Not all wind farms cause a problem with noise. Earlier examples which were built in remote parts rarely caused problems but as these sites became used developers looked more and more towards developed areas and thus came ever closer to human habitation.
There are two potential sources of noise: that from the turbine blades passing through the air at the speed of a light aircraft and that from the gearbox and generator in the nacelle (or housing). The aerofoil blade (usually three) the length of a Jumbo's wing whose tip travels at 150 miles per hour (241 Km.p.h.) and harvesting 0.6 MW or more of power inevitably makes a substantial sound. The air passing through the rotor is swept into turbulent wake vortices (which, incidentally, attract, trap and kill many bats in a process akin to the "bends" in divers) is the source of much of the sound. Within a few feet it encounters an obstruction in the form of the tower as a blade passes every one to two seconds, even at its slowest viable turning speed. This imposes a pulsating quality to the aerodynamic sound which many people find deeply disturbing.
Other periodic sounds arise as the blades sweep down into the region of wind shear so that the lowest blade position experiences both a different wind speed and varying turbulence. Some developers state categorically that, "Noise isn't a problem" but they rely on ETSU-R-97 which is not a fit instrument to assess it.
Clusters of windfarms can produce further interaction of sound periodicity as the rotors of different machines go into and out of phase creating the periodic sounds (aerodynamic or amplitude modulation) generating the "whoomph, whoomph" at one or two second intervals rising and falling in loudness - an effect which disturbs some people and which can be likened to the base "woofer" speaker in a sound system.
Furthermore, as well as the normally audible sounds machines produce a low frequency sound from vibrations which ranges from the barely audible "sub-woofer" frequencies below 200 Hz down to wavelengths which cannot be heard but often sensed as bodily discomfort (below 20 Hz) and often referred to as infrasound which is difficult to measure instrumentally but which effects ALL animals, including horses, cattle and sheep.
