Investigating alternative forms of clear speech: the effects of speaking rate and speaking mode on intelligibility

Sentences spoken "clearly" (and slowly) are significantly more intelligible than those spoken "conversationally" for hearing-impaired listeners in a variety of backgrounds [Picheny, Durlach, and Braida, J. Speech Hear. Res. 28, 96-103 (1985); Uchanski et al., J. Speech Hear. Res. 39, 494-509 (1996); Payton, Uchanski, and Braida, J. Acoust. Soc. Am. 95, 1581-1592 (1994)]. However, it is unknown whether slower speaking rates are necessary for highly intelligible speech or whether an alternative form of clear speech exists at faster (i.e., normal) rates. To investigate this question, talkers with significant public speaking experience were asked to produce clear and conversational speech at slow, normal, and quick rates. A method for eliciting clear speech was introduced that ensured the clearest possible speech was obtained at each of these speaking rates. To probe for other highly intelligible speaking modes, talkers also recorded sentences in two other speaking modes: soft and loud. Intelligibility tests indicated that clear speech was the only speaking mode that provided a consistent intelligibility advantage over conversational speech. Moreover, the advantage of clear speech was extended to faster speaking rates than previously reported. These results suggest that clear speech has some inherent acoustic properties that contribute to its high intelligibility without altering rate. Identifying these acoustic properties could lead to improved signal-processing schemes for hearing aids.