So basically,



Where R1 is the acoustic resistance of the leak through the pad. The same text suggests, "The remaining leak R1, which is variable, influences the frequency response. In order to combat this effect, the cushions are frequently made of a porous material such as foam. The acoustic resistance of the foam is chosen to be a little lower than that of the leak, so that this combination of two resistances in parallel is mainly determined by the foam, rather than the non-reproducible leak R1. ... Since the porosity of the foam is also influenced by the mechanical pressure against the ear, a further improvement is obtained by dividing the foam into two parts. The first part, which is in contact with the ear, is non-porous but soft, providing the best seal. The second part is rigid but porous, providing the well-defined low acoustic resistance".