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How do formant frequencies help identify vowels?

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Diagram of a human head in profile showing the location of vocal organs and possible places of articulation for speech sounds, with labels for lips, teeth, tongue, palate, uvula, pharynx, and vocal cords, and numbered regions corresponding to types of articulation such as bilabial, alveolar, and velar.
phonetics Diagram depicting the location of human vocal organs and possible places of articulation used for speech.

In acoustic phonetics, formants are the resonant frequencies of the vocal tract that serve as the primary acoustic factors for identifying and specifying vowels.

These frequencies characterize the shape of the vocal tract during speech production. While there is no simple, direct relationship between specific tongue positions and these frequencies, there is a strong inverse correlation between the frequency of the first (lowest) formant and the "height" of a vowel:

  • First Formant (F1): This frequency is lowest in high vowels (such as the i in "heed") and highest in low vowels (such as the a in "had"). Consequently, when phoneticians classify vowels as high or low, they are effectively describing the inverse of the F1 frequency.1
  • Higher Formants: The frequencies of the second and third formants provide additional information necessary to distinguish between different vowel qualities. Together, the first three formants account for the major part of the information used to identify speech sounds.21

In summary, these formant frequencies act as acoustic markers that define the vocal tract configuration, allowing listeners to perceive and identify distinct vowel sounds.21