vowels are segments of speech produced without any interruption of the air stream. In the production of vowel sounds, the articulation do not come very close together. Thus, the passage of the air stream is relatively unobstructed. While languages have been known to have voiceless vowels, most English vowels are phonetically voiced. That is, the sounds are produced with vibration of the vocal cords. Sometimes two vowel sounds combine to form a single gilding sound called a diphthong. The different vowel sounds are produced hard by making slight changes in the position of the tongue (front,center,back) which is raised towards the hard or soft palate, and the shape of the lips (spread,neutral, or rounded).
Therefore, when describing a vowel. it is necessary to provide information about these few four aspects ofthe articulation of the vowel.