Relatively balanced load related pressure waves from the rear surface of each rotor blade, are at a frequency of 1 per revolution of the turbine and are phase shifted by 120 degrees from each other. The superpositions of these infrasonic waves destructively interfere. This action results in a non-propagating rotor locked mode, however, the shielding (reflecting) effect of the tower as each blade passes, interrupts the balanced destructive interference for a small portion of rotor angle three times per revolution. The momentary un-balance between the destructive interfering waves, results in the generation of Tyler-Sofrin spinning mode series, which propagate into the far field. The spinning mode radiation angles, coupled with the low decay rate of infrasound, result in higher far field sound pressure levels than would be predicted for a point source. An analysis approach partially derived from Tyler-Sofrin (1962) is presented. Field microphone data including phase measurements identifying the spinning modes are also presented.
This paper discusses Blade Passing Frequency (BPF) infrasound and its harmonics. Some results on Low Frequency Noise at discrete amplitude modulated frequencies of about 20 Hz plus harmonics, is briefly discussed as a side effect of the spinning mode generation mechanism. This initial analysis focuses on Blade Passing Frequency related noise directly behind (downwind) the turbine. Atmospheric effects on the propagation of spinning modes and noise in general have not been included in this study but will clearly be required for a complete and accurate prediction of far field noise levels.
Other broadband noise generation mechanisms such as vortex shedding or turbulence are not discussed.
Narrow band infrasonic Fast Fourier Transform (FFT) measurements inside homes situated close to some wind turbine installations show clear acoustic signatures at the BPF and harmonics of the nearby wind turbines (Ref 2, Ref 4). Multiple averaged Fast Fourier Transform (FFT) measurements show similar clearly identifiable acoustic signatures of wind turbine BPF harmonics at over 125 kilometers distance from the closest wind turbine installations (Fig. 8 and Ref. 4) at Sound Pressure Levels (SPL’s) which have been measured to be within -25dB of the SPL’s recorded inside a home within 500 meters of a turbine installation (Fig. 7 Fig. 8 and Ref. 4). These measurement results spurred an in-depth analysis of the infrasonic noise generation mechanisms involved at the turbine followed by supporting close proximity phase measurements at ground level near (~170 meters) to a wind turbine (Table 1 and Fig. 5).