Purpose: A single-word identification test was used to study speech production in children and adults with Down syndrome (DS) to determine the developmental pattern of speech intelligibility with an emphasis on vowels.
Method: Speech recordings were collected from 62 participants with DS aged 4-40 years and 25 typically developing participants aged 4-7 years. Panels of 5 adult lay listeners transcribed the speech recordings orthographically, and their responses were scored in comparison with the speakers' target words.
Results: Speech intelligibility in persons with DS improved with age, especially between the ages of 4 and 16 years. Whereas consonants contribute to intelligibility, vowels also played an important role in reduced intelligibility with an apparent developmental difference in low versus high vowels, where the vowels /æ/ and/ɑ/ developed at a later age than /i/ and /u/. Interspeaker variability was large, with male individuals being generally less intelligible than female individuals and some adult men having very low intelligibility.
Conclusion: Results show age-related patterns in speech intelligibility in persons with DS and identify the contribution of dimensions of vowel production to intelligibility. The methods used clarify the phonetic basis of reduced intelligibility, with implications for assessment and treatment.