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Barriers interrupt sound propagation and create an "acoustic shadow zone ", <br />where sound levels are lower than in the respective free field. Crucial features <br />of noise barriers include: <br />-- The barriers cannot have any cracks or breaks. <br />-- The barriers must be high enough to break the line -of -sight <br />between observer and the noise source, and long enough to prevent <br />noise leaks around the ends. <br />-- The shape of the barrier can effect the amount of attenuation. ' <br />— The mass and stiffness of the barrier must be sufficient to prevent <br />bending or buckling, and it must not vibrate easily or leak air. <br />Noise Attenuation With Distances <br />PW <br />In an area which is relatively flat and free of barriers, the noise level resulting <br />from a single "point source" of noise drops by 6 decibels for each doubling of <br />distance or 20 decibels for each factor of ten in distance. This applies to fixed <br />noise sources such as industries, refrigeration /air conditioning units, and bells <br />or buzzers at schools. It also applies to individual mobile noise sources such <br />as an airplane, train or idling motor vehicle. <br />For a 'line source" of noise, such as a heavily travelled roadway, the noise <br />level drops off by a nominal value of 3.0 decibels for each doubling of distance <br />between the noise source and noise receiver. Environmental conditions such <br />as the wind direction and speed, temperature gradients, the characteristics of <br />the ground (hard or soft) and the air (relative humidity), the presence of grass, <br />shrubbery and trees combine to increase the actual attenuation achieved <br />outside of laboratory conditions to 4.5 decibels per doubling of distance. <br />However, for a "worst- case" analysis, a 3.0 decibel reduction with doubling was <br />assumed for arterials throughout this report.' <br />In its "Noise Assessment Guidelines ", the U.S. Department of Housing and <br />Urban Development uses a 4.5 decibel drop for each doubling of distance in <br />assessing roadway noise. Thus, a noise level of 74.5 decibels at 50 feet from <br />the highway centerline would be attenuated naturally to 70.0 decibels at 100 <br />feet, 65.5 decibels at 200 feet, 61.0 decibels at 400 feet and so forth. <br />IV-40 <br />