What are pure vowels? Simply stated a pure vowel is one that is produced in a way that the voice can maintain cord closure or “adduction” during phonation and can easily move between resonances from lowest note to the highest tone with little difficulty.
The Ah vowel. When vocalizing this vowel sound you want to create continuous compressions in the vocal cabinet and pharyngeal cavity through “coloring” the vowel as you ascend the scale or vocal phrase higher. Begin at a lower tone in your lowest notes pronouncing the “Ah” as in “father”.
As you ascend the scale or vocal phrase especially when approaching the “bridges” or Passagio, you want to go from an “Ah” in “Father” to an “Uh” as in “Other”. As you perform the vowel in this way, a relaxing of the larynx is maintained and the tone sounds transparent as you go higher. If you were to stay with the “Ah” as in Father on the way up, you would experience both tension and vocal cord disconnect.
In addition to the vowel shift, especially as you ascend the scale or phrase, you want to be aware of the shape of your lips, the conjecture of your head in relation to your spinal position and the position of the jaw and shoulders.
Correct Chin Placement You want your shoulders to be as relaxed as possible, not slouched, but relaxed in a natural position of rest. Imagine you’re sitting in a chair with a small child in your arms gently pulling your hands into a “cradled” position. This is how your shoulders should feel during exercise and performance as well.
Your lips should be in a semi-pursed position, in other words, slightly open as if you were about to speak the word “what”. Your head should feel as if it were “floating” or “feather light” resting gently on top of your spine. Your spine should be in a non-rigid position in line with your shoulder position and head. Your chin should be in a level position in line with your shoulders and the jaw should remain relaxed (see illustrations for proper chin placement).
Ricki Keith
http://www.articlesbase.com/music-articles/free-singing-voice-lessons-number-2-678395.html