A method for calculating the sound absorption coefcient of single- and multi-layered porous sound absorbers is developed. Existing theories for the mathematical description of porous sound absorbers are summarised. The normal sound incidence on a homogenous layer of porous material in contact with an acoustically hard wall is considered and the modelling is extended to allow for the analysis of several adjacent layers. The sound propagation in the air is described with well-known theoretical fundamentals and the wave eld in the porous absorbers is modelled with Biots theory for the wave propagation in uid-saturated porous media. Equations for determining the sound absorption coefcient are derived using two different methods and the models are adapted for the determination of the sound transmission through free-standing absorbers. The sound absorption coefcient of a sample of melamine foam is calculated and the results are validated experimentally.