Infrasound, sound waves below 20 Hz, is inaudible to humans but can cause discomfort, health issues, and even hallucinations. This article explores the sources of infrasound, its effects on the human body, scientific research on its impact, and the challenges of recording and studying these low-frequency sounds in both natural and man-made environments.
Humans with normal hearing can typically perceive sounds between 20 hertz (Hz) and 20,000 Hz. However, sounds exist below this range, known as infrasound, which are inaudible to us but still present in our environment. To detect these sounds, specialized microphones and equipment are required, often involving speeding up recordings to bring these low frequencies into the audible range.
Infrasound is prevalent in urban and suburban areas, emanating from various sources, both natural and man-made. Despite being inaudible, research suggests that infrasound can have harmful effects on human health.
Many people have experienced an unexplained feeling of discomfort or fear upon entering certain places, only to feel better upon leaving. This sensation may not be imaginary or paranormal but could be caused by infrasound.
Studies have linked infrasound exposure to a range of symptoms including headaches, fatigue, loss of concentration, mood changes, depression, sleeping disorders, panic disorders, nausea, and dizziness. One notable study found that exposure to 100 dB of infrasound around 10 Hz negatively affected the heart's contraction force, decreasing it by 9% for every 10 dB increase above that level.
Animal studies also indicate adverse effects on the heart, liver, nervous system, and lungs due to infrasound exposure. However, research is limited due to the difficulty in studying these effects and the discomfort experienced by volunteers.
In the late 1990s, Vic Tandy, an engineer in Warwick, England, experienced symptoms such as cold sweats and depression in a laboratory reputed to be haunted. He also saw a blurry gray figure that vanished when looked at directly. Investigating further, he discovered that a large fan was producing infrasound vibrations at approximately 18.9 Hz, a frequency that resonates with the human eye. Adjusting the fan reduced the infrasound and eliminated the strange experiences, suggesting that infrasound can induce hallucinations or ghostly sightings.
Infrasound waves have long wavelengths, allowing them to travel great distances with little energy loss. This means infrasound can be detected miles away from its source, sometimes even louder indoors due to phenomena like Helmholtz resonance, which can amplify low-frequency sounds inside enclosed spaces.
For example, a SpaceX satellite launch in California was detected as infrasonic vibrations over 140 miles away, traveling for more than 11 minutes before reaching distant seismographs.
A 2003 study in Liverpool Metropolitan Cathedral used a 23-foot infrasonic cannon tuned to 17.5 Hz during live music performances. The infrasonic tone, played at 90 dB, caused 22% of the audience to report feelings of sadness, chills, fear, or anxiety, indicating that infrasound can influence emotional states and reduce enjoyment.
Researchers at Goldsmith's College exposed participants to infrasound and electromagnetic frequencies (EMF) in an attempt to recreate haunted experiences. Participants exposed to infrasound reported more sensations of dizziness, tingling, out-of-body experiences, and feelings of a presence in the room compared to control groups.
Standard microphones are typically designed to capture sounds within the human audible range and often include high-pass filters that remove infrasound. Specialized equipment such as seismographs and modified microphones are necessary to accurately record infrasonic frequencies, sometimes as low as 0.5 Hz.
Devices like the Raspberry Shake 3D seismograph can measure these low frequencies but require precise time synchronization and data processing. Recording infrasound in remote areas without internet access adds complexity to data collection.
The study of infrasound is still emerging, with many unanswered questions about its sources, effects, and mitigation. Advances in technology and data analysis, including machine learning, may enable better triangulation of infrasound sources and deeper understanding of its impact on health and the environment.
Infrasound is a pervasive yet invisible part of our acoustic environment, originating from both natural phenomena and human infrastructure. While inaudible, it can influence human health, emotions, and perceptions, sometimes causing discomfort or even hallucinations. Continued research and improved recording technologies are essential to uncover the mysteries of infrasound and mitigate its potential negative effects.
Exploring the world of infrasound opens a new dimension of sound beyond human hearing, revealing a hidden layer of our environment that affects us in subtle but significant ways.
