There are some nonreflexive, nondistress sounds produced with a lowered velum and a closed or nearly closed mouth, giving the impression of a syllabic nasal or a nasalized vowel. The vocal tract is held in a fixed position. Initially comfort sounds are brief and produced in isolation, but later appear in series separated by glottal stops. Laughter appears around 4 months. Fricatives, affricates and liquids are more rarely heard, and clusters are even rarer.
Vowels tend to be low and open, at least in the beginning.
The variegated sequences are initially rare and become more common later on. Both vocal play and babbling are produced more often in interactions with caregivers, but infants will also produce them when they are alone. No other animal does anything like babbling. These underextensions and overextensions develop and change over time in an individual child's usage. In fact, the development of phonological abilities begins even earlier.