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AfNoisefModelingfA _.___.___f <br /> 9.A. - Page 107 <br /> A.MoiseExposurexlandZommunitylVoisev <br /> An individual's noise exposure is a measure of noise over a period of time.A noise level is a <br /> measure of noise at a given instant in time. The noise levels presented on Exhibit A-1 are <br /> representative of measured noise at a given instant in time,however,they rarely persist <br /> consistently over a long period of time. Rather, community noise varies continuously over a <br /> period of time with respect to the contributing sound sources of the community noise <br /> environment. Community noise is primarily the product of many distant noise sources,which <br /> constitute a relatively stable background noise exposure,with the individual contributors <br /> unidentifiable. <br /> The background noise level changes throughout a typical day,but does so gradually, <br /> corresponding with the addition and subtraction of distant noise sources such as traffic and <br /> atmospheric conditions. What makes community noise constantly variable throughout a day, <br /> besides the slowly changing background noise, is the addition of short duration single event noise <br /> sources(e.g., aircraft flyovers,motor vehicles, sirens),which are readily identifiable to the <br /> individual. <br /> These successive additions of sound to the community noise environment varies the community <br /> noise level from instant to instant requiring the measurement of noise exposure over a period of <br /> time to legitimately characterize a community noise environment and evaluate cumulative noise <br /> impacts. This time-varying characteristic of environmental noise is described using statistical <br /> noise descriptors. The most frequently used noise descriptors are summarized below. <br /> A.34•JoiseiDescriptorsv <br /> Noise levels are measured using a variety of scientific metrics. As a result of extensive research <br /> into the characteristics of transportation-related noise and human response to that noise, standard <br /> noise descriptors have been developed for use in noise exposure analyses. <br /> The noise descriptor most commonly used to describe aircraft and surface transportation noise is <br /> referred to as a"cumulative"noise descriptor. Such descriptors present the amount of noise <br /> occurring at a given location over a defined period of time in numerical terms. Depending upon <br /> the descriptor used,this period can be as brief as one hour,but is usually calculated for an <br /> annualized 24-hour period. Cumulative noise descriptors can be used to present noise exposure <br /> from a specific source, such as a roadway or an airport,to describe total noise exposure from all <br /> noise sources affecting a specific location. <br /> The noise descriptors used in this analysis are described as follows: <br /> A-Weightedd;oundd'ressureaLevek(dBA): The decibel(dB) is a unit used to describe sound <br /> pressure level. When expressed in dBA,the sound has been filtered to reduce the effect of very <br /> low and very high frequency sounds,much as the human ear filters sound frequencies.Without <br /> this filtering, calculated and measured sound levels would include events that the human ear <br /> SanfCarlosfAirportV A-3 ESAfAirportsf/130753f <br /> ALUCPf W hitef Pape/ Junef2014f <br />