>>126844456> non-12ED2While you're right in saying that the difference in pitch between notes is not equal, they still are singing with twelve notes to an octave, as you can still write the music on paper. Barbershop sings in Just Intonation, where ratios derived from the harmonic series are used to calculate the pitch of a note. When singing in a live setting, such as barbershop, humans will naturally tune the note their singing relative to the root note of the chord.
picrel