138 Building acoustics
size of objects. The energy in the incident wave will be redistributed with a directional
distribution depending both on the shape of the object and on the ratio of wavelength to
object dimensions.
Since 20 years ago, there has been a growing awareness that diffuse reflections
are very important, especially for rooms for music performances. It is realized that an
important contribution to the fame of some older concert halls, e.g. the Grosser
Musikvereinsaal in Vienna, is the diffuseness provided by numerous surface
irregularities: various types of surface decoration, columns, balconies etc. Following the
work of Schroeder (1975, 1979) on the design of artificial diffusing elements based on
number theory, a range of commercial as well as non-commercial diffusing elements are
now in use in rooms for music production and reproduction. A comprehensible treatise
may be found in Cox and D’Antonio (2004). Here we shall just give a short overview on
these types of diffuser element. In this connection a series of measurement methods are
developed to characterize the acoustic properties of such elements both in ISO (ISO
17497) and in AES (Audio Engineering Society).
4.7.1 Artificial diffusing elements
The sound scattering properties of solid bodies and surfaces is of great interest in many
areas of acoustics and the distribution of the scattered energy around structures of
various shapes for a given incident wave is well known. Such distributions are normally
given in the form of a directivity pattern for the scattered wave. In room acoustic
modelling, however, one is in most cases not interested in such a detailed pattern. A
surface property of major interest is the total amount of non-specularly reflected sound
energy in relation to the total reflected energy. In ISO 17497 Part 1 a quantity named the
scattering coefficient s is defined,^1 as one minus the ratio of the specularly reflected
acoustic energy Espec to the total reflected acoustic energy Etotal:
spec
total
1
E
s
E=−.
(4.64)
Theoretically, this quantity can take on values between zero and one, where zero means a
totally specular reflecting surface and one means a totally scattering surface. Being
measured in a reverberation room as a random incidence quantity in one-third-octave or
octave bands, it represents a direct analogue to the statistical absorption factor.
The main purpose of the artificially diffusing elements is certainly to reduce the
specularly reflected energy. However, from the point of view of the producers of such
elements one would like to have a corresponding measure characterizing the uniformity
of the reflected sound, in the same way as characterizing radiated sound from sources,
e.g. loudspeakers. There seems as yet no universal agreement concerning such a
diffusion coefficient (or factor) to characterize these so-called diffusers but there is
ongoing work e.g. inside ISO. The problem is to arrive at a single number measure
characterizing the scattering directivity pattern.
These artificial types of diffuser element constitute a hard surface with grooves or
protrusions of various shapes. The surface irregularity used may be one-dimensional or
two-dimensional, according to the task of making a diffuser working in one or two
(^1) Having the unit of 1, it should have been termed scattering factor.