Building Acoustics

Multilayer elements 291

shown in Figure 8.6 we shall have to include either a coupling element or a subsystem
between the subsystems 2 and 4. An early example of such calculations, mentioned
above, were performed by Crocker et al. (1971), where two aluminum panels of
dimensions 1.55 x 1.97 m were interconnected by a total of 50 “ties”, these being short
beams of aluminium of thickness 0.7 mm and width 25 mm. The comparison between
measured and predicted results is shown in Figure 8.12. The fit between the two sets of
data is very good but it should be noted that it is based on estimated values for internal
loss factors of the material, η equal to 0.005 in the frequency range below 800 Hz and η
equal to 0.02 above 800 Hz.
Structural connections in the form of studs are more relevant in building partitions.
In spite of being a special laboratory model, the set-up used by Brekke (1979) (see
Figure 8.5) gives results that are typical in practice. Brekke performed measurements and
predictions where the panels were interconnected by different types of stud. In this case,
the depth of the cavity was reduced to approximately 95 mm to accommodate the various
types of studs. Measured sound reduction indexes of the double panel with and without
two types of stud are depicted in Figure 8.13. Calculations for the set-up with wooden
studs were performed using a SEA model determining the coupling loss factor for the
studs by measurement. The fit between measured and predicted results were reasonably
good with a maximum deviation of 5 dB but predicted results are omitted here.

63 125 250 500 1000 2000 4000

Frequency (Hz)

0

10

20

30

40

50

60

70

80

Sound

reduction index (dB)

Without studs

Steel studs (C)

Wooden studs

Figure 8.13 Sound reduction index of experimental double panel, after Brekke (1979). 12 mm chipboards with
a cavity of depth 95 mm filled with mineral wool. See also Figure 8.5.