Civil Engineering Reference
In-Depth Information
7
10
(d)
6.5
(c)
6
(d)
5
(b)
5.5
(a)
5
0
4.5
4
(c)
−
5
3.5
3
−
10
(b)
2.5
(a)
2
−
15
1.5
1
20
−
0.5
0
0.1
1
5
0.1
1
5
Frequency (kHz)
Frequency (kHz)
Figure 9.13
Influence of the radius of the aperture on the normalized impedance
Z/Z
c
.
The configuration is the same as in Figure 9.10(a), and
s
=
0
.
1. The different curves
correspond to
R
=
(a) 0.5 mm, (b) 1 mm, (c) 2 mm, (d) 4 mm.
1
0.8
(a)
(b)
0.6
(c)
0.4
(d)
0.2
0
0.1
1
5
Frequency (kHz)
Figure 9.14
Influence of the radius of the aperture on the absorption coefficient. The
configuration is the same as in Figure 9.10(a), and
s
=
0
.
1. The different curves corre-
spond to
R
=
(a) 0.5 mm, (b) 1 mm, (c) 2 mm, (d) 4 mm.
9.3.4 Design of stratified porous materials covered by perforated
facings
From the previous examples, some trends may be noticed. For values of the open area
ratio larger than 0.2, the surface impedance of the porous layer is only slightly modified.
For lower values of the open area ratio, an increase of the absorption coefficient at low
frequencies can be obtained, but the absorption coefficient decreases at high frequencies.
More precisely, the real part of the impedance increases with the radii of the apertures and
the flow resistivity of the material close to the facing, where the velocity field is distorted.
The real part of the impedance also increases when the open area ratio decreases. The
