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.
Thanks,
feke67
Thanks,
feke67
Cheap Fix
Hello feke67,
The commercial magnetically shielded drivers that I have seen have a simple 2mm thick iron cup shaped pressing glued over the the magnet assembly.
Take a look in a typical modern television or modern 3in1 speakers.
Some have an additional magnet glued to the back of the magnet assembly.
Eric.
Hello feke67,
The commercial magnetically shielded drivers that I have seen have a simple 2mm thick iron cup shaped pressing glued over the the magnet assembly.
Take a look in a typical modern television or modern 3in1 speakers.
Some have an additional magnet glued to the back of the magnet assembly.
Eric.
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!
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.
...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.
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 http://www.diyaudio.com/forums/showthread.php?s=&postid=50348. It has been suggested that shielding the TV would be more effective than shielding the speaker.
I'll do some experiments and get back.
dave
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 http://www.diyaudio.com/forums/showthread.php?s=&postid=50348. It has been suggested that shielding the TV would be more effective than shielding the speaker.
I'll do some experiments and get back.
dave
Lligior said:
Instead of repeating it here i just did a post on the subject http://www.diyaudio.com/forums/showthread.php?s=&postid=50348.]http://www.diyaudio.com/forums/showthread.php?s=&postid=50348.[/URL]
dave
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.feke67 said:
GP.
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.
This place is great!
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. This place is great!
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