Building Acoustics

212 Building acoustics

to 5000 Hz. The procedure to substitute these data by a single number is based on the use
of a reference curve, which means that we have agreed on a suitable sound reduction
curve to be compared with our measurement data. We shall not delve into the history of
how these reference curves were developed, it is sufficient to state that there exists such
reference curves both for airborne and impact sound, these being internationally accepted
(see ISO 717 Part 1 and Part 2). The reference curve for the sound reduction index is
shown in Figure 6.4 together with a measured result, a laboratory measurement of a
double wall of 13 mm plasterboards on separate studs. The distance between the boards
is 150 mm and the void between the boards is filled with mineral wool.
To calculate the single number value RW, the reference curve is shifted in 1 dB
steps towards the measured curve until the sum of unfavourable deviations is as large as
possible but not more then 32.0 dB when using 16 measurement frequencies. An
unfavourable deviation at a given frequency occurs when the measurement result is less
than the reference value. Only the unfavourable deviations are taken into account, i.e.
high sound insulation data in the higher frequency range does not compensate for bad
insulation at low frequencies.
As is apparent from Figure 6.4, a maximum shift of 7 dB is possible in this case,
giving a sum of unfavourable deviations of 28 dB and hence an RW of 59 dB. In former
standards there existed the so-called “8 dB rule”, which was based on the non-acceptance
of very low sound insulation at one or more frequency bands. Unfavourable deviations of
maximum 8 dB was accepted, a rule that was often the determining factor when
calculating RW.

Figure 6.4 Calculation of the weighted sound reduction index RW. In this example RW is equal to 59 dB, which
is the reference value at 500 Hz. The sum of unfavourable deviations is 28 dB; see text.

63 125 250 500 1000 2000 4000

Frequency (Hz)

0

10

20

30

40

50

60

70

80

Sound

reduction index (dB)

R - double wall

Reference

Reference +7dB