Infrasound (noun, “IN-fruh-sownd”)
Infrasound refers to sound pitched too low for a human to hear.
“Infra” means below. So think of infrasound as sound so deep it’s below human detection. (Sound too high-pitched for humans to hear is called ultrasound.)
Sound is energy that moves as waves through air, water or another substance. Your ears detect sound waves as variations in air pressure. If you touch a speaker playing music, you feel these vibrations.
The length of a sound’s wave — or wavelength — determines its frequency. Shorter wavelengths come with higher frequencies. You hear high-frequency sounds as those with high pitches. Think of a singing bird, a mouse’s squeak or a whistle. Longer wavelengths come with lower frequencies. Low-frequency sounds have lower pitches. A lion’s roar, a rumble of thunder or the bass line in a song are all low pitches.
Frequency can be measured in kilohertz (kHz). This unit refers to the number of vibrations that pass a point each second. Anything below 20 kHz is too low for human ears to hear. So any sound below 20 kHz is considered infrasound.
Scientists use infrasound sensors to listen for these super low-frequency sounds. Changes in deep resonant rumblings can alert scientists to impending geological events. Underground movements can create infrasound rumbles in the atmosphere. And such sounds can provide early warnings of earthquakes or volcanic eruptions.
Humans may not be able hear these ultra-deep tones, but some animals can. For example, whales “sing” in infrasound. The long sound waves of their voices can travel many kilometers (miles) through ocean water. On land, elephants use these deep calls to communicate over long distances to their herds.
In a sentence
Scientists predicted the 2018 eruption of Hawaii’s Kilauea from changes in infrasound rumblings.
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