Babies start making speech-like vocalizations long before they start to produce recognizable words — various stages of these sounds are variously described as cries, grunts, coos, goos, yells, growls, squeals, and "reduplicated" or "variegated" babbling. Developmental progress is marked by variable mixtures of variable versions of these noises, and their analysis may provide early evidence of later problems. But acoustic-phonetic analysis of infant vocalizations is hindered by the fact that many sounds (and sound-sequences) straddle category boundaries. And even for clear instances of "canonical babbling", annotators often disagree on syllable counts, making rate estimation difficult.
In "Towards automated babble metrics" (5/26/2019), I toyed with the idea that an antique work on instrumental phonetics — Potter, Koop and Green's 1947 book Visible Speech — might have suggested a partial solution:
By recording speech in such a way that its energy envelope only is reproduced, it is possible to learn something about the effects of recurrences such as occur in the recital of rimes or poetry. In one form of portrayal, the rectified speech envelope wave is speeded up one hundred times and translated to sound pattern form as if it were an audible note.
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