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Section 5 <br />5.10 NOISE <br />The following analysis is based on a Noise Impact Analysis Report prepared for <br />the Mid Basin Centennial Park Injection Well Project by Vista Environmental in <br />October of 2015. The Mid Basin Centennial Park Injection Well Project Noise <br />Impact Analysis Report is presented in its entirety in Appendix H, <br />5.10.1 Affected Environment <br />Background <br />Noise is defined as unwanted sound. Sound becomes unwanted when it creates <br />a nuisance that interferes with normal activities, or when it causes physical harm <br />or otherwise adversely affects human health. The standard unit of measurement <br />of the loudness of sound is the decibel (dBA). The zero point on the dBA scale is <br />based on the lowest sound level that a healthy, unimpaired human ear can <br />detect. Changes of 3 dBA or fewer would only be perceptible in laboratory <br />environments. A 5 dBA increase would be readily noticeable. An increase of 10 <br />dBA would be perceived as approximately a doubling of loudness. <br />Numerous methods have been developed to measure sound over a period of <br />time, including: Equivalent Sound Level (L,,,), Community Noise Equivalent Level <br />(CNEL), Day/Night Average Sound Level (L,,,) and Maximum Noise event <br />Noise level can vary pending on the noise source and duration. Below is <br />description of Vie units of measure used in this analysis to describe the noise <br />environment. <br />• L,q: Time variations in noise exposure are typically expressed as a <br />statistical description of the sound pressure level that is exceeded over <br />some fraction of a given observation period (called L,,,). For example, the <br />noise levels exceeded on 10 percent of readings is called 1-10, the median <br />(50th percentile) reading is called L,,,r), etc, <br />• CNEL: Because community receptors are more sensitive to unwanted <br />noise intrusion during the evening and at night, state law requires that, for <br />planning purposes, an artificial dB increment penalty be added to quiet- <br />time noise levels in a 24-hour noise descriptor called CNEL. <br />• L,j, Another commonly used method is the day/night average level or Ldn- <br />The Ld, is a measure of the 24-hour average noise level at a given <br />location. It was adopted by the US, Environmental Protection, Agency <br />(EPA) for developing criteria for the evaluation of community noise <br />exposure. <br />• Lma,: The maximum noise level recorded during a noise event is typically <br />expressed as Lffllx. <br />Mid Basin Centennial Park roject Final EIR 5-144 <br />A&1�49P <br />