In adverse listening conditions, talkers can increase their intelligibility by speaking clearly [; ]. This modified speaking style, known as clear speech, is typically spoken more slowly than conversational speech [; ]. However, talkers can produce clear speech at normal rates (clear∕normal speech) with training [] suggesting that clear speech has some inherent acoustic properties, independent of rate, that contribute to its improved intelligibility. Identifying these acoustic properties could lead to improved signal processing schemes for hearing aids. Two global-level properties of clear∕normal speech that appear likely to be associated with improved intelligibility are increased energy in the 1000–3000-Hz range of long-term spectra and increased modulation depth of low-frequency modulations of the intensity envelope []. In an attempt to isolate the contributions of these two properties to intelligibility, signal processing transformations were developed to manipulate each of these aspects of conversational speech independently. Results of intelligibility testing with hearing-impaired listeners and normal-hearing listeners in noise suggest that (1) increasing energy between 1000 and 3000 Hz does not fully account for the intelligibility benefit of clear∕normal speech, and (2) simple filtering of the intensity envelope is generally detrimental to intelligibility. While other manipulations of the intensity envelope are required to determine conclusively the role of this factor in intelligibility, it is also likely that additional properties important for highly intelligible speech at normal speech at normal rates remain to be identified.
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