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NOISE AND AIR QUALITY IMPACTS <br />' In this section the differential air quality and noise impacts of <br />increased coal truck and employee traffic on SH 133 and Stevens Gulch <br /> Road are examined. Conditions associated with an increase in production <br /> levels to 1.3-million tons per year (TPY) for the year 1982 are compared to <br /> those related to current production levels of 700,000 TPY projected to 1982. <br /> NOISE <br /> Prior to a discussion of noise impacts, it is useful to define the <br /> following terms: <br /> Decibel (d B) - a unit of measurement of sound pressure. A decibel <br /> is a logarithmic scale in which each increase of 10 units <br />._• represents a ten-fold increase in intensity level. <br />_. "A". Scale of Weighting (d BA) - a sound level weighting system which <br />-- closely approximates the human hearing response by discriminating <br /> against the very low frequencies. <br /> Leq - a canposite measure of noise exposure under varying conditions <br /> for a specified time period, i. e., the "equivalent" sound <br /> level. <br />~• <br /> Ldn - a measure similar to the Leq which weights sound levels measured <br />i"' between 10 p. m. and 7 a. m. by 10 d6 to account for increased <br />L <br /> human sensitivity during those hours. <br />r.- <br /> For this assessment the comparative measure used is the 24-hour Ldn' <br /> <br />_• The U. 5. Environmental Protection Agency (EPA) proposed in 1972 that the <br /> 24-hour Ldp serve as the measure of community noise to be used by federal <br /> -18- <br />