**4.1 Technology design and experimental protocols**

We designed three binaural headphone monitoring solutions to enhance musicians' cognitive engagement in performance (**Table 1**). For each solution, we adapted the augmentation type, sound capture system, and mixing approach to the esthetic and cultural context of distinct performance situations (**Table 2**). Then, we conducted eight case studies that involved two renowned conductors of symphonic ensembles with a click track in large acoustics [7]; seven emerging music improvisers in solo or trios in separated dry rooms or overdubbing alone in small acoustics [8]; and three music students and one touring musician who recorded for a range of musical genres alone or in a duo with a click track or a soundtrack in medium-size acoustics [9]. These eight case studies were all carried out in real-life performance situations at the Paris Conservatoire (CNSMDP), Radio France, and the University of Lethbridge (ULeth).

Our mixed methods of assessing these solutions draw upon Agrawal et al. [50]'s definition of *immersion* as a psychological state that enables an individual's mental absorption in the world and in the tasks that are presented to them. Therefore, for each performance case study, we determined which auditory information would be the most important for the users to monitor in order to perform at their best, in other words, which auditory information would be "immersive enough" [51] to achieve a sense of "being there together" [52].
