Chapter 5 starts with the definitions of the note and the acoustics of sound production. Here, I first examine the acoustical underpinnings of the classical Greek writings on the subject and the impact they had on how the musical note was conceptualized. I then demonstrate that scholars of the medieval Islamic world approached their received wisdom with a skeptical eye and occasionally disagreed with their intellectual masters. These disagreements resulted in illuminating conversations about the nature of a musical note, how it should be differentiated from mere sound, and what role do acoustics of sound production play in these discussions.