Time-domain model for spherical wave reflection in a flat surface with absorber character – Application to the SOPRA measurement method

Abstract

This paper presents a developed time-domain model for the reflection of spherical waves in an absorber-like surface. The model is made to enable evaluation of measurement methods for assessing the sound absorptive properties of traffic noise barriers in direct sound field, it is thus called the Direct Field Absorption (DFA) model. In this study, the DFA model is applied to the in-situ SOPRA method for quick sound reflection index measurements on road noise barriers. The DFA model results in an impulse response (IR), based on a theoretically derived impedance of a certain absorber. The DFA IR is entered to the SOPRA formula to calculate the reflection index, $R{I}_Q$, of the absorber, which is subsequently compared to the measured $R{I}_Q$ of a wall fitted with the absorber in question. If the results are similar, it is reasonable to assume that the SOPRA measurement results are valid. But if there are any significant differences between the DFA and SOPRA sound reflection indices, possible reasons for them should be examined.

The first results are encouraging, showing that the DFA model can be a valuable tool to evaluate the results of reflection measurements. Furthermore, the DFA model could be useful for estimating the sound absorptive performance at lower frequencies of a noise barrier limited to its spatial extent in width and height, i.e., when it is physically impossible to measure. Further studies are necessary though, since the conclusions of this paper are based on only one kind of absorber.