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Chapter 4 Environmental Setting, Impacts, and Mitigation Measures <br />urban residential or semi -commercial areas (typically 55 to 60 dBA) and commercial locations (typically <br />60 dBA). People may consider louder environments adverse, but most will accept the higher levels <br />associated with more noisy urban residential or residential -commercial areas (60 to 75 dBA) or dense <br />urban or industrial areas (65 to 80 dBA). According to the City's Noise Control Ordinance, the exterior <br />noise level standard is 55 dBA from 7:00 A.M. to 10:00 P.M., and 50 dBA from 10:00 P.M. to 7:00 A.M. <br />The interior noise level standard is 55 dBA from 7:00 A.M. to 10:00 P.M., and 45 dBA from 10:00 P.M. to <br />7:00 A.M. Standards for impact noise, simple tone noise, speech, music, and any other combination are <br />5 dBA lower than the above standards, and noise levels exceeding these standards are limited to relatively <br />shorter periods of time. <br />When evaluating changes in 24-hour community noise levels, a difference of 3 dBA is a barely <br />perceptible increase to most people. A 5 dBA increase is readily noticeable, while a difference of 10 dBA <br />would be perceived as a doubling of loudness. <br />Noise levels from a particular source decline as distance to the receptor increases. Other factors, such as <br />the weather and reflecting or shielding, also intensify or reduce the noise level at a location. A common <br />method for estimating roadway noise is that for every doubling of distance from the source, the noise <br />level is reduced by about 3 dBA at acoustically "hard" locations (i.e., the area between the noise source <br />and the receptor is nearly complete asphalt, concrete, hard -packed soil, or other solid materials) and <br />4.5 dBA at acoustically "soft" locations (i.e., the area between the source and receptor is normal earth or <br />has vegetation, such as grass). Noise from stationary or point sources is reduced by about 6 to 7.5 dBA <br />for every doubling of distance at acoustically hard and soft locations, respectively. Noise levels may also <br />be reduced by intervening structures; generally, a single row of buildings between the receptor and the <br />noise source reduces the noise level by about 5 dBA, while a solid wall or berm reduces noise levels by 5 <br />to 10 dBA. The manner in which older homes in California were constructed generally provides a <br />reduction of exterior -to -interior noise levels of about 20 to 25 dBA with closed windows. The exterior - <br />to -interior reduction of newer residential units is generally 30 dBA or more. <br />Fundamentals of Environmental Groundborne Vibration <br />Vibration is sound radiated through the ground. Groundborne noise is the rumbling sound caused by the <br />vibration of room surfaces. The ground motion caused by vibration is measured as particle velocity in <br />inches per second; in the U.S., this is referenced as vibration decibels (VdB). <br />The background vibration velocity level in residential and educational areas is usually around 50 VdB. <br />The vibration velocity level threshold of perception for humans is approximately 65 VdB. A vibration <br />velocity level of 75 VdB is the approximate dividing line between barely perceptible and distinctly <br />perceptible levels for many people. Most perceptible indoor vibration is caused by sources within <br />buildings, such as operation of mechanical equipment, movement of people, or the slamming of doors. <br />Typical outdoor sources of perceptible groundborne vibration are construction equipment, steel -wheeled <br />trains, and traffic on rough roads. If a roadway is smooth, the groundborne vibration from traffic is <br />rarely perceptible. The range of interest is from approximately 50 VdB, which is the typical background <br />vibration velocity level, to 100 VdB, which is the general threshold where minor damage can occur in <br />4.8-4 City of Santa Ana Transit Zoning Code (SD 84A and SD 84B) EIR <br />