Measuring Wind Turbine Coherent Infrasound
John Vanderkooy and Richard Mann
Department of Physics and Astronomy, Department of Computer Science University of Waterloo, Waterloo, ON, Canada
Date posted: 2 October, 2014
To extract the optimum coherent infrasound signal from a wind turbine whose rotation is not precisely periodic, we use an optical telescope fitted with a photodetector to obtain reference blade passage periods, recording these together with the microphone infrasound signal. Signal processing of the quasi-periodic microphone signal is then used to obtain periodic data, which are analyzed by an appropriate length DFT to extract optimum values for the fundamental and harmonics of the coherent signal. The general procedure is similar to order domain analysis for rotating machines and is thoroughly explained and illustrated with measurements and analysis from a number of different wind farms. If several turbines are measured by a single microphone with blade passage periods obtained from several separate reference tracks, it may be possible to retrieve separate useful coherent signals from multiple turbines by appropriate processing.
Our paper shows how the coherent part of the infrasound from a single WT can be extracted from a microphone signal by using a blade passage reference track from the turbine under study.
Our analysis reveals a characteristic infrasonic pulse. We conjecture that the pulse from a single WT is caused by the interaction of the blades against the pylon, while the rather more complex background
signal relates to the radiation of the Tyler-Sofrin spinning modes.
The random component of the infrasonic signal exceeds the coherent part, and this random component is related to wind noise, which appears to be similar whether one is near or far from a wind farm.
Our paper avoids the issue of health effects from WT infrasound. Information on both sides of the controversy abounds in the literature. Read full article