Try using a baking tray (the kind you use in the owen), they are often made from iron.
Just for the record, copper is complettely useless when it comes to shielding low-frequency magnetic fields (and does not helt at all for dc-fields), copper is good for shielding out electromagnetic interference (EMI) but that is different. The shielding mechanisms are also different.
In applications where you need to shield out both magnetic fields and EMI you normally use several layers of both copper and mu-metal.
Just for the record, copper is complettely useless when it comes to shielding low-frequency magnetic fields (and does not helt at all for dc-fields), copper is good for shielding out electromagnetic interference (EMI) but that is different. The shielding mechanisms are also different.
In applications where you need to shield out both magnetic fields and EMI you normally use several layers of both copper and mu-metal.
Non-magnetic materials like copper, stainless steel, or aluminium do absolutely no good with magnets fields. Steel cans won't do much good unless you have very low stray magnetic fields. All other things being equal (size, shape, magnetic field) the effectiveness of the shield is proportional to it's permeability times its thickness. Mu-metal is about 150 times more effective than ordinary steel. A .004" of mu metal foil will give you shielding equivalent to 150 * .004 = .6" inches of 1018 steel.
A flat plate of steel may give you the shielding you want, but it may not. It depends on the strength and orientation of the field relative to the TV tube. It's quite possible that stary magnetic field will go around the edges of the flat panel and still cause a problem. The advantage of mu-metal foil is that its very flexible so you can try various shapes easily. Try that with a .6" plate of steel.
Most baking tins won't help much because they are just thin metal. Many are aluminium which does no good. If it's not atracted to a magnet its useless. One thing you might look for is an iron cooking pot. The thicker the better and again make sure its atracted to a magnet to make sure you are not dealing stainless steel (very unlikely).
A flat plate of steel may give you the shielding you want, but it may not. It depends on the strength and orientation of the field relative to the TV tube. It's quite possible that stary magnetic field will go around the edges of the flat panel and still cause a problem. The advantage of mu-metal foil is that its very flexible so you can try various shapes easily. Try that with a .6" plate of steel.
Most baking tins won't help much because they are just thin metal. Many are aluminium which does no good. If it's not atracted to a magnet its useless. One thing you might look for is an iron cooking pot. The thicker the better and again make sure its atracted to a magnet to make sure you are not dealing stainless steel (very unlikely).
Experiments I did
I need to magnetically shield my pro ribbon drivers, they use
those powerful neo magnets. I have eight tweeters (four per
cabinet) in a small line array to shield.
With all the hype going around about how great mumetal is, I
had to try this to see how well it works. People are claiming that
these thin materials are thousands times better than ordinary
sheet metal or iron of much greater thicknesses.
Mumetal comes in different thicknesses, 2 mil up to 10 mils
for foil and even thicker sheets. /expensive stuff.
The typical price for a 10 mil 12" x 15" foil is about $50.00,
but I did find one vendor that sells it for 1/2 that price.
Some also say that you can use 10 or 12 gauge sheet metal to shield, but mumetal is suppose to way better.
I used my 21" high tech gauss meter, commonly called a
computer CRT :hehe:
You can see the magnetic influence on your screen :hehe:
I took an ordinary el cheapo dome tweeter and placed it near the
monitor about 6 inches away and I got some pretty colors. I
placed the mumetal 1/2 way between the tweeter and screen
and the screen is clear now. /cool
If I move the foil closer to the magnet, the foil saturates and fails
to work. The proper air gap is required for best results.
The next test was to take an ordinary baking sheet found in your
kitchen (cookie sheet), it's about 60 mils thick metal, 12" x 18".
Guess what? It works just as well as the exotic foil at a fraction
of the cost.
I also tried a piece of sheet metal, 16 gauge for $13 and
I get similar results too.
The next test is to see how my pro ribbons will do. My pro
ribbon driver will cause my crt to change color from as far as 3
feet away, perhaps even farther. I used this distance for testing.
The mumetal barely did anything to attenuate the field of these
monster magnets when I placed the foil 1/2 way between the
driver and crt. If I get closer to the driver, saturation occurs.
The baking sheet might have done a little better or at least no
worse. But two baking sheets stacked worked much better,
double thickness.
It appears that to properly shield these monster magnets I need
to shield the inside of my speaker box, the tweeter chamber to
be exact. I need to keep some distance from the metal and the
tweeter to avoid saturation. Thicker metal allows me to get
closer to the tweeter. It appears that 10 or 12 gauge sheet
metal might do the trick vs. exotic 10 mil mumetal. I guess what
will work best is mumetal that is much thicker, maybe 50 mil ?
The cheapest price for 50 mil is $60 per sq. ft. Most places that
I've contacted seem to charge double that price.
What is going to be cheaper, some 10 or 12 gauge sheet metal
or thicker mumetal ?
All I need is to have a sheet metal shop cut plates to line the
inside walls of the tweeter chamber and it should be cheaper
than exotic mumetals.
The other issue is lack of magnetic field attenuation from the
front of the tweeter because it's exposed. I get alot of
field exiting from this point. The only way to solve this is to
cover the tweeter face plate with metal and just cut out the
opening.
Because it would be difficult to attenuate 'monster magnets' unless it's enclosed completely, I don't know if purchasing
the expensive mumetal is justified because you do have that
opening. /hehe
I also tried my PHL 8" midrange. The magnets are not as strong
as my tweeter but it's much stronger than a small tweeter obvisously. I can get decent shielding with thinner sheet metal
like 16 gauge.
I recommend that you do some experiments before finalizing
any idea on magnetic shielding. You may be surprised at the
results.
I need to magnetically shield my pro ribbon drivers, they use
those powerful neo magnets. I have eight tweeters (four per
cabinet) in a small line array to shield.
With all the hype going around about how great mumetal is, I
had to try this to see how well it works. People are claiming that
these thin materials are thousands times better than ordinary
sheet metal or iron of much greater thicknesses.
Mumetal comes in different thicknesses, 2 mil up to 10 mils
for foil and even thicker sheets. /expensive stuff.
The typical price for a 10 mil 12" x 15" foil is about $50.00,
but I did find one vendor that sells it for 1/2 that price.
Some also say that you can use 10 or 12 gauge sheet metal to shield, but mumetal is suppose to way better.
I used my 21" high tech gauss meter, commonly called a
computer CRT :hehe:
You can see the magnetic influence on your screen :hehe:
I took an ordinary el cheapo dome tweeter and placed it near the
monitor about 6 inches away and I got some pretty colors. I
placed the mumetal 1/2 way between the tweeter and screen
and the screen is clear now. /cool
If I move the foil closer to the magnet, the foil saturates and fails
to work. The proper air gap is required for best results.
The next test was to take an ordinary baking sheet found in your
kitchen (cookie sheet), it's about 60 mils thick metal, 12" x 18".
Guess what? It works just as well as the exotic foil at a fraction
of the cost.
I also tried a piece of sheet metal, 16 gauge for $13 and
I get similar results too.
The next test is to see how my pro ribbons will do. My pro
ribbon driver will cause my crt to change color from as far as 3
feet away, perhaps even farther. I used this distance for testing.
The mumetal barely did anything to attenuate the field of these
monster magnets when I placed the foil 1/2 way between the
driver and crt. If I get closer to the driver, saturation occurs.
The baking sheet might have done a little better or at least no
worse. But two baking sheets stacked worked much better,
double thickness.
It appears that to properly shield these monster magnets I need
to shield the inside of my speaker box, the tweeter chamber to
be exact. I need to keep some distance from the metal and the
tweeter to avoid saturation. Thicker metal allows me to get
closer to the tweeter. It appears that 10 or 12 gauge sheet
metal might do the trick vs. exotic 10 mil mumetal. I guess what
will work best is mumetal that is much thicker, maybe 50 mil ?
The cheapest price for 50 mil is $60 per sq. ft. Most places that
I've contacted seem to charge double that price.
What is going to be cheaper, some 10 or 12 gauge sheet metal
or thicker mumetal ?
All I need is to have a sheet metal shop cut plates to line the
inside walls of the tweeter chamber and it should be cheaper
than exotic mumetals.
The other issue is lack of magnetic field attenuation from the
front of the tweeter because it's exposed. I get alot of
field exiting from this point. The only way to solve this is to
cover the tweeter face plate with metal and just cut out the
opening.
Because it would be difficult to attenuate 'monster magnets' unless it's enclosed completely, I don't know if purchasing
the expensive mumetal is justified because you do have that
opening. /hehe
I also tried my PHL 8" midrange. The magnets are not as strong
as my tweeter but it's much stronger than a small tweeter obvisously. I can get decent shielding with thinner sheet metal
like 16 gauge.
I recommend that you do some experiments before finalizing
any idea on magnetic shielding. You may be surprised at the
results.
Thoth said:
A variation on the bucking magnets is to use a 2nd driver.
An externally hosted image should be here but it was not working when we last tested it.
In this push-push config the drivers can get within an inch or so of a CRT with out affecting it... a single driver starts screwing things up at 12-18"
dave
I've had some surplus of both mu-metal & permalloy to sell for a fellow (kept some for myself so could do some experiments). The mu-metal foil is pretty flexible -- both came in rolls.
Both can effectively be used to suck up stray magnetic fields... if they have less field to absorb they work better -- so a layer betweeb the TV & the speaker is the way to go -- a thickish chunk of steel is the price leader thou
Some info i collected http://www.planetsofta.com/mumetal.html
The cap on bucking magnets is just steel, and its purpose is to complete the magnetic circuit between the nagnet and the bucking magnet.
dave
Both can effectively be used to suck up stray magnetic fields... if they have less field to absorb they work better -- so a layer betweeb the TV & the speaker is the way to go -- a thickish chunk of steel is the price leader thou
Some info i collected http://www.planetsofta.com/mumetal.html
dhenryp said:
The cap on bucking magnets is just steel, and its purpose is to complete the magnetic circuit between the nagnet and the bucking magnet.
dave
Attachments
I purchased some of the LessEMf mu-metal foil to line a subwoofer box but I am in a quandary as to the best way to adhere it to the inside walls of the cabinet.
I am thinking it is best to apply it before the cabinet braces are glued in. That way the foil can be continuous, without interruptions, which would seem important for blocking the magnetic field. But the brace will be glued to the foil. I know I can glue the brace to the foil just fine with something like Gorilla Glue but the foil must therefore have a very good bond to the MDF cabinet.
If I did not have that worry about that, I would use contact cement to adhere the foil to the MDF. But will this form a strong enough cabinet wall/foil/ brace structure when the brace is attached to the foil with Gorilla Glue (or Elmer’s knockoff)?
I could use Gorilla Glue to attach the foil but it is an expanding-type Glue so it does not seem like the thing to use.
What would be other gluing alternatives for the foil/cabinet wall bond? It is a fairly large surface area and take a fair amount of adhesive.
I am thinking it is best to apply it before the cabinet braces are glued in. That way the foil can be continuous, without interruptions, which would seem important for blocking the magnetic field. But the brace will be glued to the foil. I know I can glue the brace to the foil just fine with something like Gorilla Glue but the foil must therefore have a very good bond to the MDF cabinet.
If I did not have that worry about that, I would use contact cement to adhere the foil to the MDF. But will this form a strong enough cabinet wall/foil/ brace structure when the brace is attached to the foil with Gorilla Glue (or Elmer’s knockoff)?
I could use Gorilla Glue to attach the foil but it is an expanding-type Glue so it does not seem like the thing to use.
What would be other gluing alternatives for the foil/cabinet wall bond? It is a fairly large surface area and take a fair amount of adhesive.
Put the Mumetal as far away from the source of the magnetic field as is practical. This way it has less field to deal with and will be more effective. If the mumetal is close to the magnet it can be oversaturated and the field will still go through it while the placement of it far away will require it to absorb less energy and it is more effective.
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