Cooing to an infant might not seem like sophisticated speech, but it turns out that baby talk is more complex than previously thought.

While it has long been known the pitch and rhythm of speech changes when mothers talk to their babies, researchers have now found the timbre of their voice changes too – a quality that reflects properties such as how velvety, raspy or nasal a sound seems.

To delve into the nature of baby talk – a mode of speech thought to help language development in infants – the team recorded 24 mothers while they talked to their children, aged between seven months and one year.

The mothers spoke to their children in 10 different native languages including English, Spanish, Mandarin and Hebrew. “We basically brought mothers into the lab and had them play with and read to their own babies, just like they would at home,” said Dr Elise Piazza, co-author of the research from the baby lab at Princeton University.



The mothers were also recorded while being interviewed by the researchers about their child’s daily routine to capture their speech when talking to adults.

Short snippets of the recordings were then analysed using a computer system to extract a measure of the timbre of the voice, independent of rhythm and variations in pitch. These “vocal fingerprints” were then fed into an algorithm-based machine-learning system to train it to distinguish between speech directed at infants and adults. The system was then tested using data from further snippets of speech – if no differences existed between speech directed at infants and adults, the system would end up assigning them by chance.



The results, published in the journal Current Biology, reveal that the system was able to differentiate between different mothers.



What’s more, about 70% of the time it was able to work out whether the sounds were from baby talk or adult chat.



As well as suggesting that timbre might be an important factor in how infants recognise voices, and when adults are talking to them, Piazza says the work could prove useful in developing educational tools for children.



“For instance maybe using a realistic timbre of [baby talk] when designing some kind of virtual [voice] that is geared towards enhancing communication skills or maybe even teaching a child a second language,” she said, adding that the approach used might also help unpick whether other types of social interactions, such as romantic conversations, involved a shift in timbre.



Dr Catherine Laing, an expert in infant language acquisition at the University of Cardiff who was not involved in the study, described the research as fascinating, adding that it opened up new avenues for research.

“Really what we need to know now is how infants respond to this – does this tone of voice actually affect the way that infants learn speech,” she said. “We know that is true for pitch – pitch range and higher pitch facilitates language development – but is the same true for these vocal fingerprints?”

And with babies able to hear in the womb and recognise their mother’s voice from birth, timbre might also play a role there, too, she said. “Maybe this is actually really important in the infant’s connection with the mother.”

Ultimately, says Piazza, the study ties into a broader body of research showing that baby talk is an important mode of communication. “It is not something to be embarrassed about at all,” she said.

