Building Acoustics

158 Building acoustics

component, which traditionally has been accomplished by filling the cavity behind the
plate partly or wholly with a porous absorber as seen from Figure 5.2 a). It is not
necessary, however, to fill the cavity wholly or partly to achieve high absorption. An
adjustment of the resistance may be achieved by gluing a thin fabric to the plate (see b)).
Such products are commercially available using panels of steel, aluminium, plaster and
wood. A combination of fabric and a porous blanket in the cavity is sometimes used to
achieve an even higher absorption.
Resonator absorbers with perforated plates or foils having holes of diameter less
than 0.5 mm are commonly referred to as microperforated absorbers (MPA) (see c)). In
this case the viscous losses in the holes give the necessary resistance without the use of
any additional fabric. Instead of using perforations with holes, thin slits are an
alternative. Commercial products are available made of steel, aluminium and plastics.

Porous

material

Panel

Fabric

Panel

Microperf.

panel

a)

b)

c)

Figure 5.2 Resonator panels based on Helmholtz type.

5.3 Measurement methods for absorption and impedance

The absorption factor, the ratio of the absorbed and the incident acoustic energy, is
determined by different methods according to the type of incident wave field. For
measurements on small specimens, i.e. where the typical dimensions are smaller than the
wavelength, the standing wave tube, also called the Kundt’s tube, is used. The absorption
factor for normal incidence is determined by measuring the maximum and minimum
pressure amplitude in the standing wave set up in the tube by a loudspeaker. This basic
technique is, as mentioned in the introduction, considered a little outdated in comparison
with more modern methods based on transfer function measurements. It may then seem a
little odd that this classical technique was implemented relatively late (1993) in an
international standard, ISO 10534–1, after being used for at least 50 years. Commercial
equipment has also been available for many decades. However, there exists a second part
of the mentioned standard, ISO 10534–2, based on using broadband signals and
measurement of the pressure transfer function between different positions in the tube.
The results from measurements of absorption factor and acoustic impedance, using
the standing wave method, obviously are meaningful only when assuming these to be