  Nonlinear coupling between eigenmodes of a system leads to spectral energy
redistribution. For multi-wavespeed chaotic billiards the average coupling
strength can exhibit sharp discontinuities as a function of frequency related
to wavevector coincidences between constituent waves of different wavespeeds.
The phenomenon is investigated numerically for an ensemble of 2D square
two-wavespeed billiards with rough boundaries and quadratic nonlinearity
representative of elastodynamic waves. Results of direct numerical simulations
are compared with theoretical predictions.
