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4.5 Noise 4.5 Noise r 4.5.1 Setting Noise Principles and Descriptors 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(dB), 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. r 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 frequencies 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. r 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 deemphasizes the frequencies below 1,000 Hz and above 5,000 Hz in a manner 6- corresponding to the human ears decreased sensitivity to low and extremely high frequencies as compared to 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 follows an international standard methodology of frequency deemphasis and is typically applied to community noise measurements. Selected representative noise sources and their corresponding A-weighted noise levels are shown in Figure 4.5-1. Noise Exposure and Community Noise 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, although they rarely persist 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 that constitute a relatively stable background noise exposure, with the individual contributors unidentifiable. The background noise level changes throughout a typical day, but does so it RMC Pacific Vernalis Quarry Mining and Reclamation Project 4.5-1 ESA/203015 Draft Environmental Impact Report May 2006 r