Infrasound is sound with a frequency too low to be detected by humans. These range from about 17 to 16 hertz, the limits of human hearing, to as low as 0.001 hertz. A sound wave that is 1 hertz has a period of one second. The range of human audibility is around 20 to 20,000Hz, with women capable of hearing slightly higher frequencies, an ability which decreases with age.
Numerous animals, including whales, elephants, rhinoceros, giraffes, okapi, and alligators, communicate using infrasound. Infrasound can be thought of as a slow, periodic rumbling - it is generated by large processes such as avalanches, volcanoes, tornadoes, ocean waves, earthquakes, and meteors. It can be generated by large chemical or nuclear explosions. Because it is one of the telltale signs of nuclear testing, infrasound is continuously monitored worldwide by the Comprehensive Nuclear-Test-Ban Treaty Organization.
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Infrasound can give humans a feeling of unease or awe. It is thought that ambient ultrasound, sound too high to be detected by humans, may be the cause of certain ghost reports. It can even cause people to hallucinate grey figures. In experiments, as many as 1/5 subjects reported such feelings when exposed to ultrasound in a laboratory setting. At the right frequency, infrasound can make human organs vibrate, causing pain. For this reason, it has been a target of weapons research, and various prototypes have been fabricated and tested.
Infrasound is at the right frequency to wreak havoc upon fragile objects like glass windows and household trinkets. With enough energy, it can even collapse walls. If a large asteroid impacted the earth or oceans, an infrasonic wave would move out from ground zero at the speed of sound, causing damage to everything in its path. The characteristic frequency of sound produced by the surf impacting the ocean is 17 cycles per second, which is just shy of infrasound, making it audible to people.
Infrasound emanates from many natural processes. Waterfalls, Antarctican ice, and even tigers emit infrasound that can be felt for miles. Infrasound can easily be picked up with acoustic instruments. One day, there may exist cybernetic implants that allow humans to sense infrasound.