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19 Data Reduction and Analysis logarithmic regression producing an equation and r2 value and plots were reviewed for "outlier" points (points seemingly out- Data Reduction side the normal range of data). Points that could be associ- To produce tire OBSI levels for each run, the leading and ated with field notes of unusual noises were dropped from the trailing edge levels were averaged together on an energy basis. data set. For each data set, the usable speed range was also For each condition (vehicle speed, pavement, and tire), the determined. The clean passby vehicle counts and speed ranges 1 /3 octave band levels between 250 and 5,000 Hz for the three for the test sites are given in Table 7. For comparison to OBSI runs were arithmetically averaged to represent that condition. and CPB data, the logarithmic regression equations for each The overall level between 400 and 5,000 Hz was then calculated. data set were used to calculate the average SPB level for Although the levels in the 400 Hz band were sufficiently low usable speed range in 5-mph steps (plots of the SPB data and so as to have minimal effect on the overall level, they were the regression curves, equations, and r2 values are given in included as data quality indicator requirements were met. Appendix D). These overall levels were then used for comparison to the over- The data from the CPB events were processed similar to all passby levels. The 60-mph samples were also reanalyzed into the SPB data. However, the data for the passby levels were shorter sample segments to assess the variation of OBSI level processed in 5-mph increments over the speed range within 50 over the standard 440-ft test section (results are provided in to 70 mph. Average levels at each speed, site, and tire configu- Appendix D). Most of the sections exhibited fairly homo- rations were calculated and used in comparison to the OBSI geneous noise levels throughout the length of the test section. and SPB data. An example of SPB and CPB levels versus speed Two of the pavement sections, Site 2 and Site 12, showed is shown in Figure 13 for Site 5, the longitudinally tined PCC notable variation. Based on the review of several passes over pavement on U.S. 30 (similar plots for all of the test sites are the same pavement section, the level variation occurred in included in Appendix D). the same locations as the vehicle traveled over the pavement. It was determined that the 4-and 5-second average levels for Data Analysis each site were appropriate for the remainder of the OBSI/ passby analysis. Once the overall SPB, CPB, and OBSI levels were estab- In processing the passby data, each event for both the 25-ft lished, data analysis was performed to investigate specific and 50-ft microphone positions was reviewed to verify that it aspects of the results. For the light vehicle overall levels, cross- was acoustically "clean." The ISO 11819-1 (21) criteria was used plots of CPB versus OBSI, SPB versus OBSI, and SPB versus to define clean passby events; only events where a single peak CPB were constructed for both test tires and for each micro- rose at least 6 dB above any surrounding data were included in phone distance. These included levels for all speeds and all sites. the analysis. The maximum level for each such event was paired For the heavy truck overall levels, plots of SPB versus OBSI and with the recorded vehicle type and speed. For the SPB events, SPB versus CPB were constructed. For each cross-plot, the data these levels were plotted against vehicle speed for both light were fit with linear regressions and best fit 1-to-1 slope lines. vehicles and heavy trucks. The data were then fit with a standard From these, the slope of the linear regression and r2 values were Table 7. Number of clean SPB events and vehicle speed range at each test site. Light Vehicles Heavy Trucks Site 25 ft Microphone 50 ft Microphone 25 ft Microphone 50 ft Microphone # Distance Distance Distance Distance # of Speed # of Speed # of Speed # of Speed Events Range Events Range Events Range Events Range (mph) (mph) (mph) (mph) 1 103 5570 44 5570 30 6070 15 6070 2 136 5070 129 5070 31 5065 31 5065 3 120 5570 120 5570 42 6065 42 6070 4 37 5565 14 5565 5 123 5570 118 5570 56 6070 54 60 70 6 123 5070 123 5070 54 5065 54 5065 7 123 5070 123 5070 56 5065 56 5065 8 108 5070 108 5070 49 5065 49 5065 9 89 5070 89 5070 42 5065 42 5565 10 136 6070 135 6070 80 5065 80 5065 11 119 6070 118 6070 52 5070 52 5570 12 126 5070 119 5070 33 5065 33 5065