A singer whose vocal resonance is even and consistently good from note to note, high or low, soft or loud, is changing the vowels semitone by and the vocal tract is constantly changing form.
Singers have to work on how they form certain vowel sounds in a performance. Some vowel sounds are naturally brighter than others. For instance the “ee” vowel sound tends to be very bright and without some modification. The sound can stick out when singing a song. This link is a vowel sound chart from Wikipedia: https://en.wikipedia.org/wiki/IPA_vowel_chart_with_audio
The art of vowel modification is to create a more pleasing sound to the ear through the tongue, jaw and lip placement. To accomplish this requires a lot of practice. The brighter vowels such as "ee" "ay" and the short "a" as in "cat" are cases in point. The "ee" vowel sound can be modified by dropping the jaw slightly to the short "i" position as in "it". Vowel modification is usually in the upper registers; for example, the "ee" as in "bee" goes to "ih" as in "it." By using rounded, closed vowel sounds, we’re able to more easily transition from chest voice to a higher register. Mariah Carey changes her vowel sounds to hit those amazing high notes.
When you come across a problem area, experiment with changing how you articulate the vowel to help make it more comfortable to sing. Singers often complain about their high notes sounding too thin or harsh. This is because as you sing higher your throat tends to close creating a thinner, harsher sound.
JOKE FOR THE DAY:
What loud instrument do you carry in your ears? Drums (ear drums).