Whilst mu metal is available in many different gauges and indeed a single layer of mu metal is suitable for most commercial applications, optimal shielding results are obtained when a shield is constructed of multiple layers of mu metal.
In theory each layer should be of a light gauge and the shield constructed of many layers [...]

What is field attenuation?
This is also known as the shielding factor (S) and is a ratio of the magnetic field strength outside of the magnetic shield (Ha) and the resultant field on the inside of the shield ie Ha/Hi (no units) or S = 20 x log(Ha/Hi) (Db). There are various formula based on the [...]

Mumetal is one of a family of three Nickel-Iron alloys widely used to produce magnetic parts and shielding.
The three alloys concerned have the following approximate compositions and well known trade names:-
36%NiFe widely known as Supra 36, Nilomag36, Radiometal 36, Magnifer36
50%NiFe widely known as Supra 50, Nilomag 50, Radiometal 50, Magnifer 50
80%NiFe 5%Mo widely known as [...]

Specifications : ASTM A-753 Alloy 4, MIL N-14411C Composition. Mu metal is a 80 % Nickel, 4.5 % Molybdenum, balance Iron soft ferromagnetic alloy which means that it does not retain a field after the removal of an external magnetizing force. Mu-metal offers exceptionally high initial and maximum permeability with minimum hysteresis loss. It also has [...]