The relative importance of spectral tilt in monophthongs and diphthongs.

Ito et al. [J. Acoust. Soc. Am. 110, 1141-1149 (2001)] demonstrated that listeners can reliably identify vowel stimuli on the basis of relative formant amplitude in the absence of, or in spite of, F2 peak frequency. In the present study, formant frequencies and global spectral tilt are manipulated independently in synthetic steady-state vowels. Listeners' identification of these sounds demonstrate strong perceptual effects for both local (formant frequency) and global (spectral tilt) acoustic characteristics. Subsequent experiments reveal that effects of spectral tilt are attenuated in synthetic stimuli for which formant center frequencies change continuously. When formant peaks are kinematic, perceptual salience of the relative amplitudes of low- and high-frequency formants (as determined by spectral tilt) is mitigated. Because naturally produced English vowels are rarely spectrally static, one may conclude that gross spectral properties may play only a limited role in perception of fluently produced vowel sounds.