However, natural quiet in parks is increasingly at risk. To study the effects of human-caused noise on visitors, volunteers at Muir Woods National Monument cataloged all sounds they heard, day and night, for a year. What they found was surprising. It was rarely quiet (Monroe et al. 2007). Parks are experiencing an on-going acoustic assault by everything from air tours to maintenance equipment. Such noise affects visitors' perceptions of solitude and tranquility and can interfere with how people rate landscapes in national parks (Weinzimmer et al. 2014). In a related study at Muir Woods, visitors found increasing levels of human-caused sounds to be unacceptable and even annoying (Monroe et al. 2007). Noisy visitors, loud talking, and other related sounds were found to substantially detract from the quality of the visitor experience. In other studies, noise has been shown to be more disturbing to visitors if it is loud, occurs in bursts, is unpredictable, or if it interferes with quiet activities such as bird watching.
Isolated areas are not exempt. In Grand Canyon, no single location is totally free of aircraft noise, and in some areas it can be heard up to 43 times in a 20-minute period. Backcountry hikers, after September 11, 2001, reported knowing that something was very wrong because there were no sounds from commercial aircraft (Bell et al. 2009). Tranquility, it turns out, even in the most remote areas of our national parks, is becoming increasingly scarce.
Published Research on the effects of noise on the visitor experience can be found on the Additional Information page.