Infrasound is sound consisting of mechanical pressure waves with frequencies below the conventional lower limit of human hearing, generally below about 20 hertz. Like ultrasound, infrasound is not a fundamentally different physical type of wave from ordinary sound. It is distinguished primarily by frequency relative to human auditory sensitivity. Infrasound can have very long wavelengths because wavelength equals propagation speed divided by frequency.
In air at roughly 343 metres per second, a 1 hertz sound has a wavelength of approximately 343 metres, while a 0.1 hertz sound has a wavelength of approximately 3430 metres. These long wavelengths influence diffraction, atmospheric propagation and interactions with structures and landscapes. Humans can sometimes perceive sufficiently strong frequencies below 20 hertz as sensations or individual pressure fluctuations, so the conventional boundary should not be treated as an absolute physiological wall.
Infrasound is generated by both natural and human activities. Natural sources include volcanic eruptions, earthquakes, large storms, ocean waves, meteors entering the atmosphere, avalanches and some atmospheric disturbances. Large explosions and powerful industrial or mechanical events can also generate infrasonic waves. Because low frequency waves can propagate long distances with relatively low attenuation under suitable atmospheric conditions, distant sources can sometimes be detected using sensitive instruments. Atmospheric pressure variations associated with large storms can contain infrasonic components, while volcanic eruptions can generate signals useful for monitoring eruptive activity. The physical source mechanism differs among these phenomena, but all can produce pressure disturbances whose spectral energy extends into the infrasonic range.
The atmosphere can guide or refract infrasound over long distances. Sound speed depends on temperature, wind and atmospheric composition, and vertical variations can bend acoustic rays. Wind direction and speed can significantly influence propagation because the effective ground relative velocity depends on the direction of travel. Temperature structures can create channels through which acoustic energy travels unusually far.
Atmospheric absorption is generally weaker at very low frequencies than at high frequencies, although propagation remains affected by geometry, turbulence and environmental conditions. These characteristics make infrasonic monitoring useful for detecting distant large scale events. Analysis often combines measurements from multiple stations so that arrival times and directions can be compared and source locations estimated.
Infrasound detection uses highly sensitive instruments capable of measuring extremely small pressure fluctuations. Microbarometers are common instruments for monitoring atmospheric pressure variations in the infrasonic frequency range. Because ordinary atmospheric pressure changes associated with weather are much larger than many infrasonic signals of interest, instruments and processing methods must distinguish coherent acoustic fluctuations from meteorological noise. Wind noise around the sensor can be particularly problematic, so porous filters, wind noise reduction systems and specialised sensor installations are used. Signal processing can identify characteristic frequency ranges, arrival patterns and coherent signals across sensor arrays. Arrays of instruments provide directional information because the same wave reaches spatially separated sensors at slightly different times.
Infrasound has an important role in atmospheric and geophysical research. The International Monitoring System associated with the Comprehensive Nuclear Test Ban Treaty includes infrasonic stations designed to detect atmospheric disturbances from large explosions and other events. Infrasound can also complement seismic, satellite and meteorological observations when studying volcanic eruptions, atmospheric waves and other large scale phenomena. Because a single signal can propagate over large distances, infrasonic observations provide information about remote events that may not be directly accessible. However, identifying the source can be difficult because different physical processes can generate similar low frequency signatures. Source attribution therefore requires comparison with other evidence and careful analysis of propagation conditions.
Infrasound also occurs in human environments. Large machinery, ventilation systems, aircraft, wind turbines, explosions and industrial processes can produce low frequency pressure fluctuations. The presence of measurable infrasound does not automatically mean that it is harmful or perceptible. Human responses depend on sound pressure level, frequency distribution, exposure duration and individual sensitivity. At sufficiently high amplitudes, very low frequency sound can be perceived through hearing or other bodily sensations, but claims about health effects require controlled evidence and should not be inferred merely from the existence of an infrasonic signal. Measurement standards and exposure assessments therefore distinguish physical presence from audibility, annoyance and established biological effects.
Infrasound differs from ordinary audible low frequency sound primarily by frequency, but the transition is not sharply identical for every person. Human hearing sensitivity decreases substantially at low frequencies, so sounds near or below 20 hertz require increasingly high sound pressure levels to become audible. The auditory system can nevertheless respond to low frequency acoustic energy under appropriate conditions. Animals have different hearing ranges, meaning a signal that is infrasonic for humans can fall within the hearing range of another species. The scientific classification therefore uses a conventional frequency boundary for human acoustics rather than implying that all organisms share the same hearing threshold.
Infrasound is important because very low frequency sound can carry information over large distances and can be generated by powerful natural and technological processes. Its long wavelengths, low frequency and atmospheric propagation characteristics make it particularly useful for monitoring large scale events. At the same time, interpreting infrasonic measurements requires careful treatment of meteorological noise, atmospheric refraction, sensor characteristics and source ambiguity. Infrasound therefore occupies an important intersection between acoustics, atmospheric physics, geophysics, environmental monitoring and planetary science, where very slow pressure oscillations can reveal events occurring far beyond the immediate measurement location.
