The term "infrasonic" applied to sound refers to sound waves below the frequencies of audible sound, and nominally includes anything under 20 Hz.
Sources of infrasound in nature include volcanoes, avalanches, earthquakes and meteorites. The eruption of the Fuego volcano in Guatamala produced infrasonic sound in excess of 120 decibels in the range below 10Hz. Measurements on Mt Erebus, an active volcano in Antarctica, found very strong ultrasonic sounds while the audible sounds were unremarkable. Sound monitors on the Sakurajima volcano of Japan measured sharp signals just before an eruption. Ocean storms and waves generate a lot of infrasound. Early studies of infrasound of hurricanes offer some hope of deciphering the infrasound signature of an approaching hurricane.
Monitoring of infrasound seems to be one of the best ways for detecting atmospheric nuclear tests. As of 2004 there were 24 such monitoring stations out of a projected total of 60. While no nuclear tests have been detected, in 2003 10 stations in the U.S. and Canada monitored the explosion of the space shuttle Columbia. A station in Fairbanks, Alaska detected the explosion of dynamite five miles from the detector.
Infrasound detectors are used in Teton Pass, Wyoming to detect the frequent avalanches and send warning signals.
A number of animals produce and use sounds in the infrasonic range. The rumbling vocalizations of elephants were measured to have frequencies as low as 14 Hz which were detectable at a range of 10 km. Observations of elephant behavior suggests that they responded to the waves through the ground before they heard them in the air - plausible since the waves would travel faster in the solid material. Whales and rhinos produce some very low frequency sounds. The flightless cassowary birds of Papua New Guinea and Australia emit low frequency calls around 23 Hz.
Auroral phenomena generate infrasound by the expansion of air accompanying the electrical discharges.
