Formant pattern and spectral shape ambiguity in vowel synthesis: The role of fundamental frequency and formant amplitude

When investigating formant pattern and spectral shape ambiguity in Klatt synthesis, an earlier study showed that the perceived vowel quality of Standard German vowel sounds can be changed by varying fundamental frequency only [Maurer et al. (2017). J. Acoust. Soc. Am. 141(5):3469-3470]. In this follow-up study, the previous original synthesis experiment was repeated twice, firstly, with fundamental frequencies (f_o) of the corresponding sounds lowered by one octave, and secondly, with different ratios of the first and second formant amplitudes. Here, the role of the f_o range and the formant amplitudes for the investigation of formant pattern and spectral shape ambiguity in vowel synthesis was further examined. The same five phonetic expert listeners that participated in the previous experiment also identified all of the newly synthesised sounds in a multiple-choice identification task. Results revealed that the perceived vowel quality only changes for f_os above 200 Hz and that, for back vowels, the ratio of the formant amplitudes used in the Klatt synthesis also affects vowel recognition. Thus, the results of the experiments confirm earlier indications of a non-systematic relation between f_o or pitch and formant patterns or spectral envelopes for vowel recognition.