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I^ 3.4 Noise i Figure 3.4-2 illustrates sound levels associated with common sound sources.The perceived loudness of sounds is dependent on many factors, including sound pressure level and frequency content. F However,within the usual range of environmental sound levels,perception of loudness is relatively predictable, and can be approximated by frequency filtering using the standardized A-weighting network.There is a strong correlation between A-weighted sound levels(expressed as dBA)and community response to noise.For this reason,the A-weighted sound level has become the standard descriptor for environmental noise assessment. All noise levels reported in this section are in terms of A-weighting. J A Generally speaking,noise from stationary (point)sources of sound, including stationary mobile 4 sources such as idling vehicles,attenuates(lessens)at a rate of 6 dB for each doubling of distance " from the source location. This represents the spherical spreading or divergence of sound over areas that are acoustical "hard," such as parking lots or large bodies of water.Acoustically"soft"sites " including agricultural crops,grass, or dense shrubbery/trees generally provides additional sound attenuation'with increased distance from the sound source;an additional 1.5 dB per doubling of distance is often assumed for such sites.Line sources such continuous traffic noise from vehicles generally attenuates at a rate of 3 dB per doubling of distance (cylindrical spreading/divergence) for hard sites and 4.5 dB per doubling of distance for soft sites. ' r: Community noise is commonly described in terms of the ambient noise level,which is defined as the all-encompassing noise level associated with a given noise environment.A common statistical tool to measure the ambient noise level is the average,or equivalent sound level(LeQ).The hourly Lam,(equivalent sound level over a 60 minute period)is the foundation of the day/night average sound level (La„)and shows very good correlation with community response to noise.The Ldn is based on the average sound level over a 24-hour day,with a+10 decibel weighting(penalty)applied to sounds during nighttime hours(10 p.m.-7 a.m.). The nighttime penalty is based on the assumption ! that people react to nighttime noise exposures as though they are twice as loud as daytime exposures. l Because the Ldn represents a 24-hour average, it tends to disguise short-term variations in the noise environment. For this reason, San Joaquin County and many other jurisdictions utilizes statistical I noise level objectives for non-transportation noise sources. Specifically, standards in terms of Leq and L,,,�,are used to assess noise generated by the on-site project sources.Please refer to the following explanations and definitions of acoustical terminology used throughout this section. Transportation Noise Sources ..a Transportation noise sources are commonly considered as traffic on public roadways,main-line r train operations,or aircraft overflights. The San Joaquin County Development Title noise standards ' applicable to transportation noise sources are specified in terms of Ldn.As a general rule,in areas where the noise environment is dominated by traffic,the Leq during the peak-hour is often equivalent 2 dBA)to the Ldn at that location(Caltrans, 1998). n R Munn&Perkins Quarry Excavation Permit 3.4-3 ESA 1211086 - DWI EIR April 2011 F