Infrasound

Infrasound is sound that is lower in frequency than 20 Hz (Hertz) or 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.

History and study

Infrasound was 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.

Sources

Infrasound sometimes results naturally from severe weather, surf, lee waves, avalanches, earthquakes, volcanoes, bolides, waterfalls, calving of icebergs, aurorae, 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 older designs of down tower 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 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.

Animal reactions to infrasound

Animals have been known to perceive the infrasonic waves going through the earth by 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.

Infrasound may also be used for long-distance communication in African elephants. These calls range from 15–35 Hz and can be as loud as 117 dB, allowing communication for many kilometres, with a possible maximum range of around . These calls may be used to coordinate the movement of herds and allow male elephants to find mates.

Human reactions to infrasound

Twenty Hz is considered the normal low frequency limit of human hearing. When pure sine waves are reproduced under ideal conditions and at very high volume, a human listener will be able to identify tones as low as 12 Hz. 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, as is Paranormal Activity.

The infrasound and low-frequency noise produced by some wind turbines is believed to cause certain breathing and digestive problems in humans and other animals close to the turbines.

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 subwoofer 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 British Association for the Advancement of Science, Professor Richard Wiseman 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." This was why Tandy had seen 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. Tandy 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

Bioacoustics

Brown note

Helmholtz resonance

Microbarom

The Hum

Ultrasound

References

"infrasound". Collins English Dictionary, 2000. Retrieved 25 October 2005, from xreferplus. http://www.xreferplus.com/entry/2657949

Gundersen, P. Erik. The Handy Physics Answer Book. Visible Ink Press, 2003.

Chedd, Graham. Sound; From Communications to Noise Pollution. Doubleday & Company Inc, 1970.

O'Keefe, Ciaran, and Sarah Angliss. "The Subjective Effects of Infrasound in a Live Concert Setting". CIM04: Conference on Interdisciplinary Musicology. Graz, Germany: Graz UP, 2004. 132–133.

Discovery's Biggest Shows aired at 8:00 pm (Indian Standard Time) on The Discovery Channel, India on Sunday, 7 October 2007

External links

Inframatics, an international infrasound monitoring organization

NOAA Infrasonics Program

US Army Space and Missile Defense Command Monitoring Research Program

Los Alamos Infrasound Monitoring Laboratory

Infrasonic and Acoustic-Gravity Waves Generated by the Mount Pinatubo Eruption of June 15, 1991, Makoto Tahira, Masahiro Nomura, Yosihiro Sawada and Kosuke Kamo

Category:Hearing Category:Acoustics Category:Seismology and earthquake terminology