A Boundary Diffuse Field Index for the characterization of diffuse acoustic field produced in reverberant rooms

A perfect diffuse acoustic field is theoretically required when performing measurements that make use of a reverberant room. From an experimental point of view, completely diffuse conditions are impossible to reach and ensuring an as effective as possible sound diffusion in the room becomes of crucial practical importance. The diffusivity indicators that are commonly used to qualify the actual sound field in a reverberant room are calculated in the room volume, away from boundaries, which is somewhat contradictory with the fact that for sound insulation and sound absorption measurements, specimens are necessarily placed at room boundaries (either a wall or the floor). This paper presents a characterization of the sound pressure field at the boundaries of a reverberant room in order to evaluate the diffusivity of the excitation at these specific locations. A boundary diffuse field index is suggested, evaluated using numerical simulations and then tested in two reverberant chambers. It is shown that this index allows evaluating the boundary diffusivity in a given reverberant room according to several relevant parameters, such as the room geometry or the sound source location. Possible implementations of this index concerning laboratory measurements are also briefly discussed.