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5.7 NOISE The Community Noise Equivalent Level (CNEL) is similar to the Ldn and uses the same day and nighttime weightings but adds an additional evening weighting of 5 dB for the hours between 7 p.m. and 10 p.m. The Ldn and CNEL values are typically within 1 dBA of each other. TABLE 5.7.1 Definitions of Acoustical Terms Term Definition Ambient Noise Level The composite of noise from all sources near and far. The normal or existing level of environmental noise or sound at a given location.The ambient level is typically defined by the Leq level. Background Noise Level The underlying ever-present lower level noise that remains in the absence of intrusive or intermittent sounds. Distant sources, such as traffic, typically make up the background.The background level is generally defined by the L90 percentile noise level. Intrusive Noise that intrudes over and above the existing ambient noise at a given location.The relative intrusiveness of a sound depends upon its amplitude, duration,frequency,time of occurrence,tonal content,the prevailing ambient noise level as well as the sensitivity of the receiver.The intrusive level is generally defined by the L10 percentile noise level. Sound Pressure Level A unit describing the amplitude of sound, equal to 20 times the logarithm to the Decibel (dB) base 10 of the ratio of the pressure of the sound measured to the reference pressure,which is 20 micropascals(20 micronewtons per square meter). A-Weighted Sound Pressure The sound level in decibels as measured on a sound level meter using the Level (dBA) A-weighted filter network. The A-weighted filter de-emphasizes the very low and very high frequency components of the sound in a manner similar to the frequency response of the human ear and correlates well with subjective reactions to noise.All sound levels in this report are A-weighted. Equivalent Noise Level (Leq) The average A-weighted noise level, on an equal energy basis, during the measurement period. Percentile Noise Level (Ln) The noise level exceeded during In percent of the measurement period,where In is a number between 0 and 100 (e.g., L9o) Community noise equivalent The average A-weighted noise level during a 24-hour day, obtained after the level (CNEL) addition of 5 decibels to sound levels from 7 p.m.to 10 p.m. and after the addition of 10 decibels to sound levels between 10 p.m. and 7 a.m. Day-Night Noise Level The average A-weighted noise level during a 24-hour day, obtained after (Ldn or DNL) addition of 10 decibels from 10:00 p.m. to 7:00 a.m. The two time periods are then averaged to compute the overall Ldn value. For a continuous noise source, the Ldn value is easily computed by adding 6.4 dB to the overall 24-hour noise level (Leq). For example,if the expected continuous noise level from the power plant were 60.0 dBA, the resulting Ldn from the plant would be 66.4 dBA. The effects of noise on people can be listed in three general categories: • Subjective effects of annoyance,nuisance, and dissatisfaction • Interference with activities such as speech, sleep, and learning • Physiological effects such as startling and hearing loss 5.7-2 SAC/371322/08234009(LEC_5.7_NOISE.DOC)