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Although a single sound level value may adequately describe environmental noise at any instant in time, community noise levels vary continuously. Most environmental noise is a conglomeration of distant noise sources which results in a relatively steady background noise having no identifiable source. These distant sources may include traffic,wind in trees, industrial activities, etc. and are relatively constant from moment to moment.As natural forces change or as human activity follows its daily cycle,the sound level may vary slowly from hour to hour. Superimposed on this slowly varying background is a succession of identifiable noisy events of brief duration. These may include nearby activities such as single vehicle passbys, aircraft flyovers, etc. which cause the environmental noise level to vary from instant to instant. To describe the time-varying character of environmental noise, statistical noise descriptors were developed. "L10" is the A-weighted sound level equaled or exceeded during 10 percent of a stated time period. The L10 is considered a good measure of the maximum sound levels caused by discrete noise events. "L50" is the A-weighted sound level that is equaled or exceeded 50 percent of a stated time period; it represents the median sound level. The "L90" is the A- weighted sound level equaled or exceeded during 90 percent of a stated time period and is used to describe the background noise. As it is often cumbersome to quantify the noise environment with a set of statistical descriptors, a single number called the average sound level or "Leq" is now widely used.The term "Leq" originated from the concept of a so-called gquivalent sound level which contains the same acoustical energy as a varying sound level during the same time period.In simple but accurate technical language,the Leq is the average A-weighted sound level in a stated time period. The Leq is particularly useful in describing the subjective change in an environment where the source of noise remains the same but there is change in the level of activity. Widening roads and/or increasing traffic are examples of this kind of situation. In determining the daily measure of environmental noise, it is important to account for the different response of people to daytime and nighttime noise.During the nighttime,exterior background noise levels are generally lower than in the daytime;however,most household noise also decreases at night,thus exterior noise intrusions again become noticeable. Further,most people trying to sleep at night are more sensitive to noise. To account for human sensitivity to nighttime noise levels, a special descriptor was developed. The descriptor is called the CNEL(Community Noise Equivalent Level)which represents the 24-hour average sound level with a penalty for noise occurring at night. The CNEL computation divides the 24-hour day into three periods: daytime(7:00 am to 7:00 pm); evening(7:00 pin to 10:00 pm); and nighttime(10:00 pm to 7:00 am). The evening sound levels are assigned a 5 dB penalty and the nighttime sound levels are assigned a 10 dB penalty prior to averaging with daytime hourly sound levels. P a. �., a . 0 "rest o_n h, sem, i. o. a It,oL,