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Infrasound
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Infrasound is sound that is lower in frequency than 20 cycles per second, the normal limit of human hearing. Hearing becomes gradually less sensitive as frequency decreases, so for humans to perceive infrasound, the sound pressure must be sufficiently high. The ear is the primary organ for sensing infrasound, but at higher levels it is possible to feel infrasound vibrations in various parts of the body.
The study of such sound waves is sometimes referred to as infrasonics, covering sounds beneath 20 Hz down to 0.001 Hz.
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Infrasound is sound that is lower in frequency than 20 cycles per second, the normal limit of human hearing. Hearing becomes gradually less sensitive as frequency decreases, so for humans to perceive infrasound, the sound pressure must be sufficiently high. The ear is the primary organ for sensing infrasound, but at higher levels it is possible to feel infrasound vibrations in various parts of the body.
The study of such sound waves is sometimes referred to as infrasonics, covering sounds beneath 20 Hz down to 0.001 Hz. This frequency range is utilized for monitoring earthquakes, charting rock and petroleum formations below the earth, and also in ballistocardiography and seismocardiography to study the mechanics of the heart. Infrasound is characterized by an ability to cover long distances and get around obstacles with little dissipation.
About infrasound
Possibly, the first observation of naturally occurring infrasound was in the aftermath of the Krakatoa eruption in 1883, when concussive acoustic waves circled the globe seven times or more and were recorded on barometers worldwide. Infrasound was also used by Allied forces in World War I to locate artillery. One of the pioneers in infrasonic research was French scientist Vladimir Gavreau, born in Russia as Vladimir Gavronsky. His interest in infrasonic waves first came about in his lab during the 1960s, when he and his lab assistants experienced pain in the ear drums and shaking lab equipment, but no audible sound was picked up on his microphones. He concluded it was infrasound and soon got to work preparing tests in the labs. One of his experiments was an infrasonic whistle.
Infrasound sometimes results naturally from severe weather, surf, lee waves, avalanches, earthquakes, volcanoes, bolides, waterfalls, calving of icebergs, aurora, lightning and upper-atmospheric lightning.
Nonlinear ocean wave interactions in ocean storms produce pervasive infrasound vibrations around 0.2 Hz, known as microbaroms.
Infrasound can also be generated by man-made processes such as sonic booms and explosions (both chemical and nuclear), by machinery such as diesel engines and wind turbines and by specially designed mechanical transducers (industrial vibration tables) and large-scale subwoofer loudspeakers. The Comprehensive Nuclear-Test-Ban Treaty Organization uses infrasound as one of its monitoring technologies (along with seismic, hydroacoustic, and atmospheric radionuclide monitoring).
Whales, elephants, hippotamuses, rhinoceros, giraffes, okapi, and alligators are known to use infrasound to communicate over distances—up to hundreds of miles in the case of whales. It has also been suggested that migrating birds use naturally generated infrasound, from sources such as turbulent airflow over mountain ranges, as a navigational aid. Elephants, in particular, produce infrasound waves that travel through solid ground and are sensed by other herds using their feet (although they may be separated by hundreds of kilometres).
Scientists accidentally discovered that the spinning core or vortex of a tornado creates infrasonic waves. When the vortices are large, the frequencies are lower; smaller vortices have higher, though still infrasonic, frequencies. These low frequency sound waves can be detected for up to 160 km (100 miles) away and can help provide early warning of tornadoes.
A number of American universities have active research programs in infrasound, including the University of Mississippi, Southern Methodist University, the University of California at San Diego, the University of Alaska Fairbanks, and the University of Hawaii at Manoa.
Animal reactions to infrasound
Concerning behavioral patterns of animals and the infrasonic effects of natural disasters, animals have been known to perceive the infrasonic waves carried through the earth from such natural disasters and can use these as an early warning. A recent example of this is the 2004 Indian Ocean earthquake and tsunami. Animals were reported to flee the area long before the actual tsunami hit the shores of Asia. It is not known for sure if this is the exact reason, as some have suggested that it was the influence of electromagnetic waves, and not of infrasonic waves, that prompted these animals to flee. Elephants have been known to hear infrasound from two and a half miles away.
Human reactions to infrasound
When testing human hearing, pure sine wave tones are perceived as less musical near the bottom of the hearing range. Between 20 and 12 Hz, such tones cease being heard as a musical tone unless they are boosted greatly in volume. Below 10 Hz it is possible to perceive the single cycles of the sound, along with a sensation of pressure at the eardrums. The dynamic range of the auditory system decreases with decreasing frequency. This compression can be seen in the equal-loudness-level contours, and it implies that a slight increase in level can change the perceived loudness from barely audible to loud. Combined with the natural spread in thresholds within a population, it may have the effect that a very low frequency sound which is inaudible to some people may be loud to others.
Infrasound has been known to cause feelings of awe or fear in humans. Since it is not consciously perceived, it can make people feel vaguely that supernatural events are taking place.
Some film soundtracks make use of infrasound to produce unease or disorientation in the audience. Irréversible is one such movie.
In music, Brian "Lustmord" Williams is known to utilize infrasound to create these same feelings.
Infrasonic 17 Hz tone experiment
On May 31, 2003, a team of UK researchers held a mass experiment where they exposed some 700 people to music laced with soft 17 Hz sine waves played at a level described as "near the edge of hearing", produced by an extra-long stroke sub-woofer mounted two-thirds of the way from the end of a seven-meter-long plastic sewer pipe. The experimental concert (entitled Infrasonic) took place in the Purcell Room over the course of two performances, each consisting of four musical pieces. Two of the pieces in each concert had 17 Hz tones played underneath. In the second concert, the pieces that were to carry a 17 Hz undertone were swapped so that test results would not focus on any specific musical piece. The participants were not told which pieces included the low-level 17 Hz near-infrasonic tone. The presence of the tone resulted in a significant number (22%) of respondents reporting anxiety, uneasiness, extreme sorrow, nervous feelings of revulsion or fear, chills down the spine and feelings of pressure on the chest. In presenting the evidence to the BA, the scientist responsible said, "These results suggest that low frequency sound can cause people to have unusual experiences even though they cannot consciously detect infrasound. Some scientists have suggested that this level of sound may be present at some allegedly haunted sites and so cause people to have odd sensations that they attribute to a ghost—our findings support these ideas."
The Ghost in the Machine
Research by the late Vic Tandy, a lecturer at Coventry University, suggested that the frequency 19 hertz was responsible for many ghost sightings. He was working late one night alone in a supposedly haunted laboratory at Warwick, when he felt very anxious and could detect a grey blob out of the corner of his eye. When he turned to face it, there was nothing.
The following day, he was working on his fencing foil, with the handle held in a vice. Although there was nothing touching it, it started to vibrate wildly. Further investigation led him to discover that the extraction fan was emitting a frequency of 18.98 Hz, very close to the resonant frequency of the eye (given as 18 Hz in NASA Technical Report 19770013810). This was why he saw a ghostly figure—it was an optical illusion caused by his eyeballs resonating. The room was exactly half a wavelength in length, and the desk was in the centre, thus causing a standing wave which was detected by the foil.
Tandy investigated this phenomenon further and wrote a paper entitled The Ghost in the Machine. He carried out a number of investigations at various sites believed to be haunted, including the basement of the Tourist Information Bureau next to Coventry Cathedral and Edinburgh Castle.
See also
External links
- - from About.com
- , Makoto Tahira, Masahiro Nomura, Yosihiro Sawada and Kosuke Kamo
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