CCP 11-13-1995 - Laserfiche WebLink

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,I ,I. perceptions of sound, ,I Traffic noise levels vary significantly over time, Traffic noise is dependent on traffic volumes, vehicle speeds, and vehicle types, Noise from vehicles comes from a variety of sources, For most cars, the primary noise source is the interaction of tires on the pavement. For trucks, the dominant noise source is usually I exhaust and engine noise. Noise is also generated by brakes, loose body components, and faulty exhaust system S, . Since traffic noise fluctuates over time, it is usually expressed in tenns of st"iistical noise level descriptors, Common traffic noise descriptors are as follows: I . L1 0 - The L1 0 is the sound level exceeded 10 percent of the time or six minutes out of an hour. . L5D - The L5D is the sound level exceeded 50 percent of the time or 30 minutes out of an hour. . Because the decibel is a logarithmic unit, decibels cannot be added and subtracted by ordinary arithrrietic. The sum of two noise sources each emitting 70 dB yields a combined noise level of73 dB. Thus, an increase I ofJ dB represents a doubling of the sound energy. Similarly, a noise source must double in strength to result in a 3 dB increase in noise levels. I A 3 dB change in noise level is considered to be the smallest noticeable change over an extended period of time for a time-varying noise SOUrce such as traffic. A change of 10 dBA is perceived by most people to be a doubling or halving of the sound level. .- The energy in a sound dissipates with distance from a source. For a point source of noise, sound levels will decrease at a rate of 6 dB per doubling of distance from the source. For a line source of noise such as a . highway, traffic noise dissipates at a rate of 3 dB per doubling of distance. A sound level measured at 65 dBA 50 feet from a roadway would be 62 dBA 100 feet from> the roadway. Sound levels c;m also be dissipated by ground and atmospheric absorption and are significantly reduced by barriers which block the line of sight between the source and the receiver. I The charts on the following pages illustrate how noise levels change with distance from the source, speed of the traffic, traffic volume and percentage of trucks in the traffic stream, Each of these charts are based . on a set of representative conditions in the Highway 96 corridor including: . Average Daily Traffic (ADT) volume of20,000. . . Operating Speed of 40 MPH. I . Distance of75 feet from the centerline of the roadway to the nearest receiver. . 5 percent trucks including 2.5 percent medium trucks (2 axles, 6 wheels) and 2.5 percent heavy I trucks (3 or more axles). . ~ Four-Lane divided road with a 24 foot median. . I I- . . 43