Numerous methods exist for predicting sound fields in enclosed spaces such as rooms, aircraft cabins, and the sea. These are generally energetic approaches which ignore phase and associated wave effects, and which describe sound reflection from surfaces by the energy absorption coefficient. Many, such as ray tracing, require long calculation times. In the work reported here, a new model is presented which is computationally efficient, includes phase, and decribes surface reflection by the angularly varying surface impedance. It is a triangular‐beam‐tracing algorithm, combined with a Biot‐theory transfer‐matrix model for laterally homogeneous, multilayer, poroelastic surfaces. The Biot model has been validated in the case of plates, porous materials, and seabeds. The combined approach is being validated in comparison with predictions by analytic and numerical approaches, and with experiment. The model is being used to study the effect of surface extended reaction on enclosed sound fields.