After various discussions on this topic I'm still not entirely convinced that Mu metal, which is very expensive, is the answer to AC induced magnetic fields but I'm open to a good answer.
Mu metal is great for strong magnetic fields particularly if the direction and strength of the field is fixed. Ferrous materials might or might not do the trick depending on their analysis [after all, the core of a transformer is ferrous] but if a ferrous material is good for a permanent magnet, then it's also good as a shield because it can't "switch" [sorry, don't know the term] fast enough. I suppose this might apply to fruit cans
Aluminium and most types of stainless steel are not useful barriers to magnetic fields [though they're ok for RFI].
Copper allows an unchanging magnetic field though
but when subjected to an AC generated magnetic field, the changes in magnetic flux induce a current in the copper with the result that the field does not penetrate the copper.
Dunno if this helps or muddies the water. What's a Hammond box made of, Aluminium?
And another edit
. The other thing that EIs do is to emit a certain amount of RFI, in which case your shield has to protect against that too, meaning it probably should be more enclosing than if you were just dealing with a magnetic field.
jules
