Infrasound

Infrasound refers to sound waves with frequencies below the lower limit of human hearing, typically under 20 Hz. Because of their long wavelengths, infrasonic waves can travel long distances through the atmosphere with relatively low attenuation, especially when guided by atmospheric temperature and wind structure in what are called ducts. They are detected by specialized sensors such as microbarographs and highly sensitive microphones, and by hydrophones for airborne or oceanic phenomena that couple into the air.

Common natural sources include thunderstorms, volcanic eruptions, earthquakes, meteors entering the atmosphere, and ocean processes that generate atmospheric pressure waves (often called infrasonic waves from ocean activity). Some animals, such as elephants and certain whales, are believed to use infrasound for long-range communication.

Detection and monitoring: Data collected by networks of sensors can be used for weather research, atmospheric science, volcanic and seismic monitoring, and detection of nuclear tests (CTBT IMS). Infrasound can complement meteorological data by providing information on distant, quasi-stationary sources and atmospheric conditions that affect sound propagation.

Health and safety: Infrasound is below human hearing and normally produces no conscious perception; at typical environmental levels it has minimal effects. However, very high-amplitude infrasonic fields, such as those near powerful industrial sources or during extreme natural events, can cause discomfort or pressure sensations in some individuals; scientific evidence on health effects is limited and ongoing. The field continues to develop with improvements in sensing technology and modeling of atmospheric ducts.