ARCHIVED REPORTS_2000 - Laserfiche WebLink

Home Browse Search

III. ENVIRONMENTAL SETTING,IMPACTS AND MITIGATION MEASURES D. NOISE given instant in time however, representative of measured nose at a g the rarely Yersist P consistently over a long period of time. Rather, community noise varies continuously over a period of time with respect to the contributing sound sources of the community noise environment. Community noise is primarily the product of many distant noise sources,which constitute a relatively stable background noise exposure,with the individual contributors unidentifiable. The background noise level changes throughout a typical day,but does so gradually,corresponding with the addition and subtraction of distant noise sources such as traffic 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 such as aircraft flyovers, vehicle passbys,sirens,etc.,which are readily identifiable to the individual. These successive additions of sound to the community noise environment vary 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: Leq: the equivalent sound level is used to describe noise over a specified period of time, typically one hour, in terms of a single numerical value. The Leq is the constant sound level that 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). Lmax: the instantaneous maximum noise level for a specified period of time. LIo: the noise level that is equaled or exceeded 10 percent of the specified time period. The Lio is often considered the maximum noise level averaged over the specified time period. Lgo: the noise level that is equaled or exceeded 90 percent of the specified time period. The Lgo is often considered the background noise level averaged over the specified time period. DNL: 24-hour day and night A-weighed 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 p.m. and 7:00 a.m. is weighted (penalized)by adding 10 dBA to take into account the greater annoyance of nighttime noise. CNEL: similar to the DNL,the Community Noise Equivalent Level (CNEL)adds a 5 dBA "penalty"for the evening hours between 7:00 p.m. and 10:00 p.m. in addition to a 10 dBA penalty between the hours of 10:00 p.m. and 7:00 a.m. SEL: a receiver's cumulative noise exposure from a single noise event. Often used to calculate Leq and Ldn values. Effects of Noise on People The effects of noise on people can be placed in three categories: Stockton Scavenger Transfer Station Erpansion III.D.3 ESA 1990190