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Infrasound is sound with a frequency too low to be detected by humans. These range from about 17 to 16 hertz, the limits of human hearing, to as low as 0.001 hertz. A sound wave that is 1 hertz has a period of one second. The range of human audibility is around 20 to 20,000Hz, with women capable of hearing slightly higher frequencies, an ability which decreases with age.

Numerous animals, including whales, elephants, rhinoceros, giraffes, okapi, and alligators, communicate using infrasound. Infrasound can be thought of as a slow, periodic rumbling - it is generated by large processes such as avalanches, volcanoes, tornadoes, ocean waves, earthquakes, and meteors. It can be generated by large chemical or nuclear explosions. Because it is one of the telltale signs of nuclear testing, infrasound is continuously monitored worldwide by the Comprehensive Nuclear-Test-Ban Treaty Organization.