Violin, cello, two clarinets doubling bass clarinet, four women’s voices, four pianos, three marimbas, two xylophones, and vibraphone with no motor. No electronics except for mics on the voices and selected other instruments. Such are the forces Steve Reich calls for in his hour-long pillar of the minimalist repertory Music for 18 Musicians. As the composer himself has pointed out, the body count is elastic, and reinforcements may be required. For what it’s worth, the roster of the out-of-the-box chamber orchestra Alarm Will Sound, which is celebrating the work’s 50th anniversary on this occasion, numbers an even 20 players. Reich’s highly technical comments on the score are precise but bewildering. Among its most distinctive features is the harmony, based on 11 chords that deconstruct and reconstitute themselves over and over. Another is its rhythm, which superimposes the constantly shifting pulse of the singing voices and instruments that ride on the breath on a steady, even metronomic pulse sustained throughout the piece by the pianos and mallet instruments. The effect is hallucinatory, well justifying the common description of minimalism as “trance music.” —Matthew Gurewitsch