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1 SUMMARY Measuring Tire-Pavement Noise at the Source The objectives of this research were to (1) develop rational procedures for measuring tire- pavement noise at the source and (2) demonstrate the applicability of the procedures through testing of in-service pavements. At the beginning of this project, a literature review was per- formed to identify and evaluate various approaches to measuring on-board tire-pavement noise and to assess if any of these methods were appropriate for development in this research. As a result of this review, two potential candidates were identified, the close proximity (CPX) and on-board sound intensity (OBSI) methods. Field-testing was conducted to assess the two candidate methods and to compare their respective ability to correlate with controlled passby measurements of a test vehicle equipped with tires of two different designs. The two at-the- source measurements correlated well with each other, and to a lesser degree, with the passby measurements. For two of the test sites, differences in sound propagation were found to con- tribute significantly to the reduced correlation of the CPX and OBSI data to the passby data. Once these differences were considered, adequate correlation to passby data was found for both methods with a slightly better correlation using the OBSI data. It was further found that the CPX results had some distortion in the 1/3 octave band spectra in comparison to both the passby and OBSI results. For these reasons, with consideration to practicality, the OBSI method was selected for further development into an at-the-source tire-pavement noise procedure. To further develop the OBSI procedure, testing was conducted to examine the effect of measurement variables on the repeatability of data obtained using the procedure. This test- ing identified the OBSI probe location in the vertical direction, vehicle speed, and vehicle loading to be the factors contributing to the variation for the ranges and parameters evalu- ated. Within reasonable limits, probe distance from the tire, probe fore/aft location, and tire inflation pressure were found not to be critical. Based on these results, parameter limits were established for the OBSI procedure. Following the initial draft OBSI test procedure, testing was performed to demonstrate the applicability of the OBSI method to characterizing the in-service noise performance of pave- ments. In this testing, OBSI, controlled passby (CPB), and statistical passby (SPB) mea- surements were made on 12 sites with different pavement structures in Iowa and California. Test data were used to examine the relationship between OBSI and both types of passby measurements. The CPB-to-OBSI comparisons indicated that site-to-site variation due to the geometric and acoustic properties was significant. Using these data, it was possible to normalize site-specific effects out of the SPB data. It was also demonstrated that the SPB results could be accurately predicted from the OBSI data for a typical site defined by the average of the sites included in the investigation. With the completion of the field measurements and data analysis, a revised draft OBSI test procedure was prepared (see Attachment).