The scattered brackets, periods, and lowercase letters that make up the “liner notes” of Nmperign’s 1998 CD, 44’38″/5 (Twisted Village), are an apt analogue to the Boston-based ensemble’s sparse, authoritative improvisations. Trumpeter Greg Kelley and soprano saxophonist Bhob Rainey don’t play many conventional notes, preferring a vocabulary that stresses twittering reed timbres, humid breaths, microtonally varied pitches, and buzzing, steely textures–Kelley often “mutes” his horn with a square of sheet metal. Their language has clear antecedents in the work of Joe Maneri (with whom Rainey studied composition at the New England Conservatory), Bill Dixon, Roscoe Mitchell, and Evan Parker–but the closer you listen, the less familiar the duo’s dialect sounds, with its volatile and idiosyncratic balance of wriggling abstraction and severely rationed lyricism. Rainey and Kelley are relentless collaborators: Nmperign has recorded or performed with percussionist Tatsuya Nakatani (once a regular member, he’s all over 44’38″/5), tape-loop manipulator Jason Lescalleet, shakuhachi player Philip Gelb, and local keyboardist Bob Falesch, among many others. And they’ve got the road stamina of a rock band: this weekend’s gig is part of a six-week cross-country tour. Rainey took up residence in Chicago for a few months earlier this year, establishing a rapport with several members of the improvised-music scene who frequent the Nervous Center–so there’s a good chance that some of them will show up to sit in. Sunday, 8 PM, Nervous Center, 4612 N. Lincoln; 773-728-5010. BILL MEYER