Properties of naturally produced clear speech at normal rates and implications for intelligibility enhancement

In adverse listening conditions, talkers can increase their intelligibility by speaking clearly[45, 55, 42]. While producing clear speech, however, talkers often reduce their speaking rate significantly[46, 55]. A recent study[28] showed that speaking slowly is not responsible for the high intelligibility of clear speech, since talkers can produce clear speech at normal rates with training. This finding suggests that acoustical factors other than reduced speaking rate are responsible for the high intelligibility of clear speech. To gain insight into these factors, acoustical properties (global, phonological, and phonetic) of conversational and clear speech produced at normal speaking rates were examined. Three global acoustic properties associated with clear/normal speech were identified: increased energy near the second and third formants, higher average and greater range of FO, and increased modulation depth of low frequency modulations of the intensity envelope. In order to determine which of these acoustical properties of clear/normal speech contribute most to its high intelligibility, signal processing transformations of conversational speech were developed. Results of intelligibility tests with hearing-impaired listeners and normal hearing listeners in noise suggest that these properties may not fully account for the intelligibility benefit of clear/normal speech. Other properties important for highly intelligible speech may not have been identified in this study due to the complexity of the acoustic database and varying talker strategies. Thesis Supervisor: Louis D. Braida Title: Henry E. Warren Professor of Electrical Engineering