Females Voice Reduced Vowel Usage Above Treble Staff.
About soprano high Cs. This would be designated as a C6 if middle C is designated as a C4. No voice can sing the vowel in “Tod” on a pitch that high. The only vowel available to any voice on that pitch is the vowel “ah” (In IPA /a/).
The reason? All vowels are recognized by the listener hearing two emphasized harmonics particular to each vowel or vowel modification. These harmonics are emphasized by the vocal tract resonances called formants. From A5 and higher the singer finds that all formants which are needed to identify vowels are lower than the pitch they are singing. Therefore the singer must emphasize the fundamental by using the vowel with the highest first formant and that vowel is /a/. The singer is able, within some restrictions, to raise the resonance pitch of the first formant to match the high fundamental. She does this by opening the mouth and jaw and in some cases spreading the lips in a kind of opened mouth smile. This is the reason that all female voices assume some form of open mouth posture when singing these high notes.
Because male voices sing an octave lower it is less necessary for them to have such extreme open mouth positions for their highest notes though some may choose to do so.