'A little more gentle, a little less hard-edged." That's how festival director Fiona Robertson sums up the difference between Sound and other contemporary music festivals.

"Gentle" isn't an adjective generally courted by the avant-garde. Typical rubrics would be substantially less comforting - "experimental", "cutting-edge", maybe "pioneering". But Robertson is right in saying that Sound is not like other festivals. It can't be, it doesn't want to be, and it is precisely in its difference that it is - whisper it - genuinely experimental, cutting-edge and pioneering.

At the heart of Sound's mission statement is inclusion. This isn't a festival that plays rarefied music in darkened rooms to select crowds of initiates. There is a role for such gatherings, and in the UK the Huddersfield Contemporary Music Festival fills it nicely, providing what is paradoxically a safe haven for Europe's hardest-edged composers and performers to annually geek out. In Aberdeen and the surrounding countryside, Sound fills a different role. From the start its ethos was about bringing new music to an audience that wouldn't otherwise choose or have the opportunity to hear it. If that has meant a little flexibility on how, where, who and what to programme, so be it.

A major figure in the history and continuing success of Sound is the composer Pete Stollery. Head of music at the University of Aberdeen and a plain-speaking Yorkshireman, Stollery's creative background is electroacoustic art - sounds recorded or generated electronically then messed about with to create music. When he moved to Aberdeen in 1991 he kept in touch with his musical community despite being in a part of the UK that didn't have much of a contemporary music "scene". He was also a bit of an evangelist, determined to spark an interest in his kind of music among his new neighbours.

"The seeds of Sound were very much about participation and breaking down barriers," he says. "Working with recorded sounds is a tangible and accessible starting point: you don't need specialist training to pick up a microphone and start listening."

In the mid-1990s Stollery started staging events at Woodend Barn, a beautiful steading-turned-arts-centre in Banchory. Fiona Robertson had recently moved back to the area (she grew up in Aberdeenshire but spent several years in Paris) and was working at Woodend. She and Stollery began to talk, and formed the idea for a contemporary music weekend. They called it Upbeat.

"We begged, borrowed and stole," says Stollery of that inaugural weekend, "but a lot of good things came out of Upbeat. Particularly the DIY approach that has remained a tenet of Sound. I don't mean DIY in a roughshod way - more that we simply use the resources available to us. We don't spend huge amounts of money getting the best contemporary music groups to come up from London; instead we convince musicians up here to play the stuff. The emphasis had to be on participation, otherwise we couldn't have made it happen."

Upbeat was 2004; the following year, the first edition of Sound proper seemed to emerge almost fully formed. The format of the 2005 festival looked remarkably similar to this year's event. It was a colossal 20 days long (this year's is 19 days), and there was a strong emphasis on community partnership: Woodend Music Society, Aberdeen Jazz, Interesting Music Promotions, Monymusk Arts, Angus Arts, Strathdee Music Club, Woodend Barn and the University of Aberdeen all pitched in. There was no single artistic director and there still isn't. Programming has always been done as a group, ensuring that different tastes are included in the mix. "It all came together rather randomly and organically," says Robertson. "It's the only way that makes sense to us."

Random, maybe, but that inaugural year was also ambitious. Sound '05 was responsible for jointly commissioning Sally Beamish to write Trance O'Nicht for percussionist Evelyn Glennie and the BBC Scottish Symphony Orchestra, while James MacMillan brought together a scratch community orchestra of musicians aged from 11 and 88 to perform his double-orchestra work Into The Ferment. Robertson remembers it fondly: "I even got my viola out and took part," she says. "I can't quite imagine doing that now!"

A fundamental challenge has been one shared by most new music festivals: what does the term "new music" actually mean? "Oh lord, the definition thing," Robertson sighs. "We still debate it all the time. A big step is acknowledging that it is difficult to define!"

The Sound team knew they wanted to keep things broad and involve all manner of musicians - folk, jazz and indie artists as well as those from the classical sphere. They're also happy to programme old music alongside the brand spanking new, something that would make other contemporary music festivals squirm. In this year's programme you'll find Mozart, Beethoven and Grieg as well as Michael Finnissy, Arvo Part and Kevin Volans. "We try not to be precious and to present things other festivals might not," Roberston says with a shrug.

For Stollery, the emphasis has always been on "sound exploration". Over the years he's happily included pop music - "just as long as it's not the commercial crap with a mindless beat. Something innovative, something new - then it's of interest to us. An 80s throwback synth band? Probably not. But the likes of Leafcutter John, King Creosote, Public Service Broadcasting - they're doing interesting things, plus they're bloody good."

One unexpected upshot of the free-form programming was that the festival rapidly took on a life of its own - "to the point where we only had a certain amount of control," says Robertson. Empowering local music promoters (such as the university and music clubs) to programme contemporary music was crucial, but suddenly Sound grew to an unruly three-week carnival, its brochure more a diffuse listings pamphlet than a carefully curated series.

Did it matter? Not really. While most festivals are keen to attract visitors for overnight hotel stays, so condense programming into a few key weekends, Sound's core audience was already at its doorstep. "The idea wasn't so much to bring people to Aberdeenshire as to bring new music to audiences here," Robertson stresses. She remembers having "some basic access" to RSNO and SCO concerts growing up, but zilch by way of contemporary music without travelling to the central belt. "That is what we wanted to change."

The organisers were under no illusions that staging contemporary music "cold" would be enough to pull in the punters, and from the start there has been an investment in education, engagement and Stollery's cheerful evangelism. This year composer Pippa Murphy co-led a residential composition course for 14 to 19-year-olds, and her blog for the festival gives an insight into the experience.

"As composers," she writes, "often the hardest stage of the task in hand is to get started ... So we did an ice-breaking, idea-generating challenge where the participants drew with charcoal on scraps of paper some 'seeds' generated from sounds I played and photos I displayed, with just 20 seconds to scribble. In groups, the participants created a graphic score with the scraps of scribbles, discussing which material looked textural, which material looked rhythmic, loud, intense, delicate, repetitive, gestural, intricate, aggressive. When arranged on the floor as a score the players played made the 'seeds' into music, and the smiles on the participants' faces were widespread. My favourite part of any composition workshop: the smiles of creators as their creations are realised…"

After a decade, the impact of Sound on Aberdeen city and shire is palpable. There's a core audience that comes to concerts knowing that they will hear something interesting. The university's music department now has an impressive five composers on staff, drawn partly by the festival's vibrant performing opportunities. Cellist Robert Irvine, artistic co-director of the contemporary music group Red Note (recently anointed Sound's associate ensemble) describes the extraordinary engagement that he routinely witnesses among Aberdeenshire audiences: people "pulling penny whistles out of their handbags and joining in, or poets suddenly starting to read poems while we improvise." Until Sound came along, says Irvine, there was "no safe place for these eccentric creative types in Aberdeenshire". Now there is.

Robertson is typically modest as she sums up the festival's achievements. "After 10 years," she says, "we feel fully established in the cultural life of the North East." But last year the festival introduced a strapline to its logo that acknowledges what it has quietly become: Scotland's festival of new music. Intentionally or not, Sound does attract audiences from the central belt and beyond. Its plethora of world premieres (19 this year) and its jovial artistic cross-filtering are too tempting to let Aberdeenshire have it all to themselves. This unique, motley new music jamboree is worth travelling for.

Sound runs until November 10. For programme details, go to www.sound-scotland.co.uk