Paul Hillier sang bass in the famous performances of Stockhausen’s psychedelic Stimmung that Gregory Rose’s Singcircle vocal troupe toured during the mid-1980s. Two decades on and Hillier’s understanding of this unique piece might even be greater than the composer’s own. He has objectivity on his side – a quality that Stockhausen himself squandered many moons ago.

Stimmung is structured around an electronically sustained drone that is sounded throughout the work’s duration. Anchored around B flat, the drone contains the complex harmonic overtone series around which Stockhausen constructs his constantly generating sound world of rarefied timbres and ethereal melodies. He provides embryonic melodic cells and fragments of text, arranged in 51 “Models”, leaving the singers to embellish this given material during the performance. It’s probably more accurate to say that Stimmung is “realised” than “performed”.

If this all sounds rather academic then the result is actually one of the most gorgeous vocal soundscapes in all 20th-century music. Hillier’s Theatre of Voices operate at such a refined level of specialised technique that radically new expressive territory emerges. Standard vocal delivery feels superseded by loops of vocalised harmonics, tactile humming and, as the piece nears its conclusion, a catchy whistled refrain that calmly draws threads together. Stockhausen’s flexi-notation blurs the supply and demand chain that usually defines composer and performer relationships. The musicians are licensed to define the micro as they transform and deliberate over the material. They converse and explore, and an enigmatic piece-specific discipline is created. A thrilling and important performance – possibly the CD highlight of 2007 so far.