Producing Vowels
If the mouth be kept wide open and no other organ be allowed to modify or interrupt the sound a vowel is produced. In speech every part of the head that can be used is brought into action to modify these uninterrupted vibrations of vocal cords and air. The lips, the cheeks, the teeth, the tongue, the hard palate, the soft palate, the nasal cavity, all coöperate to make articulate speech.
As in its mechanism, so in the essence of its modifications, the human voice is a marvel of simplicity. If the mouth be opened naturally and the tongue and lips be kept as much out of the way as in ordinary breathing, and then the vocal cords be made to vibrate, the resulting sound will be the vowel a as in father. If now, starting from that same position and with that same vowel sound, the tongue be gradually raised the sound will be modified. Try it. The sound will pass through other vowels. Near the middle position it will sound like a in fate; and when the tongue gets quite close to the roof of the mouth without touching it the vowel will be the e of feet. Others—such as the i of it—can be distinguished clearly.
Starting again from that same open position and with that same vowel sound, ah, if the tongue be allowed to lie flat, but the lips be gradually closed and at the same time rounded, the sound will pass from ah to the o of hope, then on to the oo of troop. The oa of broad and other vowels can be distinguished at various positions.
By moving lips and tongue at the same time an almost infinite variety of vowel sounds can be made.