Effect of Reflections on HVAC Systems Power-Based Acoustic Simulation

Power-based techniques are commonly used in the simulation of the sound propagation in HVAC systems. Sound is generated from supply fan, propagates through the network, experiencing multiple attenuations from different network elements, then it is radiated through the network openings. Several standards (e.g. VDI and ASHRE) describe the attenuation through different HVAC elements and the flow generated noise in other elements. These standards assume no reflections in the network. By neglecting the reflections, the calculation algorithm becomes more easy and robust, but less accurate. However, in a previous work by the authors, the reflection effect was investigated in duct networks, and the results showed that, in several cases, reflections can have a major effect and should not be ignored. This happens when the geometry is complicated with variations in the duct diameter or duct impedance. In this paper, the two-port power-based algorithm is presented after including the reflection effect. The effect of source and outlet impedances is also investigated. The algorithm is validated against FEM simulation on different duct networks built with common duct elements. The results show the effect of reflection on the overall performance of the two-port network, and the influence of diffuse field assumption between elements.