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-12- <br />Subpart 1 provides that noise standards are expressed in <br />terms of sound pressure level and apply to sounds that have a <br />duration of at least 200 milliseconds. The rule also states that <br />noise standards in Subpart 2 apply to all sources and that for <br />airports, the additional noise standards in Subpart 3 also apply. <br />The purpose of applying the rule only to sounds having a duration <br />of at least 200 milliseconds is to ensure that the standards <br />apply only to what is considered non -impulsive noise. Although <br />there is no agreed -upon definition of impulsive noise in the <br />acoustical field, it is generally agreed that noise with a <br />duration of at least two hundred milliseconds is non -impulsive <br />noise. It is reasonable to apply these standards only to <br />non -impulsive noise because there is general agreement in the ' <br />acoustical field that impulsive noise should be addressed <br />separately, from a standard -setting viewpoint, from non -impulsive <br />noise. The Agency does not intend to address impulsive noise in <br />this rule making proceeding. <br />Subpart 2 sets forth the noise standards. In order to <br />explain the proposed changes to the existing standards, is is <br />necessary to explain the nature of the existing rules. The <br />existing rules set forth limiting levels of sound depending upon <br />the activities on the land where the sound levels are received. <br />The land areas are classified according to their actual use, each <br />parcel of land being given a Noise Area Classification MC). <br />The NAC's take into account the amount of noise which is safe <br />for people to encounter in their daily lives. For example, a <br />