From The Art and Popular Culture Encyclopedia
A number of Steve Reich's early works are examples of process music, particularly a specific process called 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. 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 NymanTemplate:Fact 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. Kyle Gann (1987), on the other hand, sees many similarities between serialism and minimalism, and Herman Sabbe (1977) has demonstrated how process music functions in the early serial works of the Belgian composer Karel Goeyvaerts, especially in his electronic compositions Nr. 4, met dode tonen [with dead tones] (1952) and Nr. 5, met zuivere tonen [with pure tones] (1953). Elsewhere, Sabbe (1981) makes a similar demonstration for Kreuzspiel (1951) by Karlheinz Stockhausen. In the 1960s, Stockhausen composed several instrumental works which he called "process compositions", in which symbols including plus, minus, and equal signs are used to indicate successive transformations of sounds which are unspecified or unforeseeable by the composer. These works include Plus-Minus (1963), Prozession (1967), Kurzwellen, and Spiral (both 1968), and led to the verbally described processes of Aus den sieben Tagen (1968) and Für kommende Zeiten (1968–71) (Kohl 1978 and 1981; Hopp 1998).
Elliott Carter also uses the word "process" to describe the complex compositional shapes he began using around 1944 (Edwards 1971, 90–91; Brandt 1974, 27–28), with works like the Piano Sonata and First String Quartet, and continued to use down to the present time.
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 instruments played the section at a slower rate than the treble instruments, and the new piece created was shaped by these melodic lines drifting in and out of phase with each other.