Wind turbine noise is generated aerodynamically by the rotor blades passing through the air as well as mechanically by various components such as nacelle fans and generators. For modern, large wind turbines, i.e. upwind rotor, variable speed, blade pitch control, the aerodynamic noise from the rotor blades is considered to be the dominant source of noise (Wagner, Bareiss, & Guidati, 1996). The aerodynamic noise is characterized by infrasound, low-frequency noise, inflow-turbulence noise, and airfoil self-noise.
Infrasound
Infrasound is noise with frequencies below 20 Hz. Infrasound generated by wind turbines is characterized by tonal components at the blade passing frequency and its harmonics. The cause of infrasound is generally attributed to rapid changes in angle of attack due to the perturbed flow upwind of the turbine tower causing rapid changes in lift (Hansen, Zajamsek, & Hansen, 2014).
Low-Frequency Noise
Low frequency noise is defined as broadband sound in the range of 20 to ~200 Hz and has been attributed to the interaction between inflow turbulence and the leading edge of the rotor blade (Hubbard & Shepherd, 1991).
Dynamic stall of wind turbine blades also leads to increased levels of low-frequency noise and may add tonal components. Stall can be caused by blade tower interaction, yaw error (wind turbine is not aligned with wind direction), wind shear, or other non-uniform inflow conditions (Oerlemans, 2014).
Amplitude Modulation
Aerodynamic noise with coherent and impulsive amplitude variation by wind turbines is caused by a number of contributing factors and is defined as amplitude modulation (AM). A method of quantifying AM is examining the modulation depth of the periodic variation of sound level. This variation is also known as the modulation frequency. For wind turbines, the periodic variation is generally the blade passing frequency (BPF) and its harmonics. (Oerlemans, 2014).
References
Hansen, K., Zajamsek, B., & Hansen, C. (2014). Identification of low frequency wind turbine noise using secondary windshields of various geometries. Noise Control Engineering Journal.
Hubbard, H. H., & Shepherd, K. P. (1991). Aeroacoustics of large wind turbines. The Journal of the Acoustical Society of America, 2495-2508.
Oerlemans, S. (2014). Effect of wind shear on amplitude modulation of wind turbine noise. International Journal of Aeroacoustics, 751-728.
Wagner, S., Bareiss, R., & Guidati, G. (1996). Wind turbine noise. Springer.