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National Aeronautics and Space Administration The following abbreviations are used in this manuscript:Ĭumulative sound pressure level after passing through the A-weighting filter that approximates the response of the human hearingĪ-weighted sound pressure level exceeded for 10% of the timeĪ-weighted sound pressure level exceeded for 90% of the timeĪ-weighted sound pressure level exceeded for 95% of the timeĪ-weighted sound pressure level exceeded for 90% of the time in a 10 min sample (840 s) 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. Turbines can exhibit amplitude modulation in the low frequency region. The rate of the modulation of the dB(A) level occurs at the blade pass frequency, which is in the infrasound region. The correct term for the variation of the A-weighted level is modulation of the amplitude. In a technical sense, the general descriptor for wind turbine amplitude modulation is incorrect. Criteria have been proposed in the UK to define “excessive amplitude modulation”. The modulation of the A-weighted amplitude of the acoustic signature for wind turbines is often referred to as “amplitude modulation”. The variations in the amplitude of the sound being emitted can be significant at receiver locations both as an audible and inaudible sound. 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.

Waterfall background noise unit