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Infrasound

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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[citation needed]. One of the pioneers in infrasonic research was French scientist Vladimir Gavreau, born in Russia as Vladimir Gavronsky.[1] 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.[2][3][4]

Sources

Infrasound sometimes results naturally from severe weather, surf,[5] lee waves, avalanches, earthquakes, volcanoes, bolides,[6] waterfalls, calving of icebergs, aurorae, lightning and upper-atmospheric lightning.[7] Nonlinear ocean wave interactions in ocean storms produce pervasive infrasound vibrations around 0.2 Hz, known as microbaroms.[8] 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.[9] 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.[10] 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[vague] the actual tsunami hit the shores of Asia.[11] 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.[12]

Infrasound may also be used for long-distance communication in African elephants.[13] 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 10 km (6 mi).[14] 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.[15] 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.[16]

Some film soundtracks make use of infrasound to produce unease or disorientation in the audience. Irréversible is one such movie,[citation needed] as is Paranormal Activity.

The infrasound and low-frequency noise produced by some wind turbines is believed by some to cause 'wind-turbine syndrome' in humans and other animals close to the turbines, although there are 'mixed opinions on whether wind turbines emit infrasound and if the amount is any more than that emitted by diesel engines or waves crashing on the beach'.[17]

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.[18][19] 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."[16]

The Ghost in the Machine

Research by Vic Tandy, a lecturer at Coventry University, suggested that an infrasonic signal of 19 Hz might be responsible for some ghost sightings. Tandy 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 Tandy turned to face the grey blob, there was nothing.

The following day, Tandy was working on his fencing foil, with the handle held in a vise. Although there was nothing touching it, the blade started to vibrate wildly. Further investigation led Tandy to discover that the extractor fan in the lab was emitting a frequency of 18.98 Hz, very close to the resonant frequency of the eye given as 18 Hz by NASA.[20] 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 caused the vibration of the foil.[21]

Tandy investigated this phenomenon further and wrote a paper entitled The Ghost in the Machine.[22] 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[23][24] and Edinburgh Castle.[25][26]

See also

References

  1. ^ "Gavreau", in Lost Science by Gerry Vassilatos. Signals, 1999. ISBN 0-932813-75-5
  2. ^ *Gavreau V., Infra Sons: Générateurs, Détecteurs, Propriétés physiques, Effets biologiques, in: Acustica, Vol .17, No. 1 (1966), p.1–10
  3. ^ Gavreau V.,infrasound,in: science journal 4(1) 1968,S.33
  4. ^ Gavreau V., "Sons graves intenses et infrasons" in: Scientific Progres – la Nature (Sept. 1968) p. 336–344
  5. ^ Garces, M. (2003). "Observations of surf infrasound in Hawai'i". Geophysical Research Letters. 30 (24): 2264. Retrieved 2007-12-15. Comparison of ocean buoy measurements with infrasonic array data collected during the epic winter of 2002–2003 shows a clear relationship between breaking ocean wave height and infrasonic signal levels. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  6. ^ Garces, M. (2006). "Modeling and Characterization of Microbarom Signals in the Pacific". Retrieved 2007-11-24. Naturally occurring sources of infrasound include (but are not limited to) severe weather, volcanoes, bolides, earthquakes, surf, mountain waves, and, the focus of this research, nonlinear ocean wave interactions. {{cite journal}}: Cite journal requires |journal= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  7. ^ Haak, Hein (2006-09-01). "Probing the Atmosphere with Infrasound : Infrasound as a tool" (PDF). CTBT: Synergies with Science, 1996–2006 and Beyond. Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization. Archived from the original (PDF) on July 2, 2007. Retrieved 2007-11-24.
  8. ^ "Microbaroms". Infrasonic Signals. University of Alaska Fairbanks, Geophysical Institute, Infrasound Research Group. Retrieved 2007-11-22. The ubiquitous five second period infrasonic signals called "microbaroms", which are generated by standing sea waves in marine storms, are the cause of the low-level natural-infrasound background in the passband from 0.02 to 10 Hz.
  9. ^ Chen, C.H., ed. (2007). Signal and Image Processing for Remote Sensing. Boca Raton: CRC. p. 33. ISBN 0-8493-5091-3.
  10. ^ Goddard Space Flight Center
  11. ^ "How did animals survive the tsunami?" Christine Kenneally, December 30, 2004. Slate Magazine
  12. ^ Nature . Can Animals Predict Disaster? - PBS: posted November 2005.
  13. ^ Langbauer, W.R.; Payne, K.B.; Charif, R.A.; Rapaport, L.; Osborn, F. (1991). "African elephants respond to distant playbacks of low-frequency conspecific calls" (PDF). Journal of Experimental Biology. 157 (1): 35–46. Retrieved 2009-05-27.
  14. ^ Larom, D.; Garstang, M.; Payne, K.; Raspet, R.; Lindeque, M. (1997). "The influence of surface atmospheric conditions on the range and area reached by animal vocalizations" (PDF). Journal of experimental biology. 200 (3): 421–431. Retrieved 2009-05-27.
  15. ^ Olson, Harry F. (1967). Music, Physics and Engineering. Dover Publications. p. 249. ISBN 0486217698.
  16. ^ a b "Infrasound linked to spooky effects". msnbc.com. 2007-09-07. Retrieved 27 January 2010.
  17. ^ HowStuffWorks. Do wind turbines cause health problems?
  18. ^ Infrasonic concert, Purcell Room, London, 31 May 2003, sponsored by the sciart Consortium with additional support by the National Physical Laboratory (NPL)
  19. ^ Sounds like terror in the air Sydney Morning Herald, September 9, 2003.
  20. ^ NASA Technical Report 19770013810
  21. ^ infrasound
  22. ^ Tandy, V.; Lawrence, T. (1998). "The ghost in the machine" (PDF). Journal of the Society for Psychical Research. 62 (851): 360–364. {{cite journal}}: Unknown parameter |month= ignored (help)
  23. ^ Tandy, V. (2000). "Something in the cellar" (PDF). Journal of the Society for Psychical Research. 64.3 (860). {{cite journal}}: Unknown parameter |month= ignored (help)
  24. ^ Arnot, Chris (2000-07-11). "Ghost buster". The Guardian. London. Retrieved 2010-05-05.
  25. ^ Who ya gonna call? Vic Tandy! - Coventry Telegraph
  26. ^ Internet Archive Wayback Machine. 2007 version of Vic Tandy's Ghost Experiment webpage
  • "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
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