# Calcoli di attenuazione

 CONTINUOUS FIELD Condition of un-saturation   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 (B-H 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         A = 1 + A1 + A2 + A1 x A2 x [1 - (D1 / D2) exp2]       The principle is the same when adding other layers.