infrasound

Sound having a frequency of less than 20 Hz. Such waves cannot be heard by humans, but may be felt. Infrasonic waves are produced by explosions and by an unsteady airflow past an object. The study of infrasound is infrasonics.

Infrasound is sound with a frequency too low to be detected by the human ear. Possibly the first observation of naturally-occurring infrasound was in the aftermath of the Krakatoa eruption in 1883, when concussive acoustic waves circled the globe seven times or more and were recorded on barometers worldwide.

One of the pioneers in infrasonic research was french scientist Vladimir Gavreau, born in Russia as Vladimir Gavronsky. His interest in infrasonic waves first came about in his lab during the 1960's where he and his lab assistants experienced pain in the ear drums and shaking lab equipment, but no audible sound was picked up on his microphones.

Infrasound sometimes results naturally from ocean waves, avalanches, earthquakes, volcanoes, and meteors.

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 that migrating birds use naturally generated infrasound, from sources such as turbulent airflow over mountain ranges, as a navigational aid.

Scientists discovered by accident that the spinning core or vortex of a tornado creates infrasonic sounds. These infrasonic sound waves can be detected for up to 161km (100 miles) away, and so can help provide early warning of tornadoes.

Concerning behavioral patterns of animals and the infrasonic effects of natural disasters, it is to be noted that animals can also recognize the infrasonic waves emitted from such natural disasters and can use these as an early warning. It is not known for sure if this is the exact reason, as some have suggested that it was the influence of electromagnetic waves, and not of infrasonic waves, that prompted these animals to flee.

It has long been realized that infrasound may cause feelings of awe or fear. 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 BA, 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 film soundtracks also make use of infrasound to produce unease or disorientation in the audience.

In music, Brian "Lustmord" Williams is known to utilize infrasound to create these same feelings.