As festivals devoted to the avant wax and wane—anyone remember artsEdge at Seattle Center?—the survival and persistence of the Seattle Improvised Music Festival (SIMF) is miraculous, much like the music it champions. Created extemporaneously with few if any preconceptions of melody, harmony, rhythm, timbre, tempo, and structure, freely improvised music embodies the sound of surprise.

For the 25th anniversary of the SIMF, festival director Gust Burns enlisted two of his predecessors, founder Paul Hoskin and saxophonist Wally Shoup, as cocurators. Burns follows the format of Derek Bailey's famed Company Week during the first weekend (Thurs–Sun Feb 11–14, various venues, see for details, $5–$15 sliding scale donation), mustering musicians into new and unfamiliar duos, trios, and bigger groups. I look forward to hearing German trumpeter Birgit Uhler; much like Axel Dörner, Nate Woolley, and godfather Bill Dixon, Uhler transforms the trumpet into a burbling, crackling, old-school analog synthesizer. Also in the mix: Chicago-based cellist Fred Lonberg-Holm, whose playing ranges from atomized slivers to melodic passages that rattle and sing; and master saxophonist Jack Wright, the first person I recommend to newcomers interested in freely improvised music.

Next weekend, Shoup leads a group with torrential drummer Chris Corsano, guitar-skronker Bill Horist, and C. Spencer Yeh (Thurs Feb 18, Sunset Tavern, 9 pm, $10). Shoup, Corsano, and Horist can summon thunder of just about any magnitude. But I'm betting the presence of Yeh, who delighted me in New York several years ago playing a Stroh violin, will add an unsettling sense of the eerie to the trio's incantatory downpour.

The following night (Fri Feb 19, Chapel Performance Space, 8 pm, $5–$15 sliding scale donation) showcases Michael Johnsen. An elusive Pittsburgh-based maker of homemade electronics, Johnsen also plays the most shimmering instrument you will ever hear, the musical saw. I've been a fan of the musical saw for years and still marvel that a violin bow and a well-chosen household saw can, between the stout fingers of a sawyer, sound like angels whistling. Johnsen tailors his instruments to every performance, so let's hope he brings the saw.

Festival founder Hoskin, who launched the SIMF in 1985, curates the final evening (Sat Feb 20, Chapel Performance Space, 7 pm, $5–$15 sliding scale donation) with a tatterdemalion ensemble that includes trombonist Greg Powers, bassist Fred Chalenor, Jeffrey McGrath on trumpet, pianist Evan Gallagher, and Chris Cochrane, an unsung guitarslinger of New York's 1980s downtown scene. Jeph Jerman rounds out the group. The ex-Seattle percussionist, now living in Arizona, captivates the ears with what he simply calls "natural things"—pinecones, shells, rocks, dried mushrooms, and a stone bowl. recommended