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noted that final design may not include barriers at the locations <br />identified as approaching the 67 Leq standard. <br />Attenuation due to shielding is an important mechanism by which <br />highway sound levels are reduced. Shielding occurs when the <br />observers view of a highway is obstructed or partially obstructed <br />by an object or objects which significantly interfere with the <br />propagation of the sound waves. Shielding can be provided by <br />rows of buildings and /or existing barriers. <br />The amount of attenuation provided by rows of buildings is deter- <br />mined by the portion of the row that is occupied by the buil- <br />dings. For example, a 3.0 dBA additional attenuation is provided <br />by the first row of buildings when the structures occupy 40 to 65 <br />percent of the length of the row. A 5.0 dBA attenuation is <br />afforded when the buildings occupy 65 to 90 percent of the length <br />of the row. No attenuation is allowed for rows of houses which <br />occupy less than 40 percent of the length of the row. <br />Each successive row provides 1.5 dBA of additional attenuation <br />until a total attenuation of 10.0 dBA for all rows is obtained. <br />This is the maximum attenuation that this mechanism provides. <br />Any excess attenuation by ground effects (ie. 4.5 dBA /DD versus <br />3.0 dBA /DD) is assumed to end when the sound waves reach the <br />first row of buildings. Consequently, the shielding provided by <br />rows of buildings is only additive to the attenuation provided by <br />geometric spreading (3.0 dBA /DD). <br />Barriers interrupt sound propagation and create an "acoustic <br />shadow zone" where sound levels are lower than in the respective <br />free field. Crucial features 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- <br />sight between observer and the noise source, and long <br />enough to prevent noise leaks around the ends. <br />- The shape of the barrier can affect the amount of atten- <br />uation. <br />- The mass and stiffness of the barrier must be sufficient <br />to prevent bending or buckling and it must not vibrate <br />■ easily or leak air. <br />7PC 484 <br />