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2. Overview of Noise & Vibration 2.1 NOISE DESCRIPTORS 2.1.1 Noise Descriptors Noise is most often defined as unwanted sound. Although sound can be easily measured, the perception of noise and the physical response to sound complicates the analysis of its impact on people. People judge the relative magnitude of sound sensation in subjective terms such as "noisiness" or "loudness." Sound pressure magnitude is measured and quantified using a logarithmic ratio of pressures, the scale of which gives the level of sound in decibels (dB). The human hearing system is not equally sensitive to sound at all frequencies. Therefore, to approximate this human, frequency-dependent response, the A-weighting filter system is used to adjust measured sound levels. When sound is measured for distinct time intervals, the statistical distribution of the overall sound level during that period can be obtained. The energy-equivalent sound level (Leq) is the most common parameter associated with such measurements. The ~ metric is a single-number noise descriptor which represents the average sound level over a given period of time. Table 2-1 presents the subjective effect of changes in sound pressure levels. To provide perspective on the relative loudness of noise levels, Table 2-2 lists common sources of noise and their approximate noise levels. Table 2-1 Change In Sound Pressure Level, dB Change in Apparent Loudness + 3 dB Threshold of human perceptibility :': 5 dB Clearly noticeable change in noise level + 10 dB Half or twice as loud :': 20 dB Much quieter or louder Source: Engineering Noise Control (Bies and Hansen, 1988). ~ To account for the increased sensitivity of people to noise occurring at night, a number of noise metrics have been developed, Two of the more commonly used metrics are the Day-Night Sound Level (Ldn) and the Community Noise Equivalent Level (CNEL). The Ldn, which was developed by the United States Environmental Protection Agency, is a 24-hour average sound level (similar to a 24-hour Leq) in which a 10 dB penalty is added to the Leq occurring between the hours of 10:00 PM and 7:00 AM. CNEL, which was developed for use in the California Airport Noise Regulations, is similar to the Ldn except that a five dB penalty is also added for noise occurring during evening hours from 7:00 PM to 10:00 PM. Therefore, both the Ldn and CNEL noise metrics provide 24-hour averages of A-weighted noise levels at a particular location. The distinction is that Ldn includes a nighttime adjustment while the CNEL metric includes both an evening and a nighttime adjustment. For noise generated from vehicle traffic, CNEL and ~n can be used interchangeably because noise levels would differ between these two noise descriptors by less than one dB. Technical NoiJe Study for Mobil Oil Station/Car WaJh Expamion The Plcmning Center March 2005 . Page 7 Q:1511/1/\I-1J I ,OF\.\A,ml'JIj/; _i.....'oi\i!Repfll.,-rINALdllt 3tA8~3