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2 CHAPTER 1 Introduction Tire-pavement noise has become an increasingly important traffic noise reduction. In addition, there has been an increas- consideration for highway agencies. However, there are no ing public awareness that pavement selection can affect the widely accepted procedures for measuring solely tire-pavement resultant traffic noise levels. noise under in-service conditions. As a result, this research was An increased interest in measuring tire-pavement noise at undertaken to evaluate potential noise-measuring procedures the source has occurred parallel to the recent interest in quiet and identify or develop appropriate procedures applicable to pavements. With this type of testing, acoustic measurements light and heavy vehicles and all paved surfaces. Such proce- are made close to the tire-pavement interface with instrumen- dures will provide highway agencies with an appropriate means tation that translates with the test tire. Thus, the tire-pavement for (1) measuring and rating tire-pavement noise levels on noise of a large number of pavements can be measured in a rel- existing pavements, (2) evaluating new pavements incorporat- atively short period of time with very few restrictions on the ing noise-mitigating features, and (3) identifying design and test site. The source-level measurements require less time and construction features associated with different noise levels. fewer resources to complete relative to wayside measurements The objectives of this research were to (1) develop rational and facilitate a more direct comparison of tire-pavement noise procedures for measuring tire-pavement noise at the source generation from one site to another. However, there are no and (2) demonstrate the applicability of the procedures widely accepted procedures in the United States for tire- through testing of in-service pavements. To achieve these pavement noise source levels under in-service conditions. objectives, (1) a literature search was conducted to gain under- When this research began, tire-pavement noise source- standing of what approaches have been used in the past to level measurements in the United States primarily used two quantify tire-pavement noise source levels, (2) evaluation test- approaches, the Close Proximity (CPX) method as docu- ing was conducted to assess candidate methods and select the mented in the draft ISO 11819-2 standard document (2), and most promising one, (3) the effect of test parameters of the the On Board Sound Intensity (OBSI) method based on tech- selected method were examined to develop parameter limits, niques developed at General Motors Corporation (3). In the and (4) field tests were performed on in-service pavements to early 2000s, both of these methods were used to catalogue and demonstrate the applicability of the proposed measurement compare different pavements for their noise performance method for different pavement types. This report presents the (4, 5, 6). Other methods using different approaches of sound results of the research, the information obtained, implications pressure level measurement (7) and near field acoustical holog- for developing a rational test procedure, and the proposed test raphy (8) were also reported in the literature. As a result of the method. uncertainty inherent in the measurement procedures of tire- pavement noise source levels, development of standardized procedures was identified as a research need by the AASHTO Background Standing Committee on Research in March of 2004; the Com- Recently, there has been increased interest on the part of mittee allocated funds to develop such procedures under highway agencies to consider the use of quieter pavements to NCHRP Project 1-44. This need was also reiterated at the first mitigate traffic noise. This interest has been driven largely by Tire-Pavement Noise Strategic Planning Workshop conducted the cost and, at times, lack of public acceptance of the tradi- by the Federal Highway Administration and the Institute for tional sound wall approach to mitigating traffic noise (1) and Safe, Quiet, and Durable Highways held at Purdue University an apparent increase in the demands of the public for highway in September of 2004 (9). The research conducted in this proj-