Field Evaluation of Reflected Noise from a Single Noise Barrier (2018) / Chapter Skim
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From page 53... ...
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.
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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.
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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)
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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)
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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.
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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 passing vehicles. The results support the idea that the sound generated by a passing vehicle is made up of multiple components (consistent with Figure 1 in Chapter 1)
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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.)
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64 Figure 52. SR-71 -- differences in Ln (5 min.)
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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.)
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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 microphones in Figure 56 shows the results for the Downwind Neutral class.
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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)
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68 of the shoulder for the far-lane traffic across from the barrier.
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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)
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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.
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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.
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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)
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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 differences for BarCom03 and NoBarCom05 at I-90.
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76 microphone pair at SR-71. Positioned 400 ft.
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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.
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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.
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79 Similar results were seen at MD-5, where the lower microphones were positioned 5 ft. above the roadway plane.
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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.
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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)
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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.
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83 • The hot spots expand (grow taller and wider) when there is a barrier present.
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86 Figure 84. SR-71 -- spectrograms for motorcycle on southbound (community)
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87 Figure 85. MD-5 -- spectrograms for a southbound (Barrier-side)
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88 BarCom03 (top plot) and NoBarCom05 (bottom plot)
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89 Figure 87. I-90 -- spectrograms for a southbound (community-side)
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90 Figure 88. SR-71 -- spectrograms for 4-minute data block (09:49–09:53)
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91 Figure 89. I-24 -- spectrograms for 5-minute data block (15:56–16:01)
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92 results. This result is to be expected, as these metrics derive from similar approaches in psychoacoustics research.
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93 between sites. Direct sound from passing vehicles dominates the loudness and sharpness of sound at these locations, and the annoyance metrics -- which rely primarily on loudness -- are similarly dominated by direct sound.
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94 Figure 92. MD-5 -- comparing PA for heavy traffic (top)
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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.
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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 levels for BarRef01 and NoBarRef02.
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