What Is Infrasound? (Below 20 Hz)

Infrasound is a class of sound wave whose frequency falls below approximately 20 hertz (Hz) — the generally accepted lower limit of human hearing under the ANSI/ASA S1.1-2013 acoustic terminology standard. Because it lies below the audible range, infrasound is normally felt as pressure or vibration rather than consciously heard, though very intense infrasonic tones can occasionally be perceived as a faint pitch. The study of these frequencies, sometimes called “infrasonics,” covers a band that extends from about 20 Hz down to roughly 0.1 Hz, and in specialized geophysical monitoring, occasionally down to 0.001 Hz.

Infrasound is not a rare or exotic phenomenon — it is generated constantly, all around us, by sources ranging from ocean storms and earthquakes to wind turbines, HVAC systems, and even the low rumble elephants use to coordinate herd movements across kilometers of savanna. Because infrasonic wavelengths are very long (tens to thousands of meters) and are barely absorbed by air, they travel enormous distances with little loss of energy, bending around obstacles and buildings rather than being blocked by them. This single physical property explains why infrasound is both scientifically valuable — for tracking storms, volcanic eruptions, and nuclear tests — and, at high levels, a subject of ongoing health and environmental-noise research. This article explains what infrasound is, where it comes from, how it is measured, and what the evidence says about its effects on the human body.

What Frequency Range Counts as Infrasound?

Infrasound covers frequencies below about 20 Hz — the lower boundary of the standard human hearing range of roughly 20 Hz to 20,000 Hz (20 kHz). This 20 Hz cutoff is not a hard biological wall so much as a practical convention: hearing sensitivity declines gradually and steeply as frequency drops, so 20 Hz is where, for most healthy adults under normal listening conditions, a tone stops being perceived as a musical pitch and starts being felt as a series of separate pressure pulses.

At the opposite end of the audio spectrum sits ultrasound, defined as sound above roughly 20 kHz — the upper limit of human hearing. Infrasound and ultrasound are, in this sense, mirror images: both describe sound that exists physically and can be generated, transmitted, and measured, but that falls outside the narrow slice of frequencies human ears evolved to detect directly.

Because wavelength and frequency are linked through the speed of sound by the formula λ = c/f, infrasonic wavelengths in air are extremely long. Using 343 m/s (dry air, 20°C) as the reference speed of sound:

• At 20 Hz, the wavelength is about 17 meters (343 ÷ 20).
• At 10 Hz, the wavelength is about 34 meters.
• At 1 Hz, the wavelength is about 343 meters.
• At 0.1 Hz, the wavelength stretches to roughly 3.4 kilometers.

These very long wavelengths are the direct physical reason infrasound diffracts around buildings, mountains, and other obstacles instead of being blocked by them — a wave can only be effectively obstructed by objects comparable to or larger than its own wavelength, and few structures on Earth are kilometers wide.

Table 1: Infrasound, Audible Sound, and Ultrasound Compared

Can Humans Hear Infrasound?

Not in the ordinary sense, but the boundary is not absolute. Under laboratory conditions, using pure sine waves reproduced at very high sound pressure, trained listeners have been able to identify tones as low as about 12 Hz as a distinct pitch — several hertz below the conventional 20 Hz cutoff (a finding documented by acoustics researcher Harry F. Olson). Below roughly 10 Hz, most people stop perceiving a tone altogether and instead notice the individual pressure cycles as a pulsing sensation, sometimes felt as pressure at the eardrums or vibration in the chest, rather than as sound intensity in the conventional musical sense.

This is closely tied to how loudness perception works at low frequencies. As frequency drops from about 1,000 Hz downward, the ear’s dynamic range compresses — the gap between “just audible” and “uncomfortably loud” narrows sharply. This compression means a very low-frequency sound that is essentially inaudible to one person can register as loud, even bothersome, to another with only a modest boost in level, which is exactly why infrasound perception varies so widely between individuals and why decibel values alone don’t fully capture how “loud” a very low tone feels. This variability is one reason sound pressure level measurements of infrasound are usually paired with frequency-specific weighting rather than a single flat number — see the measurement section below.

What Natural Sources Produce Infrasound?

Infrasound is generated constantly by large-scale natural processes, most of which involve a sudden or repetitive disturbance of a very large volume of air, water, or rock:

• Earthquakes and volcanic eruptions. Seismic events and volcanic activity generate infrasound as a byproduct of the same energy release that produces ground shaking; volcano infrasound has been used since events like the 1991 Mount Pinatubo eruption to study eruption dynamics from a distance.
• Severe weather and tornadoes. The U.S. National Oceanic and Atmospheric Administration’s (NOAA) infrasonics research has shown that infrasonic arrays on the high plains can detect tornado-generating storms several minutes before a tornado actually touches down, and can also help locate avalanches in the Rocky Mountains.
• Ocean waves (microbaroms). Nonlinear interactions between opposing ocean swells inside storms generate a persistent, low-level infrasonic “hum” near 0.2 Hz, known as microbaroms — the dominant natural background signal in the 0.02–10 Hz infrasound band worldwide.
• Meteors (bolides) and lightning. Large meteors entering the atmosphere generate strong infrasonic shockwaves; the 2013 Chelyabinsk meteor produced the loudest infrasound signal recorded by the global monitoring network at that time, detected by 20 separate stations before being “dwarfed” nine years later by the 2022 Hunga Tonga–Hunga Ha’apai volcanic eruption, which registered on all 53 stations of that network.
• Surf, waterfalls, avalanches, and calving icebergs all generate broadband infrasound as a side effect of large volumes of fluid or ice being displaced rapidly.

What Man-Made Sources Produce Infrasound?

Human activity generates infrasound just as readily as nature does, usually as an unavoidable byproduct of moving large volumes of air or releasing large amounts of energy quickly:

• Wind turbines produce broadband infrasound and low-frequency noise below 20 Hz, partly from the aerodynamic pulse each blade produces as it passes the supporting tower. For a typical utility-scale turbine rotating at roughly 15 revolutions per minute with three blades, simple arithmetic (15 ÷ 60 × 3) gives a blade-pass rate of about 0.75 Hz — deep within the infrasonic band. This has made wind turbines the most publicly debated modern source of infrasound (see the health section below).
• Explosions, both chemical and nuclear, release intense, broadband infrasonic pulses, which is precisely why infrasound monitoring is one of the core technologies used to detect nuclear weapons tests.
• Sonic booms, diesel engines, HVAC and ventilation systems, industrial machinery, and rocket launches all generate significant infrasonic energy as ordinary operational byproducts.
• Large-scale audio equipment. Specially built subwoofers — an octave or more below typical commercial designs, and often around ten times the physical size — can reproduce genuine infrasound for research, cinema, or concert use, distinct from the “felt bass” of ordinary music subwoofers, which mostly operates just above 20 Hz.

Table 2: Example Infrasound Sources and Approximate Characteristics

How Do Animals Use Infrasound to Communicate?

Several large animal species rely on infrasound (or frequencies bordering on it) for long-range communication, largely because low frequencies travel farther with less atmospheric absorption than higher ones — the same physical advantage exploited by earthquake- and storm-monitoring networks.

• Elephants produce infrasonic “rumbles” in the roughly 15–35 Hz range, at source levels around 117 dB, which field studies suggest can carry through air for several kilometers — with estimates around 10 km under favorable atmospheric conditions — and even farther as ground-borne vibration detected through the feet, allowing separated herds to coordinate movement and locate mates.
• Baleen whales, including blue whales, produce some of the loudest and lowest-frequency calls in the animal kingdom. Blue whale calls have a fundamental frequency of roughly 8–25 Hz, with underwater source levels reported as high as about 188 dB (referenced to 1 micropascal, the standard underwater acoustic reference — not directly comparable to the 20-micropascal airborne reference used for ordinary decibel figures). Because water transmits low-frequency sound so efficiently, some baleen whale calls are believed capable of traveling hundreds or even thousands of kilometers.
• The Sumatran rhinoceros has been recorded producing sounds as low as roughly 3 Hz, described as bearing similarities to humpback whale song. Tigers produce roar components at 18 Hz and below, and domestic cat purring spans roughly 20–50 Hz — straddling the infrasound boundary.
• Hippopotamuses, giraffes, okapis, peacocks, and alligators have all been documented producing infrasonic or near-infrasonic vocalizations, and research has proposed that migrating birds, including homing pigeons, may use naturally occurring atmospheric infrasound (generated by wind flowing over distant mountain ranges and oceans) as a long-range navigational cue.

How Is Infrasound Detected and Measured?

Because standard measurement microphones and human ears both roll off sharply below 20 Hz, infrasound requires specialized instruments called microbarometers — essentially highly sensitive barometric pressure sensors — rather than conventional microphones. Modern research systems, including designs developed at NASA Langley, use large-diaphragm electret condenser microphones paired with wind-noise-reducing “space filters” (long perforated pipes) that average out turbulent wind pressure fluctuations so that genuine infrasonic signals can be distinguished from background gusts.

For frequency weighting — analogous to the A-weighting curve used for ordinary environmental noise measurements described in dB(A) vs dB(C) vs dB(Z) — infrasound and low-frequency noise assessments commonly use a specialized “G-weighting” curve, applied roughly across the 8–40 Hz band, to approximate how the ear’s declining sensitivity at very low frequencies affects the perceived loudness of a measured sound pressure level.

The most extensive infrasound monitoring network in the world is the International Monitoring System (IMS) operated in support of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO). It comprises 53 infrasound stations, each built from an array of eight microbarometers spread across roughly 1 to 9 square kilometers, alongside seismic, hydroacoustic, and radionuclide sensors, used to verify compliance with the global nuclear test ban. Because infrasound below about 20 Hz travels enormous distances with minimal atmospheric absorption, this same network can also detect and locate large natural events — it registered the 2013 Chelyabinsk meteor on 20 stations and the 2022 Hunga Tonga–Hunga Ha’apai eruption on every station in the network. NOAA operates a separate, complementary infrasonics research program used historically for tornado, avalanche, and severe-weather detection in North America.

Can Infrasound Affect Human Health?

This is the most actively researched — and most publicly contested — question about infrasound, largely because of the ongoing “wind turbine syndrome” debate. The scientific picture, based on the peer-reviewed literature, is nuanced rather than settled in either direction:

• At very high sound pressure levels, controlled laboratory exposures have produced measurable but modest effects. In one 1972 study, 42 young men exposed to 7.5 Hz tones at 130 dB for 50 minutes reported only drowsiness and a slight blood pressure increase. In a 1975 study, exposure to 1–20 Hz tones at levels up to 144 dB SPL produced no effects beyond mild ear discomfort.
• Some research has linked inaudible, low-level infrasound near wind turbines to reported symptoms such as ear fullness, pressure, tinnitus, annoyance, fatigue, and disrupted sleep, and one laboratory line of research has proposed that infrasound may stimulate the vestibular (balance) system in a manner resembling motion sickness. Other reviewers, however — including researchers examining the geographic distribution of reported “wind turbine syndrome” cases — have found that reported symptom clusters correlate more strongly with prior media exposure to the concept than with proximity to a turbine, consistent with a nocebo-effect explanation. Mainstream scientific and medical bodies generally treat “wind turbine syndrome” itself as an unproven, pseudomedical label rather than a confirmed diagnosis, while acknowledging that annoyance and self-reported sleep disturbance from combined audible-plus-infrasonic turbine noise remain legitimate, actively studied environmental health topics — closely related to the broader question of how noise affects sleep, focus, and health and to noise pollution generally.
• For occupational settings, the American Conference of Governmental Industrial Hygienists (ACGIH) publishes Threshold Limit Values (TLVs) specifically for infrasound and low-frequency sound: no more than 145 dB in any octave band from 1–80 Hz, and no more than 150 dB overall, considerably higher than the safe decibel levels thresholds (such as the well-known 85 dB(A) 8-hour reference used by NIOSH and OSHA) that apply to ordinary audible occupational noise and noise-induced hearing loss risk.
• Extreme exposures are a different matter entirely: vibration tests conducted for the U.S. space program, transferring low-frequency energy mechanically into cockpit seats at up to 160 dB across 0.5–40 Hz, produced motor ataxia, nausea, visual disturbance, and impaired task performance in test subjects — but these were direct mechanical (bone- and body-conducted) exposures at extreme levels, not representative of ordinary environmental airborne infrasound, since air is a comparatively inefficient medium for transferring low-frequency vibration into the human body.

Myth vs. Fact

Myth: Infrasound is a modern conspiracy theory invented to demonize wind turbines.
Fact: Infrasound has been formally studied since at least 1957, when French scientist Vladimir Gavreau traced unexplained, nausea-inducing sensations among his research team to a 7 Hz resonance excited in the ductwork of their building by a slow-speed motor — decades before wind turbines existed at utility scale. Infrasound is a genuine, well-documented physical phenomenon generated by earthquakes, ocean storms, and animals long before it became associated with wind energy; what remains actively debated in the peer-reviewed literature is specifically whether ambient infrasound at levels typical near modern wind turbines causes direct physiological harm, separate from the established fact that infrasound itself exists and can be measured.

Myth: A “brown note” — a specific infrasonic frequency — can cause involuntary loss of bowel control.
Fact: This popular claim has never been demonstrated. When the television program MythBusters tested frequencies down to 5 Hz at sound pressure levels up to 153 dB using a specially modified concert subwoofer, no such physiological effect occurred, and the show declared the claim “busted.” No published acoustic research has confirmed a “brown note” produced through airborne transmission.

Why Does Infrasound Matter?

Infrasound sits at the low-frequency foundation of the acoustic spectrum, but its practical reach is enormous. It is the basis of global nuclear-test verification and severe-weather early-warning research; it explains long-distance animal communication across landscapes and oceans; it factors into environmental-noise assessments near wind farms and heavy industry; and it connects directly to fundamental acoustic concepts covered throughout this reference — from resonance and standing waves (the mechanism behind Gavreau’s original 1957 discovery) to the sound pressure level and weighting curves used in every modern noise assessment. Understanding infrasound is really an extension of understanding frequency itself, pushed to its lowest practical extreme — the mirror image of ultrasound at the opposite end of the spectrum.

Frequently Asked Questions

What frequency range is considered infrasound?
Infrasound generally refers to sound frequencies below about 20 Hz, per the ANSI/ASA S1.1-2013 acoustic terminology standard. The field of “infrasonics” extends this down to roughly 0.1 Hz, and occasionally to 0.001 Hz in specialized geophysical monitoring.

Can humans hear infrasound at all?
Not in the normal sense, but the boundary is soft. At very high sound pressure levels under ideal test conditions, trained listeners have identified pure tones as low as about 12 Hz. Below roughly 10 Hz, people generally stop perceiving a pitch and instead feel individual pressure pulses, sometimes as a sensation at the eardrums.

Is infrasound dangerous to human health?
Controlled studies at high sound pressure levels (up to 144–150 dB) have found effects limited mostly to mild discomfort or drowsiness, and occupational exposure limits set by the ACGIH allow up to 145 dB in the 1–80 Hz octave bands. Claims linking ambient, low-level environmental infrasound (such as from distant wind turbines) to serious illness remain scientifically contested, with some researchers attributing widely reported symptom clusters to a nocebo effect rather than direct physiological harm.

What are the main natural sources of infrasound?
Major natural sources include earthquakes, volcanic eruptions, severe weather and tornadoes, ocean-wave interactions in storms (microbaroms, around 0.2 Hz), large meteors, avalanches, and waterfalls.

How do elephants use infrasound?
Elephants produce infrasonic “rumbles” around 15–35 Hz at source levels near 117 dB, used to communicate with other herd members across several kilometers of air and, through ground vibration sensed via their feet, over even greater distances — helping coordinate movement and locate potential mates.

What is the difference between infrasound and ultrasound?
Infrasound refers to frequencies below about 20 Hz, and ultrasound to frequencies above about 20 kHz — the two bands lying just below and just above the human hearing range of roughly 20 Hz to 20 kHz, respectively. Both are physically real sound waves that exist outside human audibility, differing mainly in how they propagate: infrasound travels enormous distances with little loss, while ultrasound is highly directional and attenuates quickly, which is why it is used for close-range imaging and detection instead of long-range signaling.

Soft CTA

Curious where infrasound fits in the bigger picture of acoustics? Start with the pillar guide to sound and acoustics fundamentals, or continue exploring the frequency spectrum with our guides to frequency, the human hearing range, and ultrasound.