A computer expression for predicting the shielding effectiveness of an infinite plane shield has been developed for low frequencies (below 50 kHz). It is shown that this expression, which arises as a special case of the solution of the general wave equation, is a sum of the attenuation term of Schelkunoff's transmission theory and a remainder, which is in integral but calculable form. The computed results are in excellent agreement with values obtained experimentally for aluminum, copper, and steel. Application to cases involving highly ferromagnetic materials is hampered by difficulties in determining permeability and its dependence on frequency and field strength.
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Cornelius M. Ryan (1967) studied this question.