According to composer and theorist Denis Smalley, it describes the "projection and the spreading of sound in an acoustic space for a group of listeners"[1] during a concert.
In many cases, the sound diffusion is performed by the composer themselves, whose task it is to integrate and interpret the music within the concert space.
[2] The practice was originally formulated by composer Pierre Henry and based on the diffusion of a stereo signal to multiple loudspeakers using a special mixing desk.
The aim of the diffusion is to “exaggerate the dynamic, spectral and spatial content of the musical material already present in the work.”[3] The technical limitations of magnetic tape (such as tape noise or limited spectral range) required the intervention of an engineer in the systems’ early history.
It is up to the diffusion engineer to perform the piece in such a way that the musical content is adequately interpreted as well as adjusted to the specific loudspeaker arrangement and acoustics of the respective concert hall.