Civil Engineering Reference
In-Depth Information
1.0
0.9
0.8
0.7
75
100
50
25
0.6
0.5
0.4
0.3
0.2
0.1
0.0
200
400
600
800
2000
4000
100
1000
Frequency (Hz)
Figure 5.20
Absorption factor at normal incidence for a porous material attached directly on to a hard wall.
The parameter in the graph is the thickness in mm. Dalany-Bazley model with
r
equal 10 kPa⋅s/m
2
.
1.0
0.9
0.8
50
75
25
0.7
10
0.6
5
0.5
0.4
0.3
0.2
0.1
0.0
200
400
600
800
2000
4000
100
1000
Frequency (Hz)
Figure 5.21
Absorption factor at normal incidence for a 50 mm thick porous material attached directly
on to a hard wall. The parameter in the graph is the flow resistivity in kPa⋅s/m
2
.
There is, however, a “trade-off” between thickness and flow resistance. We may
increase the absorption in the lower frequency range, for a fixed thickness, by increasing
the flow resistivity as the latter will increase the attenuation rate. But there will be an
optimum value here due to increased reflection from the front surface caused by the
