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OCR for page 118
118 Aircraft Noise: A Toolkit for Managing Community Expectations Example of Metric Display for Public Information: Cumulative Aviation Noise Percent of Time Above Natural Ambient with Existing Airport (St. George UT EIS) (162) 7-15 Relative Change in Project Noise 2020 Airport Only Percent Time Audible %TAnatural (St. George UT EIS) (163) 7-16 Best Applications Based upon the results of surveys and interviews conducted for this analysis, as well as litera- ture available in the field and author experience, several noise metrics may be identified as best practices in communicating the complexities of aircraft noise to the public. Measures that pro- vide greater understanding of the effects a proposed change may have on the level of exposure are useful in focusing attention on reality rather than feared unknown consequences. The most effective measures identified by this evaluation to achieve this purpose include: Day-Night Average Sound Level (DNL). Although DNL is not well-understood by the general public, it has been found to be the best indicator of a high degree of annoyance by several differ- ent evaluations over a long period of time. Further, the FAA has adopted DNL as its required metric in all noise evaluations for which the agency provides funding assistance. It is the only metric for which widely accepted guidance is available for land use compatibility determina- tions. Consequently, DNL is the essential metric for evaluation during noise abatement and envi- ronmental planning processes involving aircraft noise exposure assessments. Sound Exposure Level (SEL). Comparisons of SELs between existing and proposed alterna- tive conditions are able to provide the public with greater insight into what changes may occur in the future, without actually experiencing that change. The greater the array of information available to assist the public in understanding the potential changes, the better able the airport is to communicate fact rather than public fear of the unknown. Comparison of single event sound levels is one of the better tools to communicate the extremes of anticipated noise events. Number of Events Above (NA). While this metric is relatively expensive to develop, it is flex- ible enough to allow the analyst to distinguish the effects of specific noise abatement actions on specific categories of aircraft. Hence, evaluations of cost and benefit may be enhanced through greater detail. Its use to project the anticipated number of events above a threshold of public interest has been widely used to assist communities to relate the effects of alternatives using what they hear (single events), rather than relying on analysts to tell them that the DNL doesn't indi- cate they will be adversely effected. This is the single metric that responds to differences in num- bers of overflights in a way every individual can understand. Every community is unique, yet every community is interested in the same thing its quality of life. The use of the NA metric allows the individual to better relate to how any proposed development action or change of operational characteristic may impact that quality. Time Above (TA). The TA metric is best applied to evaluations of proposed changes to facil- ities or operational characteristics on schools during the school hours. When combined with an Lmax threshold equivalent to the exterior to interior attenuation plus the threshold of interest, the data may be useful in identifying those schools that should be considered for additional noise mitigation. The public's interest in the measure is usually restricted to the secondary interest in the effect of aircraft noise on the quality of life. Therefore, it may be developed to provide addi- tional understanding of existing and potential aircraft noise patterns.