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Craig Moore, Associated Press
A lone bison crosses a road ahead of a pack of snowmobilers in Yellowstone National Park, Wyo.

A few times a year, Bryson Garbett loads up his family and heads to a national park, often one in southern Utah.

Part of the draw to a place like Zion — aside from the hiking, rappelling and backcountry stargazing — is that it's quiet.

Garbett, president of a homebuilding company in Salt Lake City, is happy to swap the urban din for murmurings of frogs and birds or even flat, dead silence.

Almost always, though, that tranquility is broken by an airplane overhead or some other kind of man-made machinery.

"The immediate reaction is it just focuses everything on the noise," Bryson said. "You get over it after a while but if you run into a lot of that, you start looking for somewhere else to go."

But where?

"In many of the parks today, you can't get away from the noise," said Britt Mace, an associate professor of psychology at Southern Utah University, who has spent years studying the interaction between sounds and people in national parks.

Finding a place to rest your ears these days means looking beyond the country's flagship parks.

Some of the quietest are Great Basin in Nevada, Isle Royale in the middle of Lake Superior, North Cascades in Washington state and Big Hole National Battlefield in Montana, according to the Park Service. A few of those spots in Utah also fare pretty well — under the right conditions.

"When there's no aircraft overhead, they are among the quietest places in the continental U.S.," said Kurt Fristrup, a scientist in the Park Service's "natural sounds" office in Fort Collins, Colo.

Established eight years ago, the department's main job is preserving "soundscapes."

Those sounds — from hissing geysers and bugling elk to jazz music and battlefield artillery — are integral to the parks' character and worthy of protection like any other natural resource, said Karen Trevino, the program's director.

"The most ubiquitous source of noise in all national parks seems to be airplane noise," Trevino said.

Several years ago, Mace conducted an experiment to gauge visitors' reactions to the sound of helicopters at Grand Canyon National Park, where there are more than 86,000 commercial air tours a year. In the experiment, it didn't matter whether the helicopters were for tours, medical rescues or in aid of endangered species, Mace said.

"They just didn't like the presence of the noise," said Mace, noting that many participants had a negative emotional reaction to the sounds. "The engine sounds seem to be bothering people more than anything else."

That means airplanes, helicopters, cars, RVs, motorcycles, all-terrain vehicles, snowmobiles and even Park Service vehicles, Mace said.

There have been protracted fights about overflights in Grand Canyon. Noise has also been central to the long-running debate over snowmobiles in Yellowstone.

But other parks are also threatened, according to the Coalition of National Park Service Retirees.

Included on the group's list, released at the beginning of summer, are Mojave National Preserve, where an airport has been proposed 40 miles outside its borders; Mount Rushmore, with its motorcycle traffic and potential for a busy overflight business; and Everglades, where the roar of boats travels easily across the swampy waters.

"When people think of national parks, they think of the scenery, the wildlife and the historical icons they hold," Bill Wade, the group's council chairman, said in a statement. "Many also think of a place they can 'get away from it all' and that includes escaping ruckus of everyday life."

But how do you decide which sounds belong in a national park?

One man's happy Harley rumble is another's campground annoyance.

One visitor's breathtaking helicopter tour is a silence-shattering moment for a backcountry hiker.

And that doesn't include the effect that manmade noises have on wildlife: higher stress levels, prey that can't hear approaching predators as well and even interruptions of bats' echolocation, said Paul Bell, a psychologist at Colorado State University who works with the Park Service's sound program.

"It's a very complex problem when you start talking about sounds and noise in national parks," Bell said.

Often it comes down to a clash in values, disputes that probably won't be resolved by a single overarching Park Service policy, Bell said. And often, the sounds come from outside national parks where the agency's influence is limited.

Researchers, though, are compiling recordings from dozens of parks across the country to have comparative data if a noise conflict comes up. The natural sounds program also is developing standardized ways to measure noise in natural places to help decide what's appropriate.

It's not all about the simple pursuit of silence.

Noise can be an important part of the experience: howling wolves, roaring waterfalls and even artillery demonstrations at a place like Baltimore's Fort McHenry.

"You can't make the general statement that all noise is bad," Bell said.

Park officials are hoping to do a better job encouraging people to turn off their iPods, prick up their ears and understand what they're hearing. It may even help in spotting wildlife, Fristrup said.

"I think one of our education and interpretive challenges is to reach out to urban and suburban visitors who have become accustomed to not listening," he said.