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III. ENVIRONMENTAL SETTING,IMPACTS AND MITIGATION MEASURES D. NOISE • subjective effects of annoyance,nuisance,dissatisfaction; • interference with activities such as speech, sleep, learning;and • physiological effects such as hearing loss or sudden startling. Environmental noise typically produces effects in the first two categories. Workers in industrial plants can experience noise in the last category. There is no completely satisfactory way to measure the subjective effects of noise, or the corresponding reactions of annoyance and 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(Caltrans, 1998): • under controlled conditions in an acoustics laboratory, the trained healthy human ear is able to discern changes in sound levels of 1 dBA; • outside of such controlled conditions,the trained ear can detect changes of 2 dBA in normal environmental noise; • It is widely accepted that the averse healthy ear, however, can barely perceive noise level changes of 3 dBA; • a change in level of 5 dBA is a readily perceptible increase in noise level;and • a 10 dBA change is recognized as twice as loud as the original source.. These relationships occur in part because of the logarithmic nature of sound and the decibel system. Sound level is measured in decibels. Because the decibel scale is based on logarithms two noise sources do not combine in a simple linear fashion,but 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 of 6 to 9 dBA per doubling of distance from the source,depending on environmental conditions(i.e.,atmospheric conditions and noise barriers,either vegetative or manufactured,etc.). Widely distributed noises, such as a large industrial facility spread over many acres or a street with moving vehicles(a"line"source), would typically attenuate at a lower rate,approximately 4 to 6 dBA per doubling distance. Stork-ton Sravenger Transfer Station Expansion III.D.4 ESA 1990190