Perception of frequency-shifted speech

University Of Texas At Dallas, Richardson TX

Investigators

Abstract

With National Science Foundation support, Dr. Peter Assmann will conduct three years of research on how listeners adapt to speech that is shifted up or down along the frequency scale. Such shifts affect everyday speech communication, as listeners adjust to men, women, and children of varying ages. Recent studies have shown that intelligibility drops sharply when the spectrum envelope of speech is shifted upward by a factor of 1.5 or more, or downward by 0.7 or less. The detrimental effects of such shifts can be counteracted, to some degree, by incorporating talker-matched changes in fundamental frequency. This finding, together with predictions from a pattern recognition model, suggests that listeners are sensitive to statistical regularities in natural speech and that they may adapt to frequency-shifted speech through long-term exposure. To test these hypotheses, a speech vocoder will manipulate the spectrum envelope and fundamental frequency of natural speech. The first set of experiments will investigate the conditions that preserve the intelligibility of frequency-shifted speech and test the predictions of pattern recognition models. The second set of experiments will investigate perceptual accommodations to frequency-shifted speech following extended listening experience to determine how well these adjustments generalize across talkers, speech materials, and shift factors. Models of speech perception must explain the ability to understand frequency-shifted speech. Research on this topic may provide insights into two problems faced by hearing-impaired listeners. First, present-day cochlear implant electrode arrays cannot be inserted completely into the cochlea; they provide electrical stimulation only to the basal portion. Implant users need to accommodate to the re-mapping of the frequency spectrum provided by the device. Second, frequency shifts are used in frequency-transposing hearing aids that attempt to restore speech intelligibility for impaired listeners by shifting the spectrum into the region of better hearing. Frequency lowering provides improved speech recognition for some hearing-impaired listeners, especially after extended exposure. But the limited extent of its benefit warrants further study. Studies of the perception of frequency-shifted speech by listeners with normal hearing may provide a better understanding of the perceptual adaptations to the altered frequency mapping provided by cochlear implant processors and frequency-transposing hearing aids. Studies of the perceptual tolerance for frequency shifts may also provide a basis for improving voice quality in speech synthesis, and suggest ways to achieve a greater degree of talker independence in automatic speech recognition systems.