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11 CHAPTER 4 Evaluation of OBSI Test Parameters Introduction of tire wear and hardness were not evaluated and wheel align- ment was not evaluated except as it occurred from test vehicle As explained in Chapter 3, the OBSI method was selected as to test vehicle. Test execution variables including probe loca- a basis for developing an onboard, at-the-source measure- tion, run-to-run and day-to-day repeatability, probe configu- ment procedure for tire-pavement noise. As a portion of the ration, small variations in test speed, and reproducibility were work conducted to develop such a procedure, test variables and also measured. Reproducibility across multiple users was not measurement uncertainties were examined. Based on input assessed. from current OBSI users as well as information contained in the draft ISO CPX procedure, some pertinent variables that could affect the measurement results were identified. The sen- Measurement Sites sitivity of OBSI results to variations in pavement temperature, The initial portion of this testing was conducted at the configuration of the OBSI measurement fixture, tire infla- Minnesota DOT's MnROAD Low Volume Road facility in tion pressure, test vehicle type, test speed, and load were inves- Albertville, MN. This facility is a 2.5-mile closed loop that tigated. The intent of this investigation was to provide initial contains 20 pavement test sections. Two of these sections, a guidance on test variables and the control limits needed to fine textured AC and a random transversely tined PCC, were implement the OBSI procedure. This chapter summarizes the selected as test surfaces. Due to an extended period of rain, evaluation and results of the test parameter investigation and testing was limited to the SRTT tire and only a portion of the makes recommendations on parameter limits and controls test matrix was completed. The remainder of the testing was (additional information on the measurement sites and proto- conducted at the General Motors Desert Proving Ground col, along with a more detailed explanation of the results of this (DPG) in Mesa, AZ, on relatively smooth AC and exposed analysis, are provided in Appendix C). aggregate PCC test sections. The site location, photographs of the pavement sections, and the average 1/3 octave band spec- Description of Field Measurements trum for each surface under baseline conditions are provided in Appendix C. Parameters Evaluated Measurements were conducted to evaluate vehicle vari- Measurement Protocol ables and test execution variables on OBSI measurement results. The test matrix is shown in Table 2. The baseline test condition for each test pavement and test Environmental variables, such as air and pavement temper- tire followed the measurement protocol presented in Attach- atures, wind conditions, and moisture conditions, could not ment 1 using "full-sized" rental vehicles along with a baseline be systematically controlled for these tests. However, temper- load consisting of two people and the OBSI instrumentation. ature and wind conditions were measured throughout, and A photograph of the OBSI equipment installed on a test vehi- testing conducted over the extremes encountered was evalu- cle is shown in Figure 8. ated. All testing was conducted under dry conditions. Vehicle Ideally, the same test vehicle would have been used as the variables, including loading, tire inflation pressure, and baseline for all of the test scenarios. However, due to the relo- vehicle-to-vehicle variation, were evaluated systematically. cation of the second portion of the testing, two different base- Because of the time period of the testing, longer term variables line vehicles were used. The test vehicle used at MnROAD