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