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4.11 NOISE <br />4.11-1 <br /> <br /> <br />This section describes existing noise conditions in the project area, describes <br />criteria for determining the significance of noise impacts, and estimates the <br />likely noise that would result from implementation of the project. Where <br />appropriate, mitigation measures are recommended to reduce project-related <br />noise impacts to less-than-significant levels. A summary of the characteristics <br />of noise, vibration, and acoustical terms and definitions is provided in Ap- <br />pendix G, Noise Data, of this Draft EIR. <br /> <br /> <br />A. Fundamentals of Sound and Environmental Noise <br />Sound is technically described in terms of amplitude (loudness) and frequency <br />(pitch). The standard unit of sound amplitude measurement is the decibel <br />(dB). The decibel scale is a logarithmic scale that describes the physical inten- <br />sity of the pressure vibrations that make up any sound. The pitch of the <br />sound is related to the frequency of the pressure vibration. Since the human <br />ear is not equally sensitive to a given sound level at all frequencies, a special <br />frequency-dependent rating scale has been devised to relate noise to human <br />sensitivity. The A-weighted decibel scale (dBA) provides this compensation <br />by discriminating against frequencies in a manner approximating the sensitivi- <br />ty of the human ear. <br /> <br />Noise, on the other hand, is typically defined as unwanted sound. A typical <br />noise environment consists of a base of steady ambient noise that is the sum <br />of many distant and indistinguishable noise sources. Superimposed on this <br />background noise is the sound from individual local sources. These can vary <br />from an occasional aircraft or train passing by to virtually continuous noise <br />from, for example, traffic on a major highway. <br /> <br />Several rating scales have been developed to analyze the adverse effect of <br />community noise on people. Because environmental noise fluctuates over <br />time, these scales consider that the effect of noise upon people is largely de- <br />pendent upon the total acoustical energy content of the noise, as well as the <br />time of day when the noise occurs. The Leq is a measure of ambient noise,