Sound propagation in liquid helium near the lambda point: Thermodynamics

Abstract
A new derivation, capable of straightforward finite-frequency generalization, is given of the velocity of sound in the Pippard-Buckingham-Fairbank approximation. The first correction to this approximation is expressed in a compact form and leads to a determination of the slope of the lambda line at low pressures. A crossover function for the critical specific heat is also obtained.