Standing on the rippled hillside of Checkerboard Mesa in Zion National Park, the sound came in waves as eastbound flight after eastbound flight from McCarran International Airport in Las Vegas passed overhead about every 10 minutes, adding to the cacophony of traffic from the park road 100 or so yards away. A few days earlier, hiking in a section of wilderness in Organ Pipe Cactus National Monument, the sounds of traffic on a Mexican highway clearly in sight mingled with chirping birds and buzzing insects.

Noise is all around us, even in the National Park System. Find yourself on the Racetrack deep inside Death Valley National Park and you might be startled by military jets screaming overhead. Once, while pausing on a boardwalk deep in the swamp of Congaree National Park in South Carolina to listen to birds and insects, the slowly approaching drone of a single-engine plane overwhelmed the sounds of nature.

This noise isn't disturbing only to human visitors to the parks, but to the wildlife and, believe it or not, the plant life, according to a recent study published in the journal Science. The researchers looked at "protected areas," which include the entire National Park System and which cover more than 13 percent of the world's total land area. What they discovered was that the noise we create "doubled background sound levels in 63 percent of U.S. protected area units, and caused a 10-fold or greater increase in 21 percent, surpassing levels known to interfere with human visitor experience and disrupt wildlife behavior, fitness, and community composition. Elevated noise was also fond in critical habitats of endangered species, with 14 percent experiencing a 10-fold increase in sound levels."

Do you ever stop and realize how much human noise -- like this passing car captured in Rocky Mountain National Park -- overshadows the sounds of nature?

The study was led by Rachel Buxton, a postdoctoral researcher in the Department of Fish, Wildlife, and Conservation Biology at Colorado State University. In the study, the first of its kind, researchers focused primarily on noise exceedance of 3dB and 10dB, which represent a doubling and a ten-fold increase in noise, as well as a 50 percent and 90 percent reduction in the listening area that surrounds us; in other words, a reduction in the natural sound we hear. They found noise exceedance greater than 3 dB in 34.4 percent of all protected areas in the contiguous United States, and greater than 10 dB in 1.2 percent of those areas.

“This analyses is basically building upon maps that we published about a year-and-a-half ago. We have projected noise exposure both at natural and current sound levels for the entire continental United States," Kurt Fristrup, the Science and Technology Branch Chief in the National Park Service's Natural Sounds and Night Skies Division, said recently while discussing the paper he helped write. "We chose to focus on protected areas to kind of dive deeper into the data and try to infer what the consequences were."

In short, the consequences are concerning.

"Noise pollution causes cognitive impairment, distraction, stress, and altered behavior and physiology in ways that directly influence both wildlife and humans. Moreover, noise pollution that alters the distribution or behavior of key species can have cascading effects on ecosystem integrity," the authors wrote. "Noise pollution is often considered to be an urban problem, but expanding human development and activities in rural landscapes are extending its reach."

Sound monitoring in Olympic National Park and elsewhere in the National Park System revealed that park landscapes are not as free of human noise as you might expect/NPS

Growing Impacts Of Noise

In the National Park System, it seems to be getting more and more difficult to be surrounded only by the sounds of nature. Audible intrusions come from overflights, highways, even visitors chatting on cellphones. In Yosemite National Park, the cacophony of traffic in the scenic valley can be overwhelming when ambulances are blaring or delivery trucks are backing up to the beep beep beep of their warning systems. 

"You know, when there’s a bear jam up in Yellowstone, it can get pretty congested, also," Mr. Fristrup added as we discussed noise levels in the parks. "Pretty much anyplace where we bring lots of visitors, it’s hard to do that without bringing noise along with it. ... Great Sand Dunes National Park and Preserve in Colorado has one of the lowest background sound levels that we measured anywhere, when you’re up in the dunes, and yet it’s not far from Denver International, so, yeah, there are commercial flights going overhead very regularly.”

And while those overflights don't go unnoticed, he went on, "the question becomes, is there any role for considering potential effects on protected areas when we do air space planning? That’s a question we didn’t go anywhere near.”

Perhaps it should be a question to explore when you start to consider the growing impacts of noise: It can intrude on our experiences in the parks, scare wildlife and so cause them to burn vital calories or abandon newborns, and even affect plant life.

"Since we began working on this paper, there was another paper that came out that (said) some plants apparently sense the presence of water through vibrations and sound and, actually, it affects their growth patterns," said Mr. Fristrup. "They tend of grow towards water. That’s something we knew nothing about.”

Another impact to plants can be the noise-caused flight of seed dispersers, such as Clark's nutcrackers or squirrels, and pollinators.

"They can have big effects on the distribution of plants because of their interactions, their importance to the plants," the Park Service researcher said. “I think once upon a time, we might have told ourselves that sound wasn’t important. For example, to butterflies, because they don’t produce sound themselves. Now, it’s really clear that many organisms can have quite good hearing and pay attention to acoustical cues even if they don’t utter any kind of sound.”

What we hear, and what we don't hear because of anthropogenic noise, also can impact what we know of the nature around us.

"The other consequence of noise is it can prevent you from hearing other things, and there’s no way to habituate to that problem. If there are sounds you don’t hear, you just don’t hear them," said Mr. Fristrup. "And if you’re a male songbird, you can get around that problem by just singing more often or just shifting the frequency at which you sing. But for the incidental sounds of nature, like the foot falls of an approaching predator or for an owl the foot falls of a mouse that it’s trying to eat, once you lose those cues, you have to do something. You can’t recover that information, and there are going to be costs for whatever you have to do to compensate."

Ms. Buxton said the noise levels found in the study can be harmful to visitor experiences, human health, and wildlife.

“However, we were also encouraged to see that many large wilderness areas have sound levels that are close to natural levels,” she said. There are many opportunities for people to experience the natural soundscape across the U.S., but these relatively unimpacted systems need to be recognized and protected, she added.

Despite appearances, even deep within Congaree National Park, noise seeps in from nearby highways/Kurt Repanshek

Sanctuaries Of Silence

Back in the early 2000s, Mr. Fristrup did acoustical monitoring for the Cornell Lab of Ornithology. One assignment took him into the heart of Congaree to record bird calls with hopes of catching the audible signature of the ivory-billed woodpecker. Its call has been described as a "nasal tooting 'kent'," while it also executes a quick double knock when pecking on tree trunks.

"Even in the center of Congaree, after we stood there and listened carefully, we could still hear the distant interstates and highways," he said. Still, "(W)e got sound sequences that can’t be explained any other way than that it was an ivory-billed. In Congaree, of all the places, it was the only place where I actually heard anything, live, in person, that I thought was an ivory-bill. But I didn’t see anything. Those years back when I was working on that, I probably spent a couple hundred hours hiking around."

Some years ago, Gordon Hempton went in search of the place that is most protected from human noise, and settled on a spot in Olympic National Park.

One Square Inch of Silence is very possibly the quietest place in the United States. It is an independent research project located in the Hoh Rain Forest of Olympic National Park, which is one of the most pristine, untouched, and ecologically diverse environments in the United States. If nothing is done to preserve and protect this quiet place from human noise intrusions, natural quiet may be non-existent in our world in the next 10 years. Silence is a part of our human nature, which can no longer be heard by most people. Close your eyes and listen for only a few seconds to the world you live in, and you will hear this lack of true quiet, of silence. Refrigerators, air conditioning systems, and airplanes are a few of the things that have become part of the ambient sound and prevent us from listening to the natural sounds of our environment.

Of course, deep in the Hoh Rain Forest you're surrounded by thick, almost luxurious vegetation that helps muffle any outside noise. Mr. Fristrup surmised that finding natural sound, without noisy overlays, can be easy in Olympic "because the natural sound levels are high enough that they can mask distant noise events."

Go to parks on the Colorado Plateau, such as Canyonlands and Arches, and "the natural sound level is so low that even a very distant jet can be audible and intrude upon the scene," he said.

But if you head east, to Shenandoah, the Blue Ridge Parkway, Great Smoky Mountains, Obed Wild and Scenic River, or Mammoth Cave, it's possible to find relatively noise-free areas because of the mix of vegetation and topography.

"Generally speaking, the Eastern United States has higher noise exposure as well as higher natural sound levels, but even in those places, you can still find local environments that are shielded by terrain and vegetation and perhaps have a running brook or something else and have a relatively intact acoustical environment," said Mr. Fristrup. "I would say even near some of our large population centers, you don’t have to go very far to find places that offer a lot of relief in that way.”

Efforts To Reduce Noise

What can be done about noise levels in the parks? Well, at Zion National Park in Utah, the restriction on personal vehicles in Zion Canyon has managed vehicular sound, so to speak. By replacing cars with shuttle buses, the park has, in effect, put noise on a regular schedule. Once the buses pass a specific spot on the canyon road, there is no other vehicular traffic until the next shuttle comes through 10-15 minutes later. Visitors have noticed, and commented to park staff, about those quiet moments.

"The shuttle buses have helped reduce noise levels in the canyon (scenic drive) significantly since their inception in 2000. We do studies and monitoring periodically," said Zion spokesman John Maricano. "Controlling, mitigating noise levels, preserving soundscapes in the park, is not only pleasing to the visitor, but very important to wildlife. Sound clarity for animal communication, territory establishment, courtship and mating. ... All in keeping with preserving habitat."

It would be a shame to lose the bugle of a bull elk, or a wolf howl, to human noise.

Much the same has been done in Yellowstone, where officials crafting the winter use plan decided to "sort of group noise events as much as possible," explained Mr. Fristrup. "So instead of limiting the number of vehicles (outfitters) could send into the park, we put a limit on how many tours they could conduct … and then started putting incentives to group the vehicles. They could run bigger groups if they had quieter technology vehicles.

“The result of all that, again, we achieved a modest reduction in the noise per passenger by incentivizing the use of quieter technology vehicles, and we achieved a larger improvement in the ... noise-free intervals by creating incentives to kind of group all the traffic. Essentially, fewer tours but larger tours."

Shsssssh/NPS, Alison Taggart Barone

Sometimes, though, all it takes a is a sign asking visitors to be quiet.

“At Muir Woods, in Cathedral Grove, they decided to find out how the public would react to different methods asking them to be quieter. So, starting back eight or nine years ago, they began a series of social science surveys to assess public tolerance for that kind of signage or request, and then to find out how much a difference it makes in sound levels," Mr. Fristrup explained. “And we have two different studies now that show that just putting up signs to declare that an area is a quiet zone or asking people to be quiet reduces the noise they generate by about a factor of two.

“Just putting up a sign it’s as though there were half as many people in the park."

Traveler footnote: Audio cuts supplied by National Park Service.