Acoustic diffusion and scattering coefficients for room surfaces

This project concerns quantifying the diffuseness of sound reflections fromsurfaces by means of a diffusion coefficient. Afthough it is now acknowledged thatdiffuse reflections are important in determining sound fields within rooms, nostandardised diffusion coefficient currently exists. Definition of a universalcoefficient would permit comparison of different surfaces and aid theunderstanding of diffusion. It would also benefit diffuser designers and roomacoustic computer modellers. Previously proposed diffusion parameters for roomsurfaces are investigated and new ones developed.One approach is to parameterise the uniformity of the scattered energy measuredas a polar response; a number of such parameters have been previouslypublished. These are appraised using measured and predicted 2D and 3D polarresponses for a diverse range of sample surfaces. The situations in which theparameters succeed and fail are discussed and it is demonstrated that none isideal. A new polar response coefficient, superior to those previously published,is presented. This satisfies many criteria of the ideal diffusion measure and islikely to be standardised by the Audio Engineering Society. It is shown that theapplication of all polar response diffusion parameters is, however, limited.Two recently proposed alternative approaches to evaluating a scatteringcoefficient, which involve measuring the invariance of the energy reflected froma surface to its orientation, are discussed. One of these is a free field techniqueand the other requires reverberant conditions. Practical analysis shows that thereverberation chamber method is superior. It is likely to be standardised by ISO.An empirical reverberation chamber technique is also investigated, as is thepossibility of quantifying the diffusion efficacy of surfaces from their effect onsound field diffuseness. Both of these approaches require further research.It is concluded that to provide maAmum benefit, the choice of diffusion coefficientis application dependent