EXHIBIT 4-2_55A_BRISTOL WIDENING TECHNICAL STUDIES

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FU19DAMMALS OF MOISE Noise levels are measured on a logarithmic scale in decibels which are then weighted and added over a 24 -hour period to re- flect not only the magnitude of the sound, but also its duration, frequency, and time of occurrence. In this manner, various acoustical scales and units of measurement have been developed such as: equivalent sound levels M, ), day -night average sound levels (Lan) and Community Noise Equivalent Levels (CNEL S) . A- weighted decibels (dBA) approximate the subjective response of the human ear to a broad frequency noise source by discriminating against the very low and high frequencies of the audible spec- trum. They are adjusted to reflect only those frequencies audi- t` ble to the human ear. The decibel scale has a value of 1.0 dBA at the threshold of hearing and 140 dBA at the threshold of pain. Each interval of 10 decibels indicates a sound energy ten times greater than before, which is perceived by the human ear as being roughly twice as loud. Therefore, a 1.0 decibel increase is just audible whereas a 10 decibel increase means the sound is per - ceived as being twice as loud as before. Equivalent sound levels are not measured directly but are calcu- lated from sound pressure levels typically measured in A- weighted decibels (dBA). The equivalent sound level (Le ) is the constant level that, over a given time period, transmit the same amount of acoustic energy as the actual time- varying sound. Equivalent sound levels are the basis for both the Ldn and CNEL scales. Day -night average sound levels are a measure of the cumulative noise exposure of the community_ The Ldn value results from a summation of hourly Le 's over a 24 -hour time period with an increased weighting faOtor applied to the nighttime period be- tween 10:00 pm and 7:00 am. This noise rating scheme takes into account those subjectively more annoying noise events which occur during the normal sleeping hours. Community Noise Equivalent Levels (CNEL) also carry a weighting penalty for noises that occur during the nighttime hours. In addition, CNEL levels include a penalty for noise events that occur during the evening hours between 7:00 pm and 10 :00 pm. Because of the weighting factors applied, CNEL values at a given location will always be larger than Ld.n values, which in turn will exceed Le values. However, CNEL values are typically within one decibll of the day -night average sound level. For a "line source" of noise such as a heavily travelled roadway, the noise level drops off by a nominal value of 3.0 decibels for each doubling of distance between the noise source and noise receiver. Environmental factors such as the wind direction and speed, temperature gradients, the characteristics of the ground (hard or soft) and the air (relative humidity), the presence of 11