Infrasound, sometimes referred to as low frequency sound or (sometimes ambiguously) subsonic (subsonic being a descriptor for "less than the speed of sound"), describes sound waves with a frequency below the lower limit of human audibility (generally 20 Hz, as defined by the ANSI/ASA S1.1-2013 standard). Hearing becomes gradually less sensitive as frequency decreases, so for humans to perceive infrasound, the sound pressure must be sufficiently high. Although the ear is the primary organ for sensing low sound, at higher intensities 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 0.1 Hz (and rarely to 0.001 Hz). People use this frequency range for monitoring earthquakes and volcanoes, charting rock and petroleum formations below the earth, and also in ballistocardiography and seismocardiography to study the mechanics of the human cardiovascular system.
Infrasound is characterized by an ability to get around obstacles with little dissipation. In music, acoustic waveguide methods, such as a large pipe organ or, for reproduction, exotic loudspeaker designs such as transmission line, rotary woofer or traditional subwoofer, designs can produce low-frequency sounds, including near-infrasound. Subwoofers designed to produce infrasound are capable of sound reproduction an octave or more below that of most commercially available subwoofers, and are often about 10 times the size.
History and study One of the pioneers in infrasonic research was French scientist Vladimir Gavreau."Gavreau", in Lost Science by Gerry Vassilatos. Signals, 1999. His interest in infrasonic waves first came about in 1957 in the large concrete building that he and his research team were working in. The group was experiencing bouts of periodic and deeply unpleasant nausea. After weeks of speculation on the source of the nausea — the team was convinced that it was a pathogen or an untraced leak of noxious chemical fumes in the facility — they discovered that a "loosely poised low speed motor... was developing [these] 'nauseating vibrations'".
When Gavreau and the team attempted to measure an amplitude and pitch, they were shocked when their equipment detected no audible sound. They concluded the sound being generated by the motor was so low in pitch that it was below their biological ability to hear, and that their recording equipment was not capable of detecting these frequencies. Nobody had conceived that sound might exist at such low frequencies, and so no equipment had been developed to detect it. Eventually, it was determined that the sound inducing the nausea was a 7 cycle per second infrasound wave that was inducing a resonant mode in the ductwork and architecture of the building, significantly amplifying the sound. In the wake of this serendipitous discovery, the researchers soon got to work preparing further infrasonic tests in the laboratories. One of his experiments was an infrasonic whistle, an oversized organ pipe.Gavreau V., Infra Sons: Générateurs, Détecteurs, Propriétés physiques, Effets biologiques, in: Acustica, vol. 17, no. 1 (1966), pp. 1–10Gavreau V., infrasound, in: Science journal 4(1) 1968, p. 33Gavreau V., "Sons graves intenses et infrasons" in: Scientific Progress – la Nature (Sept. 1968) pp. 336–344 As a result of this and similar incidents, it has become routine in new architecture construction to inspect for and eliminate any infrasonic resonances in cavities and the introduction of sound-proofing and materials with specialized sonic properties.
Sources
Infrasound can result from both natural and man-made 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 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, rhinoceroses, giraffes, okapis,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 peacocks, 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,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 and the purr of felines is reported to cover a range of 20 to 50 Hz.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 2011von Muggenthaler, (2006) The Felid Purr: A Biomechanical Healing Mechanism, Proceedings from the 12th International Low Frequency Noise and Vibration Conference, pp. 189–208 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 , 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 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 singers: some vocalists, including Tim Storms, can produce notes in the infrasound range.
Man-made 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, wind turbines and 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 large horn loaded, bass reflex, sealed and transmission line loudspeakers.
Animal reaction
Some animals have been thought to perceive the infrasonic waves going through the earth, caused by natural disasters, and to use these as an early warning. An example of this is the 2004 Indian Ocean earthquake and tsunami. Animals were reported to have fled the area hours before the actual tsunami hit the shores of Asia. It is not known for sure that this is the cause; some have suggested that it may have been the influence of electromagnetic waves, and not of infrasonic waves, that prompted these animals to flee.
Research in 2013 by Jon Hagstrum of the US Geological Survey suggests that homing pigeons use low-frequency infrasound to navigate.
Human reactions
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.
From about 1,000 Hz, the dynamic range of the auditory system decreases with decreasing frequency. This compression is observable in the equal-loudness-level contours, and it implies that even 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, its effect may be 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 has also been suggested that since it is not consciously perceived, it may make people feel vaguely that odd or supernatural events are taking place.
A scientist working at Sydney University's Auditory Neuroscience Laboratory reports growing evidence that infrasound may affect some people's nervous system by stimulating the vestibular system, and this has shown in animal models an effect similar to sea sickness.
In research conducted in 2006 focusing on the impact of sound emissions from wind turbines on the nearby population, perceived infrasound has been associated to effects such as annoyance or fatigue, depending on its intensity, with little evidence supporting physiological effects of infrasound below the human perception threshold. Later studies, however, have linked inaudible infrasound to effects such as fullness, pressure or tinnitus, and acknowledged the possibility that it could disturb sleep. Other studies have also suggested associations between noise levels in turbines and self-reported sleep disturbances in the nearby population, while adding that the contribution of infrasound to this effect is still not fully understood.
In a study at Ibaraki University in Japan, researchers said EEG tests showed that the infrasound produced by wind turbines was "considered to be an annoyance to the technicians who work close to a modern large-scale wind turbine".
Jürgen Altmann of the Technical University of Dortmund, an expert on sonic weapons, has said that there is no reliable evidence for nausea and vomiting caused by infrasound.The Pentagon considers ear-blasting anti-hijack gun — New Scientist
High volume levels at concerts from subwoofer arrays have been cited as causing lung collapse in individuals who are very close to the subwoofers, especially for smokers who are particularly tall and thin.
In September 2009, London student Tom Reid died in a club of sudden arrhythmic death syndrome (SADS) after complaining that "loud bass notes" from the club's speakers were "getting to his heart". The inquest recorded a verdict of natural causes, although some experts commented that the bass could have acted as a trigger.
Air is a very inefficient medium for transferring low frequency vibration from a transducer to the human body. Mechanical connection of the vibration source to the human body, however, provides a potentially dangerous combination. The U.S. space program, worried about the harmful effects of rocket flight on astronauts, ordered vibration tests that used cockpit seats mounted on vibration tables to transfer "brown note" and other frequencies directly to the human subjects. Very high power levels of 160 dB were achieved at frequencies of 2–3 Hz. Test frequencies ranged from 0.5 Hz to 40 Hz. Test subjects suffered motor ataxia, nausea, visual disturbance, degraded task performance and difficulties in communication. These tests are assumed by researchers to be the nucleus of the current urban myth surrounding the "brown note" and its effects.ProSoundWeb: some effects of low end (bulletin board entry by Tom Danley)