Building Acoustics

Sound transmission in buildings. Flanking transmission. 329

τdiffuse=πτ(0). (9.5)

This implies that the sound reduction index for diffuse incidence will be approximately 5
dB smaller than for normal incidence.

Γ, Zc

m 1

φ m 2

φ

d

2a

Figure 9.3 Cylindrical aperture in a wall filled with a porous material and sealed by mass layers.

50 100 200 500 1000 2000 5000

Frequency (Hz)

-10

0

10

20

30

40

50

Sound reduction index (dB)

Empty

Min. wool

Tape

Min. wool + tape

Figure 9.4 Sound reduction index at normal incidence. Cylindrical aperture of radius 10 mm in a wall of
thickness 100 mm. Mineral wool of flow resistivity 5 kPa⋅s/m^2. Sealing tape of mass/area 100 g/m^2.

We shall illustrate the use of Equation (9.4) by again using a cylindrical aperture of
radius 10 mm in a wall of thickness 100 mm (see Figure 9.4). The curve giving the result
for a an open or empty aperture is the same as shown in Figure 9.2, however, now using