Measurement of Vertical Distribution of Truck Noise Sources during Highway Cruise Pass-Bys by Acoustic Beam Forming

A measurement program was completed to determine the vertical distribution of heavy-truck noise sources for pass-by events on an in-service highway for vehicles operating under cruise conditions. In addition to data on heavy trucks, some data were obtained for medium trucks and light-duty vehicles. The measurements were performed with acoustic beam forming, which provided visualization of the sound radiation of passing vehicles as well as means for calculating the vertical distribution of noise source level. The data set includes pass-by events for 125 heavy trucks, 30 medium trucks, and nine light vehicles operating nominally at 55 mph on two asphalt surfaces. The purposes of the research were to compare the source height splits assumed in the FHWA traffic noise model (TNM) to the results of this work, examine the acoustic benefit of sound walls designed to block the line of site of truck exhaust outlets, and obtain a better understanding of truck noise sources. For heavy trucks, the pass-by noise is dominated by tire noise produced by the drive axles, with other secondary sources related to the power train occurring in some cases. Noise from elevated exhaust outlets was not observed except in one or two special cases. For all vehicle types, ground level sources (tire noise) produced the highest noise levels. Furthermore, the vertical source distributions were found to be virtually identical for heavy and medium trucks. For truck and light vehicles, it appeared that the current source height splits assumed in TNM may be biased toward higher upper-source strength.