Building Acoustics

Sound absorbers 189

125 250 500 1000 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

1.1

1.2

1.3

1.4

1.5

Abso

rption

facto

r

Distance x in cm

0

20

40

60

80

100

1.20

0.60

x x

Figure 5.25 Reverberation room measurements of the absorption factor of eight mineral wool blankets, 25 mm
thick and having a total area of 5.8 m^2. Dimensions and measured configuration is shown in the sketch. Student
tutorial, NTNU.

5.5.4 Further models for materials with a stiff frame (skeleton)

In more recent years there has been development of several models using a more detailed
description of the structure of the material, the aim being to use directly measurable
material quantities. Early work on this (e.g. Zwikker and Kosten (1949)), introduced a
structure factor in addition to the flow resistivity and porosity, a factor that is now
termed tortuosity or sinuosity. In a popular way, we may say that this parameter gives us
information on the directionality of the pores in the material. In a material having straight
through pores of cylindrical shape making an angle φ with the outer surface, the
tortuosity ks is given by

(^) s 2

1

,

cos

k

φ

= (5.59)

which, as an example, gives ks equals 2 for φ equal 45°. In many applications however, a
model using these three parameters will be too simple even for materials where one
assumes isotropy and a stiff frame.