The role of lower airway resonances in defining vowel feature contrasts

Abstract

Since the voicing source is located between the lower and upper airways and has a high impedance, the resonances of the lower airway appear as pole-zero pairs in vowel spectra. These pole-zero pairs interact non-linearly with the vocal tract formants, producing narrow frequency bands within which formant structure is unstable. The broader frequency bands between lower airway resonances are thus potentially optimal for the reliable production and accurate perception of specific formant patterns. This possibility is explored from three directions. First, models of the lower airway are built and analyzed, and their effects on vowel spectra are characterized. Second, evidence for the non-linear interactions between formants and lower airway resonances is presented from a speech production study, and the relations between these non-linearities and certain distinctive feature contrasts are explored. Third, a speech perception experiment is carried out in which the identification of a vowel (which could be either [+back] or [-back]) is dependent upon the interaction of the second lower airway resonance (represented as a zero without an accompanying nearby pole) with the second formant. The results of these studies indicate that lower airway resonances do play a role in speech production and perception, and that further study is warranted. In addition, some potential applications to respiratory and vocal medicine are suggested.