Tokuma, Shinichi; (1997) The perceived quality of vowels showing formant undershoot. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Vowels in consonantal contexts are modified from their citation form by neighbouring consonants; in particular, their formant frequencies may not achieve their target values. This phenomenon is called acoustic vowel formant undershoot, and in this study vowels in /CVC/ environments are compared with steady state vowels to investigate the perceived vowel quality change caused by undershoot. Although the phonological perception of vowels showing formant undershoot has been frequently investigated, the phonetic vowel quality change caused by the formant undershoot has received little previous attention. The main study in this thesis uses a perceptual task, whereby listeners match constant /CVC/ stimuli of /bVb/ or /dVd/ to variable /#V#/ stimuli, using a schematic grid on a PC screen. The grid represents an acoustic vowel diagram, and the subjects change the F1/F2 frequencies of /#V#/ by moving a mouse. The main results of the study show that while subjects referred to the trajectory peak of the /CVC/ stimuli in vowel quality perception, their performance was also affected by the formant trajectory range of the stimuli. When the formant trajectory range was small, they selected a value between the edge and peak frequencies, while they selected a value outside the trajectory range when it was large. Some influences of low F1 frequency values on F2 matching were also found. These demonstrated phenomena are incompatible with existing vowel perception theories, and three modifications to these theories are suggested to explain these results: (1) to incorporate a psychoacoustic process of undershoot compensation; (2) to incorporate the influence of phonological processes on low-level matching; (3) to exploit a single auditory model with multiple stages, which are either open to general auditory processes or specific to speech perception. Proposals are made for further experimental work to choose between these possibilities.

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