  Finding the distribution of vibro-acoustic energy in complex built-up
structures in the mid-to-high frequency regime is a difficult task. In
particular, structures with large variation of local wavelengths and/or
characteristic scales pose a challenge referred to as the mid-frequency
problem. Standard numerical methods such as the finite element method (FEM)
scale with the local wavelength and quickly become too large even for modern
computer architectures. High frequency techniques, such as statistical energy
analysis (SEA), often miss important information such as dominant resonance
behaviour due to stiff or small scale parts of the structure. Hybrid methods
circumvent this problem by coupling FEM/BEM and SEA models in a given built-up
structure. In the approach adopted here, the whole system is split into a
number of subsystems which are treated by either FEM or SEA depending on the
local wavelength. Subsystems with relative long wavelengths are modelled using
FEM. Making a diffuse field assumption for the wave fields in the short wave
length components, the coupling between subsystems can be reduced to a weighted
random field correlation function. The approach presented results in an
SEA-like set of linear equations which can be solved for the mean energies in
the short wavelength subsystems.
