The development of monochromatic, intense, and highly directional laser light sources has made it possible to measure accurately the velocity, the frequency, and the lifetimes of thermally excited hypersonic sound waves. The technique employed is to study the spectrum of the light scattered from the thermally generated sound waves in the medium. A theory is presented which demonstrates explicitly the information contained in the intensity, the spectral positions, and spectral widths of the scattered light. An experimental system is also described to measure the spectrum. Preliminary data is presented on the velocity and lifetimes of ∼6 Gc/s sound waves in water and toluene.