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From page 54... ... 41 C H A P T E R 4 Findings and Applications FHWA Method Findings Data were collected under a fair range of meteorological classes, as shown in Table 11, with the number of equivalent five-minute period groupings by site by meteorological class. A grouping represents three or more five-minute periods with equivalent source data (based on the reference microphone level and the average speed by direction of travel) Read the entire page →
From page 55... ... 42 reason is probably due to the sampling including 1/3 octave bands ranging from 20 Hz to 10 kHz. There tends to be more variation in levels in the very low and high bands -- from sources ranging from heavy trucks on to insects -- which would affect a broadband unweighted level but get filtered out of broadband A-weighted level calculation. Read the entire page →
From page 56... ... 43 Table 12. Approximate range of differences in the Barrier and No Barrier running five-minute Leq for all locations (Barrier minus No Barrier) Read the entire page →
From page 57... ... 44 reflections off the barrier and then off the sides of the vehicles back to the microphone, especially for tractor trailer bodies. At the lower community microphones, for all of the running Leq(5min) Read the entire page →
From page 58... ... 45 weighted levels ranged from 0.5 dB lower to 1 dB higher than NoBarCom06 during both daytime and nighttime. Finding 2: The differences in Barrier and No Barrier levels are frequency-specific and vary by location and site. Read the entire page →
From page 59... ... 46 Figure 26. Sample sound pressure level spectra for BarCom03 and NoBarCom05, MD-5, Calm Inversion Group CIG-3-4, 23:15 (Leq(5min) Read the entire page →
From page 60... ... 47 Figure 27. Averages of the differences in Leq(5min) Read the entire page →
From page 61... ... 48 Figure 28. Running Leq(5min) Read the entire page →
From page 62... ... 49 Figure 30. Differences in running Leq(5min) Read the entire page →
From page 63... ... 50 Figure 31. Sample sound pressure level spectra for BarRef01 and NoBarRef02, I-24, Upwind Lapse group ULG-3-2, 13:26-13:31 (Leq(5min) Read the entire page →
From page 64... ... 51 Figure 32. Sample sound pressure level spectra for BarRef01 and NoBarRef02, SR-71, Downwind Neutral group DNG-3-2, 11:38-11:43 (Leq(5min) Read the entire page →
From page 65... ... 52 Figure 33. Averages of the differences in Leq(5min) Read the entire page →
From page 66... ... 53 The results show that while the Leq(5min) averages about 1 dB higher at BarRef01 than at NoBarRef02, the L90 and L99 at BarRef01 are much higher than at NoBarRef02. Read the entire page →
From page 67... ... 54 Figure 36. Differences in broadband A-weighted 5-min L90, L99 and Leq, I-24, BarRef1 and NoBarRef2. Read the entire page →
From page 68... ... 55 Figure 37. Differences in broadband A-weighted 5-min L90, L99 and Leq, SR-71, BarRef01 and NoBarRef02. Read the entire page →
From page 69... ... 56 Figure 38. I-24 Differences in Ln(5min) Read the entire page →
From page 70... ... 57 Finding 5: Even at the reference microphone position atop the barrier, the 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. Read the entire page →
From page 71... ... 58 Figure 42. Differences in running Leq(5min) Read the entire page →
From page 72... ... 59 Figure 43. Averages of the differences in Leq(5min) Read the entire page →
From page 73... ... 60 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. Read the entire page →
From page 74... ... 61 Figure 46. Averages of the differences in Leq(5min) Read the entire page →
From page 75... ... 62 Figure 47. Differences in broadband A-weighted 5-min L90, L99 and Leq, SR-71, BarCom03 minus NoBarCom05. Read the entire page →
From page 76... ... 63 Finding 8: Farther back from the road, but still within 100 ft, the Barrier levels are higher than No Barrier levels by 0.5 to 1.5 dB and the spectrum is changed by 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 microphones 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. Read the entire page →
From page 77... ... 64 Figure 49. Differences in running Leq(5min) Read the entire page →
From page 78... ... 65 Figure 51. Differences in running Leq(5min) Read the entire page →
From page 79... ... 66 Figure 52. Sample sound pressure level spectra for BarCom03 and NoBarCom05, I-90, Calm Neutral class, CNG-1-1, Period 15:37 (Leq(5min) Read the entire page →
From page 80... ... 67 Figure 53. Averages of the differences in Leq(5min) Read the entire page →
From page 81... ... 68 NoBarCom05 for one of the five-minute periods in the Calm Inversion meteorological class. The increase in levels in the mid-range and higher-range frequencies is similar to that observed for the I-24 and I-90 locations. Read the entire page →
From page 82... ... 69 interference with ground reflections; a dip in the 250 to 500 Hz range could be non-existent or diminished. As a result, the barrier effect would be pronounced in the 250 to 500 Hz range. Read the entire page →
From page 83... ... 70 Figure 57. Differences in A-weighted 5-min L90, L99 and Leq, I-24, BarCom03 and NoBarCom05. Read the entire page →
From page 84... ... 71 Figure 59. Differences in broadband A-weighted 5-min L90, L99 and Leq, MD-5, BarCom03 and NoBarCom05. Read the entire page →
From page 85... ... 72 Finding 10: Back at 400 ft from the road, the BarCom04 levels are typically 1 dB to 4 dB higher than at the No Barrier site, as evidenced at SR-71. Figure 61 shows the differences in the unweighted and A-weighted levels for the BarCom04 and NoBarCom06 microphone pair at SR-71. Read the entire page →
From page 86... ... 73 Figure 62. Sample sound pressure level spectra for BarCom04 and NoBarCom06, SR-71, Downwind Neutral group DNG-3-2, 11:38-11:43 (Leq(5min) Read the entire page →
From page 87... ... 74 Figure 63. Averages of the differences in Leq(5min) Read the entire page →
From page 88... ... 75 Figure 64. Differences in broadband A-weighted 5-min L90, L99 and Leq, SR-71, BarCom04 and NoBarCom06. Read the entire page →
From page 89... ... 76 higher than the levels at BarCom03. The difference was caused by insects in the vegetation behind the NoBarCom05 microphone that were not present near the BarCom03 site. Read the entire page →
From page 90... ... 77 The lower graph compares the levels at BarCom04 and NoBarCom06, both of which were 93 feet from the center of the near lane and 17 feet above the roadway surface. The levels in the frequency bands from 31.5 Hz up through 250 Hz were 0.5 dB to 1 dB higher at BarCom04. Read the entire page →
From page 91... ... 78 higher above the ground, the noise-reducing propagation effects are decreased, so the barrier-reflected noise may be partially masked or diminished. Finding 13: No effect on the sound level differences was seen as a function of traffic volume, as evidenced at all microphone pairs. Read the entire page →
From page 92... ... 79 At the I-90 location, the results show that the Calm Neutral differences were slightly greater than the Downwind Lapse differences at the community microphones. The I-24 data show that the Upwind Lapse average differences tend to be: • Both slightly less and slightly greater than the Calm Lapse average differences in the lower frequency bands, by a few tenths of a decibel; and • A few tenths of a decibel greater than the Calm Lapse average differences in the higher frequency bands (500 Hz to 4 kHz) Read the entire page →
From page 93... ... 80 Figure 68. Calm Neutral minus the Downwind Lapse average differences (Leq(5min) Read the entire page →
From page 94... ... 81 Figure 69. Differences in the Upwind Lapse average differences and the Calm Lapse average differences (Leq(5min) Read the entire page →
From page 95... ... 82 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 Downwind cases are in the afternoon measurement session and the Calm cases are in the evening session. Read the entire page →
From page 96... ... 83 Figure 70. Calm Neutral average differences minus Downwind Lapse, Downwind Neutral and Calm Inversion average differences (Leq(5min) Read the entire page →
From page 97... ... 84 Figure 71. Differences in the Calm Neutral average differences and Downwind Lapse, Downwind Neutral and Calm Inversion average differences (Leq(5min) Read the entire page →
From page 98... ... 85 Spectrogram Findings Finding 15: 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. Spectrograms (spectral time histories) Read the entire page →
From page 99... ... 86 Figure 72. SR-71 spectrograms for motorcycle on southbound (community) Read the entire page →
From page 100... ... 87 The second vehicle passby spectrogram example is from the MD-5 site in Maryland (Figure 73) Read the entire page →
From page 101... ... 88 Figure 73. MD-5 spectrograms for a pickup truck on southbound (barrier) Read the entire page →
From page 102... ... 89 Figure 74. Overlay of MD-5 pickup truck passby hot spots for levels greater than ~60 dBA: BarCom04 (hot spot now represented in gray/black) Read the entire page →
From page 103... ... 90 together in the Barrier case than the No Barrier case. As the higher levels (hot spots) Read the entire page →
From page 105... ... 92 Figure 76. SR-71 spectrograms for 4-minute block of data in the morning at 09:49: top is BarCom04; bottom is NoBarCom06. Read the entire page →
From page 106... ... 93 Figure 77. I-24 five-minute spectrograms; top to bottom: BarRef01, NoBarRef02, high mics (BarCom04 and NoBarCom06) Read the entire page →
From page 107... ... 94 Psychoacoustics Findings Finding 16: Combined psychoacoustic metrics of Unbiased Annoyance and Psychoacoustic Annoyance yield similar results to one another, while Category Scale of Annoyance does not yield useful indications. As discussed above, a set of combined psychoacoustic metrics indicating annoyance were applied to the audio recordings for each microphone. Read the entire page →
From page 108... ... 95 Figure 78. Comparing UBA, PA, and CSA (top to bottom) Read the entire page →
From page 109... ... 96 Figure 79. Comparing UBA computed for lower community microphones, close to the roadway, with upper community microphones, distant from the roadway, at CA SR-71. Read the entire page →
From page 110... ... 97 Figure 80. Comparing PA at MD-5 for heavy traffic (above) Read the entire page →
From page 111... ... 98 In other cases, the receptors may be more distant and not be impacted; nonetheless, the results of this study suggest that such receptors can experience changes in the noise environment due to the introduction of the barrier on the opposite side of the road. While it might be difficult to justify making the barrier sound-absorbing in such a case, the practitioner at least has a better idea of the nature of the phenomenon. Read the entire page →

From page 54...

... 41 C H A P T E R 4 Findings and Applications FHWA Method Findings Data were collected under a fair range of meteorological classes, as shown in Table 11, with the number of equivalent five-minute period groupings by site by meteorological class. A grouping represents three or more five-minute periods with equivalent source data (based on the reference microphone level and the average speed by direction of travel)