The call of the wild is getting harder to hear

Natural sounds are an integral, yet often overlooked, part of the way we experience national parks and other public lands. But human activities — such as roads, development, and mining and logging operations — are drowning out this soundscape in almost two-thirds of protected areas in the U.S., according to a new study published in Science.

Researchers from Colorado State University and the National Park Service listened closely and determined that humans have doubled the level of background noise in 63 percent of these protected areas, making it loud enough to disturb people and wildlife.

In 21 percent of protected areas, human activity has cranked up the background noise so much that it's 10 times louder than what the scientists estimated to be "natural" conditions.

Here's another way to think about it:

A doubling of background sound — a 3-decibel increase — makes it harder to hear the natural sounds in an area. If in pristine acoustic conditions you could hear a bird singing from 100 feet away, with anthropogenic noise you could hear it from only 50 feet away, said study leader Rachel Buxton, an acoustic ecologist at Colorado State University.

A 10-fold increase in noise reduces your listening area by 90 percent.

"We have really good research that shows how important natural sounds are for humans," said Buxton, who fell in love with natural sounds listening to seabirds on the Aleutian Islands. They enhance mood, improve memory retention and "not only that, they're just really beautiful."

In the Science study, Buxton and her colleagues analyzed the acoustic footprints of 492 national parks, monuments and other public lands around the continental U.S.

Some of these parks are known for their silence, such as Olympic National Park in Washington state. Others sit in the heart of urban areas, such as Rock Creek Park and the National Mall in Washington, D.C.

The National Park Service has collected acoustic measurements for decades. The researchers analyzed the millions of hours of recordings and related them to various landscape features that could influence sound, such as nearby roads, running water, land cover and people. Then they created a computer model that could predict noise levels for parks around the country.

The model could even simulate a scenario in which there were no cars, no airplanes and no people.

Once the researchers figured out this "natural" sound level, they were able to tell how much louder places actually were as a result of human activity. They called this measure "noise exceedance."

Perhaps unsurprisingly, the highest exceedance values were in areas with the most human neighbors, such as urban national parks and parks managed by local governments.

The good news is that compared with other ecological issues, noise pollution is a relatively easy problem to tackle, the researchers said.

Solutions include high-tech "quiet pavement," which uses an extra-porous asphalt to minimize car noise, and just putting up signs asking park visitors to please be quiet.

Noise pollution can affect wildlife in a number of ways. Many predators rely on their sense of hearing to hunt their prey, and too much background noise could mask — or even drown out — subtle sounds.

In snow-covered meadows, foxes listen for the faint rustle of voles beneath their feet and pounce when a critter gets close enough.

"If an animal is busy listening to some strange foreign noise, it may be distracted from feeding itself or avoiding a predator that's nearby," Buxton said.

Noise pollution can also have an effect on things that don't hear like we do. Spiders don't have ears, but they sense the vibrations caused by sound waves and use vibrations to communicate.

And don't forget about plants. If a noisy environment scares off all the rodents or insects that disperse seeds, the plants may cease to grow.