Building Acoustics

310 Building acoustics

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

Figure 8.27 Improvement in the impact sound insulation by a heavy floating floor, 50 mm concrete slab, 25 mm
mineral wool and 140 mm concrete basic floor. Measured data after Homb et al. (1983). Solid line – 9 dB per
octave above resonance frequency. Dashed line – predicted for an equivalent elastic unit mounting.

The solid straight line represents the predicted improvement following a frequency
dependence of 9 dB per octave above the resonance frequency, a curve that in this
example gives a slightly better estimate than what is actually achieved (see comments
below). It is also interesting to calculate the improvement in a thought experiment
assuming that the top floor floats on mounting units having the same total stiffness as the
mineral wool layer. The dashed line is calculated using Equation (8.45), where an
empirical expression by Craik (1996) is used for the loss factor of concrete slabs (see
also section 6.4.2.3),

1

0.015.

f

η=+ (8.47)

It should be stressed that the latter result is included just to give an illustration of the use
of Equation (8.45), not because we expect that there should be a good fit to measured
data applied to a continuous elastic layer.
There exists, however, a series of measurement data on heavy floating floors on
continuous elastic layers that show a smaller frequency dependence than 9 dB per octave.