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Grand Avenue Widening Project Environmental Impact Report Section 3.0 TABLE 3.6-5 PROJECTED UNMITIGATED RAILROAD NOISE LEVELS Source Distance to CNEL Contours ft 70 dB 65 dB60 dB Passenger and Freight Operations 315 600 1,150 Source: BridgeNet Consulting Services (May 2002). 3.6.2 THRESHOLDS OF SIGNIFICANCE RELATED TO NOISE City of Santa Ana Noise Criteria The Noise Element of the City of Santa Ana General Plan has established guidelines for acceptable community noise levels based on the CNEL rating scale. For residential uses, the noise levels in exterior living areas, defined as rear yards, patios and balconies, cannot exceed 65 dB CNEL. The interior noise levels attributable to exterior sources shall not exceed 45 dB CNEL in any habitable room. While there are no specific noise standards for schools, a common design goal for school classrooms is 45 dB CNEL. The State of California has also established guidelines for acceptable community noise levels based on the CNEL rating scale. These guidelines specify indoor and outdoor noise levels similar to those specified in the Noise Element, as shown in Figure 3.6-4. The specific noise level guidelines used to assess the compatibility of the proposed Grand Avenue widening project with the noise environment were: 65 CNEL exterior noise environment at existing residential uses. 45 CNEL for the interior noise environment at existing residential uses and school classrooms. Community Noise Assessment In community noise assessment, changes in noise levels greater than three dB are often identified as significant, while changes less than one dB will usually not be discernible. In the range of one to three dB, residents who are very sensitive to noise may perceive a slight change. There is no scientific evidence to support the use of three dB as a significance threshold. In laboratory testing situations, humans are able to detect noise level changes of slightly less than one dB. In a community noise situation, however, noise exposures are over a long time period and changes in noise levels occur over years, rather than the immediate comparison made in a laboratory situation. Therefore, the level at which changes in community noise levels become discernible is likely to be some value greater than one dB, and three dB appears to be appropriate for most people. As a result, a noise level change of at least 3 dBA was considered a significant increase for this noise analysis. Mitigation measures were required where the 65 dB CNEL exterior noise standard was exceeded. In the cases where there would be an increase of at least 3 dBA but the exterior noise standard was not exceeded, noise sensitive land uses already have exterior noise mitigation measures, i.e. block walls, in place. There were no instances where the interior noise standards were exceeded so no mitigation measures are necessary to address interior noise impacts. F:IPROJ-ENVIGrand eirWew Text- GrandlSection 3 SplitlSection 3.6.doc Page 3.6-16