Civil Engineering Reference
In-Depth Information
100
plate
Foam: structural
Foam: viscous
Foam: thermal
80
60
40
20
0
0
100
200
300
400
500
600
700
800
900
Frequency (Hz)
Figure 13.12
Relative
contribution
of
the
various
dissipation
mechanisms
in
the
panel - foam system.
example. It is observed that at low frequencies, the damping is dominated by losses
within the panel. At higher frequencies, the structural damping of the frame of the foam
dominates the losses followed by the viscous losses in the foam. Note that dissipation by
thermal effects is negligible. This is a consequence of the fact the foam is assumed free
at its edges and one of its faces. The results of Figure 13.12 confirm the large dissipation
seen at high frequencies and thus confirm the damping added by the foam attached to the
panel. If the foam is modelled as a solid layer, at low frequencies both the full poroe-
lastic computation and the solid models will lead to same results. However, at higher
frequencies the solid model will overestimate the space averaged quadratic velocity of
the response. This stresses the importance of viscous damping at higher frequencies. This
importance depends strongly on the nature of the foam or fibre system and its attachment
to the panel. Further discussion on this subject can be found in Dauchez
et al
. (2002)
and Jaouen
et al
. (2005).
13.9.4 Diffuse transmission loss of a plate - foam system
A transmission loss test was performed on a flat aluminium panel with attached foam.
The panel dimensions were 1
.
64 m
×
1
.
19 m
×
1
.
016 mm and the foam dimensions
were 1
.
64 m
×
1
.
19 m
×
7
.
62 cm. The foam is attached along its edges to the panel
using double-sided tape. The tests were performed at the Groupe Acoustique de
l'Universite de Sherbrooke (GAUS) transmission loss facility. The facility utilizes a
semi-anechoic - reverberant transmission loss suite. The reverberation room dimensions
are 7
.
5m
×
6
.
2m
×
3 m with a Schroeder frequency of 200 Hz and a reverberation time
of 5.3 s at 1000 Hz. The free volume of the semi- anechoic chamber is 6 m
3m
with an operational frequency range from 200 Hz to 80 kHz. The plate is secured in
a mounting window between the two chambers. The intensity technique is used to
determine the transmission loss. The technique follows closely standard ISO 15186-1:
2000. The reverberation chamber is excited using six loudspeakers and sound power
is captured using a microphone on a rotating boom. On the anechoic side, the sound
×
7m
×
