Modeling the perception of frequency-shifted vowels

A significant fact about speech perception is that intelligibility is preserved when the spectrum is shifted up or down along the frequency scale, across a fairly wide range. To study the relationship between fundamental frequency (F₀) and spectrum envelope shifts in vowel perception, we used a high-quality vocoder (STRAIGHT) to process a set of vowels spoken by 3 adult males in/hVd/context. Identification accuracy dropped by about 30% when the spectrum envelope was scaled upwards by a factor of 2.0, and in a separate condition, by about 50% when F₀ was raised by 2 octaves. However, when spectrum envelope and F₀ were both increased at the same time, identification accuracy showed a marked improvement, compared to conditions where each cue was manipulated separately. The synergy between formant frequency and F₀ was predicted by a model which accounts for the intelligibility of frequency-shifted vowels in terms of learned relationships between measured values of F₀ and formant frequencies. A second model, based on auditory excitation patterns, predicted the main effects of F₀ and spectrum envelope, but did not predict the pattern of interaction.