Effects of Noise

“In the silence I listen, I watch, I reuse, I attend, I observe. I require this silence. I search it out. The finely drawn treble song of a white-throated sparrow is part of it. Invasions of it by the noise of engines torment me. This is my solitude.”

Alice Koller - Author of The Stations of Solitude

Introduction

Every unit of the National Park Service was established for specific purposes and to protect specific resources and values. A park's purpose, resources and values are provided in enabling legislation, general management plans, and and other planning documents. Soundscape management and impact assessment is based on those purposes, values, and planning objectives as stated in Management Policy 4.9 (2006):

“Using appropriate management planning, superintendents will identify what levels of human-caused sound can be accepted within the management purposes of parks…In and adjacent to parks, the Service will monitor human activities that generate noise that adversely affects park soundscapes, including noise caused by mechanical or electronic devices. The Service will take action to prevent or minimize all noise that, through frequency, magnitude, or duration, adversely affects the natural soundscape or other park resources or values, or that exceeds levels that have been identified as being acceptable to, or appropriate for, visitor uses at the sites being monitored.”

Park Service policy is currently interpreted to apply not just to natural sound environments, but also to the “appropriate” sound environment proceeding directly from park establishment purposes. Some human sounds are entirely appropriate for purposes of interpretation and increased understanding of park resources purposes and values. Examples include interpretive talks, cultural rites or celebrations, battlefield reenactments, or living cultural histories.

Impact assessment involves determining the level of impact of various types of sound sources on park resources, purposes and values. The goal of acoustic studies in parks is to provide the data necessary to monitor and manage park soundscapes so that impacts to visitor experience, wildlife, and other resources in the park from proposed activities can be adequately assessed.

Soundscape Analysis Overview

National park soundscapes are resources to be both protected and enjoyed. The following sections briefly describe a process for assessing the impacts of existing or proposed actions on the soundscape, and other resources that may be termed “sound-sensitive.” The premise for analysis is that an action having acoustic properties will directly impact the park soundscape or sound environment. The impact can be visualized by drawing contours of measured or modeled sound metrics on the landscape, creating “soundprints.” The impact is further defined by expressing the duration and timing of sound events. Impacts on other sound-sensitive resources or values falling within the soundprints can then be determined. Sound-sensitive resources or values may include biological resources, special areas such as wilderness, cultural and historic resources, and visitor experience.

Effects on the Soundscape

Analysis begins with the identification of characteristics of an activity, or sound source that may affect the sound environment. For most sound sources, such characteristics would include the location and movement of the source, its operational features that produce sound and how the sound would be distributed over time. The sound sources themselves are defined or measured using various noise metrics. The NPS measures decibels across the frequency spectrum, divided into third-octave bands (see understanding sounds). NPS also collects audibility logging data and short sound recordings to assist in the identification of the sound sources. Such measurements allow us to calculate a variety of metrics including maximum decibels of an event, numbers of events, noise free intervals, audibility, time of audibility and the area where the sound is audible. These metrics are compared to soundscape objectives developed for each management zones. Different zones are likely to have different objectives for soundscape management. A wilderness zone soundscape should be composed mostly of sounds reflecting a natural setting, whereas a developed zone would necessarily have a large component of human and human related sounds. Impacts are described as changes in the soundscape resulting from the modeled actions, as compared to the soundscape objectives.

Effects on Visitors and Visitor Experience

Visitors to national parks often indicate that an important reason for visiting the parks is to enjoy the relative quiet that parks can offer. Americans surveyed in 1998, were asked to identify some of the most important reasons for having national parks. Seventy two percent said “Providing opportunities to experience natural peace and the sounds of nature.” This ranked as the fifth most common response. In studies of visitor preferences respondents consistently rate many natural sounds such as birds, animals, wind, and water as very pleasing. As a result, the presence of unwanted, uncharacteristic, or inappropriate sounds can interfere with or alter the soundscape resource and degrade the visitors’ experience.

Uncharacteristic sounds or sound levels affect visitors’ perceptions of solitude and tranquility and can generate high levels of annoyance. In a recent study at Muir Woods National Monument visitors showed annoyance with many sounds sources including aircraft, cell phones, vehicles, and park operations.

Visitor evaluations of annoyance tend to be multidimensional and are affected by many factors including the setting in which the sounds occur, the visitors’ recreational activities, and their expectations of quiet and solitude. Characteristics of the sound also contribute to levels of annoyance. Annoyance is related to frequency, duration, and sporadic nature of sounds. Loudness is also a factor, however research on specific causes of noise such as snowmobiles and helicopters indicate that even low levels of sound that remain audible can compromise a visitor’s enjoyment of a natural setting and generate feelings of annoyance. Click here for an Annotated Bibliography on Visitor Experience and Soundscapes.

Effects on Cultural, Archeological, and Historic Resources

Sound can affect resources, locations, events, or structures having cultural, archeological, or historic significance. In the same way that visitor experience or biological resources can be impacted, these sites are affected by sound that is out of character for the purpose of the site. Historic battlefields can be affected by aircraft, or adjacent roads. Native American cultural events or sacred sites can be affected by the noise of tour buses and passing autos, or even the nearby voices of visitors. Ideally, the cultural, religious or historic context of a site should be maintained to the greatest possible extent. Some archeological sites, set within fragile geologic structures, can be affected by vibration from low frequency emissions of aircraft and motorized vehicles. The objective for impact assessment is to determine the significance (magnitude, extent and duration) of potential impacts in objective terms.

Effects on Wildlife

Scientific studies have shown that wildlife can be adversely affected by sounds and sound characteristics that intrude on their habitats. Although the severity of the impacts varies depending on the species being studied and other conditions, research has found that wildlife can suffer adverse physiological and behavioral changes from intrusive sounds and other human disturbances. Some sound characteristics have been associated with suppression of the immune system and increased levels of stress-related hormones in animals. Studies have also shown that songbirds that live in places with increasing sound levels have to sing louder than birds in quieter environments. Birds forced to sing at a higher volume have to expend increased levels of precious energy to attract a mate or warn of predators. Bighorn sheep are less efficient at foraging for food when they are exposed to aircraft, and mountain goats often flee from the sound of helicopters and airplanes. Still other research has demonstrated that intrusive sound properties can adversely affect reproductive success in caribou and communication in whales. When these effects are combined with the other stressors faced by wildlife such as winter weather, disease, insect harassment, and food shortages, sound impacts can have important implications for the health and vitality of wildlife populations within a park. Click here for an Annotated Bibliography on the Effects of Noise on Wildlife.

¹Although National Park Service policy defines “noise” as undesirable human-caused sound. The term must be considered in the context of the range of actions that are and are not appropriate or desirable in a park. NPS considers sound and its attendant characteristics (amplitude, frequency, occurrence, duration), as it may enhance or detract from the stated purposes for which a park was created. Sounds, sound characteristics, or sound sources that detract from the purposes of a park may be said to be intrusive, inappropriate, or as an impact on the desired park soundscape.