This diagram explains the DNL (Day-Night Average Sound Level) metric used in airport noise policy, showing a bar chart of hourly LEQ sound levels (in dBA) over a 24-hour period labeled 'One Day of Events (DNL).' It highlights the 10-dB nighttime penalty applied to noise events occurring during nighttime hours, which is a standard FAA methodology for calculating community noise exposure.

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Chart depicting Day-Night Level (DNL) methodology with hourly LEQ values and nighttime penalty weighting, showing how individual aircraft events accumulate into 24-hour average noise exposure. Illustrates the regulatory metric used to assess airport noise impacts on communities.

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As the source for federal transportation grants, the Puget Sound Regional Council was charged with developing a system to meet the growing need for commercial aviation. When the search for a second airport failed, they authorized the Port of Seattle to build the Third Runway, with a mandate to develop a noise abatement and mitigation program. In their effort to stop the Third Runway, the ACC argued over every detail of the Port's efforts--including property buyouts and sound insulation. The dispute was meant to be settled by a three member Expert Arbitration Panel. This is their final report. It finds 2-1, that the Port's program was insufficient in several respects. Despite that, funding for the Third Runway was approved by the PSRC, and the 'Port Package' program, proceeded largely unchanged. Expert Arbitration Panel's final decision finding that the Port of Seattle had not shown sufficient reduction in real on-the-ground noise impacts to satisfy noise reduction conditions required for approval of a third runway at Sea-Tac International Airport. The majority decision concluded the Port's noise abatement programs were insufficient despite being impressive in scope.

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