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

13.10 Evaluation of Traffic Noise 341traffic density, (b) distances to areas of human activity, (c) finiteness of roadwaysegments, (d) presence of road gradients, (e) shielding by buildings, (f) shieldingby rows of trees, and (g) barrier attenuation.The traffic flow adjustment ( traffic ) i is rendered to account for the hourly flowof vehicle type i and to average the level over a 1-h time period as follows:( traffic ) i = 10 log (N i d 0 /S i ) − 25 dB(A) (13.15)whereN i = hourly flow rate of vehicles of type i, vehicles/hd 0 = the reference distance of 15.2 mS i = speed of the ith vehicle type, km/hDistance adjustment takes into consideration the type of ground cover (e.g.,sound will attenuate more readily over a soft surface than over a hard parking lotpavement). Field data gathered by FHWA have shown an average rate of attenuationto be approximately 4.5 dB(A) per doubling of distance over a grassy surface andan attenuation of 3.0 dB(A) per doubling of distance over a paved surface. Thesetwo rates are used by FHWA to establish a ground cover parameter called analpha factor (α). For hard sites, the alpha factor has a value of zero; for soft sites, itassumes the value 0.5. The distance adjustment distance can be generally expressedas distance = 10(1 + α) log(d 0 /d)where d is the perpendicular distance from the receiver to the center of thetravel lane. Table 13.5 provides guidance on when to use propagation rates of 3.0or 4.5.The methodology described above assumes an infinitely long road. In analyticalpractice, highways are subdivided into a series of straight segments of finite length.Table 13.5. Guidelines for Choosing Sound Propagation Rates.Situation1. All situations in which the source or the receiver are located 3 mabove the ground or whenever the line of sight a averages morethan 3 m above the ground2. All situations involving propagation over the top of a barrier3 m or more in height3. Where the height of the line of sight is less than 3manda. there is a clear (unobstructed) view of the highway, theground is hard, and there are no intervening structures, orb. the view of the roadway is interrupted by isolated buildings,clumps of bushes, or scattered trees, or the interveningground is soft or covered with vegetationPropagation rate,dB(A)3(α = 0)3(α = 0)3(α = 0)4.5 (α = 1/2)a The line of sight (L/S) is a direct line between the noise source and the observer.