Tortoise has spent nearly 25 years making music that defies description. While the Chicago-based instrumental quintet has nodded to dub, rock, jazz, electronica and minimalism throughout its revered and influential six-album discography, the resulting sounds have always been distinctly their own. As ever, Tortoise has conjured sounds on their latest album “The Catastrophist” that aren’t being purveyed anywhere else in music today. There’s a deeply intuitive interplay between the group members that comes only from two decades of experimentation, revision and improvisation. This is a free concert.