Work on the area of vowels is generally related to the following concept; 'a glottal tone modified by the action of the upper resonators of the mouth, pharyngeal and nasal cavities.' This concept usually underlies basic vowel quality. The glottal tone is that of phonation. Voice is produced as the vocal cords are set into vibration, that is into steady repetition of opening and closing movements. The rate of vibration determines the fundamental frequency of the glottal tone .