Perception and Comprehension of Synthetic Speech

An extensive body of research on the perception of synthetic speech carried out over the past 30 years has established that listeners have much more difficulty perceiving synthetic speech than natural speech. Differences in perceptual processing have been found in a variety of behavioral tasks, including assessments of segmental intelligibility, word recall, lexical decision, sentence transcription, and comprehension of spoken passages of connected text. Alternative groups of listeners—such as non-native speakers of English, children and older adults—have even more difficulty perceiving synthetic speech than young, healthy, college-aged listeners typically tested in perception studies. It has also been shown, however, that the ability to perceive synthetic speech improves rapidly with training and experience. Incorporating appropriate prosodic contours into synthetic speech algorithms—along with providing listeners with higherlevel contextual information—can also aid the perception of synthetic speech. Listener difficulty in processing synthetic speech has been attributed to the impoverished acousticphonetic segmental cues—and inherent lack of natural variability and acoustic-phonetic redundancy—in synthetic speech produced by rule. The perceptual difficulties that listeners have in perceiving speech which lacks acoustic-phonetic variability has been cited as evidence for the importance of variability to the perception of natural speech. Future research on the perception of synthetic speech will need to investigate the sources of acoustic-phonetic variability and redundancy that improve the perception of synthetic speech, as well as determine the efficacy of synthetically produced audio-visual speech, and the extent to which the impoverished acoustic-phonetic structure of synthetic speech impacts higher-level comprehension processes. New behavioral methods of assessing the perception of speech by human listeners will need to be developed in order for our understanding of synthetic speech perception to keep pace with the rapid progress of speech synthesis technology.