Speech development in infants—vowel production

Five infants ranging in age from 12 to 66 weeks at the start of the project have been recorded at biweekly intervals during half‐hour play sessions with their mothers. Fundamental and formant frequencies were derived from sound spectrograms, Fourier and Linear Predictive analysis. The data reveal differences in the infants' phonetic development as they gradually acquire the vowels of English. Their formant frequency patterns are outside the range reported by Peterson and Barney (1952) for older children but are consistent with the shorter supralaryngeal vocal tracts of these infants. The formant patterns that specify particular vowels change as the infants mature. F1 of/ae/, for example, changes from 1.0 to 0.7 kHz for one infant between 37 and 56 weeks. The infants' vowel production and their mothers' responses are consistent with the presence of an innate vocal tract length normalization perceptual mechanism that enters into the development of speech production.