FINAL REPORT. On Project Supplemental Guidance on the Application of FHWA s Traffic Noise Model (TNM) APPENDIX K Parallel Barriers

Transcription

1 FINAL REPORT On Project - Supplemental Guidance on the Application of FHWA s Traffic Noise Model (TNM) APPENDIX K Parallel Barriers Prepared for: National Cooperative Highway Research Program (NCHRP) Transportation Research Board of The National Academies March HMMH Report No. Prepared by: Research Topic Lead Bowlby & Associates, Inc. in association with Environmental Acoustics Grant S. Anderson Douglas E. Barrett

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3 Final Technical Report: NCHRP - Supplemental Guidance on the Application of FHWA s TNM Contents... K- K. Introduction... K- K. Height to-width ratio for the barriers and receiver position behind one of the barriers... K- K. Number of FHWA TNM roadways used to represent the travel lanes... K- K. Source position... K- K. Differences in the heights (top elevations) of the two barriers... K- K. Internal vertical reflecting surface... K- K. Vehicle mix (e.g., autos only vs. heavy trucks only)... K- K. Hourly volumes of vehicles... K- K. Vehicle speed... K- K. Noise reduction coefficient (NRC) of barrier surfaces... K- K. Comparison of measured and modeled levels including parallel barrier sound level increases... K- iii

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5 Final Technical Report: NCHRP - Supplemental Guidance on the Application of FHWA s TNM Appendix K K. Introduction Parallel Barriers FHWA TNM. models this three-dimensional phenomenon in a separate two-dimensional Parallel Barrier Analysis module inside the program. Input data are passed to the module from the main part of TNM, but the computed parallel barrier sound level increases are not linked back to the L eq computed by the main part of TNM. The objective of this research was to investigate the sensitivity of the parallel barrier module to a variety of factors and refine the available guidance on the use of the module. The areas that were studied relating to the sensitivity of the computed sound level increase to the input parameters include: Height-to-width ratio for the barriers and receiver position behind the barrier Number of FHWA TNM roadways used to represent the travel lanes Source position Differences in the heights (top elevations) of the two barriers Internal vertical reflecting surface Vehicle mix (e.g., autos only vs. heavy trucks only) Hourly volumes of vehicles Vehicle speed Noise reduction coefficient of barrier surfaces The research also evaluated two sets of measured and modeled data for a parallel barrier project before and after the addition of sound-absorbing panels to one of the barriers. K. Height to-width ratio for the barriers and receiver position behind one of the barriers The sound level increase due to multiple reflections between the parallel barriers is partly a function of the width-to-height ratio for the cross-section (distance between the noise barriers divided by their height). In general, the smaller the ratio, the greater the sound level increase will be. For these tests, an -lane cross-section was developed that had a barrier-to-barrier width of ft (eight -ft lanes) with -ft inside and outside shoulders in each travel direction. Each lane was modeled by an FHWA TNM Parallel Roadway, which will be referred to simply as a roadway, even though it is a distinct object different from the main FHWA TNM Roadways. Barrier heights were varied from ft to ft. The resulting width-to-height ratios are shown in Figure for heights between and ft, ranging from : for ft to : for ft. The barriers are assumed to be sound-reflecting, with a noise reduction coefficient (NRC) of.. In the FHWA TNM parallel barrier module, the points at which the sound level increases are predicted are called Analysis Locations. Their z-coordinates represent the elevation of the ear. In the main part of FHWA TNM, the points at which levels are predicted are called Receivers, where the z-coordinate represents the elevation of the ground with a height above ground entered to determine the elevation of the ear. In this discussion, for simplicity, the point at which the level is predicted will be called a receiver, with the understanding that it is located at the ear elevation, not the ground elevation. K-

6 Width-to-height ratio Final Technical Report: NCHRP - Supplemental Guidance on the Application of FHWA s TNM As illustrated in Figure, an array of receivers was modeled for six distances back from the near wall (,,,, and ft) and at four heights relative to the roadway surface: ft, ft, - ft and ft. These heights represent an exterior second-story location in an at-grade cross-section, a typical exterior first-floor receiver ft above the ground in an at-grade cross-section, a receiver alongside a -ft roadway embankment (- ft height minus an additional assumed ft drop to the ground), and a receiver alongside a -ft roadway embankment (- ft height minus an additional assumed ft drop to the ground) Barrier height, ft Figure Width-to-height ratios for the tested barrier heights for the -lane cross-section. Figure Illustration of receiver array, at heights of,, - and - ft relative to the roadway surface and distances of,,,, and ft from the near wall. Figure shows the sound level increases for autos only, and Figure shows the sound level increases for heavy trucks only. Each figure has four graphs, one for each of the four receiver heights relative to the roadway surface: ft, ft, - ft and - ft. Each line on each graph is the sound level increases for different barrier heights for a difference distance from the near wall. Both figures illustrate that the sound level increase is a function of not only barrier height (and thus width-to-height ratio), but also receiver height above or below the road, receiver distance back from the near wall, and vehicle type. For this cross-section, a : width-to-height ratio occurs between the and -ft high barrier cases. For these barrier heights, the sound level increases for autos range from. to. db over the four K-

7 Final Technical Report: NCHRP - Supplemental Guidance on the Application of FHWA s TNM receiver heights and the six distances back from the near wall. For heavy trucks, the range is. to. db for these barrier heights across the receiver array. For most of the receiver positions, this amount of sound level increase warrants attention in the barrier design process. For this same cross-section, a : width-to-height ratio occurs for a -ft barrier. For this barrier height, the sound level increases for autos range from. to. db, being greatest for the -ft receiver. For heavy trucks, the range is to. db. Depending on the mix of traffic and the receiver location, the sound level increase even for this : width-to-height ratio may warrant attention during barrier design () () () () () () (-) (-) (-) (-) (-) (-). Barrier heights, ft. Barrier heights, ft Barrier heights, ft () () () () () () Barrier heights, ft (-) (-) (-) (-) (-) (-) Figure Sound level increases for, Autos/hr/lane, -roadway cross section, varying barrier height, and NRC of. for both walls. The parallel barrier sound level increase for autos was equal to or greater than that for heights over all of the receiver positions and all of the tested barrier heights. The greatest differences were in the - ft barrier height range, with autos being as much as. db higher for the -ft high receiver. Above ft, the auto sound level increases were only. to. db greater than the heavy truck values. One set of results for both autos and heavy trucks stands out from the other: the -ft high receiver ft back from the near wall ( () ). The sound level increase is small at low barrier heights, then increases rapidly as the barrier height increases up to ft, and then decreases slightly before increasing again. For this tall receiver, the primary noise source for the lower height barriers is the direct noise from the source, and thus the sound level increase due to reflections is small. However, at a barrier height of - ft, this receiver s view of the source is blocked, but there is still a view of the far wall and the reflected sound off the far wall dominates. Once the barrier height increases so that the far wall is not visible to the receiver, then the sounds from both the direct and reflected sources are reduced by diffraction attenuation over the near wall. K-

8 Final Technical Report: NCHRP - Supplemental Guidance on the Application of FHWA s TNM Another set of results of interest is for the ft high receiver ft from the near wall ( () ) for autos. The sound level increase changes very little as barrier height increases from ft up to ft. A similar pattern is seen for the ft high receiver ft from the near wall. The results are counter to prevailing thought that the sound level increase increases as barrier height increases because more multiplereflection paths are created as the barrier height increases. This pattern appears to a lesser degree for the other distances and heights, where the sound level increase increases only slightly as barrier height increases. For the heavy trucks also, the sound level increase is not particularly sensitive to increasing barrier heights at the closer-in distances. Runs were also made for medium trucks for the -roadway case. While the results are not shown, the predicted sound level increases were very close to those for the autos. Over all of the receiver positions for all of the barrier heights, most of the values were within. to. db of the auto values. Only in a very few cases was the medium truck sound level increase as much as. db lower than that for the autos. () () () () () () (-) (-) (-) (-) (-) (-) Barrier heights, ft Barrier heights, ft Barrier heights, ft () () () () () () Barrier heights, ft (-) (-) (-) (-) (-) (-) Figure Sound level increases for, heavy trucks /hr/lane, -roadway cross section, varying barrier height, and NRC of. for both walls. In the above runs, the width-to-height ratio was varied by keeping the width constant and varying the barrier height. The following tests kept the barrier heights constant at -ft each and increased the width between the parallel barriers. Two cases were run: one where the eight roadways were left in their original positions (unchanged median width), so that the barriers moved farther from the roadway edge, and one where the median width between each travel direction was increased in order to keep the distance from each barrier to the edge of each travel direction the same. K-

9 Final Technical Report: NCHRP - Supplemental Guidance on the Application of FHWA s TNM Figure shows the results where the roadways were not moved outward and Figure shows the case where the roadways were moved outward so that the distance from the barriers to the roadway edges remained the same. In both cases, the. width-to-height ratio represents the original -ft wide separation and the -ft barrier heights. Four graphs are shown for each figure, one for each receiver height. The lines on each graph are for the different width-to-height ratios for each distance back from the near roadway. For the unchanged roadways in Figure, the sound level increases are similar for the.: and : width-to-height ratios, and then decrease as the width-to-height ratio increases. Even at :, sound level increases over db are seen for the -ft high receiver over all distances, and farther back from the near wall for the -ft receiver. At :, the sound level increases for the - and --ft receiver heights are less than db. For the moved roadways in Figure, the patterns are similar to the unmoved roadways, but there is more aviation in effect as a function of distance back. Untested in these cases is whether or not the sound level increases would occur in the real world. A : width-to-height ratio for -ft high barriers means the barriers are ft apart. Meteorological effects on the sound propagation, such as wind shear (changing wind speed with altitude) or temperature lapse rate (changing temperature with altitude) could easily have more effect on the levels over these distances due to refraction than would the reflected paths.. Width to height ratio () () () () () (). Width to height ratio (-) (-) (-) (-) (-) (-). Width to height ratio () () () () () (). Width to height ratio (-) (-) (-) (-) (-) (-) Figure Sound level increases for varying width-to-height ratio for -ft high barriers: barriers moved outward, roadways and median unchanged. K-

10 Final Technical Report: NCHRP - Supplemental Guidance on the Application of FHWA s TNM. Width to height ratio () () () () () (). Width to height ratio (-) (-) (-) (-) (-) (-). Width to height ratio () () () () () (). Width to height ratio (-) (-) (-) (-) (-) (-) Figure Sound level increases for varying width-to-height ratio for -ft high barriers: barriers and roadways moved outward, median widened. K. Number of FHWA TNM roadways used to represent the travel lanes Also investigated were the differences in modeling an -lane cross section with eight, four and two FHWA TNM parallel barrier roadways, as shown in Figure. Not a significant amount of difference was seen. The results are not displayed, although several observations can be made. For autos, the modeling as four roadways resulted in sound level increases within +/-. db of those for the -roadway model over all of the receiver locations and all barrier heights, with the exception of one instance: the () receiver for a -ft barrier height, where the sound level increase was. db. There is no apparent reason for the single case to be different from other barrier heights or other receiver locations. The -roadway model had sound level increase within -. to. db of the -roadway model, except for one case with a +. db difference. For heavy trucks, the -roadway results were +/-. db of the roadway model with the exception of three cases where the differences were. and -. db. The -roadway results for heavy trucks were generally within +/-. db of the -roadway sound level increases with the exception of two cases where the -roadway sound level increase was greater than that for the -roadway case and two cases where the -roadway sound level increases were up to a decibel greater than the -roadway model. K-

11 Final Technical Report: NCHRP - Supplemental Guidance on the Application of FHWA s TNM The trends in the differences between the auto and heavy truck sound level increases discussed earlier for the -roadway model were similar to those for the -roadway and -roadway models. Figure Cross-sections of eight, four and two FHWA TNM parallel barrier roadways, all with the same total traffic volume. K. Source position Of interest is the effect of source position in the canyon between the two parallel barriers. As shown in Figure, cases were created in the -lane cross-section representing the four far lanes by four FHWA TNM roadways and then by one roadway centered between them, and the four near lanes by four roadways and then one roadway. Separate cases were run for autos and heavy trucks and for barrier heights of,, and ft. K-

12 Final Technical Report: NCHRP - Supplemental Guidance on the Application of FHWA s TNM Figure Varying source position: far lanes (left) and near lanes (right) represented by four (top) and one (bottom) FHWA TNM roadways. Figure shows four cross-sectional views representing of the receiver array labeled with the sound level increases rounded to the nearest decibel for the four barrier heights. These results are for autos only for the case of four near roadways. In the figure, the roadways and barriers would be to the left of the receivers. Figure shows the same for the case of four far roadways. Figure then shows the differences in the sound level increases for autos between the four near roadways case and four far roadways case. A positive value means that the near roadway sound level increase is greater than the far roadway sound level increase. First, the results confirm, as shown earlier, that the sound level increase is very dependent on the receiver position behind the near wall, both in terms of distance and height. Second, the results show the relative insensitivity of the sound level increase to source position within the canyon between the two walls for many of the tested receiver positions. Source position only has a small effect on the sound level increase for the lower receiver positions, especially close to the near wall, but has a larger effect, in general, at the highest and more distant receiver positions. The effect is not sensitive to receiver height, except at the -ft high position closer to the near wall. The difference in the sound level increase between the four far roadways and for near roadways in within a range +/- for all but one of the receiver positions within ft of the near wall for the receiver heights of -, - and ft. For greater distance back and for the -ft high receiver, the differences in the near-roadway and farroadway modeling were often or more db. While not shown, the difference for heavy trucks only between these four near roadway and four far roadway cases are similar to the autos-only cases. The autos-only sound level increases are generally higher than the heavy truck-only sound level increases, with the difference between vehicle types greater for the near roadways than the far roadways, largely a function of the vertical sound energy distribution difference in the two vehicle types. K-

13 Height above road, ft Height above road, ft Height above road, ft Height above road, ft Final Technical Report: NCHRP - Supplemental Guidance on the Application of FHWA s TNM -ft High ft High ft High ft High Figure Sound level increase as function of barrier height for four near roadways with autos only in an -lane cross-section, db. K-

14 Height above road, ft Height above road, ft Height above road, ft Height above road, ft Final Technical Report: NCHRP - Supplemental Guidance on the Application of FHWA s TNM -ft High ft High ft High ft High Figure Sound level increase as function of barrier height for four far roadways with autos only in an -lane cross-section, in db. K-

15 Height above road, ft Height above road, ft Height above road, ft Height above road, ft Final Technical Report: NCHRP - Supplemental Guidance on the Application of FHWA s TNM -ft High ft High ft High ft High Figure Difference in sound level increases for autos, four near roadways minus four far roadways in an -lane cross-section, db. Modeling as four roadways in each travel direction was also compared to modeling each direction as a single roadway centered in the middle of the four-roadway group. While the results are not shown, none of the sound level increases for heavy truck-only for the four-roadway and one-roadway cases differed by more than half a decibel. For autos-only, both the near roadway and far roadway cases showed differences between modeling as four roadways or one roadway. Figure shows the differences in the sound level increase for the near lanes for four roadways vs. one roadway, with positive values meaning that the four-roadway increases were greater. Figure shows the differences for the far-lanes autos-only cases, again with a positive value meaning the four-roadway increases were greater than the one-roadway increases. K-

16 Height above road, ft Height above road, ft Height above road, ft Height above road, ft Final Technical Report: NCHRP - Supplemental Guidance on the Application of FHWA s TNM -ft High ft High ft High ft High Figure Difference in sound level increases for autos, four near roadways minus one near roadway in an -lane cross-section, in db. K-

17 Height above road, ft Height above road, ft Height above road, ft Height above road, ft Final Technical Report: NCHRP - Supplemental Guidance on the Application of FHWA s TNM -ft High ft High ft High ft High Figure Difference in sound level increases for autos, four far roadways minus one far roadway in an -lane cross-section, in db. K. Differences in the heights (top elevations) of the two barriers As the height of one of the two parallel barrier changes, there is a change is the sound level reflection pattern. Conceptually, as the height of the far wall decreases, the potential for many multiple-reflection paths decreases, which could then reduce the size of the sound level increase due to reflections. A test was made where the near wall height was held at ft and the far wall height was varied from to ft for the -roadway cross-section for autos only. Figure shows the results. In general, the sound level increase due to reflections does decrease as the far wall height decreases downward from the same height as the near wall of ft. The reduction in the sound level increase is greater for the higher receivers and the greater distances from the near wall because the actual sound level increases for the equal wall height cases are larger for these receiver positions. However, even for the relatively low far wall heights, the sound level increases can still be substantial enough to warrant possible mitigation. K-

18 Final Technical Report: NCHRP - Supplemental Guidance on the Application of FHWA s TNM Far wall barrier height, ft () () () () () () Far wall barrier height, ft (-) (-) (-) (-) (-) (-) Far wall barrier height, ft () () () () () () Far wall barrier height, ft (-) (-) (-) (-) (-) (-) Figure Sound level increases for autos only on the -roadway section with near wall height of ft and varying far wall heights, NRC=.. K. Internal vertical reflecting surface The FHWA Traffic Noise Model: Frequently Asked Questions FAQs for parallel barriers cautions about having an internal vertical reflecting surface in the analyzed parallel barrier cross sectional surface: Can TNM model more than parallel barriers? Yes, it can be modeled as a single cross section in the Parallel Barriers module. However, keep in mind that when a parallel barrier section contains two separate vertical surfaces offset on the same side of a road (i.e., a retaining wall near the edge-of-pavement and a barrier at the right-of-way), () TNM parallel-barrier accuracy is degraded somewhat for receivers on that same side of the roadway (TNM may under-compute or over-compute the noise increase), and () TNM may under-compute the noise increase for receivers on the opposite side of the roadway. Please refer to the diagram below: Traffic Noise Model: Frequently Asked Questions FAQs, FHWA web site at: / K-

19 Final Technical Report: NCHRP - Supplemental Guidance on the Application of FHWA s TNM The extent of the effect on the results may depend on the source position, the heights of the noise barriers and the internal vertical surface, the offset of the external wall from the internal vertical surface, and the receiver position. A test was created to illustrate the problem of the internal surface reflections. As shown in Figure, the cross-section on the left consisted of a -ft high near barrier and a ft high far barrier. The cross-section on the right was the same, except that a -ft high noise barrier was added offset to the left beyond the far wall. The -ft high far wall thus went from being an external vertical surface to an internal vertical surface. Figure Cross-section with -ft high far wall as an external vertical surface (left) and an internal vertical surface with a -ft noise barrier offset ft from the top of the internal vertical surface(right). Acoustically, there should be no difference in the calculated sound level increases. The -ft noise barrier offset from the -ft vertical section is not in a position to reflect sound back across to diffract over the top of the near wall. However, as seen in Figure, there are differences, ranging from. db in close to over db farther back. The results on the left side of the figure are for the -ft high far wall, while the results on the right are for the -ft high far wall with the -ft high barrier offset behind it. The -ft noise barrier was deleted so that the cross-section ended with a horizontal segment beyond the top of the -ft section. All of the sound level increases became db. The -ft noise barrier was restored and the -ft internal far wall was assigned an NRC of.. Again, all of the sound level increases became db, showing that the -ft barrier was not a cause of reflections. K-

20 Height above road, ft Height above road, ft Final Technical Report: NCHRP - Supplemental Guidance on the Application of FHWA s TNM Near Wall= ft, Far Wall= ft Near Wall= ft, Far Wall= ft atop -ft ret. wall Figure Sound level increases for external (left) and internal (right) -ft high vertical surfaces for far wall. K. Vehicle mix (e.g., autos only vs. heavy trucks only) The FHWA TNM parallel barrier module is generally not sensitive to changes in vehicle mix (the percentage of autos vs. trucks in an hourly traffic flow) once trucks are introduced into the flow. Figure shows four graphs, one for each of the four receiver heights, for the -roadway cross-section with two -ft high walls. Each line on each graph is the sound level increases for different percentages of autos for a difference distance from the near wall. In general, a +/- % change in percentage of autos changes the sound level increase by only a few tenths of a decibel, except in going from % autos to % autos, where the change in sound level increase is on the order of half a decibel. () () () () () () (-) (-) (-) (-) (-) (-) %% % % % % % % % % % % % % % % % % % % Automobile percentage, % Automobile percentage, % () () () () () () (-) (-) (-) (-) (-) (-) % % % % % % % % % % %% % % % % % % % % Automobile percentage, % Automobile percentage, % Figure Sound level increases as a function of percentage of autos in the total traffic flow, - lane cross-section with -ft high walls, NRC =.. K-

21 Final Technical Report: NCHRP - Supplemental Guidance on the Application of FHWA s TNM K. Hourly volumes of vehicles Because the parallel barrier module is predicting only a sound level increase in the one-hour L eq and not an actual one-hour L eq, the module s calculations are independent of the actual hourly volumes, but are dependent on the traffic mix, as was discussed in the previous section. Tests were made using the -roadway cross-section with two -ft high barriers. A case with one auto per roadway gave identical sound level increases as a case with, autos per roadway. The same was seen for one heavy truck compared to, heavy trucks, and for a run of, each of autos, medium trucks and heavy trucks compared to a run with just one vehicle of each type. Even when the traffic was zeroed out, sound level increases were calculated that were comparable, but not exactly equal, to the case of one each of the three vehicle types. K. Vehicle speed Parallel barrier cases were examined for speeds varying between and mph separately for autos and heavy trucks for the -roadway cross-section with -ft high barriers. The resulting sound level increases were independent of speed for each vehicle type: no change in sound level increase was predicted for a change in speed. K. Noise reduction coefficient (NRC) of barrier surfaces The FHWA TNM parallel barrier module has the capability of testing the effectiveness of changing the NRC of all or parts of one or both of the parallel barriers. NRC is a frequency-specific quantity, being the average of the sound absorption coefficients in the,,, and, Hz octave bands. Different products with difference sound absorption coefficients in these bands can still have the same NRC, yet perform differently in the field. The FHWA TNM parallel barrier module computes the diffraction attenuation of the sound passing over the near wall at a Hz frequency. As such, the application of a generic NRC will give an indication of the effect of the sound-absorbing material, but not a precise calculation. To test the parallel barrier module s application of the NRC, several cases were studied. The basic case was the -roadway cross-section for autos only with -ft barriers on either side. The was varied between. (a typically used value for concrete) and. in. increments (starting from.). Then, just the far wall was made sound-absorbing, with the same NRC variation, and then just the near wall in the same manner. Finally, the heights of both walls were varied in tandem to test the effect of NRC on difference height configurations. Figure shows the case for both walls with sound absorption. Figure is for sound absorption on the far wall only and Figure is for sound absorption on the near wall only. Each figure has four graphs, one for each receiver height. The lines on each graph show the effect of NRC on the sound level increase for each of the studied receiver distances back from the near wall. The NRC of. on the left of each graph is typically used to represent a reflective noise barrier. For all three cases, the effectiveness of the increased sound absorption is fairly linear, reducing the sound level increase as the NRC increases. For absorption on both walls, an NRC of. or higher brings the reflective barriers sound level increases down to under a decibel for all of the receiver positions except at the -ft receiver height, for which the maximum sound level increase is less than db. Sound absorption on the far wall only is also very effective for this cross-section in reducing the sound level K-

22 Final Technical Report: NCHRP - Supplemental Guidance on the Application of FHWA s TNM increases. For any given receiver position, the sound level increases with absorption on just the far wall range from to. db higher than for absorption on both walls. In contrast, absorption on just the near wall is far less effective than absorption on the far wall or both walls. For any given receiver position, the sound level increases with absorption on just the near wall range up to. db higher than absorption on both walls. The results suggest the importance of the first-order far wall reflections on the total sound level at a receiver, but also show that the program is calculating multiple reflection paths back and forth between the barriers because near wall absorption also reduces the sound level over the fully reflective case () () () () () () (-) (-) (-) (-) (-) (-) () () () () () () (-) (-) (-) (-) (-) (-) Figure Sound level increases as a function of NRC on two parallel -ft high walls on a -ft wide, -roadway cross-section, both walls with sound absorption. K-

23 Final Technical Report: NCHRP - Supplemental Guidance on the Application of FHWA s TNM Far wall NRC () () () () () () Far wall NRC (-) (-) (-) (-) (-) (-) Far wall NRC () () () () () () Far wall NRC (-) (-) (-) (-) (-) (-) Figure Sound level increases as a function of NRC on far wall of two parallel -ft high walls on an -roadway cross-section, sound absorption on the far wall only. K-

24 Final Technical Report: NCHRP - Supplemental Guidance on the Application of FHWA s TNM Near wall NRC () () () () () () Near wall NRC (-) (-) (-) (-) (-) (-) Near wall NRC () () () () () () Near wall NRC (-) (-) (-) (-) (-) (-) Figure Sound level increases as a function of NRC on near wall of two parallel -ft high walls on an -roadway cross-section, sound absorption on the near wall only. The next four figures provide a picture of how increasing the NRC reduces the sound level increases for four different pairs of equal-height barriers: Figure : -ft Figure : -ft Figure : -ft Figure : -ft K-

25 Final Technical Report: NCHRP - Supplemental Guidance on the Application of FHWA s TNM () () () () () () (-) (-) (-) (-) (-) (-) () () () () () () (-) (-) (-) (-) (-) (-) Figure Sound level increase as a function of NRC on two parallel -ft high walls on a -ft wide, -roadway cross-section. K-

26 Final Technical Report: NCHRP - Supplemental Guidance on the Application of FHWA s TNM () () () () () () (-) (-) (-) (-) (-) (-) () () () () () () (-) (-) (-) (-) (-) (-) Figure Sound level increases as a function of NRC on two parallel -ft high walls on a -ft wide, -roadway cross-section. K-

27 Final Technical Report: NCHRP - Supplemental Guidance on the Application of FHWA s TNM () () () () () () (-) (-) (-) (-) (-) (-) () () () () () () (-) (-) (-) (-) (-) (-) Figure Sound level increases as a function of NRC on two parallel -ft high walls on a -ft wide, -roadway cross-section. K-

28 Final Technical Report: NCHRP - Supplemental Guidance on the Application of FHWA s TNM () () () () () () (-) (-) (-) (-) (-) (-) () () () () () () (-) (-) (-) (-) (-) (-) Figure Sound level increases as a function of NRC on two parallel -ft high walls on a -ft wide, -roadway cross-section. K. Comparison of measured and modeled levels including parallel barrier sound level increases A comparison of measured and FHWA TNM predicted levels including parallel barrier sound level increases was made for a prior study that evaluated traffic noise barriers along both sides of State Route (SR ) in Silver Lake and Cuyahoga Falls, Ohio. The walls were both originally sound-reflecting. In response to citizen complaints in Silver Lake especially after construction of the Cuyahoga Falls barrier, Ohio DOT had an initial study conducted. After that study, absorption panels were added to Cuyahoga Falls barrier to mitigate reflections back into the Silver Lake community, and this follow-up study was then conducted. SUM--. Noise Wall After Absorption Study, State Route, Silver Lake, Ohio, Bowlby & Associates, Inc., for Ohio DOT District,.. SUM--. Noise Barrier Post Construction Study-State Route -Silver Lake Ohio, Bowlby & Associates, for Ohio Department of Transportation Office of Environmental Services,. K-

29 Final Technical Report: NCHRP - Supplemental Guidance on the Application of FHWA s TNM Photograph source: Summit County, Ohio, GIS Map Viewer Figure Aerial photograph of Silver Lake, OH study area (left) and FHWA TNM Plan View (right) (Photograph source: Summit County, Ohio, GIS Map Viewer) Included in the follow-up study were noise measurements with concurrent traffic and meteorological data collection, noise modeling with TNM.b, and administration of a follow-up survey of the affected citizens. The noise data and survey results were also compared to those in the pre-sound absorption study (where the modeling was initially done with STAMINA. and then redone with TNM.b). The data sets 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). The FHWA TNM.b runs were converted to run in FHWA TNM. for use in this research. The project study area was on the Silver Lake side of SR, shown in Figure and was divided into two analysis sections:. Two-wall area: near the center of the Silver Lake barrier where the Cuyahoga Falls barrier was also in place; both walls were essentially at-grade with the road and of nearly equal heights.. No-wall area: to the north of both the Silver Lake and Cuyahoga Falls barriers, just north of a pedestrian overpass bridge over SR. K-

30 Final Technical Report: NCHRP - Supplemental Guidance on the Application of FHWA s TNM A reference microphone was deployed in each area, and two individual study sites were chosen within each area, with a third more distant site in each area. Table describes the sites. Figure shows a close-up view of the FHWA TNM model of the No-wall area. Figure shows a close-up view of the modeled Two-wall area, including the Parallel Barrier view at the -Ref site. Figure FHWA TNM plan view of No-wall area on north end of site. Area Site Name Table SR Noise Measurement and Modeling Sites Description Distance to Reference Site (ft) Distance to Centerline of SR (ft) Two-wall -Ref Reference site feet atop eastern barrier -- Two-wall -A Vincent Rd: first- row back yard facing eastern barrier Two-wall -B Millboro Rd: second row back yard (two rows of houses between it and eastern barrier) Two-wall -C Overlook Rd: approximately sixth row front yard (six rows of houses between it and eastern barrier) No-wall -Ref Reference site to north of pedestrian overpass and to east of SR with microphone at same height above SR as -Ref, but with no -- barrier below it No-wall -A On abandoned railroad tracks behind property line for Lakeland Pkwy and east of SR, which was beginning to curve away from community; this curve required a shifting of this microphone away from the houses to maintain same distance from SR as in the two-wall area No-wall -B Lakeland Pkwy: second row back yard (two rows of houses between it and SR ) No-wall -C Silver Lake Blvd.: approximately fourth row back yard (four rows of houses between it and SR ) K-

31 Final Technical Report: NCHRP - Supplemental Guidance on the Application of FHWA s TNM Figure FHWA TNM plan view of Two-wall area in center of project (top) and Parallel Barrier view at the -Ref microphone (bottom). The results for the initial measurements, taken in June with both walls reflective, are shown in Table and Table The upper portion of Table shows the measured -min L eq at each site during the different periods. Only subsets of the sites were sampled in each period. The second portion of the table has the FHWA TNM. predicted hourly L eq based on factored-up traffic counts during the measurements. No adjustments for the reflections have been made. The third part of the table compares the measured and predicted levels. In the No-wall area, the model is predicting well at -Ref and -B. However, at -A, the TNM overprediction was. and. db. In the original study in, the FHWA STAMINA. program overpredicted by. db. When the predictions were redone with FHWA TNM.b, the overpredictions were still large. The reasons for all three models overprediction are not clear. At -A, TNM underpredicted by. to. db. If the predicted level is adjusted by the predicted minus measured differences at -Ref would reduce the underpredictions to. db or less. FHQWA TNM is slightly overpredicting at -B and underpredicting at -C, even after adjustment for the predictedmeasured reference site differences. The Two-wall models were then studied with the FHWA TNM parallel barrier module. The computed parallel barrier sound level increases were:. db at -Ref;. db at -A;. db at -B; and db at -C. The top portion of Table shows the adjusted FHWA TNM predictions, computed by adding the calculated sound level increase to the predicted levels in the previous table. The lower portion of Table shows the difference in the adjusted predicted level and the measured level at each receiver. The -Ref predictions are all now slightly closer to the measured levels, still underpredicting by. to. db. The predicted levels at -A are now higher the measured levels by. to. db, whereas they were lower before adding in the calculated sound level increase. Site -B now overpredicts the measured level by. db, and site -C still has the same. db underprediction. If the data is normalized by the -Ref predicted-measured difference, the agreement grows worse at -A and -B and improves. db at -C. K-

32 Final Technical Report: NCHRP - Supplemental Guidance on the Application of FHWA s TNM In the pre-absorption study, the inclusion of the parallel barrier sound level increase did not improve the model s results at sites -A and -B. Table SR Measured and Unadjusted Predicted L eq, Both Walls Reflective, dba Site: -Ref -A -B -C -Ref -A -B -C Period Measured -minute Leq at Each Receiver, dba..... Period Predicted -minute Leq at Each Receiver, dba Period Predicted Leq Minus Measured Leq at Each Receiver, dba Table SR Predicted L eq, Adjusted for Sound Level Increase, Both Walls Reflective, dba Site: -Ref -A -B -C -Ref -A -B -C Period Adjusted* Predicted -minute Leq at Each Receiver, dba No adj. No adj. No adj... No adj. No adj... No adj Period Adjusted* Predicted Leq Minus Measured Leq at Each Receiver, dba No adj. No adj. No adj. -.. No adj. No adj. -.. No adj * Adjustment is the TNM. parallel barrier sound level increase in db: -Ref =.; -A =.; -B=.; -C= K-

33 Final Technical Report: NCHRP - Supplemental Guidance on the Application of FHWA s TNM The results for the follow-up measurements and modeling, done in October, with the far wall soundabsorbing with an NRC as., are shown in Table and Table. The upper portion of Table shows the measured -min L eq at each site during the different periods. The second portion of the table has the FHWA TNM. predicted hourly L eq based on factored-up traffic counts during the measurements. No adjustments for the reflections have been made. The third part of the table compares the measured and predicted levels. In the No-wall area, the model is predicting within -. to +. db of the measured levels at -Ref. However, at -A, -B and -C the results are mixed. The measured levels varied substantially between periods at each site, resulting in both good and poor agreement with the modeling. The reasons for the variation in the measured levels were not clear. In the Two-wall area, the model is predicting within -. to +. db of the measured levels at -Ref. At -A, FHWA TNM underpredicted by. db and overpredicted by. db in the two periods. At -B, the model underpredicted by. and. db in the two periods. At -C, the model greatly underpredicted the levels. The Two-wall models were then studied with the FHWA TNM parallel barrier module using an NRC of. on the far wall. The computed parallel barrier sound level increase at -Ref was. db, the same as for the both walls reflective case. One would have expected this value to decrease. At -A, the sound level increase decreased from. db to. db; at -B, it decreased from. db to db; and at -C, it remained at db. The top portion of Table shows the adjusted FHWA TNM predictions, computed by adding the calculated sound level increase to the predicted levels in the previous table. The lower portion of Table shows the difference in the adjusted predicted level and the measured level at each receiver. The changes from the unadjusted levels are slight because the sound level increases were small or zero. Overall, the results of the comparisons of the measured and modeled levels in the reflective and far-wall absorptive cases were mixed. Agreement was good at the reference microphone sites for both the Nowall and Two-wall sites in each case, and at -A, the closest site behind the near wall. At the other study sites, agreement ranged from mixed at -B to poor at -A and -C. One issue was with the range in the measured sound levels at the sites, especially in the far-wall absorptive case. Site -C was deep into the community, and while care was taken regarding localized noise sources and meteorological effects on sound propagation, these factors could not be ruled out as possible causes of the sound level differences. Table SR Measured and Unadjusted Predicted L eq, Far Wall Absorptive (NRC =.), dba Site: -Ref -A -B -C -Ref -A -B -C Period Measured -minute Average Sound Level at Each Receiver, db Period Corresponding TNM Predicted L eq at Each Receiver, db K-

34 Final Technical Report: NCHRP - Supplemental Guidance on the Application of FHWA s TNM Site: -Ref -A -B -C -Ref -A -B -C Period Predicted Level Minus Measured Level at Each Receiver, db Table SR Predicted L eq, Adjusted for Sound Level Increase, Far Wall Absorptive (NRC =.), dba Site: -Ref -A -B -C -Ref -A -B -C Period Adjusted* Predicted -minute Leq at Each Receiver, dba.. No adj.. No adj. No adj. No adj. No adj. No adj. No adj. No adj No adj... No adj.. No adj.. Period Adjusted* Predicted Leq Minus Measured Leq at Each Receiver, dba.. No adj.. No adj. No adj. No adj. No adj. No adj. No adj. No adj No adj No adj. -. No adj. -. * Adjustment is the TNM. parallel barrier sound level increase in db: -Ref =.; -A =.; -B = -C = K-