From Academic Kids
A number of Steve Reich's early works are examples of process music, particularly a specific process called phase or phasing. In his 1968 work Pendulum Music, a number of microphones are connected to a number of loudspeakers, and each is allowed to swing freely above the loudspeaker it is connected to until it is still - the feedback that results from this process, as each microphone passes above its loudspeaker, makes up the music (see also Reich's short 1968 essay Music as a Gradual Process). György Ligeti's Poème symphonique (1962), in which a hundred metronomes are set to different tempos and allowed to run down is another notable example.
Process music can also be created using relatively traditional instrumental techniques - Reich's Piano Phase is an example. James Tenney is another composer who is concerned with process, such as in his tribute to Steve Reich, Chromatic Canon, in which a tone row is eventually built up and, one note at a time, from what started as a repeated open fifth, before returning by the same path.
Michael Nyman has described how the generally minimalistic tonal music associated with process music arose from the influence of and reaction against process based music of extreme determinism or indeterminism using serial, aleatoric, and stochastic methods.
Within the field of popular music, process music made its strongest early appearance in the ambient works of Brian Eno, notably his first foray into the genre, Discreet Music. On several of the tracks of this album, musicians were instructed to play a small section of Johann Pachelbel's Canon in D major in different ways. On one piece, for instance musicians played the section at different speeds, the speed determined purely by the pitch of the instrument used. Thus the bass instruents played the section at a slower rate than the treble instruments, and the new piece created was shaped by these melodic lines driftin in and out of phase with each other.
- Music as a Gradual Process PDF (http://www.finearts.uvic.ca/music/courses/mus301-old/reich.pdf) by Steve Reich