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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 loudspeaker. ==History and study== Infrasound was used by the Allies of World War I to locate artillery.〔(Wired Article, The Sound of Silence ) by John Geirland. 2006.〕 One of the pioneers in infrasonic research was French scientist Vladimir Gavreau.〔("Gavreau", in ''Lost Science'' ) by Gerry Vassilatos. Signals, 1999. ISBN 0-932813-75-5〕 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.〔 *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〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Infrasound」の詳細全文を読む スポンサード リンク
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