The expression for the normal velocity of sound in a fluid becomes indeterminate at the critical point since the coefficient of elasticity tends to zero and the specific heat at constant pressure approaches infinity. An expression has been obtained for the isentropic slope which does not become indeterminate at the critical point. Observations were made with a Pierce type variable path acoustic interferometer placed completely inside a pressure tank with provisions for external control. The isothermal variation of velocity and absorption was measured as a function of pressure at 572 kc in carbon dioxide. Above the critical temperature it was found that, as the pressure increases, the velocity decreases steadily, goes through a sharp minimum, and rises abruptly. Below the critical temperature a discontinuity in velocity was observed between liquid and vapor. The velocity in the liquid at 29°C was 208 meters/second, and in the vapor 179 meters/second. The velocity variations away from the critical point agree with previously published results. The attenuation decreases steadily with increasing pressure and shows an abrupt rise as the critical point is reached, which would be expected from critical fluctuation scattering. Using isotherm data two values of the velocity at the critical point were calculated, 154 and 146 meters/second. The measured value was found to be 141.6 meters/second.

Anderson, N. S., & Delsasso, L. P. (1951). The Propagation of Sound in Carbon Dioxide Near the Critical Point. The Journal of the Acoustical Society of America, 23(4), 423–429.