Different sound sources vary in their sound level, frequency content and temporal character. These differences are most easily described by example. First, there are steady state noise sources. A good example would be an active highway. The sound level at a listener location away from the highway may vary somewhat from moment to moment, but by small amounts. But by and large human observers perceive highway sound as near constant in amplitude.
Second, there are transient sources. These are sources present only for brief periods of time. Good examples are passing trains or aircraft. These sources rise gradually in amplitude as they approach the listener and then decay as they depart.
A third class of sound source is the impulsive one. These sources can be repetitive or random in nature. Repetitive impulse sources would be jackhammers and pile drivers. The basic difference between the two examples is the rate at which the impulses occur. A random impulsive source would be the operations in a machine shop where hammering or metal fabrication takes place. In all of these examples any one impulse lasts for only a very small fraction of a second, and the onset and offset of the impulse is very rapid.
Finally, we come to the wind turbine source. Unlike any of the previous sources described, it is neither steady state in the classic sense, nor is it transient, nor is it impulsive. Instead, it produces a distinctive broadband "swoosh-boom" sound4 with each passing blade of the turbine. Each whoosh effectively modulates the sound level, with a more gradual onset than an impulsive sound, and a less gradual one than the transient sources I mentioned. For the typical three-bladed turbine in the 1-megawatt output range the blade passage rate is nominally one per second. One can sound out the observed sound in the typical second-counting style... "whoosh" one thousand, "whoosh" two thousand, "whoosh" three thousand, and so on. So long as the wind is blowing at sufficient speed to drive the turbine the sound will be generate for lengthy periods of time without interruption.
The hours of operation of wind turbines are not dictated by diurnal patterns of human activity like many other anthropogenic sources. They are able to operate when wind conditions are favorable, day or night. This means they may become a factor during both waking and sleeping hours.
The repetitive sound character is unique and unlike any other source found in residential communities. As such it is easily identifiable. It does not blend in to other background sources that are continuous in nature.
Wind turbine noise is most prevalent in rural areas. By their very nature, large-scale wind turbine installations require vast areas of open land. Hence, any potential sound masking effect from urban and suburban sources is unlikely to be present. This means that their sound will be audible at lower levels in the rural environment. It further means that rural noise standards should be applied to these installations as opposed to suburban or urban ones where nighttime sound levels, for example, can be 15 to 20 decibels higher.