Building Acoustics

Sound absorbers 197

The quantities R, Rs and r are defined by the following

(^) s s
v
,and

P R

RRRSr

qd

Δ

==⋅=. (5.67)

The quantity qv is volume velocity (m^3 /s) of the airflow through the specimen having an
area S, which implies that the mean flow velocity is equal to qv/S (m/s). The quantity d is
the thickness of the sample in the direction of the flow. The specific airflow resistance Rs
is the linear airflow resistance of the sample, having the same dimension as the specific
impedance. In the old centimetre-gram-second (CGS) system of units, this quantity had
its own unit, the Rayl in honour of the physicist, Lord Rayleigh. One may find it still in
use but then as mks Rayls reflecting the SI system of units.

10 100

20 30 40 50 60 70 8090 200

Density (kg/m^3 )

10

100

20

30

40

50

60

70

80

90

9 8 7 6 5 4 3

Resistivity (kPa.s/m

2 )

G R

Figure 5.33 Typical data for airflow resistivity of mineral wool, glass wool (G) and rock wool (R) as a function
of density.

For a homogeneous material we may find the flow resistance per unit length, the
airflow resistivity having the symbol r. This is the quantity normally found as product
data for porous materials. Some typical data (Norwegian products) for mineral wool
types of porous material are given in Figure 5.33.
Finally, it should be mentioned that in other circumstances an inverse quantity is
used, this in order to characterize the “openness” of a porous material for airflows. This
is the quantity permeability B defined by

B (m ),^2
r

μ

= (5.68)