Infrasound and low-frequency noise
Large wind turbines have been known since the early 1980s to produce infrasound and low-frequency noise (ILFN) that is intrusive and detrimental to health, both psychologically and physiologically.
Low-frequency noise is that at the deeper-pitched end of the human conscious hearing (i.e., “audible”) spectrum, usually defined as the range from 20 Hz to 160 Hz. The full audible spectrum for humans extends up to 20,000 Hz, although for most people it is more limited, and for others it may extend down to 10 Hz or even 5 Hz.
Infrasound is that below the normal audible spectrum, i.e., <20 Hz. It is “felt” rather than heard.
ILFN travels much farther with less dissipation than higher-frequency sound. It can go around barriers and penetrate – even resonate with – walls.
Most existing noise regulations use A-weighted noise measurement, which emphasizes the range of human conscious hearing. Thus they fail to take ILFN into account. (They usually also fail to consider amplitude modulation, another characteristic that makes wind turbine noise more annoying and which itself, according to Australian acoustician Steven Cooper, may be “felt” like infrasound as it modulates at infrasonic frequencies.)
In 2011, Denmark imposed a 20 dBA limit for low-frequency (10–160 Hz) noise inside homes. A few localities have imposed limits defined by C-weighted measurements in addition to A-weighted, e.g., 50 dBC at the property line.