CRATER LAKE NATIONAL PARK, Ore. -- It's August 8th, 2010, close to midnight. A silence has fallen over the hemlock and pine forest bordering Crater Lake National Park. Then, with an explosively loud crack, a tree falls. The noise echoes for a few seconds. Nobody was there to hear it.

But the sound of that falling tree was picked up by a nearby microphone and solar-powered recorder that captured forest noises continuously for 30 days.

Over the last two years, technicians at Crater Lake National Park have set up 20 of these temporary recording stations, building a massive database of creaks, snorts, rustling, birdsong, and airplane noise. Park management will eventually analyze the data and use it to create an air-tour management plan, triggered by a Bend company's controversial proposal to offer helicopter tours of the park.

A falling tree's sound and its visual representation. It was captured by the National Park Service at Crater Lake.

"Anything you could possibly think of hearing, we probably have a recording of it," says Scott McFarland, the park's audio technician. "Everything from badgers and porcupines grunting to the wings of a mosquito flying by."

McFarland is responsible for setting up the recording stations and the detective work of combing through the data and identifying sounds. He's learned that elk like to snack on microphone windscreens. When he finishes, the park will evaluate whether helicopter tours could be routed and timed in a way that will minimize disruption to park visitors and potentially noise-sensitive species, like Pacific treefrogs and spotted owls.

Pacific tree frogs in a pond at Crater Lake National Park.

Crater Lake is not a particularly quiet park, thanks to the cars, Harleys, and snowmobiles that follow the road along the caldera's rim. But McFarland says that noise from the road doesn't carry far into the Crater Lake back country. What he does record, about 20 percent of the time, is the sound of air traffic passing overhead. In fact, airplane flyover noise is present about 30 percent of the time in virtually every part of the continental United States. McFarland says the sound of airplanes has become so constant, most people don't even hear it.

"You're acclimated to the noise. It's amazing how much we've lost the ability to hear sounds."


And while some animal species may acclimate just as humans do, scientists worry that the intrusion of human noise in wild places may be harmful. Kurt Fristrup studies wildlife acoustics and works with the National Park Service's Natural Sounds Program. He says there is evidence that the noise of air traffic makes prey species like bighorn sheep and ground squirrels nervous, and may also make it harder for predators to hunt.

"The insidious thing about aircraft noise is that it affects the quietest places," says Fristrup.

"If you go to any one place and you measure the quietest natural background sound levels, there are species that have hearing thresholds so low, they take advantage of that," he says. "Something that would seem like a subtle change in background noise, 3 decibels, can reduce a predator's search area by as much as 50 percent."

The sounds of coyotes, insects and birds at Crater Lake National Park.

About these videos

The sounds and visual representations were recorded and produced by the National Park Service's Natural Sounds Program. The image represents the sound measured over time. The Y axis shows the pitch of the sound and the X axis shows time. The brightness of the lines show sound intensity.

In 2000, Congress passed an act requiring the National Park Service and the Federal Aviation Administration to protect natural quiet and regulate commercial air tours over national parks. But in 11 years, no park has managed to finish a plan, and the Government Accountability Office has repeatedly criticized the law as vague.

A Bend-based company, Leading Edge Aviation, proposed offering 30- to 50-minute helicopter tours of Crater Lake two years ago. Private, commercial, and military aircraft already fly over Crater Lake, but they are supposed to stay above 3,000 feet.

The aviation company says its helicopters would fly at 1,000 feet or higher, and estimates the noise level of the flyovers would be about 65 decibels. It argues that given existing car traffic and their proposed flight routes, the helicopter tours would have a minimal noise impact on visitors. The FAA has approved its proposal. A decision from the park is likely several years away.

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