Brian Grey! Magnetic Shielding?

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Kelly Holdridge

Kelly Holdridge

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Heya! Dig your studio shots, would like to know more about the chicken wire/foil thing (EVERYbody does!).

I was hoping you'd share what you know about magnetic shielding, and particularly whether this foil thing will work for the same purpose. We've got a thread going in the Cave, guess this was a better place for it. Also, please check out the thread in Visitor Feedback on a possible physics/tooling forum. You sound like the guy to head it up if it's a viable idea.

Thanks, good luck with your studio!
 
Kelly Holdridge said:
Heya! Dig your studio shots, would like to know more about the chicken wire/foil thing (EVERYbody does!).

I was hoping you'd share what you know about magnetic shielding, and particularly whether this foil thing will work for the same purpose. We've got a thread going in the Cave, guess this was a better place for it. Also, please check out the thread in Visitor Feedback on a possible physics/tooling forum. You sound like the guy to head it up if it's a viable idea.

Thanks, good luck with your studio!
Click to expand...

Thanks for the great feedback! So you wanna know more about magnetic shielding do ya? :) I'll try condensing it down to the easiest way to understand as possible.

Everyone knows, or a lot of people at least, that when you pass a magnet by the leads of a volt meter, you'll see the voltage jump up for a second. The reason it does that is because the leads are cutting the magnetic lines of flux, and for a short time there is a small amount of current. You've seen those tests where someone poors metal shavings on a peice of paper resting on a magnet havn't you? And the shavings form lines all around the magnet. When you pass something metal through one of those lines it induces a voltage. Now, take that theory and try to understand the next part... When you broadcast a signal from a radio station, or TV station, what you're sending out is none other than magnetic lines of flux. The antenna on your radio or TV cuts the magnetic lines of flux, much like the leads of the voltmeter, and it runs the current through an amplifier and you get music. Understand that? Now, say you were to put metal all around your stereo, would it do anything to your reseption? Not unless you give that small amount of current some place to go. Where would you put a small amount of current? The earth loves to accept current. Another name for this is "Grounding". And since the amount of current is SO small, there's not a chance in hell you're going to fry anything. So, if you've got your radio inside a metal box that is hooked to ground, the metal box is going to CUT ALL THE MAGNETIC LINES OF FLUX before they get to the radio and send them to ground.

I hope this cleared a little bit of confusion up, and maybe someone actually learned something :) haha.

Have you ever been in a metal building and just couldn't get your radio to come in? Oh man, I have. I worked in a metal building for 2 years and couldn't get crap on the radio. Just 2 days ago I moved to a wooden building just 15 feet away and I get all the stations.

Later,
-Brian

[Edited by Brian Grey on 09-20-2000 at 05:50]
 
So take a speaker-sized plate of steel, solder on a piece of wire, and stick the other end of the wire into the ground-hole of an AC box. That would work, right? I think you're rolling your eyes right now, replying to this post to say "attach it to a grounding rod and put it in the ground." Yeah. That's what I think. You wanna make somethin' of it? Huh!? Yeah, you! Wanna step outside, huh? Okay! Fine! Let's do it, you and me, mano e mano, outside, right now!

*kelly starts towards the door, but trips on gigantic pile of neglected wires, effectively frying himself*

*Brian grins his ass off*

:D
 
Kelly Holdridge said:
So take a speaker-sized plate of steel, solder on a piece of wire, and stick the other end of the wire into the ground-hole of an AC box. That would work, right? I think you're rolling your eyes right now, replying to this post to say "attach it to a grounding rod and put it in the ground." Yeah. That's what I think. You wanna make somethin' of it? Huh!? Yeah, you! Wanna step outside, huh? Okay! Fine! Let's do it, you and me, mano e mano, outside, right now!

*kelly starts towards the door, but trips on gigantic pile of neglected wires, effectively frying himself*

*Brian grins his ass off*

:D
Click to expand...

In the situation of a speaker monitor I don't know how necessary it is to ground the metal plate. I've been thinking all day, trying to remember REAL hard what I was taught in school about this. It was 3rd semester of my two year course that I graduated from a year and a half ago.

Trial and error is the best way I can think of to learn it. I cheated and got studio monitors that were magneticaly shielded, and I don't know exactly how they did it. I do know that if you place metal that is attracted to magnets by a magnet, as many lines of flux will pass through the piece of metal as possible. Magnetic lines of flux are lazy, they like to take the shortest path possible from south to north, if a piece of metal is in their path and they can just go through it they will, thus re directing the lines away from whatever it is you're trying to protect. So in this case, where the magnetic field may not be callapsing and expanding, you may not need to ground the metal, chances are you won't.

So yes, try a big piece of metal. And preferably thick. And make sure it has lots of iron in it, in other words, something that is attracted to magnets.

Later,
-Brian

PS - you should really keep your wires clean and not in a big mess! :) haha.
 
REgarding shielding for speakers

I am under the impression that most speaker companies actually place a second magnet on top of the speaker magnet ,same size and in a certain position so that the magnetic waves cancel each other out.
 
*wiping the burn marks off his eyeballs...*

Okay. I ended up putting my monitor on TOP of my TV, and the speakers are on either side of the TV. I'm only worried about the TV getting permanently "smudged"; I did it to the family TV when I was a kid with some magnets. Okay. Don't know why I'm going into such detail...

Here's what I'm wondering: If the speakers are ON when the TV is OFF, there's no damage to the phosphorus or whatever in the TV, right? I have no plans of blasting the speakers when the TV is on, and I'm hoping that by having the TV off, the gun won't be active and the electrons won't be "distracted." Is this a correct assumption?

Lemme ask it this way: is it the electrons the gun fires that the speakers screw with, or is it the photoresponsive thingeys in the actual screen itself?
 
brian, so am i correct in thinking that you've created the biggest radio antena for home use ever? insted of grounding it you justhooked it into your radio it would pick tons of stuff up?
 
The magnets in the speaker messes with the stream of electrons in the TV and they do it regardless of whether the speakers are on or not. The big magnets in speakers are permanent magnets (as opposed to electromagnets) and are of the same type as the ones you used to smudge your parents' TV. I didn't even know that you could do that. How did you do it? Didn't you just forget one of your magnets near the TV?

Now, if I could only make my brain remember if magnetic fields from permanent magnets are stopped by a Faraday's cage in the same way as electromagnetic radiation is, I'd be happy. Hmm, net search...

BTW, what does the degauss button on computer monitors do?

/Ola
 
kristian - I'm afraid that radio antennas have to be tuned to the frequencies in order to be really effective. Just sheer size isn't enough:)
 
degausing makes yur screen go kaaachunnggzzzzzzzzzzzzz and kinda wobble around everywhere for a couple seconds. shame they didnt put those buttons on reel to reels. :)
 
Well, YEAH! :D

What I'm worried about is the combined strength of the permanent magnet and the electromagnet when the amplifier gets going. I've started to screw up the image before with different speakers/TV, and it only happened when I was using the speakers at the same time. SO... there's probably a stonger magnetic field when powering speakers.

I hope you're right, though!
 
kristian - I had that part figured out already:) I'd love an entire wall that does that at the push of a button. That would wake me up in the morning.

The magnetic field is indeed stronger when the speakers are on as you have an electromagnet going as well in the speakers. What I was aiming at was that the speakers will disturb (although not necessarily destroy, as far as I know) the TV when they're off as well. I had a look around and read that a Faraday's cage is more efficient against high-frequency electromagnetic fields and that the effect decreases down to nothing against permanent/stationary magnets. Darn. A Faraday's cage may apparently still help if your speakers don't interfere with e.g. the TV when they’re off but do when they’re on.

I really have to start studying physics again. I had forgotten how fun all this crap is.

/O
 
ola said:

Now, if I could only make my brain remember if magnetic fields from permanent magnets are stopped by a Faraday's cage in the same way as electromagnetic radiation is, I'd be happy. Hmm, net search...
/Ola
Click to expand...

A Faraday's cage will not stop a permanent magnet not it will electromagnetic radiation. For one, a sitting on your table is much stronger than the magnetic lines coming from the TV tower. They have to be that way as to not screw up the way our society lives, that is... drives cars, talks on the phone, walks around without getting zapped in the back of your head... that kind of stuff.
There's a big different you have to keep in mind. If you move a magnetic field, whatever is in it's path will cut the magnetic lines of flux. If it's paper it will cut very little lines. If it's metal it will cute some also and then that action will create a voltage. In order to do this with a permanent magnet you have to move the magnet. If you put a magnet up to foil, while you're moving it next to the foil it's magnetic lines of flux are being cut, but as soon as you stop moving it no lines are being cut. The thing about a magnetic shield in speakers is, it's thick metal that is very attracted to magnets. Like I said before, magetic lines of flux are lazy and will take the easiest (shortest) path from south to north. That means that they'll go through the metal that's attracted to them. That's the reason the piece of metal gets pulled towords the magnet, the lines of flux don't want to let it go. It's kind of like if you knew a shortcut from work to home, only the road was wavy and you had to go all over, but you had a magic car that could straiten out the road... Ok, maybe that wasn't the best analogy :) but you get the idea.
A Faraday's cage cuts very small fields of magetic lines of flux. The strength of the fields is so small it just doesn't take much to redirect them.

I don't know if this helps at all.

Later
 
OK, I'm confused.

A Faraday's cage will stop electromatnetic fields (sorry about the previous mixup of terms, I meant fields) provided that it has a not too low frequency, right? That's why e.g. radio reception (high frequency) in steel construction buildings is worse than in wood counstruction buildings, right? Thus, a Faraday's cage would be working on the magnetic fields created by the coil in speakers, provided that the frequency is high enough, right? Or...is the frequency of the electromagnetic field created by the speaker coil too low?

Or...are my above assumptions completly wrong? The above is what I was taught in school and what I read on some pages on the net but I'm not sure that they're right. Feel free to correct me or clarify any unclear things here.

/O
 
"is the frequency of the electromagnetic field created by the speaker coil too low?" Yeah, with the coil in a speaker we're talking 20HZ to 20KHZ... with radio and TV we're talking about frequencies up to 110MHZ... or there about. But what we're really talking about is strength, not as much as frequency. If you wave two leads of a DMM (digital multimeter) around in mid air you won't pick up any voltage, given you're in the middle of a field. If you wave the leads around next to a speaker magnet you'll pick up a few milivolts, maybe a bit more even. But I may be wrong there too, frequency probably does play a fairly large roll in it.
The thing about speakers... A permanent magnet has a field strength, and that strength doesn't change. When you start playing sound through it, whatever field the coil creates will make that larger field of the permanent magnet grow and collaps... So that may be why there are more problems when a speaker is on. I really dont' know too much about speaker shielding.
"Or...are my above assumptions completly wrong?"
You're not wrong at all really.
 
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