A comparative study of the physical mechanisms related to source model of FHWA's traffic noise model (TNM): Measurements versus modeling assumptions

The main purpose of this study is to identify and solve the problems existing in the source model of TNM, the FHWA official highway noise prediction model. It has already been identified in a couple of studies that TNM 2.1 over-predicts absolute noise levels. In order to solve this problem, TNM 2.5 made some changes, which lead to a significant reduction of noise prediction, thus effectively improving its accuracy. These changes and the source model were examined through numerical simulation and field measurement.; As identified in chapter 7, source model is one of the important reasons for the over-prediction of TNM 2.1. The main change from TNM 2.1 and TNM 2.5 lies at a parameter in the source model - multiplier m. The value of the multiplier m was significantly reduced, which effectively improved the accuracy of TNM. Chapter 7 developed the method to calculate this parameter and compared its value to that adopted in TNM 2.1 and 2.5 respectively. The study concluded that the value of this parameter adopted in TNM 2.1 is accurate. Therefore, there is no justification for the reduction of the value of multiplier m in TNM 2.5. The over-prediction of TNM cannot be solved from this parameter, but from other factors.; One of the factors that lead to the over-prediction that has been identified in this research work is the source directivity. The omission of the horizontal directivity makes TNM significantly over-predict absolute noise level. The omission of the vertical directivity makes TNM under-predict barrier insertion loss in some situations.; Other important parameters, equivalent source height, sub-source height and subsource energy split were also studied in this research work. It was shown that the leading parameter is the equivalent source height despite the fact that all of them are closely related to each other. The equivalent source height is the key factor that determines the accuracy of TNM.; Another factor that is omitted by TNM is the multiple reflections between vehicle body and barrier. This research work shows that the omission of this factor could lead TNM to over-predict insertion loss of barrier. This research work suggested the ray trace method could be employed for considering this factor.