A second analytical avenue that also intersects with several documents in the collection concerns the notion of sound fusion. As Moe Touizrar and Stephen McAdams point out: “Perceptual fusion occurs when sounds played by several instruments combine to give the auditory illusion of a single unified sound emanating from a single source [1].” The phenomenon of perceptual fusion lies at the heart of instrumental synthesis, a technique of writing in which, as Grisey points out, “the instrumental source disappears in favour of a synthetic timbre that is totally invented and not given a priori by the instruments [2].” This fusion – and thus synthesis – is best achieved under certain conditions. As Albert S. Bregman explains: “If, for example, the auditory system detects a series of partials with harmonic relationships, it will group them together into a single sound whose pitch it will calculate [3].” This is exactly what happens at the beginning of Partiels, which provides an ideal concrete example for considering the notion of sound fusion through the prism of performance. Four elements of the documentary material on Les Espaces Acoustiques are particularly instructive in this sense: an extract from an interview with the conductor, an extract from an interview with the two flautists in the ensemble, an extract from a rehearsal at the beginning of Partiels, and a photograph of the beginning of the score of Partiels annotated by the conductor. The remainder of this section sets out to analyse the performers’ discourse and to relate it to traces of performance activity in the course of the rehearsals.