Measuring Tire-Pavement Noise at the Source (2009)

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4Hundreds of papers have been written on the subject of tire-pavement noise. Relevant references were read, summa- rized, and grouped into categories based on the measurement types employed. Studies that dealt with measurements on pavements (not on “road-wheels” in a laboratory environ- ment) that were not limited to passby or wayside measure- ments were selected for review. Selected papers were examined for the use of a novel approach, the development of a source- level measurement approach, or the comparison of passby to source-level measurements. Papers that gave results comparing passby to source-level measurements were reviewed in the most detail. References discussing testing parameters and/or vari- ables that affect tire noise measurements were also reviewed. Nearly 100 sources of information were evaluated in this man- ner (detailed results of this work and a complete reference list are provided in Appendix A). Description of the Review The primary source of reference material was papers writ- ten for various noise conferences. The proceedings of over 100 national and international conferences and symposia have been searched for work related to tire-pavement noise measurement at the source. The Tyre/Road Noise Reference Book (10) has been used as a check ensuring that the rele- vant work prior to 2001 has been included in the search. However, material from the 1970s through 2001 was searched independently of this reference in order to ensure inclusion of all related work. In more recent years (2002 through 2005), many additional papers were published on this subject from ongoing work in Europe, the United States, and Asia. The literature deals with three general cat- egories of acoustic measurement: sound pressure level (SPL) measurements, sound intensity (SI) level measure- ments, and sound field measurements using acoustic array technology (AAT). The sound pressure level measurement approaches may be divided into three subcategories. The first of these are varia- tions of “behind the tire” (BTT) measurements where a microphone is placed directly behind the tire typically close to both the tire and pavement. This position was chosen because it was thought to reduce wind noise on the microphone. The second subcategory is the so-called “close proximity” mea- surement where several microphones are placed at various points around the tire (Figure 1). This approach evolved into the formal ISO Draft Standard procedure, ISO 11819-2 (2), which is commonly referred to as the Close Proximity (CPX) method. Use of slightly different microphone positions than those used in this draft standard was reported. The CPX approach includes techniques where the microphones are protected from airflow by trailers surrounding the test tire and those where the microphones are exposed to flow. The third subcategory is all other SPL measurements that are not included in BTT and CPX approaches and are typically unique to a single study or set of studies by an individual researcher or research group if referred to as “SPL Other.” In the SI category, the majority of the reported studies fol- low the approach developed at General Motors Corporation (GM) and documented in the relevant GM Test Procedure (11) (Figure 2) although several other unique approaches were also reported. In the AAT category, almost all of on-road work uses a near-field acoustic holography approach (NAH) and has been done largely by a single research effort at Penn State University (8). Each of the three major measurement categories has been used both for on-road and road-wheel (RW) testing. Although it was the intent of this project to develop a proce- dure for in situ measurement of tire-pavement noise at the source, some of the RW work is of interest and was included in the review. Also included in the review were a few references dealing with measurements that were important to under- standing tire-pavement noise variables as they relate to a test procedure. C H A P T E R 2 Literature Search

Remarks on Test Procedure Development In this subsection, the implications of the literature search on the selection of the candidate test method are summa- rized. This draws upon the complete discussion of the liter- ature search provided in Appendix A, which includes the citation of 85 references. Of the three overall approaches, AAT methods appear to be the furthest away from being a usable technique for rou- tine, in-service pavement noise evaluation. These techniques have never been applied to measurements in a highway environment and no comparisons to passby data have been 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- cation for following any method other than that defined in the 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 given to using trailer instead of exposed microphones. With the trailer method, concern has been expressed about reflec- tions in the enclosure. Tests to evaluate reflections have been defined; however, recent work comparing different tests and equipment have indicated some variation. An attractive alter- native to the trailer-based CPX method is the exposed micro- phone technique. With the microphones fixed to the side of the test vehicle, this approach should avoid the build up of reflections and should be less expensive to implement. How- ever, the issue of flow noise contamination of the exposed microphone remains unresolved and methods for testing for 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. An issue that remains an unknown is relating either CPX or OBSI measurements to passby levels of porous pavements. 5 Figure 1. CPX tire-pavement noise measurement configuration. Figure 2. OBSI single probe position opposite the leading edge of the tire contact patch.

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