The Application of a MEMS Microphone Phased Array to Aeroacoustics of Small Wind Turbines

Abstract

A low-cost microelectromechanical (MEMS) microphone array was developed to investigate and localize aeroacoustic sources. A 1 m by 1 m array was designed as a portable measurement apparatus capable of locating noise sources from small rotating wind turbines in conjunction with open-jet wind tunnels or field measurements. Beamforming algorithms were implemented to allow simulation of sound sources and conditions expected to be encountered in testing. Array testing with a known monopole source, multiple sources and different frequencies located the mainlobes accurately at a number of frequencies and distances. A rotating sound source was located spatially and tracked using conventional beamforming. Experimental results from the acoustic testing of a 1.3 metre rotor diameter wind turbine in an open-jet wind tunnel indicated strong evidence of trailing edge noise at freestream velocities of 4.5 m/s and 5.5 m/s. Source maps for the 5.5 m/s tests reveal that the turbine's aeroacoustic emissions are most prevalent at the outer portions of the radius, but not necessarily at the tip. The azimuthal location of the greatest sound pressure levels was found to be roughly 120° to 130°, measured from the upward vertical, for most test conditions. An analytical model confirmed this sector to be the approximate expected azimuthal location of the source in the time-averaged source maps.