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4. ENVIRONMENTAL ANALYSIS 4.5 NOISE dissatisfaction. A wide variation in individual thresholds of annoyance exists,and different _ tolerances to noise tend to develop based on an individual's past experiences with noise. Thus,an important way of predicting a human reaction to a new noise environment is the way it _ compares to the existing environment to which one has adapted: the so called"ambient noise" level. In general,the more a new noise exceeds the previously existing ambient noise level,the less acceptable the new noise will be judged by those hearing it. With regard to increases in A- — weighted noise level, the following relationships occur: • except in carefully controlled laboratory experiments,a change of 1 dBA cannot be perceived; • outside of the laboratory,a 3-dBA change is considered a just-perceivable difference; • a change in level of at least 5 dBA is required before any noticeable change in human response would be expected; and • a 10-dBA change is subjectively heard as approximately a doubling in loudness,and can cause adverse response. These relationships occur in part because of the logarithmic nature of sound and the decibel system. The human ear perceives sound in a non-linear fashion, hence the decibel scale was developed. Because the decibel scale is based on logarithms,two noise sources do not combine in a simple additive fashion, rather logarithmically. For example, if two identical noise sources produce noise levels of 50 dBA the combined sound level would be 53 dBA, not 100 dBA. NOISE ATTENUATION Stationary point sources of noise, including stationary mobile sources such as idling vehicles, attenuate(lessen)at a rate between 6 dBA for hard sites and 7.5 dBA for soft sites for each doubling of distance from the reference measurement. Hard sites are those with a reflective surface between the source and the receiver such as parking lots or smooth bodies of water. No excess ground attenuation is assumed for hard sites and the changes in noise levels with distance (drop-off rate)is simply the geometric spreading of the noise from the source. Soft sites have an absorptive ground surface such as soft dirt,grass or scattered bushes and trees. In addition to geometric spreading,an excess ground attenuation value of 1.5 dBA(per doubling distance)is normally assumed for soft sites. Line sources(such at traffic noise from vehicles) attenuate at a rate between 3 dBA for hard sites and 4.5 dBA for soft sites for each doubling of distance from the reference measurement(Caltrans, 1998). SENSITIVE RECEPTORS Some land uses are considered more sensitive to ambient noise levels than others because of the _ amount of noise exposure(in terms of both exposure duration and insulation from noise)and the types of activities typically involved.Residences,hotels, schools, rest homes, and hospitals are generally more sensitive to noise than commercial and industrial land uses. — RMC exilic vemaGs Quarry Mining and Reclamation emject 4.5-4 FSA/203015 Administrative Draft EIR Much 2004