Reso PC16-23 PC 0116 Reso CEQA - Sandpiper

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Section 4.0 — Environmental Setting and Discussion of Impacts 4.12 NOISE 4.12.1 Background Information 4.12.1.1 Fundamentals of Norse Noise is defined as unwanted sound. Sound levels are usually measured in decibels (dB) with 0 dB corresponding roughly to the threshold of hearing, Most of the sounds which we hear in the environment do not consist of a single frequency, but rather a broad band of frequencies, with each frequency differing in sound level. The intensities of each frequency add together to generate a sound. The method commonly used to quantify environmental sounds consists of evaluating all of the frequencies of a sound in accordance with a weighting that reflects the fact that human hearing is less sensitive at low frequencies and extreme high frequencies than in the frequency mid-range. This is called "A" weighting, and the decibel (dB) level so measured is called the A -weighted sound level (dBA). In practice, the level of a sound is conveniently measured using a sound level meter that includes an electrical filter corresponding to the A -weighting curve. Although the A -weighted noise level may adequately indicate the level of environmental noise at any instant in time, community noise levels vary continuously. Most environmental noise includes a conglomeration of noise from distant sources which create a relatively steady background noise in which no particular source is identifiable. To describe the time -varying character of environmental noise, the statistical noise descriptors, Lot, Lto, L5o, and Loo, are commonly used. They are the A - weighted noise levels equaled or exceeded during 1, 10, 50, and 90 percent of a stated time period. A single number descriptor called the Leq is also widely used. The Lcq is the average A -weighted noise level during a stated period of time. In determining the daily level of environmental noise, it is important to account for the difference in response of people to daytime and nighttime noises. During the nighttime, exterior background noises are generally lower than daytime levels. Most household noise, however, also decreases at night and exterior noises become more noticeable. Further, most people sleep at night and are very sensitive to noise intrusion. To account for human sensitivity to nighttime noise levels, a descriptor, DNL (day/night average sound level), was developed. The DNL, or Ld„ divides the 24-hour day into the daytime of 7:00 AM to 10:00 PM and the nighttime of 10:00 PM to 7:00 AM, The nighttime noise level is weighted to 10 dB higher than the daytime noise level to account for the increased noise sensitivity. The Community Noise Equivalent Level (CNEL) is another 24-hour average which includes both an evening and nighttime weighting. 4.12.1.2 Fundamentals of Vibration Ground vibration consists of rapidly fluctuating motions or waves with an average motion of zero. This discussion uses Peak Particle Velocity (PPV) to quantify vibration amplitude, which is defined as the maximum instantaneous positive or negative peak of the vibration wave. A PPV descriptor with units of millimeters per second (mm/sec) or inches per second (in/sec) are used to evaluate construction generated vibration for building damage and human complaints. The two primary concerns with construction -induced vibration are the potential to damage a structure and the potential City of Redwood City 71 Initial Study/Mitigated Negative Declaration Sandpiper Elementary School Facilities Expansion November 2016