Infrasound from Wikipedia, the Free Encyclopedia
Total Page：16
File Type：pdf, Size：1020Kb
Infrasound From Wikipedia, the free encyclopedia 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.[a] 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, Infrasound arrays at for reproduction, exotic loudspeaker designs such as transmission line, rotary woofer, or traditional subwoofer designs about 10 infrasound monitoring station times larger in order to octave down below their normal limit. in Qaanaaq, Greenland. Contents 1 History and study 2 Definition 3 Sources 4 Animal reactions 5 Human reactions 5.1 Infrasonic 17 Hz tone experiment 5.2 Suggested relationship to ghost sightings 6 Detection and measurement 7 Notes 8 See also 9 References 10 External links History and study Infrasound was used by the Allies of World War I to locate artillery.[1] One of the pioneers in infrasonic research was French scientist Vladimir Gavreau.[2] 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.[3][4][5] Definition Infrasound is defined by the American National Standards Institute as "sound at frequencies less than 20 Hz." Sources Infrasound can result from both natural and human sources: Natural events - infrasonic sound sometimes results naturally from severe weather, surf,[6] lee waves, avalanches, earthquakes, volcanoes, bolides,[7] waterfalls, calving of icebergs, aurorae, meteors, lightning and upper-atmospheric lightning.[8] Nonlinear ocean wave interactions in ocean storms produce pervasive infrasound vibrations around 0.2 Hz, known as microbaroms.[9] 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.[10] Animal communication - Whales, elephants,[11] hippopotamuses,[12] rhinoceros,[13][14] giraffes,[15] okapi,[16] 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 Patent for a double bass reflex 3 Hz which have similarities with the song of the humpback whale.[14] The roar of the tiger contains infrasound loudspeaker enclosure design intended to of 18 Hz and lower,[17] and the purr of felines is reported to cover a range of 20 to 50 Hz.[18][19][20] It has also produce infrasonic frequencies ranging been suggested that migrating birds use naturally generated infrasound, from sources such as turbulent from 5 to 25 hertz, of which traditional subwoofer designs are not readily airflow over mountain ranges, as a navigational aid.[21] Infrasound also may be used for long-distance capable. communication, especially well documented in baleen whales (see Whale vocalization), and African elephants.[22] The frequency of baleen whale sounds can range from 10 Hz to 31 kHz,[23] 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,[24] 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, 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,[25] 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.[26][27] Animal reactions 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.[28][29] It is not known for sure if this is the exact cause, 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.[30] Research in 2013 by Jon Hagstrum of the US Geological Survey suggests that homing pigeons use low frequency infrasound to navigate.[31] 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.[32] 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.[33] A scientist at the Sydney University Auditory Neuroscience Laboratory stated that there is a growing evidence that infrasound may affect a few people's nervous system by stimulating the vestibular system and this has shown in animal models an effect similar to sea sickness.[34] A study of 45 people Tehran University researchers stated “Despite all the good benefits of wind turbines ... this technology has health risks for all those exposed to its sound.” in particular, sleep disorder. In another study by researchers at Ibaraki University in Japan said the EEG tests showed the brain function showed that the infrasound produced by wind turbine were “considered to be an annoyance to the technicians who work in close to a modern large-scale wind turbine”.[35][36][37] 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.[38][39] 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.[38][39] 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."[33] Suggested relationship to ghost sightings Psychologist Richard Wiseman of the University of Hertfordshire suggests 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.