Sound is a pressure wave moving out from its source, something like ripples in water. Examples of sound sources the human ear can hear are a bird singing, a trumpet blowing or church bells ringing.
But there are sounds human ears cannot hear. “Infrasonic” refers to sounds below 20Hz, at the lower limit of human hearing, down to sounds of 0.1 Hz. One Hertz equals one cycle per second, so a 20 Hz sound has 20 cycles per second and 0.1 Hz has a cycle of 10 seconds.
Infrasonic sounds have some interesting aspects. Lower frequency sounds can travel considerable distances. African elephants use infrasound to communicate to a distance of about 4 km, and forest elephants can communicate up to seven km.
Whales using infrasound can communicate over hundreds of miles. Hippos, rhinos, giraffes and alligators are thought to use infrasound to communicate. While humans cannot hear infrasound, the sounds can still impact people. Infrasound may be related to human anxiety and perhaps account for apparitions. Some studies have indicated that infrasound can cause feelings of fear and awe, and may be related to insomnia.
Infrasounds are produced by storms, wind, earthquakes and tsunamis. The stories about animals running for high ground before a tsunami are explained by infrasound. Infrasonics has only begun to be studied. There are almost certainly some intriguing discoveries ahead.
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