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The impact of snow on sound propagating from a wind farm in northern Sweden has been investigated. Simultaneous acoustic and meteorological measurements, combined with daily snow observations, have been analysed for the snow season in 2013 to 2014. Such measurements are crucial since significant knowledge gaps exist, especially for conditions in cold climates, in the implementation of atmospheric boundary layer complexity in sound propagation models. The effect of snow on sound propagation is shown to be dependent on the snow quality. Moreover, snow on trees (upplega) also has an influence on sound propagation. Compared with conditions without snow on trees, the average sound level is approximately 2 dBA lower. The effect is more distinct for higher frequencies compared with lower frequencies.

Wind turbine (WT) sound annoys some people even though the sound levels are relative low compared to other man made sources. A reason for this could be due to the amplitude modulated sound – “swish and thump”. Measurements of sound immission from WTs were carried out continuously 1 year at two sites in Sweden. A method for quantifying the degree and strength of amplitude modulation (AM) is presented. AM is strongly coupled to the weather conditions. AM is observed at our two sites approximately 20 – 30 % of the operational time depending on the distance from the turbines. Continuous acoustic measurements were taken for one year at two different wind turbine sites in Sweden. The sites were covered by forest; one site is relatively flat while the other has shifting terrain. Meteorological parameters from the ground to above 120 m height were measured in parallel with the acoustics. The long measurement time capture all different sound propagation conditions that occur due to seasonal and daily variations in the weather. 

Wind turbine (WT) sound annoys some people even though the sound levels are relatively low. This could be because of the amplitude modulated “swishing” characteristic of the turbine sound,which is not taken into account by standard procedures for measuring average sound levels. Studies of sound immission from WTs were conducted continually between 19 August 2011 and 19 August 2012 at two sites in Sweden. A method for quantifying the degree and strength of amplitude modulation (AM) is introduced here. The method reveals that AM at the immission points occur under specific meteorological conditions. For WT sound immission, the wind direction and sound speed gradient are crucial for the occurrence of AM. Interference between two or more WTs could probably enhance AM. The mechanisms by which WT sound is amplitude modulated are not fully understood.

Långtidsmätningar av ljud och väder på tre platser vid vindkraftsparker i Sverige visar att vädret styr ljudutbredningen även för höga ljudkällor. Högsta ljudnivåer erhålls vid medvind då ljudvågorna böjs ner mot marken. Vid klara kvällar, nätter och morgnar kan amplitudmodulerat ljud höras på stora avstånd då väderförhållandena är olika i det stora vertikala skiktet som rotorbladen sveper igenom.En i projektet framtagen ljudutbredningsmodell, som är kopplad till en numerisk gränsskiktsmodell, tarhänsyn till vädret på en viss plats och kan beräkna den verkliga ljudnivån samt det kommande ljudklimatet vilket är användbart för operationell drift samt lokalisering av vindkraftverk.

Kontrollmätningar av ljudnivån från vindkraftverk kan göras vid vissa mätförhållandena vilka föreslås.

Ett förslag till ny formulering av riktlinjer baserat på långtidsmätningarna och är kopplat till den ljudnivå som i 90 % av tiden underskrids.

The meteorological conditions vary over the globe but also change over the day and the year and vary a lot depending on the terrain for a certain location. The meteorological parameters govern both the wind turbine emission sound levels and the sound propagation conditions and therefore gives rise to different sound immission levels. Long-time measurements of meteorological effects on sound propagation from wind turbines over forest areas have been performed at two sites in Sweden for more than two years. One site is located in the southern part with flat terrain and the other site is located in the northern part of Sweden with more hilly terrain. The aim of the project is to improve the knowledge of sound propagation from wind turbines and especially over varying terrain and weather conditions. Control measurements of wind turbine immission sound levels will be needed to see that they fulfill the noise regulations. It is therefore of most importance to be able to make representative measurements. Discussions about under what meteorological conditions the immission measurements have to be carried out, the sound metric and the impact of the guidelines are presented in this paper. 

Continuous acoustic measurements were taken for two years at two different wind turbine sites in Sweden. The sites were covered by forest; one site is relatively flat while the other has shifting terrain. Meteorological parameters from the ground to above 120 m height were measured in parallel with the acoustics. The long measurement time capture all different sound propagation conditions that occur due to seasonal and daily variations in the weather. Strict criteria were developed to avoid disturbances from ambient noise. The results show that meteorological effects on wind turbine sound propagation increase with distance and start to be important at distances somewhere between 400 – 1000 m. Variations of expected values between 6 - 14 dBA at the immission point are found depending on ground conditions and refraction. The effects are seen for all 1/3-octave bands from 63 Hz – 1000 Hz.

A comparison between the wind turbine sound measurements and the Nord2000, ISO 9613-2 and Swedish Environmental Protection Agency sound propagation models reveals that models underestimates the highest WT sound levels. This shows the importance of including meteorological effects and terrain shape in models, to give accurate predictions.  

The meteorological conditions change over the day and the year and vary a lot depending of the terrain conditions. The meteorological parameters govern both the wind turbine emission sound level and the sound propagation conditions. Long-time measurements of meteorological effects on sound propagation from wind turbines have been performed at three sites in Sweden. The measurements have been performed during 2 years. Sound propagation is studied in a forest area, over a water bay and over heterogeneous terrain. The first two sites are located in the southern part of Sweden and the third is located in the northern part of Sweden. The aim of the project is to improve the knowledge about sound propagation from wind turbines and especially over varying terrain and different weather conditions. The hub height of the studied wind turbines varies from 80-138 m. Meteorological effects increase with distance and starts to be important somewhere between 400 m – 1000 m. Variations of 7-14 dBA at the immission point are found depending on ground conditions and refraction.

Continuous measurements of wind turbine sound were performed during approximately two years at two different sites in Sweden. Meteorological parameters were measured in parallel with the acoustical measurements to see the weather influence on the sound propagation. A combined effect of refraction (bending of sound ways) and ground properties was found. This meteorological effect starts to be important somewhere between 400 – 1000 m from a wind turbine and could give a meteorological variability of the expected sound level with 7 dBA during the summer and 14 dBA during snow cover conditions. A comparison between the wind turbine sound immission measurements, ISO 9613-2 and the recommended wind turbine sound propagation model in Sweden reveals that the models underestimate the highest sound levels. Sound propagation models in general need to consider the meteorological effect for better predictions at longer propagation distances.

During the half measurement period (one year) the sound level were stored 8 times per second to capture the amplitude modulation of the sound, which according to some, increases the annoyance. It was found that amplitude modulation roughly occurs 18 % of the time at a distance of 1 – 2 km from 12 wind turbines and is more common under specific meteorological conditions such as positive sound speed gradients and positive temperature gradients.