The most important tool in your rehearsal is your brain.
Not your voice. Not your ears. Not your instrument. Your brain.
Rehearsal is mental, and the focus you bring to rehearsal is the primary determining factor of the rehearsal’s effectiveness.
I often tell my students that my hope for them is that at the end of rehearsal, their chops are fine – they could keep singing or playing. But I hope that they’re mentally exhausted. At the end of rehearsal, you should want to stop because you’re mentally tired.
Rehearsal is mental.