Mu-metal transformer shielding can specifications

[beatnik]

I am exploring the possibility of getting some mu-metal cans made for shielding audio transformers, and I am trying to figure out a method to properly spec them.

Some manufacturers asked me for the required attenuation in Tesla, and the initial field strength.
From my understanding to obtain this measurement it would require a Gaussmeter but then this figure will vary depending on each project and anyway I am not sure how the attenuation figure in Tesla translates into dB.

I am wondering if possible to just make up a ballpark figure that would lead to an useful amount of attenuation.
I am looking at shielding cans on transformers from Cinemag and Sowter and the thickness of the mumetal seems to be between 0.7-1mm.
Lundahls use much thinner shielding but maybe the C core and the way the multiple windings are arranged makes the transformer less sensitive to magnetic field pickup ?

I am inclined to think a shield 1mm thick should do the job, but then again there seem to be different kinds of mumetal, and I am not sure if different annealing processes can as well impact on the final result.
I am not experienced with this kind of parts so if you have any suggestions that would be much appreciated.

 

[CJ]

You could look in old transformer catalogs like utc and triad and peerless and steal the specs they use for single shielded, sometimes the add a layer of copper but that is usually between double or triple cans ,

I bet Mr. CMMR would have a better answer

 

[Murdock]

I think I‘ve also read somewhere that several layers of thinner sheets are better than one thick layer.

 

[Philbo King]

You can get phone apps that measure magnetic field strength using the built in Hall sensor.