Variation in A-Weighted Pink Noise
Although variation in individual bands might be important when considering tonal noise components, they will be partly levelled out when considering broadband noise typical of industrial sources, so the variation in А-weighted pink noise is analysed separately. Figure 8.37 shows the 95%
Figure 8.37 Spatial distribution of P97.5 - P2.5 predicted due to changes in moderately downwind refractive state of the atmosphere throughout the year for А-weighted pink noise propagating over “grassland" when the source height is 2 m . Reproduced from T Van Renterghem, D. Botteldooren,Variability due to short-distance favorable sound propagation and its consequences for immission assessment, J. Acoust Soc. Am.. 143: 3406-3417 (2018) with the permission of the Acoustical Society of America.
variation in А-weighted pink noise predicted from the combination of the changing moderately downwind conditions over a full year, and the influence of seven types of‘grassland’, for a source height of 2 m. The predicted overall variation for А-weighted pink noise is much less than predicted for individual 1/3 octave bands. Moreover, in general, the variation is predicted to increase with increasing distance. However, distinct zones with higher variability are predicted for specific height ranges. For a source height of 2 m (see Figure 8.37), such a zone is predicted very close to the ground. Two other zones of high variability are predicted near the source height and at heights of between 4 and 5 m. This has the important practical consequence for performing short-term (snapshot) measurements, that it might be possible to minimize the variability and hence uncertainty by careful choice of microphone height. Specifically, measuring at a height of 2 or 4 in would be less likely to produce a representative level in the short term, or would need a longer period to reduce measurement uncertainty. The predicted variation at these receiver heights could be roughly halved by different choice of microphone location.
Localized steep gradients in the predictions are not encountered if the source is significantly more elevated (see Figure 8.38). The variation increases relatively smoothly when moving closer to the ground and when going further away from the source. Overall, the predicted variation is much less than that predicted for the low source height.
Figure 8.38 Spatial distribution of P97.5 - P2.5 due to changes in moderately downwind refractive state of the atmosphere throughout the year for A-weighted pink noise propagating over “grassland” when the source height is 20 m . Reproduced from T. Van Renterghem, D. Botteldooren, Variability due to short-distance favorable sound propagation and its consequences for immission assessment, J. Acoust. Soc. Am., 143: 3406-3417 (2018) with the permission of the Acoustical Society of America.