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Ghost buster – Chris Arnot [Guardian Tuesday July 11, 2000 ]

She called herself a white witch. One of three, as it happens, who had visited a 14th-century cellar near Coventry Cathedral at different times.

Word had spread that there was a "presence" down there. Whatever it was certainly put the fear of God into witch number three. She was up the steps and through the tourist information office, which stands over the cellar, almost before staff had noticed she'd gone. Almost, but not quite.

Carole Jung, assistant manager of the tourist office at the time, noticed that the witch looked "frightened to death". And she wasn't surprised. She, too, had had first-hand experience of the apparition and felt as though she were "intruding, disturbing something" when she took tours down the cellar.

Others had been affected as well. Colour was seen to drain from the face of a visiting Canadian journalist, who said later that he was sure the face of a woman had been peering over his right shoulder.

News of these strange phenomena had spread, not only to the community of white witches but also to Coventry University. Vic Tandy was more interested than most. He is experimental officer and part-time lecturer in the school of international studies and law. He could also be described as the university's chief ghost buster.

Two years ago, he and Dr Tony Lawrence of the psychology department wrote a paper called Ghosts in the Machine for the journal of the Society for Psychical Research. They cited infrasound as the cause of apparitions seen by staff at a so-called haunted laboratory in Warwick.

Tandy has just sent in another contribution to the same magazine. It is called Something in the Cellar, and it nails the culprit which terrified a Canadian journalist and a white witch. Infrasound, again.

Infrasound, what's more, at the same level as that found in the Warwick laboratory: 18.9 Hz. As the .9 suggests, this is a very accurate reading established over a lengthy period using a sophisticated spectrum analyser from the university's department of engineering.

Infrasound is not easy to measure because it vibrates at a frequency below the level of human hearing. "Evidence from Nasa and other sources suggests that it can cause you to hyperventilate and your eyeballs to vibrate," says Tandy. Having established its presence here at a level likely to cause anxiety and apparitions, he is now trying to establish why some people are affected and not others.

Meanwhile, he is developing a device - "a sort of litmus test" - which will detect the presence of infrasound. It won't cost thousands and it won't require a 13-amp plug. "That does rather restrict your areas of research," he says with a grin. For the time being, he is using another spectrum analyser, cheaper than the one belonging to the engineering department, and battery-driven.

Still, with its stylish silver case and its laptop screen, it looks rather incongruous parked on a glass display case full of medieval bone pins, 16th-century clay pipes and other artefacts from Coventry's illustrious past. Modern science on top of ancient history.

Surely science in the 21st century will confine the ghosts to history by explaining them away. Not necessarily.

"When it comes to supernatural phenomena, I'm sitting on the fence. That's where scientists should be until we've proved that there isn't anything," says Tandy, who has had some personal experience of what it's like to feel a ghostly presence. It happened some years ago when he was designing anaesthetic machines in that "haunted" Warwick laboratory. A cleaner had already given in her notice, complaining that she had seen a grey object out of the corner of her eye and "gone all cold".

Tandy was working late one night when the grey thing came for him. "I felt the hairs rise on the back of my neck," he recalls. "It seemed to be between me and the door, so the only thing I could do was turn and face it."

It disappeared. But only to reappear in a different form the following day when Tandy, a keen fencer, was oiling his foil and changing its grip for a forthcoming tournament. "The handle was clamped in a vice on a workbench, yet the blade started vibrating like mad," he remembers.

This time it was daylight. There were other people around. Although the hairs were rising once again, he was determined to find a scientific explanation. Why did the blade vibrate in one part of room and not another? Because, as it turned out, infrasound was coming from a fairly new extractor fan.

"When we finally switched it off, it was as if a huge weight was lifted," he says. "It makes me think that one of the applications of this ongoing research could be a link between infrasound and sick-building syndrome."

Tandy has yet to establish the source of the infrasound beneath Coventry's tourist information centre. But he is coming to the conclusion that it has nothing to do with the sandstone cellar in the former Benedictine priory: "The highest readings are in the doorway and the corridor outside. That's what's resonating." It's a modern corridor, built a few years ago to provide access for tourists.

Some visitors have apparently been spooked before they have even set foot over the cellar's threshold, although they might take some convincing of that.

Especially the white witch.