The National Park Service manages "natural quiet" as a park resource. This responsibility is based in current public law, in explicit park management policy, and in visitors' reactions to park experiences. Just as parks contain many tangible features, such as animals, plants, waters, geological features, historic buildings and archeological sites, they have intangible qualities as well. These qualities include solitude, space, scenery, clear night skies, sounds of nature and natural quiet. Such qualities are increasingly rare in much of America. The scarcity of these resources and their importance to the park experience also makes them valued by park visitors.
3.1 How Important is Natural Quiet?
The concept of natural quiet and its importance as a resource is embodied in the 1916 NPS Organic Act, as amended.1 For Grand Canyon National Park (GCNP), Congress embedded the concept into two major public laws. It is also stated quite explicitly in NPS policy. Natural quiet is also very important to park managers and to a majority of park visitors.
3.1.1 Importance to the Congress
As directed by the Organic Act:
"..., the service thus established shall promote and regulate the use of the Federal areas known as national parks, monuments, and reservations hereinafter specified, except such as are under the jurisdiction of the Secretary of the Army, as provided by law, by such means and measures as conform to the fundamental purpose of such parks, monuments, and reservations, which purpose is to conserve the scenery and the national and historic objects and the wild life therein and to provide for the enjoyment of the same in such manner and by such means as will leave them unimpaired for the enjoyment of future generations."
The national parks must be managed to conserve their resources and allow their enjoyment. This section has been interpreted by the courts as providing the Secretary of the Interior with authority to determine how best to control these areas.
The United States Congress has repeatedly recognized the need to preserve the national parks in their natural state. In Section 101(b) of the Act of March 27, 1978, P.L. 95-250 The Redwood Act), 92 Stat. 166 (codified at 16 U.S.C. 1), Congress stated that:
1. NPS Organic Act, 16 U.S.C, 1
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"....The authorization of activities shall be construed and the protection, management, and administration of these areas shall be conducted in light of the high public value and integrity of the National Park System and shall not be exercised in derogation of the values and purposes for which these various areas have been established, except as may have been or shall be directly and specifically provided by Congress."
According to the legislative history of this provision, "the Secretary has an absolute duty, which is not to be compromised, to fulfill the mandate of the 1916 Act to take whatever actions and seek whatever relief as will safeguard the units of the National Park System." Furthermore, 16 U.S.C. 3 indicates that to carry out these Congressional mandates, the Secretary of the Interior "shall make and publish such rules and regulations as he may deem necessary and proper for the use and management of parks...." No limitations on the Secretary's power are noted. In the absence of specific Congressional mandate, the Secretary must determine for the National Park Service how best to protect park resources and thus how best to manage any adverse impacts.
In the case of GCNP, Congress has provided direct and explicit guidance. In the Grand Canyon National Park Enlargement Act of January, 1975, Congress stressed the importance of natural quiet in directing certain corrective actions whenever:
"... the Secretary has reason to believe that any aircraft or helicopter activity or operation may be occurring... which is likely to cause an injury to the health, welfare, or safety of visitors to the park or to cause a significant adverse effect on the natural quiet and experience of the park .. "2 (emphasis added)
Twelve years later, Congress reiterated the same concern in Public Law 100-91, The National Parks Overflights Act of 1987. A portion of that act states:
"Noise associated with aircraft overflights at the Grand Canyon National Park is causing a significant adverse effect on the natural quiet and experience of the park and current aircraft operations at the Grand Canyon National Park have raised serious concerns regarding public safety, including concerns regarding the safety of park users."3
Regarding guidance to achieve natural quiet in areas exposed to aircraft overflights, the legislative history of Public Law 100-91 provides important guidance on how a substantial restoration of natural quiet is to be achieved:
"Flight-free zones are to be large areas where visitors can experience the park essentially flee from aircraft sound intrusions, and where the sound from aircraft traveling adjacent to the flight-flee zone is not detectable from most locations within the zone."
2 Public Law 93-620, "Grand Canyon National Park Enlargement Act", 93rd Congress of the United States, January, 1975.
3 Public Law 100-91, "Aircraft Overflights Act", 100th Congress of the United States, August, 1987.
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3.1.2 Importance of Natural Quiet to the National Park Service
NPS management policy clearly articulates the value of natural quiet as a resource. Regarding the intrinsic value of the resource, NPS management policy (NPS 1988) states:
“The natural resources and values that the Park Service protects ... include plants, animals, water, air, soils, topographic features, geologic features, paleontological resources, and aesthetic values, such as scenic vistas, natural quiet, and clear night skies ... " (emphasis added)
NPS policy also tasks the agency with protecting natural quiet as a resource. Regarding protective actions to be taken, NPS policy (NPS 1988) states:
"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 the waves breaking on the shore, the howl of the wolf or the call of the loon). Activities causing excessive or unnecessary unnatural sounds in and adjacent to parks, including low-altitude aircraft overflights, 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." (Emphasis added)
These policy statements make clear the importance of natural quiet as a resource in many units of the National Park System. This resource is defined as the natural ambient sound conditions found in those units. It refers to the absence of mechanical noise, but accepts the "self-noise"4 of park visitors. This definition provides local park managers with a point of departure for developing strategies to protect this resource.
NPS-77, Natural Resource Management Guidelines (NPS 1990), also addresses the issue of protecting aesthetic values. The Guidelines define "aesthetic value" as a:
"... value, in the framework of natural resource management in the NPS, that is attributed by people to natural, unmanipulated conditions and is perceived through the senses - by seeing, hearing, touching, smelling, and tasting."
The Guidelines provide guidance on considering basic questions of aesthetics.
"To determine whether a proposed action or activity may affect resources and values important to the aesthetic experience, consideration of the following questions may be helpful. The questions can assist not only in evaluating the activities and actions that take place outside park boundaries, but also in those under the regulatory control of the NPS within park boundaries...
4 "Self-Noise" is the noise generated by the visitor--the tread of hiking boots on the trail, the creaking packframe, rattle of pots or pans, talking, etc.
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Could the action or activity be seen from the park...?
Could the action or activity be heard in the park? Where in the park would the sound be most noticeable or intrusive? From developed overlooks, headquarters areas, or trails? Would the sounds be continuous or intermittent? Are there any ways in which the effects of the sound could be mitigated or lessened... ?
Would the perceptible sight or sound change the nature or quality of the visitor's experience? In what ways...?
Does the frequency or duration of the activity or action affect the degree to which it could be perceived?
As these questions indicate, systematically looking at the effects of proposed activities or actions aims at evaluating what may be lost..."
The Guideline further notes that the courts have been reluctant to expand the regulatory control of the service for aesthetics beyond designated park boundaries. NPS managers are encouraged to look for methods other than litigation or Congressional appropriations to preserve the aesthetic integrity of parks.
3.1.3 Importance of Natural Quiet to Park Managers
In 1992 the NPS surveyed the managers of 98 parks (excluding Alaska parks) who had reported overflight problems. Although the questionnaire solicited the opinions about many aspects of aircraft overflights, three of the questions asked managers to consider the issue of natural quiet. As applied to their particular park, these questions asked for opinions about:
The importance managers ascribe to providing an opportunity for park visitors to appreciate the natural quiet of the park, 5
The degree to which managers feel aircraft activity interferes with their ability to provide this opportunity, and
The degree to which managers feel aircraft activity negatively impacts visitors' ability to appreciate natural quiet. 6
Figure 3.1 provides a summary of the responses to the first question. Although managers attached most importance to visitor enjoyment at most parks, the opportunity for natural quiet is extremely important to half the managers, and moderately to very important to roughly the other half. Figure 3.2 shows the responses to the second question. More managers believe aircraft overflights interfere with the
5 This question was asked in a context of three different opportunities: Visitor Enjoyment, Appreciation of Natural Quiet, and Appreciation of Historical Significance.
6 This question was asked in a context of five potential impacts: Normal Conversation, Natural Quiet, Historical Significance, Park Scenery, and Hear Interpretive Programs.
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opportunity for natural quiet, and to a greater degree, than believe aircraft interfere with enjoyment or historical significance. (These views, that aircraft most interfere with natural quiet, are confirmed by visitor surveys, see Section 3.1.4 and Chapters 6 and 9.)
As part of the third question, managers were also asked to rate the degree to which aircraft activity impacted four aspects of the visitors' experience besides natural quiet. Those additional aspects were normal voice conversation, historical significance of the park, enjoyment of the scenery, and the ability to hear interpretive programs. Figure 3.3 shows the managers' responses and puts the issue of natural quiet in perspective with these additional concerns. The length of each bar in the figure shows the percentage of managers who rated impact to be moderate to extreme for each aspect of the experience. In comparison with other potential impacts, natural quiet drew the highest percentage of responses.
Conclusion 3.1
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. Aircraft are judged by most managers to interfere with this opportunity, and interfere more with this opportunity than with other types of opportunities. The specific mandates and opportunities of individual parks to provide natural quiet need to be considered when estimating the severity of the effects of overflights.
3.1.4 Importance of Natural Quiet to Park Visitors
A survey of visitors to the Grand Canyon (Baumgartner et al. 1994) showed how different visitor groups felt impact from aircraft overflights. Figure 3.4 provides information about five different Grand Canyon visitor groups: frontcountry visitors, summer and fall backcountry visitors, river users in motorized boats, and river users in oar-powered boats. For all groups, more visitors reported impact in terms of interference with natural quiet, than reported interference with enjoyment or annoyance.
Figure 3.4 shows also that overflights of the Grand Canyon produce greater impacts, in terms of percent of visitors who are affected, in the backcountry than in the frontcountry. Though the reasons for this greater impact cannot be determined, it is evident that backcountry use does not provide an escape from the impacts of overflights.
Conclusion 3.2
Aircraft appear more likely to interfere with natural quiet for visitors than with visitor enjoyment or to produce annoyance. This relationship held true for Grand Canyon visitors, regardless of activity.
3.2 What is Natural Quiet?
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.
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In considering any sound environment, it is often helpful to classify the components of the environment into one of two categories: those sounds that contribute to the more or less continuous background (ambient) sound environment (such as waves breaking on the shore, or a distant waterfall), and those sounds which are intermittent in nature (such as the call of a coyote, or the passing of a flock of vocal geese). This distinction is important, because it is the ambient environment that establishes the quieter moments in the park, and provides masking to intermittent sources (such as aircraft).
3.2.1 Qualitative Assessment of Natural Quiet
Quiet itself, in the absence of any discernible source (especially man-made), is an important element of the feeling of solitude. Quiet also affords visitors an opportunity to hear faint or very distant sounds (such as animal activity, waterfalls, etc.). Such an experience provides an important perspective on the vastness of the environment in which the visitor is located, often beyond the visual boundaries determined by trees, terrain, and the like.
The range in ambient sound levels, even from indigenous sources, can vary considerably from one location to another, or time to time at any given location. At one end of the spectrum is the sound level at the base of a powerful waterfall. At the other end of the spectrum is the near absence of any perceptible sound at all. These latter conditions may be found in areas devoid of flora or fauna. In the middle is an array of sound conditions which vary from moment to moment, hour to hour. During non-inclement weather conditions, these variations result from three factors in natural environments:
Lulls in the wind or interludes between animal sounds create intervals where the quiet of a sylvan setting is quite striking. 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.
3.2.2 Quantitative Assessment of Natural Quiet
To provide a quantitative perspective on the quiet found in many parks, Figure 3.5 shows sound levels for a range of park and non-park settings in the form of an "acoustic thermometer." Values at the bottom of the thermometer are very quiet. In comparison, values at the top of the thermometer are much noisier. The sound levels shown on the thermometer are measured in decibels.
The "A-weighted sound level" title over the scale refers to an internationally recognized measurement standard that accounts for the different sensitivity of the human ear to different frequencies (pitches) of sound. This standard is used to assure that two different sounds which seem equally loud to a human observer will have very nearly the same measured sound level, in decibels.7
7. Technically, the specified ranges of Figure 3.5 may be thought of as identifying commonly occurring equivalent levels, Leq, for the identified location and condition.
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The thermometer in Figure 3.5 is divided into two ranges. The upper half shows sound levels of typical non-park settings, and the lower half shows the ranges of sound levels measured in parks. At the high end of the non-park environments are outdoor settings in downtown areas of large, busy cities, such as New York, Chicago, or Washington, DC. Further down the scale are daytime suburban settings out-of-doors in areas around these cities, and even further down are out-of-doors suburban settings at night.
In the lower range are sound level environments found in parks. This range includes sound levels that most people who live in urban or suburban environments rarely encounter during their normal daily routines. At the upper end of this range are areas along a major river, such as the Colorado River in GCNP. In the middle of the range are scenic overlook areas where a few visitors may congregate at any one time, and areas along remote backcountry trails where encounters with other visitors are infrequent.
At the very bottom of the range are extremely quiet areas of parks or wildernesses generally devoid of vegetation or major terrain features which might generate noise from the wind or might support insect or animal populations. In the absence of wind, these locations have ambient levels very near the human threshold of hearing. Such environments may be found in places like Death Valley National Monument or in the crater of Haleakala National Park (where they have been measured with specialized instrumentation). To put the lower half of the figure in some perspective, sound levels in the 20 to 30 decibel range would be found late at night inside a single family residence, with all windows closed, no internal noise sources operating (such as heating or ventilating systems) and no local traffic in the vicinity.
Some perspective on how quiet the natural environment of a park can be may be gained by comparing the two ranges in Figure 3.5. The relatively large sound level range (of roughly 40 decibels) that can be found between a busy downtown area and the suburbs at night, can also be found in park areas, but lying entirely below the lowest of the common outdoor sound levels in suburban environments. In such quiet park areas, it is not surprising that even relatively quiet aircraft can be heard at great distances.
Conclusion 3.3:
The quiet afforded in park settings is virtually in a range of its own, well below that which we experience in our normal daily routine.
3.3 What Are the Characteristics of Natural Quiet?
Generally low sound levels, but with considerable variability over both time and location, characterize the ambient sound environments in many national parks. The rise and fall of the wind in a coniferous forest can change the ambient sound level over a matter of minutes at a single location. Likewise, the synchronized activity of insects such as crickets can produce substantial changes in the ambient sound environment as well.
From one location to the next, the proximity of vegetation and water, the local insect population (and its normal diurnal activity patterns), and the location's susceptibility to winds can give rise to large differences in ambient sound levels. Figures 3.6 through 3.8 show a range of ambient sound levels measured during the summer and fall of 1992 at a number of diverse locations in three parks. (Horonjeff et al. 1993) The bars in each figure show the range in sound levels observed 90 percent of the time during the measurements (5 percent of the time levels were higher than shown, and 5 percent of the time levels were lower than shown).
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Looking at the three figures as a whole, a range of almost 50 decibels may be observed between the levels at Havasu Creek on the Colorado River and the lowest levels along Sliding Sands Trail in the crater of Haleakala National Park. Such a range is indicative of the differences to be found across the remote visitation areas of the park system. Within a single park the range can be almost as large. Generally speaking, at locations dominated by water noise, the range in ambient levels will generally be smaller (the 2 to 10 decibel ranges, as shown in Figure 3.6) than in other areas due to the consistency of water flow. In other areas the range is generally 15 to 20 decibels (Figures 3.7 and 3.8).
The relationship between these quiet settings and aircraft overflights is shown in Figure 3.9. The vertical axis of this figure shows the sound level in decibels, and the horizontal axis shows the passage of time over a 50 minute period. In the upper left portion of the figure, a 20-minute portion of a sound level trace obtained at the Great Meadows National Wildlife Refuge is presented. Great Meadows is located in a suburban area approximately 25 miles northwest of Boston, MA. The trace shows an ambient environment of 45 to 50 decibels, largely controlled by wind interacting with deciduous woods and distant road traffic. The trace also shows four single-engine propeller aircraft overflights which protrude 20 to 25 decibels above the ambient level, and are clearly audible in the ambient of Great Meadows.
To the right in the figure is a 30-minute trace showing helicopter and ambient sound levels at Haleakala National Park. The extreme quiet of the crater is exemplified by the ambient sound levels ranging from 7 to 27 decibels. The distant air tour helicopter at the beginning of the trace rises about 20 decibels above the ambient, and is clearly audible. The maximum level of this helicopter overflight is almost 10 decibels below the ambient at Great Meadows. Thus, the same helicopter overflight at Great Meadows would have been completely inaudible in that ambient environment.
Later in the Haleakala trace, a commercial jet aircraft overflew the crater and the maximum level exceeded the ambient by approximately 50 decibels. This event would have been noticeable in the Great Meadows environment as well as at Haleakala, but the protrusion of the sound event above the ambient is more pronounced at Haleakala, and is likely to be perceived as a greater intrusion. Immediately following the commercial aircraft overflight, a second air tour helicopter entered the crater area and began circling approximately one mile away. The maximum levels were about 30 decibels above the ambient and the aircraft was clearly audible. The same helicopter event in the presence of the Great Meadows ambient noise would have likely gone unnoticed.
Another important observation to be made from Figure 3.9 is the dynamics of the ambient sound level. While the difference between maximum and minimum ambient levels is different for the two environments shown, for each ambient the sound level consistently returns to within 2 or 3 decibels of the lowest levels every few minutes. Hence, in the absence of any other sounds, the visitor has a consistently recurring opportunity to experience and appreciate the quietest levels the particular location has to offer.
Conclusion 3.4:
Extremely low ambient sound levels in many parks means that visitors to remote sections of those parks are likely to hear aircraft, even if aircraft sound levels are very low.
3.4 Why is it Difficult to Preserve Natural Quiet?
There are four primary reasons why natural quiet is difficult to preserve:
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With aircraft activity on the rise in many parks, without FAA assistance in regulating the increases, the amount of uninterrupted time available for visitors to notice, appreciate and contemplate the quiet of the park will decrease proportionately. While more flights do not necessarily mean higher sound levels, there are two inevitable outcomes of increased flights. First, if increasing numbers of flights are routed along a limited number of existing flight corridors, then the length or number of quiet interludes between flights will decrease. If additional flight corridors are opened to serve the rising demand, then new land areas will be affected and interludes of natural quiet will be reduced in new (and perhaps previously unaffected) areas as well.
With the exception of water-related sources, there are few naturally occurring sound sources in many parks which continually generate sound levels capable of masking the sound of nearby aircraft. Sources that do provide a predictable and constant level of masking sound generally do so to a fairly localized geographic area. High ambient levels from ocean surf, or from river rapids are usually limited to distances well under a mile, and therefore are of limited value in protecting large areas of a park from aircraft audibility.
Conclusion 3.5
Preserving natural quiet is difficult because many park areas experience very low levels of ambient sound and aircraft are consequently audible at considerable distances (several miles).
3.5 Aircraft Overflight Effects on Natural Quiet
When visitors can hear the sound of aircraft, natural quiet does not exist. Specific areas within specific parks provide the opportunity to experience natural quiet. Such areas, however, are likely to have very low ambient sound levels and hence, intruding sounds will be more easily heard. For these areas, actions are necessary to preserve the natural quiet resource. The NPS recognizes that achieving natural quiet will not always be possible at these locations. There are locations where intruding sounds cannot be eliminated. Local street traffic, other visitor-generated mechanical noises as well as aircraft can eliminate natural quiet. On the other hand, studies have shown that visitor judgment of the importance of natural quiet varies, probably as a function of the type of visitor activity, (see Figure 3.4), and hence, from the visitor perspective, natural quiet is not equally important in all locations or for all visitor activities.
In developing an approach to preserve natural quiet, the NPS recognizes the following five important facts:
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1. Natural quiet is a resource for preservation within the NPS mandate.
2. The human auditory system is an excellent mechanism for determining the presence or absence of natural quiet. No readily available electronic device can duplicate human hearing for identifying audible sounds produced by non-natural sources.
3. The difficulty of preserving natural quiet is directly related to how quiet it is. If natural quiet, natural ambient sound, is relatively loud, as along a beach with pounding surf, or near a waterfall, then intruding non-natural sounds will have to be comparably loud to be heard. On the other hand, in a remote park location with no wind or water, or one with little or no vegetation or wildlife, even very quiet intruding non-natural sounds will be audible.
4. Humans are not always aware of sounds that are audible. Humans, when engaged in any number of activities, may have their attention focused on the activity and not be aware that a new sound has become audible. Visitors who for the first time view the Grand Canyon at Lipan Point are not very likely to remember hearing any aircraft. Only about 30 percent of the visitors interviewed reported hearing aircraft, (see Chapter 6) even though roughly 90 percent of them could have.
5. Park settings can provide levels of natural quiet so quiet that there is no sound to be heard except that generated by the listener - the sounds of walking, breathing, heart pumping, and blood flowing, (Figures 3.5 through 3.9).
The NPS studies discussed in this chapter demonstrate that:
A natural sound environment, and especially the extreme quiet found in many parks, is a resource valued by both park management and visitors.
The very low sound levels in many parks allow non-indigenous sounds, such as aircraft, to be clearly audible even at great distances.
The complexity of the issue strongly suggests that a system-wide homework is required With flexibility to define unique park problems and solutions because it would:
(1) carefully consider the resource values to be preserved and the types of experiences desirable to provide visitors, as well as consider the interests of the locally affected parties, and
(2) recognize both the visitation opportunities and variable visitor sensitivities in concert with the physical characteristics unique to each park location.
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