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 “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 England a wind energy facility was approved with conditions including a definition of unacceptable AM: any change, upon complaint, outside the dwelling, in LA_eq,125ms of >3 dB in any 2-second period ≥5 times in any minute with LA_eq,1min ≥28 dB and such excess occurring in ≥6 minutes in any hour, which is to say that a 125-ms pulse of 3 dBA or greater (3 dB being the difference in noise level detectable by the human ear) can not occur in any 2-second period five or more times in six or more minutes of any hour, when those minute-long average noise levels are 28 dBA or more.