The main hypothesis in this study is that vowel length plays a key role in speech production and synthesis. In noisy, less-than-ideal situations, vowels are regarded as "unmarked," whereas consonants are comparatively valued as conveyors of important information. However, I argue that the duration of vowels is more important in characterizing speech tempo. Vowel length differs in proportion to tempo. When we change tempo, the length of consonants is not affected very much, while the length of vowels is considerably altered.