Dominique Michel wrote:
. . .
If it is 50 or 100Hz, only mu-metal will do something. With copper or
metal, you will only get Foucault's current in the shield, and those
currents will make a new EM field on the other side of the shield.
Successive metal shields can even be a very good mean to transport at
distance a low frequency magnetic field. So, only mu-metal will be
effective in that case.

That's absolutely right, although you might get by with a good layer of
steel for mains-frequency shielding. I was thinking only of RF
interference. If you're having problems with mains-frequency hum from
the power supply transformer's field or the field from the conductors
between the transformer and the filter, you'll need some highly
permeable material. Steel might be adequate, but mu-metal is better.

Roy Lewallen, W7EL