Noise level is expressed in decibels (dB), using a logarithmic scale. A difference of 3 dB is the smallest that can be detected by the human ear, while a noise that is 10 dB louder than another is perceived to be twice as loud, although it is physically 10 times higher in pressure. An increase in noise level of 6 dB or more causes widespread annoyance and disruption. The usual measurement is in dB(A), which emphasizes the frequencies easily heard (consciously) by humans. A quiet rural night may have an ambient sound level of 20-30 dB(A). Fifteen hundred feet from an industrial wind turbine, the sound level may be 45-70 dB(A), at least four to sixteen times as loud.
Another measurement is dB(C), which includes lower frequencies that are not so much heard as felt and have adverse medical and psychological effects. Lower-frequency sounds more easily penetrate walls and windows and are a significant component of wind turbine noise. Yet another measurement is dB(G), which includes very-low-frequency infrasound, which recent research shows the inner ear to be sensitive to.
Noise measurements may be expressed as L10, the level exceeded 10% of the time (generally taken as the level which will be found annoying), L90, the level exceeded 90% of the time (generally taken as the background ambient level), and Leq,x, the average level over x time. The Ldn, or day-night average over 24 hours, with 10 dB added to the night-time levels, is used to compare noise levels before and after a new source is added to the environment. Following ANSI standards, 5 dB should be added to recorded levels of unfamiliar sounds and 10 dB should be added in rural areas where there is an expectation of peace and quiet. Studies of wind turbine noise find that “high annoyance” occurs at levels 20-30 dB lower than with other noises. Furthermore, in predicting noise levels multiple sources must be considered (not just the nearest turbine), and “line source” decay, which is half the rate of “point source” decay, must be used for facilities in a line, as on a mountain ridge.
Note that the swishing or thumping sound of wind turbines in time with their rotation frequency, which is associated with higher annoyance, is not usually reflected in the above measures, owing to the relative brevity of the peaks. This characteristic noise is called “blade swish” or “blade thump” and sometimes referred to with the nonspecific term “amplitude modulation”. It is likely caused primarily by different air densities and/or wind speeds between the top and bottom of the sweep area of the blades. In the rulings allowing the Den Brook Wind Farm in England to proceed, conditions included consideration of excess amplitude modulation upon complaint as any change, outside the dwelling, in Leq,125ms of >3 dBA in any 2-second period 5 times or more in any minute with Leq,1min ≥28 dBA and such excess occurring in 6 or more of the minutes in any hour.