Acoustic Beamforming: Mapping Sources of Truck Noise (2009)

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

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