Calculations are carried out to describe the propagation of an acoustic wave which absorbs energy from an external radiation flux applied instantaneously at the gas boundary. This external radiation is considered to be far out of equilibrium with the gas; therefore, emission of radiation by the gas may be neglected. Two analyses are carried out: (1) a perturbation calculation with the radiation affecting the acoustic wave only through second‐order terms, i.e., radiation only slightly perturbs the acoustic wave; (2) an analysis for the transparent limit with the radiation producing a first‐order interaction with the acoustic wave, i.e., the acoustic wave is appreciably changed.