Suppose two monochromatic waves of different amplitude propagating in the same direction (either same sense or opposed); then (except for phase difference pi/2) the resulting mean energy flow is not the sum (case of same sense) or the difference (opposed sense) of the interfering beams. Where the energy of the separate beams is gone? Depending on the phase difference between the waves the resulting energy flows change but (except for phase difference pi/2) the problem is that the total energy in space is not the sum of the two energies. In the case where the two amplitudes are equal, standing waves can be understood as an energy redistribution, but when amplitudes are different it seems there is energy creation or anihilation...