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14 Dunlop producing higher levels than the SRTT. They were, lop tire at the DPG, the ranges in level between baselines were however, similar to the differences found for the passby sites 1.1 and 1.0 dB for the AC and PCC pavements, respectively, (see Chapter 5), which ranged from 0.9 dB to 3.1 dB with an with the levels showing a decreasing trend with an increase in average difference of 2.0 dB. temperature. The results indicate a decrease of 1 dB in the over- all OBSI level measured with the Dunlop tire with an air tem- Environmental Variables perature increase of about 18F. This corresponds to 1 dB decrease in level for a 48.6F increase in pavement tempera- Three-run series of baseline configuration measurements ture. For the data of Figure 10, the r2 values for noise-to- were performed at the completion of each set of tests for each temperature regressions for the Dunlop tire were 0.76 and parameter. Over the day of testing at MnROAD, air tempera- 0.82. The spectra for temperature changes increased or ture ranged from about 66.2F to 74.3F for the baseline con- decreased with temperature in a uniform manner. figurations and pavement temperature varied from 79.9F to 107.2F for the AC pavement and from 78.1F to 98.6F for the Systematic Vehicle and Test PCC pavement. The temperature fluctuation throughout the Execution Variables day in Mesa was greater than that in Minnesota and, unlike the MnROADs testing, the baselines at the DPG were acquired Measurement parameters including probe location in the over multiple days. During baseline measurement runs at the vertical and fore/aft directions, probe distance from tire side- DPG site, the air temperature ranged from about 99.0F to wall, vehicle test speed, vehicle loading, and tire inflation 102.9F and pavement temperature varied from 99.0F to pressure were evaluated incrementally for both the SRTT and 141.8F for the AC pavement and from 95.4F to 131.4F for Dunlop tires. Three-run sets of baseline repeats, as discussed the PCC pavement over the four-day testing period. The rela- for the evaluation of environmental variables, were con- tionship between overall A-weighted OBSI levels and the mea- ducted prior to and after each series of tests for each param- sured air temperatures are plotted in Figure 10 for the SRTT eter. For some of the parameters, no difference in level within and Dunlop test tires on both DPG AC and PCC pavements, the established measurement uncertainty could be deter- along with a linear regression for each data set. mined. The most sensitive parameters were found to be vari- The data indicated no clear correlation between the SRTT ation of probe location in the vertical direction, vehicle speed, and air/pavement temperature. A slight downward trend with tire inflation pressure, and vehicle loading, as summarized in an increase in air and pavement temperature was found, but r2 Table 4 for the range of parameter values defined in Table 2. values were very low (0.0 to 0.4). In addition, the range in over- The trends noted in Table 4 only apply within the range of all levels for the SRTT tire was only slightly higher than the the parameter variations measured in this testing. Given the standard deviation of the consecutive baselines: 0.5 and 0.3 dB limited number of data points for each parameter, only for the AC pavements, and 0.6 and 0.5 dB for the PCC pave- linear relationships were considered. The range of slopes ments at MnROAD and the DPG, respectively. For the Dun- reported for any one parameter reflects the range found for 1.0 Difference from Average Baseline Level, dBA SRTT AC 0.8 SRTT PCC Dunlop AC Dunlop AC 0.6 Dunlop PCC 0.4 SRTT AC 0.2 Dunlop PCC 0.0 SRTT PCC -0.2 -0.4 -0.6 85 90 95 100 105 Air Temperature, F Figure 10. OBSI level versus air temperature for DPG pavement sections.