Civil Engineering Reference
In-Depth Information
1
0.8
0.6
0.4
Measurements, with screen
Prediction, with screen
Prediction, screen removed
0.2
0
0.5
1
1.5
2
Frequency(kHz)
Figure 11.12
The absorption coefficient of the material represented in Figure 11.10.
The parameters for the different layers are given in Table 11.4. Measurements reproduced
from Allard
et al
. (1987).
Table 11.5
The parameters used to predict the sound absorption of a felt - screen -
foam system.
Material
Thickness,
φ
σ
α
∞
ρ
1
(N s/m
4
)
(kg/m
3
)
h
(mm)
(
µ
m)
(
µ
m)
23
×
10
3
Felt (1)
19
0.99
1.4
64
131
66
137
×
10
3
Screen (2)
0.08
0.08
model
model
model
10
.
9
×
10
3
Foam (3)
27
0.99
1.02
100
130
8.8
of Chapter 9 can also be used within the TMM framework to solve problems involving
perforated facings.
11.7.2 Materials with impervious screens
The surface impedance of porous layers faced by impervious screens has been studied by
Zwikker and Kosten (1942) at normal incidence. The Biot theory has been used by Bolton
(1987) to predict the surface impedance of these materials at oblique incidence. Similar
work has been performed by Lauriks
et al
. (1990) with the transfer matrices. The use of
these matrices gives more flexibility to the method because a stratified porous material
can be represented in the same way as only one porous layer by a transfer matrix.
A plastic foam surfaced with an impervious screen is represented in Figure 11.14.
The screen is modelled as a perfectly flexible membrane and is represented by a 4
4
transfer matrix to account for shearing by the foam. The surface impedance of the system
is presented in Figure 11.15. The parameters that characterize the foam are presented in
Table 11.6. The thickness of the screen is equal to 25
×
m, and the mass per unit area
is equal to 0
·
02 kg m
−
2
. The flexural stiffness and the stiffness related to an increase of
µ
