Vowel productions of 2 Mandarin-speaking children were audio recorded in their homes with picture naming tasks once every 3 months, from birth to 9 years old. The present study is the ninth year of a longitudinal observation. Major findings in this stage are: 1) The trend of decrease in formant values was continuously observed in the boy subject. As for the girl subject, it is not until nine years old, the obvious decrease in formant values was found, especially in F1; 2) F1 values are more stable than F2 values in both subjects. They appeared to acquire jaw movement sooner than tongue movement; 3) Throughout these 9 years, the variability of F1 is around 200-300Hz, and the variability of F2 is 500-700Hz in both subjects. No trend of decrease was found; 4) The trend of shrinkage in F1-F2 vowel area continues from 7 to 9 years old for the boy subject, but not for the girl subject; 5) There is a clear decline in fundamental frequencies at 8-9 years of age in the boy subject. Longitudinal data of vowel formant values from the same group of subjects provides important references for assessment and treatment of articulation disorders in children.

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