Editor’s note: Rick James is, without doubt, one of North America’s premier experts on wind turbine noise. Unlike the great majority of noise engineers who have sold their souls and ethics to the wind energy industry, Mr. James can’t be “bought.” Together with Rob Rand and Steve Ambrose, Rick has exposed the deceit and mendacity of wind company acoustic consultants — as in their fraudulent use of A-weighted noise measurements, for instance.
We all owe these three gentlemen a huge debt of gratitude.
As the blade passes the tower, the low frequency noise and infrasound is generated at a frequency related to the hub’s rotation and number of blades. These pressure pulsations appear as tones during analysis, but are not heard as tones by most people. Instead, they may feel the pressure changes as pulsations, internal organ vibrations, or as a pain (like ear aches or migraines).
This frequency is called the Blade Pass Frequency, often abbreviated as BPF.
For modern utility-scale wind turbines, this frequency is at 1Hz or lower. A three-bladed wind turbine with a hub rotation of 20 revolutions per minute (rpm) has a BPF of 1Hz. This means there is a pressure pulsation emitted into the community once every second. At 15 rpm the BPF is 0.75 Hz; and at 10 rpm, 0.5 Hz.
When wind turbine blades rotate past the tower, a short pressure pulse occurs, producing a burst of infrasound. When analyzed, the result is a well-defined array of tonal harmonics below 10 Hz.
For impulsive sound of this type, the harmonics are all “phase-correlated.” This means the peaks of each occur at the same time. Thus, the peaks add together in a linear fashion, with their individual maximum sound pressures all coinciding.
Thus, for an impulse having 4 equal amplitude harmonics (BPF, 2nd, 3rd and 4th) each of the same amplitude, the peak level is +12 dB. Ten equal harmonics would produce a peak level of +20 dB.