Identification of frequency-shifted vowels.

Within certain limits, speech intelligibility is preserved with upward or downward scaling of the spectral envelope. To study these limits and assess their interaction with fundamental frequency (F0), vowels in /hVd/ syllables were processed using the STRAIGHT vocoder and presented to listeners for identification. Identification accuracy showed a gradual decline when the spectral envelope was scaled up or down in vowels spoken by men, women, and children. Upward spectral envelope shifts led to poorer identification of children's vowels compared to adults, while downward shifts had a greater impact on men's vowels compared to women and children. Coordinated shifts (F0 and spectral envelope shifted in the same direction) generally produced higher accuracy than conditions with F0 and spectral envelope shifted in opposite directions. Vowel identification was poorest in conditions with very high F0, consistent with suggestions from the literature that sparse sampling of the spectral envelope may be a factor in vowel identification. However, the gradual decline in accuracy as a function of both upward and downward spectral envelope shifts and the interaction between spectral envelope shifts and F0 suggests the additional operation of perceptual mechanisms sensitive to the statistical covariation of F0 and formant frequencies in natural speech.