Stochastic Music

Stochastic music applies probability theory and statistical models to composition. The composer does not specify each event individually but instead defines the distribution from which the events are drawn — a cloud of pitches, a density of attacks, a glissando field. Iannis Xenakis used this approach to write 'Metastaseis' (1953-54), in which the string section produces a continuous mass of glissandi whose collective shape is the musical material, and 'Pithoprakta' (1955-56), built from clouds of pizzicato attacks distributed by statistical model.

Xenakis trained as an architect and worked in Le Corbusier's studio in Paris in the 1950s before turning to composition. His mathematical and engineering background shaped his rejection of the integral serialism that dominated the post-war European scene. His 1963 book 'Formalized Music' is the methodological summary. The Philips Pavilion at the 1958 Brussels World's Fair — designed in Le Corbusier's studio with significant Xenakis contribution, and the venue for Varese's 'Poeme Electronique' — is the architectural counterpart to the music.

Stop trying to follow a melodic line. Track the shape of the mass — how dense it is, how it expands or contracts, where the texture thins. Glissandi in 'Metastaseis' function as the edges of large surfaces; the pizzicato attacks in 'Pithoprakta' function as a granular field whose density is the parameter.

'Metastaseis' (1953-54) for the glissando-mass side. 'Pithoprakta' (1955-56) for the pointillist density side. 'Nomos Alpha' (1965-66) for solo cello shows the same thinking compressed to one instrument.

Xenakis used pencil-and-graph-paper methods for his early stochastic pieces, then moved to computer-assisted composition in the 1960s with his UPIC system, which let him draw musical scores graphically and have the machine sonify them.