It is shown that the conventional theory of thermoelasticity is insufficient to explain the behavior of pulsed laser ultrasonic signals from materials with defects. A theoretical model of thermoelasticity is proposed that takes into account the thermal excitation of defect states by laser radiation with their subsequent recovery to the initial state with a certain relaxation time. The linear model of thermoelasticity in terms of slow dynamics is developed. The difference in the behavior of ultrasonic signals from dielectrics and metals under pulsed laser excitation is analyzed and explained. It is shown that in the linear approximation of the thermoelastic model, slow dynamic effects can be taken into account by introducing an effective thermal coefficient.