Acoustic phenotypes for speech-genetics studies: reference data for residual /з/ distortions

An eventual genetic account of at least one subtype of child speech disorders may require the use of acoustic markers to phenotype children and family members. Acoustic markers have the potential for sensitivity and specificity that are not available using auditory-perceptual procedures such as phonetic transcription. Prior reports on the use of acoustics in speech-genetics research have described methods to complete large-scale acoustic analyses, addressed relevant technical and linguistic sampling issues, and provided an acoustic reference database for /s/ production in adolescents. The present paper addresses additional methodological issues and demonstrates how reference data might be used for speech-genetics studies of another class of frequently persisting speech errors - distortions of the English rhotics /r/ and /з/. Findings support the need to adjust acoustic reference data by age and gender, and to subgroup reference data by rhotic phoneme and phonetic context. We describe a z score procedure that accommodates these methodological needs. A companion paper uses these data to develop an acoustic phenotype marker of residual /з/ distortions for speech-genetics research.