Low-frequency sound velocity near the critical point of xenon

By measuring the resonant frequency of an azimuthal mode in a horizontal annular resonator, it was possible to determine the sound velocity in xenon at {~} 1 kHz without difficulties caused by gravity-induced density gradients near the critical point. It was shown that the resulting velocity data along the critical isochore above Tc and in the coexisting vapor below Tc correspond to the thermodynamic (zero-frequency limiting) values. The experimental data were fitted with a theoretical curve based on the linear-model parametric equation of state, and the resulting value of 0.08 for the critical exponent α is in good agreement with the value determined previously from specific-heat data.