Infrasound is sound that vibrates at frequencies below 20 hertz, the conventional lower limit of human hearing. Despite its reputation as “silent” sound, infrasound is actually audible to humans at sufficient intensity, with researchers measuring hearing thresholds down to 1.5 Hz. It surrounds us constantly, generated by everything from ocean waves and volcanic eruptions to highway traffic and industrial ventilation systems.
How Infrasound Differs From Normal Sound
Sound is a pressure wave moving through air (or water, or solid material). What distinguishes one sound from another is its frequency, measured in hertz, which is the number of wave cycles per second. Normal human conversation sits around 250 to 6,000 Hz. A bass guitar hits roughly 40 to 400 Hz. Infrasound occupies the territory below 20 Hz, where individual wave cycles stretch so long that your ears stop processing them as pitched tones.
At these low frequencies, the wavelengths become enormous. A 1 Hz sound wave is about 340 meters long. This means infrasound behaves differently from higher-pitched sound in important ways: it passes through walls, bends around obstacles, and travels vast distances with far less energy loss. A volcanic eruption’s infrasound can circle the globe. This persistence is what makes infrasound both fascinating and useful for monitoring distant events.
Natural Sources of Infrasound
The natural world is full of infrasound. Volcanoes are prolific generators, producing signals across a wide range, with most falling between 0.5 and 4 Hz. The largest eruptions create even deeper vibrations: massive Plinian eruptions (the kind that produce towering ash columns) generate frequencies as low as 0.1 Hz, and the thermal energy from major eruptions can excite the atmosphere into gravity waves oscillating at just 0.003 to 0.005 Hz, with periods of 200 to 300 seconds per cycle.
Earthquakes produce infrasound in a broadband range, typically between 1 and 3 Hz, as seismic energy couples into the atmosphere above the epicenter. Severe storms generate infrasound in two distinct bands, one between 0.02 and 0.08 Hz and another between 0.5 and 2.5 Hz. Tornadoes emit infrasound between 0.5 and 10 Hz, and researchers have explored whether detecting these signatures could improve early warning systems. Lightning strikes produce characteristic compression-then-rarefaction pulses, mostly between 0.2 and 2 Hz.
Even the ocean is a constant source. Colliding ocean waves generate a near-continuous infrasonic hum called microbaroms, peaking around 0.2 Hz. Wind flowing over mountain ranges creates what scientists call mountain-associated waves, at frequencies between 0.01 and 0.1 Hz, lasting minutes to hours. Meteors entering the atmosphere generate infrasound between 0.2 and 3 Hz. The aurora borealis produces signals between 0.01 and 0.1 Hz.
Man-Made Sources
Industrial and urban environments produce significant infrasound. Ventilation systems, industrial fans, compressors, pumping stations, and air conditioning systems all generate low-frequency vibrations. Heavy transport vehicles, diesel locomotives, jet and rocket engines, and gas turbines contribute as well. Even acoustic barriers along highways can create secondary infrasound as vehicles pass by.
Wind turbines have received particular public attention as an infrasound source, though the levels they produce at typical residential distances are well below what humans can perceive. More on that below.
Animals That Communicate With Infrasound
Elephants are the best-known infrasound communicators. They produce low-frequency rumbles that travel far beyond what their herds can see, and they detect these signals both through their ears and through vibrations in the ground picked up by their feet. How far these calls carry depends heavily on atmospheric conditions, particularly temperature layers near the ground and wind patterns. In favorable conditions (typically around dawn and dusk, when temperature inversions form), elephant calls travel much farther than during the heat of the day. Infrasound plays a central role in elephant reproduction, helping males and females locate each other across large distances, as well as in coordinating group movement, avoiding predators, and managing social relationships.
Other animals sensitive to infrasound include whales, which use extremely low-frequency calls to communicate across ocean basins, and pigeons, which appear to navigate partly using infrasonic cues from the environment.
How Infrasound Affects the Human Body
At high enough intensities, infrasound produces distinct physical sensations. When infrasound reaches levels near or above the hearing threshold, people feel vibrations in their chest, head, or body. In studies, the level at which participants felt vibration in the head was about 6 to 15 decibels above their hearing threshold, depending on frequency. Still higher levels were needed before the sound became unpleasant or distracting enough to interfere with concentration.
The inner ear’s balance system is sensitive to acoustic stimulation, particularly at low frequencies. Specialized low-frequency sensing organs in the vestibular system can respond to infrasound, and in people with certain inner ear conditions (such as a small opening in the bone surrounding the semicircular canal), this sensitivity is amplified. Sound energy that would normally go to the hearing organs gets redirected toward the balance organs, potentially causing dizziness or vertigo at infrasonic frequencies.
In a healthy ear, however, this effect requires substantial sound pressure levels, far above what most people encounter in daily life.
The “Ghost Frequency” at 19 Hz
In the 1990s, engineer Vic Tandy was working in a laboratory where staff reported feelings of unease, cold sweats, and even glimpses of shadowy figures at the edges of their vision. Tandy discovered that a newly installed fan system was generating infrasound at 19 Hz. This frequency happens to be close to the resonant frequency of several parts of the human body, including the eyeball. Vibrations in his chest were causing breathing difficulties and a sense of dread, while resonant vibrations in his eyes may have produced the visual disturbances that staff interpreted as ghostly apparitions.
The finding, while based on a single case, became widely cited as a possible explanation for some reported hauntings and feelings of unease in enclosed spaces with unidentified mechanical vibration sources.
Wind Turbines and Health Concerns
Public concern about wind turbine infrasound has driven considerable research. The current scientific picture is fairly clear: annoyance from audible wind turbine noise increases with sound level, but infrasound specifically does not appear to be the cause. At typical distances between wind farms and homes, infrasound levels are too low for human perception.
A large study involving over 1,300 survey respondents found that about 5% attributed health symptoms to wind farm infrasound. But when tested, people who reported symptoms were unable to distinguish noise samples containing infrasound from those without it, and they did not find infrasound-containing samples more annoying. Physiological measurements of heart rate, heart rate variability, and skin conductance (markers of stress) showed no association with wind turbine infrasound exposure. Soft or inaudible infrasound did not trigger reactions in the vestibular system either.
The consensus from the research is that the overall loudness and rhythmic amplitude changes of wind turbine sound are what drive annoyance, not the infrasonic component specifically. Long-term health effects appear to be related to the annoyance itself rather than to any direct physical mechanism of low-frequency sound at the levels wind turbines produce.
Infrasound in Global Security Monitoring
One of the most significant practical applications of infrasound is nuclear test detection. The Comprehensive Nuclear-Test-Ban Treaty Organization operates the only global infrasound monitoring network in existence. When fully operational, it will consist of 60 array stations spread across 35 countries. Both atmospheric and shallow underground nuclear explosions generate infrasound waves that these stations can detect, working alongside seismic monitoring to identify and analyze possible nuclear tests. The same network also picks up volcanic eruptions, large meteorite entries, rocket launches, and other high-energy events, making it a valuable tool for understanding large-scale atmospheric disturbances worldwide.