Speech intelligibility of dysarthric speech: human scores and acoustic-phonetic features

We investigated speech intelligibility in dysarthric and non-dysarthric speakers as measured by two commonly used metrics, ratings through the Visual Analogue Scale (VAS) and word accuracy (AcW) through orthographic transcriptions. To gain a better understanding of how acoustic-phonetic correlates could be employed to obtain more objective measures of speech intelligibility and a better classification of dysarthric and non-dysarthric speakers, we studied the relation between these measures of intelligibility and some important acoustic-phonetic correlates. We found that the two intelligibility measures are related, but distinct, and that they might refer to different components of the intelligibility construct. The acoustic-phonetic features showed no difference in the mean values between the two speaker types at the utterance level, but more than half of them played a role in classifying the two speaker types. We computed an acoustic-phonetic probability index (API) at the speaker level. API is moderately correlated to VAS ratings but not correlated to AcW. In addition, API and VAS complement each other in classifying dysarthric and non-dysarthric speakers. This suggests that the intelligibility measures assigned by human raters and acoustic-phonetic features relate to different constructs of intelligibility.