Civil Engineering Reference
In-Depth Information
5
40
θ
= 0
θ
= 0
35
θ
=
p
/3
θ
=
p
/3
4.5
30
θ
=
p
/6
θ
=
p
/6
25
4
20
15
3.5
10
5
3
0
−
1
−
5
2.5
−
15
2
−
20
0.1
1
5
0.1
1
5
Frequency(kHz)
Frequency(kHz)
Figure 9.17
=
0,
π/
6,
π/
3of
a layered material made up of the two porous materials of Table 9.1 covered by a facing
of thickness
d
Normalized Impedance
Z/Z
c
at an angle of incidence
θ
=
1 mm, an open area ration
s
=
0
.
005 and circular apertures of radius
R
=
0
.
5 mm.
= 0
θ
=
π
/3
θ
θ
1
=
π
/6
0.8
0.6
0.4
0.2
0
0.1
1
5
Frequency (kHz)
Absorption coefficient
A
0
for the same configuration as in Figure 9.17.
Figure 9.18
9.4.4 Evaluation of the surface impedance at oblique incidence
for the case of square perforations
At oblique incidence, Equation (9.54) becomes
sin
2
πna
k
0
a
sin
θ
2
D
−
1
φ(L)
Z(B
)
=
Z
0
,n
(B)
πn
sin
θ
2
k
0
D
2
n
=
0
,
±
1
±
2
...
−
