Acoustic diffusion and scattering coefficients for room surfaces
This project concerns quantifying the diffuseness of sound reflections from surfaces by means of a diffusion coefficient. Afthough it is now acknowledged that diffuse reflections are important in determining sound fields within rooms, no standardised diffusion coefficient currently exists. Definition of a universal coefficient would permit comparison of different surfaces and aid the understanding of diffusion. It would also benefit diffuser designers and room acoustic computer modellers. Previously proposed diffusion parameters for room surfaces are investigated and new ones developed. One approach is to parameterise the uniformity of the scattered energy measured as a polar response; a number of such parameters have been previously published. These are appraised using measured and predicted 2D and 3D polar responses for a diverse range of sample surfaces. The situations in which the parameters succeed and fail are discussed and it is demonstrated that none is ideal. A new polar response coefficient, superior to those previously published, is presented. This satisfies many criteria of the ideal diffusion measure and is likely to be standardised by the Audio Engineering Society. It is shown that the application of all polar response diffusion parameters is, however, limited. Two recently proposed alternative approaches to evaluating a scattering coefficient, which involve measuring the invariance of the energy reflected from a surface to its orientation, are discussed. One of these is a free field technique and the other requires reverberant conditions. Practical analysis shows that the reverberation chamber method is superior. It is likely to be standardised by ISO. An empirical reverberation chamber technique is also investigated, as is the possibility of quantifying the diffusion efficacy of surfaces from their effect on sound field diffuseness. Both of these approaches require further research. It is concluded that to provide maAmum benefit, the choice of diffusion coefficient is application dependent.