Introduction 

The group of sounds called "Class 1 sounds" are vowels, and are distinguished as sounds that are made with an open vocal tract. These include A, E, I, O and U.

Vowels are produced when the voice is activated and the vocal tract is open. After the breath stream acquires the vibrations imparted by the vocal folds the shape of the vocal tract then produces certain changes in the sound.

Physiology

Physical characteristics of producing vowel sounds include:

• Closure of the vocal folds
• Contraction of muscles needed to initiate movement of the air stream
• Activation of the voice
• Movement of the tongue to a specific location
• Changes in the size of the mouth opening

In all vowel sounds the vocal tract remains open while voicing occurs, while the shape of the vocal tract determines which vowel sound is produced.