Everything about Infrasound totally explained
Infrasound is
sound with a
frequency too low to be heard by the human
ear. The study of such sound waves is sometimes referred to as
infrasonics, covering sounds beneath the lowest limits of human hearing (20
hertz) down to 0.001 hertz. This frequency range is utilized by
seismographs for monitoring
earthquakes. 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; the
frequency of the muzzle blast from firing was noticeably different than that produced by the explosion, allowing the two sources to be discriminated.
One of the pioneers in infrasonic research was French scientist
Vladimir Gavreau, born in Russia as Vladimir Gavronsky.
(External Link
) His interest in infrasonic waves first came about in his lab during the 1960s where 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
sprites.
Nonlinear ocean wave interactions in
ocean storms produce pervasive infrasound around 0.2
Hertz known as
microbaroms.
Infrasound can also be generated by man-made processes such as
sonic booms,
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,
hippopotamuses,
rhinoceros,
giraffes,
okapi, and
alligators are known to use infrasound to communicate over distances up to hundreds of miles, as in the case of the whale. 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.
(External Link)
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 beneath human hearing, frequencies. These infrasonic sound waves can be detected for up to 160 km (100 miles) away, and so 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.
(External Link) It isn't 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.
(External Link) Elephants have been known to hear infrasound from two and a half miles away.
Human reactions to infrasound
Infrasound has been known to cause feelings of awe or fear in humans. Since it isn't 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 wouldn't 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 can't 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.
Further Information
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