The Flow of (u) is a piece of music for three voices by Kenneth Gaburo, composed in 1974.
The piece consists of "one note sung by three singers for twenty-three minutes" on u whose goal is "to sing in tune to such an extent that … this would consist of phase-coherent waves of sound."
This, however, proves impossible, as singers must breathe, beating occurs between nearly in tune pitches, and the frequency spectrum or harmonics: "Gently changing balances between the voices would result in very subtle glissandi of the harmonics of the spectrum being heard."
The piece creates a "(sonic) universe in this (linguistic) 'grain of sand'".
[1] Unlike other minimalist works in which, compared to serial music, "a certain level of articulational subtlety is eschewed in favor of an overall psycho-acoustic effect," in Gaburo's piece "the focus is even more intense, and the attention to dynamic shaping given to the lines … is here transferred to the micro-level".