  The dependence on chopper frequency of the effective thermal diffusivity and
effective thermal conductivity in photoacoustic experiments is discussed. The
theoretical model of a two-layer structure at rear-surface illumination in the
high frequency limit is considered. It is shown that the effective thermal
diffusivity presents ``resonance'' while the effective thermal conductivity
sharply changes its magnitude and sign. Such ``resonant'' behavior strongly
depends on the surface thermal conductivities associated with the interface
thermal contacts.
