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6 CHAPTER 1 Background 1.1 Introduction United States, source height distributions were last measured in the mid-1990s using the technology available at that time Heavy trucks are significant contributors to overall traffic (2, 3). Those measurements were made when the current reg- noise levels. At highway speeds, the noise level produced by ulated level had been in place for 8 years. They do not reflect heavy trucks is about 10 dB greater than that of light vehicles changes in the fleet that may have occurred in the subsequent (1). As a result, every heavy truck in the traffic flow contributes decade. the same amount to the average noise levels as 10 light vehi- A second factor challenging the current treatment of truck cles. Because of their contribution, a thorough understanding noise is recent research that has been performed in Europe. of trucks as a noise source is crucial to the prediction and mit- From these studies (4, 5), tirepavement noise has been found igation of traffic noise. In addition to the overall noise level of to make a much larger contribution, 63% to 84%, to total truck passbys, the location and relative strength of the princi- truck noise at highway speeds. From the European work (6) pal noise sources (e.g., exhaust, powertrain, tirepavement, and some research in the United States, a strong dependence and aerodynamic) on individual trucks is important. For mod- on truck tire type has also been determined. The more aggres- eling and abatement of traffic noise, the barrier performance of sive tread tires often used on the drive axles are found to be sound walls depends on the assumed distribution of source as much as 10 dB louder than those used on the steering or heights. In some states, highway sound walls are designed so trailer axles. With such large differences between tire types, the top of the exhaust stack is obscured from sight under the it is expected that tirepavement noise could be quite pro- assumption that exhaust noise is a major source. For abating nounced for some trucks, depending on tire selection. truck noise through quieter pavements, the amount of expected The third set of observations comes from research work noise reduction depends on the contribution of tirepavement conducted in conjunction with the application of quieter noise relative to other sources such as powertrain, exhaust, and pavements. In a number of recent projects, reductions in traf- possibly aerodynamic noise. The current treatment of truck fic noise have been measured consistent with the reduction of noise for highway conditions is simplistic, placing about 50% of tirepavement source levels when pavement modifications the source strength at a height of 12 ft (3.7 m) and the other half have been made (79). Even statistical passby measurements at ground level. for trucks have shown reductions with pavement modifica- Recently, a number of observations have challenged the tions almost as great as indicated by tirepavement noise current treatment of trucks and led to the need for new source level reductions (10). The reduction in truck noise research. First, truck noise levels are one of the very few noise source levels goes beyond that which would be predicted sources regulated at the federal level in the United States. Over based on the current 50-50 split between tirepavement and the past few decades, the regulated noise level has been incre- other, elevated noise sources on trucks. mentally lowered to the point where trucks are expected to meet the same level requirement as light vehicles under engine 1.2 Heavy Truck Noise Sources noisedominated test procedures. In achieving this lower level of noise performance, engine and exhaust noise have been Noise from heavy trucks originates from a variety of sources: addressed. As older trucks are replaced, these sources are expected to play less of a role in total truck noise emissions Exhaust stack outlet under highway conditions than tirepavement noise. In the Muffler shell