The overall noise level of a wind turbine is proportional to the sixth power of the blade velocity. Analysis indicates that the noise source is essentially dipole-like. Further, it can be deduced that the noise is caused by the pulsating forces exerted by the blades on the air flowing through the turbine. Two discrete-frequency noise sources can be considered: one is the propeller-like noise caused by the pressure field of the moving rotor blades, and the other is the blade pulsating noise caused by aerodynamic interference. The distance between the moving and stationary blades is a significant factor in interference noise.
When this distance is small, changes in potential flow and wake will have an impact, and the blades may also act as sound barriers, amplifying the sound radiation generated by lift pulsations on adjacent blade rows. This effect depends on the ratio of the wavelength of the sound wave associated with the lift pulsations to the size of the blade acting as a barrier. In the frequency range where this ratio is greater than 2, the change in radiation intensity caused by this effect is most significant. Therefore, this effect will be stronger when a noisy blade has the same number of blades upstream and downstream, and each blade in these two rows simultaneously encounters a rotor blade, forming sound barriers on both sides of the noise source.
As the distance between the moving and stationary blades increases, the decrease in the effect of potential flow interference is much faster than the decrease in the effect of wake velocity changes, and the effect of the blades as a sound barrier also decreases with increasing distance. Therefore, at least three parameters affect the magnitude of interference noise: the blade shape of the velocity field waveform (i.e., blade load), the distance between blade rows, and the radiating area of the blades as a sound source. Very small spacing can produce two acoustic effects. If the force pulsation established by the stationary blade interference field on the moving blade makes the moving blade a sound source, then the stationary blade acts as a sound barrier.
