The piano starts: tinktinktinktinktinktinktinktinktinktinktinktinktinktinktinktink. In a few seconds the rest of the ensemble joins in, half as fast: bRAAH, bRAAH, bRAAH, bRAAH, bRAAH, bRAAH, bRAAH, bRAAH. You’re probably hearing a saxophone in your head right now, if you got to know Terry Riley’s In C from the landmark 1968 Columbia recording—but it could be any sound, since Riley didn’t specify which or how many instruments are needed for his piece. All he provided in his score was 53 melodic cells, from one to 17 notes long, and a few simple instructions. Each musician plays each cell for as long as he or she likes before moving to the next, fitting it all against the piano’s grid of steady eighth notes—and the communal result is an irresistible bubbling, teeming with intricate detail on the surface but slowly morphing underneath as F sharps and later B flats start to infiltrate and adulterate the title key. Riley himself will be in Seattle this weekend for a performance of In C, outdoors on First Avenue, to celebrate the unveiling of MIRROR, Doug Aitken’s LED installation at SAM. Leading the ensemble of 35 or so musicians is trombonist Stuart Dempster, who played not only on that recording but in In C ’s premiere nearly 50 years ago. Seattle Art Museum, 1300 First Ave., 654-3121, seattleartmuseum.org. Free. 6:30 p.m. Sun., March 24.