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  /  curiosity   /  When sound is “infra”
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Ferdinando Catalano - Miryadi Assistenza Familiare

by Ferdinando Catalano

A brief journey into the unsettling world of infrasound

Whenever we hear something, the organ doing most of the work is the tympanic membrane. Responsible for the perception and analysis of sound, the membrane vibrates at the same frequency as the acoustic wave it picks up. It then analyzes the sonic impulse and transmits the resultant acoustic sensation to our brain – what we call “sound.”
Unlike synthetic organs such as the eye, which generates colors (among other things) by mixing different types of light after the signal reaches the brain, the ear is analytical- that is, it is able to distinguish the timbre of sound as soon as it is perceived, even of the same frequency. In other words, if an orchestra is playing, not only are we are able to perceive the wide range of sound that comprises the music, we can even distinguish the violin from the clarinet playing the same note.
However, some sound waves cannot be perceived by the human ear – these fall outside what is called the “audibility threshold,” which ranges from roughly 20 Hz to 20,000 Hz. In other words, the eardrum fails to transmit acoustic waves slower than 20 vibrations per second (known as infrasound) or faster than 20000 vibrations per second (ultrasound).
Ultrasounds are well known in this day and age for their use in muscular therapy, as well as high-powered cleaning for industrial equipment, jewelry, etc.

What about infrasound?

Infrasound is more of an enigma.
They are known to be caused by man-made objects such as diesel engines, wind turbines, subwoofer speakers, or chemical and nuclear explosions. Perhaps the most famous example of human-generated infrasound is the sonic booms produced by high-speed aircraft. They can be found in nature, as well, produced by phenomena such as thunder, wind, earthquakes, volcanic eruptions, avalanches, waterfalls, and iceberg shearing.
In fact, scientists have discovered, largely by accident, that a tornado vortex creates infrasonic waves. These infrasound can be detected up to about 161 km (100 miles) away, and therefore can provide early warning of an oncoming tornado.
Some animals, such as whales and elephants, are known to use infrasound to communicate with each other. Elephants, with their trumpets, produce infrasound that other groups can perceive up to several kilometers across the ground and through their legs.
Animals are well known for their ability to seemingly know when disaster is about to strike; this is also thanks to infrasound. Many animals are able to perceive the infrasonic waves produced by natural disasters, which propagate through the ground ahead of the event itself, and use these signals as an early warning A recent example was given during the 2004 tsunami in the Indian Ocean: the animals had evacuated the area long before the tragedy struck the coasts of Asia.

Human reaction to infrasound

It is believed that infrasound, although not audible to the human ear, may be the cause of seemingly unexplained anxiety, sadness, and chills, and in fact infrasound have been shown to cause feelings of awe or fear in humans. The infrasound produced by air conditioners and airplanes, for example, can cause vertigo, nausea and headache.
Since they are not consciously perceived, it is possible to make people feel a vague effect of supernatural sensation, in association with the events that are taking place. For example, some movie soundtracks make use of infrasound to create discomfort or disorientation in the audience, such as in the Irréversible and Paranormal Activity films. The heart also produces infrasound around 20 Hz.

A disturbing experiment on infrasound

On May 31, 2003, a team of researchers from the United Kingdom carried out an experiment in which around 700 people were exposed to music mixed with sounds of a frequency of 17 Hz, produced by woofers mounted along a plastic sewer pipe of length of 7 meters.
The experimental concert took place in the Purcell Room during two shows each consisting of four musical pieces. Unbeknownst to the listeners, two of the pieces in each concert had a 17 Hz sound in the background.
A significant number of respondents (22%) reported a sense of anxiety, discomfort, nervousness, feelings of horror or fear, chills along the spine and a feeling of pressure on the chest in the presence of infrasound.
In addition, some data suggests that prolonged exposure to infrasound could potentially be dangerous for living organisms, to the point that it was used as a weapon by the American army during the Gulf War.
These findings have chilling implications: it’s not hard to imagine these sounds being used to bring about worryingly possible apocalyptic scenarios. For example, an entire country could be kept anxious and on edge if an antenna were to flood it with low-intensity infrasonic waves that interact with people’s sensory system. No one would know why everyone was suddenly so sad and short-tempered, taking psychological warfare to a horrifying new level.
Throughout human history, there has been no shortage of unscrupulous people, so all we can do is hope that spreading knowledge of these awful potentialities leads to the development of a suitable defense, and pray that no one has yet harnessed the sinister power of infrasound.

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