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-6- <br />methodologies for describing noise have replaced older, outdated rl <br />methodologies, and advances in instrument technology have made <br />noise assessment more practicable and reliable. It is desirable <br />to update the rules to incorporate the improvements which have <br />been made in noise description and in noise instrument <br />technology. <br />The Agency's review of the noise rules also led it to <br />conclude that there is a need to revise the Noise Area <br />Classifications established in the existing rules. Experience <br />since 1974 in administering the existing rules has shown that the <br />existing classification system contains some ninety land use <br />activities which makes the application cumbersome because the user <br />may classify a particular land use in more than one Noise Area <br />Classification. In addition, many of the existing land uses are <br />ambiguous and not well defined, which has made interpretation of <br />the rule difficult in certain cases. There is, therefore, a need <br />to amend the list of Noise Area Classifications so that the rule <br />will be less susceptible to conflicting interpretations. <br />The Agency's review of the noise rules also led it to <br />conclude that there was a need to add another descriptor (Ldn) <br />for airports. Airport operators have commented that the noise <br />descriptors in the existing rules (L10 and L50) are not <br />appropriate for measuring airport noise. While the proposed <br />change from the existing descriptor to an energy based standard <br />(Ley) will address some of the airport operators' concerns, they <br />continue to contend that the Ldn standard is a more fair standard <br />