2014-069 - Final Environmental Impact Report No. 2014-01

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HARBOR BLVD. MIXED USE TRANSIT CORRIDOR PLAN FINAL FIR CITY OF SANTA ANA 5. Environmental Analysis NOISE hearing system is not equally sensitive to sound at all frequencies. Therefore, to approximate the human, frequency dependent response, the A- weighted filter system is used to adjust measured sound levels. The normal range of human hearing extends from approximately 0 dBA to 140 dBA. Unlike linear units such as inches or pounds, decibels are measured on a logarithmic scale, represented by points on a sharply rising curve. Because of the physical characteristics of noise transmission and perception, the relative loudness of sound does not closely match the actual amounts of sound energy. Table 5.9 -1, Change in Sound Pressure Level dB, shows the subjective effect of changes in sound pressure levels. Table 5.9 -1 Change in Sound Pressure Level, dB Chanqe in Apparent Loudness ±3dB Threshold of human perceptibility ± 5 dB Clearly noticeable change in noise level ± 10d13 Half or twice as loud ± 20 dB Much quieter or louder Soumz: Bies and Hansen 1988. Sound is generated from a source, and the decibel level decreases as the distance from that source increases. Sound dissipates exponentially with distance from the noise source. This phenomenon is known as spreading loss. When sound is measured for distinct time intervals, the statistical distribution of the overall sound level during that period can be obtained. The energy- equivalent sound level (Ley is the most common parameter associated with such measurements. The Ley metric is a single- number noise descriptor of average sound level over a given period of time. For example, L50 is the noise level that is exceeded 50 percent of the time: half the time the noise exceeds this level and half the time it is less than this level. This is also the level that is exceeded 30 minutes in an hour. Similarly, the 42, Los, and L25 values are exceeded 2, 8, and 25 percent of the time or 1, 5, and 15 minutes per hour. Other values typically noted during a noise survey are the L. and L. These values are the minimum and maximum root mean square noise levels obtained over the measurement period. Because community receptors are more sensitive to unwanted noise intrusion during the evening and at night, state law requires that, for planning purposes, an artificial dB increment be added to quiet time noise levels in the CNEL /Lao. Psychological and Physiological Effects of Noise Physical damage to human hearing begins at prolonged exposure to noise levels higher than 85 dBA. Exposure to high noise levels affects the entire system, with prolonged noise exposure in excess of 75 dBA increasing body tensions, thereby affecting blood pressure and functions of the heart and the nervous system. Extended periods of noise exposure above 90 dBA would result in permanent cell damage. The ambient or background noise problem is widespread and generally more concentrated in urban areas than in outlying, less developed areas. Elevated ambient noise levels can result in noise interference (e.g., speech Page 5.9 -2 PlaceWorkr