Sound propagation in rigid-frame porous materials is governed by the effective density and the effective bulk modulus of the fluid in the pore space. These quantities, for which analytical expressions have already been derived by other researchers, are frequency-dependent, complex and non-linear. However, because of the complexity of these expressions, it is difficult to obtain physical insight into the acoustic behaviour of the porous materials and to determine the dominant mechanism for sound absorption for a given material at a given frequency. Alternatively there are very simple expressions. In this paper the relationships between the complicated and relatively simple models are studied, and simple non-dimensional expressions for the characteristic impedance and wavenumber for sound propagation in rigid-frame porous materials are derived using the concepts of acoustic mass, stiffness and damping. An upper bound for thermal losses in a rigid-frame material is presented, and a simple rule of thumb is given for the required flow resistivity, porosity and tortuosity for a given thickness of porous material.