  We analyse the interaction of exciton and exciton-polariton condensates in
semiconductor microcavity with a coherent acoustic wave. An analytical solution
for the dispersion of excitations of coupled condensate-phonon system is found
in the approximation of k-independent interactions. Accounting for k-dependence
results in a stronger modification of the dispersion, and even in the
appearance of the "roton instability" region.
