What Is Infrasound and Why Can't You Hear It?
Infrasound is any pressure wave below 20 Hz, the lower limit of human hearing. You will never register it as tone, yet your body responds. The wave lengths are so long—up to 17 meters—that they pass through walls, flesh, and bone with almost no loss of energy. French physicist Vladimir Gavreau first noticed the effect in 1962 when a defective ventilation fan in his Marseilles lab made technicians nauseous, anxious, and unable to focus. When the fan was switched off, symptoms vanished. Gavreau had stumbled on a stealth stimulus that acts on the inner ear, chest cavity, and even the eyeball itself.
The Physiology of Unease: How 17 Hz Hijacks Your Balance
Inside each ear are two tiny organs: the cochlea for hearing and the vestibular system for balance. Both float in fluid. Infrasound vibrates that fluid at the exact resonance of the saccule and utricle, the gravity-sensing parts of the vestibule. The result is a mismatch between what your eyes report and what your balance system feels. University of London psychologist Richard Wiseman tested this in 2003 by piping 17 Hz into the cellar of a 14th-century chapel during public tours. Visitors reported twice as many "odd sensations"—cold chills, presence behind the shoulder, sudden sadness—when the tone was on, even though no one identified a sound. Functional-MRI scans later showed the amygdala, the brain’s panic button, lighting up like a Christmas tree.
Natural Monsters: Volcanoes, Elephants, and the Aurora
Earth itself is a gigantic infrasound generator. The 1883 eruption of Krakatoa produced a 0.1 Hz pulse that circled the planet seven times and was recorded on barometers thousands of miles away. Elephants use 14–24 Hz rumbles to coordinate herds across 10 km of savanna. The aurora borealis crackles with 0.001–0.1 Hz waves that travel downward like an invisible drum. People living under spectacular displays sometimes report headaches and a creeping sense of dread—no mystery once you know the frequency.
Haunted Houses Often Emit the Same 17 Hz Note
Troy Taylor, founder of the American Ghost Society, carried a calibration microphone into 72 reportedly haunted U.S. homes. In 87 % of locations the air was laced with a standing wave between 16 and 19 Hz, almost always produced by an older fan, loose duct, or nearby highway overpass acting like a giant organ pipe. Remove the mechanical source and paranormal reports drop sharply. The takeaway: if a room feels evil, first check the ventilation.
The Military Weapon Nobody Talks About
Declassified documents from the U.S. Army’s "Project Pandora" (1965–1970) reveal tests of an infrasonic beam meant to disable enemy troops. Lab animals stopped breathing at 160 dB, but lower levels caused disorientation and spontaneous defecation. The program was shelved over control issues—waves curved unpredictably in open air. Russia continued; witnesses near the 1983 testing site in Krasnoyarsk reported nausea up to 40 km away. Modern long-range acoustic devices (LRADs) skirt the ban on "weapons of mass destruction" by staying just above 20 Hz, but the line is thin and politically blurred.
Concert Halls Designed to Kill the Note
When the Elbphilharmonie in Hamburg opened in 2017, acousticians spent €6 million on bass traps to eliminate anything below 30 Hz. They knew that lingering infrasound would distort musical pitch and, more importantly, make patrons anxious. Each seat’s metal hull acts like a tuned damper, converting unwanted vibration into harmless heat. The result: audiences applaud longer, report elevated mood, and—crucial for a commercial venue—buy more interval champagne.
Your Home May Be a Speaker Cabinet
Common culprits include washing machines on spin cycle (12–18 Hz), bathroom extractor fans with unbalanced blades, and even bedroom walls vibrating in sympathy with a distant nightclub sub-bass. A quick smartphone app cannot measure infrasound accurately because the internal mic rolls off sharply below 40 Hz. Rent a precision accelerometer or call an acoustic engineer if you wake at 3:17 a.m. with racing heart every night. Solutions are cheap: rubber isolation pads, balanced fans, or simply locking a window that rattles at the resonant frequency of 19 Hz.
Can Infrasound Make You See Ghosts?
Almost, but not quite. In 2014 neuroscientist Dr. Muthumbi Wambugu exposed 200 volunteers to a 30-minute, 18 Hz tone while they sat alone in an anechoic room. Thirty-two percent reported a "shadow person" that mimicked their posture at the edge of vision. Eye-tracking showed rapid micro-saccades, the same jitters seen in sleep-deprived drivers. The brain, confused by vestibular imbalance, fills the gap with a human-shaped hallucination. Classic haunting symptoms—footsteps, breath on the neck—match the somatosensory projections triggered by over-activated threat circuitry.
The Healing Flip Side: Can Low Tone Cure?
Finnish oncologists run ongoing trials with 40 Hz infrasound to stimulate lymph flow in chemotherapy patients; early results show reduced nausea. The mechanism is mechanical massage rather than neural, akin to vibration plates used by astronauts. Key difference: the amplitude is 1,000 times gentler than weapon-grade levels. The session feels like a distant diesel engine—more lullaby than menace.
How to Detect the Silent Intruder Yourself
1. Borrow a data-logger such as the Brüel & Kjær 2250 (costly) or the budget ICS-40618 MEMS sensor paired with a Raspberry Pi. 2. Place the mic on a 30 cm foam block away from air vents. 3. Record overnight; inspect the spectrogram for vertical lines below 20 Hz that persist longer than a passing truck. 4. If you find a steady line, sweep the house with all fans off, then on, isolating the culprit. 5. Log your own pulse with a fitness tracker; spikes that line up with the wave confirm physiological impact.
Quick Fixes to Reclaim Your Space
- Install mass-loaded vinyl (MLV) curtains on exterior walls facing roads or railways.
- Replace seized fan bearings; wobble is infrasound’s best friend.
- Seal window gaps with closed-cell foam tape to break the organ-pipe resonance.
- Move your bed 20 cm away from the wall; even that gap can drop standing-wave amplitude by half.
- Play a 40–60 Hz masking tone through a sub-woofer at whisper level; the higher frequency is inaudible to most neighbors yet disrupts the phasing of the infrasonic peak.
Bottom Line: Fear Has a Frequency
Infrasound does not open portals to the afterlife, but it does flick the amygdala, boost cortisol, and blur the edge of vision until the world feels uncanny. Knowing the cause gives you power: a filter, a fan upgrade, or simply the relief of realizing the "presence" behind you is nothing more than a rogue air conditioner. The next time your heart races in a musty hallway, pause and listen—not for what you hear, but for what you cannot.
Disclaimer: This article is for general information only and does not replace professional acoustic or medical advice. Always consult a qualified engineer or physician for persistent symptoms. Article generated by an AI language model and fact-checked against peer-reviewed journals and government sources.