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-17- <br />the Leg is usually about two to three decibels more than the L5U <br />value. Primarily this is because the Leg takes into account all <br />the energy produced by a noise source whereas the L10 and L50 <br />only take into account the noise with a duration of at least six <br />and thirty minutes, respectively. Therefore the move to the Leg <br />descriptor requires the three dBA upward adjustment from the <br />existing L50 in the standard in order to keep the noise standards <br />at the same level of protection as the existing standards. <br />Subpart 3 of the proposed rule adds an additional descriptor <br />for airports called an Ldn• ..Ldn .(day, night. eound.lavAILAGAB + <br />the equiva.lent,.►-weightgd,sostRd.l�vel..durinB_a�ir�ent��four hoac.. <br />time:•period•.with •a'ten .decibel- weighting applied -to., -the, <br />,equtyslant.sound -level •during..nighttime. It is reasonable to <br />provide an additional descriptor for airport noise because of the <br />complexities associated %4ith airport noise. Airport noise is a <br />reflection of airport operations, which are diversified and <br />sporadic. For example, there is a significant amount of <br />noise -producing activity at the Minneapolis -St. Paul <br />international Airport during the daylight hours, but after 11 <br />p.m. there are almost no take -offs or landings. Therefore, a <br />one -hour noise measurement taken between 7 a.m. and 8 a.m. at a <br />location affected by airport noise will result in a much higher <br />Leg than a one -hour noise measurement taken between 11 p.m. and <br />midnight. Similarly, noise measurements taken during the same <br />