Building Acoustics

290 Building acoustics

The effect of such a lining of 12 mm chipboard attached to a lightweight brick
wall of 87 mm thickness in shown in Figure 8.11. The cavity depth is 50 mm and filled
with a porous absorber. The uppermost three curves show the sound reduction for the
brick wall alone together with the result when adding the lining attached to the wall with
wooden studs and steel profiles, respectively.
The lower curves show the improvement by the lining when using the two types of
studs together with predicted results using Equation (8.16), assuming the studs are
infinitely stiff. Obviously, using the steel profiles one has not got a perfectly stiff
connection and furthermore, the ratio u 2 /u 1 is frequency dependent.
Finally, dealing with acoustical linings we shall call attention to the achievable
improvement in practice, being normally limited by flanking transmission. In most cases,
the flanking transmission will have a greater influence by airborne sound transmission
than by impact sound transmission. The improvement offered by a lining is therefore
often higher for impact sound than for airborne sound.

63 125 250 500 1000 2000 4000 8000

Frequency (Hz)

0

10

20

30

40

50

60

70

80

Sound

reduction index (dB)

Meas. ex/ties

Pred. ex/ties

Meas. w/ties

Pred. w/ties

Figure 8.12 Sound reduction index of double aluminium panel with (w/ties) and without tie beams (ex/ties).
Predicted results using SEA modelling. Adapted from Crocker et al. (1971).

8.2.2.2 Lightweight double leaf partitions with structural connections

A double leaf partition with structural connections, in the form of point or line
connection between the leaves, should be suited to modelling by SEA. In the model