Vowel acoustic space development in children: a synthesis of acoustic and anatomic data.

PURPOSE This article integrates published acoustic data on the development of vowel production. Age specific data on formant frequencies are considered in the light of information on the development of the vocal tract (VT) to create an anatomic-acoustic description of the maturation of the vowel acoustic space for English. METHOD Literature searches identified 14 studies reporting data on vowel formant frequencies. Data on corner vowels are summarized graphically to show age- and sex- related changes in the area and shape of the traditional vowel quadrilateral. CONCLUSIONS Vowel development is expressed as follows: (a) establishment of a language-appropriate acoustic representation (e.g., F1-F2 quadrilateral or F1-F2-F3 space), (b) gradual reduction in formant frequencies and F1-F2 area with age, (c) reduction in formant-frequency variability, (d) emergence of male-female differences in formant frequency by age 4 years with more apparent differences by 8 years, (e) jumps in formant frequency at ages corresponding to growth spurts of the VT, and (f) a decline of f0 after age 1 year, with the decline being more rapid during early childhood and adolescence. Questions remain about optimal procedures for VT normalization and the exact relationship between VT growth and formant frequencies. Comments are included on nasalization and vocal fundamental frequency as they relate to the development of vowel production.