Infrasound is sound with a frequency too low to be detected by the human ear. The study of such sound waves is sometimes referred to as infrasonics, covering sounds from the lower limit of human hearing (about 16 or 17 hertz) down to 0.001 hertz. This frequency range is the same one that seismographs use for monitoring earthquakes. Infrasound is characterized by an ability to cover long distances and get around obstacles with little dissipation.
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. Infrasound was also used by Allied forces in World War I to locate artillery; the frequency of the muzzle blast from firing was noticeably different than that produced by the explosion, allowing the two sources to be discriminated.
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 1960s 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. He concluded it was infrasound and soon got to work preparing tests in the labs. One of his experiments was an infrasonic whistle. 
Infrasound sometimes results naturally from ocean waves, avalanches, earthquakes, volcanoes, and meteors. Infrasound can also be generated by man-made processes such as explosions, both chemical and nuclear, and by specially-designed mechanical transducers (industrial vibration tables) and large-scale subwoofer loudspeakers. The Comprehensive Nuclear-Test-Ban Treaty Organization uses infrasound as one of its monitoring technologies (along with seismic, hydroacoustic, and atmospheric radionuclide monitoring).
Whales, elephants, hippopotamuses, 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. 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.  Elephants, in particular, produce infrasound waves that travel through the ground and are sensed by other herds using their feet(even though they may be separated by upto a few kilometres)
Scientists accidentally discovered that the spinning core or vortex of a tornado creates infrasonic waves. When the vortices are large, the frequencies are lower; smaller vortices have higher frequencies. These infrasonic sound waves can be detected for up to 161 km (100 miles) away, and so can help provide early warning of tornadoes.
A number of American universities have active research programs in infrasound, including the University of Mississippi, Southern Methodist University, the University of California at San Diego, the University of Alaska Fairbanks, and the University of Hawaii at Manoa.
Animal and human reactions to infrasound
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. A recent example of this is the 2004 Indian Ocean earthquake. Animals were reported to flee the area long before the actual tsunami hit the shores of Asia. 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. Elephants have been known to hear infrasound from two and a half miles away.
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 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 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".
The Ghost in the Machine
Research by the late Vic Tandy, a lecturer at Coventry University, suggested that the frequency 19 hertz was responsible for many ghost sightings. He was working late one night alone in a supposedly haunted laboratory at Warwick, when he felt very anxious, and could detect a grey blob out of the corner of his eye. When he turned to face it, there was nothing.
The following day, he was working on his fencing foil, with the handle held in a vice. Although there was nothing touching it, it started to vibrate wildly. Further investigation led him to discover that the extraction fan was emitting a frequency of 18.98 Hz, very close to the resonant frequency of the eye (given as 18 Hz in NASA Technical Report 19770013810). This was why he saw a ghostly figure — it was an optical illusion caused by his eyeballs resonating. The room was exactly half a wavelength in length, and the desk was in the centre, thus causing a standing wave which was detected by the foil. 
Vic investigated this phenomenon further, and wrote a paper entitled The Ghost in the Machine. He carried out a number of investigations at various sites believed to be haunted, including the basement of the Tourist Information Bureau next to Coventry Cathedral  and Edinburgh Castle. 
In addition, infrasound created by predators like the tiger (in their grunts) 'freezes' their prey in their tracks (due to fear caused by the infrasound) and aides the predators in catching their prey.Thus, it may be concluded that the phenomena of infrasound causing emotional imbalances is observed in the case of other animals too.
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- Infrasound: Air Seismology - from About.com
- Boom car noise is an offense to homeowners and recreation seekers
- An article about supposed infrasonic weapons
- Inframatics, an international infrasound monitoring organization
- National Center for Physical Acoustics
- Infrasound Laboratory, University of Hawaii
- Southern Methodist University infrasound studies
- NOAA Infrasonics Program
- US Army Space and Missile Defense Command Monitoring Research Program
- Laboratory for Atmospheric Acoustics, University of California, San Diego
- Los Alamos Infrasound Monitoring Laboratory
- University of Alaska Infrasonics Page
- Infrasonic and Acoustic-Gravity Waves Generated by the Mount Pinatubo Eruption of June 15, 1991, Makoto Tahira, Masahiro Nomura, Yosihiro Sawada and Kosuke Kamo
- Deadly Sounds — Dr. Vladimir Gavreau
- How did animals survive the Tsunami?
- Can animals predict disaster?