Notes on the sound field above a porous material

In numerical simulations a porous material with a rigid skeleton is modelled as an equivalent fluid by considering its acoustic complex properties such as sound velocity and equivalent density. In other cases, to reduce computational time, the behavior of a porous material is modeled as a surface acoustic impedance that depends on frequency, porous material properties (e.g. airflow resistivity, porosity, tortuosity, thickness, etc.), its backing surface, and on the characteristics of the sound field impinging on the porous material (e.g. type of wave front, distance from the porous material surface and the sound source). In turn, the simpler way to model the surface acoustic impedance is to give a constant value over the surface of the porous material for a given frequency. However, the surface acoustic impedance is also affected by the incidence angle for extended reaction materials. In this work, by using numerical and experimental results, a comparison among a complete model for porous materials and approximate models will be reported in terms of sound pressure and particle velocity. Practical information will be drawn up for those interested in modelling porous materials.