The relationship between the vocalizations of early infants and their mothers was investigated by acoustic analysis. The subjects were eight infants aged 40–70 days and their mothers. The infants were sufficiently developed to utter pleasure vocalization. The acoustic characteristics of the voices of the infants and mothers while communicating with each other analyzed in terms of pitch, duration, latency and melody types. The results were as follows: 1) Correlation was significant between the average fundamental frequencies of the infants' voices and those of their mothers. Motherese and infants' vocalizations were similar in pitch. 2) There was little individual difference in the duration of infants' vocalizations, but a considerable individual difference in the duration of motherese. No correlation was observed in the duration of the vocalizations of the infants and mothers. 3) The latencies of infant vocalization and motherese showed large differences among the infant‐mother pairs. There was a tendency for the latency of the motherese to affect the number of utterances by the infant. 4) Significant correlations in the melody types were observed in three mother‐infant pairs. These findings indicate that there are significant correlations in the vocal exchanges between infants and their mothers.
It is commonly understood that ambient noise and sounds produced by a pregnant woman herself are propagated into the body and reach the unborn child in the uterus. However, it would be unethical to study the propagation of sound to the fetus directly, so the present study, which aimed to clarify this process from an acoustical point of view, used the stomach as a model of the womb. The following points were demonstrated:
For sound waves in the stomach, no interference such as occurs outside the body was observed. However, in the range 2 to 3 kHz, a resonance peak was visible, which was probably due to the gastric air space.
Observing an average spectrum of songs showed that the sound pressure level (SPL(dB)) in the medium to lower range (below 3 kHz) tended to be higher in the stomach than outside.
Adult attribution of the infant vocalizations was investigated by perceptual rating experiments for 717 voice samples recorded from seven infants at 2, 6, 9, 12, and 17 months of age. Perceptual rating was performed using nine vocalization- and emotion-related reference words by 15 normal heating adult listeners. By a principal factor analysis, three factors representing the emotional contrast of crying/frightened/sad versus laughing/pleased/happy, shout/surprising versus secret-talk/calm, and speaking versus singing were studied. Even at 2 months of age, significant individual differences were observed in the extracted factor scores, although three were less variability compared to those obtained from the elder infants’ vocalizations. These results suggest that the ability to express emotion through vocalization seems to be developing during the observed months of age, although infants even at 2 months of age can express some aspects of emotion through vocalization.
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