By definition, Infrasound is the range of audio frequencies that lie below 20 Hz. It means that the sound wave is making upto 20 oscillations a second, which is the limit of ‘normal’ human hearing. Any frequency above that is audible to the human ears and can be heard as a low ‘audible’ hum extending to a shrill sound upto the frequency of 20,000Hz, which is the upper limit of human hearing. In this article we are concerned only about the frequencies below the lower range of human hearing.
The ear is the primary sensing unit of infrasound, but at higher intensities, it can be picked up by various parts of the body. Infrasounds exists almost everywhere in nature, a few examples being severe weather, wind blowing strategically through the forests and hills, surf, avalanches, earthquakes, waterfalls, lightning and upper atmospheric lightning, non linear ocean waves, communications between animals( elephants, whales, giraffes etc.) , roar of a tiger (18Hz) to name a few. Infrasound is also generated by natural man made sources like sonic booms, explosions, machinery such as diesel engines, wind turbines, subwoofer loud speakers etc.
Why is this all important? As scientifically studied, the sound frequencies below 20Hz affect our internal body organs. Each organ is susceptible to these subsonic frequencies and starts to vibrate when their critical frequency is reached. These silent mysterious sounds affect our internal organs in out abdominal or cranial cavity. One study suggested that infrasound causes a feeling of awe and fear in humans. Since, these feelings are not consciously perceived; it can make people feel that supernatural events are taking place. A most interesting study in this regards was conducted by the late Vic Tandy, lecturer at Coventry University that the frequency of 19Hz was responsible for the resonance of the human eye balls, which gave an effect of a grey blob moving at his peripheral vision. This frequency has been responsible for a majority of ghost sightings around the globe.
First Experiment: Vic Randy was working late one night in a supposedly haunted laboratory in Warwick, when he felt very anxious and detected a grey blob out of the corner of his eye. When he turned to face it, he saw nothing. The next day, as he was working on his fencing foil, held by a vice at its handle, he noticed that the blade started to vibrate wildly. Further investigations led him to discover that an extraction fan in the room was emitting a frequency of 18.98 Hz, very close to the resonant frequency of the eye. This was why; he saw the ghostly figure, which was an optical illusion caused by his eyeballs resonating. The room was exactly half the wavelength in length, with the desk being at the centre, thereby causing a standing wave which was detected by the foil. By calculations, it turns out that the room was 8.7 m wide which is half the wavelength of 19Hz.
This phenomenon was further investigated by Tandy and he wrote a paper on the same, entitled, “The Ghost in the machine”. He carried out a number of investigations at various sites thereby proving his theory for existence of ghosts to be just an optical illusion of the eyes.
Professor Richard Wiseman, of the British association for advancement of science, says that low frequency sounds can cause people to have unusual experiences even though they cannot consciously detect infrasound. These frequencies are present in the so called haunted sites, temples, caves and cause people to have odd sensations, which they attribute to a ghost or a deity (if in a temple).