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Grand Avenue Widening Project Environmental Impact Report Section 3.0 3.6 NOISE Based on the findings of the IS, the proposed Grand Avenue widening may or. would result in potentially significant adverse noise impacts, specifically related to long term increases in noise and the potential exposure of sensitive receptors to extreme noise levels. The analysis in the following sections focuses on the existing conditions in the study area, the analysis methodology, thresholds of significance, the potential noise impacts of the Grand Avenue widening alternatives related to increases in noise levels and sensitive receptors, and mitigation. The analysis of the potential noise impacts of the proposed Grand Avenue widening is documented in the Noise Assessment for Grand Avenue Widening EIR City of Santa Ana (BridgeNet Consulting Services, May 9, 2002). The findings of this technical analysis are summarized in the following sections. The complete noise technical report is included in Appendix G of this EIR. The noise study determined the existing noise levels local to the project site, predicted the noise levels that would exist with the proposed project and analyzed the noise impacts of the proposed road widening alternatives on adjacent noise sensitive land uses. These with -project noise levels were then compared with applicable City and State noise criteria and mitigation measures identified. 3.6.1 EXISTING SETTING RELATED TO NOISE Noise Definitions Sound is technically described in terms of the loudness (amplitude) of the sound and frequency (pitch) of the sound. The standard unit of measurement of the loudness of sound is the decibel (0). Decibels are based on the logarithmic scale. The logarithmic scale compresses the wide range in sound pressure levels to a more usable range of numbers in a manner similar to the Richter scale used to measure earthquakes. For example, in terms of human response to noise, a sound 10 dB higher than another is judged to be twice as loud and sound 20 dB higher is four times as loud. Everyday sounds normally range from 30 dB (very quiet) to 100 dB (very loud). Because the human ear is not equally sensitive to sound at all frequencies, a special frequency - dependent rating scale was developed to relate noise to human sensitivity. The A -weighted decibel scale (dBA) performs this compensation by discriminating against frequencies in a manner approximating the sensitivity of the human ear. Community noise levels are measured in terms of the A -weighted decibel (dBA). Table 3.6-1 provides examples of various noises and their typical A -weighted noise levels. Sound levels decrease as a function of distance from the source as a result of wave divergence, atmospheric absorption and ground attenuation. As the sound wave form travels away from the source, the sound energy is dispersed over a greater area, thereby dispersing the sound power of the wave. Atmospheric absorption also influences the sound levels received by the observer. The greater the distance traveled, the greater the influence and the resultant fluctuations. The degree of absorption is a function of the frequency of the sound as well as the humidity and temperature of the air. Turbulence and gradients of wind, temperature and humidity also play an important role in determining the degree of attenuation. In cases where receptors are far away from potential adverse noise sources, intervening topography can have a substantial effect on perceived noise levels. F.IPROJ-ENVIGrand eirlNew Text - GrandlSection 3 SplitlSection 3.6.doc Page 3.6-1