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8.A. - Page 90 <br /> Section 4: Evaluation of Environmental Impacts <br /> 4.12 - NOISE <br /> Would the project result in: <br /> Potentially Less Than Less Than No <br /> Significant Significant with Significant Impact <br /> Impact Mitigation Impact <br /> Incorporation <br /> a) Exposure of persons to or generation of noise <br /> levels in excess of standards established in the <br /> local general plan or noise ordinance, or applicable <br /> standards of other agencies? <br /> b) Exposure of persons to or generation of excessive <br /> groundborne vibration or groundborne noise <br /> levels? <br /> c) A substantial permanent increase in ambient noise <br /> levels in the project vicinity above levels existing ❑ ❑ ® ❑ <br /> without the project? <br /> d) A substantial temporary or periodic increase in <br /> ambient noise levels in the project vicinity above ❑ ❑ ® ❑ <br /> levels existing without the project? <br /> e) For a project located within an airport land use plan <br /> or, where such a plan has not been adopted, within <br /> two miles of a public airport or public use airport, <br /> would the project expose people residing or <br /> working in the project area to excessive noise <br /> levels? <br /> f) For a project within the vicinity of a private airstrip, <br /> would the project expose people residing or <br /> working in the project area to excessive noise <br /> levels? <br /> Redwood City is affected by several different sources of noise, including automobile, rail, and air traffic, sports events, <br /> commercial and industrial activity, and periodic nuisances such as construction. <br /> Fundamentals of Sound and Environmental Noise <br /> Noise can be defined as unwanted sound. Sound (and therefore noise) consists of energy waves that people receive <br /> and interpret. Sound pressure levels are described in logarithmic units of ratios of sound pressures to a reference <br /> pressure, squared. These units are called bels. In order to provide a finer description of sound, a bel is subdivided <br /> into ten decibels, abbreviated dB. To account for the range of sound that human hearing perceives, a modified scale <br /> is utilized known as the A- weighted decibel (dBA). Since decibels are logarithmic units, sound pressure levels cannot <br /> be added or subtracted by ordinary arithmetic means. For example, if one automobile produces a sound pressure <br /> level of 70 dBA when it passes an observer, two cars passing simultaneously would not produce 140 dB. In fact, they <br /> would combine to produce 73 dBA. This same principle can be applied to other traffic quantities as well. In other <br /> words, doubling the traffic volume on a street or the speed of the traffic will increase the traffic noise level by 3 dBA. <br /> Conversely, halving the traffic volume or speed will reduce the traffic noise level by 3 dBA. A 3 dBA change in sound <br /> is the level where humans generally notice a barely perceptible change in sound and a 5 dBA change is generally <br /> readily perceptible. <br /> 26 California Department of Transportation. Basics of Highway Noise: Technical Noise Supplement. November 2009. <br /> Redwood City Housing Element Initial Study 51 <br />