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

13.10 Evaluation of Traffic Noise 343It is commonly observed that large trucks become noisier as they travel uphill inlower gears. The FHWA recommends that the 1-h average levels for heavy trucksbe increased as a function of roadway gradient 1 in the following manner:1. 0–2%: grade = 0 dB(A)2. 3–4%: grade =+2 dB(A)3. 5–6%: grade =+3 dB(A)4. Over 7%: grade =+5 dB(A).If there are one or more rows of buildings present between an observation pointand a road, an adjustment for the shielding, shielding , by these buildings must beestimated. A rule of thumb can be used in the following manner: if 40–65% ofthe length of the first row of buildings is occupied by the building themselves,then subtract 3 dB(A) from the average sound level. If 65–90% of the length ofthe row is occupied by the buildings, subtract another 3 dB(A) from the averagesound level; and if the percentage exceeds 90%, the buildings may be treated asnoise barriers. Each successive row of buildings adds an additional 1.5 dB(A) tothis adjustment, up to a maximum reduction of 10 dB(A). Any excess groundattenuation stops at the first row.A second type of shielding, also referred to as shielding , can occur from thepresence of trees between the road and the receiver. The tress may be dense enoughto virtually disallow a direct view of the road. For a 30-m (100 ft) belt width, FHWAsuggests an adjustment of –5 dB(A). For an additional 30-m belt width there is anadditional –5 dB(A) adjustment for a total of –10 dB(A). When this adjustment isused, both distance adjustment and a segment adjustment should be used on thebasis of the propagation rate of 3 dB(A) per doubling of distance from the linesource.Barrier attenuation adjustment barrier can be computed using the path-differenceprocedure described in the last chapter. According to the FHWA model, this shouldbe computed by using an incoherent line source model separately for each vehicletype. The attenuation is computed for a series of paths, defined by the angleswith the perpendicular to the source-to-receiver line, and the results are combinedvia numerical integration. This calculation is usually achieved with the use of acomputer program such as the FWHA STAMINA program.The total 1-h average sound levels for each vehicle type can be obtained bysumming up the various adjustments:(L eq ) i = [(L 0 ) E ] i + ( traffic ) i + ( distance ) i+ ( grade ) i + ( shielding ) i + ( barrier ) i (13.18)The 1-h average levels for automobiles, medium trucks, and heavy trucks may nowbe combined to yield the total 1-h average level.1 When a road changes its elevation n meters (or feet) for every 100 meters (or feet) horizontal travel,the road is said to have an n percent gradient.