There are many possible explanations for hauntings, not least that humans are highly suggestible creatures, especially when we want to believe. But some ghost sightings might actually be the result of sounds — sound waves that vibrate just below our range of hearing, dubbed the "fear frequency."
Sound is basically mechanical energy in the form of a pressure wave with crests and troughs: vibrations create a disturbance in the surrounding air and ripple outward, like tossing a pebble in a pond. Frequency measures how many crests occur within one second in a wave. The unit of measurement is called a Hertz (Hz), and 1 Hz is equivalent to 1 vibration per second. A plucked guitar string might vibrate 500 times per second, causing surrounding air particles to vibrate at the same frequency, so the sound wave's frequency would be 500 Hertz.
The typical range for human hearing runs from 20 Hz to 20,000 Hz (20 kHz), although this varies from person to person, and shrinks as we age. Under ideal lab conditions, some people can pick up sounds as low as 12 Hz — well into the "infrasound" range. But even when we don't consciously hear such sounds, they may induce feelings of anxiety, especially at higher intensities. This has led some people to dub infrasound in the 18.9 Hz range — i.e., just a tad below the threshold for human hearing — the "fear frequency."
We can thank a British engineer named Vic Tandy for associating this so-called fear frequency with ghostly visitations. He was a real-life ghost buster, thanks to his own personal experience with a suspected "ghost" while working late one night at the (supposedly haunted) Warwick Laboratory. He inexplicably felt the hairs on his neck prickle, as if in fear, and caught the barest glimpse of a grey blob-like figure out of the corner of his eye. It vanished when he turned his head to look at it directly.
Being a sensible sort, he cast about for a logical explanation, and he found one in the phenomenon of resonant frequency. Every material object has a natural resonant frequency at which it vibrates. If there is another object nearby that is sensitive to the same frequency, it will absorb the vibrations (sound waves) emanated from the other object and start to vibrate in response. The effect is known as "sympathetic resonance." It's why running your damp finger along the rim of a crystal wine glass produces a faint hum, and why a chord struck on one piano will be echoed by a piano in another room.
While working on a fencing foil the next day in the lab, Tandy noticed that the blade began vibrating even though nothing was touching it. It turned out that the lab's extractor fan was emitting a resonant frequency of around 18.98 Hz, also roughly the same resonant frequency as the human eye.
He concluded that the grey blob he'd seen was an optical illusion, the result of his eyeballs resonating at just that frequency. Ditto for his feelings of anxiety and fear. "When we finally switched it off, it was as if a huge weight was lifted," he told the Guardian back in 2000.
Tandy died in 2005, but others have carried on his work. Psychologist and paranormal debunker Richard Wiseman and a few UK colleagues conducted their own mass infrasound experiment in May 2003 via a public concert they called Infrasonic. Some 700 people showed up for the two performances, featuring two pieces of music that contained the critical 17 Hz tones at a volume right at the edge of human hearing. (Other pieces without those tones served as controls, since the audience didn't know which of the pieces had those near-infrasonic tones.)
The result: It wasn't a slam dunk in terms of hard evidence — there's a lot of subjectivity at play, and scientists still aren't sure why infrasound affects some people and not others — but a good 22% of the audience reported feeling anxious, uneasy, fearful, pressure on the chest, or a chill down the spine. As Wiseman told the British Association for the Advancement of Science when he reported his results, "These results suggest that low frequency sound can cause people to have unusual experiences even though they cannot consciously detect infrasound."
A 2008 experiment led by British psychologist Christopher French proved even more intriguing. With colleagues from University of London College, he built a "haunted" room rigged up with infrasonic generators (as well as sources of electromagnetic pulses); 79 brave Londoners volunteered to spend some time inside.
"Most people reported at least some slightly odd sensation, such as a presence or feeling dizzy, and some reported terror, which we hadn't expected," French told Scientific American in 2008. But he stopped short of claiming that low-frequency electromagnetic fields or infrasound were the direct cause of such feelings; suggestibility seems to also play a role.
Incidentally, infrasound is also associated with the infamous "brown note": sound frequencies between 5 and 9 Hz that are rumoured to make people lose control of their bowels. Those rumours appear to stem from the early days of the U.S. space program, when astronauts reported adverse effects from vibration tests. Subsequent studies however, including a 2005 investigation by TV's Mythbusters, haven't shown any such effects.
French, CC., Haque, U., Bunton-Stasyshyn, R., Davis, R. (2009) "The "Haunt" project: An attempt to build a 'haunted' room by manipulating complex electromagnetic fields and infrasound," Cortex 45 (5): 619 — 629.
St-Pierre, LS; Persinger, MA. (2006) "Experimental facilitation of the sensed presence is predicted by the specific patterns of the applied magnetic fields, not by suggestibility: re-analyses of 19 experiments," International Journal of Neuroscience 116 (9): 1079 — 96.
Tandy, Vic and Lawerence, Tony. (1998) "Ghosts in the machine," Journal of the Society for Psychical Research 62 (851): 360-364.
Tandy, V. (2000) "Something in the cellar," Journal of the Society for Psychical Research 64(3): 860.
Wiseman, Richard. Paranormality: Why We See What Isn't There. London: Spin Solutions Ltd., 2011.