Robert O’Neal of Epsilon Associates, one of the study’s authors, said the work was commissioned by wind farm operator NextEra Energy because of the frequency with which concerns about low-frequency sound and infrasound are raised during the process of obtaining permits for wind energy farms.
He said Epsilon Associates conducted a literature search, to determine what work had been done on the specific subject; a field measurement program; and a comparison of the results with standards set by the American National Standards Institute (ANSI) and the Acoustical Society of America (ASA).
"The takehome message," O’Neal said, "is that for the factors considered–audibility, disturbance, and vibration or rattle, all sound levels for low-frequency noise and infrasound were below criteria [specified by standards]."
A paper presented on the work before the ASA in April says flatly, "Wind farms with Siemens SWT-2.3-93 and GE 1.5sle wind turbines at maximum noise at a distance more than 1,000 feet from a residence do not pose a low frequency noise or infrasound problem."
The normal human hearing range covers frequencies from 20 Hertz (Hz) to 20,000 Hz, although humans can perceive infrasound down to a few Hertz if the sound level is high. Low-frequency sound is defined as sound at frequencies between 12.5 Hz and 200 Hz, and infrasound at frequencies below 20 Hz.
According to O’Neal:
– Low-frequency sound and infrasound are found everywhere and are generated by sources such as traffic, fans and the wind: they are "not unique to wind turbines."
– While a typical area in downtown Boston generated sound levels between 60 and 70 decibels (dB), even a quiet rural area in Illinois showed levels of 40 to 65 dB for low-frequency sound and higher decibel levels of infrasound.
The field measurements in the study were carried out on two types of wind turbines found on NextEra’s Horse Hollow wind farm in Texas. Measurements were taken at 1,000 and 1,500 feet and the interiors and exteriors of four homes approximately 1,000 feet from turbine strings.
The study has been peer-reviewed and was published in the March-April 2011 issue of Noise Control Engineering Journal.
By Tom Gray,www.awea.org/blog/