A longitudinal analysis of the spectral peaks of vowels for a Japanese infant

This paper describes a longitudinal analysis of the vowel development of a Japanese female infant between the ages of 4 and 60 months. The speech data were natural spontaneous speech recorded for at least an hour per month. Vowels were extracted from the data according to their phoneme labels by two transcribers. The two lower spectral peaks of the vowels were estimated by linear predictive coding method. Analyses of the mean values of the spectral peaks suggest that the values change with age according to the articulation positions of the tongue and the rapid development of the vocal tract. The values change greatly until the age of 18 months. In addition, the distribution of the two spectral peaks for individual vowels shows that the vowel space expands with age, and this also continues until the age of 18 months. Another investigation of the vowel distributions based on discriminant analysis confirms that the differentiation of the spectral peak distributions also continues until around the age 18 months in terms of a physical phenomenon. These results suggest that the infant becomes able to produce discriminable vowels corresponding to the rapid increase in the vocal tract length at around that age.