I am not sure the transformer magnetic models apply here as obviously the speakers magnet is not designed to couple with a magnetic circuit path. I looked up mumetal and it has a saturation of .77 tesla and a permeability of 2.4e5. Where as steel is supposed to be about .8 tesla at 1e3 pereability. I guess this the part where i get the books out and start trying to remember this stuff. I was hopeing that someone had tried this (Planet10 seems to have) and could say something like "Yea its easy" or "Stop wasting your time." Outside of the mumetal camp I guess there is no experience with this.
:) Guess I have been reading too many threads where it turns out the unlikely and somewhat ridiculus is commonplace. :)
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