The “devil’s frequency” most commonly refers to 18.9 Hz, a specific infrasound frequency just below the range of human hearing that can cause feelings of dread, chills, and even visual hallucinations. The term also sometimes refers to the tritone, a dissonant musical interval banned from medieval compositions and nicknamed “diabolus in musica.” These are two very different phenomena, but both earned their sinister reputations for the same reason: they make people deeply uncomfortable in ways that are hard to explain in the moment.
The 18.9 Hz “Ghost Frequency”
In the late 1990s, a British engineer and lecturer named Vic Tandy was working late in a research lab that had a reputation for being haunted. Staff reported cold chills, unease, and shadowy figures in their peripheral vision. Tandy himself experienced a growing sense of dread one evening, along with a gray shape at the edge of his sight that vanished when he turned to look at it directly.
The explanation turned out to be mechanical, not supernatural. Tandy happened to be a competitive fencer, and when he clamped a fencing sword into a vice the next day, the blade’s tip began vibrating wildly on its own. That was the clue. A newly installed ventilation fan in the lab was pumping out a steady sound wave at 18.9 Hz, just below the 20 Hz threshold where human hearing begins. Nobody could hear it, but everyone in the room was being physically affected by it. Once the fan was turned off, the hauntings stopped.
Tandy spent years investigating this further. He measured infrasound levels in supposedly haunted locations around the UK, including underground cellars in Coventry and the famous Mary King’s Close in Edinburgh. In those Edinburgh tunnels, areas that visitors described as “haunted” had infrasound levels 200 times higher than areas visitors found unremarkable. The pattern held: where the low-frequency sound was strongest, people reported a strong sense of presence, cold chills, and apparitions.
Why 18.9 Hz Affects the Body
The reason this particular frequency is so unsettling comes down to resonance. Every physical object has a natural frequency at which it vibrates most easily. A wine glass shatters when a singer hits its resonant frequency. The human body works the same way. Much of the body’s soft tissue, including the chest cavity, resonates at or near 19 Hz.
When infrasound at this frequency hits your chest, it can interfere with your breathing rhythm. That disruption triggers the same physical cascade you’d feel during a panic attack: shallow breathing, a racing heart, cold sweat, and a mounting sense of dread. Your body is responding to a real physical stimulus, but because you can’t hear the sound causing it, your brain has no rational explanation. The result feels supernatural.
The eyes are especially vulnerable. Research conducted for the U.S. Department of Defense found that the resonant frequency of the human eyeball is approximately 18 Hz, and the primary movement is rotational. When infrasound vibrates the eye at or near this frequency, it can cause the kind of smeared peripheral vision that people interpret as a shadowy figure or ghostly apparition. The effect disappears when you look directly at it because your central vision compensates differently than your peripheral vision does.
Symptoms of Infrasound Exposure
The eerie feelings Tandy experienced in his lab are just one slice of what infrasound can do. A CDC investigation of a building where employees reported strange symptoms found that over half of workers attributed health complaints to unusual sounds or vibrations. The most common symptoms were headaches (reported by 19 people), anxiety (14 people), and lightheadedness (12 people). Workers also reported nausea, vomiting, chest pressure, ear pain, fatigue, and vertigo.
Earlier controlled studies found similar results. Workers exposed for just 15 minutes to simulated infrasound at 5 and 10 Hz reported fatigue, ear pressure, poor concentration, drowsiness, and the perception of vibration inside their own organs. At higher intensities, people reported a sense of pressure in the middle ear starting at around 127 to 133 decibels. Breathing difficulties, the sensation of body sway, and tinnitus have all been documented at infrasound levels between 100 and 120 decibels.
At everyday exposure levels, the most consistently reported effect is simply annoyance. The kind of infrasound produced by a distant wind turbine or industrial equipment is rarely intense enough to cause the dramatic symptoms seen in lab settings, but it can create a persistent low-grade discomfort that’s hard to pin down.
Where Infrasound Shows Up
You encounter infrasound more often than you might think. Natural sources include volcanic eruptions, ocean waves, earthquakes, and strong wind patterns. On the human-made side, the list includes industrial ventilation systems (like the fan in Tandy’s lab), wind turbines, aircraft, heavy machinery, and military operations such as artillery fire and missile launches. Human exposure to these frequencies is increasing as industrialization spreads.
Only a handful of countries have set indoor limits for infrasound. Denmark recommends that infrasound inside homes not exceed 85 decibels, with low-frequency noise kept to 25 decibels during the day and 20 decibels at night. Australia follows a similar model. Japan considers infrasound problematic when levels reach 92 decibels or above. Most countries, however, have no specific regulations, which means exposure levels in workplaces and near industrial sites often go unmonitored.
The Other Devil’s Frequency: The Tritone
The term “devil’s frequency” sometimes refers to something entirely different: a musical interval called the tritone. In music theory, a tritone is the interval between two notes separated by three whole steps, such as C to F-sharp. It produces a harsh, unstable, clashing sound that the human ear finds naturally unpleasant.
From at least the 18th century onward, the tritone carried the Latin nickname “diabolus in musica,” or “the devil in music.” Popular legend holds that the medieval Catholic Church banned the interval outright, believing it could literally summon the devil. The reality is less dramatic. Medieval music theorists discouraged the tritone primarily for practical reasons: it sounded extremely dissonant, clashed with the harmonic rules composers followed at the time, and was notoriously difficult for singers to hit accurately. There’s no solid evidence of a formal ecclesiastical ban, but the interval’s reputation as something dark and forbidden stuck.
That sinister quality is exactly why modern composers lean into it. The tritone is a staple of horror film scores, heavy metal, and jazz. It creates tension precisely because it sounds unresolved, like something bad is about to happen. When you hear a two-note combination in a movie that makes your skin prickle, there’s a good chance it’s built on a tritone.
How the Two “Devil’s Frequencies” Differ
The 18.9 Hz infrasound frequency and the tritone operate on completely different principles. Infrasound is a physical phenomenon: a pressure wave below the threshold of hearing that vibrates your body tissue and triggers real physiological symptoms. You don’t need to be aware of it for it to affect you. The tritone, by contrast, is a fully audible musical relationship between two notes. Its unsettling quality is perceptual and cultural, rooted in how your brain processes dissonance.
What they share is the ability to produce discomfort that feels irrational. A tritone makes you uneasy because your ear expects resolution that never comes. Infrasound at 18.9 Hz makes you uneasy because your body is reacting to something your conscious mind can’t detect. Both have been called the devil’s work for centuries, and both turn out to have perfectly clear explanations grounded in physics and biology.