CONTINUOUS FIELD
 Condition of unsaturation
 To determine the induction inside the material (Bin), we use the following formula: Bin = k x Ho x D / t
(where k is chosen between 1 and 1.5 depending on the other Ho)
Then we verify that Bin <2/3 Bsat
 Calculation of attenuation (Simplified)
 For an infinite cylinder whose field is perpendicular to the axis: At = μ x t / D
 For a sphere:
A = 4/3 x μ x t / D
 More generally:
A = k x μ x t / D
k is a factor depending on the shape of the shield, its
orientation relative to the field and its degree of opening.

ALTERNATING FIELD
 Alternating fields will create Eddy currents generating a field which opposes the interfering field.
 The result will be determined by a parameter called the skin effect: delta = [2,54.105 x ro / (µ x f)]1/2
(ro = resistivity of the material / f = frequency
 This skin effect will then be used in a chart giving coefficients as: Aac = p x Adc
(Aac: alternating field Attenuation)
(Adc: constant field Attenuation)

Ho: Outfield
At or A: transverse attenuation or attenuation
μ: permeability
t: thickness of the material
D: diameter of the shield

CAUTION
 All formulas are based on:
 The conservation of flux (field lines in the air are concentrated in the thickness of the shielding material)
 The principle that the ferromagnetic material deflects the magnetic field lines in a ratio of a given area (in a perpendicular field). However, this ratio depends on the permeability of the material and the size of the screen (a small mumetal cylinder acts differently on its environment than a pure iron screen 2 meters in diameter.
 Permeability is not the same:
 Depending on the field (BH curve)
 According to the thickness
 According to the frequency
 Although this is only one approach, these formulas have solved thousands of designs.

MULTILAYER
 The formula for an infinite cylinder with an axis perpendicular to the field with 2 layers of diameters D1 and D2 (D1 <D2) is:
A = 1 + A1 + A2 + A1 x A2 x [1  (D1 / D2) exp2]
 The principle is the same when adding other layers.
