Moms adjust the timbre of their voices when talking to their babies

Voices carry so much information. Joy and anger, desires, comfort, vocabulary lessons. As babies discover their world, their mother’s voice is an especially powerful tool. One way for mothers to use this tool is to speak in the often ridiculous and sometimes condescending language of baby.

Also called “mom”, this is a high-pitched, exaggerated language, full of short, slow sentences and great vocal impulses. And when faced with a little human, pretty much everyone – not just mothers, fathers, and grandparents – does it instinctively.

Now a study has come up with another way mothers modulate their voice during baby talk. Instead of focusing on changes such as pitch and rhythm, researchers focused on the timbre, “color” or quality of a sound.

The tone is a bit nebulous, a kind of “know when you hear it”. For example, the timbre of a reed clarinet differs from that of a grandiloquent trumpet, even when both instruments strike the same note. The same goes for the vocals: when you hear the song “Hurt” you don’t need to check if it’s Trent Reznor from Nine Inch Nails or Johnny Cash singing it. The voiceprints make it obvious.

It turns out the stamp is not set in stone. People – mothers in particular – change their tone, depending on whether they are talking to their baby or an adult, scientists report online Oct. 12 in Current biology.

For the study, 12 English-speaking moms brought their babies to a Princeton lab. The researchers recorded the women talking or reading to their babies aged 7 to 12 months, and talking with an adult.

An algorithm sorted the stamp data derived from both infant and adult directed speech, and used that input to create a mathematical classifier. Based on speech snippets, the classifier could then tell if a mother was speaking with an adult or with her baby. The differences in timbre between speech intended for babies and adults were obvious enough that a computer program could tell them apart.

Similar timbre changes were also evident in other languages, the researchers found. These baby-focused changes occurred in 12 different women who spoke Cantonese, French, German, Hebrew, Hungarian, Polish, Russian, Mandarin, or Spanish – a consistency that suggests this aspect of baby language is universal.

Mathematically defined, these timbre changes were consistent across females and across languages, but it’s still unclear what vocal qualities caused the change. “It probably combines several characteristics, such as brightness, respiration, purity or nasality,” says Elise Piazza, study co-author, cognitive neuroscientist at Princeton University. She and her colleagues plan to study these attributes to see if babies pay more attention to some of them.

It is not yet known whether babies perceive and use their mother’s stamp information. Babies recognize their mother’s voice; they may also recognize their mother’s baby patch. Babies can distinguish differences in timbre between musical instruments, so they can probably detect differences in timbre in spoken language, Piazza explains.

The work “highlights a new signal that mothers are using implicitly,” says Piazza. The purpose of this signal is not yet clear, but researchers suspect that the change in timbre may emotionally engage babies and help them learn language.

People can’t just reserve stamp changes for babies, Piazza points out. Politicians speaking to voters, high school teachers speaking to a classroom, and lovers whispering to each other can all shift their stamp to convey… something.

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