Evaluation of wind turbine noise levels and impact studies

Abstract

Measured A‐weighted sound levels at 125‐ft distance for individual wind turbines with 20‐ to 120‐kW power ratings are typically in the range 65–75 dB at moderate to high power output conditions (20–30‐mph wind speeds). Tonelike sounds in the 300‐ to 1000‐Hz frequency range often are clearly audible. Cyclical fluctuations of 10 dB in low‐frequency noise levels are propagated by some downwind‐type turbines. The random aerodynamic rotor noise sounds like a roar, the gear box noise sounds like a whine, and the low‐frequency noise fluctuations sound like “thump‐thump” or “whoosh‐whoosh.” All of these wind turbine noises are propagated from existing wind farms to residential areas and are judged intrusive and annoying. Measurements and predictions of wind turbine noise submitted with applications for wind farm development have often contained errors which understated the noise levels by 3–10 dB. These errors were due to noise measurements at minimal wind speeds and turbine power, and faulty modeling procedures. Simple analytical expressions have been developed which quickly and accurately predict the noise levels for large turbine arrays.