Building Acoustics

342 Building acoustics

in Chapter 6 (see sections 6.5.2.1 and 6.5.4). The plasterboard had a mass per unit area of
9.5 kg/m^2 , a critical frequency of 3250 Hz and a loss factor of 0.1. The flow resistivity of
the porous absorber was 10 kPa⋅s/m^2 (density 20 kg/m^3 ).
In the same way as shown in Figure 9.11, where the plenum is without an absorber,
measured and predicted results for the case when an absorber is added, is shown in
Figure 9.13. We have also included predicted results using the one-dimensional model as
shown in Figure 9.12. Comparing with the results for the case without any absorber, both
measured and predicted results using the modal theory show that the absorber has a
greater influence in the low frequency range than in the middle and high frequency
ranges. This is in fact surprising and an explanation is not readily at hand. As also seen,
the one-dimensional model completely fails at frequencies below approximately 400 Hz,
giving a reasonable fit with the measured data just over a couple of octaves.

63 125 250 500 1000 2000 4000

Frequency(Hz)

0

10

20

30

40

50

60

70

80

Sound

reduction index (dB)

Wall alone

w/ceil., ex/ absorbent

w/ceil., 4 cm abs.

Figure 9.14 Sound reduction index of lightweight double leaf partition and apparent sound reduction index
including suspended ceiling transmission path. Sound reduction indexes for suspended ceiling path are
measured data from Figure 9.11 and Figure 9.13.

9.2.3.4 Apparent sound reduction index with suspended ceiling

Apart from being able to predict the sound reduction index for the transmission path
across a suspended ceiling, our main interest will be the overall sound insulation between
two rooms; i.e. the resulting apparent sound reduction index. We shall use the case of the