Terminology can be a tricky thing when it comes to a festival such as Counterflows.

Sound art, experimental, free, avant-garde? Contemporary, new, noise, downright weird? The opaque descriptors bandied about by the culture industry and by writers like myself are constantly trying to pin down the unpindownable.

Many promoters resort to "experimental", but Alasdair Campbell, principal programmer behind Counterflows, doesn't like the term: it's too vague, he says, and too forbidding for the uninitiated. In any case, shouldn't all music be experimental in one way or another?

Instead Campbell has settled on "underground" to best describe next month's festival. "'Underground' seems eternally fashionable. It's dark glasses, Lou Reed, Andy Warhol," he says. It's smoky basement jazz bars in Krakow; chess games and cigarettes, and all things nocturnal, whimsical, covert. The posters read "A festival exploring international networks of underground music". Their design features overlapping circles: maybe the programming skirts their parameters, maybe it's what thrives in the intersection of cultural Venn diagrams.

Either way, the lack of easy definitions is exactly right. Counterflows is a festival about music and performance art that falls in between the cracks. You'll not find much straight-up rock or folk or jazz or classical on the bill, though what you will find probably borrows from some or all of these. Campbell grew up listening to his dad's record collection in which Sibelius sat alongside Louis Armstrong. In the 1980s he ran Iona Records on Stockwell Street, a beacon for Glaswegians of eclectic musical taste, and later produced Stirling's intrepid Le Weekend festivals.

Counterflows launched last year as a three-way collaboration between Glasgow, London and Berlin, with partnership from Café Oto – epicentre of marginal music-making in deepest Dalston – and the uber-hip Ausland co-op in Prenzlauer Berg, a "territory for experimental music, performance and art". Funding was gathered from Germany's Goethe Institute, Creative Scotland and the British Council: the notion of cultural cross-fertilisation was integral from the start.

This year the tri-part collaboration continues in spirit, but the festival drops its Berlin and London editions to take up sole residency in venues in Glasgow. Campbell was keen to involve the Glad Café, the south side's bustling new hub of performance art and community events that was itself directly modelled on Café Oto.

And it's at the intimate Glad Café where the weekend culminates with a rare UK appearance by the roguish grand-master of European free jazz, Peter Brötzmann. The 71-year-old German painter/saxophonist has been unleashing his ear-splitting improvisations since the 1960s.

"Hearing him is about as full-on an experience as you're likely to get from any live performance," says Campbell. "It's playing from the heart or soul or guts or somewhere; you'll not hear anyone like him."

In the 1960s and 1970s Brötzmann embodied the fierce politicisation of European jazz, which Campbell describes as "more angry, more soulful, perhaps more radical" than British equivalents such as guitarists Derek Bailey or saxophonist Evan Parker. Nowadays Brötzmann still drives home the decibels, but a certain lyricism has crept in, too. "Maybe he's mellowed with age," says Campbell, "though he'd probably hate me for saying that."

Add the fact Brötzmann is appearing with the wildly innovative drummer Paal Nilssen-Love in the Glad Café and there is potential for some serious sonic intensity. "Noisy free jazz in a tiny venue? This should be almost nostalgic!" says Campbell. "Brötzmann is still making art that is totally relevant, and he's still doing it better than anyone else." There's a point here about longevity, to do with the ability of artists working at the margins of industry to simply keep on going. "Just doing your thing never goes out of date," says Campbell.

Another veteran on the bill is New York-based composer, film-maker and multimedia adventurer Phill Niblock, who presents his powerful electronic drones and immersive audiovisuals with Berlin/Amsterdam saxophonist Thomas Intermit (CCA, Saturday). There's a Finish strand from film-maker Sami Sanpakkila (CCA, Saturday) and the power duo Jarse (Mono, Friday). From Japan comes folk singer-songwriter Kan Mikami (St Andrews in the Square, Friday) and sonic installation artist Rei Nakajima (Glad Café, Sunday).

That said, much of the festival centres on local voices. Glasgow drummer/multi-instrumentalist Alex Neilson appears in several guises, most notably at the Southside Studios fronting his new trio Death Shanties with saxophonist Sybren Renema and visuals from artist Lucy Stein.

Scottish experimental hip-hop duo Hector Bizerk are at Mono on Friday, and on Sunday afternoon there's a family workshop at the Glad Café and a walkabout with King's Park Brass. "I love the community aspect of brass bands," says Campbell. "They're truly versatile musicians, and operate with a real spirit of community mentoring. It's not necessarily an obvious comparison, but look deeper and there is a significant shared ethos between the experimental improvising community and the makeup of a brass band."

From Berlin's Ausland comes electroacoustic composer Annette Krebs, who teams up with Glasgow-based visual artist Janie Nicoll for this year's official Counterflows residency. They'll present the outcomes of seven days of intensive talking, trying and testing at the CCA on Saturday afternoon.

"Actually they don't have to produce anything at all," says Campbell. "The point was to see what might come from putting together two fascinating artists, and the interesting thing to witness here will be the process."

Fundamentally, that process is what this festival is about. With much of the material improvised and almost all of it essentially indescribable, the intrigue of the unknown looms large over the programme. Stalwarts like Brötzmann and Niblock are reliable, but by no means predictable.

Counterflows comes at a good time for experimental arts in Glasgow, says Campbell, who is also working with Ilan Volkov and the BBC Scottish Symphony Orchestra to curate the forthcoming Tectonics (May 11-12). Their cross-genre remits overlap, but Campbell doesn't see the two festivals as conflicting: the more platforms for this music, the more chances for audiences to bypass the rhetoric and try it for size.

Counterflows runs from April 5-7. www.counterflows.com