Convergence of modes in exterior acoustics with infinite elements

The radiated sound power is seldom computed by modal decomposition, since modal quantities are uncommon in exterior acoustics. We apply the finite and the infinite element method (FEM and IFEM) in order to discretize an unbounded fluid-filled domain and to obtain the system matrices that are independent of frequency. From these system matrices of mass, damping and stiffness we compute frequency-independent normal modes as right eigenvectors of a state space eigenvalue problem. By increasing the polynomial order of radial interpolation in the domain of the infinite elements, normal mode eigenvalues converge and lead to reliable results for the radiated sound power in exemplary load cases. However, the additional degrees of freedom may also provide mathematical artifacts or spurious modes, which might falsify the calculated sound power in case of modal superposition. By application of the Modal Assurance Criterion (MAC), significant and converged modes are identified and their contribution to the total radiated sound power is investigated.