Assessment and improvement of a highway traffic noise prediction model with Leq(20 s) as the basic vehicular noise

Abstract

A highway noise prediction model which considers 20s continuous equivalent sound level measure as the basic vehicular noise has been developed and is called the Leq(20s) model. The Leq(20s) model is believed to provide accurate predictions by measuring the sound level of individual vehicle without assuming the vehicle noise source as a point source. To verify the rationality of Leq(20s) model, a mathematical derivation was performed based on the Federal Highway Administration (FHWA) traffic noise prediction model in this study. The derivation process indicated two defects in the Leq(20s) model. One was that given the lack of a revision item for finite length road, the Leq(20s) model cannot be applied to some special roads, such as in predicting the traffic noise level of a receiving point which is located at the tunnel portal of a highway. The other was that the Leq(20s) model had some theoretical deviations from the model of mathematical derivation from the FHWA model. When the speed of a vehicle ranged from 20km/h to 120km/h, the deviations ranged from 0.80dB(A) to 0.13dB(A). The deviations could be reduced if the constant item of −22.55dB(A) in the Leq(20s) model was revised to −22dB(A). To compare the accuracy of the Leq(20s) measure with the measure of the mean energy emission level, which was used as the basic vehicular noise in the FHWA model, a series of simultaneous measurements of a 5s continuous equivalent sound level [Leq(5s)] and the maximum emission levels of individual vehicles was conducted. The measurements of Leq(5s) were compared with the calculations based on the hypothesis of monopole and dipole sources from the maximum emission levels of individual vehicles measured on roadsides. Result indicated that the mean value of measured Leq(5s) was close to that calculated from the maximum emission level with the hypothesis of monopole source for light vehicles. However, for medium and heavy vehicles, the mean value of measured Leq(5s) was between the two values calculated from the maximum emission level with the hypothesis of monopole and dipole sources. A mean error of 1.2–1.6dB(A) was obtained for Leq(5s) when the mean energy emission level was employed in the FHWA model for medium and heavy vehicles. The accuracy of traffic noise prediction can be improved by considering Leq(20s) as the basic vehicular noise instead of the mean energy emission level.