noted below, community noise is an immission problem (i.e., what people hear).2
Today, the widely used criterion for assessing community noise levels in the United States is the day-night average sound level, or DNL (see Appendix A for definition). It is the sound pressure level averaged over 24 hours with the amplification of the measuring systems increased by 10 decibels during the nighttime hours. Since 1974, when the U.S. Environmental Protection Agency’s (EPA) “Levels Document” and related documents were published, DNL and an exposure-effect relationship showing the percentage of respondents on social surveys who say they are “highly annoyed” by noise from various sources have generally been accepted as overall indicators of the impact of community noise. This exposure-effect relationship was first described in a classic analysis by Schultz (1978), who synthesized 12 major social surveys of reactions to transportation noise. The Schultz curve, which describes the results, essentially illustrates the percentage of the population predicted to be highly annoyed as a function of noise level (see Chapter 2 for further discussions of community and building noise criteria).
Noise in Quiet Areas
Areas in the United States that are relatively free of transportation noise and noise from most other sources are often used for recreation and are places where people value the absence of noise and the opportunity to hear “natural” sounds, such as the flapping of a bird’s wings or wind rustling through trees. However, noise from aircraft, off-road vehicles, and other sources sometimes intrudes on these quiet environments. The DNL metric is generally inadequate to describe the “soundscape” in such areas.
FIGURE 1-1 Comparison of A-weighted sound levels in common outdoor environments. Source: Miller (2003).
Complaints about aviation noise have a long history. In an introduction to a review of current activities by the Federal Aviation Administration (FAA) related to aircraft noise, Burleson (2005) points out that 2003 was the 100th anniversary of flight and the 92nd anniversary of the first editorial complaining about aircraft noise.3 The most serious problems arose in the late 1950s when commercial jet aircraft came into service.
In the past 50 years, considerable progress has been made in reducing noise emissions from aircraft—mainly through the introduction of high bypass ratio engines, which were driven by a desire to reduce noise emissions and increase fuel efficiency. A 2001 U.S. General Accountability Office (GAO) report stated: “We currently estimate that the airlines’ costs directly attributable to complying with the transition to quieter aircraft noise standards ranged from $3.8 billion to $4.9 billion in 2000 dollars” (GAO, 2001). The transition, over a period of 35 years, led to a 95 percent reduction in the number of people impacted by aircraft noise in the United States (PARTNER, 2004).
Despite this progress, there are still noise issues around most of the nation’s commercial airports. In a report to Congress in 2000, a survey of the nation’s 50 busiest commercial airports indicated that noise was the number one concern for 33 airports and was of some degree of concern in areas around 49 of the 50 airports (GAO, 2000).
Emission and immission are defined in Appendix A. Briefly, emission is the sound directly emitted from a noise source essentially unaffected by the immediate environment around the source. Immission is the sound the receiver hears after it has traveled along a sound transmission path and has been affected by it.
Burleson, C. Aviation and the Environment: Navigating the Future. Presentation at an NAE-sponsored workshop, Technology for a Quieter America. Washington, DC., September 1, 2005.