I was at a wind energy workshop speaking about noise issues. There I was asked several questions about low-frequency and infra- sound and its impact on health. Because of the questions at the workshop, I decided to write this note.
In this blog, I will highlight what is known about infrasound and low-frequency sound issues and the conclusions of an expert panel of physicians, otolaryngologist, and physicists with expertise in noise, vibrations and safety. The panel was organized by the American Wind Energy Association (AWEA) and Canadian Wind Energy Association (CANWEA). Let us start with the conclusions:
“1. Sound from wind turbines does not pose a risk of hearing loss or any other adverse health effect in humans.
2. Subaudible, low frequency sound and infrasound from wind turbines do not present a risk to human health.
3. Some people may be annoyed at the presence of sound from wind turbines. Annoyance is not a pathological entity.
4. A major cause of concern about wind turbine sound is its fluctuating nature. Some may find this sound annoying, a reaction that depends primarily on personal characteristics as opposed to the intensity of the sound level.”
The conclusions clearly state that the sounds generated by wind turbines are not harmful to human health. Next, let us dig deeper.
What is low frequency and infra sound?
Low frequency is sound with a frequency range of 10Hz to 200Hz. Infrasound corresponds to frequency below 20 Hz, the lower limit of audible frequency at normal sound amplitude. Note at sufficiently high volume (amplitude) infrasound is audible; therefore the exact frequency that defines infrasound is not precise.
Why focus on low frequency and infra sound?
The speed of sound in air is about 340 m/s. The product of frequency and wavelength is equal to the speed of sound. For example, if frequency is 20 Hz, then the wavelength is 17m, which is large. As it turns out most of the common noise (due to traffic, airports) inside a house is low frequency because walls and other barriers easily block higher frequency noise. This low frequency noise can be annoying if the noise level is high, as measured by dBA.
At 20 Hz, a noise level of 80 dBA is barely audible. At lower frequencies, the noise level has to be even higher for it to be audible. As a result, in almost all situations in which the turbine is outside a radius of 150m from a residence, the low frequency sound is barely audible because the noise level in terms of dBA is in the range of 50 to 60 dBA. At such low noise level, the sound may be annoying, but has no impact on health.