Civil Engineering Reference
In-Depth Information
5
(c)
0
4.5
4
−
5
3.5
3
−
10
2.5
(
b)
2
(a)
−
15
1.5
1
0.5
−
20
0
0.1
1
5
0.1
1
5
Frequency (kHz)
Frequency (kHz)
Figure 9.11
Normalized Impedance
Z/Z
c
for the three configurations represented in
Figure 9.10: (a) porous layer, (b) porous layer
+
air gap, (c) porous layer
+
resistive
screen. The effect on the reactance is negligible.
1
(a)
(b)
0.8
0.6
(c)
0.4
0.2
0
0.1
1
5
Frequency (kHz)
Figure 9.12
Absorption coefficients
A
0
for the three configurations represented in
Figure 9.10: (a) porous layer, (b) porous layer
+
air gap, (c) porous layer
+
resistive
screen.
In the third example, the same porous material as in the first example is covered by a
facing of thickness
d
=
1 mm, perforated by circular holes of radius
R
, with an open area
0
.
1. The normalized impedance and the absorption coefficient are represented
in Figures 9.13 and 9.14 for radii
R
equal to 0.5, 1, 2 and 4 mm. The distance
D
between
the holes increases with
R
if
s
is constant, and the resistive effects in the porous material
also increase. As a consequence, the real part of impedance strongly increases with
R
.
The imaginary part of impedance, which is related to inertial effects, also increases with
R
, because the added length given by Equation (9.18) is proportional to
R
. The effect is
small at low frequencies, the contribution of the added length to the imaginary part of
impedance being equal to
ε
e
ρ
0
ω/s
.
=
ratio s
