More than 40 years ago, psychologists found signs that children living in noisy places were having trouble learning to read. They suspected that the noise interfered with language learning. Now, their suspicions have been confirmed, this time in the lab.

The original experiment, published in 1973, looked at children living in four unique apartment buildings in New York City. The Bridge Apartments, on the Manhattan side of the George Washington Bridge, sit directly on top of Interstate 95.

At the time of the study, the researchers wrote, “open highway vents and vertical surfaces of the buildings produce[d] high noise levels and an ‘echo chamber’ effect.” On the bottom floor of the buildings, the noise was as loud as having a blender on all the time — 85 decibels.

On higher floors in the building, where the noise was less loud, the children had fewer reading difficulties. But the apartments on the top floor also had higher rents, and family income is associated with reading ability. Over the years, scientists have wondered whether noise really is linked to language difficulties.

Now, lab experiments have borne it out: A new study finds that toddlers have trouble learning words when there’s too much background noise.

“A lot of studies are done in very quiet environments, which isn’t really what kids experience out in the real world,” said study author Brianna McMillan, a graduate student at University of Wisconsin, Madison.

Learning in the lab

For the lab study, McMillan had 106 children between ages 22 and 30 months sit in a booth and learn the relationship between some made-up words played through speakers and some made-up objects presented on a television screen. The words were read by a female voice and, in some cases, the kids heard background noise of two other men talking too. McMillan adjusted how loud that background talking was to see whether toddlers could still learn the new words.

After that, McMillan tested whether the toddlers had learned the words by seeing how much more they looked at the correct object compared with a distractor object while hearing the newly learned words.

While children who learned the words with quieter background noise looked at the correct object two-thirds of the time, children who learned the words in the louder background noise only looked at the correct object half of the time — the same as if they hadn’t been cued to look at either object. The findings were published Thursday in Child Development.

Real-life analog

In this case, the louder background noise, at 60 decibels (dB), is the average volume of an adult conversation, while the quieter background noise was set at 55 dB. The “target” speech of the female voice was set slightly louder, at 65 dB.

Rochelle Newman, professor of speech sciences at the University of Maryland, who wasn’t involved in the study, said that’s important. A 5 dB difference, Newman said, is the amount of difference that scientists have measured in day care settings during story time. Even when children are supposed to be quiet and listening to a reader, there can be a lot of extra noise.

“Obviously, if you make something noisy enough, all of us are going to fail, and that wouldn’t be very interesting,” Newman said. “But the important part is that where the children are failing is actually at a noise level that they’re experiencing on a common basis.”

Newman has been studying language learning in noise as well. She published a paper in 2014 that didn’t find any language learning problems with noise, but her study was slightly different. The children were 5-6 months older, and the background noise used was different as well.

“We used babble sounds of nine different people talking. That’s more like what you get at a crowded restaurant or a crowded day care,” Newman said. The new study, by contrast, had just two men in the background talking at the same time, which is “more like what would happen in a home environment if, say the other parent was talking to another child, or the television is on,” Newman said.

Delays in learning language can affect children for years afterward. “We already know that, when they get to school, children who have a weaker vocabulary don’t necessarily catch up,” Newman said.

But McMillan said parents shouldn’t worry too much. “First and foremost, it’s important to remember that kids are really resilient and really good at learning language,” she said. In her study, she found that children who heard the new words spoken in a quiet setting before trying to learn the object-word relationship in the noisy background had no problems.

Newman agrees. “It’s not that everything needs to be in quiet, but that at least some of the day the children should have an opportunity to hear language where there aren’t lots of other sounds in the background,” such as TVs, radios, loud toys, or media devices.

Republished with permission from STAT. This article originally appeared on July 21, 2016.