Simulation of duct breakout transmission loss and applications

The noise propagation path of greatest concern in heating, ventilation, and air conditioning ductwork is airborne from the air handler to the duct entrance to a room. However, there is sometimes an important secondary noise path called duct breakout. Sound inside the duct vibrates the duct walls which then radiate sound to the environment. This secondary path is of consequence if there is no barrier, like a drop ceiling, between the ductwork and the receiver. The primary metric that ASHRAE has used for assessing this propagation path is breakout transmission loss. In this research, the breakout transmission loss is determined using coupled structural-Acoustic finite element analysis. For an ideal measurement of the breakout transmission loss, an anechoic termination should be attached to the end of the duct. In that case, both the sound power propagating down the duct and the breakout power can be measured directly. However, this ideal measurement is difficult in practice and so it is normally approximated. The input sound power is either estimated by running the duct into a reverberation room and adding a correction factor, or by sampling sound pressure inside the duct. However, both the ideal and approximate approaches can be simulated. The breakout transmission loss is determined using finite element simulation for both approaches. Simulation is then compared with published measurement results for unlined rectangular cross-section ductwork. The results demonstrate the appropriateness of the approximate measurement methods though there are differences with an ideal measurement at low frequencies.