Infrasound, sometimes referred to as low-frequency sound, 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 intensities it is possible to feel infrasound vibrations in various parts of the body.
The study of such sound waves is referred to sometimes 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. In music, low frequency sounds, including near infrasound, can be produced using acoustic waveguide methods, such as a large pipe organ or, for reproduction, a transmission line based loudspeaker.
History and study
Infrasound was used by the Allies of World War I to locate artillery. One of the pioneers in infrasonic research was French scientist Vladimir Gavreau. His interest in infrasonic waves first came about in his laboratory during the 1960s, when he and his laboratory assistants experienced pain in the ear drums and shaking laboratory equipment, but no audible sound was picked up on his microphones. He concluded it was infrasound caused by a large fan and duct system and soon got to work preparing tests in the laboratories. One of his experiments was an infrasonic whistle, an oversized organ pipe.
Infrasound can result from both natural and human sources:
- Natural events - infrasonic sound sometimes results naturally from severe weather, surf, lee waves, avalanches, earthquakes, volcanoes, bolides, waterfalls, calving of icebergs, aurorae, meteors, lightning and upper-atmospheric lightning. Nonlinear ocean wave interactions in ocean storms produce pervasive infrasound vibrations around 0.2 Hz, known as microbaroms. According to the Infrasonics Program at the NOAA, infrasonic arrays can be used to locate avalanches in the Rocky Mountains, and to detect tornadoes on the high plains several minutes before they touch down.
- Animal communication - 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. In particular, the Sumatran Rhinoceros has been shown to produce sounds with frequencies as low as 3 Hz which have similarities with the song of the humpback whale. The roar of the tiger contains infrasound of 18 Hz and lower, and the purr of felines is reported to cover a range of 20 to 50 Hz. 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. Infrasound also may be used for long-distance communication, especially well documented in baleen whales (see Whale vocalization), and African elephants. The frequency of baleen whale sounds can range from 10 Hz to 31 kHz, and that of elephant calls from 15 Hz to 35 Hz. Both can be extremely loud (around 117 dB), allowing communication for many kilometres, with a possible maximum range of around 10 km (6 mi) for elephants, and potentially hundreds or thousands of kilometers for some whales. Elephants also 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. These calls may be used to coordinate the movement of herds and allow mating elephants to find each other.
- Human created sources - Infrasound can be generated by human processes such as sonic booms and explosions (both chemical and nuclear), or by machinery such as diesel engines and wind turbines and by specially designed mechanical transducers (industrial vibration tables). Certain specialized loudspeaker designs are also able to reproduce extremely low frequencies; these include large-scale rotary woofer models of subwoofer loudspeaker, as well as transmission line loudspeakers whose use of a long sound-absorbent acoustic duct "folded" within the enclosure allows reproduction of frequencies down to as low as 5 or 7 Hz at sensitivities of (in some cases) 96 dB. British company TDL's professional speaker range contained several models whose responses reached as low as 7 Hz, with sensitivity of 96 dB.
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 hours 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 may have been the influence of electromagnetic waves, and not of infrasonic waves, that prompted these animals to flee.
20 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.
One study has suggested that infrasound may cause feelings of awe or fear in humans. It also was suggested that since it is not consciously perceived, it may make people feel vaguely that odd or supernatural events are taking place.
Infrasonic 17 Hz tone experiment
On 31 May 2003, a group 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 the 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."
Suggested relationship to ghost sightings
Psychologist Richard Wiseman of the University of Hertfordshire thinks that the odd sensations that people attribute to ghosts may be caused by infrasonic vibrations. In 1998, Vic Tandy, experimental officer and part-time lecturer in the school of international studies and law at Coventry University, and Dr. Tony Lawrence of the psychology department wrote a paper called "Ghosts in the Machine" for the Journal of the Society for Psychical Research. Their research 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 vice. 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. 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.
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.
Detection and measurement
NASA Langley has designed and developed an infrasonic detection system which can be used to make useful infrasound measurements at a location where it was not possible previously. The system comprises an electret condenser microphone PCB Model 377M06, having a 3-inch membrane diameter, and a small, compact windscreen. Electret-based technology offers the lowest possible background noise, because Johnson noise generated in the supporting electronics (preamplifier) is minimized. The microphone features a high membrane compliance with a large backchamber volume, a prepolarized backplane and a high impedance preamplifier located inside the backchamber. The windscreen, based on the high transmission coefficient of infrasound through matter, is made of a material having a low acoustic impedance and sufficiently thick wall to insure structural stability. Close-cell polyurethane foam has been found to serve the purpose well. In the proposed test, test parameters will be sensitivity, background noise, signal fidelity (harmonic distortion), and temporal stability.
The microphone design differs from that of a conventional audio system in that the peculiar features of infrasound are taken into account. First, infrasound propagates over vast distances through the Earth’s atmosphere as a result of very low atmospheric absorption and refractive ducting that enables propagation by way of multiple bounces between the Earth’s surface and the stratosphere. A second property that has received little attention is the great penetration capability of infrasound through solid matter – a property utilized in the design and fabrication of the system windscreens  Thus the system fulfills several instrumentation requirements advantageous to the application of acoustics: (1) a low frequency microphone with especially low background noise, which enables detection of low-level signals within a low-frequency passband; (2) a small, compact windscreen that permits (3) rapid deployment of a microphone array in the field. The system also features a data acquisition system that permits real time detection, bearing, and signature of a low-frequency source.
The Comprehensive Nuclear-Test-Ban Treaty Organization Preparatory Commission uses infrasound as one of its monitoring technologies, along with seismic, hydroacoustic, and atmospheric radionuclide monitoring. The loudest infrasound recorded to date by the monitoring system was generated by the 2013 Chelyabinsk meteor.
- Brown note
- Clear-air turbulence
- Demonstrated infrasonic weapon
- Helmholtz resonance
- The Hum
- Wired Article, The Sound of Silence by John Geirland. 2006.
- "Gavreau", in Lost Science by Gerry Vassilatos. Signals, 1999. ISBN 0-932813-75-5
- *Gavreau V., Infra Sons: Générateurs, Détecteurs, Propriétés physiques, Effets biologiques, in: Acustica, Vel .17, No. 1 (1966), p.1–10
- Gavreau V.,infrasound,in: Science journal 4(1) 1968,S.33
- Gavreau V., "Sons graves intenses et infrasons" in: Scientific Progress – la Nature (Sept. 1968) p. 336–344
- Garces, M.; Hetzer C., Merrifield M., Willis M. and Aucan J. (2003). "Observations of surf infrasound in Hawai’i". Geophysical Research Letters 30 (24): 2264. Bibcode:2003GeoRL..30xOCE5G. doi:10.1029/2003GL018614. Retrieved 15 December 2007. "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."
- Garces, M.; Willis, M. (2006). "Modeling and Characterization of Microbarom Signals in the Pacific". Retrieved 24 November 2007. "Naturally occurring sources of infrasound include (but are not limited to) severe weather, volcanoes, bolides, earthquakes, mountain waves, surf, and, the focus of this research, nonlinear ocean wave interactions."
- Haak, Hein (1 September 2006). "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 on 2 July 2007. Retrieved 24 November 2007.
- "Microbaroms". Infrasonic Signals. University of Alaska Fairbanks, Geophysical Institute, Infrasound Research Group. Retrieved 22 November 2007. "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."
- "NOAA ESRL Infrasonics Program". Retrieved 10 April 2012.
- Katharine B. Payne, William R. Langbauer, Elizabeth M. Thomas: Infrasonic calls of the Asian elephant (Elephas maximus), Behavioral Ecology and Sociobiology, Volume 18, Number 4, pp. 297–301, 1986, doi:10.1007/BF00300007
- William E. Barklow: Low‐frequency sounds and amphibious communication in Hippopotamus amphibious, Journal of the Acoustical Society of America, Volume 115, Issue 5, pp. 2555–2555 (2004)
- E.K. von Muggenthaler, J.W. Stoughton, J.C. Daniel, Jr.: Infrasound from the rhinocerotidae, from O.A. Ryder (1993): Rhinoceros biology and conservation: Proceedings of an international conference, San Diego, U.S.A. San Diego, Zoological Society
- E. von Muggenthaler, P. Reinhart, B. Lympany, R.D. Craft: Songlike vocalizations from the Sumatran Rhinoceros (Dicerorhinos sumatrensis), Acoustic Research Letters ARLO 4(3), July 2003, pp. 83–88, DOI 10.1121/1.1588271. Also cited by: West Marrin: Infrasonic signals in the environment, Acoustics 2004 Conference
- E. von Muggenthaler, C. Baes, D. Hill, R. Fulk, A. Lee: Infrasound and low frequency vocalizations from the giraffe; Helmholtz resonance in biology, proceedings of Riverbanks Consortium on biology and behavior, 1999. Also work by Muggenthaler et al cited by Nicole Herget: Giraffes, Living Wild, Creative Education, 2009, ISBN 978-1-58341-654-9, p. 38
- E. Von Muggenthaler: Infrasound from the okapi, invited presentation, student competition award, proceedings from the 1992 American Association for the Advancement of Science (A.A.A.S) 158th conference, 1992
- Work by Muggenthaler et al, also referred to in: The Secret Of A Tiger's Roar, ScienceDaily, 1 December 2000, American Institute of Physics, Inside Science News Service (1 December 2000), Retrieved 25 December 2011
- Von Muggenthaler, E., Perera, D. (2002), The cat's purr: a healing mechanism?, In review, presented 142nd Acoustical Society of America International Conference, 2001.
- Work by Muggenthaler et al, referred to in: David Harrison: Revealed: how purrs are secret to cats' nine lives, The Telegraph, 18 March 2001, Retrieved 25 December 2011
- von Muggenthaler, (2006) The Felid Purr: A Biomechanical Healing Mechanism, Proceedings from he 12th International Low Frequency Noise and Vibration Conference, p. 189-208
- Goddard Space Flight Center
- 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". The Journal of Experimental Biology 157 (1): 35–46. Retrieved 27 May 2009.
- Richardson, Greene, Malme, Thomson (1995). Marine Mammals and Noise. Academic Press. ISBN 978-0-12-588440-2.
- 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". The Journal of Experimental Biology 200 (3): 421–431. Retrieved 27 May 2009.
- Chen, C.H., ed. (2007). Signal and Image Processing for Remote Sensing. Boca Raton: CRC. p. 33. ISBN 0-8493-5091-3.
- Elizabeth Malone, Zina Deretsky: After the tsunami, Special Report, National Science Foundation, version of 12 July 2008, downloaded 26 December 2011
- "How did animals survive the tsunami?" Christine Kenneally, 30 December 2004. Slate Magazine
- Nature. Can Animals Predict Disaster? – PBS: posted November 2005.
- Knight, Kathryn (2013). Disappearing homing pigeon mystery solved. The Company of Biologists. Retrieved 2013-01-31
- Olson, Harry F. (1967). Music, Physics and Engineering. Dover Publications. p. 249. ISBN 0-486-21769-8.
- "Infrasound linked to spooky effects". MSNBC. 7 September 2007. Retrieved 27 January 2010.
- Infrasonic concert, Purcell Room, London, 31 May 2003, sponsored by the sciart Consortium with additional support by the National Physical Laboratory (NPL)
- Sounds like terror in the air Sydney Morning Herald, 9 September 2003.
- NASA Technical Report 19770013810
- Tandy, V.; Lawrence, T. (April 1998). "The ghost in the machine.". Journal of the Society for Psychical Research 62 (851): 360–364.
- Tandy, V. (July 2000). "Something in the cellar.". Journal of the Society for Psychical Research 64.3 (860). Archived from the original on 2011-09-29.
- Arnot, Chris (11 July 2000). "Ghost buster". The Guardian (London). Retrieved 5 May 2010.
- Who ya gonna call? Vic Tandy! – Coventry Telegraph
- Internet Archive Wayback Machine. 2007 version of Vic Tandy's Ghost Experiment webpage
- Development and installation of an infrasonic wake vortex detection system By Qamar A. Shams and Allan J. Zuckerwar, NASA Langley Research Center, Hampton VA USA, WakeNet-Europe 2014, Bretigny, France.
- NASA Langley Researchers Nab Invention of the Year for Infrasound Detection System By Joe Atkinson, 2014, NASA Langley Research Center
- Paul Harper (20 February 2013). "Meteor explosion largest infrasound recorded". The New Zealand Herald (APN Holdings NZ). Retrieved 31 March 2013.
- "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, 1970.
- O'Keefe, Ciaran, and Sarah Angliss. The Subjective Effects of Infrasound in a Live Concert Setting. CIM04: Conference on Interdisciplinary Musicology. Graz, Austria: 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
- 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 15 June 1991, Makoto Tahira, Masahiro Nomura, Yosihiro Sawada and Kosuke Kamo
- Sub-surface windscreen for the measurement of outdoor infrasound Qamar A. Shams, Cecil G. Burkett and Toby Comeaux NASA Langley Research Center, Allan J. Zuckerwar Analytical Services and Material, and George R. Weistroffer Virginia Commonwealth University