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Impact Noise Transmission

The impact noise transmitted into a room will depend not only on the direct transmission through the separating floor, but also on the flanking transmissions by all walls linked to this separating element. Last parasite transmissions (such as leakage around a constructive element or through a duct or opening) will also pollute the result.

The impact noise transmitted into a room usually is measured using an impact machine on the floor of one room (labeled “emission room”); the mean sound pressure level is measured in the receiving room. The relevant measurement standard usually is ISO 140-7 [43] or ASTM 1007 [44].

The impact noise level is directly measured in the receiving room. However, this sound level value is sensitive to the amount of acoustic absorption inside the receiving room: This would eventually mean that according to the fittings in the room, one would pass or fail the impact noise criteria. In order to avoid this inconvenience, the usual rule is to standardize the result by the reverberation time.

The standardized impact noise level LnT is given by

where L1 is the mean sound pressure level in the reception room, expressed in dB, T is the reverberation time in the receiving room, expressed in s, and T0 is the reference reverberation time (usually 0.5 s).

The presence of a ceiling underneath the floor will of course affect the results. Some standards do take this into account (e.g., ASTM E1007 [44]).

The good news is the tapping machine is reproductive enough. More to the point, it does manage to provide useful data for impact noise prediction, for example, using standard ISO 15712-2 [45]. Now for the bad news: While it is a useful tool for the acoustician, it has trouble correlating with the eventual annoyance from impact sound. This is due to the fact that the generated impact spectrum is poor in the low-frequency range (which is not that surprising, as it was initially developed to try to imitate the impacts from high-heeled shoes). Unfortunately, one may have experienced that a bare-footed person walking on tile does not generate any greater impact noise than walking on a carpet, as this mainly is a low-frequency problem; yet the tapping machine will make a sizable difference out of it. In order to introduce this kind of impact, some countries have been using a rubber ball or even a small tire [46] as an impact sound source. Such a procedure has been investigated [47, 48] and is now considered in an EN/ISO project [49].

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