I have been reading everyone's posts and appreciate everyone's input. First
of all, I don't study singing in a formal program, I just take lessons
privately, so I am not familiar at all with IPA other than having heard
about it on Vocalist. But at least I know now that what I am talking about
is an [i] sound!
My voice teacher has been the person who tells me that I need to keep the
back of my tongue down for this vowel. He has explained that the back part
of the tongue rises slightly and touches the molars in the back for this
vowel. However, my tongue rises up A LOT on [i], especially at the top of my
range. I can see it myself when I sing in front of a mirror. I have been
struggling with this for awhile and am wondering if there is some
physiological reason why I cannot keep the back of my tongue down for this
vowel. As I said before, this only happens on this one vowel. My teacher
tells me this is an issue of tension and inadequate support.