Objective and subjective evaluation of reflecting and diffusing surfaces in auditoria

Cox, TJ

Abstract

The performance of reflectors and diffusers used in auditoria have been
evaluated both objectively and subjectively.
Two accurate systems have been developed to measure the scattering
from surfaces via the cross correlation function. These have been used to
measure the scattering from plane panels, curved panels and quadratic residue
diffusers (QRDs). The scattering measurements have been used to test
theoretical prediction methods based on the Helmholtz-Kirchhoff integral
equation. Accurate prediction methods were found for all surfaces tested. The
limitations of the more approximate methods have been defined. The
assumptions behind Schroeder's design of the QRD have been tested and the
local reacting admittance assumption found to be valid over a wide frequency
range. It was found that the QRD only produces uniform scattering at low
frequencies. For an on-axis source the scattering from a curved panel was as
good as from a QRD. For an oblique source the QRD produced much more
uniform scattering than the curved panel.
The subjective measurements evaluated the smallest perceivable change
in the early sound field, the part most influenced by reflectors and diffusers. A
natural sounding simulation of a concert hall field within an anechoic chamber
was used. Standard objective parameters were reasonable values when compared
to values found in real halls and subjective preference measurements. A
difference limen was measured for early lateral energy fraction (.048 ± .005); .
inter aural cross correlation (.075 ± .008); clarity index (.67 ± .13 dB); and
centre time (8.6 ± 1.6ms). It was found that; (i) when changes are made to
diffusers and reflectors, changes in spatial impression will usually be larger than
those in clarity; and (ii) acousticians can gain most by paying attention to lateral
sound in auditoria. It was also found that: (i) diffuse reflections in the early
sound field are not perceived differently from specular reflections; and (ii) the
initial time delay gap is not significant to listener preference.

Citation

Cox, T. Objective and subjective evaluation of reflecting and diffusing surfaces in auditoria. (Thesis). University of Salford

• Thesis Type: Thesis
• Deposit Date: Sep 19, 2011
• Publicly Available Date: Sep 19, 2011
• Award Date: Jan 1, 1992