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.
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 sub-woofer 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 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.
Further investigation of the phenomenon revealed that sound waves of this low frequency may appear rather frequently under natural conditions. Infrasound arises when strong gusts of wind clash with chimneys or towers. These heavy basses penetrate even through very thick walls. Such sound waves start rumbling in tunnel-shaped corridors. This is the reason why people often come across ghosts in long serpentine corridors of ancient castles looking like tunnels. Strong winds are quite typical of Great Britain.
However, other scientists call the idea into question. Physicists studying effect of infrasound upon the human body say that volunteers participating in their experiments complain of weariness, high pressure in the eyes and in the ears, but never mention hallucinations or ghosts. At that, physicists say that drivers also have no optical illusions when cars overcome the air drag at a really high speed and the level of infrasound waves is very high.
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