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Grand Avenue Widening Project Environmental Impact Report Section 3.0 Time -weighted refers to the fact that noise which occurs during certain sensitive time periods is penalized for occurring at these times. The evening time period, defined as 7 PM to 10 PM is penalized by five, and the night time period, defined as 10 PM to 7 AM, is penalized by 10 dB. These penalties and time periods were selected to attempt to account for increased human sensitivity to noise during the quieter periods of a day, where sleep is the most probable activity. A CNEL noise level may be reported as a CNEL of 60 dBA, 60 dBA CNEL or 60 CNEL. Typical noise levels in terms of the CNEL scale for different types of communities are shown in Figure 3.6-5. L(N), or L%, is a statistical method of describing noise which accounts for the variance in noise levels throughout a given measurement period. L(N), where N equals a percentage, is a way of expressing the noise level exceeded for a percentage of time in a given measurement period. For example, since 15 minutes is 25 percent of 60 minutes, L(25) is the noise level that is exceeded for fifteen minutes of a sixty minute measurement period. The L(N) metric is the most commonly used for noise ordinance standards. For example, most daytime city, state and county noise ordinances use an ordinance standard of 55 dBA for 30 minutes per hour or an L(50) level of 55 dBA. In other words, these noise ordinances require that for residential uses, a piece of equipment not on the property will not be allowed to generate a noise level of 55 dBA for more than thirty minutes in any hour. As the noise level limit increases, the time the noise is allowed to occur within any hour is reduced. Assessing Existing Noise Levels The existing traffic noise levels in the Grand Avenue project area were established in terms of the CNEL metric by modeling the road for the current traffic and speed characteristics. The noise levels modeled in this report were computed using the Highway Noise Model published by the FHWA ("FHWA Highway Traffic Noise Prediction Model," FHWA-RD-77-108, December 1978). Traffic data used to project existing noise levels were derived from City of Santa Ana 2001 traffic counts. The traffic mix in terms of truck percentages and time distributions for Grand Avenue and I-5 used in the analysis are based on measurements for roads in southern California and are considered typical for arterials in this area (Orange County Environmental Management Agency, Traffic Census, 1975). Railroad Noise The "Assessment of Noise Environments Around Railroad Operations" (Wyle Laboratories Report WCR 73-5, July 1973) was used to model the train noise levels in the Grand Avenue project area, associated with rail operations on the railroad tracks approximately 1,200 feet west of this area. The noise generated by a train pass -by can be divided into two components: the noise generated by the engine or locomotive, and the noise generated by the railroad cars. The characteristic frequency of a train engine is different than for train cars. The effective radiating frequency is 1,000 hertz (Hz) for locomotive engines and 2,000 Hz for the part of the noise generated by the cars. The noise generated by a train engine is the result of the mechanical movements of the engine parts, the combustion process, the horn if used, and, to a lesser extent, the exhaust system. The noise F.•IPROJ-ENVIGrand eirWew Text - Gran&Section 3 SplitlSection 3.6.doc Page 3.6-19