As part of the ongoing “Impossible Music” exhibition at Tufts University Art Galleries, the Tufts New Music Ensemble — with special guest John McDonald — will perform a selection from Tom Johnson’s “Imaginary Music” series (1974-1975, 1993). Containing a total of 104 scores, these drawings of musical symbols or loose suggestions of notes promote non-linear interpretations, and Johnson used them as tools to imagine new musical possibilities. Free and open to the public with no tickets required. On view until April 20, “Impossible Music” brings together sculptures, sounds, scores, video, and live performances to explore the radical, interdisciplinary potential of conceptual and experimental music.