CLASSICAL Sinfonia 21, Wigmore Hall Jonathan Harvey's `Hidden Voice' leaves Stephen Johnson straining to hear

Jonathan Harvey's Hidden Voice, premiered by Sinfonia 21 at St John's Smith Square on Thursday, illustrated one of the classic problems that have faced post-war composers. Instrumental music works best when it meaningfully subverts our expectations. For centuries, composers have known, more or less, what audiences' expectations were: well-behaved melodies and harmonies that should do certain things; large-scale forms that were normally based on one of a small number of patterns, for example "sonata form". The second work in Sinfonia 21's programme, Shostakovich's Second Piano Concerto, showed that a composer could still use sonata form as an effective formal background in 1957; the audience's enthusiastic reaction suggests that it still works for them as such in 1996.

But pity Jonathan Harvey: making a complete break with past forms means that somehow you have to supply your own background. Harvey tried it in a programme note which identified the title's "hidden voice" as a line, passed between muted solo strings, which "sings of a secret longing - for the original language of childhood, for the divine." An elastic figure for horns, cellos and bells was "the voice of the Master, or teacher", who eventually "dissolves". In the background is the idea of the process of enlightenment, and "inner self/outer self/guide are the basic personae of the piece."

In the interval, conductor Martyn Brabbins gave us his reaction to Hidden Voice: "Like a glass of champagne - fizzy, bubbly, slightly intoxicating": so much for spiritual enlightenment. Talking to Brabbins, Harvey did give one helpful piece of advice: concentrate on the solo string line as a continuing line, a line that aspires to "Brahmsian" expression. And yes, on a second hearing, Hidden Voice did make more sense. The impression was less of a sequence of more or less attractive gestures, and more of a kind of continuity. The swirling string tremolos were more atmospheric the second time round; on the other hand, the "ritualistic" woodwind sounded even more like second-hand Messiaen on repetition.

Perhaps Brabbins got closer to the essence: in some ways Hidden Voice was like drinking a glass of champagne. At its best it was fizzy, and slightly intoxicating; and, as with champagne, the experience stops simply because there's nothing left in the glass. Hidden Voice could have ended at almost any point; it could have gone on twice, three times as long. Credit to Sinfonia 21 for evidently putting a good deal of effort and devotion into the music.

But then came the Shostakovich (soloist Piers Lane), and the public premiere of "Sketch No 2" for viola and strings by the 17-year-old Benjamin Britten, a work that goes through some fairly intense harmonic adventures later on, but never quite obscures the background of strophic song form - result, dramatic tension. Whatever Harvey's mystical beliefs may tell him, Hidden Voice could have done with a little of that.