America's protected natural areas are polluted, by noise

There are thousands of parks, refuges and wilderness areas in the U.S. that are kept in something close to their natural state. But one form of pollution isn't respecting those boundaries: human-made noise.

New research based on recordings from 492 protected natural areas reveals that they're awash in noise pollution.

Researchers from Colorado State University spent years making the recordings, setting out microphones in natural areas across the country. They caught all sorts of wildlife sounds, such as rutting elk and howling wolves. But they were also after "background" sound — wind, rain, birdsong, flowing streams and rivers, even bubbling mud pots in Yellowstone National Park.

They compared the decibel level of this natural background with the kind of intrusive noisiness from human activity. And they've discovered that in two-thirds of the places they studied, the median decibel level of human-made sound was double the normal background sound. These were sounds that came from within the area, such as road traffic, as well from as outside, such as passing jets or mining and logging equipment.

And in more than 20 percent of the areas studied, the unnatural sounds were 10 times as loud as the natural background. "So, fairly shockingly high levels of noise pollution within protected areas," says Rachel Buxton, lead author of the study, which appears in the journal Science.

Buxton, an ornithologist, points out that adding just a few decibels of artificial background sound, like of a jet plane, means it's harder to hear what's around you: a stream, or wind through the trees, or wildlife. Even with an increase in unnatural sound of three decibels, "if you could hear a sound before at 100 feet, now you can only hear it from 50 feet," she explains.

The National Park Service is well aware of the problem. They have a scientist dedicated to studying noise in parks: biologist Kurt Fristrup. He says people who are used to urban soundscapes might not know what they're missing.

What sound looks like

A model was developed to understand relationships between measured sound levels and variables such as climate, topography, human activity, time of day and day of year. In general, the brighter the spot, the greater the sound intensity. (Graphic source: National Park Service)

"It can make for an immersive experience when you listen carefully enough," he says, "to hear all those sounds that the animals around you are also attending to."

For people, listening to natural sound may be optional. But animals have to listen — for danger from predators, for example. Or conversely, predators such as owls also need to hear their prey, like the rustling of a mouse in the leaf litter. Hearing well means surviving, Fristrup says.

"For these animals, it's literally a matter of life and death if they miss these subtle sounds of nature: the sound of a foot-fall or the sound of another animal breathing," he says.

The researchers found some good news: Most national parks and larger wilderness areas are quieter than U.S. Forest Service or other federally protected lands or state parks. That's because those parks are generally larger and more remote. And not surprisingly, the protected areas farthest from urban centers are the quietest.

The team says there are solutions: keeping roads and road traffic to a minimum, for example, or controlling where and when overflights take place.