I was just writing an email about this to someone, and decided to share what I wrote since it might help some of you. It's about what happens with the EE vowel in the upper ranges.

Even though the EE vowel has a low first formant, it still undergoes a modification, but it all depends on how you are trying to sing it. If you're only using a really light intensity, you don't have to modify it much -- it can stay more true to the original vowel sound. However, when you start using much greater intensity like you're belting, it needs to modify, otherwise you will get tight in the larynx. For men, this modification is especially prominent in the first bridge range of F4-Ab4 (C5-Eb5 for women). The more you try to belt it, the more necessary it is. It modifies a bit towards a French U. This is a slightly darker version of the regular EE vowel. You can think of it as a rounded EE. It allows the larynx to sit a little lower and actually allows you to sing with much greater chest activity if you desire to. If you try to use too much of this in the second bridge range, though, it will make things harder. For the second bridge of A4+ (E5+ for women), you gradually start to open the mouth more, and it begins to shade towards IH a little bit (but will still sound like EE to your listener).

Listen to Brent Smith in this acoustic performance of "Bully" for WAAF. Each time he sings an EE vowel in the chorus, especially for the word "bully", he does it with the French U. It's not a normal spread, smiley EE type sound -- it's rounded. This is so the larynx sits more comfortably. With this approach, he could still sing the EE with even more intensity than he used there and it would still sound exactly like the vowel.

https://www.youtube.com/watch?v=R7IrEA7VX1I

In this Manowar rehearsal of "Heart of Steel", you can hear Eric Adams doing the same thing. Listen to "steel" at 1:59, 3:48, 4:15 and 4:49. When you modify the vowel correctly, the voice picks up this very particular kind of resonance. You can hear this "whistling" overtone come into the sound. This is what I call a "vertical" resonance rather than a "horizontal" resonance, because it feel like it goes straight up and down into the head instead of sideways out the mouth (like a normal spoken EE would).

http://www.dailymotion.com/vid.....blic_music

In this performance of "All the Man I Need" by Whitney Houston, you can hear the same thing again. She gives the most perfect example of how this vowel should sound. There are lots of EE vowels in the chorus of this song on "need" and "he". She's using a lot of the French U here. You can see even the way she shapes her mouth is different than a normal spoken EE. It's the same thing operatic tenors do. Listen to the "need" from 2:24-2:30. Then listen to all the EE vowels from 2:55-3:24, but especially the "he" at 2:59, "he" at 3:18, "need" at 3:20, and EE at 3:22. Then listen more from 3:25-3:52 to all the EE vowels. Especially listen and LOOK at the "he" at 3:39, "he" at 3:46, "need" at 3:48, and "need" at 3:49. You can see the proper mouth shape more clearly at those times.

https://www.youtube.com/watch?v=xmIp8j2cjDQ

In this performance of "Stay" by Sugarland, you can hear Jennifer Nettles often using the French U. Especially listen to all the EE vowels in the chorus. You see she keeps her mouth in a very neutral position when she sings it.

https://www.youtube.com/watch?v=k2r6QUZ9UmI

She does the same in this performance of "Hello Again". Listen to "sleep" 0:34, "think" at 1:06, "we" at 1:48, and "think" at 2:43. Great technique, btw.

https://www.youtube.com/watch?v=LuJCo0CgWIY