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Mums alter the timbre of their voice when speaking "motherese" - or baby talk - say scientists in the US.

Timbre refers to the unique quality of a sound and is why a piano sounds different to a violin, even when playing the same note.

Experiments at the Princeton Baby Laboratory found women use different timbres when talking to adults and babies.

The same vocal shift was found across women speaking 10 languages.

Dr Elise Piazza said: "It's so consistent across mothers, they all use the same kind of shift to go between those modes."

'Vocal footprint'

Many of the traits of baby talk, such as differences in speed and pitch, are thought to help infants develop language skills, but this is the first time a shift in timbre has been discovered.

When you describe a voice as nasal or hoarse, gravelly or velvety, then you are talking about its timbre.

The mums were recorded while they interacted with their child, aged between seven and 12 months, and to the adult researchers.

The scientific team then took "vocal fingerprints" by measuring the spectrum of sounds within the recordings.

The results on 12 English-speaking mums, published in the journal Current Biology, showed a unique speech pattern was directed at infants.

'Highly reliable'

A computer programme was trained to spot the difference and it could then find it - in less than one second - in mums speaking other languages.

The difference was found in 12 non-English speakers communicating in languages including Hebrew, Mandarin and Cantonese.

Dr Piazza told the BBC: "There is wide-ranging research showing infants learn better from infant-directed versus adult-directed speech.

"Specifically they can segment words into syllables better and they can learn novel words better and that probably encompasses these timbre features."

The study did not look at dads or grandparents, but the researchers anticipate similar timbre adjustments.

Prof Jenny Saffran, from the University of Wisconsin-Madison, commented: "This is the first study to ask whether [mothers] also change the timbre of their voice, manipulating the kinds of features that differentiate musical instruments from one another.

"This is fascinating because clearly speakers are not aware of changing their timbre, and this new study shows that it is a highly reliable feature of the way we speak to babies."

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