A survey establishes that measurable frequency noise comes from wind turbines.It has been assumed that such noise has not been a major factor contributing to annoyance as the blade passing frequency is in the order of one hertz,a point at which the human auditory system is relatively insensitive.But according to researchers the blade passing frequency modulates well audible,higher frequency sounds,and thus creates periodic sound.Residents near wind turbines often observe that,often in late afternoon or evening,the turbine sound acquires a distinct'beating'character,the rhythm of which is in agreement with the blade passing frequency.
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