Vowels are speech sounds produced without obstruction in the vocal tract. They take on different qualities through subtle changes in the shape of the vocal tract caused by the positioning of the lips, tongue, and palate. For example, the vowel sound /i:/ as in "bean" is produced with the tongue raised towards the front of the mouth and lips spread, while /u:/ as in "boon" has the back of the tongue raised and lips rounded. Vowels also vary in terms of lip rounding, position, and length.