A comparative study of duct breakout transmission loss: Finite element simulation versus legacy measurements

The primary noise path in HVAC applications is the direct air path from the air handler to the room. However, there exists a secondary noise path known as duct breakout, where sound propagating in the airpath vibrates the surrounding ductwork which in turn radiates noise. This propagation path is especially important for rectangular duct cross-sections with no sound absorptive lining inside the duct. This study determines the duct breakout transmission loss using coupled structural-acoustic finite element analysis. The simulation method is validated against legacy measurement data for unlined and lined rectangular and cylindrical cross-sections. Simulation results generally agree with measurement data. Procedures for measuring the breakout noise in laboratories are also investigated using the simulation model. Ideally, the test duct should have an anechoic termination so that reflections from the duct end do not impact the measurement. However, anechoic terminations are difficult to design, build, and qualify in practice, so approximate measurement methods are used to determine the breakout transmission loss. The simulation results confirm that the approximate measurement methods are suitable over most of the frequency range though there are discrepancies at low frequencies.