Outside Edge: Nine Bulgarian grandmothers sing to the North Sea. Nick Curtis reports

Unless you're a rural Bulgarian, you may not have heard of the Bistritsa Babi. An amateur group upholding a centuries-old tradition of Bulgarian folk music, the Babi ('grandmothers' or 'wise women') come from the hill village of Bistritsa. James Lingwood, who is responsible for bringing them to Britain says: 'They've released one cassette, but I don't think it's been widely distributed here. They are reasonably well known amongst folkloric specialists. But anyone who hears their kind of music never forgets it.'

The nine tanned, blue-eyed women, ranging in age from 50 to their early seventies, have only performed once outside Bulgaria, at a folk festival in Dijon. Until tonight, when their music - antiphonous and polyphonous drones over which dramatic lead parts are sung - will be matched with the roar of the North Sea at high tide on the Northumberland coast.

It all started two years ago. The producers Artangel commissioned Welsh artist Bethan Huws to create a piece. Lingwood, a director of Artangel, points out that the briefs for their commissions are very open, aimed at ambitious, unusual collaborative work. So when Huws traced a snippet of singing heard on the radio to Bulgaria and eventually to the amateur singing grannies of Bistritsa, and then decided they should address the North Sea at Craster in a symphonic dialogue, Lingwood wasn't that surprised.

There's a refreshing - or alarming - sense of 'let's-do-the-antiphony-right-here' to the performance. The Babi arrived on Monday, and Lingwood immediately whisked them off in a van to Craster. 'It was epic. They love singing, so instead of having the radio on I'd suddenly have this beautiful Bulgarian lament behind me. It was the closest I've felt to being on tour, only with eight grannies instead of eight heavy metal guitarists in a van: I don't think they'll start trashing hotel rooms or anything.'

It's also not totally clear what, how or where they'll sing. Most of their historic songs relate to nature, the seasons and rural life, such as 'Vai Dudole' (for rain) and 'Sultz Saide' (for the sunset). Living in the mountains, they've never had to sing for the sea before, and they're very excited about working out a vague repertoire. They'll sing in traditional costumes too. Bethan Huws scouted coves and cliffs for acoustics and mise-en-scene, but when you're dealing with the sea and wind, such things are liable to change.

The 100-odd would-be Babi fans who can attend each concert must book in advance with a coach that will deliver them to a coastal spot suitable for the evening's weather. Or, in the event of torrential rain, to the tiny Creighton Hall in Embleton. None of these variables seems to bother the Babi in the slightest, though. As one spokes- grandmother says: 'We believe in the power of song. For us it is not theatre, it is life.'

The Bistritsa Babi perform Fri 6.15pm and Sat 7.45pm (high tide); coaches from Alnwick are bookable on 0665 576001.