A longitudinal acoustic study of a phonetic disorder affecting the production of voiced stops.

Voicing is considered as less salient acoustically and more complex to produce than other phonetic features, and frequently causes speech errors in disordered speech. However, it is not straightforward to study voicing: the phonetic vs. phonological nature of the speech errors is difficult to determine precisely, and so is the characterization of some productions as voiced vs. unvoiced, especially in clinical applications. This study investigated the implementation of the voicing feature in a single-case longitudinal clinical study. The study was based on a 9-months longitudinal tracking of a 11-year-old boy showing difficulties in the production of voiced stops affecting both spoken and written language. Three recording sessions have been carried out three months apart, which included simultaneous acousting and nasometric recordings on a naming task. Additionally, over the 9-months tracking a speech therapy was conducted, with the purpose of improving the voiced/unvoiced distinction via specific exercices using proprioceptive and tactile biofeedback. The results of the perceptual and acoustical analyses point out to a benefical effect of the therapy - the subject showing an improvement in the production of voiced stops. Moreover, our analyses have highlighted some utterances qualified as « intermediate », meaning that their characterization as voiced vs. unvoiced could not be clearly established. VOT measures on these productions revealed a specific phonetic pattern. Furthermore, some specific acoustic phenomena will be discussed, which could result from compensatory strategies spontaneously used by the subject (nasalization, epenthesis, ...). This study contributes to attest the direct benefits of using acoustic analyses in clinical practice, such as diagnosis and evaluation of therapy effectiveness. However, our results also raise the question of the use of VOT as the primary cue to distinguish between voiced and unvoiced productions in clinical context.