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Why Baby's Listening to Babble

Tots can't separate the important parts of sounds, says new study

TUESDAY, June 12, 2001 (HealthDayNews) -- If you think your baby's listening to everything -- the dog barking, the radio blaring, the kettle whistling -- but you, you could be right.

She can't focus on you because she probably is listening to everything, says a study in a recent issue of The Journal of the Acoustical Society of America. Babies can't focus on important sounds the way adults can, the study says. Instead, they listen more generally, unselectively, to a range of sounds surrounding them.

The problem, says the study's lead author, Lynne Werner, is that this "makes babies more susceptible to background noise. It makes it harder for them to pick out sounds." If you want your baby to attend to you, she adds, "you might not want to keep the TV on all day."

Researchers have known for some time that babies don't hear as well as adults, says Werner, who is a professor of speech and hearing science at the University of Washington in Seattle. A sound that adults can hear must be played much more loudly for a baby to hear, and this is true until the child is 6 years old or even older.

But why? Some researchers think a child's brain may not be up to speed yet so that sounds aren't processed completely. Others think that such young children may not have learned to focus their attention.

Werner and her colleagues compared 7-to-9 month-old babies with adults. They measured how loudly you had to play two different types of sounds before the two groups could hear them. The first sound was a tone something like a dial tone, which has a very narrow frequency; the second was a sound like static that contained lots of frequencies.

Compared with the adults, the babies needed louder levels of both types of sounds before they could hear them. But they were relatively better at hearing the static-like noises than the tones, and that difference was significant.

It suggests, Werner says, that the problem is not in hearing but in attending to more specific sounds. For example, the difference between "ba" and "pa" is that one consonant is voiced (the vocal cords are used) and the other unvoiced (no vocal cord involvement). Adults can focus on the crucial difference; babies can't.

Babies are so busy listening to all the sounds in the syllables that they can't attend well to what matters, she says. They simply haven't learned what matters and how to focus on it. "By being selective in what they listen to, adults increase their capacity to understand," Werner says.

What this means practically is that other noises are very distracting to babies; all noises register the same. "If you actually watch moms, they'll tend to talk a little louder. It's as if they know babies need a little more," Werner says. Similarly, some moms repeat their words to babies or lean down to look the babies in the eyes as they talk, encouraging focus. "It's natural that we do that."

In the actual experiment, children and adults had to respond when they heard a tone or a noise. The kids' response was rewarded with toys lighting up or otherwise being activated; adults simply motioned to indicate they had heard a sound.

Outside monitors who couldn't hear the tones or noises watched for signs that a baby had heard the sound; usually the baby turned his head in the direction of the noise. In this way, researchers could measure how loudly a tone or noise had to be played before a baby or adult could hear it.

In one set of experiments, the sounds were played with background noise. There was a 10-decibel gap between when the adults and children heard the tone and a 5-decibel gap between when the adults and children heard the noise. In the second set, the sounds were played in quiet. Here there was a 14-decibel gap between the two groups when the tone was played and a 7-decibel gap when the tone was played. For a yardstick, the sound of rustling leaves is 20 decibels.

What To Do

If you're interested in learning more about how babies hear and acquire language, try Kids Ears. For everything you ever wanted to know about acoustics, including how the ear actually works, click here or take a look at this site that explains everything about decibels.

You can catch up on the latest research concerning children and hearing at HealthDay.

SOURCES: Interviews with Lynne Werner, Ph.D., professor of speech and hearing science, University of Washington, Seattle, Wash.; May 2001 The Journal of the Acoustical Society of America
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