Civil Engineering Reference
In-Depth Information
1
(a)
0.8
(b)
0.6
(c)
0.4
0.2
(d)
0
0.1
1
5
Frequency (kHz)
Figure 9.9
Influence of the open area ratio on the absorption coefficient
A
0
of an
isotropic porous material covered by a facing. Same material and same facing as in
Figure 9.8. Open area ratio
s
=
(a) 0.4, (b) 0.1, (c) 0.025, (d) 0.005.
porous layer
e
1
e
1
air gap
screen
e
e
e
(a)
(b)
(c)
Three different configurations: (a) facing
+
porous layer, (b) facing
+
Figure 9.10
+
+
+
air gap
porous layer, (c) facing
resistive screen
porous layer.
For the three configurations, the porous material of the porous layer is the same as in
the first example. The thickness
e
of the porous layer is equal to 2 cm in configuration (a),
to 1.9 cm in configurations (b) and (c). The thickness
e
1
of the air gap in configuration (b)
and the resistive screen in configuration (c) is equal to 1 mm. The screen is isotropic, with
the following parameters:
σ
=
0
.
5
×
10
6
Nm
−
4
s,
α
∞
=
1
.
5,
φ
=
0
.
013 mm,
=
0
.
05 mm. The thickness
d
of the facing is equal to 1 mm. The radius
R
of the
holes and the perforation ratio are equal to 0.5 mm and 0.1, respectively. The predicted
normalized impedance and absorption coefficient are represented in Figures 9.11 and 9.12
for the three configurations.
The real part of the impedance strongly depends on the flow resistivity of the mate-
rial in contact with the facing because the mean length of the macroscopic molecular
trajectories in the screen in configuration (c), and in the air gap in configuration (b), is
much larger than the thickness
e
1
. It may be noticed that the inertial effects under the
facing do not depend on the flow resistivity of the material in contact with the facing.
The imaginary parts of the impedances for the three configurations are nearly equal. The
largest absorption coefficient
A
0
at low frequencies is obtained with the resistive screen,
but the real part of the impedance is too large for
A
0
to be close to 1.
=
0
.
95,
