You know what you want to hear when you head to a park. Ideally, you'd like to hear birds singing and maybe the sound of a rushing stream or waterfall. You might catch the wind rustling through branches overhead or the crackle of animals darting through the brush.

But depending on where you are, those natural noises may be drowned out by cars and airplanes, kids yelling and industrial sounds.

Noise pollution created by humans doubles the ambient natural background noises at the majority of protected areas around the country, according to a new study published in the journal Science. Researchers at Colorado State University worked with National Park Service engineers, who collected more than 1.5 million hours of recordings from 492 protected sites across the United States including small urban parks as well as national parks.

The researchers analyzed the recordings and determined which sounds were natural and which were created by people. Then, using algorithms and detailed maps of the U.S., they created a model that predicted estimated noise levels throughout the country.

They found a lot of vehicle traffic, air traffic, industrial noise, and just general people noise, such as talking and sounds from mechanized equipment like watercraft.

"We found an enormous range of sounds from a number of different sources," says study co-author George Wittemyer, associate professor in the Department of Fish, Wildlife and Conservation Biology at Colorado State University.

Many protected areas were twice as loud as they should be

The researchers found that 63 percent of the protected areas were twice as loud as they should be. Although this study didn't look at impact, there has been a lot of research around the harmful effects of noise pollution on wildlife. Noise can be frightening and threatening.

Wittemyer cites the example of a fox hunting a vole in a snowy meadow. The fox can't see the vole, but he listens intently for the sound of the rodent underneath the thick snow cover.

"The fox is triangulating the sound of vole feet, which is quite a subtle sound," Wittemyer says. "That listening process and the accuracy that it takes requires high-level silence. Without silence, it can be a matter of life or death for a lot of these species."

Watch the amazing process as a fox catches a vole in the snow:

Undisturbed natural soundscapes

Not all areas are affected equally by noise pollution, the researchers found. Parks in urban areas, obviously, tend to be the loudest. On the other hand, researchers found that some of the big federal wilderness lands were very quiet.

About one-third of the protected sites that the researchers analyzed remained at undisturbed natural soundscapes.

"There are protected areas in every state that are at near natural sounds, and in every state there are protected levels that are really loud. There's a real diversity of conditions out there," Wittemyer says. "I don’t know that we'd say it's beyond repair. There's no simple prognosis how to resolve noise pollution."

Although the researchers suspected that noise pollution would be a pervasive issue, Wittemyer says they were surprised at how overreaching it was.

"The most important next step is for people to get out and pay attention to the noise levels. If they're disturbed, they need to figure out what the problem is and hopefully resolve it, and if they have nature areas they need to work and keep it that way," Wittemyer says. "Once we recognize the value of the natural soundscape, we can work much harder to protect it."

Mary Jo DiLonardo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.