What is infrasound?

From Wiki Answers we have this:

Infrasound is audio frequencies below the level of human hearing at 20Hz (Hertz)
Infrasound can be found right through nature, from Thunderstorms, being next to a stormy beach, Volcanic erruptiopns, Atomic weapons, Wildlife, to faulty electrical equipment: It is in fact a deep base rumble or hum.

Other sources of infrasound include earthquakes, pounding surf, waterfalls, calving of glacial ice, tidal waves, aurora borealis (0.1 – 0.01 Hz), solar flares, solar winds, hurricanes, thunderstorms, the jet stream (30-40Hz),
winds in caverns (20-30 Hz.), etc.

Infrasound is still a new science and the effects on the human body are still not understood. Severe damage can be cause when the audio frequency matches that of the human body: During world war II, Adolf Hitler’s engineers used infrasound to stir up anger in the large crowds that had gathered to hear Hitler. The result was a nation filled with anger and hatred and experimented with a weapon that would kill the enemy, just by sound (Song, Kate Bush) Experiment Iv lyrics)) Currently the US Government tried to invent similar equipment during the Gulf war (was actually just loud Heavy metal)

In Nature, It has been known that certain whales are able to stun their prey with powerful blasts of inaudible sounds. Called ‘gunshots,’ whales focus these powerful blasts at large squid and other fish to paralyze and catch them. In some instances, they have been known to burst their prey apart by tonal projection alone.”

Even though you cannot hear infrasound, it’s effects can be felt. These range from feeling uneasy, to being watched, to actually seeing things that are not there: Certain Paranormal investigators look at Infrasound as a possible explanation for Paranormal events.

Man-made structures, such as engines, cars, buses, trains, motorcycles, and airplanes also produce infrasound. John Cody also noted that pilots exposed to infrasonic vibrations of jet chassis experience a reduction in “vision, speech, intelligence, orientation, equilibrium, ability to accurately discern situations, and make reasonable decisions.”

Studies show the different ways in which infrasound affects the human body. As infrasound pitches, or cycles per second, decrease, deadly effects on the body increase. Infrasound disrupts the normal functioning of the middle and inner ear, leading to nausea, imbalance, impaired equilibrium, immobilization, and disorientation. Exposure to even mild doses of infrasound can lead to illness. Increased intensities of infrasound can result in death.
These are a few examples of low frequency and their effects:

12 Cycles Per Second (Hz) – Walt Disney and his artists accidentally experienced infrasound on one occasion. A cartoon sound effect was slowed from 60 cycles per second to 12 cycles per second via a tape-editing machine and was amplified through the theater system. The resulting tone, though brief in duration, produced in the entire crowd nausea that lingered for several days.

1 – 10 Cycles Per Second (Hz) – “Lethal infrasonic pitch lies in the 7 cycle range. Small amplitude increases affect human behavior in this range. Intellectual activity is first inhibited, blocked, and then destroyed. As the amplitude is increased, several disconcerting responses have been noted. These responses begin a complete neurological interference. The action of the medulla is physiologically blocked, its autonomic functions cease.” (source; the Sonic Weapon of Vladimir Gavreau, by Gerry Vassilatos)

7 Cycles Per Second (Hz) – The most profound effects at this infrasonic level occur here. Seven Hz “corresponds with the median alpha-rhythm frequencies of the brain. It is also commonly alleged that this is the resonant frequency of the body’s organs and hence organ rupture and death can occur at high-intensity exposures.” (source; ‘Acoustic Trauma: Bioeffects of Sound,’ by Alex Davies)

Lionel Rudd, an acoustics and engineering expert from Sudbury Ont. has recently observed how similar the list of adverse health effects from industrial turbines is to the list of known effects from infrasound. He was able to use a low frequency sound measuring device to document surprisingly high levels of infasound at large distances from turbines. Infrasound has the ability to follow land contours through hills and valleys without much attenuation. It also penetrates residential housing structures easily making the noise inside a home at night seem much louder and more disturbing.