Gradient evaluation of /k/-likeness in typical and misarticulated child speech

Phonetic transcription is an important instrument in the evaluation of misarticulated speech. However, this instrument is not sensitive to fine acoustic-phonetic detail – information that can provide insight into the processes underlying speech production [1]. An objective and fine-grained measure of children’s efforts at producing a specific speech target would be clinically valuable, both in assessment and when monitoring progress in therapy. Here, we describe the first steps towards such a measure.This study describes the perceptual and acoustic evaluation of children’s successful and inaccurate efforts at producing /k/. A corpus of 2990 recordings of isolated words, beginning with either /tV/ or /kV/, produced by 4-8-year-old children, was used. The recordings were labelled with regards to whether they were a) correct productions, b) clear substitutions (i.e. [t] for /k/, or [k] for /t/), or c) intermediate productions between [t] and [k].In the perceptual evaluation, 10 adult listeners judged 120 typical and misarticulated productions of /t/ and /k/ with regards to a scale from “clear /t/” to “clear /k/”. The listeners utilized the whole scale, thus exhibiting sensitivity to sub-phonemic detail. This finding demonstrates that listeners perceive more detail than is conveyed in phonetic transcription. However, despite their experience of evaluating misarticulated child speech, the listeners did not discriminate between correct productions and clear substitutions, i.e. they did not distinguish successful productions of [t] for /t/ from cases where [t] was a misarticulated production of /k/ (and vice versa).In order to explore the existence of covert contrasts, i.e. sub-perceptual differentiation between correct productions and clear substitutions, acoustic analysis was performed. Here, a frequently described approach [2] to the analysis of voiceless plosives was compared to more recent methods. We report on the performance of the different methods, regarding how well they modelled the human evaluations, and to their sensitivity to covert contrast.