Building Acoustics

Sound transmission 239

loss factor is frequency independent, the impact sound pressure level Ln in the receiving
room (see section 6.2.2), will be frequency independent.
Using these presumptions, we shall use Equations (6.21), (6.58) and (6.67) to
calculate the normalized impact sound pressure level giving

(^) n 10 lg 2 c 82 dB.
f
L
mη

⎛⎞

≈⋅⎜⎟⎜⎟+

⎝⎠

(6.74)

Measurements of the loss factor in practice have shown that it is slightly frequency
dependent. Following Craik (1996), we may, as a rough estimate for heavy floors of
concrete etc., write

1

0.015.

f

η=+ (6.75)

Figure 6.20 Normalized impact sound pressure level of a 140 mm thick concrete slab and a 260 mm thick
hollow core concrete slab. Laboratory measurement data together with calculated results for the concrete slab.

An example is given in Figure 6.20 showing results of laboratory measurements on
a concrete slab of thickness 140 mm. Comparing with calculated results we have used
Equation (6.74) together with Equation (6.75) for the loss factor. The mass per unit area
of the slab is 320 kg/m^2 and the critical frequency is 120 Hz. As seen from the figure

63 125 250 500 1000 2000 4000

Frequency (Hz)

0

10

20

30

40

50

60

70

80

Impact sound p

ressu

re level

Ln

(dB)

140 mm concrete

Calculated

260 mm hollow concrete

260 mm hollow concrete