Comparison of pressure release boundary condition and in-situ method for determining acoustic free velocity

Acoustic free velocity is a key property of a sound source, and together with source impedance, it fully characterizes the acoustic properties of the source. This study compares two methods for determining acoustic free velocity: the pressure release boundary condition and the in-situ inverse method. As the pressure release condition is not feasible in practice, the in-situ inverse method is used instead. The equivalence of both methods is demonstrated through finite element analysis (FEA) simulations using a duct similar to an HVAC duct with a theoretical source resembling an air handling unit (AHU). The source is reconstructed at a virtual interface inside the duct, which is discretized into smaller surfaces, each representing a 1D source in terms of acoustic free velocity and source impedance. In the pressure release method, the boundary condition is applied at the virtual interface to obtain the acoustic free velocity. In the in-situ method, it is determined using an inverse procedure. The acoustic free velocities obtained from both methods are compared and used to predict the far-field pressure for validation.