As I've mentioned, beefy Hoffmann steel electrical equipment boxes,
with fully formed/welded corners and a tight cover, typically can't
reduce an external 60 Hz field by even half. Thin steel lining a room
will be much, much worse.
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.
[/QUOTE]
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.
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.
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.
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.
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
