2014-028 - Approving General Plan Amendement No. 2014-01

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City of Santa Ana -Park View at Town and Country Manor Draft EIR Noise 4.5 - Noise 4.5.1 - Introduction This section describes the existing noise setting and potential effects from project implementation on the site and its surrounding area. 4.5.2 - Existing Conditions Acoustical Terminology Sound is mechanical energy transmitted by pressure waves in a compressible medium such as air. Noise is generally defined as unwanted sound. Sound is characterized by various parameters that describe the rate of oscillation of sound waves, the distance between successive troughs or crests, the speed of propagation, and the pressure level or energy content of a given sound wave. In particular, the sound pressure level has become the most common descriptor used to characterize the loudness of an ambient sound level. The unit of sound pressure, a ratio of the faintest sound detectable by a keen human ear, is called a decibel (dB). A dB is a unit of measurement that indicates the relative intensity of a sound. The zero point on the dB scale is based on the lowest sound level that the healthy, unimpaired human ear can detect. Changes of 3 dB or fewer are only perceptible in laboratory environments. Audible increases in noise levels generally refer to a change of more than 3 dB, as this level has been found to be barely perceptible to the human ear in outdoor environments. Sound levels in dB are calculated on a logarithmic basis. An increase of 10 dB represents a 10 -fold increase in acoustic energy, while 20 dB is 100 times more intense, and 30 dB is 1,000 times more intense. Each 10 -dB increase in sound level is perceived as approximately a doubling of loudness. Because sound or noise can vary in intensity by over one million times within the range of human hearing, a logarithmic loudness scale similar to the Richter scale used for earthquake magnitude is used to keep sound intensity numbers at a convenient and manageable level. Since the human ear is not equally sensitive to all sound frequencies within the entire spectrum, noise levels at maximum human sensitivity are factored more heavily into sound descriptions in a process called "A- weighting," written as dBA. This scale gives greater weight to the frequencies of sound to which the human ear is most sensitive. Any further reference to decibels in this report written as dB should be understood to be A- weighted values. Time variations in noise exposure are typically expressed in terms of a steady -state energy level equal to the energy content of the time- varying period (called Leq), or, alternately, as a statistical description of the sound pressure level that is exceeded over some fraction of a given observation period. Finally, 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 a 24 -hour noise descriptor called the Community Noise Equivalent Level (CNEL). Michael Brandman Associates 4.5 -1 H\Cl t(PN- JN)b327b32]003MMVB2]0030 Sec0 Noi�.doc