Building Acoustics

174 Building acoustics

As mentioned several times already, it is possible to obtain a sufficiently high
resistance just by making the holes or slits small enough, which means a dimension of
less than 0.5 mm. Traditionally, the hole-perforated models have been called micro-
perforated absorbers or MPA due to the fact that these were the first on the market.
However, as the same effect may be achieved by other shapes of perforations the term
MPA comprises a larger range of products. We shall give two examples of data for such
products, the first is 0.6 mm thick steel panels perforated by circular holes having
diameter of 0.46 mm placed with a c-c distance of 5 mm. This corresponds to a rate of
perforation of approximately 0.7%.

Figure 5.14 Absorption factor of microperforated plates (Gema Ultramicro® suspended 200 mm from a hard
surface). Dotted curve – measured in a reverberation room. Solid curve – predicted.^

Measured results, i.e. product data from the producer, are obtained in a standard
reverberation room test, here with a cavity depth of 200 mm. These one-third-octave
band data are shown in Figure 5.14 together with a prediction using the same type of
calculation method as shown in Figure 5.12. Here, however, we have calculated a mean
value over all incidence angles to compare with the diffuse field data from the
reverberation room test. The prediction method for these absorption data is given in
section 5.7.
Data allowing one to calculate the resonance frequency of resonator panels having
perforations of other shapes than the circular holes may be found in the literature. We
shall restrict ourselves to one important type, depicted in Figure 5.11 b). These absorbers
are denoted “slatted panels”, if they are constructed from parallel slats; which implies
that the panel thickness normally is 9–10 mm or more. However, products of this type
are often thin metal panels perforated by long slots. In that case the notion “slotted
panels” may be more appropriate. As for the common slatted panels the width of the
slots may be from some 5–10 mm upwards, which implies that a resistance fabric or
porous material must be added. As for the calculation of resonance frequency, Equation
(5.39) still applies but the end correction will be given by

100 1000

200 400 600 800 2000 4000

Frequency (Hz)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

ption facto

r

r

A

bso