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9 correct results and agrees with measurements from established The scope of work for this project was divided into nine methods. The work of Crewe et al. (19) provides some of tasks that generally define and demonstrate a technology that demonstration. Their test vehicle was, however, rather for truck noise source localization, plan and execute mea- simple--a minivan whose dominant noise source was tire surements to better understand truck noise source distri- pavement noise. Two non-tire noise sources were identified: butions under actual operating conditions, and document the vehicle's horn and a speaker placed in the top near part of the results throughout the project. These tasks are detailed the engine compartment. The distribution of noise sources on in Chapter 2. heavy trucks is, in general, considerably more complex and The interim report for the project, submitted in July 2007 in requires additional technique verification. accordance with Task 4, presented the results of the literature search, development of the experimental design, and proof-of- concept tests (Tasks 1 through 3). Upon an NCHRP review 1.4 Objective and Scope and approval of the interim report, the roadside test plan of Research was developed and implemented in Tasks 5 and 6. This final The objective of this study was to use acoustic measure- report incorporates the interim report material revised per the ments and noise source mapping techniques to accurately NCHRP comments (Tasks 1 through 4), includes the results of identify, locate, and quantify the noise sources on typical com- the roadside truck testing (Tasks 5 and 6), summarizes the key mercial truck and tractor-semitrailer combinations operating findings of the study (Task 7), and identifies the future research in the U.S. roadway environment. and testing needs (Task 8).