Auditory, Visual, and Audiovisual Speech Intelligibility for Sentence-Length Stimuli: An Investigation of Conversational and Clear Speech

Previous investigators have shown that the use of clear speech improves the auditory speech intelligibility of talkers. In the present study, the differences in speech intelligibility for sentences spoken conversationally and in a clear manner were investigated under three different experimental conditions: Auditory-only, visual-only, and audiovisually. Six talkers were videotaped while saying a list of 17 sentences twice: first while using conversational speech and then while using clear speech. The recorded stimuli were randomized and presented to subjects under one of the three experimental conditions, A broadband noise was mixed with the audio signal for the auditory-only and the audiovisual conditions. An auditory, visual, and audiovisual speech intelligibility score was obtained for the tokens of conversational and clear speech spoken by individual talkers. Overall, in each experimental condition, speech intelligibility improved significantly for the tokens of clear speech. However, for the auditory-only and the visual-only conditions there was a significant interaction between talker and manner of speech. In those sensory modalities, the speech intelligibility of some talkers improved when they used clear speech. The results suggest that for an individual talker there is not a direct association in the amount of improvement provided by the use of clear speech across sensory modalities