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L4. ENVIRONMENTAL ANALYSIS 4.5 NOISE L 4.5 NOISE 4.5.1 SETTING NOISE PRINCIPLES AND DESCRIPTORS Introduction Noise is defined as unwanted sound. Sound,traveling in the from of waves from a source,exerts a sound pressure level (referred to as sound level)which is measured in decibels(0), with zero dB corresponding roughly to the threshold of human hearing,and 120 to 140 dB corresponding to ` the threshold of pain. Pressure waves traveling through air exert a force registered by the human ear as sound. Sound pressure fluctuations can be measured in units of hertz(Hz), which correspond to the frequency of a particular sound. Typically, sound does not consist of a single frequency,but rather a broad band of frequencies varying in levels of magnitude(sound power). When all the audible frequencies of a sound are measured, a sound spectrum is plotted consisting of a range of frequency spanning 20 to 20,000 Hz. The sound pressure level,therefore,constitutes the additive force exerted by a sound corresponding to the sound frequency/sound power level spectrum. The typical human ear is not equally sensitive to all frequencies of the audible sound spectrum. As a consequence, when assessing potential noise impacts, sound is measured using an electronic filter that de-emphasizes the frequencies below 1,000 Hz and above 5,000 Hz in a manner corresponding to the human ears decreased sensitivity to low and extremely high frequencies instead of the frequency mid-range. This method of frequency weighting is referred to as A- weighting and is expressed in units of A-weighted decibels(dBA). Frequency A-weighting L follows an international standard methodology of frequency de-emphasis and is typically applied to community noise measurements. Some representative noise sources and their corresponding A-weighted noise levels are shown in Figure 4.5-1. NOISE EXPOSURE AND COMMUNITY NOISE L An individual's noise exposure is a measure of noise over a period of time. A noise level is a measure of noise at a given instant in time. The noise levels presented in Figure 4.5-1 are representative of measured noise at a given instant in time, however, they rarely persist L consistently over a long period of time. Rather,community noise varies continuously over a period of time with respect to the contributing sound sources of the community noise environment. Community noise is primarily the product of many distant noise sources, which L constitute a relatively stable background noise exposure, with the individual contributors unidentifiable. The background noise level changes throughout a typical day,but does so gradually, corresponding with the addition and subtraction of distant noise sources such as traffic and atmospheric conditions. What makes community noise constantly variable throughout a day, RMC Pacific Vemalis Quarry Mining and Reclarnmon Project 4.5-1 ESA 1203015 i Achni istrafive Draft EW March 20M L