Something very strange started to happen in the middle of the Xasax Saxophone Ensemble's performance of The Mouth, the Feet, the Sound by Italian composer Salvatore Sciarrino.

The four players were positioned at four corners of the vast atrium of Edinburgh's Royal Museum, with a 1,000-strong audience sitting all over the floor, between fishponds, and on balconies. The sounds the quartet produced were completely unlike anything associated with the conventional repertoire of the saxophone: a mist of quiet clicks and calls replaced recognisable notes, creating a mobile, indefinable musical texture.

But then, from one end of the hall, an even weirder noise began, like a torrent of raindrops or a distant flock of birds. It was as if the building itself was responding in sympathy with the saxophone quartet, echoing and amplifying these tiny sounds, and haunting the space.

The reason for this acoustic phenomenon was revealed in a moment of theatrical magic. Dozens of saxophonists, previously hidden off-stage, solemnly started to process through the space, flooding the hall with their own sounds, a collection of sighs and breaths. Over 130 saxophonists, each clad in all-black outfits, made a pilgrimage through the museum following a continuous, circular course around the quartet and through the audience.

The saxophones invaded the hall with their gentle sounds and gestures until the very end of the piece, when the quartet suddenly played loud, repeated notes, abruptly silencing the chorus of saxophones.

The Mouth, the Feet, the Sound was a bewildering piece, both musically and theatrically - more an installation or sonic environment than a conventional performance - but the Xasax's interpretation, with professional and amateur players from all over Scotland, was a unique, beguiling experience.