Professor
Ph.D., University of Alberta (Canada), 1985
Speech Perception and Psychoacoustics

Email: assmann@utdallas.edu
Phone: 972-883-2435
Office: GR 4.126

Visit personal webpage

About Peter Assmann

My research investigates the mechanisms and perceptual strategies used by listeners to recognize speech under adverse conditions. Speech communication has an extraordinary resistance to distortion: intelligibility is preserved when a substantial portion of the spectrum is eliminated by filtering, when substantial segments of the waveform are deleted or replaced by silence, or when the signal is embedded in background noise. Research in my lab has centered around a series of experimental investigations of the distortions introduced by a competing voice, narrow bandpass filtering, spectral flattening, local time reversal, and frequency shifts. I have been developing theoretical and computational models of speech perception that can account for the extraordinary resilience of human speech communication to channel distortions and predict the effects of such distortions on intelligibility. My current research project is funded by a 3-year grant from the National Science Foundation to examine the effects of frequency shifts on speech perception. The ability to understand frequency-shifted speech is a fundamental skill in everyday speech communication, because listeners must adapt to the frequency shifts that result from differences in larynx and vocal tract size across talkers. Speech intelligibility drops when the spectrum is shifted up in frequency by more than 150%, or down by less than 70%. These limits on intelligibility may reflect listeners' exposure to statistical regularities in natural speech. To test this hypothesis, we are using a high-quality speech vocoder to study the perceptual adaptations to frequency-shifted speech and determine whether these adjustments generalize across talkers, speech materials, and shift factors.

There are three reasons why the perceptual adaptation to frequency-shifted speech deserves careful study. First, present-day cochlear implant electrode arrays cannot be inserted completely into the cochlea, and 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. Adaptation to frequency-shifted input may require long-term exposure, leading to gradual improvements in speech comprehension. Second, frequency shifts are used in frequency-transposing hearing aids which 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. Third, listeners with normal hearing are able to adapt to frequency shifts associated with differences in vocal tract size. This ability must by explained by models of speech perception, and can provide insights into the perception of frequency-shifted speech by hearing-impaired listeners.

Research Interests

Listeners with normal hearing can communicate successfully even when speech is produced under extreme conditions: for example, when the speaking rate is as high as 400 words per minute; when competing voices and other types of background noise are present; and when the signal is distorted by narrow bandpass filtering. Current research in our laboratory considers how listeners achieve this by looking at auditory, perceptual, and cognitive processes that intervene between the production of speech and its recognition. We are developing and testing models of auditory and phonetic analysis to describe how information is extracted from speech under adverse listening conditions.

Recent Publications

Assmann, P.F. and Katz, W.F. (2005). Synthesis fidelity and vowel identification. Journal of the Acoustical Society of America 117(2), 886-895.

Stickney, G.S., Zeng, F.G., Litovsky, R., and Assmann, P.F. (2004). Cochlear implant speech recognition with speech maskers. Journal of the Acoustical Society of America. 116(2), 1081-1091.

Assmann, P.F. and Summerfield, A.Q. (2004). The perception of speech under adverse conditions. Speech Processing in the Auditory System. Chapter 5, Volume 14, Springer Handbook of Auditory Research. S. Greenberg, Ainsworth, W.A.., Popper A.N., and Fay, R.R. (Eds.). New York : Springer Verlag.