I have a section in a vocal piece where I want singers to hum. I'm going to include "mmm" in my music notation, but is there a good way to more clearly express the intent to hum in the notation?
Elaine Gould's excellent book Behind Bars includes a section on humming which gives three options for notation:
Humming is indicated by a verbal instruction hum or bocca chiusa, abbrev: b.c. (Italian, 'mouth closed'). It is important to place the instruction above the stave, so that the singer does not articulate it as if it were sung text (a). Where there is frequent alternation with text, two other methods of prescribing humming are useful. To give mm in place of text requires no instruction and is immediately conspicuous (b). Alternatively, the closed-mouth symbol '+' may be substituted, placed above the stave; this symbol should be defined at its first appearance (c).
I've created some short snippets to illustrate these three options.
In my personal experience, I think I've seen option (b) most often, but all three should be clear to the singer as to what is intended.