Field Evaluation of Reflected Noise from a Single Noise Barrier (2018)

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42 and the meteorological station. Table 13 gives each micro- phone’s position, distance to the edge of nearest travel lane, and height above roadway grade. Figure 26 shows pictures of each microphone at its measurement position. Figure 27 shows cross-sections at the Barrier and No-Barrier sites. The cross-section at the No-Barrier site is virtually iden- tical to the cross-section at the Barrier site: the ground eleva- tion is 0.5 ft. lower at BarRef01 compared to NoBarRef02 and 1 ft. higher at BarCom03 compared to NoBarCom05. A concrete median barrier is present at both the Barrier and No-Barrier sites. The microphones were set at 10 ft. and 20 ft. above the roadway elevation to minimize the shielding of the direct noise coming from tires on vehicles traveling on the northbound side of I-75. Measurements were conducted on Tuesday, November 15, 2016, from approximately 17:00 to 21:00. The field crew observed no insect or other natural sounds audible at night—only highway noise—and no significant differences were noted between the highway noise at BarCom04 and NoBarCom06. Traffic was steady for the first 1.5 hours. Traf- fic eased after approximately 18:40. The volume of heavy trucks remained high throughout the measurement period. Even when no traffic was immediately in front of the micro- phone, the background level of the sound environment was still set by distant traffic noise. No other contributors to the background level were audible. I-70, South Vienna, Ohio (Location OH-2, Sound-Absorbing Barrier) As shown in Figure 28, the I-70 location is a six-lane free- way. Approximately 900 ft. long, the 17-ft.-high barrier is off- set 80 ft. from the edge of the nearest travel lane. The barrier is a precast concrete post-and-panel design and uses the same highway-side proprietary rubber-chip sound-absorbing face as does the I-75 barrier. Sound-absorption test results were not available for this barrier; however, results for a compara- ble panel from the same company had the sound-absorption coefficients shown in Table 12. Figure 29 shows a site map of the measurement positions. Table 14 gives each microphone’s position, distance to the edge of nearest travel lane, and height above roadway grade. Figure 30 shows pictures of each microphone at its measurement position. Octave Band Center Frequency (Hz) Sound-Absorption Coefficient 125 0.09 250 0.25 500 1.25 1,000 0.74 2,000 1.03 4,000 1.07 NRC 0.80 Table 12. Sound-absorption coefficients and NRC for a barrier like the I-75 and I-70 barriers. Source: Satellite image © 2016 Google Earth Figure 25. I-75—microphone positions.

43 The tree and underbrush cover at this site restricted how far back the microphones could be placed. Instead of put- ting one set of microphones closer to the road, where the direct near-lane traffic noise would dominate, both sets of microphones were placed at the same distance from the road but at different heights. BarCom03 was positioned approxi- mately 11 ft. above road grade, as it was located 5 ft. above ground on a hill approximately 6 ft. above the road elevation. BarCom04 was positioned approximately 10 ft. above ground on the same pole as BarCom03. Similarly, NoBarCom05 was positioned approximately 11 ft. above road grade, as it was located 5 ft. above ground level on a tripod that was at the top of a sloped area approximately 6 ft. above road grade. NoBarCom06 was approximately 10 ft. above ground on the top of a hill and close to NoBarCom05. The meteorological station was located near NoBarRef02 with its sensors positioned 5 ft. and 15 ft. above ground. Figure 31 shows cross-sections at the I-70 Barrier and No- Barrier sites. The cross-section at the No-Barrier site is like the Barrier site, with the ground elevation being a foot lower at BarRef01 compared to NoBarRef02. No concrete median barrier is present at either site. The microphones were set at 11 ft. and 16 ft. above the roadway elevation to study potential effects of height above roadway on reflected noise. Measurements were conducted on Wednesday, Novem- ber 16, 2016, from approximately 14:30 to 18:30. There was almost no cloud cover and the temperature dropped swiftly as the sun went down during the period when the measure- ments were taken. Winds were calm after sunset, and because most of the leaves had fallen off the trees in this tree/brush line, wind noise through the foliage was not audible. Traffic flow was heavy and steady for the first 2 hours of data collec- tion. Traffic was noticeably reduced for the last 30 minutes of data collection. Few individual pass-by candidates were observed because gaps of 6 to 7 seconds between vehicles were rare. Vehicles were audible to the west for several sec- onds longer than to the east, likely because of the hill to the east. Louder heavy trucks could be heard for 20 seconds to the west if no other loud vehicle was in front of the microphones. No other noise sources (such as insects or birds) were audible at the microphones. At BarCom03 and BarCom04, there was no influence from local traffic on the overpass that was approximately 400 ft. away. Likewise, there was no sense of reverberant noise from I-70 traffic passing under the overpass. No differences were discernible between the sound being heard at the community microphones at the Barrier and No-Barrier sites. I-270, Grove City, Ohio (Location OH-3, Sound-Absorbing Barrier) As shown in Figure 32, the I-270 location is a six-lane freeway. The barrier is approximately 2,000 ft. long and 10 ft. high, and is offset 12–20 ft. from the edge of the nearest travel lane. The barrier is a precast concrete post-and-panel design that uses a proprietary porous, free-draining con- crete composed of a specially processed wood fiber aggregate blended with a Portland cement binder on the highway side. Microphone Name Side of Road (NB, SB) Distance from Edge of Nearest Travel Lane (ft.) Height Above Roadway Plane (ft.) BarRef01 NB 25 10 (12 ft. above ground, near midpoint of barrier) NoBarRef02 NB 25 10 (11.5 ft. above ground) BarCom03 SB 50 10 (9 ft. above ground) BarCom04 SB 100 20 (19 ft. above ground) NoBarCom05 SB 50 10 (10 ft. above ground) NoBarCom06 SB 100 20 (19 ft. above ground) Table 13. I-75—microphone positions.

44 Source: Research team, NCHRP Project 25-44 Figure 26. I-75—BarRef01 (top left), NoBarRef02 (top right), BarCom03 (middle left), BarCom04 and meteorological station (middle right), NoBarCom05 (bottom left), and NoBarCom06 (bottom right).

Figure 27. I-75—cross-sections at Barrier site (top) and No-Barrier site (bottom). Source: Research team, NCHRP Project 25-44 Figure 28. I-70—No-Barrier view (top left), Barrier view (top right), roadway close-up (bottom left), and highway-side Barrier close-up (bottom right).

46 Source: Satellite image © 2016 Google Earth Figure 29. I-70—microphone positions. Microphone Name Side of Road (EB, WB) Distance from Edge of Nearest Travel Lane (ft.) Height Above Roadway Plane (ft.) BarRef01 WB 40 11 (11 ft. above ground, near midpoint of barrier) NoBarRef02 WB 40 11 (12 ft. above ground) BarCom03 EB 75 11 (5 ft. above ground) BarCom04 EB 75 16 (10 ft. above ground) NoBarCom05 EB 75 11 (5 ft. above ground) NoBarCom06 EB 75 16 (10 ft. above ground) Table 14. I-70—microphone positions.

47 Source: Research team, NCHRP Project 25-44 Figure 30. I-70—BarRef01 (top left), NoBarRef02 (top right), BarCom03 and BarCom04 (middle left), NoBarCom05 and NoBarCom06 (middle right), and the meteorological station with NoBarRef02 (bottom).

48 Exact sound-absorption test results for this barrier were not available; however, results for a similar panel from the same company had the sound-absorption coefficients shown in Table 15. Figure 33 shows a site map of the measurement posi- tions. No microphone was placed at BarRef01 given the barrier’s location at the edge of the pavement and concerns that reflected noise off the truck bodies would bias the data, making meaningful comparisons to the NoBarRef02 data difficult. Table 16 gives each microphone’s position, distance to the edge of the nearest travel lane, and height above roadway grade. Figure 34 depicts each microphone at its measurement position. Figure 35 shows cross-sections at the Barrier and No-Barrier sites. The cross-sections are similar at the BarCom03 and NoBarCom05 locations adjacent to I-270, but they are dif- ferent at the BarCom04 and NoBarCom06 positions farther from I-270. At these more distant positions, higher micro- phone elevations were chosen to minimize possible ground Figure 31. I-70—cross-sections at Barrier site (top) and No-Barrier site (bottom). effects differences between the two sites. At BarCom04, the microphone was located 19 ft. above the ground and 15 ft. above the road, while at NoBarCom06, the microphone was located 9 ft. above the ground while also being 15 ft. above the road. The median was grassy at both sites, with no concrete median barrier. The end of a row of one-story houses parallel to I-270 was located approximately 40 ft. behind NoBarCom06. The data suggest that reflected noise off the houses in this row raised the NoBarCom06 levels, although no note of reflected noise was made during the field review as part of the location selec- tion process or during the measurements. Measurements were conducted on Friday, November 18, 2016, from approximately 00:00 (midnight) to 04:00. A goal of the late-night measurements was to obtain data on indi- vidual vehicle pass-bys in addition to the 5-minute averages. Results on individual pass-bys are presented in the section on spectrograms. Truck traffic seemed to dominate the road late at night. A surge in traffic was apparent just after 02:00 and traffic

49 Source: Research team, NCHRP Project 25-44 Figure 32. I-270—No-Barrier view (top left), Barrier view (top right), roadway close-up (bottom left), and highway-side Barrier close-up (bottom right). Octave Band Center Frequency (Hz) Sound-Absorption Coefficient 125 0.30 250 0.50 500 1.09 1,000 0.84 2,000 0.90 4,000 0.88 NRC 0.85 Table 15. Sound-absorption coefficients and NRC for a barrier like the I-270 barrier. remained steady after 03:30, yielding few or no potential pass- bys during those periods. As observed from the BarCom04 position during the data collection, traffic volumes were rela- tively low, with several candidate single-vehicle pass-by events occurring during any 30-minute measurement period. Heavy trucks typically were audible for 10 to 15 seconds to the east if other vehicles were not nearby. At BarCom04, the sound of the heavy trucks (and sometimes the autos) passed through the tree line along the property line boundary to the east. No dif- ferences could be discerned in the characteristics of the noise at the NoBarCom06 position compared to the BarCom04 position. Winds were calm throughout the data collection. Skies were clear, and temperatures ranged in the low to mid- dle 40s (°F). No insect sounds or wind-in-trees sounds were audible during the measurement period.

50 Source: Satellite image © 2016 Google Earth Figure 33. I-270—microphone positions. Microphone Name Side of Road (EB, WB) Distance from Edge of Nearest Travel Lane (ft.) Height Above Roadway Plane (ft.) BarRef01 n/a n/a n/a NoBarRef02 WB 30 (set back farther than barrier offset) 5 (5.5 ft. above ground) BarCom03 EB 30 5 (7 ft. above ground) BarCom04 EB 110 15 (19 ft. above ground) NoBarCom05 EB 30 5 (7 ft. above ground) NoBarCom06 EB 110 15 (9 ft. above ground) Table 16. I-270—microphone positions.

51 Source: Research team, NCHRP Project 25-44 Figure 34. I-270—NoBarRef02 (top), BarCom03 (middle left), BarCom04 and meteorological station (middle right), NoBarCom05 (bottom left), and NoBarCom06 (bottom right).

52 Figure 35. I-270—cross-sections at Barrier site (top) and No-Barrier site (bottom).

53 Findings: Sound-Reflecting Barriers At the sound-reflecting barrier locations, data were collected under several meteorological classes. Table 17 shows the num- ber of equivalent 5-minute period groupings by site by meteo- rological class. A grouping represents three or more 5-minute periods with equivalent source data (based on the reference microphone level and the average speed by direction of travel) and meteorological class. Reference will be made to the meteo- rological class as appropriate in the discussion of the findings. Based on the data collected and analysis procedures described above, the findings are outlined individually below. Sound-Reflecting Barrier Finding 1: Measured Broadband Equivalent Sound Levels Measured broadband unweighted sound pressure levels and A-weighted sound levels are generally higher at the Barrier microphones than at the No-Barrier microphones. As a first step in analyzing the data, the running Leq (5 min.) for each microphone pair at Barrier and No-Barrier sites were graphed and level difference plots were developed. These graphs and analysis give an overall picture of the measured levels in terms of unweighted and A-weighted sound levels. The graphs reflect the data before any grouping of the 5-minute periods by source and meteorological class equivalence. As an example, the BarRef01 and NoBarRef02 levels at I-24 are shown in Figure 36 (unweighted). Then, Figure 37 shows the differences in both the unweighted and A-weighted levels for this microphone pair. A positive value means the Barrier level was higher than the No-Barrier level. Appendix E pre- sents similar graphs for all microphone pairs at all locations. Table 18 presents the approximate range of differences in the Barrier and No-Barrier running Leq (5 min.) for these pairs. These ranges are termed approximate because the data displayed are before any attempt to group the 5-minute periods into equivalent periods based on source and meteorological class. Because the MD-5 data were collected in two separate periods, ranges for both daytime and nighttime are shown for MD-5. Level differences are shown for both unweighted and A-weighted data. In general, the ranges in the differences were greater for the unweighted levels than for the A-weighted levels, probably because the sampling included one-third octave bands ranging from 20 Hz to 10 kHz. Greater variation in levels often occurs in the very low and high bands (e.g., from sources such as heavy trucks or insects), which would affect a broad- band unweighted level but would be filtered out of broadband A-weighted level calculations. Each location is discussed below. I-24 For virtually all the running Leq (5 min.) periods, the unweighted and A-weighted BarRef01 levels were higher than the NoBarRef02 levels by 0.5 to 1.5 dB. These higher levels were expected because the BarRef01 microphone was located halfway between the barrier and I-24. For most of the running Leq (5 min.) periods at the com- munity lower microphones, the unweighted and A-weighted BarCom03 levels were higher than the NoBarCom05 levels by 0.0 to 1.0 dB, with some differences as much as 1.5 dB. For most of the running Leq (5 min.) periods at the upper microphones, the unweighted and A-weighted BarCom04 levels were higher than the NoBarCom06 levels by 0.0 to 0.5 dB, with some differences as much as 1.0 dB. For the other periods, the A-weighted levels at NoBarCom06 are 0 dB to 0.5 dB higher than the BarCom04 levels. SR-155 (Briley Parkway) In general, the running Leq (5 min.) decreased over time as the traffic decreased from the evening rush hour into the later evening. At the lower microphones, the unweighted sound pressure levels at BarCom03 typically were higher than at C H A P T E R 5

54 Meteorological Class Number of Equivalent Groupings I-24 SR-155 (Briley Pkwy.) I-90 SR-71 MD-5 Upwind Lapse 31 -- -- -- -- Calm Lapse 13 4 -- -- -- Calm Neutral 4 4 4 -- 10 Calm Inversion -- 31 -- -- 15 Downwind Lapse -- -- 12 -- 15 Downwind Neutral -- -- -- 6 7 Total 48 39 16 6 47 Table 17. Number of equivalent groupings, by location and meteorological class. 83 84 85 86 87 88 13 :1 3 13 :2 3 13 :3 3 13 :4 3 13 :5 3 14 :0 3 14 :1 3 14 :2 3 14 :3 3 14 :4 3 14 :5 3 15 :0 3 15 :1 3 15 :2 3 15 :3 3 15 :4 3 15 :5 3 16 :0 3 16 :1 3 16 :2 3 16 :3 3 16 :4 3 16 :5 3 17 :0 3 17 :1 3 So un d Pr es su re L ev el , d BZ Time BarRef01 NoBarRef02 Figure 36. I-24—running Leq (5 min.), unweighted sound pressure levels (dBZ), BarRef01 and NoBarRef02. -3 -2 -1 0 1 2 3 13 :1 3 13 :2 3 13 :3 3 13 :4 3 13 :5 3 14 :0 3 14 :1 3 14 :2 3 14 :3 3 14 :4 3 14 :5 3 15 :0 3 15 :1 3 15 :2 3 15 :3 3 15 :4 3 15 :5 3 16 :0 3 16 :1 3 16 :2 3 16 :3 3 16 :4 3 16 :5 3 17 :0 3 17 :1 3 D iff er en ce in L ev el , d B Time dBA dBZ Figure 37. I-24—differences in A-weighted and unweighted levels (dB), running Leq (5 min.), BarRef01 minus NoBarRef02.