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7.B. - Page 84 Redwood City Chapter 4.Environmental Impact Analysis • Day-Night Level (Lan).Similar to CNEL,this noise descriptor adds a 10-dB penalty to all nighttime noise events between 10:00 p.m.and 7:00 a.m.However, Lan does not add the evening 5-dB penalty. Because of the logarithmic decibel scale,sound levels from different noise sources cannot be added directly to give a combined noise level. Instead,the combined noise level produced by multiple sources is calculated logarithmically.If one bulldozer produces a noise level of 80 dBA,then two bulldozers would generate a combined noise level of 83 dBA,not 160 dBA.If a steady stream of cars on a roadway causes an Leq noise level of 60 dBA at the nearest home and occasional trucks (by themselves) cause 50 dBA,then the noise caused by the combined traffic (cars plus trucks) would be 60.4 dBA. People generally perceive a 10-dBA increase in a noise source as a doubling of loudness.An average person would perceive a 70 dBA sound level as being twice as loud as a 60 dBA sound.People generally cannot detect differences of 1 to 2 dBA between noise levels of a similar nature(e.g.,an increase in traffic noise compared to existing traffic noise). However,under ideal listening conditions,some people can detect differences of 2 or 3 dBA.Under normal listening conditions, most people would likely perceive a 5 dBA change in sounds of a similar nature.When the new sound is of a different nature than the background sound(e.g.,backup alarms compared to quiet residential sounds),most people can discern the new noise even if it increases the overall Leq noise by less than 1 dBA. When distance is the only factor considered,sound levels from isolated point sources of noise typically decrease by about 6 dBA for every doubling of distance from the noise source.When the noise source is a continuous line (e.g.,vehicle traffic on a highway),sound levels decrease by about 3 dBA for every doubling of distance.Attenuation rate is used to describe the rate at which the intensity of a sound signal declines as it travels outward from its source. For traffic noise studies,an attenuation rate of 4.5 dBA per doubling of distance is often used when the roadway is at ground level and the intervening ground is effective in absorbing sound (e.g.,ground vegetation,scattered trees,and clumps of bushes).When the roadway is elevated, 3 dBA of noise attenuation per doubling of distance is used because the sound-absorbing effects of the intervening ground are limited. Ambient Noise Environment The existing noise environment in the project area is characteristic of an urban environment with the local traffic,aircraft overflights,pass-by trains,and commercial and industrial noise sources. Vehicles traveling on Middlefield Road and Woodside Expressway(State Route [SR] 84) are likely the dominant noise source in the project area.The ambient noise level measured near the Middlefield Road/Charter Street intersection was about 71 dBA CNEL,as shown in Table 4.5-1.The Caltrain trains traveling along the rail lines south of the site of the proposed modifications also contribute to the existing noise environment.The average daily noise levels along the railroad lines range from 61 to 70 dBA CNEL (City of Redwood City 2010a). As shown in Table 4.5-1,background noise level measurements were conducted in the project area at four locations.One 24-hour long-term measurement was conducted on June 19 and 20, 2012 near the southwest fa�ade of the multifamily units located north of the Middlefield Road and Charter Street intersection,approximately 50 feet from the centerline of Middlefield Road.Three 15-minute short-term measurements were conducted between the hours of 11:00 am and 12:00 pm on June Addendum No.2 to the Environmental Impact Report for 4 39 September 2012 the Redwood City Costco Wholesale Project ICF 00154.12