Infrasound, or low frequency sound, is the name given to sounds lower than 20hertz in frequency. This range is below what experts concur is the normal limit of human hearing. Infrasound often comes from manmade sources like sonic booms, explosions, and wind turbines. Natural events such as avalanches, lightening strikes, and meteors can also produce infrasonic sounds that are undetectable to the human ear.
Some animals are able to perceive infrasonic waves and have been known to use them as early warnings during natural disasters. During the 2004 Indian ocean earthquake and tsunami many animals purportedly departed from the area before the tsunami hit.
Despite being inaudible, Infrasound can affect humans in strange ways. Studies shows that for some, it can cause extremely disquieting feelings such as anxiety revulsion, and inexplicable fear. Infrasound may also cause chills, pressure in the chest, and feelings similar to sea sickness.
These strange reactions have led some scientists to theorize that the eerie sensations people attribute to ghosts may actually be caused by infrasound. Some experts speculate that many supposed ghost sightings may be caused by resonant vibrations that affect the eyeballs.
Because of the effect infrasound has on people, filmmakers are now experimenting with low frequency sound to induce higher levels of fear in audiences. The 2007 horror movie "Paranormal Activity," for instance is believed to contain low frequency sound waves designed to unnerve audiences.
