Acoustic radiation modes and normal modes in exterior acoustic problems

Acoustic radiation modes are eigenvectors of the real and symmetric acoustic impedance matrix Z, which is usually computed by the boundary element method (BEM) matrices G and H at the surfaces of inner obstacles in an unbounded fluid-filled domain. Application of the finite element method (FEM) and the infinite element method (IFEM) allows the computation of the acoustic radiation modes as well, but also normal modes can be computed as right eigenvectors of a state-space eigenvalue problem. Modal superposition of both radiation modes and normal modes leads to accurate results of the radiated sound power. However, normal modes additionally provide modal sound pressure distributions in the whole computational domain and can therefore be used to calculate frequency response functions. In this work, modal superposition and reduction in exterior acoustics are presented and discussed.