The annoying character of wind turbine sound is primarily due to amplitude modulation, which causes the sound level to go up and down continually, by a range of up to 6 decibels, making it very hard to ignore. The sound also has an azimuthal variation; even though it may be quieter to the side of a turbine, the increased amplitude modulation may actually make the sidewind case more annoying than being directly downwind. And, unlike the situation with regard to continuously occurring sound (fans, busy highways), it is very difficult to become accustomed to uneven sound. In fact, many residents have reported being more annoyed with turbine sound over time rather than less. The effect is particularly pronounced with very large turbines featuring relatively low rotation rates, where the amplitude modulation is at its greatest.
The frequency content of the turbines is also of considerable importance. Larger turbines generate noticeably more low frequencies than do so smaller ones. This is due to the fact that low frequency sound is better generated by movement over larger areas, corresponding to the longer wavelength of such sound. In general, there is a shift of about one one‐third octave band of the spectrum between mid‐sized (600 KW) and very large turbines (2.5 MW). This is corresponds to about a 4‐5 dB change in each 1/3 octave band, a clearly noticeable difference. This shift, coupled with the higher amplitude modulation, accounts for the fact that larger turbines are perceived to be more noticeable than smaller ones, and are usually found to be more annoying. An examination of wind turbine annoyance shows, in fact, shows very few locations where turbines of 750 KW or less are found to be annoying.