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-18- <br />hours <br />on different days may y produce radically different numbers <br />due to the day-to-day differences in the numbers of take -offs and <br />landings. Because of this, a one -hour descriptor does not <br />accurately describe the total or composite noise produced by <br />an <br />airport. This makes it difficult to assess airport noise and to <br />use predictive computer modeling using only an hourly Leq. The <br />airport operators have commented to the Agency that an Ldn <br />standard is a more useful descriptor of airport noise because it <br />takes into account both the noisy hours and the quiet hours. The <br />Agency agrees that an Ldn standard would be useful in assessing <br />airport noise and is amenable to adding such a descriptor to <br />airports, but aaly so.lpng, the hourly <br />applied -so that the Agency'can continue to•determina-whoth". <br />operatioua_at..busy.hours are -creating. noise levelB..WbiCh..ar,_ <br />annoying gr .04M.W .to, People .af"acted by the .aircraft-nedea„ <br />Therefore, the Agency's proposal to add an Ldn of 63, 68, and 78 <br />decibels for NAC-1, NAC-2, and NAC-3, respectively, is <br />seasonable. <br />Part 7010.0050 Noise Area Classifications <br />The Agency is proposing to amend the existing rule <br />establishing Noise Area Classifications in order to simplify it. <br />The existing rule sets forth four NAC's and ninety land use <br />activities. When determining what numerical noise standard <br />applies to a given land area, it is necessary to categorize the <br />