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4.8 Noise fragile buildings. Table 4.8-2 (Human Response to Different Levels of Groundborne Vibration) describes the general human response to different levels of groundborne vibration velocity levels. Vibration Velocity Level Human Reaction 65 VdB I Approximate threshold of perception for many people. 75 VdB Approximate dividing line between barely perceptible and distinctly perceptible. Many people find that transportation -related vibration at this level is unacceptable. 85 VdB I Vibration acceptable only if there are an infrequent number of events per day. SOURCE; HMMH, 2006 Existing Ambient Daytime Noise Levels According to the Noise Element of the City of Santa Ana General Plan, the primary source of noise within the City is traffic noise from major roadways and freeways within the area from mobile sources such as automobiles, buses, trucks, and vehicles associated with construction equipment transport. Secondary noise sources in the City include aircraft operations, railroad operations, and construction activities. Also, stationary sources such as industrial activities, public gatherings, activities in open areas, and the use of equipment in unenclosed spaces, produce a significant amount of noise. Existing daytime noise levels were monitored at twelve locations around the project area, which are depicted in Figure 4.8-1 (Noise Monitoring Locations), in order to identify representative noise levels at various areas. The noise levels were measured using a Larson -Davis Model 814 precision sound level meter, which satisfies the American National Standards Institute (ANSI) for general environmental noise measurement instrumentation. The average noise levels and sources of noise measured at each location are identified in Table 4.8-3 (Existing Noise Levels around the Proposed Transit Zoning Code [SD 84A and SD 84B] Area). As shown, the daytime noise levels range from a high of 77.0 dBA to a low of 52.9 dBA and are characteristic of a typical urban area. IM Existing Local Noise Environment The Atchison, Topeka, and Santa Fe Railroad (AT&SF) tracks, which are used by Amtrak and Metrolink for commuter train services during most of the day, run along the eastern edge of Santa Ana and through the eastern portion of the Transit Zoning Code (SD 84A and SD 84B) area. Rail transit traveling at grade typically produces a noise level of 80 dBA at a distance of 50 feet from the tracks while rail transit stopped at a station typically produces a noise level of 65 dBA at a distance of 50 feet (HMMH 2O06). In addition, the sounding of train horns, which is required when approaching at -grade crossings with streets and highways for safety reasons, represents the highest noise levels associated with train services. The sounding of a train horn can generate noise levels in excess of 110 dBA at a distance of 100 feet. City of Santa Ana Transit Zoning Code (SD 84A and SD 848) EIR 4,8-5