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20 100 Dunlop 25ft CPB SRTT 25ft CPB 95 Dunlop 50ft CPB Hvy Trucks Sound Pressure Level, dBA SRTT 50ft CPB 25ft SPB 90 Lt Vehicles 25ft SPB Lt Vehicles 50ft SPB Hvy Trucks 50ft SPB 85 Hvy Trucks 25ft SPB Lt Vehs Hvy Trucks 50ft SPB 25ft SPB 80 Lt Vehs 50ft SPB 75 70 65 40 45 50 55 60 65 70 75 80 Speed, mph Figure 13. SPB and CPB levels versus speed for heavy and light vehicles at 25 and 50 ft for Site 5 (longitudinally tined PCC pavement). determined as was the offset, standard deviation, and average Results and Discussion deviation from the 1-to-1 line resulting in data similar to that shown in Table 1. Additionally, the difference between OBSI The primary results of these measurements are presented in and CPB levels was calculated for each site, each speed, and this section (more complete results, including the remainder each tire for those cases where the corresponding data or both of the cross-plots, spectral comparisons of OBSI and CPB lev- types were available (for the 25-ft and 50-ft data). The average els, and level versus speed plots for the CPB and SPB data are and deviations were also determined for these data. The differ- given in Appendix D). ence between the 25-ft and 50-ft passby levels was also calcu- lated for each passby event for which the corresponding data Normalized SPB and CPB Data versus OBSI were available. The differences between the OBSI and CPB data were used In order to demonstrate the applicability of the OBSI data to to develop normalization coefficients to account for site-to-site in-service pavements, a series of cross-plots were considered in geometry and propagation differences. The need for site nor- which the relationships between OBSI and the passby data malization has been noted in previous work (15). Earlier SPB could be quantified. The first step was to develop the normal- studies on LA 138 reported site biases ranging from -0.6 to ized relationship between the CPB and OBSI data for all sites 1.4 dB relative to the reference site factors for both the 25-ft and speeds for each tire using the analysis discussed in the pre- and 50-ft microphone locations (15). The measurements con- vious section. The effect of the normalization can be seen by ducted in Phase I displayed site-specific effects of up to 4 dB comparing the raw cross-plots of CPB levels versus OBSI for even for measurement distances of 25 ft. These effects were evi- the SRTT tire in Figure 14 to the normalized results shown in denced both by propagation testing (see Appendix B) and cor- Figure 15. responding differences between the OBSI and CPB. Due to the In Figure 14, the data points from any one site tend to fall traffic volumes at the measurement sites, propagation tests below or above the regression line and the 1-to-1 line. Ignoring could not be made. As a result, site normalization factors these offsets, the points for each site tend to follow a constant were determined by first determining the average difference slope similar to the regression and 1-to-1 lines. When the data between OBSI and CPB levels at each site and overall average are normalized as shown in Figure 15, these offsets collapse to for the 12 test sites. The average OBSI/CPB difference for each follow a 1-to-1 slope with considerably less scatter. In this exam- site was then subtracted from the average of all sites to deter- ple, the slope of the regression is decreased from 1.31 to 1.06 and mine the normalization factor for each site. These factors were r2 value is increased from 0.91 to 0.96 with normalization. The then applied to the SPB data on a site-by-site basis, and back to 1-to-1 offset remains virtually the same with normalization the CPB data for confirmation. The normalization coefficients (24.2 dB with it and 24.3 dB without), however, the standard were also applied to the heavy-truck SPB data. deviation about the line is reduced substantially from 1.4 to 0.6.
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21 88 PCC 1 86 PCC 2 84 PCC 3 Sound Pressure Level, dBA AC 4 82 PCC 5 AC 6 80 AC 7 78 AC 8 AC 9 76 PCC 10 PCC 11 74 AC 12 72 1-to-1 Line (24.3 dB offset) 70 Linear 68 90 92 94 96 98 100 102 104 106 108 110 OBSI Level, dBA Figure 14. Controlled vehicle passby levels at 25 ft versus OBSI level for the SRTT at all test sites and speeds--raw data. Normalization produced similar effects on the data from the was virtually unaffected while the scatter was reduced as shown Dunlop tire and both tires for the 50-ft microphone locations in Figures 16 and 17 for the 25-ft, light vehicle SPB data and the as indicated in Table 8. SRTT OBSI data. The effect on the plot metrics for 50-ft data In each case, the value of 1-to-1 offset remained virtually and the Dunlop tire are given in Table 9. unchanged while the standard deviations are reduced by more As noted in Table 9, the reduction in scatter for the SPB data than 50%. This finding confirms that the normalization does is not as pronounced as it was for the CPB data (see Table 8) not affect the relationship between the CPB and OBSI data but partially due to the appreciable scatter was seen between the it reduces the scatter attributed to site-to-site variation, and CPB and SPB data as illustrated in Figure 18 for the SRTT data therefore, the coefficients were also applied to SPB data. Site at 25 ft. In this plot, any site bias is effectively cancelled out leav- normalization produced a similar effect on the SPB data as it did ing only the ability of the test tire to replicate the behavior of the on the CPB data. Invariably, the value of the 1-to-1 line offset SPB data that spans many different tires and other light vehicles. 88 PCC 1 86 PCC 2 PCC 3 84 Sound Pressure Level, dBA AC 4 82 PCC 5 AC 6 80 AC 7 78 AC 8 AC 9 76 PCC 10 74 PCC 11 AC 12 72 1-to-1 Line (24.2 dB offset) 70 Linear 68 90 92 94 96 98 100 102 104 106 108 110 OBSI Level, dBA Figure 15. Controlled vehicle passby levels at 25 ft versus OBSI level for the SRTT at all test sites and speeds--normalized data.
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22 Table 8. Metrics for CPB 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 1.06 1.17 0.95 1.41 0.97 1.38 0.97 r2 0.91 0.96 0.85 0.94 0.86 0.96 0.86 0.96 Offset, dB 24.3 24.2 24.8 24.7 31.4 31.0 31.7 31.3 Std Dev, dB 1.4 0.6 1.4 0.6 1.9 0.6 2.0 0.5 Avg Dev, dB 1.1 0.6 1.1 0.5 1.6 0.4 1.6 0.4 88 US 30 BD 86 US 30 TT 84 US 30 UT Sound Pressure Level, dBA US 30 AC 82 US 30 LT LA138 S1 80 LA138 S2 78 LA138 S4 LA138 S5 76 Moj S1 74 Moj S2 Redding 72 1-to-1 Line (21.9 dB offset) 70 Linear 68 90 92 94 96 98 100 102 104 106 108 110 OBSI Level, dBA Figure 16. Statistical light vehicle passby levels at 25 ft versus OBSI level for the SRTT at all test sites and speeds--raw data. 88 US 30 BD 86 US 30 TT US 30 UT 84 Sound Pressure Level, dBA US 30 AC 82 US 30 LT LA138 S1 80 LA138 S2 78 LA138 S4 LA138 S5 76 Moj S1 74 Moj S2 Redding 72 1-to-1 Fit (21.8 dB offset) 70 Linear 68 90 92 94 96 98 100 102 104 106 108 110 OBSI Level, dBA Figure 17. Statistical light vehicle passby levels at 25 ft versus OBSI level for the SRTT at all test sites and speeds--normalized data.