The lowest frequency (first harmonic) of a periodic signal, such as the tone of the human voice. The partials above the fundamental frequency are called harmonics of the fundamental frequency, and the class of sound harmonic. Harmonic sounds are periodic, with their period being the same as its fundamental, and they have a definite pitch. Such sounds are relatively rare in the natural environment, but most musical instruments are designed to produce harmonic or near harmonic sounds. A recent (US) study of healthy fullterm infants (about 4-10 months) and children with severe-to-profound hearing loss (about 25-30 months) revealed no significant changes in F₀ with improvements in pre-language non-distressed vocal development in the fullterm infants. The hearing loss infants presented with a higher F₀ variability, regardless of age. The findings suggested that the development of F₀ may be dependent on audition. In another (Japanese) study, healthy infants and their parents were followed-up from the infant’s birth until about 5 years. Here, it was found that the infant’s F₀ decreases with age, the apparent difference with the US sample possibly due to differences in the linguistic rearing environment and methodology. The Japanese study also reported that parental F₀ is high when using infant-directed speech before the onset of two-word utterances. After the emergence of the utterances, parental F₀ decreases and more or less attains the same value as that for adult-directed speech. The F₀ of the cries made by newborns and infants with various medical problems have been examined for their diagnostic and prognostic value for a number years. Despite many studies, there is as yet no unanimous agreement as to its value in this respect.
See Adolescent voice change, Crying, Frequency, Intensity, Larynx, Pitch, Vocalic resonance