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Grand Avenue Widening Project Environmental Impact Report Section 3.0 3.6.3 METHODOLOGY RELATED TO NOISE Noise Assessment Metrics Community noise is generally not steady state and varies with time. Under conditions of non -steady state noise, some type of statistical metric is necessary to quantify human response to noise. There are several rating scales for the analysis of adverse effects of community noise on people. They are designed, to account for the lmown effects of noise on people. Based on these effects, the observation has been made that the potential for a noise to impact people is dependent on the total acoustical energy content of the noise. A number of noise scales have been developed to account for this observation. The description, analysis and reporting of community noise levels around communities is made difficult by the complexity of human response to noise and the number of noise metrics that have been developed for describing noise impacts. Each metric attempts to quantify noise levels with respect to community response. Most metrics use the A -weighted noise level to quantify noise impacts on humans. A -weighting is a frequency correction that correlates the overall sound pressure levels with the frequency response of the human ear. Noise metrics can be divided into two categories: single event and cumulative. Single event metrics describe the noise levels from an individual event such as an aircraft flyover or perhaps a heavy equipment pass -by. Cumulative metrics average the total noise over a specific time period, which is typically 1 -hour or 24 -hours for community noise assessments. For the proposed Grand Avenue widening, cumulative noise metrics, described below, were used. Cumulative Noise Metrics There are several rating scales for the measurement of community noise. These rating scales account for the parameters of noise that have been shown to contribute to the effects of noise on humans; the variety of noises found in the environment; the variations in noise levels that occur as a person moves through the environment; and the variations associated with the time of day. They are designed to account for the known health effects of noise on humans described earlier. Based on these effects, the observation has been made that the potential for a noise to impact people is dependent on the total acoustical energy content of the noise. A number of noise scales have been developed to account for this observation. Two predominate noise scales are the Equivalent Noise Level (LEQ) and the CNEL, which are described as follows: LEQ is the sound level corresponding to a steady-state sound level containing the same total energy as a time -varying signal over a given sample period. LEQ is the "energy" average noise level during the time period of the sample. LEQ can be measured for any time period, but is typically measured for 15 minutes, one hour or 24 -hours. The noise measurement survey for the Grand Avenue widening project was conducted in terms of the LEQ metric. CNEL is similar to LEQ but it is for 24 hours, and applies a weighting factor which places greater importance on noise events occurring during the evening and night hours when sleep disturbance is a concern. CNEL is a 24-hour, time -weighted annual average noise level. F: WROJ-ENMGrand eirlNew Text - GrandlSection 3 SplitlSection 3.6.doc Page 3.6-18