Sound absorbers 159
independent of the size of the specimen, which is normally quite small. The homogeneity
of the material is therefore an important factor. If we wish to extrapolate the results to
larger areas and other angles of incidence we also need to know whether we may assume
that the material is locally reacting or not, which means whether or not we expect that the
impedance is a function of the angle of incidence.
Traditionally, measurement of the absorption factor of larger specimens is
performed in a reverberation room. One then determines the average value over all
angles of incidence under diffuse field conditions. The product data normally supplied by
producers of absorbers are determined according to the international standard ISO 354,
which specifies measurement conditions for reverberation room testing. The area
required for measurement is 10–12 square metres and there are requirements as to shape
of the area. The reason for these requirements is that the absorption factor determined by
this method always includes an additional amount due to the edge effect, which is a
diffraction phenomenon along the edges of the specimen. This effect makes the specimen
acoustically larger the geometric area, which may result in obtaining absorption factors
larger than 1.0. Certainly, this does not imply that the energy absorbed is larger than the
incident energy (!).
In the literature one may find a number of laboratory measurement methods for
determining absorption factors as a function of incidence angle, applying relatively large
specimens. None of these is yet standardized. There have recently been efforts put into
the development of similar in situ techniques, i.e. methods for measurements of
absorption and impedance both inside buildings and outside. These methods are mainly
based on the same principle as used in ISO 10534–2, which implies the specified two-
microphone method is extended to spherical wave fields. References to most of these
methods may be found in Dutilleux et al. (2001). We shall not treat them here any further
apart from one suggested by Mommertz (1995).
This method determines the reflection factor for a surface based on using a single
microphone placed near to the surface of the specimen. The idea is to use MLS signals in
a subtraction technique; the impulse response measured placing the microphone in a free
field is subtracted from the impulse response measured near to the surface. Doing this,
one is left with a signal representing the reflection. One obvious prerequisite is that the
configuration (loudspeaker source and microphone) is identical in both impulse response
measurements. With certain modifications this method is implemented in an ISO
standard for determining absorption factors for road surfaces (see ISO 13472–1). Later
efforts have been to make a reference measurement near to a very hard surface, i.e. a
totally reflecting surface, instead of a measurement in the free field. The swept sine
technique (SS) may of course be just as applicable as using MLS signals.
5.3.1 Classical standing wave tube method (ISO 10534–1)
Using this method the specimen is placed at one end of a tube (see Figure 5.3). A
loudspeaker is used to create a standing wave field in the tube, a field that is detected by
a probe microphone. To fulfil the requirement having only plane wave propagation in the
tube, the linear dimension of the cross section must be less than the wavelength. More
specifically, the frequency range of the measurements extends upwards to the frequency
of the first cross mode of the tube, which for tubes of circular cross-section will
approximately be given by 0.586⋅c/D, where c is the speed of sound and D the diameter
of the tube. For a 10 cm diameter tube this frequency will be approximately 2000 Hz.