If you couldn't tell, I'm really strapped for time this week (work, work, and, uh, work!), so I'm going to let AMG's D. J. Hoek lead you through this one:
Following intensive study of West African music at the University of Ghana in 1970, Steve Reich returned to the United States with a heightened interest in polyrhythm and the desire to compose his first large-scale work. The resulting composition, Drumming (1971), requires more performers, employs greater variety in instrumentation, and — at ninety minutes in length — is longer than any of Reich's earlier works. This more expansive approach represents the culmination of Reich's experiments in phase shifting and positions Drumming as one of the first masterpieces of musical minimalism.
Although Reich's time in Ghana introduced him to the music and traditions of another culture, his studies there were most important in reinforcing his own predilection with repetition and slowly changing patterns. While the shifting polyrhythms of his compositions at the time seemed foreign to Western art music, Reich found similar manners of organization in African drumming, and this sense of musical heritage encouraged him to continue his experiments.
In Drumming, a single rhythmic pattern provides the source material for the entire composition. Although this sort of economy also characterizes most of his earlier pieces, Reich develops this cell not only through rhythmically shifting repetitions, but also through varying timbral combinations.
Each of the four parts that comprise Drumming — performed in sequence without pause — features a particular instrumental-vocal ensemble: Part One is written for bongos and male voice; Part Two is for marimbas and female voices; Part Three is for glockenspiels, piccolo, and whistling; and Part Four, the composition's apex, includes all the instrumental and vocal forces from the previous sections. These instrumental combinations would prove significant: the instrumentation for mallet percussion appears in a number of Reich's later works, and the textless vocalizations recur in Music for Mallet Instruments, Voices, and Organ (1973) and Music for 18 Musicians (1976), as well as in many works by fellow minimalist Philip Glass. By melding the crisp precision of percussion with the inherent lyricism of human voices (which are at times extended by whistling and the use of the piccolo), Reich creates smooth transitions between sections of contrasting orchestration while maintaining the timbral and textural interest that enhances Drumming's fundamental rhythmic momentum.