75C - PH - CENTENNIAL RSCCD STUDY

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EXHIBIT 2 NOISE BACKGROUND AND MODELING DATA Source. Bier, David A. and Colin E. Hansen. 2009. Engineering Now Conhol. Theory andPradioa. 4th ed. New York: Span Press. Frequency The human ear is not equally sensitive to all frequencies. Sound waves below 16 Hz are not heard at all, but are "felt" more as a vibration. Similarly, though people with extremely sensitive hearing can hear sounds as high as 20,000 Hz, most people cannot hear above 15,000 Hz. In all cases, hearing acuity falls off rapidly above about 10,000 Hz and below about 200 Hz. When describing sound and its effect on a human population, A -weighted (dBA) sound levels are typically used to approximate the response of the human ear. The A -weighted noise level has been found to correlate well with people's judgments of the "noisiness" of different sounds and has been used for many years as a measure of community and industrial noise. Although the A -weighted scale and the energy equivalent metric are commonly used to quantify the range of human response to individual events or general community sound levels, the degree of annoyance or other response also depends on several other perceptibility factors, including: ■ Ambient (background) sound level ■ General nature of the existing conditions (e.g., quiet rural or busy urban) ■ Difference between the magnitude of the sound event level and the ambient condition ■ Duration of the sound event ■ Number of event occurrences and their repetitiveness ■ Time of day that the event occurs Duration Time variation in noise exposure is typically expressed in terms of a steady-state energy level equal to the energy content of the time varying period (called LQ, or alternately, as a statistical description of the sound level that is exceeded over some fraction of a given observation period. For example, the L50 noise level represents the noise level that is exceeded 50 percent of the time, half the time the noise level exceeds this level and half the time the noise level is less than this level. This level is also representative of the level that is exceeded 30 minutes in an hour. Similarly, the L2, Ls and L25 values represent the noise levels that are exceeded 2, 8, and 25 percent of the time or 1, 5, and 15 minutes per hour, respectively. These "n" values are typically used to demonstrate compliance for stationary noise sources with many cities' noise ordinances. Other values typically noted during a noise survey are the Ln,;,, and L... These values represent the minimum and maximum root -mean -square noise levels obtained over the measurement period, respectively. Because community receptors are more sensitive to unwanted noise intrusion during the evening and at night, state law and many local jurisdictions use an adjusted 24-hour noise descriptor called the Community Noise Equivalent Level (CNEL) or Day Night Noise Level (LQ. The CNEL descriptor requires that an artificial increment (or "penalty" of 5 dBA be added to the actual noise level for the hours from 7:00 PM to 10:00 PM and 10 dBA for the hours from 10:00 PM to 7:00 AM. The Lai, descriptor uses the same methodology except that there is no artificial increment added to the hours between 7:00 PM and 10:00 PM. Both descriptors give roughly the same 24-hour level, with the CNEL being only slightly more restrictive (i.e., higher). The CNEL or L& metrics are commonly applied to the assessment of roadway and airport -related noise sources. Nowmber2016 Page 3 75C-222