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grass, shrutybery, and trees, combine to increase the actual <br />attenuation achieved outside laboratory conditions to 4.5 deci- <br />bels per doubling of distance. <br />1 <br />In an area which is relatively flat and free of barriers, the <br />l sound level resulting from a single "point source" drops by 6 <br />decibels for each doubling of distance or 20 decibels for each <br />r factor of ten in distance. This applies to fixed noise sources <br />I and mobile noise sources which are temporarily stationary such as <br />an idling truck or other 'heavy duty equipment operating within a <br />confined area. <br />The noise levels adjacent to line sources increase by 3.0 dBA <br />with each doubling in the traffic volume (provided that the speed <br />and truck mix do not change). From the mathematical expression <br />relating increases in the number of noise sources (motor vehi- <br />cles) to the increase in the adjacent noise level, it can be <br />shown that a 26 percent increase in the traffic volume will cause <br />a 1.0 dBA increase in adjacent noise levels. <br />1 <br />1 75C -5121 <br />