A novel photoacoustic measurement technique is introduced, which is based on the phase perturbation of an external acoustic wave that propagates through a region of material irradiated by an optical beam. The mechanism of the perturbation is the local change in acoustic velocity due to heating of the irradiated sample. Because the acoustic phase perturbation may be calibrated directly, it is found that the absolute optical absorption coefficient of the irradiated sample may be determined using the technique. Both the experimental phase detection system and a theoretical analysis of the acoustic perturbation are discussed. Finally, an experiment is described in detail, where the measurement technique is used to determine the absolute optical absorption coefficient of a liquid.