Developmental progressions in the harmonic structure of infant-directed speech

Caregivers speak differently to infants than to adults early in life. Previous studies of this special register, “motherese,” have sought to identify key differences in voice quality between infant-directed and adult-directed speech. Acoustic properties most salient to infants appear to reside in exaggerations of prosodic structure, but these findings are limited by speech analysis techniques that do not fully capture the acoustic structure of the maternal voice or the way motherese changes over the course of development. In this study, multitaper analysis was used to measure developmental progressions in the complete harmonic structure of the maternal voice. Samples of adult- and infant-directed speech were extracted from home audio recordings of 20 mothers collected monthly from 0-24 months using LENA technology. Multitaper analysis was used to calculate the time-varying amplitude and phase of every harmonic component, the residual noise component, and the spectral envelope, permitting a complete statistical analysis of source and resonance properties. Key differences and developmental changes between adult- and infant-directed registers were found not only in the fundamental frequency, but in the overall structure of the harmonic spectrum, indicating that manipulations of the whole voice source may be employed to create motherese.