A computer program based on the transfer matrix formalism has been developed which treats acoustic multi-layer structures of up to five different layers, namely any combination of the following layer types: Continuous media (gaseous or liquid), Barrier, Porous Barrier. Honeycomb and Fibrous Felt (or Open Cell Foam). The calculations yield: Sound Pressure Differences across the structure, Normal Specific impedance and Absorption Coefficient, all as a function of frequency and for any given angle of incidence of sound. The Sabine Absorption Coefficient can also be calculated. Calculations and measurements were found to be in good agreement.