Apart from the usual sound mode known from classical fluids characterized as waves of density (or pressure), He II supports additional sound modes due to its two-fluid nature.
The usual sound mode is realized by the two components oscillating in phase, reproducing the density wave, and is called "first sound". Its velocity varies continuously between He I and He II, from 180 m/s at 4.2 K to 240 m/s at low temperatures. However, if the normal and superfluid components oscillate in anti-phase, an undamped wave of temperature (entropy) is created, with density and pressure kept constant. This wave is called "second sound" and is well-defined in He II between the lambda point and cca. 0.8 K.
First sound = waves of pressure/density
Second sound = waves of temperature/entropy
Additionally, a "third sound" exists in the superfluid film and is related to quantum evaporation and condensation. Finally, "fourth sound" propagates inside superleaks and other environments where the normal component cannot move due to viscosity. Note also that the term "zero sound" is sometimes used for oscillations of the Fermi surface in Fermi liquids such as 3He, but is not relevant to He II.
