MUSIC / THE PROMS: Too authentic for its own good: A 19th-century hall is not ideal for 18th-century music, writes Nicholas Williams

HOW far do you go to achieve musical authenticity? Period instruments and costumes? The question has been asked often enough, yet with no clear answer. The Hanover Band's performance of Bach's F minor harpsichord concerto at Monday's Prom - inaudible, at least from block L of the stalls - did throw some light on the question.

Radio 3 listeners were probably unaware of any acoustical problem. But for others at the back of the hall, or high up in the gallery, the music was nothing more than a faint aural disturbance somewhere in the middle distance. Still, the strings managed to articulate the first movement rhythms with impressive unanimity, and in the slow movement Anthony Halstead stitched together a sequence of harpsichord arabesques like ornate embroidery. But the 19th-century interior swallowed up the 18th-century sound. Performance space, like authentic technique, is rarely ideal. In an imperfect world, you make do with what's available.

Perhaps that's why many musicians prefer the recording studio, where everything can be directed to capturing the ultimate sound. Even so, things go missing. One lesson to emerge from the Albert Hall experience on Monday, in a concert not otherwise remarkable for programming flair, was to do with concerto form itself. Instruments, like people, can behave strangely in public places. In the harpsichord concerto, it was a curious matter that the music carried further in the slow movement than in the outer movements, where the instrument went full tilt against the orchestral tutti, yet the whole ensemble made a puny noise.

In the 5th Brandenburg Concerto, flute and violin soloists imparted a more penetrating quality not only to the solo harpsichord itself, but also to the entire string orchestra. Admittedly there was also plenty of strong, forward playing, with the first Allegro's long, central buildup - like a playful anticipation of Philip Glass. With the 2nd Brandenburg for a quartet of soloists - recorder, oboe, trumpet and violin - it was the period oboe, expertly played by Anthony Robson, that was the equal of the trumpet. Even the recorder, in duet, imparted a bright haze to the trumpet's highest notes and, in the Elysian rhapsody of the slow movement, filled the hall's vast spaces with its languid song.

How the details of Bach's forms compensate for these inequalities creatively is a fascinating conjecture. No doubt scholars can provide some answers. Perhaps Monday's lesson was that no great harm is done in trying to get authenticity, but there must be limits.