We began our journey the way most visitors to Denali National Park and Preserve do — in a car, driving down the bumpy and intermittently paved park road and looking out the window, hoping for a glimpse of bear or moose. The roadside scenery was spectacular, and the gray sky and cold drizzle seemed to encourage me to linger in the sanctuary of the warm vehicle even as the open wilderness of the park beckoned.

After a dozen bumpy miles we pulled off to the side of the road and stepped out of the car into the elements, ready to continue our journey on foot. Carrying packs with 30 pounds of gear, we walked for miles up a dry, unnamed riverbed. Every mile, the sounds of the road and civilization faded, and the sounds of the landscape and our own footsteps became more pronounced and detailed.

My guide was Davyd Betchkal, a biochemist by training who had a passion for recording bands when he was a college student at the University of Wisconsin in the early 2000s. From that, he branched out into making field recordings and has been a collector of sounds ever since. Betchkal and his assistant are the two people tasked by the National Park Service with documenting the natural soundscapes of Alaska's national parks, an area that covers over 54 million acres and accounts for about 60 percent of all the land the park service manages nationwide. In the Lower 48 there are three people with his job, monitoring the soundscape of parks and other lands managed by the park service.

"This place looks pretty similar to how it might have looked a hundred years ago," Betchkal said, looking out over a lush green boreal forest filled with spruce and birch trees. "But it probably doesn't sound even like it did 20 years ago. So we are basically trying to document what the level of quietude and noise is in these areas, and whether or not natural processes are impacted by human activities."

Plane noise

During 2000, about 364,000 people visited Denali. By 2015 that number had climbed by 50 percent to 560,000. And the overwhelming majority of those visitors get to the park by way of a noisy internal-combustion engine. During our first six-hour hike we stopped numerous times to note when we could hear the sound of a passing Bush plane, more often than not ferrying tourists from the nearby town of Healy on a sightseeing tour of North America's highest peak.

In the late 1990s and early 2000s, scientists devised a system that divided up Denali along a 20-by-20-kilometer grid, called the Long Term Ecological Monitoring grid. The grid helped improve the statistical validity of previous monitoring systems, making the science more rigorous and better able to withstand scrutiny. A large part of Betchkal's job is to record at least a month's worth of audio from sites identified on the grid, collect baseline data and recommend mitigation measures to reduce the impact of human-made sounds in the park.

"In our data sets," Betchkal said, "the sound we hear most often is the sound of aircraft. The background level in Alaska is about two to five aircraft events per day. In a rare case you might get to a place that has less than one event per day. There are still places in Denali that you can go and have 24 hours without a combustion engine, and that's kind of an interesting premise. My basic understanding is that in the early 1990s that wasn't an uncommon experience. A lot of things have changed, especially in this part of the world, since the 1990s."

Aviators are generally supportive of the soundscapes program, even with the changes they've had to make to their operations.

"I think the mitigation efforts have been successful," said Tom George, the Alaska regional manager for the Aircraft Owners and Pilots Association, or AOPA, a private aviation group. "One of our early concerns with the program was that there wasn't enough data for us to tell if we were meeting the voluntary standards. The question now is whether those standards are going to be revised."

60-decibel limit

Denali has a backcountry management plan that recognizes both the need for quietude and the need for people to be able to enjoy the park from the air. And park rules prohibit vehicles or machinery that exceeds a noise level of 60 decibels, though officials have the discretion to penalize "unreasonable" noise.

"There are several types of enjoyment," said Denali's science team leader, Dave Schirokauer. "And sometimes they come into conflict." The management plan separates the park into zones, some of which call for the expectation of 50 aircraft events per day, and others for one.

The sound-monitoring station we were hiking to was located in the northern part of the park, far above treeline, 4,500 feet up a rocky, rugged mountain along Healy Ridge. The station itself is simple, made up of a microphone and a few different weather instruments that provide some context to the acoustic data being recorded. It's powered by lithium batteries, and a solar panel helps recharge the batteries.

"I hope it's still working," Betchkal said, an acknowledgment of the rough environment in which he works. Wind can topple equipment, a river can change course and swallow up solar panels, snow can bury a station. He keeps a badly damaged microphone windscreen in his office as a reminder of another challenge, bears.

"That's one of the more unique recordings I've made," he said.

When we get to the station, the view is spectacular. The park road is a faint ribbon in the distance, the only sign of human existence besides the station and ourselves. Far below us, the creek, no longer dry, rumbles through the steep valley walls, making undulating sounds like you might expect to hear at a beach.

Betchkal explains the physics behind the phenomenon, trying to get me to understand the way the sound waves propagate in the valley, stretching and compressing not unlike the siren of a passing ambulance, amplifying themselves and creating the sound of waves.

Quietest place in Alaska

The most noise-free site that Betchkal has documented in Alaska is at Walker Lake in Gates of the Arctic National Park and Preserve, where a station recorded 14 straight days without the sound of an airplane. The quietest spot, averaged over a month, is in Denali's Dunkle Hills, southwest of Cantwell, where the average sound level was 15.6 decibels during the winter of 2012. A normal conversation at 3 feet is around 60-65 decibels, or about 60-70 times louder than Dunkle Hills.

Oftentimes on our hike to the station at Healy Ridge, Betchkal stops, clasping his hands around his hiking pole, closing his eyes, listening. He would stand motionless for minutes at a time, soaking in the sounds of wilderness.

"When I'm in a remote part of the backcountry I feel like I'm swallowed up by the world, that it's wrapped its arms around me and I feel very connected again," he said. "I have a great feeling of relation, and that's hard to describe exactly but I feel quite a bit more alive because of that. I feel stimulated, and I feel almost like I'm part of something that's much bigger. That feeling is almost like weightlessness."

That feeling is more than just sentiment. As Betchkal explains, when we are in a quiet landscape we can hear sounds from over a far greater area than when we are in a noisy environment.

"We call this idea listening-area reduction," he said. "It's sort of a fancy way of saying that what you can detect and how far away you can detect sounds changes with how loud the background level is. You get into a louder environment, say you're close to a creek, or you're in a car or something, it's a lot more difficult to hear those faint sounds.

"Everyone kind of knows that, but what's neat about it is there's actually a really tidy relationship. You increase the background ambiance, the noise around you, by 3 decibels, and you cut your listening area down by half. Up at Healy Ridge, it's about 40 decibels. The other day I was in an area up on a mountaintop where it was 22 decibels, quite a bit quieter, and it means that my listening area on Healy Ridge is about 128 times smaller that it was up on that mountaintop.

"That's oftentimes one of the reasons why, when people see a beautiful vista at the top of a mountain, you've also got this feeling of real space that comes with the acoustic part of it. You're part of a huge volume that you can interact with, with your ears as well, and that helps give you this impression of vast grandeur."

Back up on Healy Ridge, we cracked open waterproof Pelican cases holding the sensitive equipment only to find that the batteries had given out during the past month. Later that day, back in Betchkal's lab, we found out that the station had recorded about two weeks of data, not quite the unbroken month that he had hoped for but still useful nonetheless.

How wilderness sounds

Sifting through all the data that Betchkal and his assistant collect is a time-consuming process. During 2015 he collected over 800 days of audio in a number of parks across Alaska, and since the program started in 2001 the project has generated almost 13 years worth of audio in Denali, Gates of the Arctic, Katmai and other areas managed by the park service. To make the job easier he uses a computer program that creates a spectrogram, a visual representation of the data. Using this, he can quickly scan through the data, looking for animal sounds or the sounds of human-made noise that stand out from the ambient background.

For Betchkal, this study is more than just a catalog of the sounds of Alaska's national parks.

"This study is really a study of wilderness," he said. "Frame it from an Alaskan point of view. We're rich. We have a rich heritage of sounds that we can still protect here. We can get away from the sounds of our own engines. When I tell my colleagues down in the Lower 48 about having whole days without a combustion engine, they just, they're flabbergasted, that doesn't happen anymore, it's gone. It doesn't exist."

Out of 4,655 days of audio Betchkal and his predecessors have recorded in Alaska, only 100 of them are noise-free — with the bulk of those in the far northern part of the state, including little-visited Noatak National Preserve and Gates of the Arctic National Park. In Denali, he estimates he's only captured about 25 noise-free days, all of them in summer when the ambient natural sounds rise to a level where they can drown out far-away airplane and truck noises.

"It's easy to forget that the world has changed a lot in this century that these parks have been around," he said. "What will the world be like when all we can remember is (the sound of) combustion engines?

"I believe that society struggles with balancing freedoms — that's one of the reasons I feel that the force has started to be applied to the opposite side of the lever — the one that pushes back against the prevalence of noise and tries to conserve the few remaining places where silence can be sought.

"I feel like it is my work to reinvigorate the awareness of quietude — and in order to do that, quietude has to exist."