Investigation of the effect of neglecting reflections in Power-based sound in ducts

Abstract

In high frequency sound propagation inside ducts, the modal density is very high that the sound starts to propagate in rays. The acoustic performance of a duct network can be simulated using power-based models. The application of such situations is for HVAC systems and large silencers for power generation. Several standards are available for the analysis of HVAC systems using the same technique, such as ASHRAE and VDI. For each element, the flow-generated sound power inside the element is added to the input sound power and the output sound power is calculated by subtracting the Insertion Loss of the element. The attenuated sound energy can be either dissipated inside the element or reflected back to the system. Standards always assume that no energy is reflected and all the attenuation happens inside the element. This assumption is investigated in this paper. Several standard HVAC elements are considered, calculating the amount of energy dissipated inside the element and that reflected back. It was found that if all the reflected energy is considered, this will affect the output power from the system, especially in the highly reflective elements. The investigation was done using the Finite Element Method with the ray tracing technique.