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 “whoosh … whoosh”, which is called amplitude modulation (AM).
Most existing noise regulations – if they exist at all – use long-term average measures of sound levels. Thus they 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 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) can not occur five or more times in any 2-second period in 6 or more minutes of any hour.