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Appendix H Equivalent Sound Level Time-varying sound levels are often described in terms of an equivalent constant decibel level. The equivalent sound level(Leq)is the average of sound energy occurring over a specified time period. In effect,Leq is the steady-state sound level that in a stated time period would contain the same acoustical energy as the time-varying sound that actually occurs during the same period. Equivalent sound levels (Leq)are often used to develop single-value descriptions of average sound exposure over various periods of time. Such average sound exposure values often include additional weighting factors for annoyance potential attributable to time of day or other considerations. The Leq data used for these average sound exposure descriptors are generally based on A-weighted sound- level measurements. Day-Night Average Sound Level Average sound exposure over a 24-hour period is often presented as a day-night average sound level (L&). Ld„values are calculated from hourly Leq values,with the Leq values for the nighttime period(10:00 p.m. to 7:00 a.m.) increased by 10 dB to reflect the greater disturbance potential from nighttime noises. Community Noise Equivalent Level The community noise equivalent level(CNEL)is also used to characterize average sound levels over a 24-hour period,with weighting factors included for evening and nighttime sound levels. Leq values for the evening period(7:00 p.m. to 10:00 p.m.)are increased by 5 dB,while Leq values for the nighttime period (10:00 p.m. to 7:00 a.m.)are increased by 10 dB. For given set of sound measurements,the CNEL value will usually be about 1 dB higher than the Ld„ value. In practice, CNEL and Ld„are often used interchangeably. Percentile-Exceeded, Maximum, and Minimum Sound Level The sound level exceeded during a given percentage of a measurement period is the percentile-exceeded sound level(Lx). Examples include Lio, 1,5o,and Lyo. Lia is the A-weighted sound level that is exceeded 10%of the measurement period, L50 is the level exceeded 50%of the period, and so on. 1_50 is the median sound level measured during the measurement period. L90,the sound level exceeded low 90%of the time,excludes high localized sound levels produced by nearby sources such as single car passages or bird chirps. L90 is often used to represent the background sound level. L50 is also used to provide a less conservative r. assessment of the background sound level. tr H-3