Civil Engineering Reference
In-Depth Information
connected structure. A typical example of the latter type in buildings is the so-called
flanking transmission
.
10
0
-10
-20
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
-30
-40
0.01
0.1
1
10
f
/
f
c
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
f
c
.
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.
