Abstract
This paper discusses noise associated with wind turbines that have rotors upwind of the support structure. It also discusses methods for sound level prediction and presents estimated noise levels for the Boeing MOD-2 wind turbine operating in both the upwind and downwind configurations. The predominant source of noise for the downwind rotor configuration of the MOD-1 turbine appeared to be fluctuating airloads arising from interaction of the rotor and the wake trailed by the tower support structure. Upwind rotor configurations, such as the MOD-2 turbine, did not experience the same magnitude of airload fluctuations as the blade passed upstream of the tower, and other noise sources dominated. Results indicated that upwind rotor configurations may be advantageous from an acoustical standpoint. Predictions for the MOD-2 turbine indicated that the noise signature would be of a broad band nature. Although noise measurements had not been made on the MOD-2 turbine to the date of the publication, observers indicated that the predominant noise is a swishing sound characteristic of a broad band noise source. Noise levels between 60 and 65 dBA were predicted at a distance of 200 ft, similar to those near a freeway with moderate traffic at an equivalent distance. The existing empirical broad band methodology lacks a rigorous analytical understanding that must be developed from an adequate data base to accurately quantify broad band, nonperiodic noise associated with the MOD-2 turbine.