Abstract
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.