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Monochromatic hypersonic sound waves are efficiently generated by spatially periodic impulsive heating of a material with two interfering laser pulses. Their evolution can be monitored optically and the acoustic velocity and attenuation in the GHz frequency range can be measured. This method is particularly well adapted when external transducers are difficult to use: mechanical contact, strong attenuation, etc. This paper critically analyzes the use of the method and demonstrates its application to the study of a polymer material (PMMA) undergoing the glass‐rubber phase transition.