Have you ever wondered what a ghost sounds like? Engineer Vic Tandy may already know. In the early 1980s, Tandy was working in a laboratory designing medical equipment. Word began to spread among the staff that the labs might be haunted, something Tandy put down to the constant wheeze of life-support machines operating in the building.

One evening he was working on his own in the lab when he began to feel distinctly uncomfortable, breaking into a cold sweat as the hairs on the back of his neck stood on end. He was convinced that he was being watched. Then, out of the corner of his eye, Tandy noticed an ominous grey shape drifting slowly into view, but when he turned around to face it, it was gone. Terrified, he went straight home.

The next day Tandy, a keen fencer, noticed that a foil blade clamped in a vice was vibrating up and down very fast. He found that the vibrations were caused by a standing sound wave that was bouncing between the end walls of the laboratory and reached a peak of intensity in the centre of the room. He calculated that the frequency of the standing wave was about 19hz (cycles per second) and soon discovered that it was produced by a newly installed extractor fan. When the fan was turned off, the sound wave disappeared.

The key here is frequency: 19hz is in the range known as infrasound, below the range of human hearing, which begins at 20hz. Tandy learned that low frequencies in this region can affect humans and animals in several ways, causing discomfort, dizziness, blurred vision (by vibrating your eyeballs), hyperventilation and fear, possibly leading to panic attacks.

A more recent investigation took place in an allegedly haunted 14th-century pub cellar in Coventry, where people have reported terrifying experiences for many years, including seeing a spectral grey lady. Here Tandy also uncovered a 19hz standing wave, adding further evidential weight to his theory.

In an interesting parallel, researchers have recorded that, prior to an attack, a tiger's roar contains frequencies of about 18hz, which might disorientate and paralyse their intended victim. Is this the sound of fear itself?