When the conservationist John Muir was campaigning to preserve national parks more than a century ago, he likened the sound of locomotives near the Grand Canyon to "mere beetles and caterpillars."
"The noise they make is as little disturbing as the hooting of an owl in the lonely woods," Muir wrote in "The Grand Cañon of the Colorado" in 1902.
A lot has changed since then, said Kurt Fristrup, senior scientist of the National Park Service's Natural Sounds and Night Skies Division, based in Fort Collins, Colo.
Today, as the sounds of wind, waterfalls or birds and the howls of distant wolves compete with the growing intrusion of vehicles, air tours and energy development, the Park Service is hoping to gauge how much its visitors value natural soundscapes.
The agency earlier this month announced it would be conducting its first-ever national survey of how noise pollution affects visitation at national parks and the economies of gateway communities.
"Currently, the NPS has no information about the value that visitors hold for preserving natural sound conditions in national parks," the agency said in an Aug. 9 Federal Register notice. "Nor does NPS have any information of how human-caused sound conditions affect the likelihood of visitation to national parks."
While the survey is not set to begin until 2015, the agency next year hopes to conduct focus groups that will help test the questions that will be used in the national survey.
The agency apparently needs approval from the White House to proceed.
The Park Service said it is working with economists and noise specialists at the Department of Transportation's Volpe National Transportation Systems Center to develop a model that could calculate the economic benefits of varying acoustical conditions in the park.
The results could help inform park decisionmaking, such as how to manage helicopter overflights, vehicle access and speed limits, how to heat and cool visitor centers, and when to use chain saws or other motorized equipment to conduct landscaping.
"In addition to parsing out the extent to which visitors value being able to hear the sounds of nature, the study will provide other useful information such as how acoustic conditions affect the likelihood of repeat visitation to national parks," the agency said in a summary of the survey.
Previous studies have suggested that "robust park soundscapes" can bolster the economies of surrounding communities through increased tourism, the agency said.
In 2012, national parks, along with wildlife refuges and other Interior Department lands, saw more than 417 million visits that contributed $45 billion to local economies and supported 372,000 jobs, Interior reported earlier this summer.
While soundscapes are just one of parks' natural features, "NPS is interested in investigating and evaluating the likelihood and extent of potential reductions in visitation, if any, due to noise."
At a recent public forum on natural quiet in Denver, Fristrup said previous surveys at the Grand Canyon suggest park visitors are nearly universally irritated at the sound of helicopter tours.
Visitors who hiked the Hermit Trail in the early morning, before the air tours began, used the words "silence" and "tranquility" to describe the experience, but responses soured considerably once the air tours began, he said.
"It's not that some fraction of the population is annoyed; it's that everyone experiences a loss of enjoyment or a loss of this superlative opportunity to enjoy park resources," he said.
While affecting people, artificial noise can disrupt wildlife behavior and functions, such as mating rituals and predator-prey dynamics, as well as nocturnal species, biologists say.
The Park Service has taken steps to minimize noise impacts through travel management plans and noise action plans at units including Zion National Park (Land Letter, April 1, 2010).
In addition, 17 parks are currently conducting ongoing base-line acoustic monitoring, an agency spokesman said.
In some parks, data are being translated into policy.
For example, Rocky Mountain National Park in Colorado is working with Denver International Airport to utilize "continuous descent technology," which he said uses less fuel and generates less noise, while consolidating flight paths above the park's heavily trafficked Trail Ridge Road, Fristrup said. While the noise is now greater along the road corridor, the presence of vehicle traffic dilutes its impact and backcountry areas are kept even quieter, he said.
Just last week, Yellowstone National Park finalized a winter use plan that will reduce the number of snowmobile groups that enter the park, a "historic event" that could end decades of conflict, Fristrup said (Greenwire, Aug. 23).
One the biggest improvements to natural acoustics happened at Zion in 1997, when the Park Service banned private vehicles from the park's main roads, eliminating an estimated 1,300 cars that drove through Zion Canyon each day.
But public awareness of their impacts is also key, the agency said.
The agency this week set up an experimental sign in Rocky Mountain that tells drivers the noise level of their vehicles as they pass.
Frank Turina, a manager for the Park Service's soundscapes program, said the sign aims to educate motorcyclists, whose engines generate the most complaints among park visitors. The agency plans to set up three more signs in other parks next summer, he said.
But when it comes to curbing the most ubiquitous noise in national parks -- aircraft noise -- the agency is more limited.
The Park Service is working with the Federal Aviation Administration to tailor flight plans over national parks as authorized under the National Parks Air Tour Management Act.
But threats persist in places like Crater Lake National Park, which the environmental group Oregon Wild last week ranked as the second most threatened place in the state, in part due to the threat of helicopter tours disturbing natural quiet.