Infrasound is sound with a frequency lower than 20 Hz. Human hearing registers sounds from roughly 20 to Hz 20,000, though under certain circumstances the body will hear/feel sound at a lower frequency than 20Hz. Though the human ear does not normally register infrasound, these inaudible waves effect our everyday lives.

In Nature:

Infrasound is produced naturally by severe weather and other forms of nature. Storms, thunder, volcanic activity, earthquakes, avalanches, tsunamis, waves, wind, and even the aurora borealis emit these sound waves. ⁽¹⁾ Because many animals can detect sounds of lower frequencies than humans, they are able to sense oncoming natural phenomenon. This explains why many animals move to higher ground or act oddly before a storm.

Animals also use infrasound to communicate. Whales, hippopotamuses, giraffes, elephants and alligators are among the group that do so. These animals are able to send messages over thousands of miles in this manner. Homing pidgeons and other birds are also able to sense these sounds. They use infrasound in combination with magnetic fields to create a mental map of their surroundings.

Human encouters:

Infrasound is utilized by humans to locate earthquakes, specific types of rock, petroleum, and even artillery/nuclear weaponry. Low frequencies are also created by man-made objects. These noises are theorized to create disturbances for creatures that can register infrasound. The sounds may cause the animals to become disoriented, causing whales to beach themselves or pidgeons to lose their bearings. ⁽²⁾ Infrasound also causes irritation to human beings. In studies such as that of Professor Richard Wiseman, Richard Lord (acoustic scientist), and Vic Tandy, human exposure to infrasound may increase paranoia, physical discomfort and even nausia. ⁽³⁾⁽⁴⁾

In Tandy's situation, halucinations and supernatural "sightings" were later found to be linked to low frequency readings in the area. Infrasound is also the factor behind mysterious humming, called "The Hum," in certain areas that are audible to some people yet nonexistent to others. The hums are often emitted by man-made or natural sources. For example in Kokomo, Indiana, the humming was later pinpointed in a local factory. A fan producing a 10 Hz noise was the source of the scare.

***Authors note: This post took a little bit more research than usual ^^; It was interesting, though! Enjoy

Sources:

1.) http://www-dase.cea.fr/public/dossiers_thematiques/infrasons/description_en.html

2.) http://web.archive.org/web/20041102004955/http://pao.gsfc.nasa.gov/gsfc/educ/science/2000/04-07-00.htm

3.) http://www.msnbc.msn.com/id/3077192/#.UJV9R2-unBY

4.) http://www.coventrytelegraph.net/news/coventry-news/page.cfm?objectid=12722447&method=full&siteid=50003

5.) http://www.theindychannel.com/news/possible-source-found-for-kokomo-hum