A method has been developed to predict random incidence sound absorption coefficients of porous materials, with various different facings (slots, perforated board, light panels). Materials are assumed to be locally reacting, with properties determined by their static flow resistivity. The effect of facings is modelled by adding the reactance of the facing to the normal acoustic impedance of the material. The random incidence coefficients are predicted from normal incidence impedance using diffraction theory. Within certain limits good agreement is obtained between predictions and experiments.