Capable of being triggered by nothing more than the wind passing over walls, the sound waves cannot be heard. But scientific tests have revealed that they have effects on the human body that can account for the wraith-like appearance of ghosts and even the feelings of cold and terror that accompany them.

The explanation emerged after a chance discovery by a university academic who found himself involved in a haunting in a laboratory. One night as Vic Tandy worked alone he began sweating despite feeling cold and then he noticed a figure in the room. ``The hair was standing up on the back of my neck _ I was terrified.''
The explanation emerged the following morning. Mr. Tandy, a fencing enthusiast, had left a foil clamped in a vice. ``When I returned, I noticed that the free end of the blade was frantically vibrating up and down.''
A trained engineer, he realized that the blade might be receiving energy from very low frequency sound waves filling the laboratory _ so low that they could not be heard.

Tests duly revealed the existence of a ``standing wave'' trapped in the lab which reached a peak in intensity next to his desk. ``It turned out to be caused by a new extraction fan ... When the fan's mounting was altered, the ghost left with the standing wave.''

Working with Dr Tony Lawrence of the university's school of health, he has now discovered the significance of this rate of vibration. In research published in the latest issue of the Journal of the Society for Psychical Research they reveal that ``infra-sound'' around this frequency has been linked to a whole host of physiological effects _ including breathlessness, shivering and feelings of fear.