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Pages 83-90

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From page 83...
... 83 C H A P T E R 1 2 12.1 Research Approach 12.1.1 Basic Concepts and Use of the FHWA TNM Parallel Barriers Module Sound levels behind a barrier can increase when there are multiple reflections of the sound between the barrier and a second barrier parallel to it on the opposite side of the road, forming a vertical wall "canyon," as shown in Figure 55. Figure 56 shows in a schematic overhead view that these multiple reflections are a three-dimensional phenomenon, with reflected sound reaching the receiver before vehicles pass by it and after they pass as well.
From page 84...
... 84 Figure 55. Highway with parallel noise barriers on opposite sides of the road.58 Figure 56.
From page 85...
... 85 Figure 59. Illustration of receiver array at heights of 15, 5, –5 and –15 ft relative to the roadway surface and distances of 25, 50, 100, 150, 200, and 300 ft from the near wall.
From page 86...
... 86 below the road, receiver distance back from the near wall, and vehicle type. For the studied cross section, a 10:1 width-to-height ratio resulted in sound-level increases for automobiles ranging from 1.0 to 6.5 dB.
From page 87...
... 87 12.2.5 Internal Vertical Reflecting Surface The FHWA TNM FAQ for parallel barriers cautions analysts about having an internal, vertical, reflecting surface in the analyzed parallel barrier cross-sectional surface. (See Figure 60.)
From page 88...
... 88 of automobiles changes the sound-level increase by only a few tenths of a decibel, except in going from 100% automobiles to 95% automobiles, where the change in sound-level increase is on the order of 0.5 dB. 12.2.7 Hourly Volumes of Vehicles The FHWA TNM parallel barrier module is only predicting a sound-level increase in the 1-hour Leq and not an actual 1-hour Leq.
From page 89...
... 89 from the initial and follow-up studies provide field data and FHWA TNM runs for a reflective parallel wall situation (before absorption) and for a situation with a near side reflective wall and a far-side, sound-absorbing wall (after absorption)
From page 90...
... 90 In general, a parallel barrier analysis would begin with a review of the highway plans and proposed noise barriers to identify areas where multiple sound reflections might occur, namely, where there are barriers and/or vertical retaining walls on both sides of the road. Several representative sites or cross sections would then be selected for study, such as the following: • Different cross section types (e.g., cut, fill, and at grade)

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