A number of Steve Reich's early works are examples of process music, particularly a specific process called phase. 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 louspeaker 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 twelve-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.
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