Acoustic features of infant vocalic utterances at 3, 6, and 9 months

Recordings were obtained of the comfort-state vocalizations of infants at 3, 6, and 9 months of age during a session of play and vocal interaction with the infant's mother and the experimenter. Acoustic analysis, primarily spectrography, was used to determine utterance durations, formant frequencies of vocalic utterances, patterns of f0 frequency change during vocalizations, variations in source excitation of the vocal tract, and general properties of the utterances. Most utterances had durations of less than 400 ms although occasional sounds lasted 2 s or more. An increase in the ranges of both the F1 and F2 frequencies was observed across both periods of age increase, but the center of the F1-F2 plot for the group vowels appeared to change very little. Phonatory characteristics were at least generally compatible with published descriptions of infant cry. The f0 frequency averaged 445 Hz for 3-month-olds, 450 Hz for 6-month-olds, and 415 Hz for 9-month-olds. As has been previously reported for infant cry, the vocalizations frequently were associated with tremor (vibrato), harmonic doubling, abrupt f0 shift, vocal fry (or roll), and noise segments. Thus, from a strictly acoustic perspective, early cry and the later vocalizations of cooing and babbling appear to be vocal performances in continuity. Implications of the acoustic analyses are discussed for phonetic development and speech acquisition.