CHAPTER 5
Sound absorbers
5.1 INTRODUCTION
In the preceding chapter on room acoustics, we presupposed that an absorption factor
and an accompanying equivalent absorption area could characterize the relevant sound
absorbing surfaces. We did not, however, consider the kind of material parameters
determining these quantities. Here we shall aim at giving a theoretical basis for the
functioning of so-called acoustic materials; sound absorbing materials and constructions
having their primary applications in controlling the acoustic conditions in rooms in
buildings. This knowledge is in fact not only applicable in rooms but also for designing
proper acoustic conditions in transport, e.g. for passengers and personnel in trains and
buses and also for designing special devices for sound reduction, e.g. silencers for air-
conditioning units. We shall in this chapter also deal with measurement and prediction
methods for acoustic absorption, including how one measures the material parameters
that determine the absorption.
The functioning of absorbing materials is linked to the behaviour of sound waves at
the interface between two media (see Chapter 3). When a sound wave hits such a
boundary it will normally be diffracted; a part of the energy will be deflected in a
direction different from that of the incidence wave. If the boundary surface is large
compared with the wavelength one characterizes the process as reflection. If the opposite
is true, the word scattering is used. In many cases, we shall also be interested in what is
happening on the other side of the boundary, i.e. we shall be concerned with the energy
transmitted through the boundary surface. As in Chapter 3, we shall limit the treatment to
simple cases of reflection and we shall, furthermore, assume plane wave incidence.
With reference to the preceding chapter on room acoustics, we shall remind the
reader that the primary task of acoustic absorbers placed in a room is to ensure that only
a controlled part of the sound energy is reflected back into the room. Seen from inside
the room we want the rest of the energy, originating from whatever source, to be
absorbed, which normally means that the energy is transformed into heat. It should be
pointed out that according to internationally accepted conventions we class all non-
reflected energy as absorbed energy. Seen from inside the room, an open window is a
strongly absorbing surface even if no energy is dissipated in this surface, only
transmitted out of the room.
The absorption factor of a given surface, defined by the ratio of the absorbed
energy to the incident energy, may be determined by different measurement methods. For
normal incidence the so-called Kundt’s tube or standing wave tube may be used. This
technique has as its background the determination of the absorption factor by scanning
the maximum and minimum values of the standing wave set up in the tube. This classical
method is considered to be a little outdated compared to methods based on modern signal
analysis techniques. However, standing wave tube measurements are normally used for
testing on small specimens, usually in development projects. Large-scale measurements
for product data are normally performed in a laboratory reverberation room measuring
the absorption factor by diffuse sound incidence, i.e. measuring the average value for all
angles of incidence.