312 Building acoustics
The last term only applies to frequencies above fz, a frequency that is determined by
the ratio between the impedance of the top floor, specified by its mass and bending
stiffness and the mass impedance of the hammer. At sufficiently high frequencies, the
frequency dependence will be as high as 18 dB per octave, a result completely
determined by the specific mass of the hammer. Testing such floors, however, is
normally performed using additional loads of the order of 20–25 kg per unit area
simulating the weight of furniture etc. This diminishes the effect of the hammer mass.
Two examples of the improvement gained by using lightweight floating floors are
shown in Figure 8.29. In both cases the top floor is made of 22 mm thick chipboard,
surface mounted on 15 mm plastic foam in the one case, line mounted by 22 x 95 mm
beams on 25 mm stiff mineral wool in the second case. The primary floor is a 140 mm
thick concrete slab in the first case and 200 mm lightweight concrete in the second. The
difference in weight of the primary floor has negligible influence on the results as the
mass in the latter case is just some 20 % less than in the case of the concrete floor.
The frequency fz (see Equation (8.48)), will be approximately 600 Hz and we
observe that the frequency dependence above fz is very close to 18 dB per octave. We
shall note that the measured data for the floor on plastic foam include the effect of a thin
floor covering on top of the chipboard but this gives a contribution of maximum 5 dB (at
2000 Hz).
Figure 8.29 Impact sound improvement of lightweight floating floors, 22 mm chipboard surface mounted on 15
mm plastic foam (80 kg/m^3 ) and line mounted on 25 mm mineral wool (100 kg/m^3 ). Measured results from
Homb et al. (1983).
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)nChipbd + plastic foam
Chipbd + min. wool
18 dB/octave