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23 Table 9. Metrics for light vehicle SPB versus OBSI relationship. Cross-Plot 25 ft Microphone Distance 50 ft Microphone Distance Metrics SRTT Dunlop SRTT Dunlop Raw Norm Raw Norm Raw Norm Raw Norm Slope 1.31 0.92 1.29 0.93 1.32 0.91 1.32 0.91 r2 0.95 0.88 0.89 0.87 0.84 0.87 0.84 0.89 Offset, dB 21.9 21.8 23.9 23.8 28.5 28.3 30.5 30.3 Std Dev, dB 1.2 0.9 1.4 0.9 1.7 0.9 1.9 0.9 Avg Dev, dB 1.2 0.8 1.2 0.7 1.5 0.7 1.7 0.6 The metrics for the SPB versus CPB cross-plots for the 25-ft and of the increase in the relative contributions from engine/ 50-ft and SRTT and Dunlop data are given in Table 10; the stan- exhaust noise as the speed decreases as noted in the Refer- dard deviations range from 0.7 to 1.0. Since the relationship of ence Energy Mean Emission Levels (REMELs) database (22 ). the OBSI data to the SPB data is linked to the correlation of the This would have the effect of causing higher overall levels at CPB results to the SPB data, the scatter between the normal- the lower speeds than levels due to tires alone resulting in a ized SPB and OBSI data was similar to the scatter reported in decreased slope. Comparing Tables 9 and 11, the offsets Table 8 between the CPB and OBSI data. between the SPB and OBSI data for trucks are 8.9 to 9.3 dB less In applying this approach to heavy trucks, the normalization than for light vehicles indicating that trucks are much louder coefficients developed for the light vehicles were applied than light vehicles on average throughout the data range of the directly in the SPB to OBSI comparison. Arguably, the effect of measurements. Further comparing the standard deviations for site-to-site variations may be different for trucks than light light vehicles and heavy trucks, it is seen that values for trucks vehicles due to differences in effective source height. However, are not much larger than those of light vehicles, typically no as with the light vehicles, normalizing the SPB data produced more than 0.2 dB. This suggests that the SPB levels could be a reduction in the deviations about the 1-to-1 line with mini- estimated from OBSI data measured with either test tire with mal change in offset for each microphone distance and test tire. almost the same confidence for both light vehicles and heavy Cross-plots of the SPB versus OBSI data for the SRTT mea- trucks. sured at 25 ft are shown in Figures 19 and 20 for the raw and normalized data, respectively. Prediction of SPB Data from OBSI Data The metrics for 50 ft and the Dunlop tire are shown in Table 11. Unlike the light vehicle data, normalizing the truck The applicability of OBSI data to assessing the noise per- data did not result in the regression line slope more closely formance of in-service pavement is demonstrated by how well approaching 1. Instead, the slope decreases even more because SPB levels can be predicted from OBSI data. To demonstrate 90 PCC 1 Lt Vehicle Statistical Passby Level, dBA 88 PCC 2 86 PCC 3 AC 4 84 PCC 5 82 AC 6 AC 7 80 AC 8 AC 9 78 PCC 10 76 PCC 11 AC 12 74 1-to-1 Line 72 (27 dB offset) Linear 70 70 72 74 76 78 80 82 84 86 88 90 Controlled Passby Level, dBA Figure 18. Statistical light vehicle passby levels at 25 ft versus controlled vehicle passby level for the SRTT at all test sites and speeds.
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24 Table 10. Metrics for CPB versus light vehicle SPB relationship. Cross-Plot 25 ft Microphone Distance 50 ft Microphone Distance Metrics SRTT Dunlop SRTT Dunlop Slope 0.94 1.02 0.92 0.94 r2 0.93 0.96 0.94 0.96 Offset, dB 2.7 1.0 2.6 1.1 Std Dev, dB 0.9 0.7 1.0 0.8 Avg Dev, dB 0.7 0.6 0.7 0.7 100 PCC 1 98 PCC 2 96 PCC 3 Sound Pressure Level, dBA AC 4 94 PCC 5 AC 6 92 AC 7 90 AC 8 AC 9 88 PCC 10 86 PCC 11 AC 12 84 1-to-1 Line 82 (12.9 dB offset) Linear 80 96 98 100 102 104 106 108 110 112 114 116 OBSI Level, dBA Figure 19. Statistical heavy-truck passby levels at 25 ft versus OBSI level for the SRTT at all test sites and speeds--raw data. 100 PCC 1 98 PCC 2 96 PCC 3 Sound Pressure Level, dBA AC 4 94 PCC 5 AC 6 92 AC 7 90 AC 8 AC 9 88 PCC 10 86 PCC 11 AC 12 84 1-to-1 Line (12.9 dB offset) 82 Linear 80 96 98 100 102 104 106 108 110 112 114 116 OBSI Level, dBA Figure 20. Statistical heavy truck passby levels at 25 ft versus OBSI level for the SRTT at all test sites and speeds--normalized data.
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25 Table 11. Metrics for heavy truck SPB versus OBSI relationship. Cross- 25 ft Microphone Distance 50 ft Microphone Distance Plot SRTT Dunlop SRTT Dunlop Metrics Raw Norm Raw Norm Raw Norm Raw Norm Slope 1.13 0.84 1.14 0.85 1.17 0.76 1.22 0.79 r2 0.82 0.89 0.77 0.85 0.81 0.79 0.78 0.77 Offset, dB 12.9 12.9 14.9 14.9 19.6 19.2 21.6 21.0 Std Dev, dB 1.4 0.9 1.6 1.0 1.6 1.2 1.8 1.2 Avg Dev, dB 1.2 0.7 1.3 0.8 1.3 1.0 1.5 1.1 this applicability in a less abstract manner than cross-plots, the ior is almost identical to that for the light vehicles with an aver- offsets for the 1-to-1 lines from Tables 8 and 10 can be sub- age difference of 0.1 dB and standard deviations of 0.7 and tracted from the OBSI data for each tire. This yields a predicted 0.8 dB for the normalized SPB data and 1.3 and 1.5 dB for the SPB level based on either the SRTT or Dunlop tire at whatever uncorrected SPB data. OBSI test speed is selected. Depending on which offset is For light vehicles at the 50-ft microphone positions (Fig- selected, the SPB levels for light vehicles or trucks at 25 ft or ure 23), the results are similar to the 25-ft results except that a 50 ft can be predicted. Further, the predicted SPB levels can be larger variance occurs between the predicted SPB and the compared to both the raw and normalized SPB levels. This is uncorrected data. The normalized SPB maintains an average illustrated for the primary test speed of 60 mph in Figures 21 difference from the predicted of 0.1 dB with a standard devi- through 24. ation of 0.7 and 0.8 dB. For the uncorrected data, the standard For light vehicle and the 25-ft microphone distance (Fig- deviations are 2.0 and 2.2 dB depending on the tire. This larger ure 21), several features are noted. First, there is virtually no variance of the uncorrected 50-ft data is expected based on the difference whether the predicted SPB levels are generated from average difference in level between the 25-ft and 50-ft micro- the SRTT or Dunlop tires. Second, the normalized (measured) phone. These differences were found to vary as much as 3.6 dB SPB values compare quite well to the predicted levels with an between sites (see Appendix D). average difference of only 0.1 dB and standard deviations of The same trends are seen for the trucks at 50 ft (Figure 24) 0.8 dB. Third, when site-to-site differences are not normalized, with the exception of a larger standard deviation (0.9 to the variation between the predicted and measured SPB levels 1.0 dB) between the predicted and normalized SPB data. varies more with standard deviations of 1.2 and 1.4 dB depend- This SPB prediction methodology was applied to OBSI ing on the tire. For heavy trucks at 25 ft (Figure 22), the behav- and SPB data obtained at other vehicle speeds and resulted 100 SPB - Predicted Based on SRTT SPB - Predicted Based on Dunlop 95 SPB - Measured Raw Data SPB - Measured Normalized Data Noise Level, dBA 90 85 80 75 70 4 6 7 8 9 1 2 3 5 10 11 12 AC AC AC AC AC C C C C C C AC PC PC PC PC PC PC Figure 21. Predicted SPB based on SRTT and Dunlop tires and measured light vehicle SPB levels at 60 mph and 25 ft--raw and normalized.
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26 110 SPB - Predicted Based on SRTT SPB - Predicted Based on Dunlop 105 SPB - Measured Raw Data SPB - Measured Normalized Data Noise Level, dBA 100 95 90 85 80 1 2 3 4 5 6 7 8 9 10 11 12 PC C PC C PC C AC PC C AC AC AC AC C C AC PC PC Figure 22. Predicted SPB based on SRTT and Dunlop tires and measured heavy truck SPB levels at 60 mph and 25 ft--raw and normalized. in essentially the same findings (details are provided in do not include differences that may be encountered from tire- Appendix D). to-tire variation. For each case, two standard deviations are In general, SPB can be predicted from OBSI by subtracting the given: one for the normalized SPB data and one for the uncor- offset values established in this research using either of the two rected SPB data. The first of these can be thought of as the stan- test tires. The offsets appropriate for each tire, vehicle type, and dard deviation that would be expected for an average of sites microphone distance are provided in Table 12 along with the with the same pavement. The second standard deviation is that standard deviations expected for such predictions. which should be applied to a specific site for those that are geo- In applying these values using different test tires (SRTT or metrically and acoustically in the range of the sites included in Dunlop design), it should be realized that standard deviations this research. For either the average or site-specific case, the off- 90 SPB - Predicted Based on SRTT SPB - Predicted Based on Dunlop 85 SPB - Measured Raw Data SPB - Measured Normalized Data Noise Level, dBA 80 75 70 65 60 4 6 7 8 9 1 2 3 5 10 11 12 AC AC AC AC AC C C C C C C AC PC PC PC PC PC PC Figure 23. Predicted SPB based on SRTT and Dunlop tires and measured light vehicle SPB levels at 60 mph and 50 ft--raw and normalized.