Precision measurements of wind turbine noise using a large aperture microphone array

Experiments are described with a large microphone array (40 m scale) recording wind turbine noise. The array comprised 42 purpose-designed low-noise microphones simultaneously sampled at 20 kHz. Very high quality, fast, meteorological profile data was available from nearby 80 m masts and from the turbine nacelle, giving wind speed, wind direction, and turbulence data. A speaker was mounted at the base of the turbine tower, for determining the spatial characteristics of coherence, and for compensating for local wind variations. This speaker emits a continuous dual tone (allowing continuous time-of-flight at each microphone). An experiment was also run recording the sound from a continuous tone speaker mounted near the tip of a turbine blade, allowing testing of signal processing to correct for the very substantial Doppler shift. These various experiments are targeted at obtaining very high spatial and temporal resolution acoustic images of the sound emitted from turbine blades. An overview of some of the first results from this work will be given