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