Building Acoustics

Sound absorbers 185

increased flow resistivity. This is easily demonstrated by Figure 5.21 showing again the
absorption factor for a 50 mm porous absorber at normal incidence, now where the flow
resistivity is varied in the range 5 to 75 kPa⋅s/m^2. An increased absorption at the lower
frequencies is obtained at the cost of a decreased absorption at the higher frequencies
when making the flow resistivity too high. Some product data shown in Figure 5.33 may
serve as an indication of the relationship between flow resistivity and the density.
We may, however, obtain a substantial increase in the absorption at lower
frequencies by mounting the absorber at a certain distance from a wall or ceiling. This is,
in fact, common practice when mounting absorbing ceiling panels. One will not obtain
fully the same absorption as when applying the same total thickness of the material, but
the effect is good. An example is shown in Figure 5.22, giving the absorption factor for
three different combinations of a porous absorber and the cavity behind. The model used
here is the one by Mechel using a flow resistivity of 10 kPa⋅s/m^2. As shown in section
5.5.2, this model gives a more correct result in the lower frequency range than the
Delany-Bazley model but this is not important here. Making a comparison with the
absorption offered by a 25 mm thick absorber placed directly against a hard wall (see
Figure 5.20), the combination of a 25 mm absorber and a 75 mm cavity gives a
substantial increase at low frequencies. The drawback of such a combination is that we
get standing wave phenomena in the cavity, thereby a reduced effect in certain frequency
ranges. We have seen these phenomena before when treating resonance absorbers (see
e.g. Figure 5.12). The locations of these minima will, however, depend on the angle of
incidence. For the absorption factor data measured by diffuse sound incidence this effect
will hardly be noticeable, except when the ratio of cavity depth to the thickness of the
porous sample is very large.

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

Abso

rption facto

r

100 mm

50 mm + 50 mm air

25 mm + 75 mm air

Figure 5.22 Absorption factor at normal incidence for combinations of a porous absorber with an airspace in
front of a hard wall. Mechel’s model for the porous absorber having r = 10 kPa⋅s/m^2.

5.5.3.2 Angle of incidence dependency. Diffuse field data

We have, to make it simple, used the condition of normal sound incidence in most
illustrations. This is, however, not the angle of incidence giving the maximum