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71 CHAPTER 4 Conclusions and Recommendations 4.1 Conclusions 4.2 Recommendations Overall, the results of the study validated the beamform- Based on the key findings and conclusions for the proj- ing measurement technique in the truck noise application. ect, the following future research and testing needs can be It is confirmed that the measurement system developed in identified: the course of the project performed effectively in mapping and localizing typical noise sources for stationary and mov- Conduct nationwide roadside truck noise measurements ing trucks in actual road conditions in a wide frequency on a wider range of pavement types in multiple states to range from 250 to 2000 Hz. Sound distribution images and establish a new truck noise source database (source height maps obtained during truck passbys permitted examination distributions and spectral content) for traffic noise mod- of the time histories and spatial distributions of sources, as els. Additional analysis of subsampling of the full beam- well as an analysis of the noise paths from the engine, ex- forming microphone array may be necessary, based on haust, muffler, tires, and other body components for various the data obtained in the current project, to expand the trucks. technique, simplify the array, and speed up data collec- Statistical analysis of the vertical distribution of noise tion for wider scale measurements at a practical degree of sources indicated that for the majority of truck passbys effort. measured at highway speeds on an in-service highway, tire The noise source distributions for trucks obtained in the pavement interaction was the dominant source generating current study, although based on a relatively small sam- sound close to the pavement. A small proportion of heavy ple of the truck population, can be applied (if deemed trucks, however, exhibited significant noise generation in the appropriate) as interim source height adjustments to the area of the vertical exhaust stack, dominating at low frequen- reference emission levels in the FHWA Traffic Noise cies and elevations around 3.6 m (12 ft). These results are in Model. general agreement with the conclusions of the Caltrans study Traffic noise prediction models updated using the noise that used a commercial beamforming microphone array. source distributions obtained in the current study can serve The two studies provided essentially similar results in terms as a resource for state and federal agencies to examine the of sources identified, their relative contributions, and lack of effectiveness of highway noise mitigation strategies, such as higher elevation sources except in a few cases. the use of quieter pavements or barrier design. For the noise prediction modeling purposes, the current Novel information obtained for noise source distributions study indicated that a simple system of two uncorrelated on trucks, as well as the measurement technique developed sources, one located near the pavement and another at the in the course of the study, are recommended to truck man- exhaust stack elevation, can generally be used for simulating ufacturers for further studies of potential source- or path- statistical vertical distributions (mean and maximum) of truck targeting treatments. noise sources. The beamforming measurement technique developed dur- The results of the proof-of-concept testing have been pub- ing the course of the study is also recommended for use lished in two presentations (33, 34), and additional publica- in the analysis of noise-generating mechanisms and noise tions of the major results are anticipated. abatement measures for automobiles, buses, and motor-

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72 cycles, as well as other noise sources such as construction types and terrains on noise generated by different vehicle equipment, etc. types, including the effects on vertical noise source distri- The approach developed in the study can be used for ex- bution and tirepavement interaction. ploring in greater detail the effects of varying pavement