A formant is a broad resonance produced by the pharynx and oral cavity. By following the peak frequencies as a series of vowels are elided, one can identify the lowest three formants for each sung vowel. The third formant (2300-2700Hz) doesn't move much in frequency as the vowel changes. You may also notice a fourth formant, known as the singer's formant.
Play vowel elision
The following files sound different simulated vowel sounds with a fundamental frequency of 110Hz.