All around the world, parents talk differently to babies than they do to adults. With their young kids, parents switch into a mode of communication known to linguists as infant-directed speech (aka baby talk), a form of speech featuring long pauses and a roller coaster of pitch changes.
While some parents may feel a bit silly using baby talk, they shouldn’t: Babies not only prefer listening to these exaggerated contours, but they also learn new words more easily from them. By highlighting the structure of speech, such as the differences between the vowels “a” and “o,” it helps babies translate a torrent of sound into meaningful units of language.
Although scientists know a lot about the changes in rhythm and pitch in infant-directed speech, we know much less about the role of timbre, or tone color, which includes the breathiness, roughness or nasality in a voice.
The timbre of an instrument (whether buzzy, warm or twangy) affects how we experience music. Mothers change their overall timbre when speaking to babies, almost as if they’re morphing their voice into a different instrument to address these unique little listeners.
Timbre is a complex acoustic feature that helps us distinguish the unique flavors of sounds around us. For example, Barry White’s silky-smooth voice sounds different from Leonard Cohen’s gravelly one or comedian Gilbert Gottfried’s nasally one even if they’re all singing the same note. Contorting the shape of your vocal tract (which goes from your vocal cords all the way up to your lips) results in different resonances, allowing celebrity impersonators and voice-over artists to change their overall timbre.
Because timbre refers to a more complex collection of features than pitch, rhythm or volume, it is a less well-understood property of sounds. But we do know that timbre provides an important pointer to different sound sources, thus helping us identify people, animals and objects based on their characteristic auditory “fingerprints.” Which raised the question: Do mothers unconsciously change their overall fingerprints when talking to their babies, perhaps to signal that an important source of speech, which is highly relevant for learning, is coming their way?
In the Princeton Baby Lab, where researchers study how children learn, English-speaking mothers were recorded while they played with and read to their 7- to 12-month-old babies, and while they spoke to an adult researcher. (To keep the overall pitch range fairly consistent across participants, only mothers were tested, but future projects hope to include fathers and other caregivers.) The researchers came up with a mathematical formula for the timbre fingerprint of each mother’s voice and found that adult-directed and infant-directed speech had consistently different fingerprints.
Then, in a second sample of non-English-speaking mothers, the same timbre shift was found to be highly consistent across nine diverse languages (Spanish, French, Russian, Polish, Hungarian, German, Hebrew, Mandarin and Cantonese). This suggests that these timbre shifts may represent a universal form of communication with infants.
Being able to identify baby talk across various languages could help researchers and educators to improve outcomes such as vocabulary and success in school.