216 Building acoustics
Figure 6.6 Measurement of impact sound pressure level caused by the standard tapping machine. The
laboratory situation is illustrated in the left figure; the figure to the right shows how flanking transmission may
contribute in a field situation.
In the laboratory situation, where the task is to determine the radiated power from
the actual test specimen, the receiving room is equipped with elastic layers in such a way
that other surfaces are structurally separated from the primary one. In a building, on the
other hand, flanking constructions may contribute but normally this is a lesser problem
than in the case of airborne sound.
Finally, it should be mentioned that the impact sound pressure level measured in
the field might, in an analogous way to the airborne sound pressure level difference in
Equation (6.11), be referred to a standard reverberation time T 0. The quantity in question
is then called standardized impact sound pressure level
(^) ni
0
''10lg,T
T
LL
T
⎛⎞
=+⋅⎜⎟
⎝⎠
(6.23)
where T 0 for dwellings is equal to 0.5 seconds.
6.2.2.1 Single number rating and adaptation terms for impact sound
As for airborne sound insulation, we may characterize the impact sound insulation by a
weighted normalized impact sound pressure level Ln,w for a building element and by L'n,w
when measured in a building. As the quantities measured represent the radiated sound
power, a high impact sound isolation implies that the values of Ln are low. When
comparing measured data with the reference curve unfavourable deviations are
characterized by the measured data being greater than the corresponding reference data.
Apart from this difference, the procedure is the same as for airborne sound insulation.
The reference curve is shifted in steps of 1.0 dB against the measured one until the sum
of unfavourable derivations is as large as possible but no greater than 32 dB.
An example showing laboratory data for a wood joist floor is given in Figure 6.7
together with the reference curve. When shifting the reference curve by 5 dB as shown
we obtain a normalized impact sound pressure level Ln,w of 65 dB as the sum of
unfavourable deviations then becomes 27 dB. Shifting the reference by 4 dB only, one
gets a deviation sum of 33 dB, which is larger than the limit. It should be mentioned that
Li A L'
i