Measurement of sound scattering coefficients of surfaces in a non-diffuse sound field: theoretical and experimental investigation

There are some methods for characterizing sound scattering properties of surfaces. Standardized methods propose measurement of scattering coefficients in free of diffuse sound field. Another method has been recently proposed. The idea is that in some cases sound decay of a non-diffuse sound field depends on the scattering properties of surfaces. So the scattering coefficients can be defined by means of analysis of measured decay curves. A rectangular room with only one absorbing wall is appropriate for proposed method. Tested scattering elements should be placed on the reflecting walls and their scattering coefficient can be found from comparison of decay curves measured in the room with and without the elements. In present paper the theory on reverberation in the rectangular room with specular and diffuse reflecting walls is developed. Analytical expression for the sound decay law is obtained. It is shown that under certain conditions the decay law depends only on the scattering coefficient of the walls with tested elements, which means that the scattering coefficient can be found by fitting of measured decay curve by the analytical curve. Moreover due to the sound field anisotropy the dependence of the scattering coefficient on the angle of incident sound wave can be determined. Considered method has some limitations, which are analyzed in this paper as well. The proposed method was investigated in the scale model experiment. The objectives were to verify theoretic results and find the dependence of measured decay curve on number of scattering elements. The results show that (1) proposed theory agrees with the experiment; (2) the sound scattering coefficient can be measured in the non-diffuse sound field; (3) it is possible to determine the angle dependence of the scattering coefficient by the investigated method.