We present a comparative analysis of two different recordings of Gerard Grisey's Partiels (1975) to investigate perceptual fusion in orchestration. This interdisciplinary study is an intermediary step of a broader project which aims to study the perceptual features of spectral music compositional strategies. The starting point of this investigation is the exploration of the notion of instrumental synthesis proposed by Gerard Grisey (1946 - 1998). Additive synthesis allows sound creation by adding sine waves with different frequencies and amplitudes in time. However, instrumental synthesis affords the emergence of a new orchestral quality by "synthesizing" it with orchestral sounds (Hasegawa, 2021). This new quality engenders a state that, according to Grisey, is "a hybrid being of our perception, a sound which, without being yet a timbre, is already not quite a chord" (Grisey, 2019b). Far from being a fixed formula, though, each recording of Partiels leads to different sensations in terms of perceptual fusion. That being the case, one might argue that an overlooked aspect of auditory fusion pertains to the domain of interpretation. Thus, how could we take into account those unwritten features of the musical realization? We propose an audio-based approach to address this question.