It has been suggested that some ghost sightings may be induced by the effect of infrasound on witnesses. It is already known that infrasound could make people feel uncomfortable. Its also suggested that they might experience visual hallucinations as well.
In particular, it was suggested that 19 Hz infrasound might cause a visual disturbance. As a result, paranormal researchers have been looking for infrasound in haunted locations ever since.
Infrasound is just like audible sound (a compression wave going through the atmosphere) except that the frequency is too low for people to hear it. It is generally defined as below 20 Hz.
Infrasound has few natural sources but they include ocean waves, avalanches, earthquakes and certain wind conditions (eg. storms, hurricanes and wind shear around mountain ranges). There are also artificial sources of infrasound. Anything with an engine in it can induce infrasound, particularly any form of transport.
Typically, given the few sources, background infrasound is not very loud. However, if it enters a resonant ‘cavity’ it may be amplified. A resonant cavity, in this case, is a closed volume of space whose dimensions cause the waves to bounce backwards and forwards (or resonate). A room in a building may be of a suitable specific dimensions (which depends on the wavelength of the infrasound) to cause such resonance.
One confounding problem with infrasound is that it operates at the same frequencies as EIFs (experience-inducing fields) that could produce magnetic hallucinations. Thus if there are any suitable metal objects in the area, they may vibrate in sympathy with the infrasound (or they could even be its source eg. an engine). Thus, when infrasound is suspected as the source for a haunting, the possibility of magnetically-induced hallucinations must be eliminated.
Several experiments have demonstrated that low-frequency sound waves, known as infrasound, can cause phenomena that people typically associate with ghosts. This includes feelings of nervousness and discomfort as well as a sense of a presence in the room. The sound waves may also vibrate the human eye, causing people to see things that are not there. Usually, these waves have frequencies of less than 20 Hz, so they are too low-pitched for people to actually perceive. Rather than noticing the sound itself, people notice its effects.