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CHAPTER 7 Noise Metrics and Community Response National aircraft noise policy and public perception are often different in the view of what conditions constitute an adverse noise impact. The EPA and FAA have adopted the DNL as the defining metric for the description of aircraft noise impacts. The Federal Interagency Committee on Urban Noise (FICUN) (152) found in 1980 that DNL is the best descriptor of community impact. The finding was reconfirmed by the subsequent Federal Interagency Committee on Noise (FICON) (8) in 1992. Several studies of public reaction to noise have found the metric to best correlate to the number of persons highly annoyed by transporta- tion noise. Figure 7-1 (150) reproduces the Schultz and Fidell Curves, which relate the per- centage of persons who consider themselves to be highly annoyed by noise to the DNL decibel level. Although DNL takes into account every aircraft noise event experienced by the airport neigh- bor, it can be experienced only in cumulative terms. Many airports responding to the surveys and interviews conducted for this assessment, as well as the authors' experience on numerous 14 CFR Part 150 and EIS evaluations, have indicated that individuals respond more positively to single event noise levels that they directly experience. As airport managers communicate with neighbors and the general public on noise issues, it is appropriate that they be acquainted with the variety of noise measurements and metrics available to respond to public comment. In dis- cussions with the public, it is essential to use graphic representations to demonstrate the con- cepts of noise. Several examples are provided for the more common of the metrics used in the United States. For complex situations, the reader also may consider various demonstration tools that have been developed to convey some of the complexities of noise and its associated metrics. Among these tools are the FAA's designated computational model for noise evaluations, the Inte- grated Noise Model (INM) ( models/inm_model/) (153), the Interactive Sound Information System (ISIS) (http://www. (120) and the Noise Model Simulation (NMSIM) (http://www.wylelabs. com/products/acousticsoftwareproducts/nmsim.html) (154). A number of derivative graphical interfaces are commercially available from aviation and acoustic consultants to display the results of noise simulations. There are three types of noise metrics: those that express noise cumulatively as a function of total energy experienced over a set period of time, those that express the noise levels experi- enced during a discreet aircraft operation, and those that are a hybrid of the other two. The follow- ing sections will address each and how they might be used in describing the patterns of current or changing aircraft noise levels to the public. The tools identified throughout this document provide illustrative examples of ways each metric is used, including several animations devel- oped to aid public understanding. Table 7-1 provides a summarization of the utility of various metrics that are discussed in subsequent paragraphs for functions that are subject to public scrutiny. 111