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CORRESPONDENCE - 75A (COMMENT)
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CORRESPONDENCE - 75A (COMMENT)
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City Clerk
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75A
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10/15/2019
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Chapter Environmental Setting, Impacts, and Mitigation Measures <br />commercial, industrial, and civic uses and their environs. Specific adjacent (off -site) uses include the <br />following: <br />■ North: Single-family residential, office, and commercial uses, as well as I-5 <br />■ East: Commercial and residential uses. I-5 is located immediately adjacent to portions of the <br />Transit Zoning Code (SD 84A and SD 84B) area <br />■ South: Institutional (including educational), commercial, industrial, and residential uses <br />■ West: Residential and commercial uses with open space located further to the west <br />Fundamentals of Sound and Environmental Noise <br />Sound is technically described in terms of amplitude (loudness) and frequency (pitch). The standard unit <br />of sound amplitude measurement is the decibel (dB). The decibel scale is a logarithmic scale that <br />describes the physical intensity of the pressure vibrations that make up any sound. The pitch of the <br />sound is related to the frequency of the pressure vibration. Because the human ear is not equally sensitive <br />to a given sound level at all frequencies, a special frequency -dependent rating scale has been devised to <br />relate noise to human sensitivity. The A -weighted decibel scale (dBA) provides this compensation by <br />discriminating against frequencies in a manner approximating the sensitivity of the human ear. <br />Noise, on the other hand, is typically defined as unwanted sound. A typical noise environment consists <br />of a base of steady "background" noise that is the sum of many distant and indistinguishable noise <br />sources. Superimposed on this background noise is the sound from individual local sources. These can <br />vary from an occasional aircraft or train passing by to virtually continuous noise from, for example, <br />traffic on a major highway. Table 4.8-1 (Representative Environmental Noise Levels) lists representative <br />noise levels for the environment. <br />Several rating scales have been developed to analyze the adverse effect of community noise on people. <br />Because environmental noise fluctuates over time, these scales consider that the effect of noise upon <br />people is largely dependent upon the total acoustical energy content of the noise, as well as the time of <br />day when the noise occurs. The L, is a measure of ambient noise, while the Ld„ and CNEL are measures <br />of community noise. Each is applicable to this analysis and defined as follows: <br />■ L,g, the equivalent energy noise level, is the average acoustic energy content of noise for a stated <br />period of time. Thus, the L,9 of a time -varying noise and that of a steady noise are the same if they <br />deliver the same acoustic energy to the ear during exposure. For evaluating community impacts, <br />this rating scale does not vary, regardless of whether the noise occurs during the day or the night. <br />■ L , , the Day -Night Average Level, is a 24-hour average Le7 with a 10 dBA "weighting" added to <br />noise during the hours of 10:00 P.M. to 7:00 A.M. to account for noise sensitivity in the nighttime. <br />The logarithmic effect of these additions is that a 60 dBA 24 hour L, would result in a <br />measurement of 66.4 dBA Ld,,. <br />■ CNEL, the Community Noise Equivalent Level, is a 24-hour average L,q with a 5 dBA <br />"weighting" during the hours of 7:00 P.M. to 10:00 P.M. and a 10 dBA "weighting" added to noise <br />during the hours of 10:00 P.M. to 7:00 A.M. to account for noise sensitivity in the evening and <br />4.8-2 City of Santa Ana Transit Zoning Code (SD 84A and SD 84B) EIR <br />
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