Noise with prominent impulses is more annoying than continuous types of noise (without impulses or tones) with the same equivalent sound pressure level. Therefore an adjustment KI is added to the measured LAeq, if prominent impulses are present in the noise, to adjust for the extra annoyance due to the impulses.
Impulsive sounds are characterised by a sudden onset, which makes them more prominent than continuous noise types, and makes it easier to identify the sound source.
The adjustment to LAeq for impulses depends on how prominent the impulse characteristic is perceived through the continuous part of the noise.
Measurements according to this Nordtest method give as the main result a measure for the prominence of impulsive sounds at the immission point. The method aims at predicting the prominence of impulsive sounds in correspondence with average subjective judgements. Based on the predicted prominence, P, a graduated adjustment, KI, to the measured LAeq is defined.
The method is based on the presumption that the annoyance increases with increasing audibility (perceived prominence) of the impulses. The audibility of the impulses is expressed by the prominence, which shall exceed a certain limit before an adjustment is made to LAeq. Below this limit no adjustment is made. When the prominence rises, the adjustment increases.
The prominence P is defined by a logarithmic measure based on the level difference and onset rate of the A weighted sound pressure level with time weighting F. In practice, the logarithmic scale together with time weighting F sets an upper limit for the adjustment.