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8.B. - Page 68 <br /> 4.12 NOISE <br /> 4.12.1 Existin� Settin� <br /> 4.12.1.1 Background Information <br /> Noise is defined as unwanted sound. Sound levels are usually measured in decibels (dB)with 0 dB <br /> corresponding to the threshold of hearing.30 Most of the sounds which we hear in the environment <br /> do not consist of a single frequency,but rather a broad band of frequencies,with each frequency <br /> differing in sound level. The intensities of each frequency add together to generate a sound. The <br /> method commonly used to quantify environmental sounds consists of evaluating all of the <br /> frequencies of a sound in accordance with a weighting that reflects the fact that human hearing is less <br /> sensitive at low frequencies and extreme high frequencies than in the frequency mid-range. This is <br /> called"A"weighting, and the decibel level so measured is called the A-weighted sound level (dBA). <br /> In practice,the level of a sound is conveniently measured using a sound level meter that includes an <br /> electrical filter corresponding to the A-weighting curve. <br /> Although the A-weighted noise level may adequately indicate the level of environmental noise at any <br /> instant in time, community noise levels vary continuously. Most environmental noise includes a <br /> conglomeration of noise from distant sources which create a relatively steady background noise in <br /> which no particular source is identifiable. To describe the time-varying character of environmental <br /> noise,the statistical noise descriptors, Loi> Lio> Lso> and L90, are commonly used. They are the A- <br /> weighted noise levels equaled or exceeded during one percent, 10 percent, 50 percent, and 90 percent <br /> of a stated time period. A single number descriptor called the Leq is also widely used. The Leq is the <br /> average A-weighted noise level during a stated period of time. <br /> In determining the daily level of environmental noise,it is important to account for the difference in <br /> response of people to daytime and nighttime noises. During the nighttime, exterior background <br /> noises are generally lower than daytime levels. However, most household noise also decreases at <br /> night and exterior noises become very noticeable. Further, most people sleep at night and are very <br /> sensitive to noise intrusion. To account for human sensitivity to nighttime noise levels, a descriptor, <br /> DNL (day/night average sound level),was developed. The DNL divides the 24-hour day into the <br /> daytime of 7:00 AM to 10:00 PM and the nighttime of 10:00 PM to 7:00 AM. The nighttime noise <br /> level is weighted to 10 dB higher than the daytime noise level. The Community Noise Equivalent <br /> Level (CNEL) is another 24-hour average which includes both an evening and nighttime weighting. <br /> 4.12.1.2 Applicable Noise Standards and Policies <br /> Based on the City's General Plan, Table 6 shows the noise levels considered consistent with specific <br /> land uses. For residential uses, including the proposed project, outdoor noise levels of up to 55 <br /> decibels are considered acceptable and up to 65 decibels are permitted only after a detailed analysis <br /> of noise reduction requirements are made. <br /> 3o The zero on the decibel scale is based on the lowest sound level that the healthy,unimpaired human ear can <br /> detect. Michael Thill,Illingworth&Rodkin Inc. E-mail dated December 7,20ll. <br /> 1410 Valota Road Residential Project 56 Initial Study <br /> Redwood City January 2012 <br />