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5 reported. The measurement systems are not standardized and require acquisition and manipulation of many channels of acoustic signals. Considering the similarity of results pro- vided by sound intensity and AAT mapping, there appears to be no advantage in pursuing AAT technology over the sim- pler OBSI methods. For SPL measurements, a lot of different approaches have been cited in the literature. Some of the early work using BTT methods displayed some limited level of correlation to passby measurement for trucks. However, more recent research work has shown that the noise region at the front of the tire is equally important to overall tire noise and that there is little correlation between the front and the rear of the tire. Of the remainder of the SPL methods, there appears to be no justifi- Figure 1. CPX tire-pavement noise measurement cation for following any method other than that defined in the configuration. ISO CPX draft standard. Comparisons between passby and onboard measurements using other SPL methods show about the same degree of correlation as seen with CPX methods. Regarding the CPX approaches, some consideration should be Remarks on Test given to using trailer instead of exposed microphones. With Procedure Development the trailer method, concern has been expressed about reflec- In this subsection, the implications of the literature search tions in the enclosure. Tests to evaluate reflections have been on the selection of the candidate test method are summa- defined; however, recent work comparing different tests and rized. This draws upon the complete discussion of the liter- equipment have indicated some variation. An attractive alter- ature search provided in Appendix A, which includes the native to the trailer-based CPX method is the exposed micro- citation of 85 references. phone technique. With the microphones fixed to the side of Of the three overall approaches, AAT methods appear to the test vehicle, this approach should avoid the build up of be the furthest away from being a usable technique for rou- reflections and should be less expensive to implement. How- tine, in-service pavement noise evaluation. These techniques ever, the issue of flow noise contamination of the exposed have never been applied to measurements in a highway microphone remains unresolved and methods for testing for environment and no comparisons to passby data have been it are not defined. As with the CPX method, the OBSI method using the GM methodology has been used extensively for in situ highway pavement noise measurements. This method has been shown to correlate reasonably well with both controlled passby (CPB) data and CPX data. Unlike the test tires specified in the ISO CPX draft standard, tires used today in OBSI testing seemed to be somewhat arbitrary relative to "typical" tire noise as little data comparing OBSI to statistical passby (SPB) for light vehicles has been reported. Further, the use of con- sumer tires for standardized testing is problematic as tire sup- pliers discontinue production of these tires, as has been experienced both by users of the CPX and OBSI methods. International availability of test tires has also been an issue as some test tires used in Europe are not available in the United States and vice versa. For the onboard procedure to be used by highway agencies in the United States, the selection and availability of test tires must be considered regardless of the test procedure used. Figure 2. OBSI single probe position opposite the An issue that remains an unknown is relating either CPX leading edge of the tire contact patch. or OBSI measurements to passby levels of porous pavements.

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6 Differences between CPX to CPB or SPB relationships have Prior to this study, there had been no research to compare been reported in some European studies. Also, one study sug- OBSI to CPX and both to CPB within the same study. Such gested that porosity played a role in CPB and OBSI data information is necessary in order to assess the technical mer- measured for two test surfaces, one slightly porous and one its of both approaches and to determine if there is a technical non-porous (12). Differences may also exist in the way in which advantage in one of the approaches that should be considered these two methods respond to porous pavement and how they along with other, non-technical issues. The evaluation testing relate to passby levels. of these two methods is reported in Chapter 3.