Infrasound - Wikiparency

# Infrasound **Infrasound** is [[Sound|sound]] with frequency below about 20 Hz, the accepted lower edge of human hearing.[^up17-1] It is not a different kind of wave from the sound in the middle of a piano keyboard — it obeys the same wave equation and travels at the same [[Speed_of_sound|speed of sound]] — only its wavelength is enormous. A 2 Hz wave in air, moving at 343 m/s, has a wavelength of about 171 m, longer than the buildings, wind turbines and vehicles that produce it, which is one reason it registers in the body as a pressure or a rumble rather than as a pitch. The article's microsim, *Infrasound: waves longer than buildings*, sets a wave alongside a wind-turbine tower and a house and lets the reader change its frequency, the propagating medium and the listening distance to see the scale and the reach of the wave together. Infrasound sits at the low end of a spectrum most of which cannot be heard at all: below roughly 20 Hz, humans stop perceiving oscillation as a tone and, at high enough amplitude, start perceiving it as a felt vibration or a sensation of pressure. Its long wavelength gives it an unusual property among audible sounds — air absorbs it far more weakly than it absorbs higher frequencies, so infrasound generated by a storm, a volcano or a nuclear test can be detected thousands of kilometers from its source. ## History and study Systematic study of infrasound followed the events large enough to generate it. The 1883 eruption of Krakatoa produced a pressure wave that barographs around the world recorded as it circled the globe several times, an early demonstration that very-low-frequency sound could travel such distances with so little loss. Twentieth-century interest grew with the recognition that explosions, volcanic eruptions, meteors, ocean waves and the atmosphere's own turbulence all generate infrasound, and that a network of sensitive microbarometers could listen for all of them at once. ## Sources Natural and human sources of infrasound span an enormous range of scale. Ocean waves interacting with each other generate a nearly continuous "microbarom" hum near 0.2 Hz, present in the background of the atmosphere worldwide.[^garces2013] Severe weather, volcanic eruptions, earthquakes, meteors entering the atmosphere, and avalanches all radiate infrasound as a byproduct of the violent motion of large volumes of air, rock or ice. Elephants communicate over kilometers using calls with strong content down to about 14–35 Hz, near the boundary between audible sound and infrasound and low enough that human observers standing nearby feel more than they hear.[^garces2013] Among engineered sources, a modern wind turbine sheds infrasound near 1 Hz from the periodic passage of its blades past the tower, and a large diesel engine or an industrial fan can produce strong tones in the same low range.[^garces2013] *Try: in the microsim, set the medium to air and slide frequency down toward 1–2 Hz to see the wavelength stretch past the house and the turbine tower; the tower in the model itself sheds a slow rotor tone near the frequencies described above.* ## Animal reaction Several species are reported to react to infrasound in ways not fully explained by direct hearing. Elephants use very low frequencies in their calls, which some researchers interpret as long-range communication that exploits infrasound's low atmospheric absorption. Homing pigeons and some migratory birds have been proposed to sense infrasound from ocean waves or mountain ranges as a navigational cue, though the mechanism remains debated. Reports of unusual animal behavior before large earthquakes are sometimes attributed to infrasound or ground-borne vibration preceding the shaking felt by people, but the evidence for a specific infrasound trigger is not settled. ## Human reactions Human sensitivity to infrasound is a matter of degree rather than a hard cutoff at 20 Hz: at high enough sound pressure levels, frequencies well below 20 Hz can be felt, and some listeners report an ability to perceive tones down to about 10 Hz or lower as a vague pulsation rather than a pitch. Reported physiological effects of strong infrasound exposure include feelings of unease, pressure in the chest or ears, fatigue and, at high levels associated with some industrial or military settings, more severe complaints; the evidence linking moderate-level infrasound (of the kind produced by wind turbines, for example) to health effects is contested and remains an active area of study. ### Hygienic standards in the workplace Occupational health standards for infrasound exposure exist in a number of countries, typically expressed as maximum sound pressure levels in specific low-frequency bands averaged over a working shift, following the same decibel logic used for audible noise. *Citation needed: the specific numeric limits vary by jurisdiction and standards body, and no single figure is verified here.* ### Brown note The "brown note" is a popular legend describing an infrasonic frequency claimed to cause involuntary loss of bowel control in anyone who hears it. It has been treated on television (notably tested, and not confirmed, by the MythBusters program) and has no basis in a verified acoustic mechanism; it survives as internet and broadcast folklore rather than as an established acoustic effect. ### Infrasonic 17 Hz tone experiment A widely reported 2003 experiment at a London concert hall played music that included a nearly inaudible 17 Hz tone from a large pipe during some pieces and not others; audience members reported unusual sensations, including unease and shivers, more often during the pieces that carried the tone, which the experimenters offered as evidence that infrasound near the edge of hearing could affect people who could not consciously detect it. *Citation needed: the original experiment (associated with physicist Richard Lord and reported via the British Association for the Advancement of Science and the National Physical Laboratory) is described from secondary summaries here rather than the primary study.* ### Suggested relationship to ghost sightings Some researchers, notably Vic Tandy, proposed that infrasound near 19 Hz, produced incidentally by equipment such as extractor fans, could explain some reports of a "haunted" feeling, blurred vision and unease in a room, on the grounds that 19 Hz is near the resonant frequency of the human eyeball and could distort vision without being consciously heard.[^tandy1998] The proposal is a hypothesis about one possible contributing factor in specific documented cases, not a general explanation for reports of hauntings. ## Detection and measurement Infrasound is measured with microbarometers — sensitive pressure sensors, rather than conventional microphones — because standard microphone diaphragms are poorly matched to such long wavelengths and low frequencies. Arrays of several microbarometers spread over hundreds of meters to kilometers let analysts determine the direction a wave arrived from by timing its arrival at each sensor, the same principle used in seismic arrays. ### Infrasound for nuclear detonation detection Infrasound monitoring is one of four technologies (with seismic, hydroacoustic and radionuclide monitoring) in the International Monitoring System built to verify compliance with the Comprehensive Nuclear-Test-Ban Treaty. The system operates a global network of 60 certified infrasound stations, each typically using an array of four or more microbarometers, designed to detect the low-frequency pressure wave from a nuclear explosion anywhere on Earth and distinguish it from other sources such as volcanoes, meteors and industrial explosions.[^ctbto] *Try: push the distance slider out toward 1,000 km at low frequency and watch the absorbed-dB bars stay small next to the 200 Hz comparison wave — the same weak absorption that lets the treaty's infrasound network listen across continents.* ## In popular culture Infrasound appears frequently in fiction and film as a source of dread, disorientation or invisible danger, often drawing loosely on the ghost-sighting hypothesis or the brown-note legend rather than on the measured, mostly benign physics of very-low-frequency sound. ## Minnesota *This section is specific to Wikitube.* A transformer at a Minnesota substation hums audibly at 120 Hz — not infrasound, but a useful comparison inside the same low-frequency neighborhood, since it comes from the same physics of a vibrating structure radiating a wave far longer than the object producing it. See [[Mains_hum]] for the mechanism. ## See also - [[Aeolian_sound]] - [[Mains_hum]] - [[Sound]] - [[Vibration]] - [[Speed_of_sound]] - [[Noise_control]] ## References [^up17-1]: OpenStax, *University Physics Volume 1* (2016), ch. 17.1 "Sound Waves": the audible range for human hearing is about 20 Hz to 20,000 Hz; sound below 20 Hz is called infrasound and sound above 20,000 Hz is called ultrasound. Also 17.2 "Speed of Sound," Table 17.1 and Eq. 17.7 (speed of sound in air 331 m/s at 0 degrees C, 343 m/s at 20 degrees C). [^garces2013]: Garcés, Milton (2013). "On infrasound standards, Part 1: Time, frequency, and energy scaling." *InfraMatics* 2, 13–35. https://doi.org/10.4236/inframatics.2013.22002 — ocean-swell microbaroms near 0.2 Hz, wind-turbine blade-pass tones near 1 Hz, elephant call content in the 14–35 Hz range. [^tandy1998]: Tandy, Vic; Lawrence, Tony R. (1998). "The ghost in the machine." *Journal of the Society for Psychical Research* 62, 360–364. Proposed link between a 19 Hz standing wave from a fan and reports of a haunted feeling in one documented room. [^ctbto]: Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO). "Infrasound Monitoring." https://www.ctbto.org/verification-regime/monitoring-technologies-how-they-work/infrasound-monitoring — describes the International Monitoring System's global infrasound station network.  **Microsim — three.js (Wikitube framework):** *Infrasound* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/acoustics/Infrasound.html" data-title="Infrasound"></div> *Built from `MICROSIM_GUIDE/specs/acoustics/sims/Infrasound.json`; part of the [[PORTAL_Acoustics|Acoustics portal]] spine (section sims and See-also variants).*  ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Infrasound) : [Wikitube](https://en.wikitube.io/wiki/Infrasound) - skeleton pinned to revision 1362773588 (2026-09-11).