Effect of Speaking Rate on Recognition of Synthetic and Natural Speech by Normal-Hearing and Cochlear Implant Listeners

Abstract

Most studies have evaluated cochlear implant (CI) performance using "clear" speech materials, which are highly intelligible and well articulated. CI users may encounter much greater variability in speech patterns in the "real world," including synthetic speech. In this study, the authors measured sentence recognition with multiple talkers and speaking rates, and with naturally produced and synthetic speech in listeners with normal hearing (NH) and CIs. NH and CI subjects were asked to recognize naturally produced or synthetic sentences, presented at a slow, normal, or fast speaking rate. Natural speech was produced by one male and one female talker; synthetic speech was generated to simulate a male and female talker. For natural speech, the speaking rate was time-scaled while preserving voice pitch and formant frequency information. For synthetic speech, the speaking rate was adjusted within the speech synthesis engine. NH subjects were tested while listening to unprocessed speech or to an eight-channel acoustic CI simulation. CI subjects were tested while listening with their clinical processors and the recommended microphone sensitivity and volume settings. The NH group performed significantly better than did the CI-simulation group, and the CI-simulation group performed significantly better than did the CI group. For all subject groups, sentence recognition was significantly better with natural speech than with synthetic speech. The performance deficit with synthetic speech was relatively small for NH subjects listening to unprocessed speech. However, the performance deficit with synthetic speech was much greater for CI subjects and for CI-simulation subjects. There was significant effect of talker gender, with slightly better performance with the female talker for CI subjects and slightly better performance with the male talker for the CI simulations. For all subject groups, sentence recognition was significantly poorer only at the fast rate. CI performance was very poor (approximately 10% correct) at the fast rate. CI listeners are susceptible to variability in speech patterns caused by speaking rate and production style (natural versus synthetic). CI performance with clear speech materials may overestimate performance in real-world listening conditions. The poorer CI performance may be because of other factors besides reduced spectro-temporal resolution, such the quality of electric stimulation, duration of deafness, or cortical processing. Optimizing the input or training may improve CI users' tolerance for variability in speech patterns.