3.4 Meteorological effects on the propagation of noise

One factor the mine has no control over is the influence meteorological conditions have on the propagation of noise, particularly over large distances (greater than 500m). Some understanding of these effects is critical if the mine is to effectively manage noise impacts. Of most importance is downwind propagation and the effects of temperature inversions, which both cause noise to be ‘bent’ back towards the ground, thus increasing noise levels.

3.4.1 Wind effects

Steady light to moderate winds produce higher noise levels downwind, and lower noise levels upwind, than in still air.

In general (and depending on the amount and type of local vegetation), a steady, gentle breeze of less than about 1.5 metres per second can increase noise levels without increasing background noise levels. On the other hand, winds of higher velocity tend to increase background levels due to turbulence or movement of trees and shrubs, and obscure other noise sources. Downwind, wind velocities up to about 1.5 metres per second can enhance noise levels by around 5 dBA relative to still conditions, assuming flat topography between source and receiver, and more if shielding is provided by natural topography. Conversely, noise levels upwind may be reduced by a similar amount.

It should be noted that noise enhancement due to wind effects is extremely site specific and significant variations from the ‘typical’ changes in level can be expected.

3.4.2 Temperature Inversion effects

Air temperature normally decreases with altitude (as shown in B in figure below), a condition known as ‘temperature lapse’. A ‘temperature inversion’ occurs when a layer of air has its temperature increasing with altitude, or at the boundary between a lower cool layer and a higher warm layer (as shown in A in figure below).

Effect of temperature inversion on the propagation of sound-normal sound propagation (A) and propagation with a temperature inversion (B).

In winter, temperature inversions are normally associated with drainage flows, where cool air flows down to areas of lower ground level, creating a light breeze. This ‘drainage flow’ is affected by the topography, and as such the extent of this effect depends on the depth of the inversion layer.

Wind and temperature inversion effects generally apply to all noises, including airblasts. Temperature inversions appear to affect low-frequency sound more than higher frequency sound. This is possibly because, over relatively large distances, the higher frequency sounds are readily attenuated by other effects (such as atmospheric absorption). Since temperature inversions normally appear at night and disperse an hour or two after sunrise in the summer period, noisy events (particularly blasting) should be planned around these periods. In areas which are prone to severe inversions, excessively noisy activities should be avoided on overcast days when possible.

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