“Flat” vowel spectra revisited in vowel synthesis

Abstract

Some studies of natural and of synthesized vowel sounds indicate “flat” vowel-related spectral envelopes or envelope parts in terms of vowel-related frequency ranges with harmonics equal in amplitude. The present investigation addresses this question in a vowel synthesis experiment in which sounds related to series of harmonics, multiples of 200 Hz in frequency and equal in amplitude, were created. Thereby, for various frequency ranges, the number of harmonics was increased stepwise from a single lower harmonic to an increasingly broader harmonic series, and, inversely, it was also decreased from a broad series of harmonics to a single higher harmonic. The entire frequency range of investigation was 0.2-4 kHz. Vowel recognition was investigated by means of a listening test in which five phonetic expert listeners were asked to assign the synthesized sounds to Standard German vowel qualities. The results of the experiment reveal that synthesized sounds with frequency bands of series of two or more equal-amplitude harmonics allow for a perceptual differentiation of the Standard German vowels /i-y-e-ɛ-a-ɔ-o/. Methodological issues concerning future investigations as well as implications for the acoustics and perception of vowels are discussed.