Chicagoan Adam Sonderberg started Longbox Recordings in 1993 to release his own work, but over time the label has emerged as an important outlet for electroacoustic improvisers from around the globe, including European artists like Boris Hauf and Francisco Lopez. Longbox keeps a strong focus on the midwest, though, and this performance serves as a release party for albums by a pair of area acts. Percussionist JON MUELLER (who used to play in the rock band Pele) and synthesizer player JIM SCHOENECKER deliver a glacial symphony of ringing tones on The Interview, a gorgeous 31-minute piece that glides and hovers within a narrow pitch range. Schoenecker employs a series of long tones–often just one, never more than two–that undulate slowly and vary in loudness; some paired tones create pleasantly harsh beats of interference, but the sounds are usually pure and clean. Mueller bows metal bowls to create high-pitched drones, and when he does use a conventional drum he rattles it instead of striking it. During one passage Schoenecker’s gray and muffled synths merge with Mueller’s buzzing percussive sounds to evoke a windstorm rising and falling in intensity.

Sonderberg uses similarly unconventional percussion in his own project, CIVIL WAR, a trio with bassoonist Katherine Young (an editorial assistant at the Reader) and violist Amy Cimini (who now lives in New York). On their new album, The Brutality of Fact, they also employ long tones, but with much richer harmonic depth and detail, and their improvisations have a wonderfully ghostly quality. Set alongside Sonderberg’s rustling percussion and Cimini’s extended technique–muted pizzicato, string rubbing, and bow thwacks–Young’s low notes can sound like distant foghorn blasts; on other pieces her breathy puffs and purring notes stay in the background, letting Cimini’s dry plucking stutter over the low-end hum. Sonderberg alternates between bowed drones and brittle clatter, complementing the moods and colors that his partners create. Impressively, the group avoids any single free-improv idiom, casually calling up everything from contemporary classical music to electronic sounds using only breath and friction.

Jon Mueller & Jim Schoenecker play fourth, Carol Genetti plays third, Civil War plays second, and Jonathan Chen plays first. Sat 1/7, 8 PM, Spareroom, 2416 W. North,, $5. All ages.