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Infrasound: sound below 20 Hz
Infrasound is sound with a frequency below about 20 hertz, under the usual range of human hearing, which runs from about 20 Hz to 20,000 Hz. Volcanic eruptions and large explosions produce it, and scientists monitor it to detect them; wind turbines produce it too, usually at or below the background level. At everyday levels it goes unnoticed.
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Where infrasound comes from
| Source | Notes |
|---|---|
| Volcanic eruptions | Monitored with infrasound sensors, because the waves travel far through the atmosphere. |
| Large explosions | Infrasound stations are used to detect them, including nuclear tests. |
| Wind turbines | Measured near wind farms; usually at or below the background level. |
Infrasound, audible sound and ultrasound
Sound is a pressure wave, and the frequency is how many times a second it repeats. People hear roughly 20 Hz to 20,000 Hz; below that is infrasound and above it ultrasound. The edge at 20 Hz is not sharp: sensitivity falls steadily towards it, which is why the deepest notes in the bass test are felt more than heard on most equipment. For how frequency and loudness relate, see hertz vs decibels.
Can humans hear infrasound?
Not as a tone. Hearing is usually said to start around 20 Hz; below that, sound at a high enough level is felt as pressure or vibration rather than heard as pitch, and it has to be far louder than an audible sound to be noticed at all.
Is infrasound from wind turbines harmful?
Large studies have not found that it is. Health Canada's wind turbine study found turbine infrasound was often below background levels and turbine noise was not linked to measured stress hormones, blood pressure, heart rate or sleep; a Finnish government study found infrasound caused no measurable changes in the body and listeners could not detect it. Annoyance from turbine noise is a documented effect.
How is infrasound measured?
With sensors built to respond below 20 Hz, like the ones volcano observatories and test-ban monitoring stations use. It is outside what this site's decibel meter is built for.
What is the opposite of infrasound?
Ultrasound: sound above about 20,000 Hz, too high for people to hear, used by bats, dolphins and medical imaging.
Sources: USGS, infrasound at Yellowstone; HyperPhysics; Infrasound; Health Canada, Wind Turbine Noise and Health Study; Yle, on the Finnish VTT infrasound study.