The Vowels : An Overview
There are five vowels in English but 15 vowel sounds. The vowels are named A, E, I, O, U and the pronunciation of each of these vowel names makes what is called a long vowel. Each long vowel has what has traditionally been called a short vowel. The "long and short" sounds of vowels constitute 10 of the vowel sounds.
Vowels can be classified as simple or complex. A simple vowel is a sound with no detectable change from beginning to end. The short vowels are simple vowels. The long vowels are more complex. They are characterized by a continual movement of the tongue, which changes the sound quality. For this reason, in many phonetic alphabets the long vowels are represented by two sound symbols, representing the beginning and end of the changing vowel sound. Changing vowel sounds (long vowels) are called diphthongs, and they are produced by a gliding movement of the tongue from the first part of the diphthong to the second.
Vowels are also characterized as tense or lax. The tense vowels (long vowels A, E, O, and U, which correspond to # 3, 1, 11, and 9 on page 191 in Well Said) are formed by tensing the muscles of the tongue and face. The lips are spread for A and E, rounded for O and more pursed for U. The muscles are in a more relaxed position for all other vowels, which are also shorter in duration except for the dipthong /ay/ (number 13 on page 191.) The front part of the tongue is bunched forward and is in a very tense position for the second sound of this diphthong.
The starting tongue position for all vowels is low in the mouth with the tip of the tongue resting very low against the back of the lower teeth. Practice "brushing" the back of your lower teeth with the tip of the tongue, and as it moves downward in this "brushing" motion you will feel exactly where the tip of the tongue should be as you begin to pronounce any of the vowels.