Vowel exercises that use semi-occluded lip positions.
Here are some exercises that I have used to help students develop a vocal tone this is rich yet easy to produce. They are all based on the concept that good vocal tone is a combination of efficient and clear phonation assisted by resonance adjustments that compliment the phonation while simultaneously enhancing the tone through a balanced vowel resonance.
They are not given in order of preference. All of these are most effective if used with a siren sound that takes the voice from the bottom of its range to as high as is comfortable.
a. lip trills
b. tongue trills
c. back of the hand placed very near slightly puckered lips almost closing and sealing the mouth opening
d. blowing air through loosely closed lips so they puff up the cheeks and then adding tone to this configuration. The lips will often tickle when doing this.
e. a variation of the puffed lips. Place a clean folded handkerchief over the first finger of the hand, place it horizontally to the lips of the mouth, and breath air through the handkerchief cloth. Then produce phonated sound through the cloth.
f. A variation of [e]. Place a folded paper towel over your first finger and place the finger horizontally at your lips close enough so that the lips create a breath seal around the handkerchief/towel covered finger. You will discover that you can exhale out through the mouth and the breath passes through the fibers of the handkerchief or paper towel. Phonate a siren sound through this handkerchief/towel. It is a very effective way to warm up the voice without making a loud sound yet you are singing at near full voice.
g. semi-occluded sustainable consonant sounds such as v, voiced th, l, m, n, ng, and even z.
Normally the vowel /a/ is used during these semi-occluced sirens but other vowels are also beneficial if for no other reason than that they make the singer aware that it is position of the tongue that creates the vowel so the tongue must be put into the necessary position for the desired vowel without changing the semi-occluded position of the lips. Obviously this cannot work with semi-occluded consonants that require the tongue be in position against the teeth or lips such as lip trills, voiced th, l, m, n, ng, and z.
Try these out. I would be interested in which of them seem to be most helpful to you.
I have used them all with my students.