Wind Turbine Sound in Cold Climates

Sammanfattning: The increase in the number of wind turbines (WTs) in populated areas in cold climates increases the number of people potentially being affected by WT sound. Outdoor sound propagation is strongly dependent on meteorological conditions, however, limitations in the knowledge exist regarding the implications of meteorological conditions in cold climates. Long-term acoustic and meteorological measurements were conducted in the vicinity of two wind farms in northern Sweden, to investigate the effect of snow and low-level wind maxima on WT sound, to analyse the occurrence of amplitude modulation and to evaluate selection methods for WT sound measurements. Different selection methodologies were applied to the acoustical data. The simplest method only includes a minimum rotational frequency of the WTs, while the most comprehensive method additionally includes criteria based on spectral resemblance, temporal variation of the sound level, amplitude modulation and wind speed. The effect of snow on WT sound depends on the snow quality. Snow on trees lowers the sound level by ca. 2 dBA. Low-level wind maxima below hub height reduce the sound level near the surface. Since this effect is increasing with increasing strength of the low-level wind maximum, the WT sound is assumed to be partly trapped above the low-level wind maximum. Amplitude modulation was shown to be dependent on atmospheric stability and was most common for very stable conditions. Moreover, a clear difference between the occurrences of amplitude modulation for the two crosswind sectors was observed. The choice of selection method needs to be taken into account when comparing different studies since it affects the results and conclusions. The studies emphasise to include the effects of individual meteorological conditions of a site in the formulation of guidelines on WT sound.