In 1987, Congress ordered that natural quiet be restored to the Grand Canyon -- the only national park with such a mandate. But that's been a tough order to fulfill.
For one thing, air tours have been flying over the canyon for almost 80 years. And for nearly two decades, air tour operators have resisted rules to restrict them. Now, both tour operators and park managers have begun working together. The first challenge is trying to measure natural quiet.
This summer, the park is compiling 50 days' worth of data of natural sounds from four different sites in the park, using tripods with microphones attached to a computer that records 10 seconds of sound every two minutes. The park will also compile 50 days of data in the winter. The goal is to have natural quiet in 50 percent of the park at least 75 percent of the time.
But measuring natural quiet isn't easy. The recording devices pick up sounds from a nearby airport where many air tour flights take off and land everyday, and from tour helicopters overhead.
That doesn't matter so much in areas where people expect to hear noise, such as around hotels. But in the back country, expectations are different. That's why people like Dick Hingson of the Grand Canyon Trust want severe restrictions on overflights.
"You are out there on that trail," says Hingson, "and you find you're hearing the reverberating buzz and whip-whop of the helicopters. Over and over again, all day long, all the way down to the Colorado River."
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