Vic Tandy, a lecturer in law and international relations at Coventry University, believes that extremely low-frequency sound — ‘infrasound’ — may explain hauntings. Twenty years ago, after he and co-workers had been experiencing an unsettling presence in their office, he identified a standing sound wave of ~18.5hz from a ventilation fan as the culprit; the lower limit of human hearing is around 20hz. The sense of presence disappeared when the fan was shut off. A ‘haunted’ 14th century pub cellar in Coventry was shown to have an 18.9hz peak when its soundscape was spectrally analyzed. Other research has shown that infrasound around this frequency can cause nausea, fear and panic. Extraordinarily, the human eyeball has a resonant frequency of 18hz, which might explain visions of ghostly presences.

Storms and waves pounding the shoreline can produce infrasound effects travelling hundreds if not thousands of miles. In a 1968 study, a U.S. town receiving infrasound from a storm over 1500 miles distant demonstrated increased traffic accidents and school absenteeism. The military has been interested in possible applications and claims powerful effects to human subjects from low-frequency and low-intensity pulses.

Recently, a team of North Carolina animal researchers has shown that, prior to attacking, tigers stun their intended prey with a roar containing frequencies around 18hz. Some dolphin clicks have long been suspected to have a role in their hunting, but a recent Hawaiian study has filmed them emitting low-frequency ‘bangs’ while chasing and catching fish; these may disorient the fish, damage their hearing, or even paralyze and kill. Researchers have observed Atlantic dolphins emit a buzz which makes buried eels jump out of the sand. Tandy speculates that human sensitivity to infrasound may be an evolutionary vestige of times when we were prey to big cats.