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from these operations was measured to be 72.7 dBA (Leq) at 200 feet, whereas the average sound level used for this analysis was 76.7 dBA (average of 73.5 and 80 dBA). Significance of Project Noise Lei,els Various studies of community responses reveal the following implications of changes to ambient noise levels (San Joaquin County, September 1989): A three dBA difference in noise is considered just barely noticeable; - An increase or decrease of at least 5 dBA is required before any noticeable change in community response would be expected; and A 10 dBA increase is subjectively heard as a doubling in loudness and would almost certainly cause adverse change in community response. This information is expanded upon in Table 3.6-6. The maximum, worst-case project noise levels (as shown on Table 3.6-5) would increase ambient noise levels from 6 to 22 dBA at the four receptors evaluated. As shown in Table 3.6-6, the high increase above ambient would be considered a significant impact. The ambient noise levels during the entire Phase I operation would increase from four to seven dBA at two of the receptors but would be lower than ambient at the two other receptors. Comparison of the resulting noise levels to noise standards shown in Tables 3.6-2 and 3.6-3 show that the worst-case noise levels would be in the "unacceptable" range, while the average Phase I noise levels would be within the "acceptable" noise levels. Because of the estimated community response to the noise level increases and the exceedence of accepted guidelines, there would be potentially significant noise impacts at receptors adjacent to the site only when the extraction activities are along the borders of the site closest to each receptor. 3.6.3 Mitigation Measures The following measures are recommended to mitigate potential noise impacts to less than significant levels: 91