Laserfiche WebLink
8.A. - Page 91 <br /> Section 4: Evaluation of Environmental Impacts <br /> Noise consists of pitch, loudness, and duration; therefore, a variety of methods for measuring noise has been <br /> developed. According to the California General Plan Guidelines for Noise Elements, the following are common metrics <br /> for measuring noise: <br /> LEQ (Equivalent Energy Noise Level): The sound level corresponding to a steady -state sound level containing <br /> the same total energy as a time - varying signal over given sample periods. LEQ is typically computed over 1 -, 8- <br /> , and 24 -hour sample periods. <br /> CNEL (Community Noise Equivalent Level): The average equivalent A- weighted sound level during a 24 -hour <br /> day, obtained after addition of five decibels to sound levels in the evening from 7:00 P.M. to 10:00 P.M. and after <br /> addition of ten decibels to sound levels in the night from 10:00 P.M. to 7:00 A.M.. <br /> LDN (Day -Night Average Level): The average equivalent A- weighted sound level during a 24 -hour day, obtained <br /> after the addition of ten decibels to sound levels in the night after 10:00 P.M. and before 7:00 A.M.. <br /> CNEL and LDN are utilized for describing ambient noise levels because they account for all noise sources over an <br /> extended period of time and account for the heightened sensitivity of people to noise during the night. LEQ is better <br /> utilized for describing specific and consistent sources because of the shorter reference period. <br /> Fundamentals of Environmental Groundborne Vibration <br /> Vibration is sound radiated through the ground. The rumbling sound caused by the vibration of room surfaces is called <br /> groundborne noise. The ground motion caused by vibration is measured as particle velocity in inches per second, and <br /> in the U.S. is referenced as vibration decibels (VdB). <br /> The background vibration velocity level in residential and educational areas is usually around 50 VdB. The vibration <br /> velocity level threshold of perception for humans is approximately 65 VdB. A vibration velocity level of 75 VdB is the <br /> approximately dividing line between barely perceptible and distinctly perceptible levels for many people. Sources within <br /> buildings such as operation of mechanical equipment, movement of people, or the slamming of doors causes most <br /> perceptible indoor vibration. Typical outdoor sources of perceptible groundborne vibration are construction equipment, <br /> steel - wheeled trains, and traffic on rough roads. If a roadway is smooth, the groundborne vibration from traffic is rarely <br /> perceptible. The range of interest is from approximately 50 VdB, which is the typical background vibration velocity level, <br /> and 100 VdB, which is the general threshold where minor damage can occur in fragile buildings. <br /> The general human response to different levels of groundborne vibration velocity levels is described in Table 5 (Human <br /> Reaction to Vibration). <br /> 27 California Governor's Office of Planning and Research. General Plan Guidelines. 2003. <br /> 52 Redwood City Housing Element Initial Study <br />