Interference patterns from two flutes playing the same tune in the same place produce "acoustic shadows" that give the impression of an invisible ring of massive pillars, research has shown.

The ancient builders of Stonehenge may have marked the location of these unseen mystical objects with real blocks of solid stone, it is claimed.

Other megalithic stone circles in the British Isles and France possibly owe their origins to the same auditory illusion, according to US "archaeoacoustics" expert Steven Waller.

Myths and legends of "Pipers' Stones" and "invisible towers" are said to support the theory.

Mr Waller, who describes himself as an independent scholar, presented his findings at the world's biggest science conference in Vancouver, Canada.

He conducted tests on himself and with blindfolded volunteers which showed how real the acoustic shadow effect can be. When two sets of sound waves meet, they reinforce and cancel each other out at different points.

Where the peaks and troughs of the waves cancel out, the sound becomes softer. To a blindfolded listener walking in a circle around the sound source, it is as if a series of solid objects is standing in the way of what can be heard.

Mr Waller, from La Mesa, California, told the annual meeting of the American Association for the Advancement of Science (AAAS): "What was really interesting was that when I walked around this pair of flutes to experience the interference pattern myself, I did feel that pattern of quiet and loud and quiet and loud, but the impression that I got when I walked through these quiet zones where the sound waves were cancelling each other out was the experience of being sheltered from the sound by some object that was blocking the sound.

"As I walked in a complete circle, I experienced all these objects in a ring that gave a mental vision of a Stonehenge-like structure."