Predicting Wind Turbine Noise
An Untypical Industrial Source
Although wind turbines are considered as industrial infrastructure, the behaviour of their sound emissions is quite different from those of typical industrial sound sources. The fact that the sound source is relatively high means that knowledge about sound propagation from low height sound sources is not necessarily applicable. In addition, sound propagation from a wind turbine is influenced by the complex flow fields which appear upwind and downwind from the turbine. While direct sound contributions interacting with ground reflections might be the dominant propagation effects close to a turbine, at longer ranges accurate predictions require advanced sound propagation models that account for height-dependent refraction.
Specific characteristics of the sound fields produced by wind turbines, including amplitude modulation (see Chapter 3, section 3.6), have led to stringent legal limits to prevent noise annoyance reactions at nearby dwellings. This implies that the propagation distances of concern might exceed 1 km from the infrastructure and care is required in predicting the resulting low levels. In many countries, IS09613-2 is used for impact assessment during the planning of wind turbine projects. But some care is needed.
12.1 1.2 Complex Meteorologically Induced
Complex Meteorologically Induced Propagation Effects
In contrast to IS09613-2, full-wave numerical simulations allow in-depth investigations of sound propagation from wind turbines. Such research has led to some counter-intuitive predictions. For example, up to a few hundred metres from the mast, upwind sound pressure levels are predicted to be higher at all frequencies than at the same distance downwind [67], whereas, generally, for low-height sound sources, the opposite is true.
Up to roughly 1 km downwind from the turbine, higher levels are observed as expected. Beyond that distance, mainly wake-induced sound pressure level amplification is observed [67,68]. Especially in very stable atmospheric conditions, long and persistent wakes appear, leading to more pronounced wake amplification effects. In contrast, if the incoming turbulent intensity is strong, wake amplification is poorer. The presence of the turbulent wake has been shown to increase amplitude modulation significantly downwind, and its magnitude increases with distance [69].
There may be more intense refraction effects around noise sources present in the upper emission quadrant of the rotor plane. Wind velocity depletion near hub height means that higher sources are more subject to downward refraction than lower sources which are influenced, initially, by an upward- refracting zone in the atmosphere. Relatively near to the turbine, directivities of the aero-acoustic sources present at the blades are thought to be the main drivers for amplitude modulation, while at larger distances, refraction effects become dominant, depending on source position, as a result of the height-dependent refraction [70].
