Infrasound is sound with a frequency too low to be detected by the human ear (less than approximately 20 hertz). Infrasound is characterized by an ability to cover long distances and get around obstacles with little dissipation.
Whales, elephants rhinoceros, giraffes, okapi, and alligators are known to use infrasound to communicate over varying distances of up to many miles as in the case of the whale, for instance. It has also been suggested [1] that migrating birds use naturally generated infrasound, from sources such as turbulent airflow over mountain ranges, as a navigational aid.
It has long been realized that infrasound may cause feelings of awe or fear. Since it is not consciously perceived, it can make people feel vaguely that supernatural events are taking place. In a controlled experiment published in September, 2003, people at a concert were asked to rate their responses to a variety of pieces of music, some of which were accompanied by infrasonic elements. The participants were not aware of which pieces included the infrasound. Many participants (22%) reported feelings of anxiety, uneasiness, extreme sorrow, nervous feelings of revulsion or fear and chills down the spine which correlated with the infrasonic events. In presenting the evidence to the British Association, the scientist responsible 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".
Infrasound sometimes results naturally from avalanches, earthquakes, volcanoes, and meteors.
Some film soundtracks also make use of infrasound to produce unease or disorientation in the audience. Irréversible is one such movie.
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