Civil Engineering Reference
In-Depth Information
Table 11.6
The parameters used to predict the surface impedance of the material
represented in Figure 11.14.
φ
σ
α
∞
ρ
1
ν
s
Material
Thickness,
E
(N s/m
4
)
(kg/m
3
)
h
(mm)
(
µ
m)
(
µ
m)
( a)
22
×
10
3
294
×
10
3
Foam
20
0.98
1.9
87
146
30
0.2
0.18
6
4
2
0
−
2
−
4
−
6
ReZ - Measurements
ReZ - Prediction
ImZ - Measurements
ImZ - Prediction
−
8
−
10
0.8
1
1.2
1.4
1.6
1.8
2
2.2
2.4
Frequency (kHz)
30
◦
of foam with an
Figure 11.15
Surface impedance
Z
at an angle of incidence
θ =
impervious facing, bonded on to a rigid impervious wall.
area have been neglected, while the frequency dependence of the viscous force and of the
bulk modulus of the air in the foam have been calculated using the model of Section 5.9.2.
The agreement between prediction and measurement is not very good. Nevertheless, the
predicted and measured impedances present similar behaviour. A resonance appears close
to 2000 Hz and the imaginary part of the impedance increases quickly with frequency. It
may be noticed that the measurement is difficult to perform far from the resonance, the
reflection coefficient being very close to 1.
Next, the example of a porous material made up of several porous layers with
an embedded impervious screen wall is presented. The structure is represented in
Figure 11.16. It is made up of a carpet (1, 2), an impervious screen (3) and a fibrous
layer (4). The carpet is modelled as a porous material made up of two layers, because
the fibres are grouped in small bundles fixed to an impervious surface at the lower face,
and are more regularly distributed at the upper face. The screen is 3 mm thick and has
a surface density of 6 kg/m
2
. Because of the mass density and thickness of the screen
