Horizontal directivity of sound emitted from wind turbines

Abstract

Field measurements of noise generated from two different wind turbines, one with an upwind rotor and one with a downwind rotor, have been performed. To examine the radiation characteristics of wind turbine noise, some receiving points were set around each wind turbine and the apparent A-weighted sound power levels were calculated from the obtained data at 200 ms intervals under various wind conditions. Wind turbine operational data were collected at 1 s intervals along with corresponding acoustic data. Additionally, a simple empirical formula for the sound directivity was proposed, assuming the directivity pattern of aerodynamic and mechanical sound to be bi- and omnidirectional, respectively. The results showed that the horizontal directivity of the A-weighted sound pressure level at the ground level for the two different wind turbines is almost the same, whereas the frequency dependence of the sound directivity is different for the individual wind turbines. Furthermore, obtaining data of the rotor rotational speed, output power, and nacelle direction is strongly recommended to assess the characteristics of noise emission, such as the changes in the sound power level, sound directivity, and tonal components of wind turbine noise.