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A video detailing how measured data and noise models can be used for impact assessment.
5. Impact Assessment
A key component of protecting the acoustic environment is assessing and predicting potential impacts from actions and activities that occur both inside the park and outside the park boundaries. The National Environmental Policy Act (NEPA) requires federal agencies to assess the impacts of major Federal actions significantly affecting the quality of the human environment. Director's Order 12 (DO 12) [PDF] and the National Park Service NEPA Handbook [PDF] provide detailed guidance on conducting impact analyses. This section provides guidance on analyzing impacts from noise within the framework outlined in DO 12 and the NEPA Handbook.
This priority is expressed in Management Policies 8.2.3 which states that the natural ambient sound level needs to be considered when assessing impacts to the acoustic environment. In a NEPA context (see NEPA Handbook, section 4.4 [PDF]), impacts should be assessed based on conditions that exist at the time of the proposed action. In national parks, sound levels are constantly fluctuating throughout the day, with periods of high levels of noise, and periods of relative quiet. Some locations in national parks are naturally loud (e.g. near a waterfall), while others are naturally very quiet. Even in areas where noise is dominant, there are periods when sound levels drop, and acoustic conditions approach natural ambient sound levels. Measurements in many national parks indicate that the acoustic conditions approach natural ambient levels for periods throughout the day, and for most of the night due to a lack of noise sources. These moments of relative quiet are important for visitors and wildlife and they need to be protected. However, special consideration must be given to areas and times where acoustic conditions approach the natural ambient sound level. During these times, impacts to the acoustic environment from a noise source will be greatest.
5.1 Describing the Affected Environment
Using data collected at the park and the NPS noise model, environmental impact analyses should thoroughly describe the acoustic conditions, incorporating temporal and spatial variation. The following steps provide a process for this task.
- Identify the “no-noise” condition. The no-noise condition is the time period when no noise is present and only natural sounds exist. These periods represent natural conditions and are particularly important for preserving the integrity of the acoustic environment. The no-noise condition is the natural ambient as calculated from data collected at the site or modeled. Those levels should be disclosed in a description of the affected environment.
- Identify the hourly time audible for noise. The no-noise condition occurs throughout the day, however it is often degraded by noise from a variety of sources. The time audible indicates the percentage of the hour in which noise is audible. The hourly time audible can be calculated for each location where data were collected. For example, at Pictured Rocks National Lakeshore, the time audible at 10:00 am is 80% (Figure 1). That means that the no-noise condition occurred at that site on average for 20% (or 12 minutes) of that hour. The hourly time audible for each site where data were collected should be disclosed in a description of the affected environment .
- Identify the average noise free interval (NFI). NFI is the average length of time between human-caused noise events (when only natural sounds were audible). It describes the average length of time of the no-noise condition when it occurs. NFI is calculated from data collected during on-site listening sessions. The on-site listening data also includes the average number of intervals that occur per hour. The average number of no-noise intervals and the average length of each interval should be disclosed in a description of the affected environment.
- Identify the difference between the existing ambient and the no-noise condition. The existing ambient represents the median sound level (L50) and includes both natural sounds and noise. When there is no noise present, the L50 is the same as the no noise condition. When noise occurs, the L50 increases. If the noise is loud, the difference between the L50 and the no-noise condition will be greater. If the noise is quieter, the difference will be smaller. This difference should be disclosed in a description of the affected environment.
5.2 Assessing Impacts from NEPA Alternatives
Assessing impacts to the intrinsic value of the acoustic environment
Every park has unique acoustic conditions that contribute to the overall environment at the park. The acoustic environment is a natural resource with intrinsic value of its own – without human or wildlife related reference points. When appropriate, impact analyses should address the acoustic environment. Impacts to the acoustic environment should be assessed by examining the effects of noise from a proposed action on both the natural and existing ambient sound levels. The measure for natural ambient is needed to understand the difference between natural ambient, the existing ambient, and the ambient resulting from each alternative.
Planners should review the DO 12 handbook when determining whether, how, and where these analyses should be incorporated in NEPA documents. When determining whether to carry noise impacts forward for detailed analysis and when determining the significance of the impact, the park’s purposes and park context or management zones (e.g., backcountry/front country, developed/natural) should be considered. More rigorous noise analysis should be prepared when the acoustic environment is a fundamental resource or value of a park and when the alternatives evaluated would result in meaningful adverse impacts to the acoustic environment.
For example, if a proposed action will generate a noise level of 65 dB, the natural ambient sound level is 30 dB, and the existing ambient is 40 dB, then the impact to the acoustic environment is based on a change of 35 dB for natural ambient and 25 dB for existing ambient. The impact intensity is based on this change and other factors such as management priorities, management objectives, the presence of sensitive resources (e.g. threatened and endangered species, traditional culture properties or values), and visitor expectations.
Assessing the behavioral, physiological, and ecological consequences of noise
The goal of impact analysis is to present sound level data in a way that is meaningful and allows park managers to protect the acoustic environment. This is done by considering sound level measurements in terms of "functional effects," the behavioral, physiological, and ecological consequences of noise. Some examples of functional effects are:
- Audibility (can you hear the sound or not?)
- Speech interference (is the noise loud enough to interfere with communication?)
- Sleep interruption (is the noise loud enough to wake someone up in the middle of the night?)
- Reduction in listening area for humans and wildlife (is the intensity and duration of the sound loud enough to mask important auditory cues?)
These effects can be used as metrics for assessing impacts. For example, if a noise source is so loud it wakes you up in the middle of the night, it has an impact. If noise interferes with a visitor's ability to have a conversation or to listen to a ranger give an interpretive talk, it also has an impact. See section 5.4 for more information on functional effects.
Assessing Impacts to Park Resources and Values
In addition to quantifying impacts to the acoustic environment, NEPA analyses should also assess the effects of noise on other park resources and values.
5.3 Addressing Noise Impacts in NEPA Documents
5.3.1 Acoustic Environment
NEPA documents should contain a section titled “Acoustic Environment” that assesses noise impacts from proposed actions in the Affected Environment and Impacts chapters. It will describe the impacts to the acoustic resource itself by quantifying change from both natural and existing ambient. The affected environment section should characterize the natural ambient conditions and existing acoustic conditions based on on-site listening, off-site listening, sound level measurements, or models. The impacts section should consider how possible future conditions would affect the acoustic environment.
5.3.2 Wildlife
Effects of noise on wildlife can be addressed in the acoustic environment section of a NEPA document, or it can be incorporated into the wildlife impact analysis. If addressed in the wildlife section, the document should cross-reference the acoustic environment analysis.
A growing number of studies indicate that wildlife, like humans, are stressed by a noisy environment. Sound, just like the availability of nesting materials or food sources, plays an important role in the ecosystem. Activities such as finding desirable habitat and mates, avoiding predators, protecting young, and establishing territories are all dependent on the acoustical environment. In order to continue with these activities, animals are forced to adapt to increasing noise levels or avoid high noise areas. When these effects are combined with other stressors such as winter weather, disease, and food shortages, noise impacts can have important implications for the health and vitality of wildlife populations within a park. Careful consideration of the impacts of human-generated noise on wildlife is a critical component of management for healthy ecosystems in our parks.
Sound propagates efficiently in water, and not surprisingly, most marine organisms depend on sound for basic life functions. For example, dolphins use sound to locate food, large whales use sound to communicate with mates across great distances, crabs use sound to find suitable habitat, groupers use sound in elaborate mating displays and many organisms respond to the cues produced by predators. The addition of noise in the ocean (e.g. ship noise, seismic air guns, pile-driving) masks biologic signals from predators, mates, habitat (Radford et al. 2014; Hatch et al. 2012; Ellison et al. 2012), disrupts behaviors such as migration, foraging (Aguilar Soto et al. 2006; Jung & Swearer 2011; Goldbogen et al. 2013; Voellmy et al. 2014), and in some cases causes permanent and temporary hearing loss and increases stress (Mooney et al. 2009; Anderson et al. 2011; André et al. 2012; Rolland et al. 2012). These behavioral, physiological and ecological changes not only have significant impacts to individual animals, but can have both population and ecosystem level consequences. A synthesis of two decades of research documenting the effects of noise on wildlife (Shannon et al. 2016) provides a detailed review of the effects of noise on wildlife, as documented in the scientific literature. For more information, also see our reference section and website.
When considering the effects of noise on wildlife, consider both terrestrial and marine organisms. Adopting mitigation strategies to reduce unnecessary underwater noise produced by human activities can potentially increase resilience to other stressors that are more difficult to reduce (e.g. climate change).
Monitoring Long Term Trends
To continue to track and understand long-term trends in ocean noise, the NPS has launched a network of underwater acoustic monitoring sites. Learn more about this network here.
5.3.3 Visitor Experience
Visitor experience is also affected by noise. Impacts to visitors can be addressed in the acoustic environment section or the visitor experience section. If addressed in the visitor experience section, the document should cross-reference the acoustic environment analysis. It is important to clearly articulate how changes in the acoustic environment may affect visitor use. An effective way to do this is noting the percent of time the noise source will be heard or percent time above specific sound levels that might affect the visitor experience (e.g., 52 dB for interpetive programs).
Our world is getting noisier. With dramatic increases in traffic, the explosion of digital gadgets and our increasing capacity to reach once-remote areas, solitude is a diminishing commodity. Not surprisingly, the American public comes to parks with natural quiet in mind. They come for the soothing effect of a gurgling stream, a delicate bird song, or the rustle of leaves on a fall day. From the awe-inspiring thunder of a waterfall to the gentle rustle of leaves in the breeze, natural sounds have a subtle but profound impact on visitors. In fact, in a survey of the American public, 95% responded that one of the most important reasons for preserving national parks is to provide opportunities to experience natural peace and the sounds of nature (Haas & Wakefield, 1998).
Visitor reaction to noise is complex, and may have unexpected consequences beyond annoyance. In one study, participants rated the scenic quality of national park landscapes more negatively when noise was also played in the background (Weinzimmer et al. 2014). Many additional studies have been conducted to quantify the effects of noise on human natural experience. Our reference section gives a detailed account of these findings.
5.3.4 Cultural Resources
The effects of noise on cultural resources must also be considered in NEPA documents. Section 106 of the National Historic Preservation Act, as amended in 2004, states that "an adverse effect is found when an undertaking may alter, directly or indirectly, any of the characteristics of a historic property that qualify the property for inclusion in the National Register in a manner that would diminish the integrity of the property's location, design, setting, materials, workmanship, feeling, or association." It also states, "adverse effects on historic properties include, but are not limited to... introduction of visual, atmospheric or audible elements that diminish the integrity of the property's significant historic features" §800.5(a)(2),
Based on these requirements, agencies must assess whether noise intrusions would result in adverse effects to cultural resources. Impacts to cultural resources can be addressed in the acoustic environment section or the cultural resources section. If addressed in the cultural resources section, the document should cross-reference the acoustic environment analysis.
The specific sounds associated with our cultural heritage not only teach us about the past, they connect us to distant times and places in a way that few other things can. The acoustical environment of national park cultural sites, therefore, is an important part of the ambiance and helps create meaningful connections. The silence of an empty cell on Alcatraz Island hints at the sense of isolation of a former inmate. Cannon fire or "Taps" at a Civil War battlefield conjures images of both pride and sadness. Every unit within the national park system has its own acoustic environment that is both unique to and appropriate for that particular place. From the brassy horns of New Orleans jazz music to hypnotic Native American drumming, no two are the same. Unwanted or inappropriate sounds, such as aircraft, vehicles, and construction equipment, can detract from the experience. With this in mind, the National Park Service manages park units to protect those cultural sounds considered fundamental to the park's purposes, and mitigates extraneous noise. See Management Policies Section 5.3.1.7 for further guidance on cultural soundscape management.
Minute Man National Historic Park provides a good example of how parks can protect cultural soundscapes. Minute Man NHP has worked with NSNSD to develop an Acoustic Resource Management Plan, which ensures the cultural soundscape of the Revolutionary War is protected.
5.3.5 Wilderness
Many parks contain areas that are designated, proposed, or managed as wilderness. Preserving the acoustic environment and natural sounds of such areas are critical to effective wilderness management and can have important effects on wilderness character. Natural soundscapes and the absence of noise are crucial components of the wilderness qualities of solitude and naturalness. Noise is one of the most common and pervasive human influence on the primeval character of wilderness. Noise from roads, park operations and maintenance activities, or aircraft overflights are common intrusions to wilderness or other remote protected areas. Impacts to wilderness areas can be addressed in the acoustic environment section or the wilderness section. If addressed in the wilderness section, the document should cross-reference the acoustic environment analysis.
Addressing park soundscapes is often critical to protecting the quality and character of wilderness. A loud or persistent noise source can be a prominent “imprint of man’s work” that can make a backcountry user or camper instantly aware that they are not alone. Human-caused noise, especially low frequency sound from industrial developments, vehicles, and machinery, can travel for miles in certain terrain. Wilderness should be essentially unhindered and free from modern human control or manipulation and retain its primeval character and influence. When addressing wilderness resources in compliance and planning documents, the importance of natural sounds and potentially adverse effects of noise on wilderness character should be considered. Additional information on incorporating sounds and wilderness into NEPA documents can be found in “Keeping it Wild 2: An Updated Interagency Strategy to Monitor Trends in Wilderness Character Across the National Wilderness Preservation System [PDF]."
In a 2016 Journal of Forestry article (McKenna et al. 2016), the National Park Service and partners at the U.S. Department of Transportation describe a framework for assessing impacts of air tour noise on wilderness character. Using a case study of noise in the Haleakalā Wilderness in Haleakalā National Park, the paper presents a tiered approach to impact assessment, where sound level, audibility, and spatial extent of noise events are considered to evaluate current conditions and establish a threshold not to be exceeded.
5.4 Functional Effects
In addition to describing noise impacts in terms of changes in sound level, it is important to discuss how noise affects the behavior, physiology, and other characteristics of visitors and wildlife. These measures, known as “functional effects,” describe the consequences of noise and provide a more complete and comprehensive understanding of impacts than merely stating how the sound level changed as a result of the actions being assessed. Functional effects examined by NPS include:
Time audible is the percentage of time during a specified time period that noise can be heard. For example, 25% time audible means that noise could potentially be heard in specified areas for 25% of the day, or three hours during a 12-hour day –not necessarily consecutive hours, but spaced throughout the day. Time audible or "audibility" is one of the ways NPS measures or characterizes the acoustic environment in national park units. It is a measure that correlates well with visitor complaints of excessive noise and annoyance. Most noise sources are audible to humans at lower levels than virtually all wildlife species. Therefore, percent time audible is also a protective proxy for wildlife. In some cases, a sound may be above natural ambient sound levels, but still not be audible due to the frequency of the sound. Similarly, some sounds that are below the natural ambient can be audible. Time audible can be calculated by either a trained observer (on-site listening) or by making high-quality digital recordings (for later playback).
Deviation from natural ambient is the difference between the average existing sound level and the natural ambient condition. This metric reports the difference between the average hourly sound level, including all natural and human-caused sounds, and the hourly natural ambient. It represents the extent to which human-caused sounds raise the natural ambient sound levels. This metric does not provide information on event duration or timing, nor does it mean that human-caused sounds cannot be heard at or below the ambient sound level. It means that the sound levels produced by human sources are above the natural ambient sound level. Deviation from natural ambient can be used to identify reductions in listening area and alerting distance.
Reduction in listening area quantifies the loss of hearing ability to humans and wildlife as a result of an increase in ambient sound level. Under natural ambient conditions a sound is audible within a certain area around visitors or wildlife. If the ambient level is increased due to a noise event, the area in which the sound is audible decreases.
Figure 2 illustrates the relationship between increased ambient and listening area reduction. Under natural ambient conditions, an owl perched in the tree may be able to hear a mouse scurrying through the brush anywhere within an area of 100 m2 of the perch (the largest ½ sphere). If a noise event increases the ambient level by 3 dB, the area in which the owl can hear a mouse would decrease by 50 percent to approximately 50 m2 (the listening area is reduced from the largest ½ sphere to the middle ½ sphere in the Figure). Similar reduction would occur for visitors and their ability to hear natural sounds or interpretive programs.
Noise free intervals (NFI) are time periods during which only natural sounds are audible. This metric represents the periods of time a park visitor could expect to experience the soundscape without noise. NFI are also important for wildlife as they provide a period of recovery from the behavioral and physiological responses to noise. NFI data are expressed as maximum NFI, minimum NFI, and median NFI during specified periods of time. NFI are calculated from on-site listening data and sound pressure data collected at parks.
Speech interference represents the amount of time during which noise may interfere with human speech. The potential for speech interference from a noise depends on the distance between the speaker and listener and the acceptable level of intelligibility. The percentage of time or number of minutes that speech may be adversely affected by noise during a specified time period (e.g. hour, day) is calculated from the sound pressure data collected at parks. As shown in Table 2, different types of conversation can be differentially affected by noise. Some of the types include:
General Conversation occurs between two or more people standing approximately 2 m apart speaking at normal conversational volume. Hikers and visitors viewing scenic vistas in the park would likely fall into in this category.
Interpretive Programs This type of conversation occurs during interpretive programs conducted by park staff or other groups (schools, tours, etc.). Interpreters typically speak in a "raised voice" with approximately 10 m between the speaker and the furthest participants.
Rock Climbers/Canyoneers/Mountaineers This type of conversation occurs between technical rock climbers or between climbers and belayers in climbing areas. Climbers, canyoneers, and mountaineers appreciate and value a natural setting when climbing and effective communication is critical for this and other recreational uses. Typically, the distance between rock climbers, canyoneers, or mountaineers ranges from less than 1 m to more than 50 m.
Sleep Interruption In 1997, the Federal Interagency Committee on Aircraft Noise (FICAN) issued a report [PDF] on sleep interruption. The report contains a model for estimating the probability of awakening due to a noise event based on the intensity of the sound. The likelihood for sleep interruption at campsites, lodges, employee housing, and other areas can be calculated from sound pressure data collected at the park. FICAN acknowledges that this criterion may underestimate the probability of awakening in campgrounds or other temporary residences, where people are typically more prone to disturbance. Therefore, the actual likelihood of sleep interruption for areas within national parks may be greater than those calculated by the model.
5.5 Impact Assessment Tools
Impact assessment can be as simple as subtracting the natural ambient from the existing to determine the amount of noise being introduced into an environment, or reviewing daily spectrograms (See section 3.1, visual analysis) to visually identify patterns. Or, in the case of a proposed action that would affect the acoustic environment, impact assessment could include comparing the estimated or modeled noise to both the natural and existing ambients, as the new action could increase or decrease the existing ambient. If the task at hand warrants a more complex assessment, tools such as those described below will be useful.
Attenuation Calculator
Computerized modeling software can be computationally intensive and expensive to run. Many projects require less precision, and more simplified approaches may be appropriate. NSNSD has developed a user-friendly tool called Attenuation Calculator which quickly estimates noise propagation. Based on the basic physical properties of sound in a medium (air or water), the program can predict the sound levels of several common transportation sources at given distances. This program can also be used as a scoping tool to determine if additional modelling may be required. The results can be exported to mapping software in order to spatially examine the noise impact (Figure 3).
Noise Source Modeling
Noise modeling software allows users to estimate the landscape level noise impacts based on inputs such as route, vehicle type and numbers, and environmental conditions. Modeling is often used to determine the impacts a specific noise source (such as a construction project, road expansion, or aircraft overflights) may have on a specific area, or the park as a whole. Models allow decision makers to estimate what noise exposure would be like under a variety of conditions.Noise models incorporate existing sound conditions and landscape features such as terrain, ground cover, and vegetation cover, and add a specific noise source to the landscape. Models such as Noise Modeling Simulation (NMSim) provide the ability to evaluate the impacts of different noise source scenarios and assess mitigation options (Figure 4). NSNSD has traditionally relied upon the FAA's Integrated Noise Model (INM) to estimate aircraft overflights noise impacts.
NSNSD often uses iterative analyses to compare the aggregate noise footprint of given traffic or noise scenarios. This process begins with a model described above to provide the base layers for this analysis. This dynamic visualization can show parks different scenarios that may help in determining appropriate noise mitigation techniques, including alternative traffic routes. In addition, graphics produced from iterative analyses are a valuable resource for public scooping meetings (Figure 5). Park managers can easily show stakeholders the different scenarios, and with public input, the graphics change right away, showing how their input changed the noise footprint in a particular area.
SPreAD-GIS and Sound Mapping Toolbox
SPreAD-GIS is a GIS tool for modeling anthropogenic noise propagation in natural ecosystems. More information on SPreAD-GIS can be found here. Sound Mapping Tools is an ArcGIS toolbox designed to provide free, spatially-explicit tools for modeling sound propagation across a landscape. Potential applications include assessing noise impacts from off-road vehicles, energy development, or traffic noise. More information about Sound Mapping Tools can be found here.
SPreAD-GIS is a GIS tool for modeling anthropogenic noise propagation in natural ecosystems. More information on SPreAD-GIS can be found here. Sound Mapping Tools is an ArcGIS toolbox designed to provide free, spatially-explicit tools for modeling sound propagation across a landscape. Potential applications include assessing noise impacts from off-road vehicles, energy development, or traffic noise. More information about Sound Mapping Tools can be found here.
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Last updated: January 29, 2024