Building a Faraday's cage ?

I'll back you up whenever you're right. Whenever.

John

In article , John Larkin wrote: [....]

This disagrees strongly with what I have seen. A mu-metal shield can reduces the 60Hz by about a factor of 10 per layer. For a distant 60Hz source. ie: the 3 layer shield cans reduce the 60Hz field from about

150nTp-p to about 0.1nTp-p.

An open structure such as the steal 2x4s in the walls of the building I work in, reduces the earths field from about 50,500nT to about 39,000nT when measured in the middle of a nearly empty office.

The OP is worried about a local source for the 60Hz not a distant one. He has a field from a romex in the wall. This is quite a different matter than the distant source case.

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kensmith@rahul.net   forging knowledge

In article , Phil Allison wrote: [...]

.... and I still ask how this explains the 180 degrees observation.

I assume that even where you live, adding two dipoles yelds a dipole and that reciprocity holds.

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kensmith@rahul.net   forging knowledge

It doesn't disagree "strongly". If my mild-steel box attenuates a bit less than 2:1, and your mu-metal shield attenuates 10:1, that's right in the ballpark of the permeability ratio of the metals. The price ratio of the metals will be a lot higher. The OP suggested lining his studio with sheets of thin steel, not inch-thick, seamless mu-metal.

Consider a cubic box exposed to a far-field hum source. Assume the field is vertical, with flux entering the top of the box and exiting the bottom. The flux has a choice of flowing through the metal sides or of penetrating and flowing through the air inside. The situation is not at all simple, but in general it acts like a resistive voltage divider, where current (flux) flows through a path inverse to the path's resistance (reluctance.) If the box is big and its walls are thin, the air path isn't very well "shorted" so the flux short-cuts through the inside of the box.

Mu-metal and metglas can have permeability from tens to hundreds of times that of cheap sheet steel, and most magnetic shields are designed for a relatively fat wall thickness. Metglas can hit mu of

1e6 if handled right, but is only available in thin foils.

That's only about 20% attenuation for a lot of steel. I'd guess that there are spots within the structure where the field is actually concentrated above the 50K nt. The downside of any non-uniform magnetic structure is that it sucks in ambient field and can actually concentrate it in places.

True, near-field is even more complex. He should replace the offending romex with MX or something like that, twisted wiring inside a close-fitting steel jacket. But first he should scan the e and h fields to find out what's actually where and which fields are giving him problems. It could still be electric.

John

In article , John Larkin wrote: [....]

As I understood it, he was making a 2 meter by 2 meter by 1 meter box and had accepted the idea that it would be a double layer shield.

This is how I was thinking about it when I wrote the responce where I explained why a two layered shield worked better.

IIRC Mumetal is 50000 "tin plate" is 2000 so the OP may have to go with a

3 layer design for distant sources.

The metal of one of the shield cans I mensioned is under 0.03" thick.

[....]

The metal of a steel 2x4 is actually quite thin. They have a "U" shaped cross section. The sides of the "U" are the 2" face and the base of the U is the 4" side. The 4" side has a lot of holes in it for conduit to be run etc.

I did a fairly extensive survey of the building before we set up in it. The field is always less than the 50K when you are away from magnetic objects. The field strength above the roof was very eratic. In the outside, you have to get atleast 20 feet from the building before you get anything like a normal field.

I assumed that he didn't have a net current in the romex. If he does, the house may burn down and solve the problem.

Elsewhere in the thread, he stated that the noise dropped to zero with a

180 degree rotation of the pickup WRT the wall. It would require a stack of coincidences about 50 feet tall for that to be purely an magnetic pickup.

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kensmith@rahul.net   forging knowledge

"Ken Smith"

** More asinine crapology from a know nothing PITA !!

The attenuation of low level, AC frequency, magnetic fields buy a tin plate shield is virtually zero.

Using two is still close to zero.

However, one thin layer of Mumetal or similar material attenuates that same field by a factor of about 100.

That is WHY Mumetal is used to make shields for on scope tubes and cans for mic transformers.

Thin steel simply does NOT work !!!!

** Or a more complicated hum field that you are naively assuming.

The OPs story is entirely bollocks.

........ Phil

Given equal thickness and geometry, the shielding ratio goes directly with the permeability ratio. Figure permeability of a couple of K for mild steel, 10K for good steel, 50-200K for mu-metal.

Given your idea of "thin", and the volume to be shielded, 100:1 shielding may be optimistic for mumetal.

John

More rude nonsense from PA.

To OP: He is optimistic bay more than a factor of 10 here. Unless the OP is still reading along, I'm done.

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kensmith@rahul.net   forging knowledge

"John Larkin"

** Shame how magnetic permeability varies with flux density and is a not simple number.

Shame you are picking numbers out of the sky .

** Is this the manic John Larkin speaking ?

Or has the autistic pedant been let out again ?

........ Phil

"Ken Smith"

** You were done like a dinner a long time ago.

....... Phil

No, these are pretty good permeability numbers. To be more precise, the exact alloy and annealing would have to be specified. There's plenty of that on the web for anybody who's interested.

Where did your "100:1" shielding number come from? As others have mentioned, that's optimistic for most practical geometries.

When I talk numbers, I do so as an engineer. Emotional states don't affect quantitative reality, unless you're an audio tech, of course.

John

"John Larkin"

** Nope - just convenient figures YOU picked out of the sky with no reference level.

However, even using * YOUR* thin air figures -

the ratio of "mild steel" to Mu-metal *IS* 100:1.

I'd call that " Game Set and Match" .

** You have completely missed the point.

Manics generally do that.

They also suffer from grandiose notions.

...... Phil

Or 30:1, depending on the alloys.

This isn't tennis, this is physics. Reality just is, it doesn't keep scoce. The shielding ratio between mild steel and anything else depends on the permeability ratio, which is a hard fact to personalize.

Dang, you always snip my best lines.

John

"John Larkin = an admitted Manic "

** Now you ARE squirming like a cut worm.

** I used *YOUR* figures - ASSHOLE.

The ones YOU picked out of thin air.

** What you posted is not reality nor physics.

Just a manic person being obnoxious and grandiose.

You have completely missed my and the original points.

Manics generally do that.

They also suffer from grandiose notions.

They are also insufferable PITA.

Even worse when simultaneously affected by ASD.

...... Phil

Ooh, crabby today. But consider: If you're always angry, and I'm having fun, which of us is crazy?

John

Phyllis is on the rag, again.

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Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

Yeah, it's sad. And he never talks about it.

John

That's because he spends all his time campaigning to be the "Troll Queen of all Usenet newsgroups". :(

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Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

I think "crazy" is misapplied a lot - there's "crazy like a fox", and "crazy like a loon", so I guess it's pretty much "take your pick". :-)

"He's cry-zy. They're all cry-zy. 'cept you 'n me; an' sometimes I got me doubts about you!"

Cheers! Rich