Theatre of Voices's performance of Stockhausen's meditative masterpiece Stimmung began as a ritualised processional: the six singers, resplendent in suits of white and beige walked onto stage to sit round a table in whose centre shone a glowing orb. The atmosphere was that of a weird musical seance as they bowed to each other and intoned the single, hypnotic chord that would underpin the 90-minute performance.

Stimmung is Stockhausen's evocation of a spirituality at once ancient and modern. The notes of the chord are based on the harmonics of the overtone series, the naturally occurring spectrum of vibrations that makes up every note we hear. Stockhausen overlays the chord with mysterious incantations from distant religious traditions and cultures. As well as calling out "Shiva" and "Vishnu", the singers also intone the days of the week as magic words, turning "Saturday" into a strange, cosmic riff. The effect is part shamanistic drone and part ultra-minimalist soundscape - even if, as so often in Stockhausen's music, there is something kitschy about its pretensions to spiritual transcendence.

But Theatre of Voices, prepared by their founder and director, Paul Hillier, performed with an intensity that made for a unique sonic experience. At the heart of their interpretation was a feat of musical alchemy: they made audible the harmonics that are usually hidden around individual notes. By changing the vowel sound of a held pitch, they created a chorus of ethereal, bell-like sounds that soared octaves above the chord they were actually singing. There was even the odd outburst of humour - I swear I heard the exchange "Salami? Come on!" two-thirds of the way through - and the singers managed an expressive richness as well as technical sophistication. There were longueurs but then the whole piece was, literally, one gigantic longueur, a vast extension of a single chord into a seemingly infinite span of time, an eternity glimpsed in Theatre of Voices's performance.