Building Acoustics

Sound transmission 217

the standard demands that the measured data should be given to one decimal place but
that is not used in this example.
The “8 dB rule” mentioned above was formerly also used for impact sound
insulation. This has been superseded by an adaptation term having the symbol CI where
the index signifies “impact”. The reason for introducing this term is that the impact
sound pressure level Ln,w does not take sufficient account of level peaks at low
frequencies, especially for wood joist floors. There is clear evidence that the unweighted
impact level of the tapping machine is more representative of the A-weighted impact
levels caused by walking for all types of floors. The adaptation term CI is therefore given
as the difference between the unweighted sum Ln,sum of the normalized impact levels and
the weighted impact sound pressure level Ln,w, such that

/10

I n,sum n,w 15 dB where n,sum 10 lg^10

Lj

j

CL L L

⎛⎞

=−− =⋅⎜⎟

⎜⎟

⎝⎠

∑. (6.24)

Figure 6.7 Calculation of normalized impact sound pressure level Ln,w. Example of laboratory measurement of a
wood joist floor. Ln,w is equal to 65 dB and the sum of unfavourable deviations is 27 dB; see text.

The sum is either taken in one-third-octave bands in the frequency range 100 Hz to 2500
Hz or in octave bands in the frequency range 125 to 2000 Hz. An extended range
including the one-third-octave bands 50, 63 and 80 Hz is also used. The term is then
denoted CI, 50 –2500.

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)

Ln - Wood joist floor

Reference

Reference + 5 dB