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