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

Chapter: Chapter 5 - Findings: Sound-Reflecting Barriers

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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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Suggested Citation:"Chapter 5 - Findings: Sound-Reflecting Barriers." National Academies of Sciences, Engineering, and Medicine. 2018. Field Evaluation of Reflected Noise from a Single Noise Barrier. Washington, DC: The National Academies Press. doi: 10.17226/25297.
×
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55 NoBarCom05 by measures from a few tenths of 1 dB to just over 2 dB; however, the A-weighted sound levels at BarCom03 generally were 1.5 dB to 2 dB lower than the NoBarCom05 levels during the first 3 hours of the measurement and 2 dB to 3 dB lower during the last hour. For the upper microphones, the results were different. The differences in the unweighted sound pressure levels at BarCom04 and NoBarCom06 varied from positive to negative over most of the measurement period and became generally negative (NoBarCom06 higher than BarCom04) later in the evening. The A-weighted sound levels at NoBarCom06 tended to be slightly higher than at BarCom04 during the early part of the measurement, with the difference increasing as the mea- surement period moved later into the evening. Insect and frog noise from trees near NoBarCom05 and NoBarCom06 became major sound contributors starting early in the evening. I-90 For the reference microphones, both the unweighted and A-weighted running Leq (5 min.) at BarRef01 were on the order of 0 dB to 0.5 dB above the NoBarRef02 levels for the first 2 hours of measurement (13:00 to 15:00). For the second half of the measurement period (15:00 to 17:20), this difference increased to a range of 0.5 dB to 1.0 dB. The BarRef01 micro- phone was located atop the barrier, and the barrier was just off the shoulder. It is speculated that the slightly higher levels at BarRef01 could be due to multiple sound reflections occur- ring. These sound waves reflected first off the barrier and then off the sides of the vehicles (e.g., tractor-trailer bodies), then reached the microphone. At the lower community microphones, for all the running Leq (5 min.) periods, the BarCom03 unweighted sound pres- sure levels were on the order of 0 dB to 1.5 dB higher than the NoBarCom05 levels. For the A-weighted sound levels, the BarCom03 levels were on the order of 0.4 dB to 1.3 dB higher than the NoBarCom05 levels. At the upper and slightly more distant microphones, for most of the running Leq (5 min.) periods, the BarCom04 unweighted levels ranged from 0.7 dB lower than NoBarCom06 to 1.5 dB higher. The A-weighted levels ranged from 0.2 dB to 1 dB higher. At all the microphones, the Leq (5 min.) dropped slowly as time passed, on the order of 1 dB to 2 dB over the 4-hour period. During the same time period, the differences in measured sound levels between the Barrier and No-Barrier microphone pairs increased on the order of 0.5 dB. SR-71 For the reference microphones, the unweighted running Leq (5 min.) at BarRef01 were on the order of 0 dB to 1.8 dB higher than the NoBarRef02 levels, averaging roughly 1 dB higher. The A-weighted levels at BarRef01 were on the order of 0 dB to 1 dB higher than NoBarRef02, averaging roughly 0.5 dB. Higher levels at BarRef01 were expected because the microphone was positioned between the barrier and the road. For the microphones just off the shoulder on the opposite side from the barrier, little evidence of reflection was seen in these broadband data. The unweighted running Leq (5 min.) at BarCom03 ranged from 0.9 dB lower to 1 dB higher than those at NoBarCom05. The A-weighted running Leq (5 min.) at BarCom03 ranged from 0.7 dB lower to 0.5 dB higher than those at NoBarCom05. With these microphones so close to the far lanes of traffic, relative to the distance from BarCom03 to the barrier, little increase in level due to reflections was expected. At the distant community microphones, for virtually all the running Leq (5 min.) periods, the unweighted and A-weighted Microphone Pair Type of Level Approximate Range of Level Differences by Location (dB) I-24 SR-155 (Briley Pkwy.) I-90 SR-71 MD-5 (Day) MD-5 (Night) BarRef01 minus NoBarRef02 Unweighted 0.5 to 1.5 n/a 0 to 1 0 to 1.8 -1 to 1.8 -2 to 2.2 A-weighted 0.5 to 1.5 n/a 0 to 1 0 to 1 -0.5 to 0.5 -1.5 to 0.7 BarCom03 minus NoBarCom05 Unweighted 0 to 1 0 to 2 0 to 1.5 -0.9 to 1 -1 to 2 -2 to 1.5 A-weighted 0 to 1 -3 to 1 0.4 to 1.3 -0.7 to 0.5 0.5 to 2.4 0 to 1.5 BarCom04 minus NoBarCom06 Unweighted 0 to 0.5 -1 to 1 -0.7 to 1.5 0 to 4 -0.5 to 1.8 -1 to 2 A-weighted -0.5 to 0.5 -1.5 to 0.5 0.2 to 1 1 to 3.9 -0.5 to 1 -0.5 to 1 Table 18. Approximate range of differences in the Barrier and No-Barrier running 5- minute Leq for all sound-reflecting barrier locations (Barrier minus No-Barrier).

56 BarCom04 levels were higher than the NoBarCom06 levels. The unweighted levels ranged mostly from 0 dB to 4 dB higher than the levels at NoBarCom06. The A-weighted lev- els ranged from 1 dB to nearly 4 dB higher than the levels at NoBarCom06. For both the unweighted and A-weighted levels, the average difference was 2.1 dB higher at BarCom04. MD-5 For the reference microphones, the running Leq (5 min.) at BarRef01 and NoBarRef02 are roughly comparable. Unweighted levels at BarRef01 ranged mostly from 2 dB below NoBarRef02 levels to 2.2 dB above them. A-weighted levels were within ±0.5 dB of each other during the afternoon session. Because of frog noise near NoBarRef02, however, its evening A-weighted levels generally were higher than the BarRef01 levels. The research team expected little difference in the levels because the BarRef01 microphone was positioned atop the barrier, although reflections off the vehicle bodies might increase its levels, as was discussed for the I-90 location. For the lower community microphones opposite the bar- rier, the daytime unweighted running Leq (5 min.) at Bar- Com03 ranged from 1.0 dB lower to 2 dB higher than those at NoBarCom05. The daytime A-weighted levels at BarCom03 generally ranged from 0.5 dB to 2.4 dB higher than those at NoBarCom05. In the evening, the unweighted levels at the two microphones were roughly within –2 dB to 1.5 dB of each other. The BarCom03 A-weighted levels ranged mostly from 0 dB to 1.5 dB higher than the NoBarCom05 levels. For most of the running Leq (5 min.) periods during both the afternoon and evening, the BarCom04 levels were higher than the NoBarCom06 levels. The unweighted Leq (5 min.) generally ranged from 0.5 dB lower to 1.5 dB higher dur- ing the day and –1 dB lower to 2 dB higher during the eve- ning. The A-weighted levels ranged from 0.5 dB lower to 1 dB higher than the NoBarCom06 levels during both day- time and nighttime. Sound-Reflecting Barrier Finding 2: Frequency-Specific Differences in Barrier and No-Barrier Levels The differences in the levels at the Barrier microphones and the No-Barrier microphones are frequency specific and vary by location and site. Clear examples exist of enhanced levels opposite the Barrier compared to the corresponding No-Barrier position. To see the differences by frequency band more clearly, graphs were developed that show the differences in Leq (5 min.) between comparable microphones for an average of all the equivalent 5-minute periods in a particular meteorological class, with their error bars. The error bars are ± one standard deviation for each average value. Each graph shows the averages of the average level differ- ences for the A-weighted sound level, the unweighted sound pressure level, and the one-third octave band sound pressure levels from 20 Hz to 10 kHz. Across the one-third octave band frequencies, the trends are generally similar in these individual groups of equivalent periods, with some differences likely related to background noise and the uniqueness of vehicle noise sources in the 5-minute periods in each group. Figure 38 shows a sample plot of sound pressure level spec- tra for the MD-5 BarCom03 and NoBarCom05 microphones. Figure 39 shows the frequency-based average level difference graph for all the equivalent groups in the Calm Inversion mete- orological class. Higher levels exist at BarCom03 in the bands from 200 Hz to 500 Hz (with the maximum difference, 5 dB higher, occurring at 250 Hz and 315 Hz) and in the 800 Hz to 2.5 kHz bands (with differences ranging from 0.5 dB to 1 dB higher). The 4-kHz band is 6 dB higher at NoBarCom05 than at BarCom03 due to localized frog noise. The large barrier effect in the 200 Hz to 500 Hz bands seen in Figure 39 is evidence of a loss of some of the ground effects in these bands, which is indicated by the “dip” in the No-Barrier spectrum in Figure 40. More is said on this effect in Sound-Reflecting Barrier Finding 8. A complete location-by-location presentation of all the results is provided in Appendix E. Sound-Reflecting Barrier Finding 3: Effects Between the Barrier and the Road Sound levels are higher and spectral content changes at a position between the barrier and the road, compared to the No-Barrier site, as evidenced at I-24 and SR-71. At I-24 and SR-71, the BarRef01 microphones were set midway between the roadway and the noise barrier. As was described in Chapter 3, the SR-71 barrier is 13 ft. tall, con- sisting of a 7-ft. concrete block wall atop a 6-ft.-high berm and located 50 ft. from the center of the near travel lane; the BarRef01 microphone was set 25 ft. from the center of the near travel lane and 10 ft. above the roadway plane. At I-24, the bar- rier is 19 ft. tall and is located approximately 96 ft. from the center of the near travel lane. The BarRef01 microphone was set 51 ft. from the center of the near travel lane (33 ft. from the edge of the shoulder) and 45 ft. in front of the barrier at a height of 10 ft. above the roadway plane. At both locations, the microphones were in the path of reflected sound.

57 25 30 35 40 45 50 55 60 65 70 75 dB A d BZ 2 0 2 5 3 1. 5 4 0 5 0 6 3 8 0 1 00 1 25 1 60 2 00 2 50 3 15 4 00 5 00 6 30 8 00 1 k 1. 25 k 1. 6k 2k 2. 5k 3. 15 k 4k 5k 6. 3k 8k 10 k So un d Pr es su re L ev el , d BZ 1/3 Octave Band Frequency, Hz BarCom03 NoBarCom05 Figure 38. MD-5—sample sound pressure level (dBZ) spectra, BarCom03 and NoBarCom05, Calm Inversion Group CIG-3-4 at 23:15, Leq (5 min.). -10 -8 -6 -4 -2 0 2 4 6 8 10 dB A d BZ 2 0 2 5 3 1. 5 4 0 5 0 6 3 8 0 1 00 1 25 1 60 2 00 2 50 3 15 4 00 5 00 6 30 8 00 1 k 1. 25 k 1. 6k 2k 2. 5k 3. 15 k 4k 5k 6. 3k 8k 10 k D iff er en ce in L ev el , d B 1/3 Octave Band Frequency, Hz All CIG Groups Figure 39. MD-5—averages of the differences in A-weighted and unweighted levels (dB), Leq (5 min.) é one standard deviation, BarCom03 minus NoBarCom05, for all Calm Inversion groups.

58 -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 41. I-24—differences in A-weighted and unweighted levels (dB), running Leq (5 min.), BarRef01 minus NoBarRef02. 78 79 80 81 82 83 84 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 Le ve l, dB A Time BarRef01 NoBarRef02 Figure 40. I-24—running Leq (5 min.), A-weighted sound level (dBA), BarRef01 and NoBarRef02. Figure 40 shows the ungrouped running 5-minute A-weighted Leq data for BarRef01 and NoBarRef02 at I-24. (See Figure 36, in a previous section, for the unweighted Leq data for the same periods at I-24.) Figure 41 plots the differences between the A-weighted and unweighted levels for the same periods at I-24. Figure 42 plots the differences for the SR-71 reference microphones’ running 5-minute Leq data. At both the I-24 and SR-71 locations, the BarRef01 A-weighted and unweighted levels are higher than at NoBarRef02 by 0.5 dB to 1.5 dB. Figure 43 and Figure 44 show typical sound pressure level spectra, respectively, for I-24 (an Upwind Lapse 5-min. period) and SR-71 (a Downwind Neutral 5-min. period). The broadband A-weighted and unweighted levels appear on the left side of each graph. Both sets of spectra show that the Barrier and No-Barrier differences are frequency specific. These spectral differences are better seen by plotting the differences in the one-third octave bands. Figure 45 plots the I-24 Barrier/No-Barrier differences averaged over all of the Upwind Lapse meteorological class data groupings. In general, the BarRef01 levels are roughly 0.9 dB to 1.3 dB higher than the NoBarRef02 levels across the entire spectrum. At 25 Hz, the difference is 2 dB; at 200 Hz and 250 Hz, it is approximately 0.5 dB. Similarly for SR-71, Figure 46 plots the averaged differences between one-third octave bands for the Downwind Neutral meteorological class. The BarRef01 levels are higher than the NoBarRef02 levels across virtually the entire spectrum, except for 8 kHz and 10 kHz. The BarRef01 levels are higher by 3 dB at 31.5 Hz, 2.5 dB at 125 Hz, and 1.5 dB at 2.5 kHz. In the range from 400 Hz to 1.25 kHz, the differences are less than 0.5 dB.

59 -3 -2 -1 0 1 2 3 9: 00 9: 10 9: 20 9: 30 9: 40 9: 50 10 :0 0 10 :1 0 10 :2 0 10 :3 0 10 :4 0 10 :5 0 11 :0 0 11 :1 0 11 :2 0 11 :3 0 11 :4 0 11 :5 0 12 :0 0 12 :1 0 12 :2 0 12 :3 0 12 :4 0 12 :5 0 13 :0 0 13 :1 0 13 :2 0 D iff er en ce in L ev el , d B Time dBA dBZ Figure 42. SR-71—differences in A-weighted and unweighted levels (dB), running Leq (5 min.), BarRef01 minus NoBarRef02. 40 45 50 55 60 65 70 75 80 85 90 dB A d BZ 2 0 2 5 3 1. 5 4 0 5 0 6 3 8 0 1 00 1 25 1 60 2 00 2 50 3 15 4 00 5 00 6 30 8 00 1k 1. 25 k 1. 6k 2k 2. 5k 3. 15 k 4k 5k 6. 3k 8k 10 k So un d Pr es su re L ev el , d BZ 1/3 Octave Band Frequency, Hz BarRef01 NoBarRef02 Figure 43. I-24—sample sound pressure level (dBZ) spectra, BarRef01 and NoBarRef02, Upwind Lapse group ULG-3-2, 13:26-13:31, Leq (5 min.).

60 40 45 50 55 60 65 70 75 80 85 90 dB A d BZ 2 0 2 5 3 1. 5 4 0 5 0 6 3 8 0 1 00 1 25 1 60 2 00 2 50 3 15 4 00 5 00 6 30 8 00 1k 1. 25 k 1. 6k 2k 2. 5k 3. 15 k 4k 5k 6. 3k 8k 10 k So un d Pr es su re L ev el , d BZ 1/3 Octave Band Frequency, Hz BarRef01 NoBarRef02 Figure 44. SR-71—sample sound pressure level (dBZ) spectra, BarRef01 and NoBarRef02, Downwind Neutral group DNG-3-2, 11:38-11:43, Leq (5 min.). -3 -2 -1 0 1 2 3 4 5 dB A d BZ 2 0 2 5 3 1. 5 4 0 5 0 6 3 8 0 1 00 1 25 1 60 2 00 2 50 3 15 4 00 5 00 6 30 8 00 1k 1. 25 k 1. 6k 2k 2. 5k 3. 15 k 4k 5k 6. 3k 8k 10 k D iff er en ce in L ev el , d B 1/3 Octave Band Frequency, Hz All ULG Groups Figure 45. I-24—averages of the differences in sound pressure levels (dB), Leq (5 min.) é one standard deviation, BarRef01 minus NoBarRef02, for all Upwind Lapse groups.

61 Sound-Reflecting Barrier Finding 4: Background Levels Between the Barrier and the Road The background sound pressure level is elevated in the presence of the noise barrier at the microphone position between the barrier and the road. Evidence exists that background level increased at the BarRef01 position in front of the barrier at both I-24 and SR-71. For I-24, Figure 47 presents the L90 (5 min.) and L99 (5 min.) for BarRef01 and NoBarRef02, in terms of overall A-weighted sound levels and unweighted sound pressure levels. The upper graphs are L90 (A-weighted on the left and unweighted on the right). The lower graphs are L99 (A-weighted on the left and unweighted on the right). Figure 48 presents the differences in L90 (5 min.) and L99 (5 min.) along with Leq (5 min.), com- puted as BarRef01 minus NoBarRef02 for the A-weighted sound levels. The results show that, at I-24, whereas the Leq (5 min.) aver- age about 1 dB higher at BarRef01 than at NoBarRef02, the L90 and L99 at BarRef01 are much higher than at NoBarRef02. -3 -2 -1 0 1 2 3 4 5 dB A d BZ 2 0 2 5 3 1. 5 4 0 5 0 6 3 8 0 1 00 1 25 1 60 2 00 2 50 3 15 4 00 5 00 6 30 8 00 1k 1. 25 k 1. 6k 2k 2. 5k 3. 15 k 4k 5k 6. 3k 8k 10 k D iff er en ce in L ev el , d B 1/3 Octave Band Frequency, Hz All DNG Groups Figure 46. SR-71—averages of the differences in sound pressure levels (dB), Leq (5 min.) é one standard deviation, BarRef01 minus NoBarRef02, for all Downwind Neutral groups. Figure 47. I-24—L90 (5 min.) and L99 (5 min.), BarRef01 and NoBarRef02: A-weighted and unweighted levels.

62 This effect on these two descriptors is evidence of an increase in the background level in front of the barrier that could be attributed to the presence of reflected sound reaching the microphone in addition to the direct sound from the pass- ing vehicles. The results support the idea that the sound generated by a passing vehicle is made up of multiple components (con- sistent with Figure 1 in Chapter 1). For each vehicle, both direct and reflected sounds are sustained through a pattern of rising, peak, and falling levels that incorporate components generated as the vehicle approaches, passes, and recedes. When multiple vehicles are present, the pattern of sustained sound generated by each vehicle overlaps with the patterns generated by other vehicles. This overlap has the effect of elevating the background noise level and reducing the time during which the background level might decrease. The spec- trogram findings (discussed later in this section) also support this hypothesis. For the SR-71 data, Figure 49 presents the differences in L90 (5 min.) and L99 (5 min.) along with Leq (5 min.), computed as -2 -1 0 1 2 3 4 5 6 7 8 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 Le ve l D iff er en ce , d B Time L90 L99 Leq Figure 48. I-24—differences in A-weighted levels (dB), 5-min. L90, L99, and Leq, BarRef01 and NoBarRef02. -6 -4 -2 0 2 4 6 8 10 9: 00 9: 10 9: 20 9: 30 9: 40 9: 50 10 :0 0 10 :1 0 10 :2 0 10 :3 0 10 :4 0 10 :5 0 11 :0 0 11 :1 0 11 :2 0 11 :3 0 11 :4 0 11 :5 0 12 :0 0 12 :1 0 12 :2 0 12 :3 0 12 :4 0 12 :5 0 13 :0 0 13 :1 0 13 :2 0 So un d Le ve l D iff er en ce , d B Time L90 L99 Leq Figure 49. SR-71—differences in A-weighted levels (dB), 5-min. L90, L99, and Leq, BarRef01 and NoBarRef02.

63 BarRef01 minus NoBarRef02 for the A-weighted sound levels. Much like the I-24 data, the results show that whereas the Leq (5 min.) average about 0 dB to 1 dB higher at BarRef01 than at NoBarRef02, the L90 (5 min.) and L99 (5 min.) at BarRef01 are much higher than at NoBarRef02. At BarRef01, the L90 (5 min.) exceeds that at NoBarRef02 by as much as 4 dB, and the L99 (5 min.) by as much as 7 dB. These differences are compel- ling evidence of an increase in the background level in front of the barrier. This increase could be attributed to the pres- ence of reflected sound from the passing vehicles reaching the microphone in addition to the direct sound. The result is a sustained sound that keeps the background level from dropping off during gaps between vehicles. The graphs in Figures 47–49 only show the differences for the broadband A-weighted sound levels and unweighted sound pressure levels. Figure 50 broadens the analysis of the I-24 data to include the individual one-third octave bands using color shading. In Figure 50, the differences in broadband A-weighted sound levels (dBA) and unweighted sound pressure levels (dBZ) are shown in columns at the far left, followed by the one-third octave band differences in columns running from left to right. As detailed in Figure 51, each band’s data col- umn comprises eight smaller, distinct data columns. The first seven data columns show the differences in sound pressure level values for seven statistical descriptors (Ln)—specifically L1, L5, L10, L33, L50, L90, and L99, in order—and the eighth col- umn shows the values for Leq. Figure 50 graphs time vertically (increasing from top to bottom) with each row representing the starting minute of a running 5-minute period. Although the individual rows are too small to be discerned in Figure 50, the color shad- ing allows the relative values to be perceived in the aggregate. Brown shading indicates that the BarRef01 levels exceed the NoBarRef02 levels, whereas blue shading means that NoBarRef02 is higher. Small variations exist in the total num- ber of rows per data column (reflecting data excluded for reasons discussed in the sections on data collection and pro- cessing). Each vertical block represents approximately 4 hours of data collection. Altogether, Figure 50 uses color shading to show more than 57,000 sound pressure level differences. In Figure 50, vertical brown streaks can be seen on the right sides of the data columns, representing L90 (5 min.) and L99 (5 min.), in the frequency bands from 400 Hz through 2 kHz for most of the sample period and through 5 kHz for the first half of the sample period. These brown streaks mean that the BarRef01 background levels are higher than the NoBarRef02 background levels, evidence of a sustaining of a vehicle’s pass-by noise due to the creation of an image source for each vehicle as the sound reflects off the barrier. In contrast, Figure 50. I-24—differences in Ln (5 min.) by one-third octave frequency bands, BarRef01 and NoBarRef02. Figure 51. Order of statistical levels for a single one-third octave band, itself consisting of eight columns of data.

64 Figure 52. SR-71—differences in Ln (5 min.) by one-third octave frequency bands, BarRef01 and NoBarRef02. -3 -2 -1 0 1 2 3 9: 00 9: 10 9: 20 9: 30 9: 40 9: 50 10 :0 0 10 :1 0 10 :2 0 10 :3 0 10 :4 0 10 :5 0 11 :0 0 11 :1 0 11 :2 0 11 :3 0 11 :4 0 11 :5 0 12 :0 0 12 :1 0 12 :2 0 12 :3 0 12 :4 0 12 :5 0 13 :0 0 13 :1 0 13 :2 0 D iff er en ce in L ev el , d B Time dBA dBZ Figure 53. I-90—differences in A-weighted and unweighted levels (dB), running Leq (5 min.), BarRef01 minus NoBarRef02. the vertical blue streaks in the 8-kHz band are evidence of elevated background levels at the NoBarRef02, likely attrib- utable to insect noise in the vegetation behind this position. A similar pattern is seen in Figure 52 for SR-71 between BarRef01 and NoBarRef02. Vertical brown streaks are found on the right sides of the data columns, representing L90 (5 min.) and L99 (5 min.), in the frequency bands from 500 Hz through 4 kHz for nearly all the sample period, and through 5 kHz for the first half of the sample period. These brown streaks mean that the BarRef01 background levels are higher than the NoBarRef02 background levels. The BarRef01 levels are also seen to be higher in the 20 Hz to 31.5 Hz bands across most of the descriptors for most of the measurement period. The reason for that difference in those very-low-frequency bands is not apparent. Sound-Reflecting Barrier Finding 5: Reference Microphone Position Atop the Barrier Even at the reference microphone position atop the barrier, the sound level can be slightly higher than at the equivalent No-Barrier position, as evidenced at I-90. However, little difference was seen at MD-5. The I-90 and MD-5 noise barriers are located close to the edge of the shoulder of the road. The BarRef01 microphones were placed 5 ft. directly above the top of the barriers. Figure 53 shows the differences in the unweighted and A-weighted running Leq (5 min.) between BarRef01 and NoBarRef02 at I-90. In general, both the unweighted

65 sound pressure levels and A-weighted sound levels are higher at the Barrier microphones than at the No-Barrier microphones. For the I-90 reference microphones, both the unweighted and A-weighted running Leq (5 min.) at BarRef01 are on the order of 0 dB to 0.5 dB above the NoBarRef02 levels for the first 2 hours of measurement (13:00 to 15:00). For the second half of the measurements (15:00 to 17:20), this difference increased to a range of 0.5 dB to 1.0 dB. The slightly higher levels at BarRef01 could be due to sound reflections off the barrier and then off the sides of the vehicles, especially heavy truck trailers. The MD-5 reference microphones yielded mixed results. Figure 54 shows the running Leq (5 min.) at BarRef01 and NoBarRef02 to be roughly equal. Unweighted levels at BarRef01 ranged mostly from 1.5 dB below NoBarRef02 levels to 2 dB above. A-weighted levels were within ±0.5 dB of each other during the afternoon session. Due to frog noise near NoBarRef02, however, its evening A-weighted levels generally were about 1 dB higher than the BarRef01 levels. Little difference in the levels was expected because the BarRef01 microphone was positioned atop the barrier, although reflections off the vehicle bodies might increase its levels, as noted above for I-90. Figure 55 presents the Calm Neutral meteorological class spectral difference plot for the I-90 reference microphone. In general, the BarRef01 levels are 0 dB to 1 dB higher than the NoBarRef02 levels at 400 Hz and below. Above 400 Hz through 3.15 kHz, these levels are 0.5 dB to 1 dB higher than the NoBarRef02 levels. Above 4 kHz, the No-Barrier levels are higher, likely due to localized insect noise. These results -3 -2 -1 0 1 2 3 12 :0 0 12 :3 0 13 :0 0 13 :3 0 14 :0 0 14 :3 0 15 :0 0 15 :3 0 16 :0 0 19 :4 9 20 :1 9 20 :4 9 21 :1 9 21 :4 9 22 :1 9 22 :4 9 23 :1 9 D iff er en ce in L ev el , d B Time dBA dBZ Figure 54. MD-5—differences in A-weighted and unweighted levels (dB), running Leq (5 min.), BarRef01 minus NoBarRef02. -3 -2 -1 0 1 2 3 4 dB A d BZ 2 0 2 5 3 1. 5 4 0 5 0 6 3 8 0 1 00 1 25 1 60 2 00 2 50 3 15 4 00 5 00 6 30 8 00 1 k 1. 25 k 1. 6k 2k 2. 5k 3. 15 k 4k 5k 6. 3k 8k 10 k D iff er en ce in L ev el , d B 1/3 Octave Band Frequency, Hz All CNG Groups Figure 55. I-90—averages of the differences in sound pressure levels (dB), Leq (5 min.) é one standard deviation, BarRef01 minus NoBarRef02, for all Calm Neutral groups.

66 are like the reference microphone results at I-24, where that microphone was placed between the barrier and the road. The corresponding plot for the MD-5 reference micro- phones in Figure 56 shows the results for the Downwind Neutral class. In general, the BarRef01 levels vary little com- pared to NoBarRef02 from 500 Hz through 6.3 kHz. Below 500 Hz, the BarRef01 levels generally were less than 1 dB above those at NoBarRef02. The Downwind Neutral time periods were in the afternoon before the high-frequency frog noise began at NoBarRef02. No evidence existed of the sustaining or elevating of the background noise level at BarRef01 for either I-90 or MD-5 based on the difference graphs for L90 (5 min.) and L99 (5 min.), both in terms of the broadband A-weighted levels and the one-third octave bands. The supporting figures and discus- sion are provided in Appendix E. Sound-Reflecting Barrier Finding 6: Effects at Lower Microphones Near the Roadway Edge Near the edge of the road for the lower-height microphones, the BarCom03 levels are roughly the same as the NoBarCom05 levels, being slightly higher in the very-low-frequency bands, as evidenced at SR-71. At the SR-71 location, the BarCom03 and NoBarCom05 microphones were located just off the shoulder of the road, 25 ft. from the center of the near travel lane, at heights of 10 ft. above the roadway surface. Of all the studied locations, these community microphones were the closest to the road on the side opposite the barrier. Figure 57 shows the differences in the broadband unweighted and A-weighted levels for BarCom03 and NoBarCom05 for SR-71. For these broadband measures, little evidence of reflec- tion is seen. The unweighted levels at BarCom03 range from 0.9 dB lower to 1 dB higher than those at NoBarCom05. The A-weighted levels at BarCom03 range from 0.7 dB lower to 0.5 dB higher than those at NoBarCom05. With these micro- phones so close to the far lanes of traffic (relative to the dis- tance from BarCom03 to the barrier), little increase in level due to reflections was expected. Figure 58 shows the averages of the differences in the BarCom03 and NoBarCom05 levels for all the Downwind Neu- tral groups. The levels at BarCom03 are generally higher than or the same as those at NoBarCom05. The levels in the frequency bands from 20 Hz through 125 Hz are 0.5 dB to 1.5 dB higher at BarCom03. From 160 Hz through 1.6 kHz, the levels are different by only 0.5 dB or less. From 2 kHz through 5 kHz, the BarCom03 levels are 0.5 dB higher than NoBarCom05. The NoBarCom05 levels are less than 1.5 dB higher than BarCom03 at and above 6.3 kHz. Sound-Reflecting Barrier Finding 7: Background Levels Near the Edge of the Road for the Lower-Height Microphones Near the edge of the road for the lower-height microphones, some evidence exists of an increase in the background A-weighted sound level on the order of 1 dB to 1.5 dB at BarCom03, as seen at SR-71. Figure 59 presents the differences in L90 (5 min.) and L99 (5 min.) along with Leq (5 min.) for the background A-weighted sound levels, computed as BarCom03 minus NoBarCom05 along SR-71. Some evidence exists of the elevated background level at BarCom03 compared to NoBarCom05 even though these two microphones are close to the edge -3 -2 -1 0 1 2 3 4 dB A d BZ 2 0 2 5 3 1. 5 4 0 5 0 6 3 8 0 1 00 1 25 1 60 2 00 2 50 3 15 4 00 5 00 6 30 8 00 1 k 1. 25 k 1. 6k 2k 2. 5k 3. 15 k 4k 5k 6. 3k 8k 10 k D iff er en ce in L ev el , d B 1/3 Octave Band Frequency, Hz All DNG Groups Figure 56. MD-5—averages of the differences in sound pressure levels (dB), Leq (5 min.) é one standard deviation, BarRef01 minus NoBarRef02, for all Downwind Neutral groups.

67 -3 -2 -1 0 1 2 3 9: 00 9: 10 9: 20 9: 30 9: 40 9: 50 10 :0 0 10 :1 0 10 :2 0 10 :3 0 10 :4 0 10 :5 0 11 :0 0 11 :1 0 11 :2 0 11 :3 0 11 :4 0 11 :5 0 12 :0 0 12 :1 0 12 :2 0 12 :3 0 12 :4 0 12 :5 0 13 :0 0 13 :1 0 13 :2 0 D iff er en ce in L ev el , d B Time dBA dBZ Figure 57. SR-71—differences in A-weighted and unweighted levels (dB), running Leq (5 min.), BarCom03 minus NoBarCom05. -4 -3 -2 -1 0 1 2 3 4 dB A d BZ 2 0 2 5 3 1. 5 4 0 5 0 6 3 8 0 1 00 1 25 1 60 2 00 2 50 3 15 4 00 5 00 6 30 8 00 1 k 1. 25 k 1. 6k 2k 2. 5k 3. 15 k 4k 5k 6. 3k 8k 10 k D iff er en ce in L ev el , d B 1/3 Octave Band Frequency, Hz All DNG Groups Figure 58. Averages of the differences in Leq (5 min.) é one standard deviation (dB), BarCom03 minus NoBarCom05 for all Downwind Neutral groups, SR-71. -4 -3 -2 -1 0 1 2 3 4 5 6 9: 00 9: 10 9: 20 9: 30 9: 40 9: 50 10 :0 0 10 :1 0 10 :2 0 10 :3 0 10 :4 0 10 :5 0 11 :0 0 11 :1 0 11 :2 0 11 :3 0 11 :4 0 11 :5 0 12 :0 0 12 :1 0 12 :2 0 12 :3 0 12 :4 0 12 :5 0 13 :0 0 13 :1 0 13 :2 0 So un d Le ve l D iff er en ce , d B Time L90 L99 Leq Figure 59. SR-71—differences in A-weighted levels (dB), 5-min. L90, L99, and Leq, BarCom03 minus NoBarCom05.

68 of the shoulder for the far-lane traffic across from the bar- rier. The Leq (5 min.) averages about ± 2 dB compared to NoBarCom05; however, the L90 at BarCom03 are, on aver- age, about 1 dB higher than NoBarCom05, and the L99 at BarCom03 average approximately 1.5 dB higher. For both descriptors, there are also many times when the NoBarCom05 levels are higher than the BarCom03 levels. This variation in level differences is likely related to the 50-ft. setback of the barrier from the center of the near lane on the opposite side of the highway and differences in traffic from one data block to the next. Regarding the bar- rier setback, the near-lane traffic would tend to cause the highest levels at the nearby BarCom03 and NoBarCom05 microphones. Next, in expected order of significance, would be the far-lane traffic, then any far-lane reflections, and then any near-lane reflections. Regarding traffic, because some difference exists in distance between the Barrier and No-Barrier sites, the exact same vehicles do not pass each microphone in each 5-minute period. Also, differences in vehicle operation (e.g., speed and lane changes) could exist at the two sites. Nonetheless, it is interesting that, on average, the trend is for the L90 and L99 to be higher at the Barrier microphone. Some members of the field team observed that they could sense the “presence” of the bar- rier at the Barrier microphone. Figure 60 presents the spectral Ln differences for BarCom03 and NoBarCom05. Not much difference is seen between the descriptors for the two microphones, which is consistent with the A-weighted sound level graphs. A slight increase is appar- ent in the L90 and L99 values in the 1 kHz to 3.15 kHz bands, as evidenced by the brown streaks on the right side of the data columns for those bands. Sound-Reflecting Barrier Finding 8: Effects Farther Back from the Road, but Still Within 100 ft. Farther back from the road, but still within 100 ft., the levels at the Barrier microphones are higher than those at the No-Barrier microphones by 0.5 to 1.5 dB. The spectrum is changed even more in some of the frequency bands between 250 Hz and 630 Hz and in some of the bands above 800 Hz, as evidenced at I-24, I-90, and MD-5. At the I-24 location, the BarCom03 and BarCom04 micro- phones were set back 84 ft. from the center of the near travel lane and at heights of 5 ft. and 15 ft. above the roadway plane, with NoBarCom05 and NoBarCom06 at corresponding positions. At the I-90 location, the BarCom03 and NoBarCom05 microphones were set back 69 ft. from the center of the near travel lane and placed at a height of 10 ft. above the roadway plane. BarCom04 and NoBarCom06 were set back 93 ft. from the center of the near travel lane and placed at a height of 17 ft. above the roadway plane. At the MD-5 location, the BarCom03 and BarCom04 microphones were set back 80 ft. from the center of the near travel lane and also placed at heights of 5 ft. and 15 ft. above the roadway plane. Figure 61 shows the differences in the unweighted and A-weighted levels for the lower-height BarCom03 and NoBarCom05 microphones at I-24. For most of the running 5-minute Leq periods, the BarCom03 levels, both unweighted and A-weighted, are higher than the NoBarCom05 levels by 0.0 to 1.0 dB, with some differences as much as 1.5 dB. Figure 60. Differences in Ln (5 min.) by one-third octave frequency bands: SR-71, BarCom03 minus NoBarCom05.

69 -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 61. I-24—differences in A-weighted and unweighted levels (dB), running Leq (5 min.), BarCom03 minus NoBarCom05. -3 -2 -1 0 1 2 3 13 :0 0 13 :1 0 13 :2 0 13 :3 0 13 :4 0 13 :5 0 14 :0 0 14 :1 0 14 :2 0 14 :3 0 14 :4 0 14 :5 0 15 :0 0 15 :1 0 15 :2 0 15 :3 0 15 :4 0 15 :5 0 16 :0 0 16 :1 0 16 :2 0 16 :3 0 16 :4 0 16 :5 0 17 :0 0 17 :1 0 17 :2 0 D iff er en ce in L ev el , d B Time dBA dBZ Figure 62. I-90—differences in A-weighted and unweighted levels (dB), running Leq (5 min.), BarCom03 minus NoBarCom05. For I-90 at BarCom03 and NoBarCom05, Figure 62 shows the differences in the unweighted and A-weighted levels. For all the running 5-minute Leq periods, the BarCom03 unweighted sound pressure levels are on the order of 0 dB to 1.5 dB higher than the NoBarCom05 levels. For the A-weighted sound levels, the BarCom03 levels are on the order of 0.4 dB to 1.3 dB higher than the NoBarCom05 levels. For MD-5, Figure 63 shows the differences in the unweighted and A-weighted levels for the lower-height BarCom03 and NoBarCom05 microphones opposite the barrier. The daytime unweighted levels at BarCom03 ranged from 1.0 dB lower to 2 dB higher than those at NoBarCom05. The A-weighted levels at BarCom03 ranged from 0.5 dB to 1.5 dB higher than those at NoBarCom05. In the evening, the unweighted levels

70 at the two microphones were comparable. The BarCom03 A-weighted levels ranged mostly from 0 dB to 1 dB higher than the NoBarCom05 levels. Figure 64 presents an example of the sound pressure level spectra for BarCom03 and NoBarCom05 (microphones lower and closer to the far side of the road) at I-90 for a Calm Neutral period. The BarCom03 levels are noticeably greater in the 250 Hz to 500 Hz one-third octave bands. The differences in the Barrier and No-Barrier levels can be seen in the following figures. Figure 65 shows the dif- ferences in level between the BarCom03 and NoBarCom05 microphones at I-24 for an average of all the Upwind Lapse groups. Both microphones were placed 5 ft. above the road- way plane. In general, the BarCom03 levels are equal to or slightly greater than the NoBarCom05 levels over most of the frequency range through 4 kHz. The increase is less than 1 dB in the bands from 31.5 Hz to 250 Hz, and on the order of 1 dB to 2 dB in the bands from 315 Hz to 1 kHz. Above 4 kHz, the levels at NoBarCom05 are higher than the levels at BarCom03. That high-frequency difference was caused by insects in the vegetation behind the NoBarCom05 micro- phone that were not present near the BarCom03 site. Figure 66 shows the averages of the averages of the dif- ferences in the BarCom03 and NoBarCom05 levels for the I-90 location for all the measured Calm Neutral groups. The BarCom03 and NoBarCom05 microphones were both located 69 ft. from the center of the near travel lane and 10.4 ft. above the roadway surface. The levels in the fre- quency bands from 20 Hz through 80 Hz were 0.5 dB to 1 dB higher at BarCom03. For 1 kHz and higher, the BarCom04 levels were approximately 1 dB to 2 dB higher than those at NoBarCom05. The most noticeable differences were in the 250 Hz to 500 Hz bands, where the levels ranged between 2.5 dB and 5 dB higher, peaking at 400 Hz. The next location with microphones at two heights is MD-5. All the groupings of 5-minute periods that were judged equivalent for traffic parameters at the MD-5 loca- tion fell into four meteorological classes: Downwind Neutral and Downwind Lapse (daytime) and Calm Neutral and Calm Inversion (evening). The potential difference by meteoro- logical class is addressed under Sound-Reflecting Barrier Finding 14 in this chapter. Typical sound pressure level spectra are shown in Figure 67 for BarCom03 and NoBarCom05 for one of the 5-minute periods in the Calm Inversion meteoro- logical class. The increase in levels in the mid- and higher-range frequencies is like that observed at the I-24 and I-90 locations. Figure 68 shows the level difference averages for all the Calm Inversion groups at the MD-5 location. The results for early-evening Calm Neutral class evening periods are similar. The graph shows the higher BarCom03 levels in the bands from 200 Hz to 500 Hz (with the maximum at 5 dB higher at 250 Hz and 315 Hz). At 4 kHz, the level at NoBarCom-5 is 6 dB higher than at BarCom03 due to loud, localized frog noise. A possible explanation for the barrier effect being promi- nent in the low-frequency range (250 Hz to 500 Hz) for BarCom03 at I-90 (Figure 66) and MD-5 (Figure 68) is that direct and reflected sound take different propagation paths. At both the Barrier and No-Barrier sites, the direct sound likely experienced ground effects and wave interference that caused a dip in sound level in that frequency range, as is seen in the sample spectra in Figure 64 and Figure 67. The reflected sound at the Barrier site experienced a different propagation path than the direct sound, with different ground effects and -4 -3 -2 -1 0 1 2 3 4 12 :0 0 12 :3 0 13 :0 0 13 :3 0 14 :0 0 14 :3 0 15 :0 0 15 :3 0 16 :0 0 19 :4 9 20 :1 9 20 :4 9 21 :1 9 21 :4 9 22 :1 9 22 :4 9 23 :1 9 D iff er en ce in L ev el , d B Time dBA dBZ Figure 63. MD-5—differences in A-weighted and unweighted levels (dB), running Leq (5 min.), BarCom03 minus NoBarCom05.

71 35 40 45 50 55 60 65 70 75 80 85 dB A d BZ 2 0 2 5 3 1. 5 4 0 5 0 6 3 8 0 1 00 1 25 1 60 2 00 2 50 3 15 4 00 5 00 6 30 8 00 1 k 1. 25 k 1. 6k 2k 2. 5k 3. 15 k 4k 5k 6. 3k 8k 10 k So un d Pr es su re L ev el , d BZ 1/3 Octave Band Frequency, Hz BarCom03 NoBarCom05 Figure 64. I-90—sample sound pressure level (dBZ) spectra, BarCom03 and NoBarCom05, Calm Neutral class, CNG-1-1, Period 15:37, Leq (5 min.). -4 -3 -2 -1 0 1 2 3 4 5 6 dB A d BZ 2 0 2 5 3 1. 5 4 0 5 0 6 3 8 0 1 00 1 25 1 60 2 00 2 50 3 15 4 00 5 00 6 30 8 00 1 k 1. 25 k 1. 6k 2k 2. 5k 3. 15 k 4k 5k 6. 3k 8k 10 k D iff er en ce in L ev el , d B 1/3 Octave Band Frequency, Hz All ULG Groups Figure 65. I-24—averages of the differences in Leq (5 min.) é one standard deviation (dB) for all Upwind Lapse groups, BarCom03 minus NoBarCom05.

72 -4 -3 -2 -1 0 1 2 3 4 5 6 dB A d BZ 2 0 2 5 3 1. 5 4 0 5 0 6 3 8 0 1 00 1 25 1 60 2 00 2 50 3 15 4 00 5 00 6 30 8 00 1 k 1. 25 k 1. 6k 2k 2. 5k 3. 15 k 4k 5k 6. 3k 8k 10 k D iff er en ce in L ev el , d B 1/3 Octave Band Frequency, Hz All CNG Groups Figure 66. I-90—averages of the differences in Leq (5 min.) é one standard deviation (dB) for all Calm Neutral groups, BarCom03 minus NoBarCom05. 25 30 35 40 45 50 55 60 65 70 75 dB A d BZ 2 0 2 5 3 1. 5 4 0 5 0 6 3 8 0 1 00 1 25 1 60 2 00 2 50 3 15 4 00 5 00 6 30 8 00 1 k 1. 25 k 1. 6k 2k 2. 5k 3. 15 k 4k 5k 6. 3k 8k 10 k So un d Pr es su re L ev el , d BZ 1/3 Octave Band Frequency, Hz BarCom03 NoBarCom05 Figure 67. MD-5—sample sound pressure level (dBZ) spectra, BarCom03 and NoBarCom05, Calm Inversion Group CIG-3-4, at 23:15; Leq (5 min.).

73 wave interference such that a corresponding dip in the 250 Hz to 500 Hz range could be nonexistent or diminished. As a result, the barrier effect would be pronounced in the 250 Hz to 500 Hz range. Sound-Reflecting Barrier Finding 9: Background Levels Farther Back from the Road but Still Within 100 ft. Farther back from the road, but still within 100 ft., the background levels increase in the bands from 630 Hz through 3.15 kHz, as evidenced at I-90 and MD-5, but not at I-24. For I-24, Figure 69 presents the differences in L90 (5 min.) and L99 (5 min.) along with Leq (5 min.) for the A-weighted sound levels, computed as BarCom03 minus NoBarCom05. Little evidence exists for an elevated background noise level at BarCom03 compared to at BarRef01, which is not too unexpected given the dominance of the direct sound from the nearby vehicles. A possible factor contributing to this result is the large offset distance of the barrier from the edge of the road. For I-90, Figure 70 presents these same differences. In this case, strong evidence exists of an elevated background noise level at BarCom03. The Leq (5 min.) at BarCom03 average about 0.5 dB to 1 dB higher than the Leq (5 min.) at NoBarCom05; however, the L90 (5 min.) at BarCom03 range from 1 dB to 2 dB higher and the L99 (5 min.) at BarCom03 range from 1 dB to 4 dB higher. For MD-5, Figure 71 presents the same differences, com- puted as BarCom03 minus NoBarCom05. Evidence exists of the elevated background level at BarCom03 during the daytime hours (on the left). Although the Leq (5 min.) average -8 -6 -4 -2 0 2 4 6 8 dB A d BZ 2 0 2 5 3 1. 5 4 0 5 0 6 3 8 0 1 00 1 25 1 60 2 00 2 50 3 15 4 00 5 00 6 30 8 00 1 k 1. 25 k 1. 6k 2k 2. 5k 3. 15 k 4k 5k 6. 3k 8k 10 k D iff er en ce in L ev el , d B 1/3 Octave Band Frequency, Hz All CIG Groups Figure 68. MD-5—Averages of the differences in sound pressure level (dB), Leq (5 min.) é one standard deviation, BarCom03 minus NoBarCom05, for all Calm Inversion groups. -6 -4 -2 0 2 4 6 8 10 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 Le ve l D iff er en ce , d B Time L90 L99 Leq Figure 69. I-24—differences in A-weighted levels (dB), 5-min. L90, L99, and Leq, BarCom03 and NoBarCom05.

74 -6 -4 -2 0 2 4 6 8 10 13 :0 0 13 :1 0 13 :2 0 13 :3 0 13 :4 0 13 :5 0 14 :0 0 14 :1 0 14 :2 0 14 :3 0 14 :4 0 14 :5 0 15 :0 0 15 :1 0 15 :2 0 15 :3 0 15 :4 0 15 :5 0 16 :0 0 16 :1 0 16 :2 0 16 :3 0 16 :4 0 16 :5 0 17 :0 0 So un d Le ve l D iff er en ce , d B Time L90 L99 Leq Figure 70. I-90—differences in A-weighted levels (dB), 5-min. L90, L99, and Leq, BarCom04 and NoBarCom06. -6 -4 -2 0 2 4 6 8 10 12 :0 0 12 :3 0 13 :0 0 13 :3 0 14 :0 0 14 :3 0 15 :0 0 15 :3 0 16 :0 0 19 :4 9 20 :1 9 20 :4 9 21 :1 9 21 :4 9 22 :1 9 22 :4 9 23 :1 9 So un d Le ve l D iff er en ce , d B Time L90 L99 Leq Figure 71. MD-5—differences in A-weighted levels (dB), 5-min. L90, L99, and Leq, BarCom03 and NoBarCom05. about 1 dB higher than at NoBarCom05, the L90 (5 min.) at BarCom03 range from 2 dB lower to 8 dB higher than at NoBarCom05, averaging approximately 3 dB higher. Almost all these daytime L90 (5 min.) are higher at BarCom03 than at NoBarCom05, evidence of an increase in the back- ground level due to reflected sound off the barrier. As was noted under “Data Collection and Analysis,” the daytime recordings from MD-5 were free from contamination. Con- sequently, none of these levels were edited for contamina- ting sounds. One possible reason for the increase in background lev- els at the I-90 and MD-5 Barrier microphones—and not at the I-24 Barrier microphone—is that the barriers at I-90 and MD-5 sit just at the edge of the shoulder, whereas the barrier at I-24 is set back nearly 100 ft. The pass-by signal is more likely to be sustained by the reflected sound at the close-in barriers, which elevates the background level. In the evening at the MD-5 location, the clear trend was for the L90 (5 min.) and L99 (5 min.) at NoBarCom05 to grow larger relative to BarCom03 as time continued to pass. This

75 trend is a clear result of the increased level and constant nature of the frog and insect noise. Figure 72 illustrates the one-third octave band Ln dif- ferences for BarCom03 and NoBarCom05 at I-90. Similar graphs for the other locations are provided in Appendix E. The brown color in the 250 Hz to 500 Hz bands indicates an increase in all the Ln descriptors and means the Barrier levels are higher than the No-Barrier levels. The blue color means the No-Barrier levels are higher. The vertical brown streaks on the right sides of the data columns in the frequency bands from 630 Hz through 3.15 kHz indicate that the BarCom03 background levels are higher than the NoBarCom05 background levels. Sound-Reflecting Barrier Finding 10: Increase in Broadband and Spectral Levels at 400 ft. from the Road At 400 ft. from the road at the SR-71 location, the BarCom04 broadband levels are typically 1 dB to 4 dB higher than the levels at the No-Barrier site. Spectral levels are 2 dB to 4 dB higher than the No-Barrier levels, except for those likely affected by ground effects in the 100 Hz to 250 Hz bands. Figure 73 shows the differences in the unweighted and A-weighted levels for the BarCom04 and NoBarCom06 Figure 72. I-90—differences in Ln (5 min.) by one-third octave frequency bands: BarCom03 and NoBarCom05. -1 0 1 2 3 4 5 6 9: 00 9: 10 9: 20 9: 30 9: 40 9: 50 10 :0 0 10 :1 0 10 :2 0 10 :3 0 10 :4 0 10 :5 0 11 :0 0 11 :1 0 11 :2 0 11 :3 0 11 :4 0 11 :5 0 12 :0 0 12 :1 0 12 :2 0 12 :3 0 12 :4 0 12 :5 0 13 :0 0 13 :1 0 13 :2 0 D iff er en ce in L ev el , d B Time dBA dBZ Figure 73. SR-71—differences in A-weighted and unweighted levels (dB), running Leq (5 min.), BarCom04 minus NoBarCom06.

76 microphone pair at SR-71. Positioned 400 ft. from the road, these microphones were the most distant in the study. For nearly all the running 5-minute Leq periods, the unweighted and A-weighted BarCom04 levels are higher than the NoBarCom06 levels. The unweighted BarCom04 levels range from 0 dB to 5.5 dB higher than the NoBarCom06 levels, and the A-weighted levels range from 1 dB to 3.7 dB higher. For both unweighted and A-weighted cases, the average difference was 2.1 dB higher at BarCom04. At the SR-71 location, all the groupings of 5-minute peri- ods that were judged equivalent for the reference Leq and for average speeds fell into one meteorological class: Downwind Neutral. Figure 74 presents the sound pressure level spectra for BarCom04 and NoBarCom06. The BarCom04 levels are higher in all one-third octave bands except 100 Hz to 200 Hz, where the levels at NoBarCom06 are higher. Terrain differences between the two sites could have affected results below 500 Hz. Differences below 500 Hz likely can be attributed to a combi- nation of terrain differences and barrier effects. A simplified FHWA TNM 2.5 analysis showed that, for some of the frequen- cies below 500 Hz, the BarCom04 sound levels should be lower due to ground effects. More information is provided with the spectrogram results for Site SR-71 in Appendix E. Figure 75 shows the averages of the differences in the dis- tant BarCom04 and NoBarCom06 levels for all Downwind Neutral groups. The levels in the frequency bands from 20 Hz through 80 Hz were 2 dB to 4 dB higher at BarCom04 com- pared to those at NoBarCom06. From 315 Hz through 8 kHz, the BarCom04 levels ranged from 1.5 dB to 3 dB higher than those at NoBarCom06. From 100 Hz through 250 Hz, the lev- els at NoBarCom06 ranged from 0 dB to 3 dB higher than the levels at BarCom04, peaking at 200 Hz. Sound-Reflecting Barrier Finding 11: Ln Descriptors at 400 ft. from the Road At 400 ft. from the road at the SR-71 location, all the Ln descriptors were higher at the BarCom04 site, not just the background levels. For the SR-71 location, Figure 76 presents the differences in L90 (5 min.) and L99 (5 min.), along with Leq (5 min.) for 25 30 35 40 45 50 55 60 65 70 75 dB A d BZ 2 0 2 5 3 1. 5 4 0 5 0 6 3 8 0 1 00 1 25 1 60 2 00 2 50 3 15 4 00 5 00 6 30 8 00 1k 1. 25 k 1. 6k 2k 2. 5k 3. 15 k 4k 5k 6. 3k 8k 10 k So un d Pr es su re L ev el , d BZ 1/3 Octave Band Frequency, Hz BarCom04 NoBarCom06 Figure 74. SR-71—sample sound pressure level (dBZ) spectra, BarCom04 and NoBarCom06, Downwind Neutral group DNG-3-2, 11:38-11:43, Leq (5 min.).

77 the A-weighted sound levels, computed as BarCom04 minus NoBarCom06. The figure starts 45 minutes into the 4-hour measurement period because audible roofing nail gun noise at NoBarCom06 contaminated the measured levels during this period; that data was deleted from this Ln analysis. The results for this microphone pair differ from those for most of the other microphone pairs across the various study locations because these microphones were located at the farthest distance from the road. During the first 23 minutes shown on Figure 76, the BarCom04 Leq (5 min.) ranged from 2.5 dB to 3.8 dB higher than that for NoBarCom06. During this time, the meteorological class was Calm Neutral and the L90 (5 min.) and L99 (5 min.) differences ranged from 2 dB to 5 dB higher at BarCom04 than at NoBarCom06. During the last 3 hours, the Leq (5 min.) differences became more variable, ranging from 0.5 dB to 2.5 dB higher at BarCom04. During this period, L90 (5 min.) differences also became more variable, ranging from 0 dB to 3.5 dB higher at BarCom04. The L99 (5 min.) became even more variable, with the BarCom04 values ranging from 1 dB lower than those at NoBarCom06 to 5.4 dB higher. During this time period, the meteorological class was Downwind Neutral. On aver- age, and over the full measurement period, the BarCom04 Leq (5 min.), L90 (5 min.), and L99 (5 min.) were 1.7 dB, 2.0 dB, and 2.1 dB higher than at NoBarCom06, respectively. Taken together, these results suggest that the overall levels from the traffic noise are higher at the Barrier site. Because the traffic is 400 ft. away, however, there is less overall rise and fall to the levels compared to the patterns observed at other locations (and based on measurements taken closer to the road). Consequently, at SR-71, there was little chance for lulls in the noise under the studied traffic flows. It is possible that -8 -6 -4 -2 0 2 4 6 8 dB A d BZ 2 0 2 5 3 1. 5 4 0 5 0 6 3 8 0 1 00 1 25 1 60 2 00 2 50 3 15 4 00 5 00 6 30 8 00 1k 1. 25 k 1. 6k 2k 2. 5k 3. 15 k 4k 5k 6. 3k 8k 10 k D iff er en ce in L ev el , d B 1/3 Octave Band Frequency, Hz All DNG Groups Figure 75. SR-71—averages of the differences in sound pressure levels (dB), Leq (5 min.) é one standard deviation, BarCom04 minus NoBarCom06, for all Downwind Neutral groups. -2 -1 0 1 2 3 4 5 6 9: 45 9: 55 10 :0 5 10 :1 5 10 :2 5 10 :3 5 10 :4 5 10 :5 5 11 :0 5 11 :1 5 11 :2 5 11 :3 5 11 :4 5 11 :5 5 12 :0 5 12 :1 5 12 :2 5 12 :3 5 12 :4 5 12 :5 5 13 :0 5 13 :1 5 13 :2 5 So un d Le ve l D iff er en ce , d B Time L90 L99 Leq Figure 76. SR-71—differences in A-weighted levels (dB), 5-min. L90, L99, and Leq, BarCom04 and NoBarCom06.

78 nighttime measurements taken when the traffic flow is much lower might show background level elevation at a distant site across from a barrier. Figure 77 presents the spectral Ln differences for BarCom04 and NoBarCom06. A pattern can be seen of higher broadband A-weighted levels at BarCom04 in the low and mid-to-upper bands, with higher NoBarCom06 levels in the 100 Hz to 250 Hz bands and in the highest-frequency bands. This pattern applies across most of the Ln descriptors, not just L90 (5 min.) and L99 (5 min.). Terrain differences between the Barrier and No-Barrier microphone sites may have affected the results below 500 Hz; readers are asked to refer to the SR-71 spectrogram results in Appendix E for more information. Sound-Reflecting Barrier Finding 12: Higher Microphone Compared to Lower Microphone The increase in sound levels due to reflections decreased by 1 dB to 2 dB going from a lower microphone to a higher microphone, as evidenced at I-24, I-90, and MD-5. At I-24, the BarCom03 and NoBarCom05 microphones were positioned 5 ft. above the roadway plane. The BarCom04 and NoBarCom06 microphones were positioned 15 ft. above the roadway, at the same distance back as the lower micro- phones. Figure 78 shows the differences in measured sound levels between the microphone pairs for an average of all the Upwind Lapse groups. The top graph shows the differences in levels between BarCom03 and NoBarCom05. In general, the BarCom03 levels equal or slightly exceed the No BarCom05 levels over most of the frequency range through 4 kHz. The increase is less than 1 dB in the frequency bands from 31.5 Hz to 250 Hz, and on the order of 1 dB to 2 dB in the bands from 315 Hz to 1 kHz. Above 4 kHz, however, the levels at NoBarCom05 are higher than the levels at BarCom03. This difference was caused by insects in the vegetation behind the NoBarCom05 microphone that were not present near the BarCom03 site. The lower graph compares the levels at BarCom04 and NoBarCom06. The results show that the BarCom04 levels in the frequency bands from 20 Hz through 1.25 kHz equal or slightly exceed the levels at NoBarCom06. At 31.5 Hz to 63 Hz, the levels at BarCom04 are approximately 1 dB higher than those at NoBarCom06. At 1.6 kHz and above, the NoBarCom06 levels are higher than the BarCom04 levels, ranging from a fraction of 1 dB at 1.6 kHz to 2.5 dB in the 6.3 kHz band. The higher levels at NoBarCom06 in the high- frequency bands also can be attributed to insect noise in some vegetation behind this microphone. For I-90, Figure 79 shows the averages of the differences in the Barrier and No-Barrier microphone levels for all the Calm Neutral groups. The upper graph compares the levels at BarCom03 and NoBarCom05 (the lower-height micro- phones), both of which were 69 ft. from the center of the near travel lane and 10.4 ft. above the roadway surface. The lev- els in the frequency bands from 20 Hz through 80 Hz were 0.5 dB to 1 dB higher at BarCom03. For frequency bands at 1 kHz and higher, the BarCom04 levels were approximately 1 dB to 2 dB higher than those at NoBarCom05. The most noticeable differences were in the 250 Hz to 500 Hz bands, where the levels ranged from 2.5 dB to 5 dB higher, peaking at 400 Hz. The lower graph compares the levels at BarCom04 and NoBarCom06, both of which were 93 ft. from the center of the near lane and 17 ft. above the roadway surface. The lev- els in the frequency bands from 31.5 Hz through 250 Hz were 0.5 dB to 1 dB higher at BarCom04. From 1.25 kHz to 3.15 kHz, the BarCom04 levels were approximately 0.5 dB higher. The most noticeable differences were in the 315 Hz to 630 Hz bands, where the levels ranged from 1.5 dB to 3 dB higher, peaking at 400 Hz. Figure 77. SR-71—differences in Ln (5 min.) by one-third octave frequency bands, BarCom04 and NoBarCom06.

79 Similar results were seen at MD-5, where the lower micro- phones were positioned 5 ft. above the roadway plane. Graphs for those data are provided in Appendix E, as well as graphs and details of the Ln analysis at I-24, I-90, and MD-5. Although this idea requires further consideration, one pos- sible conclusion that can be drawn from this finding is that barrier reflection effects may be more pronounced closer to the ground due to sound-reducing propagation effects. Closer to the ground, shielding from median barriers/vehicles and ground effects can reduce sound levels at various frequencies. For receivers higher above the ground, the noise- reducing propagation effects are decreased, so the barrier- reflected noise may be partially masked or diminished. For the lower microphone with a barrier present, both the direct and reflected sound propagation path can be affected by these sound-reducing propagation effects. For the ground effect, however, there are differences in the ground type affect- ing the direct and barrier-reflected paths. For the direct path, the sound that reaches the receiver is affected by both the road surface (not sound-absorbing) and the ground adjacent to the road (more sound-absorbing than pavement), particu- larly when the microphone is not immediately adjacent to the shoulder. For the barrier-reflected path, the sound that reaches the receiver is primarily affected by the road surface. Particu- larly at frequencies absorbed by the ground adjacent to the road, the effect of the barrier could be quite pronounced. For the tall microphone, on the other hand, both the direct and barrier-reflected propagation paths are elevated, whereas the effects of shielding and ground interactions are reduced, which reduces the spectral difference between the direct and reflected sound. Sound-Reflecting Barrier Finding 13: No Effect of Traffic Volume No effect on the sound level differences was seen as a function of traffic volume, as evidenced at all microphone pairs. For all sites and among all groups within a meteorological class, little correlation was found between changes in traf- fic volume and changes in the differences in Leq (5 min.). Although the Calm Inversion group showed a roughly 300% change in factored hourly traffic volume across all the equiv- alent groups, meaningful conclusions about correlations between traffic volumes and the differences in Leq (5 min.) could not be established. -4 -3 -2 -1 0 1 2 3 4 D iff er en ce in L ev el , d B 1/3 Octave Band Frequency, Hz dB A d BZ 2 0 2 5 3 1. 5 4 0 5 0 6 3 8 0 1 00 1 25 1 60 2 00 2 50 3 15 4 00 5 00 6 30 8 00 1k 1. 25 k 1. 6k 2k 2. 5k 3. 15 k 4k 5k 6. 3k 8k 10 k All ULG Groups -4 -3 -2 -1 0 1 2 3 4 D iff er en ce in L ev el , d B 1/3 Octave Band Frequency, Hz dB A d BZ 2 0 2 5 3 1. 5 4 0 5 0 6 3 8 0 1 00 1 25 1 60 2 00 2 50 3 15 4 00 5 00 6 30 8 00 1k 1. 25 k 1. 6k 2k 2. 5k 3. 15 k 4k 5k 6. 3k 8k 10 k All ULG Groups Figure 78. I-24—averages of the differences in sound pressure levels (dB), Leq (5 min.) é one standard deviation, BarCom03 and NoBarCom05 (top) and BarCom04 and NoBarCom06 (bottom), for all Upwind Lapse groups.

80 In general, the range in speeds for each vehicle speed class also was too small to address any relationship between vehicle speeds and sound level differences. The ranges in volumes and speeds in the studied 5-minute equivalent periods are shown in Table 19. Sound-Reflecting Barrier Finding 14: Effects of Meteorological Classes Slight differences exist in the sound level differences for different meteorological classes; however, there are no clear trends, as evidenced at I-90, I-24, and MD-5. Data collected at greater distances from the road might tell more. As was shown in Table 17, data were collected under a fair range of meteorological classes across all the locations studied. Examining the results, it became apparent that too many differences existed across the locations for meaningful comparison of meteorological class results from one location to another. Comparison of difference meteorological class results at the same location has more validity. The following comparisons were made: • I-90—Calm, Neutral, and Downwind Lapse; • I-24—Upwind Lapse, Calm Lapse, and Calm Neutral; and • MD-5—Downwind Lapse, Downwind Neutral, Calm Neutral, and Calm Inversion. The results are summarized below, followed by more detailed discussions and supporting figures. • At the I-90 location, the Calm Neutral differences were slightly greater than the Downwind Lapse differences at the community microphones; • At the I-24 location in the lower frequency bands, the Upwind Lapse average differences tend to be both slightly less and slightly greater than the Calm Lapse average dif- ferences by a few tenths of 1 dB; • At the I-24 location in the higher frequency bands (500 Hz to 4 kHz), the Upwind Lapse average differences tended to -4 -3 -2 -1 0 1 2 3 4 5 6 dB A d BZ 2 0 2 5 3 1. 5 4 0 5 0 6 3 8 0 1 00 1 25 1 60 2 00 2 50 3 15 4 00 5 00 6 30 8 00 1k 1. 25 k 1. 6k 2k 2. 5k 3. 15 k 4k 5k 6. 3k 8k 10 k D iff er en ce in L ev el , d B 1/3 Octave Band Frequency, Hz All CNG Groups -4 -3 -2 -1 0 1 2 3 4 5 6 dB A d BZ 2 0 2 5 3 1. 5 4 0 5 0 6 3 8 0 1 00 1 25 1 60 2 00 2 50 3 15 4 00 5 00 6 30 8 00 1k 1. 25 k 1. 6k 2k 2. 5k 3. 15 k 4k 5k 6. 3k 8k 10 k D iff er en ce in L ev el , d B 1/3 Octave Band Frequency, Hz All CNG Groups Figure 79. I-90—averages of the differences in sound pressure levels (dB) for all Calm Neutral groups, Leq (5 min.) é one standard deviation, BarCom03 and NoBarCom05 (top) and BarCom04 and NoBarCom06 (bottom).

81 be a few tenths of 1 dB greater than the Calm Lapse average differences; and • At the MD-5 location (ignoring the frog noise at 4 kHz at the No-Barrier microphones), for the lower community microphones (BarCom03 and NoBarCom05), the Calm Neutral differences were as follows: – 1 dB to 1.5 dB greater than all three of the other classes at 125 Hz; – 0.5 dB to 1.0 dB less than all three of the other classes at 200 Hz; – About 1 dB greater than the two Downwind classes at 250 Hz; – 1 dB to 1.5 dB less than the two Downwind classes at 400 Hz through 630 Hz; and – About 0.5 dB less than the two Downwind classes at 1 kHz through 3.15 kHz. • At the MD-5 location, for the upper community micro- phones (BarCom04 and NoBarCom06), the Calm Neutral differences were as follows: – 1 dB to 2.5 dB greater than all three of other classes at 125 Hz; – 0.5 dB to 1.0 dB less than all three of the other classes at 200 Hz; – About 1 dB less than the Calm Inversion class at 63 Hz and 100 Hz; – One-half decibel or less different compared to all three other meteorological classes at the rest of the frequency bands. At the SR-71 location, all of the equivalent-period data fell in a single meteorological class (Downwind Neutral) but some data also were collected during Calm Neutral conditions earlier in the measurement. Examin- ing these data, the broadband A-weighted data for SR-71 (shown in Figure 76) indicates that the differences in mea- sured sound levels between BarCom04 and NoBarCom06 were 1.5 dB to 2 dB greater during Calm Neutral condi- tions than during Downwind Neutral conditions (see Figure 76). I-90 Calm Neutral and Downwind Lapse Comparison Figure 80 compares the differences in sound levels for the Downwind Lapse and Calm Neutral classes for BarCom03 versus NoBarCom05 and BarCom04 versus NoBarCom06 at I-90. The data values are the average Calm Neutral differ- ences minus the average Downwind Lapse differences for each frequency band. The data show that the Calm Neu- tral average differences tend to be slightly greater than the Downwind Lapse average differences across the frequency spectrum. For both microphone pairs, the Calm Neutral differences are greater than the Downwind Lapse differ- ences by 1 dB or less up though 2.5 kHz with one exception, at 25 Hz, where the difference is 2 dB). At 3.15 kHz and above, the Calm Neutral differences range from 0.5 dB to 2.0 dB higher. Location Range of Two-Way Factored Hourly Volume * (vph) Range in Average Speeds (mph) I-24 5,700 to 8,212 67 to 72 for Upwind Lapse groups 68 to 72 for Calm Lapse groups 69 to 71 for Calm Neutral groups I-90 4,779 to 5,488 66 to 71 for Downwind Lapse groups 68 to 70 for Calm Neutral groups SR-71 3,628 to 3,764 66 to 76 for Downwind Neutral groups MD-5 400 to 2,936 58 to 63 for Downwind Lapse groups 58 to 64 for Downwind Neutral groups 58 to 63 for Calm Neutral groups 58 to 64 for Downwind Neutral groups * Total two-way volume averaged across the periods in that grouping and factored up to 1 hour. Table 19. Ranges in volumes and speeds in the studied 5-minute equivalent periods.

82 -3 -2 -1 0 1 2 3 D iff er en ce in L ev el , d B 1/3 Octave Band Frequency, Hz dB A d BZ 2 0 2 5 3 1. 5 4 0 5 0 6 3 8 0 1 00 1 25 1 60 2 00 2 50 3 15 4 00 5 00 6 30 8 00 1k 1. 25 k 1. 6k 2k 2. 5k 3. 15 k 4k 5k 6. 3k 8k 10 k CNG - DLG -3 -2 -1 0 1 2 3 dB A d BZ 2 0 2 5 3 1. 5 4 0 5 0 6 3 8 0 1 00 1 25 1 60 2 00 2 50 3 15 4 00 5 00 6 30 8 00 1k 1. 25 k 1. 6k 2k 2. 5k 3. 15 k 4k 5k 6. 3k 8k 10 k D iff er en ce in L ev el , d B 1/3 Octave Band Frequency, Hz CNG - DLG Figure 80. I-90—differences in average sound pressure levels (dB), Calm Neutral average differences minus Downwind Lapse average differences, Leq (5 min.), BarCom03 minus NoBarCom05 (top) and BarCom04 minus NoBarCom06 (bottom). I-24 Upwind Lapse, Calm Lapse, and Calm Neutral Comparisons Figure 81 compares the differences in levels for the Upwind Lapse and Calm Lapse classes for the four community micro- phone positions at the I-24 location. The data values are the average Upwind Lapse differences minus the average Calm Lapse differences for each frequency band. MD-5 Downwind Lapse, Downwind Neutral, Calm Neutral, and Calm Inversion Comparison For the MD-5 data, the results are shown by microphone pair. The data for the Downwind cases are from the afternoon measurement session and the data for the Calm cases are from the evening session. In Figure 82 and Figure 83, the top graph shows the differences in the Calm Neutral and Down- wind Lapse average difference; the middle graph compares Calm Neutral to Downwind Neutral; and the bottom graph compares Calm Neutral to Calm Inversion. Figure 82 shows the results for the lower microphones (BarCom03 minus NoBarCom05). Figure 83 shows the results for the higher microphones (BarCom04 minus NoBarCom06). Sound-Reflecting Barrier Finding 15: Spectrograms Show Increased and Sustained Sound Levels Barrier reflections cause sound levels to increase over a broad range of frequencies and cause higher sound levels to be sustained for a longer period of time, as evidenced by the spectrogram results at all locations. In this study, spectrograms (spectral time histories) were generated for pass-by events involving individual vehicles or groups of vehicles as well as for samples of highway traffic noise at each measurement site. This section provides examples of the spectrograms, which compare data collected at the Barrier sites with data collected at an equivalent position at the No-Barrier sites to visualize the effects of barrier-reflected noise. Examples of the vehicle pass-by events are shown first, followed by example 5-minute data blocks of highway traf- fic noise. Comparing the Barrier to the No-Barrier sites, the spectrogram data reveal that: • The hot spots (highest sound levels) get hotter (sound lev- els increase) when there is a barrier present.

83 • The hot spots expand (grow taller and wider) when there is a barrier present. The presence of a noise barrier causes sound levels to increase over a broad range of frequencies and causes higher sound levels to be sustained for a longer period of time. These observations apply to vehicles traveling on either side of the road, for a range of distances from the road and heights above the road, and for the vehicle types examined (autos, heavy trucks, and motorcycles). Evidence exists that the bar- rier effect is more pronounced at distances farther from the road. It is assumed that the path-length difference between direct and reflected sound is one of the variables control- ling the strength of the effect seen from barrier reflections. At farther distances, the path-length difference is compara- tively smaller, allowing both the direct and reflected sound to contribute to the overall sound level. With larger path- length differences, as is the case near the highway, the direct sound would be more dominant than the reflected sound and would therefore contribute more to the overall sound level; by comparison, the reflected sound would contribute little (because it must travel farther than the direct sound). Such observations led to the development of the Barrier Reflec- tions Screening Tool, which estimates the barrier reflection effect based on sound propagation path lengths (described further in the documentation for the downloadable Barrier Reflections Screening Tool). The first vehicle pass-by spectrogram example is from the SR-71 site in California (Figure 84). For this example, results are shown for the distant microphone pair, BarCom04 (top plot) and NoBarCom06 (bottom plot), located 400 ft. from the road. The pass-by event is a motorcycle traveling south- bound, adjacent to the community side, going by the Barrier site at approximate event time ∼ 12:10:25 and the No-Barrier site at ∼ 12:10:50. The barrier effect can be seen clearly when comparing the two spectrograms. For the Barrier site, the hot spots are hotter, wider, and taller for a broad range of frequencies. It is particularly noticeable for frequencies from 250 Hz to 2.5 kHz. Given the differences in the terrain over the long distance, a simplified FHWA TNM 2.5 analysis was conducted to determine how the terrain differences would affect the comparison. Based on the FHWA TNM analysis -3 -2 -1 0 1 2 3 dB A d BZ 2 0 2 5 3 1. 5 4 0 5 0 6 3 8 0 1 00 1 25 1 60 2 00 2 50 3 15 4 00 5 00 6 30 8 00 1k 1. 25 k 1. 6k 2k 2. 5k 3. 15 k 4k 5k 6. 3k 8k 10 k D iff er en ce in L ev el , d B 1/3 Octave Band Frequency, Hz ULG minus CLG -3 -2 -1 0 1 2 3 dB A d BZ 2 0 2 5 3 1. 5 4 0 5 0 6 3 8 0 1 00 1 25 1 60 2 00 2 50 3 15 4 00 5 00 6 30 8 00 1k 1. 25 k 1. 6k 2k 2. 5k 3. 15 k 4k 5k 6. 3k 8k 10 k D iff er en ce in L ev el , d B 1/3 Octave Band Frequency, Hz ULG minus CLG Figure 81. I-24—differences in average sound pressure levels (dB), Upwind Lapse average differences and Calm Lapse average differences, Leq (5 min.) é one standard deviation, BarCom03 minus NoBarCom05 (top) and BarCom04 minus NoBarCom06 (bottom).

84 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 dB A d BZ 2 0 2 5 3 1. 5 4 0 5 0 6 3 8 0 1 00 1 25 1 60 2 00 2 50 3 15 4 00 5 00 6 30 8 00 1k 1. 25 k 1. 6k 2k 2. 5k 3. 15 k 4k 5k 6. 3k 8k 10 k D iff er en ce in L ev el , d B 1/3 Octave Band Frequency, Hz CNG - DLG -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 D iff er en ce in L ev el , d B 1/3 Octave Band Frequency, Hz dB A d BZ 2 0 2 5 3 1. 5 4 0 5 0 6 3 8 0 1 00 1 25 1 60 2 00 2 50 3 15 4 00 5 00 6 30 8 00 1k 1. 25 k 1. 6k 2k 2. 5k 3. 15 k 4k 5k 6. 3k 8k 10 k CNG - DNG -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 dB A d BZ 2 0 2 5 3 1. 5 4 0 5 0 6 3 8 0 1 00 1 25 1 60 2 00 2 50 3 15 4 00 5 00 6 30 8 00 1k 1. 25 k 1. 6k 2k 2. 5k 3. 15 k 4k 5k 6. 3k 8k 10 k D iff er en ce in L ev el , d B 1/3 Octave Band Frequency, Hz CNG - CIG Figure 82. MD-5—Differences in average sound pressure levels (dB),Calm Neutral average differences minus Downwind Lapse (top), Downwind Neutral (middle), and Calm Inversion (bottom) average differences, Leq (5 min.), BarCom03 minus NoBarCom05.

85 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 dB A d BZ 2 0 2 5 3 1. 5 4 0 5 0 6 3 8 0 1 00 1 25 1 60 2 00 2 50 3 15 4 00 5 00 6 30 8 00 1k 1. 25 k 1. 6k 2k 2. 5k 3. 15 k 4k 5k 6. 3k 8k 10 k D iff er en ce in L ev el , d B 1/3 Octave Band Frequency, Hz CNG - DLG -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 dB A d BZ 2 0 2 5 3 1. 5 4 0 5 0 6 3 8 0 1 00 1 25 1 60 2 00 2 50 3 15 4 00 5 00 6 30 8 00 1k 1. 25 k 1. 6k 2k 2. 5k 3. 15 k 4k 5k 6. 3k 8k 10 k D iff er en ce in L ev el , d B 1/3 Octave Band Frequency, Hz CNG - DNG CNG - DNG -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 dB A d BZ 2 0 2 5 3 1. 5 4 0 5 0 6 3 8 0 1 00 1 25 1 60 2 00 2 50 3 15 4 00 5 00 6 30 8 00 1k 1. 25 k 1. 6k 2k 2. 5k 3. 15 k 4k 5k 6. 3k 8k 10 k D iff er en ce in L ev el , d B 1/3 Octave Band Frequency, Hz Figure 83. MD-5—Differences in average sound pressure levels (dB), Calm Neutral average differences and Downwind Lapse (top), Downwind Neutral (middle), and Calm Inversion (bottom) average differences, Leq (5 min.), BarCom04 minus NoBarCom06.

86 Figure 84. SR-71—spectrograms for motorcycle on southbound (community) side: Barrier site (BarCom04) at } 12:10:25 (top), No-Barrier site (NoBarCom06) at }12:10:50 (bottom). conclusions, the differences seen from 500 Hz to 2.5 kHz can be attributed to the barrier reflections. Below 500 Hz the dif- ferences may or may not be attributable to the barrier. The second vehicle pass-by spectrogram example is from the MD-5 site in Maryland (Figure 85). For this example, results are shown for the high-microphone pair, BarCom04 (top plot) and NoBarCom06 (bottom plot), located 75 ft. from the road. The pass-by event is a pickup truck traveling south- bound, adjacent to the Barrier side, going by the Barrier site at ∼ 20:09:20 and the No-Barrier site at ∼ 20:09:35. The barrier effect can be seen clearly when comparing the two spectro- grams. The darkest red areas (highest sound levels) fill in more and become wider and taller with the barrier present. The red centers around 800 Hz or 1,000 Hz. The same effect occurs in

87 Figure 85. MD-5—spectrograms for a southbound (Barrier-side) pickup truck: Barrier site (BarCom04) at }20:09:20 (top), No-Barrier site (NoBarCom06) at }20:09:35 (bottom). An additional vehicle follows the heavy truck. the surrounding frequency bands, stepping through various colors of the spectrum. The effect is clarified in Figure 86, in which the highest levels seen in Figure 85 are extracted and overlaid. The trace from the barrier site (in gray) is taller (representing a broader frequency spread) and wider (representing more time duration) than the trace from the No-Barrier site (in red). The intensifying and expanding hot spots indicate that the barrier is causing higher sound levels at frequencies that contribute most to the overall sound level, causing these levels to be sustained for a lon- ger period. The third vehicle pass-by spectrogram example is from the I-90 site in Illinois (Figure 87). For this example, results are shown for the microphone pair closest to the road,

88 BarCom03 (top plot) and NoBarCom05 (bottom plot), located about 52 ft. from the road. The pass-by event is a heavy truck traveling southbound, adjacent to the com- munity side, going by the Barrier site at ∼ 13:29:36 and the No-Barrier site ∼ 13:29:43. The event is identified by Doppler Effect, with a distinct yellow/orange band (around 62 dBA), shifting over time from 160 Hz to 125 Hz. The barrier effect can be seen clearly when comparing the two spectrograms. For a broad range of frequencies, at the Barrier site the hot spots are wider and taller than those for the No-Barrier site. The tallest darkest red band (corresponding to the highest sound level band), centered around 1,000 Hz in the top plot of Figure 87, is both wider and taller than the corresponding band in the bottom plot. The same effect occurs in the sur- rounding frequency bands, stepping through various colors of the spectrum. This difference indicates that the barrier is causing higher sound levels at frequencies that contribute most to the overall sound level and causing these levels to be sustained for a longer period for each vehicle pass-by event. The next two examples show longer periods of traffic noise. Figure 88 shows a 4-minute block of data starting at 9:49 for the SR-71 site in California. For this example, results are shown for the distant microphone pair, BarCom04 (top plot) and NoBarCom06 (bottom plot), located 400 ft. from the road. The spectrograms show a clear difference between the Barrier and No-Barrier sites. As with the pass-by data, the clean data blocks show that hot spots are both wider and taller for a broad range of frequencies, particularly for 500 Hz and up, the range to which barrier effects can be attributed (based on the FHWA TNM 2.5 analysis for terrain differences at this site). In the FHWA Method analysis of the overall A-weighted equivalent sound level, several clean data blocks were examined, and the difference between Barrier and No-Barrier A-weighted equivalent sound levels ranged from 1.3 dB to 3.3 dB. In the spectrogram, the 4-minute block at 9:49 shows the 3.3 dB dif- ference. (The two “blips” in the spectrogram for BarCom04 at ∼ 09:50:10 and ∼ 09:52:30 are the result of vehicles on a side road passing close to the microphone.) Figure 89 shows a 5-minute block of clean data starting at 15:56 for the I-24 site in Tennessee. This figure presents results for two pairs of microphones, in order from top to bottom: BarRef01 (33 ft. from road), NoBarRef02 (33 ft. from road), BarCom04 (66 ft. from road, high microphone), and NoBarCom06 (66 ft. from road, high microphone). The spectrograms provide a clear indication that BarRef01 (reference microphone on the barrier side of the highway but without the barrier present) recorded more occurrences of higher sound levels (shown in dark red) than did BarRef02 (reference microphone on the barrier side of the highway, but without the barrier present). In addition, the higher sound level events are broader in frequency and time. Vehicles trav- eling eastbound (on the Barrier side of the road) dominate the sound levels, and during the 5-minute block, single events can be tracked from the Barrier site to the No-Barrier site about 15 to 20 seconds later. Across the road from the barrier, the high microphones (BarCom04 and NoBarCom06) also indicate that the higher sound level events are broader in fre- quency and time with the barrier present on the opposite side of the highway (as is the case for BarCom04). Vehicles travel- ing westbound (on the community side of the road) domi- nate the sound levels, and single events can be tracked from the No-Barrier microphone site to the Barrier microphone site about 15 to 20 seconds later. The barrier effect trends are less obvious for the microphones located on the community side, but they can be seen by focusing on a series of events and noticing that multiple consecutive events are more blended together in the Barrier case than in the No-Barrier case. As the higher levels (hot spots) broaden, they blend together more. Sound-Reflecting Barrier Finding 16: Psychoacoustic Metrics of UBA, PA, and CSA Combined psychoacoustic metrics of UBA and PA yield similar results, whereas CSA does not yield useful indications. As discussed, a set of combined psychoacoustic metrics indicating annoyance were applied to the audio recordings for each microphone. Three such annoyance metrics were tested. Of these, UBA and PA consistently showed similar 10 seconds Figure 86. MD-5—Overlay of pickup truck pass-by hot spots for levels greater than ∼60 Dba, BarCom04 (hot spot color adjusted to gray/black for comparison) and NoBarCom06 (hot spot represented in orange/red).

89 Figure 87. I-90—spectrograms for a southbound (community-side) heavy truck: Barrier site (BarCom03) at }13:29:36 (top), No-Barrier site (NoBarCom05) at }13:29:43 (bottom).

90 Figure 88. SR-71—spectrograms for 4-minute data block (09:49–09:53), BarCom04 (top), NoBarCom06 (bottom).

91 Figure 89. I-24—spectrograms for 5-minute data block (15:56–16:01) for Calm Lapse group CLG-6-1: BarRef01, NoBarRef02, and high microphones (BarCom04 and NoBarCom06) (top to bottom).

92 results. This result is to be expected, as these metrics derive from similar approaches in psychoacoustics research. Both metrics are dominated by loudness (measuring total energy and accounting for masking) and sharpness (an indicator of high-frequency spectral content). The CSA was ineffective at indicating any differences at all sites and microphone loca- tions. This result is most likely due to the simplicity of the CSA (a simple linear combination of psychoacoustic metrics); it was derived from listening studies based on simplified prod- uct noise. Figure 90 presents the time series and histograms of UBA, PA, and CSA for the upper community microphones. Sound-Reflecting Barrier Finding 17: Annoyance Metrics Annoyance metrics show differences between Barrier and No-Barrier sites at moderate distances, but the results are contra-indicative. The psychoacoustic metrics applied to the audio record- ings do not show a positive correlation with higher annoy- ance at the Barrier sites. The results from microphones located close to the roadway show no statistical difference Figure 90. SR-71—comparing UBA (top), PA (middle), and CSA (bottom) at upper community microphones, BarCom04 and NoBarCom06.

93 between sites. Direct sound from passing vehicles domi- nates the loudness and sharpness of sound at these loca- tions, and the annoyance metrics—which rely primarily on loudness—are similarly dominated by direct sound. When simple descriptive statistics are applied to the UBA and PA time series results, however, cases are found in which the mean values from the Barrier sites differ significantly from those at the No-Barrier sites. The cases for which the mean values of annoyance differ to a statistically significant extent tended to occur in record- ings from the higher microphones, and the effect was more pronounced for those microphones located at moderate distances from the roadway (see Figure 91). At SR-71, the microphones BarCom03 and NoBarCom05 are positioned 5 ft. above and close to the roadway, whereas BarCom04 and NoBarCom06 are positioned 15 ft. above and far from the roadway. Unfortunately, to the extent that the mean val- ues of annoyance showed significant differences, they were contra-indicative: the annoyance metrics at Barrier sites tended to have lower values than those at the No-Barrier sites. An explanation for why this tended to occur was not developed in this work. Sound-Reflecting Barrier Finding 18: Annoyance Metrics as a Function of Traffic Volume Annoyance metrics are less effective in heavy, constant traffic, but show differences in lighter traffic with separated pass-bys. In relation to traffic volume, the annoyance metrics com- puted from the recordings at MD-5 are of particular note. The recordings made in the afternoon, with continu- ous heavy traffic, do not show clear differences between the sites. At night, however, when the sound signals con- sisted mostly of individual vehicle pass-by events, the recordings show further differentiation as the traffic becomes lighter. This effect is demonstrated in Figure 92. These results may indicate that the psychoacoustic met- rics, as applied here, are more applicable to complexi- ties of individual vehicle events than to the general sonic mash of heavy traffic. The annoyance metrics’ dependence on loudness is revealed in the gradual decrease in traffic toward midnight. Figure 91. SR-71—comparing UBA computed for lower community microphones BarCom03 and NoBarCom05 close to the roadway (top), with upper community microphones BarCom04 and NoBarCom06 distant from the roadway (bottom).

94 Figure 92. MD-5—comparing PA for heavy traffic (top) and light, decreasing traffic (bottom).

95 Findings: Sound-Absorbing Barriers Compared to Sound-Reflecting Barriers This chapter summarizes the results of the Phase 2 noise measurements at sound-absorbing barriers. It also compares the results of the Phase 2 measurements to those obtained at the sound-reflecting barriers described in Chapter 5. The detailed results of the sound-absorbing barrier measure- ments and analysis are provided in Appendix B. Site photo- graphs for the Phase 2 measurement locations are provided in Appendix G, which can be downloaded from the NCHRP Research Report 886 webpage. The comparison of results included differences in the following: • Broadband unweighted and A-weighted 5-minute equiva- lent sound pressure levels—Leq (5 min.); • One-third octave band 5-minute equivalent sound pres- sure levels for equivalent meteorological conditions at Barrier and No-Barrier sites; • A-weighted L90 (5 min.) and L99 (5 min.) statistical descrip- tors, compared to Leq (5 min.) at Barrier and No-Barrier sites; • One-third-octave band statistical descriptors, compared to Leq (5 min.) at Barrier and No-Barrier sites; and • Spectrograms for multi-minute blocks of data and spec- trograms, difference spectrograms, and comb-filtering analysis of individual pass-by events. The research team studied five locations with sound- reflecting barriers in Phase 1 and three locations with sound-absorbing barriers in Phase 2. Because of some issues with locations BA3 (SR-155, known as Briley Parkway, in Nashville, Tennessee) and OH-3 (I-270, in Grove City, Ohio), the results for these locations are not included in the com- parisons. Nonetheless, the I-270 results (presented in detail in Appendix B and online Appendix G), are briefly summarized. Reviewing the microphone positions and cross-sectional sketches in Chapter 2 and Chapter 3 may help in under- standing the results. Comparison Finding 1: Differences in Broadband Levels at the Reference Microphones Differences in broadband levels at the reference microphones between the road and the barrier are seen for the sound-absorbing barriers (with NRCs of 0.80). For the sound-reflecting barriers, the measured broadband unweighted sound pressure levels and A-weighted sound lev- els generally were higher at the Barrier microphones than at the No-Barrier microphones. This finding was consistent for the reference microphones located midway between the bar- rier and the road for both the I-24 and SR-71 barriers. To a lesser degree, this finding also held for the I-75 and I-70 sound-absorbing barriers for the reference microphone located midway between the barrier and the road. The results suggest that noise is being reflected off these sound-absorbing barriers. Both barriers are made of the same product, which is specified as having an NRC of 0.80, meaning that each panel is not 100% absorptive. Simple image source modeling with FHWA TNM 2.5 showed that reflections off a barrier with an NRC of 0.80 should have little effect on the total A-weighted sound level at the reference microphone position halfway between the barrier and the road. To review, for the I-24 sound-reflecting barrier, Figure 37 (in Chapter 5) shows the differences in the unweighted and A-weighted running Leq (5 min.) for BarRef01 and NoBar- Ref02. For virtually all the running 5-minute Leq periods, the unweighted and A-weighted BarRef01 levels are higher than the NoBarRef02 levels by a range of 0.5 dB to 1.5 dB. Figure 93 shows similar differences for BarRef01 and NoBarRef02 at the sound-reflecting SR-71 barrier. The unweighted levels at BarRef01 are on the order of 0 dB to 1.8 dB higher than the NoBarRef02 levels, averaging approxi- mately 1 dB higher. The A-weighted levels at BarRef01 are on C H A P T E R 6

96 the order of 0 dB to 1 dB higher than NoBarRef02, averaging approximately 0.5 dB. For the sound-absorbing I-75 barrier location, Figure 94 shows the differences in the unweighted and A-weighted lev- els for BarRef01 and NoBarRef02. For virtually all the run- ning 5-minute Leq periods, the unweighted BarRef01 levels are higher than the unweighted NoBarRef02 levels by a range of 0.4 dB to 2.6 dB, averaging approximately 1.4 dB higher; the A-weighted levels are higher by a range of 0.2 dB to 1.8 dB, averaging approximately 1.4 dB higher. Figure 95 shows the differences in sound levels between microphones BarRef01 and NoBarRef02 for the sound- absorbing barrier at I-70. Over all the running 5-minute Leq periods, the unweighted sound pressure levels at BarRef01 range from 0.1 dB lower than those at NoBarRef02 to 1.5 dB higher, averaging approximately 0.7 dB higher. The A-weighted sound levels at BarRef01 range from 0.2 dB lower than those at NoBarRef02 to 1.0 dB higher, being approximately 0.5 dB higher. The differences are smaller than those found at the I-75 sound-absorbing barrier, but not zero. -3 -2 -1 0 1 2 3 9: 00 9: 10 9: 20 9: 30 9: 40 9: 50 10 :0 0 10 :1 0 10 :2 0 10 :3 0 10 :4 0 10 :5 0 11 :0 0 11 :1 0 11 :2 0 11 :3 0 11 :4 0 11 :5 0 12 :0 0 12 :1 0 12 :2 0 12 :3 0 12 :4 0 12 :5 0 13 :0 0 13 :1 0 13 :2 0 D iff er en ce in L ev el , d B Time dBA dBZ Figure 93. SR-71—differences in A-weighted and unweighted levels (dB), running Leq (5 min.), BarRef01 minus NoBarRef02. -3 -2 -1 0 1 2 3 17 :1 2 17 :2 2 17 :3 2 17 :4 2 17 :5 2 18 :0 2 18 :1 2 18 :2 2 18 :3 2 18 :4 2 18 :5 2 19 :0 2 19 :1 2 19 :2 2 19 :3 2 19 :4 2 19 :5 2 20 :0 2 20 :1 2 20 :2 2 20 :3 2 20 :4 2 20 :5 2 21 :0 2 D iff er en ce in L ev el , d B Time dBA dBZ Figure 94. I-75—differences in A-weighted and unweighted levels (dB), running Leq (5 min.), BarRef01 minus NoBarRef02.

Next: Chapter 6 - Findings: Sound-Absorbing Barriers Compared to Sound-Reflecting Barriers »
Field Evaluation of Reflected Noise from a Single Noise Barrier Get This Book
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TRB's National Cooperative Highway Research Program (NCHRP) Research Report 886: Field Evaluation of Reflected Noise from a Single Noise Barrier analyzes the characteristics of sound reflected from a noise barrier to the opposite side of a highway. State departments of transportation (DOTs) periodically receive complaints from residents about increases in traffic noise that residents believe are the result of noise reflected from a new noise barrier added across the roadway from them. Currently available analytical tools are limited in their ability to evaluate reflected noise and some of the subtle changes in the quality of sound that can occur when it is reflected. Therefore, it is a challenge for DOTs to determine conclusively if complaints about reflected noise are the result of actual or perceived changes in noise characteristics, and to identify locations where absorptive surface treatments could be beneficial.

The study compares reflected noise from sound-reflecting barriers and from barriers with a sound-absorptive surface. It examines both the levels and frequencies of reflected noise to better understand how reflected noise is experienced by communities.

The full report, which includes four detailed appendices, is 27 MB and may take time to download. It is accompanied by several appendices, a tool, and a guide:

A presentation file that summarizes the research also is available on the report project page.

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