Frequency Shifts and Vowel Identification

To study the effects of frequency shifts on vowel identification, a high-quality vocoder (STRAIGHT) was used to process a set of vowels in /hVd/ syllables spoken by two adult males, two adult females, and two children. Vowel identification accuracy was reduced when the spectrum envelope was shifted upward by a factor of 2.0, or downward by a factor of 0.6. Upward shifts produced a smaller decline for the vowels of adult males compared to the vowels of adult females and children, while downward shifts led to a smaller drop for children's vowels compared to those of adults. In several conditions, the drop in accuracy with spectrum envelope shifts was counteracted by introducing corresponding upward or downward shifts in fundamental frequency (F0). The results suggest that listeners' prior exposure to statistical regularities in natural speech is an important factor in the identification of frequency-shifted vowels.