Building Acoustics

314 Building acoustics

governing equations for two plates connected and stiffened by beams. In several cases,
Brunskog and Hammer’s prediction model fits well to measured results. We shall present
one example (see Figure 8.30), where the construction is depicted in the insert; a
platform structure of two 22 mm matched boards connected by wooden beams of
dimensions 67 x 220 mm, the cavity partly filled by mineral wool of thickness 120 mm
and density 20 kg/m^3. The measured results are taken from an earlier work by Bodlund
(1987). It is worth noting that the general frequency dependence is quite different from
the ones found for heavy floor constructions (see Figure 6.20).
We shall now return to the case of the impact sound improvement of a lightweight
floating floor combined with such a lightweight primary floor as opposed to a
combination with a heavy floor. The floating floor is here a combination of 22 mm thick
chipboards and 13 mm plasterboards surface mounted on 25 mm stiff mineral wool. It is
combined with two different primary floors; one being a 200 mm thick lightweight
concrete floor of density 1300 kg/m^3 , the other a wood joist floor. The latter is a platform
structure of 48 x 198 mm beams with 22 mm chipboard on top and combined with a
ceiling of 2 x 13 mm plasterboards. As evident from Figure 8.31 we get, in the case of
the heavy primary floor, a frequency dependency of 18 dB per octave just as shown in
Figure 8.29. As expected, however, we do not see this effect using the wood joist floor.

63 125 250 500 1000 2000 4000

Frequency (Hz)

0

10

20

30

40

50

60

70

80

Reduction in impact sound level,

'

L

(dB)n

Concrete floor

Wood joist floor

18 dB/octave

Figure 8.31 Impact sound improvement of floating floor, 22 mm chipboard and 13 mm plasterboard on 25 mm
mineral wool. Measurements on two different primary floors, 200 mm lightweight concrete and a wood joist
construction.