Effects of the wave front on the acoustic reflection coefficient

In this work a comparison between the plane and spherical reflection coefficient is given for different types of porous materials, backed by a hard and impervious surface, both theoretically and experimentally. For this purpose also a method to determine the spherical reflection coefficient in free-field conditions, starting from sound pressure measurements, is presented. It is based on the propagation model over porous material surface given by Allard. It requires an optimization technique to assess non- acoustical properties of the porous material, if they are unknown, and a free-field and hard floor calibration to take into account the sound source directivity. It is shown that the differences in the reflection coefficient can be significant if the distance between the sound source and the material surface is small compared to the wavelength. The differences depend on the porous material properties, its thickness and the incidence angle of the sound waves.