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4.5 Noise v gradually, corresponding to the addition and subtraction of distant noise sources such as traffic b. and atmospheric conditions. What makes community noise constantly variable throughout a day, besides the slowly changing background noise, is the addition of short-duration, single-event noise sources(e.g.,aircraft flyovers, motor vehicles, sirens), which are readily identifiable to the individual. These successive additions of sound to the community noise environment varies the community ` noise level from instant to instant requiring the measurement of noise exposure over a period of time to legitimately characterize a community noise environment and evaluate cumulative noise impacts.This time-varying characteristic of environmental noise is described using statistical noise descriptors. The most frequently used noise descriptors are summarized below: Lam: the equivalent sound level is used to describe noise over a specified period of time, 6. typically one hour, in terms of a single numerical value. The L�is the constant sound level which would contain the same acoustic energy as the varying sound level,during the same time period (i.e., the average noise exposure level for the given time period). L the instantaneous maximum noise level for a specified period of time. L50: the noise level that is equaled or exceeded 50 percent of the specified time period. The L50 represents the median sound level. L90: the noise level that is equaled or exceeded 90 percent of the specified time period.The L90 is sometimes used to represent the background sound level. DNL: 24-hour day and night A-weighted noise exposure level which accounts for the greater sensitivity of most people to nighttime noise by weighting noise levels at night ("penalizing" nighttime noises). Noise between 10:00 PM and 7:00 AM is weighted (penalized)by adding 10 dBA to take into account the greater annoyance of nighttime noises. CNEL: similar to the DNL the Community Noise Equivalent Level(CNEL)adds a 5 dBA "penalty"for the evening hours between 7:00 PM and 10:00 PM in addition to a 10 dBA ` penalty between the hours of 10:00 PM and 7:00 AM As a general rule, in areas where traffic dominates the noise environment, the L.during the peak- hour is generally equivalent to the DNL at that location(Caltrans, 1998). Effects of Noise on People The effects of noise on people can be placed into three categories: • Subjective effects of annoyance, nuisance,dissatisfaction; • Interference with activities such as speech, sleep, learning; and • Physiological effects such as hearing loss or sudden startling. Environmental noise typically produces effects in the first two categories. Workers in industrial plants can experience noise in the last category. There is no completely satisfactory way to .. measure the subjective effects of noise,or the corresponding reactions of annoyance and r RMC Pacific Vernalis Quarry Mining and Reclamation Project 4.5-3 ESA 1203015 Dreg Environmental Impact Report May 2006 r