Building Acoustics

Sound transmission in buildings. Flanking transmission. 341

Figure 9.13 Sound reduction index Rcl of suspended ceiling of 9.5 mm plasterboard with 40 mm thick porous
absorber. Measured results and predicted results according to a modal theory, reproduced from Mechel (1995).
Predicted results by a one-dimensional model by Mechel (1980): Equation (9.18) with sS and sR equal to 0.5.

63 125 250 500 1000 2000 4000

Frequency (Hz)

0

10

20

30

40

50

60

70

80

Sound

reduction index (dB)

Measured

Modal analysis

1 - dim. model

9.2.3.3 Damped plenum (cavity)

In the paper by Mechel (1995) an example is also given for a case where a 40 mm thick
porous absorber is placed above the suspended ceiling, comparing measured and
predicted result using his modal theory. However, before showing these results we shall
use the full one-dimensional model, expressed by Equation (9.18), to predict the total
sound reduction index and the single components included in the model as well.
Figure 9.12 shows the predicted sound reduction index Rcl using the full one-
dimensional model, setting both sS and sR equal to 0.5, together with the results
according to Equation (9.20). The latter result includes the reduction index of the
absorber but the attenuation inside the plenum is not included. The purpose for doing so
is to show the effect of the attenuation. As expected, the difference between these two
curves exhibits a maximum when the height of the plenum, which here is 39 mm, is
approximately equal to one half wavelength. A comment to be added here is that the
attenuation is calculated for the fundamental mode only, i.e. plane wave propagation in
the plenum.
In addition, the sound reduction index of the plasterboard and the porous absorber
are shown separately, using material data given by Mechel (1995) and models presented