Yellowstone National Park

Sound Survey

Presidents' Day Weekend, 2000


Report compiled by:

Greater Yellowstone Coalition
National Parks Conservation Association


The opportunity to experience natural sounds and silence is rare in our modernized world. National parks are among the last refuges where people can experience natural quiet. Current use of snowmobiles in Yellowstone National Park undermines visitors' opportunities to hear natural sounds and quiet as part of their park experience. Snowmobiles emit significant amounts of noise at higher frequencies than automobiles. This combination of volume and pitch makes snowmobile noise quantitatively and qualitatively different from other vehicle use in Yellowstone National Park.

Despite strong policy guidance on noise impacts in parks and the appropriateness of using the human ear to measure natural quiet, the Park Service has failed to collect useful data on noise pollution in Yellowstone National Park. Anecdotal reports document degradation of natural quiet up to 15-20 miles into the backcountry (Yochim, 1999). Thus, current snowmobile use degrades large portions of Yellowstone Park for those seeking natural quiet.

Policy Requirements

Natural quiet has been recognized as an integral and irreplaceable resource of parks, which must be protected.

The National Park Service will strive to preserve the natural quiet and the natural sounds associated with the physical and biological resources of the parks (for example, the sounds of the wind in the trees or of waves breaking on the shore, the howl of the wolf, or the call of the loon.). Activities causing excessive of unnecessary unnatural sounds in and adjacent to parks…will be monitored and action will be taken to prevent or minimize unnatural sounds that adversely affect park resources or values or visitors' enjoyment of them. (National Park Service Management Policies of 1988).

The National Park Service further emphasized noise policy in a 1995 report to Congress, "Preserving natural quiet is an integral part of the mission of the NPS. This is confirmed in law, policy, and the beliefs of NPS managers." Report to Congress, p.76) The Park elaborated:

Parks and wildernesses offer a variety of unique, pristine sounds not found in most urban or suburban environments. They also offer a complete absence of sounds that are found in such environments. Together, these two conditions provide a very special dimension to a park experience… Quiet itself, in the absence of any discernible source (especially man-made), is an important element of the feeling of solitude…In considering natural quiet as a resource, the ability to hear clearly the delicate and quieter intermittent sounds of nature, the ability to experience interludes of extreme quiet for their own sake, and the opportunity to do so for extended periods of time is what natural quiet is all about. (NPS Report to Congress on Effects of Aircraft Overflights on the National Park System, 1995, p.78).


In developing an approach to preserve natural quiet, the NPS outlined several "important facts." The first two are: " 1. Natural quiet is a resource for preservation within the NPS mandate; and 2. The human auditory system is an excellent mechanism for determining the presence or absence of natural quiet. No available electronic device can duplicate human hearing for identifying audible sounds produced by non-natural sources." (NPS Report to Congress on Effects of Aircraft Overflights on the National Park System, 1995, p.85).

"Percent time audible" methodology utilizes the human ear to determine the percentage of time during which a targeted sound was audible. This methodology, used in other parks such as Grand Canyon to assess impacts of overflight noise, fits well with Park policy on natural quiet (cited above). In short, the human ear is the best way to measure whether artificial noise, in this case, snowmobiles, disrupts natural sounds and natural quiet.

For the Yellowstone percent time audible study, volunteers were transported by snowcoach to trailheads between Madison Junction and Old Faithful. Volunteers skied or snowshoed in pairs to 13 study sites marked on topographic maps. Listening sites were located in the Lower, Midway and Upper Geyser Basins at well-known attractions accessible by trail in winter. Distances from the road ranged from approximately 0.5 miles to 2.5 miles. Percent time audible data were collected for 20 minutes of each hour for the four-hour period between 9:00AM and 1:00PM. Data were collected at the same sites and during the same time period on Saturday, February 19, and Sunday, February 20, by different teams of investigators.

The listener and data recorder stood or sat quietly and noted noise present and the time when dominant noise changed or disappeared. The listener identified to the recorder what sounds were present, according to codes. Three codes were used for percent time audible data collection: "S" was marked if snowmobiles were audible; "O" if no snowmobiles were audible but other human sounds were present; and "N" if no human sounds were present. Snowmobile sound took precedence over all other sounds, and other human generated sounds were recorded over natural sounds. Only when natural sounds were the only noise present was "N" recorded on data sheets. Volunteers recorded these codes and time of duration for each sound source throughout the twenty minute data collection period. Natural sounds present at each site also were recorded.

Data were compiled by Mountain West GIS Co-op using a spreadsheet program and the ratio of percent time with snowmobiles audible to percent time with natural sounds was determined for each collection period. Percent time audible for all collection periods for each site was calculated and data for Saturday and Sunday were averaged, creating a final percent time audible for snowmobile noise for each listening site. A final map showing percent time audible by site was prepared by Mountain West GIS Co-op in Bozeman, MT.

Thirteen sites were surveyed. Eleven sites had percent time audible for snowmobiles above 70% (Table 1), eight of those with 90% or more of the time with audible snowmobile sounds. Goose Lake had 41% time audible due to high winds that drowned out all other sounds. Lone Star Geyser recorded no snowmobile noise, most likely due to its topographical position in a river canyon at lower elevation than the road. Lone Star Geyser is approximately 1.5 miles from the road. The Nez Perce Creek site is over two miles from the road and had 92% time audible for snowmobiles. Variation between sites seemed to be due to topographical variation, wind speed and direction.

Most sites had a mixture of snowmobile noise and ambient sounds, yet natural sounds often were rendered inaudible because of snowmobile noise. Ambient sounds heard included: geese, ravens, Clark's nutcrackers and other bird calls; waterfalls, rivers and streams; wind; geysers, mudpots, and other thermal features. Listeners described the snowmobile noise as a "constant whine" that escalated to a roar when several machines passed in a group, machines were accelerating or revving in parking lots, or when an exceedingly loud machine passed. At Old Faithful, snowmobile noise was present 100% of the time surveyed (between 10:00AM and noon). Noise at Old Faithful consisted of a constant whine punctuated by louder, higher-pitched sounds both from the road and parking areas.

This percent time audible study for snowmobile noise in Yellowstone National Park on a busy winter weekend illustrates the problem encountered by winter visitors: it is virtually impossible to escape snowmobile noise. The only visitors who have any chance of finding quiet and experiencing natural sounds are those capable of skiing significant distances. Day visitors to Old Faithful and the geyser basins would find it exceedingly difficult to access an area not susceptible to snowmobile noise. Ambient natural sounds are abundant, yet overwhelmed by constant snowmobile noise. Volunteers for this study routinely commented that the noise was constant and of a pitch and intensity that was extremely disturbing and detracted from their enjoyment of park values.


Yellowstone National Park is degraded by snowmobile noise at levels that conflict with park policy and visitor expectations. Current winter management of the park makes it impossible for visitors to access natural sounds and quiet which they seek in a winter experience. Visitors in the most famous areas are inundated with snowmobile noise over 90% of the time. The average visitor watching and waiting for Old Faithful over Presidents' Day weekend could hear snowmobiles the entire time. Even visitors who are physically able to ski find that snowmobile noise follows them far into the backcountry.


We recommend that the Park Service adopt a winter management plan that complies with park policy on natural quiet. Current winter use renders it nearly impossible for the average visitor to experience natural sounds and quiet at many popular locations in Yellowstone. Implementation of a visitor mass transportation plan using quieter vehicles in lower numbers is necessary. The snowcoach-only plan outlined in The Citizens' Solution for Winter Access to Yellowstone, supported by the Greater Yellowstone Coalition and National Parks Conservation Association, would restore natural quiet to our first national park.

Literature Cited