Building Acoustics

Sound absorbers 177

model has to be modified. Kang and Fuchs (1999) have presented measured and
predicted results for such microperforated membranes and an example is given in Figure
5.16.
In this case the membrane has a thickness of 0.11 mm and a surface weight of 0.14
kg/m^2 and it is mounted at a distance of 100 mm in front of a rigid wall in a reverberation
room. The diameter of the holes was 0.2 mm and the perforation rate was 0.79%. The
measured results are presented together with predicted results using two slightly different
models, the one by the authors and the other using a commercial software package
WinFlag™. Both models represent the impedance of the perforated membrane as a
parallel combination of the impedance of the membrane itself and the impedance
represented by the perforations. For the latter, Kang and Fuchs use the approximations
given by Maa (see e.g. Maa (1987)), whereas the other uses the Equation (5.31) directly.

Figure 5.16 Absorption factor for a microperforated membrane mounted against a rigid wall at a distance of
100 mm. Measured (•) and predicted results (solid line) reproduced from Kang and Fuchs (1999). Dashed line –
predicted results using the software WinFlag™.

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5.5 Porous materials

Modelling porous materials is still an active area of research due to the different fields of
application. These are certainly not limited to the design of absorbers for use in room
acoustics, but spans from materials for application in silencers over to the modelling of
sound propagation in complex porous structures, which could be geological formations
on the sea bottom as well as human tissue.
Traditionally, the porous materials used as sound absorbers have been of mineral
wool fibre, either rock wool or glass wool. Later developments have been on cellular
plastic foam materials, e.g. polyurethane, polyester etc. There are literally hundreds of
different cellular foam materials on the market but only a few are actually suited as
acoustic absorbers. The built-up structure of most porous materials is too complicated for
a direct modelling of characteristic impedance and propagation coefficient based on the
geometry of the frame or “skeleton” structure. This applies even if we assume that the