Amplitude modulation
A characteristic of wind turbine noise that makes it more annoying than other noises at the same average sound level is its pulsing or throbbing “whoomph … whoomph”, which is called amplitude modulation (AM).
Most existing noise regulations – when they exist at all – use long-term average measures of sound levels, and thus fail to take AM into account. (They usually also fail to consider infrasound and low-frequency noise, another characteristic that makes wind turbine noise more intrusive. In addition, Australian acoustician Steven Cooper[1] notes that AM may be “felt” like intrusive infrasound as it modulates at infrasonic frequencies.)
In England a wind energy facility was approved with conditions including a definition of unacceptable AM: any change, upon complaint, outside the dwelling, in LAeq,125ms of >3 dB in any 2-second period ≥5 times in any minute with LAeq,1min ≥28 dB and such excess occurring in ≥6 minutes in any hour. Or in other words: When the minute-long average noise level is 28 dBA or more, a 125-millisecond spike of 3 dBA or greater above the average noise level (3 dB being the difference in noise level detectable by the human ear[1]) can not occur five or more times in any 2-second period in 6 or more minutes of any hour.