Infrasonic refers to sound waves with frequencies below 20 hertz (Hz), the lower threshold of typical human hearing. These ultra-low-frequency waves are produced by everything from volcanic eruptions to ocean swells, and while you can’t hear most of them in the conventional sense, they can travel extraordinary distances and, at high enough intensities, produce physical sensations in the human body.
How Infrasound Differs From Audible Sound
Sound is a pressure wave moving through air (or water, or solid ground). What distinguishes one sound from another is its frequency, meaning how many wave cycles pass a given point each second. Human hearing spans roughly 20 Hz to 20,000 Hz. Anything below 20 Hz falls into the infrasonic range.
That 20 Hz cutoff is a useful shorthand, but it’s not a hard wall. Researchers have measured human hearing thresholds down to about 1.5 Hz when the sound is loud enough. At those frequencies, you don’t “hear” in the usual sense. Instead, you feel pressure changes in your ears and chest, or perceive a pulsing, rumbling quality that’s more physical than auditory. The lower the frequency, the more sound pressure is required for any perception at all.
Infrasonic waves also behave differently from higher-pitched sound. Because their wavelengths are enormous (ocean swells generate infrasound around 0.5 Hz, corresponding to wavelengths on the order of a kilometer), they pass through walls, terrain, and atmospheric layers with very little energy loss. This is why infrasound from a major event can be detected on the other side of the planet.
Natural Sources of Infrasound
Nearly every large-scale geophysical event produces infrasound. Volcanic eruptions, earthquakes, meteor impacts, tornadoes, lightning, and even the aurora borealis all generate waves below 20 Hz. Ocean swells are a constant, global source.
The most dramatic historical example dates to August 26, 1883, when the Krakatoa volcano in Indonesia erupted with enough force that its infrasonic pressure wave registered on barometers in London, over 10,000 kilometers away. Twenty-five years later, the Tunguska asteroid explosion in Siberia, estimated at 1,000 times the power of the Hiroshima bomb, was similarly detected thousands of kilometers from the blast site.
Thunder offers a more everyday example. When lightning strikes several kilometers away, the sharp crack has already been absorbed by the atmosphere. What reaches you is the low rumble: the infrasonic portion of the original sound spectrum, because only the lowest frequencies survive over distance.
Human-Made Sources
Modern environments are full of infrasound. Wind turbines, heavy industrial machinery, mining operations, trains, aircraft, and dense traffic all produce low-frequency waves, typically in the 1 to 10 Hz range for transportation and urban sources. Ventilation systems in large buildings are another common culprit, particularly when ductwork or mechanical components create resonant vibrations.
Rocket launches and even satellite re-entries generate significant infrasound. On the military side, chemical and nuclear explosions produce distinctive infrasonic signatures, which is why a global network of monitoring stations exists specifically to listen for them.
How Infrasound Affects the Body
At everyday exposure levels, infrasound from traffic or building ventilation is too weak to cause harm. But at higher intensities, the body responds in noticeable ways, because certain organs and structures have resonant frequencies that fall within the infrasonic range.
The vestibular system, the balance-sensing apparatus in your inner ear, is most sensitive to infrasound around 7 Hz. Frequencies below about 40 Hz can also affect the semicircular canals, the chest cavity, and the abdominal cavity. This explains the cluster of symptoms that shows up in case reports of high-intensity infrasound exposure: dizziness, vertigo, nausea, ear pressure, chest pressure, a sensation of vibrations running through the body, difficulty concentrating, and fatigue. Frequencies in the 50 to 60 Hz range (technically low-frequency sound rather than infrasound) fall in the chest’s resonance range and can cause whole-body vibration that produces discomfort even when the sound isn’t consciously perceived as loud.
A well-known investigation by a CDC/NIOSH team documented exactly these symptoms among employees in an administrative building. Workers near an industrial flare system reported vertigo, nausea, chest pressure, and emotional discomfort during loud noise events, even though the sound levels were well within occupational safety limits for conventional noise. The issue was the low-frequency content, which standard noise measurements can miss entirely.
The “Haunted Room” Effect
One of the most widely cited infrasound stories involves a 1998 report by researcher Vic Tandy, who was working in a factory laboratory where multiple employees described uneasy feelings and even seeing shadowy figures. Tandy himself reported glimpsing an apparition in his peripheral vision. He eventually traced the cause to a standing wave at 18.9 Hz generated by a newly installed extractor fan.
Tandy proposed that 18.9 Hz might be close to the resonant frequency of the human eyeball, causing it to vibrate and produce visual disturbances. Later researchers challenged this explanation, noting that eyeball vibration would be expected to distort the entire visual field rather than just peripheral vision, and that such vibration couldn’t plausibly produce complex, sustained hallucinations. The visual effects Tandy described haven’t been reliably reproduced in controlled settings. Still, the link between infrasound and a generalized feeling of unease or discomfort is well documented, even if the precise mechanism behind visual disturbances remains uncertain.
Animals That Communicate With Infrasound
Several species have evolved to exploit the long-range properties of infrasound. Elephants are the best-known example. Adult males live apart from female herds, and when females enter their fertile period, they broadcast infrasonic calls that travel through both air and ground vibrations, allowing males to detect them from kilometers away.
Baleen whales communicate across hundreds of kilometers using infrasonic frequencies that propagate efficiently through ocean water. Giraffes, once believed to be largely mute, were eventually found to produce low-pitched murmurs at frequencies so low that humans have difficulty detecting them without instruments.
How Infrasound Is Monitored
The Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) operates a worldwide network of infrasound monitoring stations. These stations use instruments called microbarometers, which measure tiny fluctuations in air pressure caused by passing infrasonic waves. Because infrasound travels vast distances with little energy loss, this network can detect atmospheric nuclear explosions anywhere on Earth.
The same stations routinely pick up volcanic eruptions, large meteorite entries, rocket launches, and severe weather events. In practice, infrasound monitoring has become a multipurpose tool for tracking everything from illegal mining blasts to the trajectories of space debris re-entering the atmosphere.
Exposure Limits and Safety Standards
Workplace exposure limits for infrasound exist, but they’re less standardized than limits for conventional noise. In the United States, the only occupational limits specifically covering low-frequency noise and infrasound are set at 145 decibels for individual frequency bands between 1 and 100 Hz, and 150 decibels for the overall sound pressure level across that same range. These are extremely high thresholds, well above what most workers would encounter.
Several European countries have established lower, more protective indoor guidelines. Germany, Poland, the Netherlands, and the United Kingdom each set their own frequency-specific limits for indoor low-frequency noise, covering ranges as low as 8 Hz. These guidelines are designed to address comfort and health rather than just preventing acute harm, reflecting a growing recognition that chronic exposure to even moderate infrasound in workplaces and homes can cause persistent annoyance and physical symptoms.