Predicting insertion loss of large duct systems above the plane wave cutoff frequency

If the dimensions of a silencer or muffler component are small compared to an acoustic wavelength, plane wave propagation can be assumed. This is not the case for HVAC (heating, ventilation, and air conditioning) duct systems, and large diesel engine mufflers commonly used in ship and generator sets. For such applications, the wave behavior in the inlet and outlet ducts is three-dimensional. In this paper, the finite element method is utilized to simulate large duct systems with an aim to predict the insertion loss. The boundary condition on the source side is a diffuse field applied by determining a suitable cross-spectral force matrix of the excitation. At the termination, the radiation impedance is calculated utilizing a wavelet algorithm. Simulation results are compared to published measurement results for HVAC plenums and demonstrate good agreement.