ARCHIVED REPORTS_2000 - Laserfiche WebLink

Home Browse Search

III. ENVIRONMENTAL SETTING,IMPACTS AND MITIGATION MEASURES D. NOISE This section addresses temporary construction-phase noise impacts and operational noise impacts. This analysis uses typical construction equipment noise levels to estimate corresponding noise levels at the nearest sensitive receptor locations. Long-term operation-phase impacts are based on estimates of noise increases from project-related noise sources. SETTING NOISE PRINCIPLES AND DESCRIPTORS Noise Background Noise is defined as unwanted sound. Sound,traveling in the form 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. 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 each measured Hz and corresponding sound power level. The audible sound spectrum consists 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 (20 to 20,000 Hz). 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 ear's 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 follows an international standard method of frequency de-emphasis and is typically applied to community noise measurements. In practice,the level of a sound source is conveniently measured using a sound level meter that includes an electrical filter corresponding to the A-weighting curve. Some representative noise sources and their corresponding A-weighted noise levels are shown in Figure III.D-1. All of the noise levels reported herein are A-weighted unless otherwise stated. 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 III.13-1 are Stockton Scavenger Transfer Station Expansion III.D.1 ESA 1990190