Abstract
The operation of a wind turbine results in a series of pulses where there is a significant instantaneous increase in the amplitude of the pressure signal, dependent upon the wind speed at the turbine blades. The variations in the amplitude of the sound being emitted can be significant at receiver locations both as an audible and inaudible sound. The modulation of the A-weighted amplitude of the acoustic signature for wind turbines is often referred to as “amplitude modulation”. Criteria have been proposed in the UK to define “excessive amplitude modulation”. In a technical sense, the general descriptor for wind turbine amplitude modulation is incorrect. The correct term for the variation of the A-weighted level is modulation of the amplitude. The rate of the modulation of the dB(A) level occurs at the blade pass frequency, which is in the infrasound region. Turbines can exhibit amplitude modulation in the low frequency region. The differences between amplitude modulation and modulation of the amplitude occurring at an infrasound rate are discussed in the context for an environmental assessment of a wind farm with respect to permit conditions and a simplified method of assessment with respect to the Modulation Index.
Highlights
The operation of a wind turbine generates non-steady broadband sound that exhibits fluctuations at the blade pass frequency rate
The operation of a wind turbine gives rise to a variation in the noise as the blades rotate such that the noise is not a constant noise, but is subject to changes in the amplitude of the noise signal
The introduction of wind turbines as an alternative means of power generation commenced with small-scale units upon which various socio-acoustic and environmental studies were undertaken to develop acoustic criteria
Summary
The operation of a wind turbine generates non-steady broadband sound that exhibits fluctuations at the blade pass frequency rate. The blade pass frequency occurs between 0.65 and 0.8 Hz. The blade pass frequency occurs in the infrasound region, i.e., less than 20 Hz. In an ideal situation, the angle of the turbine blades (the blade pitch) is adjusted to be at the most efficient angle for extracting power from the wind. Start-up of power generation, or at high wind speeds (where the blades are intentionally feathered to depower the turbines), the blades are not positioned at an ideal/efficient angle, and under those scenarios have been found to generate higher levels of pulsations [1] that correspond to a higher level of annoyance [2,3]. Period being the reciprocal of the blade pass frequency (e.g., for a three-bladed turbine operating at 17 rpm, the blade pass frequency is 0.85 Hz, which gives a pulse every 1.18 s)
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