Wind turbine noise source characteristics measured with a large microphone array

A large 40 m scale microphone array was designed to record the noise from a wind turbine. The objective was to acoustically image the noise source characteristics across the entire diameter of the turbine at a spatial resolution of 1 m at 1/3 octave resolution. This allows simultaneous definition of the spatial, temporal, and spectral properties of the generated sound. 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. 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. We describe the significant challenges in imaging with such a large array. High resolution image results are given as well as time-resolved and spectrally-resolved turbine noise directivity patterns.