Infrasound is sound that is lower in frequency than 20 Hz (Hertz) or cycles per second, the "normal" limit of human hearing (Leventhall, Pelmear & Benton, 2003 ). While human hearing may not necessarily pick up the sound through hearing, many times the body can detect and feel the effects of lower level sound, through vibrations, sensations of fear or other emotions, and even visual hallucinations which can be an explanation for paranormal type experiences. 

Early investigators of the paranormal and supernatural recognized that vibrations were a component in some reported haunt and poltergeist cases. Harry Price for example included a bowl of mercury in his personal ghost hunting kit for the detection of tremors in a room or passage (Price, 1974a). Price was also aware of the ability of certain notes and sounds to cause a sympathetic vibration in other objects. For example, he observed that in one case a particular pealing of nearby church bells caused the wires of a piano in a haunted house to vibrate in sympathy leading to the residents reporting that ghostly music was at times being played by unseen hands (Price, 1974b).

Oddly enough, in recent years many new paranormal investigators have seemed to abandon infrasound as a plausible explanation for paranormal type experiences. In most cases, infrasound is never even considered or measured. This could be simply a means of financial inability to utilize the proper technical equipment to measure infrasound on a location. 

I have also come across a few paranormal research teams that have misinterpreted the early research of infrasound and the paranormal, assuming that lower levels of sound are how spirits communicate. There is no data that has ever suggested such a thing, although I am not completely opposed to the possibility. This of course relies on the assumption that spirits exist and that it also communicates. A more logical assumption (based on research), is that low level sounds effect the body in various ways, giving various sensations that can attribute to paranormal experiences.

A ridiculous infrasonic amplifier to pick up inaudible sounds for communicating with spirits.

Reported paranormal experiences that have been frequently linked to infrasound exposure include psychological; such as a sense of presence and foreboding: Psycho-physiological, caused by the vibration of body organs and cavities and Physical; the infrasound creating secondary observable effects upon the structures within a location, leading to movement of objects and anomalous sounds. Such claims are rarely upon empirical observations of infrasound but instead draw upon similarities between the witness reports of paranormal experiences and the reported effects of infrasound exposure in the civilian studies and restricted NASA / military research programs (ParaScience,org.uk).

Research by Vic Tandy, a lecturer at Coventry University, suggested that an infrasonic signal of 19 Hz might be responsible for some ghost sightings. Tandy 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 Tandy turned to face the grey blob, there was nothing.

The following day, Tandy was working on his fencing foil, with the handle held in a vise. Although there was nothing touching it, the blade started to vibrate wildly. Further investigation led Tandy to discover that the extractor fan in the lab was emitting a frequency of 18.98 Hz, very close to the resonant frequency of the eye given as 18 Hz by NASA. This was why Tandy had seen 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 caused the vibration of the foil. Tandy investigated this phenomenon further and wrote a paper entitled The Ghost in the Machine (Wikipedia).

SOURCES:

Wikipedia - http://en.wikipedia.org/wiki/Infrasound 

ParaScience - http://www.parascience.org.uk/articles/infra.htm