Building Acoustics

184 Building acoustics

100 1000

200 400 600 800 2000 4000

Frequency (Hz)

0.0

0.1

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0.3

0.4

0.5

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1.0

A

bso

rption facto

r

100 75 50 25

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.

100 1000

200 400 600 800 2000 4000

Frequency (Hz)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

A

bso

rption facto

r^25

10

5

50 75

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