Effects of Noise
Introduction
The following sections briefly describe a process for assessing the impacts of existing or proposed actions on the acoustic resources and soundscapes, 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 and acoustical environment. Sound-sensitive resources or values may include biological resources, special areas such as wilderness, cultural and historic resources, and visitor experience.
The NPS Natural Sounds Program begins acoustic analysis by indentifying sound sources and their characteristics 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 and Noise section). 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 should be composed mostly of sounds reflecting a natural setting, whereas a developed zone is likely to have a large component of human and human related sounds.
Effects on Wildlife
Functioning ecosystems depend on natural acoustical environments. Many animals, insects, and birds decipher sounds to find desirable habitat and mates, avoid predators and protect young, establish territories, and to meet other survival needs. 
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, and not all species have the ability to adapt in this way. 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.
Effects on Cultural, Archeological, and Historic Resources
The primary mission for many national parks is to protect the resources and values related to the culture, ethnic heritage, and history of a group or a place. Many locations in national parks are significant because of the meaning, memories, and experiences that people associate with them. Cultural resources include tangible materials such as structures and artifacts, as well as intangible aspects of cultural expression: oral traditions, music, and community rituals. Visitors to cultural and historic units of t
he National Park System want to better understand and embrace America's heritage in a direct and personally meaningful way.
An appropriate acoustical environment is an important element in how we experience the cultural and historic resources in the national parks. Visitors want to immerse themselves in the historic time period or cultural expressions associated with a site. The quiet of a civil war battlefield can evoke reflection and feelings of reverence; silence and the sound of gulls heard from a jail cell on Alcatraz Island can elicit feelings of empathy, despair or isolation. Unwanted or inappropriate sounds, such as aircraft, watercraft, vehicles, construction equipment, and energy development can detract from the overall enjoyment of their experience. The National Park Service manages park units to protect those cultural and historic sounds they consider fundamental to the purposes and values for which the parks were established.
Effects on Visitors
Visitors also appreciate natural sounds. Many natural sounds such as gurgling streams, bird songs, or the rustling of leaves on a fall day can have a calming and relaxing effect. Other sounds such as the chirp of crickets or a gentle breeze through a forest can trigger memories of pleasant past experiences.
Visitors to national parks often indicate that an important reason for visiting the parks is to enjoy the relative quiet that parks can offer. In a 1998 survey of the American public, 72% of people identified opportunities to experience natural quiet and the sounds of nature as an important reason for having national parks. 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 2005 & 2006 study at Muir Woods National Monument visitors showed annoyance with many noise sources including aircraft, cell phones, vehicles, and park operations.
Visitor evaluations of annoyance 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 rate of occurrence, 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.