Sound transmission 229
connected structure. A typical example of the latter type in buildings is the so-called
flanking transmission.
Figure 6.15 Radiation index by resonant radiation from plates of steel or aluminium. Calculated from
expressions by Leppington et al. (1982).
Hence, we cannot use these data when a sound field is driving the plate in a forced
vibration pattern which is not “natural”. This is illustrated by the data in Figure 6.16,
which are collected from a series of measurements by Venzke et al. (1973) on panels of 4
mm thick aluminium. The radiation factor is measured using two different types of
excitation: directly by an electrodynamic exciter and by a diffuse sound field,
respectively, the latter is used in a standard sound insulation measurement. For the
former we have compared the results by calculations according to the Equations (6.48),
which shows that the fit between these data is quite good for frequencies above some
400–500 Hz. Similar results are also reported by others (see e.g. Macadam (1976)).
As shown, the radiation factor will be larger for the case of sound field excitation
than for a mechanical excitation in the frequency range below the critical frequency fc.
The wave field in the plate will partly be determined by the sound pressure distribution
imposed by the sound field, a forced vibration field, partly by the free waves originating
from the edges of the finite plate. Of these partial wave types, the non-resonant (forced)
and the resonant one, the former will be dominant when it comes to sound radiation. This
implies, when we shall be able to predict the sound transmission through a panel or wall,
which we will treat later, one must take both the resonant and the non-resonant radiation
into account.
0.01 0.1 1 10
f/fc-40
-30
-20
-10
0
10
10 lgσ (dB)Plate dimensions
0.5 x 2 m, h = 4 mm
0.5 x 2 m, h = 2 mm
1 x 2 m, h = 2 mm
2 x 2 m, h = 2 mm