Out-of-tune string quartets; late-night chill-out gigs; city-wide humming: Aberdeen’s Sound Festival is hardly your average (some would say po-faced) contemporary classical festival.

For a start, it stretches (some might say sprawls) across a full month, and takes in the broadest range of art music being created now: jazz, electronics, hardcore instrumental, choral and world music have all featured this year. ‘It’s contemporary music in its widest sense,’ says organiser Fiona Robertson. ‘We attempt to be as flexible as possible, while trying to put together a programme that’s still coherent.’

There’s not even the traditional structure of an artistic director and minions to do their bidding: the trio of Robertson, Pete Stollery (professor at Aberdeen University) and Mark Hope (chairman of Banchory’s Woodend Barn) simply work things out between themselves. ‘We just call ourselves the festival working group,’ says Robertson. ‘It’s an unusual model, but it works well for us. It allows different input from different people.’

The eclectic, organic feel to the festival programme is quite intentional – ‘It’s curated, but not too much,’ says Robertson – and encourages groups and concerts already happening in the north-east to get involved in sound. Music clubs, for example, have been persuaded to put on a piece of new music, only to find it goes down well and go on to commission more. Free gigs in Aberdeen’s airy art gallery have encouraged passing punters to dip their toes into more serious paying events.

But lest the eclecticism should shoot off in too many directions, it’s all kept in focus with themed collections of events. Last year, an opera bus took listeners the length and breadth of Aberdeenshire for a non-stop day of cutting-edge music theatre in a lighthouse, a chilly farm and a city-centre yuppie flat. This year, there was a film weekend, and also microtonal music on the menu – that’s music using intervals smaller than a semitone for anyone in the know, or for anyone else, pieces that slide and jangle (even grate, in the nicest sence), using their out-of-tune-ness for remarkably expressive effect.

In one microtonal highlight, Swiss-born and Istanbul-resident Julien Jalal Eddine Weiss slid all over his Middle Eastern qanun zither, flicking miniature handles to alter the pitches of the instrument’s strings – almost imperceptibly, but enough to subtly alter the mood of the traditional tunes he was playing. At times it was like the wildest free-form jazz, at others like the precise purity of Bach.

And an astonishing concert in the beautiful King’s College Chapel showed off two brand new microtonal string quartets specially commissioned for the occasion, played with remarkable passion by the Edinburgh Quartet. It shouldn’t really have worked – tuning the four instruments slightly apart from each other should have produced an unholy racket. Instead, it was vivid, otherworldly and exotically expressive, both in the evocative nature portraits of Geoff Palmer’s String Quartet no.5 (Haec dies) and the tongue-in-cheek The Wedding at Cana by Christopher Fox.

Elswhere, the festival had requisitioned city-centre bar Musa for a series of late-night gigs, including a relaxed, ambient set from pianist and laptop wizard Ross Whyte – the bar’s Clangers projections, we were assured, were coincidental, but they fitted in perfectly with Whyte’s retro yet bang-up-to-date sounds.

And that city-wide humming? That’s still to come, from Thursday 14 to Saturday 30 November, when Aberdeen Uni lecturer Suk-Jun Kim unleashes a sound installation bringing together childhood tunes as remembered by city residents.

Next year, says Robertson, there’s the possibility of collaborating with a similarly laid-back contemporary classical event in Clermont-Ferrand, central France, and new music from both French and Scottish composers. ‘And we’re doing a theme on traditional music – not just Scottish, but from all over the world,’ she says. ‘We seeing if we can push traditional things quite far.’ Judging by the ambition and scale of this year’s offerings, it’s likely to be essential listening for anyone serious about the cutting edge of music.

This year’s Sound festival runs until 23 November, with some events continuing until 30 November.