György Ligeti’s eightieth-birthday bash at the Barbican was characteristically provocative. The stage was shared in turn – and on strictly equal terms – by pianist Pierre-Laurent Aimard playing Ligeti’s ferociously complex Etudes, and by some singer-drummer Pygmies brought in from the rainforests of the Congo. No allowances were made for the latter: it became immediately clear that, despite their dramatically differing provenance, these musics were, in terms of melodic and rhythmic sophistication, on a par; the segues between them seemed so unforced that they might have been designed to go together.

The founder of this feast was a French-Israeli ethnomusicologist named Simha Arom, for whom the concert marked the culmination of a forty-year crusade. That had begun when, as a visiting horn player hired to create a brass band in the Central African Republic, he had looked out of his hotel window in Bangui and heard some Pygmy musicians in the garden below. ‘It was a shock,’ he told me in 2003. ‘It was a polyphony which made my spine tingle. How could these people play such complex music without a conductor? For me, that was as deep a musical experience as first hearing the music of Bartók. I sensed that this music existed in us all, like some Jungian archetype.’

He got to know the musicians, learned their language, and began to analyse how they made their music. ‘I noticed that they knew instantly when a wrong note was played. That meant they had rules. And if you have rules, you have a theory. But their theory was implicit – they didn’t know they had one, because they couldn’t express it in words. I made it my job to discover that theory, to establish the grammar of their music.’ To notate it, he first recorded the full ensemble, then played the tape through headphones to each musician in turn, getting them to perform their particular part for him to record. Meanwhile Ligeti had come to adore this intricately layered music thanks to some recordings he’d heard by the British anthropologist Colin Turnbull. Under its influence Ligeti had written similarly constructed pieces for Aimard to play, and the Barbican concert was the hall’s birthday present to him.