Acoustic diffusion and scattering coefficients for room surfaces

Hargreaves, TJ

Abstract

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.

Citation

Hargreaves, T. Acoustic diffusion and scattering coefficients for room surfaces. (Thesis). University of Salford

• Thesis Type: Thesis
• Deposit Date: Aug 17, 2011
• Publicly Available Date: Aug 17, 2011
• Award Date: Jan 1, 2000