ABSTRACT
While surface scattering elements have been used accidentally or by design in rooms for centuries, it is only in recent decades that a concerted effort has been made into developing methods for measuring and characterizing the scattering from these surfaces. Without measurements of the scattering produced by surfaces, it is impossible to confidently design and apply diffusers. Consequently, this chapter starts by describing methods for measuring the scattering produced by a surface. This mainly concentrates on measurements in terms of polar responses, as this is the primary way that the scattering from surfaces has been measured for diffuser design and evaluation. The polar response of a source, like a loudspeaker, can be determined by measuring
the sound energy distribution on a polar arc or hemisphere surrounding the source. With care, this concept can be translated from loudspeaker measurements, to be used for backscattering from architectural surfaces. While polar responses tell designers much about how a surface reflects sound, they contain a considerable amount of data and a different polar response is required for each frequency band and angle of incidence. This is one of the reasons why the focus of recent years has been on developing single figure parameters, which condense the polar data and allow characterization in terms of diffusion coefficients. The reflection from a surface can also be characterized using a scattering coeffi-
cient, which is different from a diffusion coefficient. The differences between the coefficients will be discussed in some depth later in this chapter, but for now, it is opportune to give the contrasting definitions of diffusion and scattering coefficients. Diffusion coefficient (d), is a measure of the uniformity of the reflected sound. The
purpose of this coefficient is to enable the design of diffusers, and to also allow acousticians to compare the performance of surfaces for room design and performance specifications. Scattering coefficient (s), is a ratio of sound energy scattered in a non-specular
manner to the total reflected sound energy. The purpose of this coefficient is to characterize surface scattering for use in geometrical room modelling programs. Both the coefficients are simplified representations of the true reflection behaviour.
It is necessary to come up with simple metrics, rather than trying to evaluate the reflection characteristics for all possible source and receiver positions, because the amount of data soon becomes too large to deal with. The coefficients attempt to represent the reflection by a single parameter, maximizing the information carried by that single number. The difference between diffusion and scattering coefficients is the emphasis on which information is most important to be preserved in the data reduction. For diffuser designers, it is the uniformity of all reflected energy which is most
important; for room acoustic modelling, it is the amount of energy scattered away from specular angles. The difference between the definitions may appear subtle, but it is significant. In this chapter, diffusion and scattering coefficients are described after a discussion
of the direct ways of measuring the scattered energy from surfaces. For example, given the source and receiver positions, what are the best ways of obtaining the scattered and total sound fields? The chapter concludes by describing some other techniques for characterizing surfaces which have yet to find great favour, but may in the future be useful techniques.
