BRYCE CANYON NATIONAL PARK, Utah — The rock towers called Silent City can't be reached by trail, and they aren't on the visitors' map of this remote desert park. To get there, a hiker must veer into a dry streambed and labor uphill, gasping in the thin air 7,500 feet above sea level.

But for Gordon Hempton, America's foremost guru of quiet and a connoisseur of natural sounds, a place called Silent City is irresistible. So he scrambles up the slope, his $1,500 sound meter in hand.

He soon returns: Silent City failed miserably to live up to its name. "There was continuous (aircraft) noise," Hempton reports. "One helicopter was 48 (decibels), the other was at 42, and the jet was at 41. Those figures seem low, but they're the loudest thing going."

Americans visit the national parks expecting tranquility, a little respite from the hurly-burly of everyday suburban life. But unless they're among the few who venture into the backcountry, they don't always find the peace and quiet they seek. Even at isolated parks, the sounds of civilization — the internal-combustion engine, the car alarm, the jackhammer — intrude on the visitor's ear. Too often lost in the hubbub are the wind in the trees and the singing birds.

Even the National Park Service agrees that parks are often easy on the eye but hard on the ear. Park Service officials are so concerned that this fall they're expected to ask all park superintendents to find ways to protect what's known as the soundscape, the blend of natural sounds unique to every park. During the last year, the Park Service has banned or restricted motorized playthings such as snowmobiles and personal watercraft, partly because of the noise they make.

"All of a sudden, places that look the same as 100 to 200 years ago don't sound like they did," says Wes Henry, a Park Service natural resources specialist who is leading a federal effort to restore quiet to the parks. "It sneaked up on us."

But the bans on various recreational vehicles have outraged riders, and the Park Service has aroused ridicule with controversial new goals for park noise levels.

"If you walk, you make noise," says Christine Jourdain, executive director of the American Council of Snowmobile Associations. If the Park Service truly wants quiet, she suggests, "they need to put a lock on the front door and not let anyone in, including themselves."

Take away the sounds of civilization and many parks are so silent that visitors hear nothing but the blood surging in their ears. At Bryce, for example, when the helicopters are grounded and the air is calm, the decibel level is too low to register on many sound meters. One spot in California's Mojave National Preserve is so isolated that it's "quieter than a sound recording studio," says Mike Reynolds, Mojave's chief of resource management.

City Sounds Intrude

But many other parks are less suitable as recording studios than as sites for recording urban noise:

Acadia National Park in Maine, at 40,000 acres of mountain and forest, is relatively small. So the noise of carloads of sightseers rolling down park roads is hard to escape, some visitors say. "It's one of the most beautiful parks, but if you go up (certain paths) , you hear in the background constant automative traffic," says hiker David Gutierrez, a consultant from Bedford, N.H.

Petersburg National Battlefield in Virginia commemorates the Civil War, but the loud noises from a new car-crushing mill next door are unmistakably 21st century. "It keeps people from going back in time in their minds when there's all that clanking and banging going on," superintendent Michael Hill says.

Yosemite National Park in California has some of the most jaw-dropping scenery in the world, but some visitors only want to party. "They bring equipment, and they don't leave their (camp) sites," says campground ranger Jack Salvinger. Rangers are constantly telling campers to shut off their motor home generators, whining devices that power TVs and microwaves.

Cape Cod National Seashore in Massachusetts includes within its borders the Provincetown Municipal Airport, serving surrounding small towns. At a recent park program, "They had to stop the historical re-enactment because a plane was going overhead," says Eileen Woodford, northeastern regional director for the National Parks Conservation Association. "The audience couldn't hear the explanation of what they were doing."

At Bryce, there are the usual noises that accompany America on vacation: the thump of car doors being slammed, the roar of diesel bus engines, the sobs of cranky children. This year, the thud of construction equipment has been added to the list as the park renovates its visitor center and the road leading to the scenic overlooks.

Bryce also has something many parks don't: at least two companies that offer, for several hundred dollars a ride, helicopter tours of the park and several other outfits that offer tours in propeller planes.

The aircraft don't fly directly over the park's main road or the overlooks where tourists ooh and aah over the scenery. But in a recent weekend visit to Bryce, during what is traditionally one of the park's busiest months, the whup-whup-whup of helicopters and the rumble of propeller planes were easily heard from several scenic overlooks, as were the distant roar of jets overhead.

A 1998 study by the Federal Aviation Administration found that from the Queen's Garden Trail, a popular short hiking path, from 1 to 2 p.m. on a summer day, aircraft could be heard up to 88% of the time. The same study found one-quarter of visitors were annoyed by aircraft noise.

Hikers Most Irritated

It's the hikers who've trudged into the backcountry who are most irritated by aircraft sounds, park officials say. "They've exerted a fair amount of energy getting into a ... remote location, and to have a helicopter flying over them is fairly disruptive," park superintendent Fred Fagergren says.

Among the annoyed on a recent sunny day is David Lane, a doctor from Tucson. He couldn't see the choppers that buzzed by his campsite in the backcountry, but he couldn't help but hear them. "The helicopter was not a nice noise. I would've rather not heard it," he says.

But many other visitors asked about helicopter noise respond with a variation of "Helicopters? What helicopters?"

"Didn't bother me at all," says Becky Vincent, who owns a cleaning business and is visiting from Spanish Fork, Utah, with her husband and kids. "It's not like they're right over you, hovering."

For those who can afford it, the rides are exhilarating. "You see waterfalls and arches, a whole panorama you wouldn't know is there," says Jane Toffel, a teacher from Essex, England, who viewed Bryce from the air with her husband. The noise, she adds, "doesn't compare with the sound of voices. That's what's really intrusive."

Under the Park Service's controversial new policies on sound, the views of visitors such as Lane, Vincent and Toffel are only part of the equation. The agency argues that the natural soundscape is a precious resource that should be preserved for its own sake, independent of public opinion.

"The sounds of a canyon are fundamentally part of what that canyon is," says Jacqueline Lowey, a special assistant to the director of the Park Service. "It's part of what that resource is. ... Our responsibility is to manage resources, and you can't do that just on the basis of what visitors say."

Critics flatly refuse to entertain that idea. "The fundamental issue of noise is: How many people does it bother?" says Steve Bassett, the president of the United States Air Tour Association. "We have a problem with someone who simply says, 'It's a resource, and therefore, we can do anything we want.' "

Anti-Noise Measures Unpopular

Already the Park Service has taken far-reaching and, in some quarters, highly unpopular steps that help preserve that resource. In March, the Park Service banned personal watercraft such as the popular Jet Ski at 13 parks and allowed superintendents at 21 more to decide the fate of the powerful craft. Since then, at least one park, Assateague Island National Seashore in Maryland, has opted to exclude them. And in April, the Park Service said it planned to ban snowmobiles at many parks.

Individual parks have all moved to reduce the dull roar that now pervades many of them:

Grand Canyon National Park in Arizona bought a "quiet" helicopter for rescue missions and other tasks.

Haleakala National Park in Hawaii convinced air tour companies to keep their aircraft from crossing the park's volcanic crater, which is naturally nearly silent.

Biscayne National Park in Florida, which is 95% water, will soon propose a wide-ranging sound-management plan, the first park to do so. The plan will probably bar the use of internal-combustion boat engines in some areas.

In a development closely watched at Bryce, President Clinton signed a law in April that finally gives the Park Service a say about the skies above its own land. Before the law passed, only the FAA could regulate aircraft, no matter where they were. The law allows the FAA and Park Service officials to limit or even ban sightseeing flights in the airspace above national parks.

Officials at Bryce will soon start monitoring sound levels throughout the park, the first step toward regulating aircraft and other sources of noise. They also admit they could probably do more to crack down on the noise the park itself produces, such as the growling of garbage trucks.

For a fanatic of quiet like Hempton, that day can't come too soon. If he had his way, sightseeing aircraft wouldn't just be limited, they'd be banned, and commercial jets would swerve around Bryce and other parks to avoid contaminating them with noise pollution. Quiet, he argues, is a scarce and restorative commodity, and America's national parks should provide it.