THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

342 13. Criteria and Regulations for Noise ControlFigure 13.7. Subdivision of a road for noise analysis purposes with segment ends Aand B defining the angles with respect to the normal from the observer to the road. In(a) an angle clockwise to the observer is positive and an angle counterclockwise is negative.In (b) the angles are measured from the normal line to the extension of the roadsegment.The 1-h average sound levels are computed separately for each segment, and thenthey are combined in the end. Such subdivision of the roadway for analyticalpurposes should be executed (a) where the traffic volumes or speeds change (e.g.,at an exit or entry ramp or a road fork), (b) where the ground cover changessignificantly, (c) where a curved road is being analyzed as a series of straightsegments, and (d) where the vertical gradient of the road changes. With referenceto Figure 13.7, the finite roadway segment adjustment (which depends on theground cover) is defined as∫ φ 21 segment = 10 logπ (cos φ)a dφ (13.16)φ 1where φ 1 and φ 2 are angles in radians at the receiver, as shown in Figure 13.6;φ 1 is the angle to the left end of the segment and the angle φ 2 to the right end ofthe segment. If an angle is measured counterclockwise from the normal line, it isassigned a negative value; if measured clockwise, a positive value. For a hard siteEquation (3.16) reduces to segment = 10 log φ 2 − φ 1π(13.17)In the case of a soft site, Equation (13.16) needs to be evaluated by numerical integration.For the general case of a “soft” site and an infinitely long roadway (whereφ 1 =−π/2 radians and φ 2 =+π/2 radians), the adjustment is −1.2 dB(A).