Webster's Dictionary defines infrasonic, or infrasound, as "1: having or relating to a frequency below the audibility range of the human ear. 2: utilizing or produced by infrasonic waves or vibrations." Theories on Infrasound and its Effect on Humans

Infrasound waves hug the ground, and travel for long distances without losing strength. Not much amplitude is needed to produce negative effects in the human body, and even mild infrasound exposure requires several hours, or possibly even days, to reverse symptoms.

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

Depending on the pitch, infrasound can cause physical pressure,  fear, disorientation, and negative physical and mental symptoms.

Please note there are varying degrees in the infrasound range and those that cause the most negative and damaging effects is not a common occurrence. It is something that can often be noticed physically, but in low levels the vibrations may not noticeably felt, but nonetheless it may still affect you.

Infrasound disrupts the normal functioning of the middle and inner ear, and may lead to nausea, imbalance, impaired equilibrium, immobilization, and disorientation. Exposure to even mild doses of infrasound can lead to illness in some people.

The Infrasound frequency can appear frequently under natural conditions. Infrasound may arise when strong gusts of wind clash with chimneys or towers. I’m sure you’ve all experienced this at one time or another during your life. This heavy base sound penetrates even through very thick walls. Such sound waves start rumbling in tunnel-shaped corridors.

Long pipe organs, such as those found in churches and cathedrals produce infrasound. In one UK study, the extreme bass frequencies instilled strange feelings at a concert hall. Effects were "extreme sense of sorrow, coldness, anxiety, and even shivers down the spine." (source; Organ Music Instills Religious Feelings,' by Jonathan Amos, 9/8/2003)

Some cars are equipped with a boom type device initiated with a  button. These devices generate large amplitude pressure/low frequency noise. When the “burp button” is used, it activates a low-band pass-filter which forces all of the amplifier's power through the sub-woofer speakers at frequencies lower than a certain number of Hertz. At extremely low frequencies, it becomes infrasound. Thus you feel the blast of noise, as well as hear it. Which may explain why some humans like booming subwoofers in their vehicles and/or homes, and it is not just a “cool and popular” status meaning as we as a society have come to believe. We’ve all tried to block the sound from these cars when next to them stopped at a traffic light. Rolling up the wondow to block the sound doesn’t work, because you still continue to feel the vibrations and of course, how much it annoys us.

Infrasonic vibrations can also be pleasantly stimulating in mild levels. The effects of brief, mild exposure can give a feeling of invigoration for hours. While a person may feel euphoric, the body is being subjected to an elevated heart rate, elevated blood pressure, and a release of endorphins which is similar to the "fight or flight" adrenaline response. Feeling the effects of high-intensity/low-frequency sound can be addictive,  partially due to the release of endorphins in the body.

Interestingly, 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. 100 Cycles Per Second (Hz) - At this level, a person experiences irritation, "mild nausea, giddiness, skin flushing, and body tingling." Following this, a person undergoes "vertigo, anxiety, extreme fatigue, throat pressure, and respiratory dysfunction." (Source; the Sonic Weapon of Vladimir Gavreau, by Gerry Vassilatos)

 There are still a lot of theories about infrasound in question, but it is important that people and paranormal investigators in particular are aware of its existence and affects.

Physicists studying the effects of infrasound upon the human body have reported that  participants in their experiments complained of weariness,  pressure in the eyes and in the ears, but there was never a mention of hallucinations or ghosts. More results from other studies revealed the following.

Infrasound Toxicological Summary, November 2001 - "When male volunteers were exposed to simulated industrial infrasound of 5 and 10 Hz and levels of 100 and 135 dB for 15 minutes, feelings of fatigue, apathy, and depression, pressure in the ears, loss of concentration, drowsiness, and vibration of internal organs were reported. In addition, effects were found in the central nervous system, the cardiovascular system, and the respiratory system. Synchronization phenomena were enhanced in the left hemisphere. Visual motor responses to stimuli were prolonged, and the strength of the effect was reduced. Heart rate was increased during the initial minutes of exposure. Heart muscle contraction strength was reduced. Respiration rate was significantly reduced after the first minute of exposure." (Sorry, I failed to note the citation for this paragraph of information)

Human reactions to infrasound

20 Hz is considered the normal low frequency limit of human hearing. When pure sine waves are reproduced under ideal conditions and at very high volume, a human listener will be able to identify tones as low as 12 Hz. Below 10 Hz it is possible to perceive the single cycles of the sound, along with a sensation of pressure at the eardrums.

Infrasound has been known to cause feelings of awe or fear in humans. Since it is not consciously perceived, it can make people feel vaguely that supernatural events are taking place.

The Ghost in the Machine

Research by Vic Tandy, a lecturer at Coventry University, suggested that the frequency 19 Hz 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, the blade 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.

Tandy 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.