Superfluid Two-Phase Sound

The properties of the sound wave propagating in a superfluid two-phase system (liquid helium and its vapor) are investigated. At a frequency sufficiently low so that the superfluid helium and its vapor are in quasistatic equilibrium, there can propagate only one sound mode whose wave-guide-like velocity depends strongly on the vapor properties as well as the velocity of second sound. Near the $\lambda$ point, the mode probes the critical properties of the specific heat of the superfluid.