Infrasound is sound below roughly 20 Hz, and it is one of the most misunderstood subjects in audio. Popular accounts describe it as an inaudible force that rattles organs, induces panic or, in the case of the so-called brown note, empties bowels. Laboratory work paints a plainer and more interesting picture. Infrasound can be heard and felt at high enough levels, it can contribute to unease, and its documented harms are mostly about annoyance at ordinary levels and discomfort at extreme ones. This article separates what has been measured from what has been repeated for decades, with attention to the way low-frequency sound is used in experimental and drone music.
What infrasound is
A 2004 review by Henrik Møller and Christian Pedersen in the journal Noise & Health defines infrasound as sound below 20 Hz, with sound from 20 to 200 Hz called low-frequency sound. The authors call the 20 Hz boundary conventional and note that there is no sharp change in hearing at that point.
Infrasound is not rare. The Comprehensive Nuclear-Test-Ban Treaty Organization, which runs a global monitoring network, lists natural sources such as volcanic eruptions, earthquakes, meteors, storms and auroras, and human-made ones such as mining and chemical explosions, aircraft and rocket launches. Its network is planned to include 60 array stations in 35 countries. What matters for health and perception is the level, not the mere presence of a component below 20 Hz.
Can people hear it?
The idea that infrasound is inaudible is a common error. Geoff Leventhall’s review “What is infrasound?”, indexed on PubMed, states that infrasound is clearly audible, with hearing thresholds measured down to 1.5 Hz, and that the notion of inaudibility below 20 Hz is incorrect. The catch is level. Thresholds rise steeply as frequency falls, so a tone that is easy to hear at 1 kHz may need very high sound pressure at 5 Hz.
Møller and Pedersen describe how the experience changes. Tonal sensation and pitch fade out around 20 Hz, and below about 10 Hz individual cycles can be perceived and counted. Loudness also grows unusually fast: at 4 Hz a change of about 1 dB was reported to move a sound from inaudible to clearly detectable, so a signal a few decibels above threshold can seem quite intense. At levels around 20 to 25 dB above threshold, vibration can be felt in the lower back, thighs and calves, and pressure in the chest and throat. The authors also report that thresholds are the same whether only the ears or the whole body is exposed, and that deaf subjects in one study had similar vibrotactile thresholds, which supports the ear as the primary sensing organ.
Where the brown note came from
The brown note is a hypothetical infrasonic frequency said to cause loss of bowel control, and Wikipedia’s entry classes it as an urban myth. It also summarizes the likely origin: vibration tests run for the US space program on seats mounted on vibration tables, covering 0.5 to 40 Hz, with peak levels reaching 160 dB at 2 to 3 Hz. Subjects experienced loss of coordination, nausea, visual disturbance and degraded task performance. Researchers cited there assume these studies are the root of the legend.
The published review literature repeats the point that the level was the story, not the frequency. N. Broner’s 1978 review in the Journal of Sound and Vibration (hosted PDF) concluded that the effects of low-frequency noise resemble those of higher-frequency noise and that the danger from infrasound had been greatly overrated. Citing earlier human-tolerance data, Broner reported short exposures tolerated up to about 145 dB between 0 and 50 Hz, with chest-wall vibration and fatigue, and estimated that a fatal level would be at least about 174 dB, which he considered unlikely to be reached by any practical weapon.
Testing the legend
In February 2005 the television program MythBusters tried to produce the effect. According to Wikipedia’s summary, the team used twelve modified Meyer Sound 700-HP subwoofers in four stacks of three with their tuning ports blocked, and test frequencies went as low as 5 Hz, reaching 120 dB at 9 Hz and up to 153 dB above 20 Hz. Subjects reported anxiety, shortness of breath and slight nausea, which the hosts attributed to rapid air movement in the lungs, and the show declared the myth busted.
This is entertainment television, not a peer-reviewed study, and it should be read as a demonstration. Its result is consistent with the literature: very high levels of low-frequency sound are unpleasant and can produce physical sensations, but a magic note that triggers one specific response has never been documented.
Broner also rejected related claims about resonance. He judged that the idea that infrasound resonates organs for therapeutic benefit rests on single incidents and is speculative, and that claims linking it to brain tumors or to alpha-wave synchronization had no research basis. Because the body absorbs very little acoustic energy from air, he noted, very high levels are needed to match what direct vibration produces.
Infrasound and unease
If the physical claims are overstated, the psychological ones are more interesting, though the evidence is thin. In 1998, Vic Tandy and Tony Lawrence published “The Ghost in the Machine” in the Journal of the Society for Psychical Research. As summarized by Wikipedia, Tandy noticed a blade vibrating in a laboratory, traced the source to a newly installed extractor fan, and concluded that infrasound at around 19 Hz can produce effects such as fear and shivering. The paper is a case study in a journal of psychical research, not a controlled trial.
A more controlled test came in 2003. On May 31, 2003, in two counterbalanced concerts at the Purcell Room in London, infrasound was present during different pieces and neither performers nor audience knew when. Composer Sarah Angliss’s account names psychologists Richard Wiseman and Ciaran O’Keeffe as designers and National Physical Laboratory engineers as the source of the tone, which was produced with a pipe designed to resonate near 17.5 Hz. Audience members reported more unease and a sense of presence when infrasound was playing, but the page calls the result tantalizing and inconclusive and notes that publicity about ghosts may have primed listeners.
Taken together, these studies suggest a modest, suggestible effect that deserves replication, not proof that infrasound produces ghosts or terror.
What the evidence supports
Several points emerge. Infrasound is audible at sufficient level, and it is felt in the body as pressure and vibration before it is perceived as pitch. Extreme levels produce discomfort and physical symptoms. At everyday levels, the most solidly documented effect in Broner’s review is annoyance, even when A-weighted levels are low. Leventhall’s PubMed abstract also observes that investigations of low-frequency noise complaints often fail to measure significant noise, and that adverse publicity over the past decades rests mainly on media exaggeration and misunderstanding.
Why it matters for drone and experimental music
Listeners who meet low-frequency sound through heavy drone, such as the Sunn O))) tradition covered in the guide to drone metal, can use the findings above. A sober reading follows from the research: bass-heavy listening is a physical experience, and the documented effects of loud low-frequency sound are annoyance and discomfort, not mystical transformation. The folklore of the brown note belongs with the broader history of sound as a weapon, discussed in the glossary entry on sonic warfare. Related phenomena such as the hum and the Mosquito device show how sound at the edges of hearing becomes a subject for both science and rumor, a theme the site also explores in the sonic limits of sound.
