Microbarometers have been used to quantify the infrasonic emissions (0.05Hz to 20Hz) from five wind farms in Victoria, Australia. The wind farms measured include; Macarthur wind farm (140 turbines type Vestas V112 3MW); Cape Bridgewater (29 turbines type MM82 2MW); Leonards Hill (2 turbines type MM82 2MW); Mount Mercer (64 turbines type MM92 2MW), and; Waubra (128 turbines 3 types of Acciona Windpower 2MW).
Upwind indoor measurements at the Macarthur wind farm during an unplanned shutdown from full power and subsequent startup to 30% load has shown that stationary turbines subject to high winds emit infrasound pressure below 8 Hz at levels similar to the infrasound emissions at blade pass frequencies and harmonics.
The stationary V112 turbine infrasound emissions are caused primarily by blade and tower resonances excited by the wind. It is apparent from the mismatch of resonances and blade pass frequency components that Vestas have carefully designed this unit to minimise fatigue of the wind turbine.
Short range (up to 2km) measurements from the Leonards Hill wind farm have shown the determination of attenuation rate with distance to be problematic due to interference between the two turbines. A model to explain the unexpected attenuation results at Leonards Hill has demonstrated that the commonly observed amplitude modulation of blade pass tones is the result of changing phase between turbine rotor speed and changes in wind speed.
Long range measurements from two different wind farms over a distance of 80km have shown that infrasound below 6Hz has a propagation loss approximating 3dB per doubling of distance.
Les Huson, L Huson & Associates, Woodend, Victoria, Australia
6th International Conference on Wind Turbine Noise, Glasgow, 20-23 April 2015