Adiabatic and isothermal sound waves: the case of supercritical nitrogen
  The acoustic sound dispersion of nitrogen in its liquid and supercritical phases has been studied by Inelastic X-Ray Scattering. Approaching supercritical conditions, the gradual disappearance of the positive sound dispersion, characteristic of the low temperature liquid, is observed. In the supercritical state, evidence for a crossover between adiabatic and isothermal sound propagation regimes is inferred by an analysis of the dynamic structure factor based on generalized hydrodynamics. 
F. Bencivenga, A. Cunsolo, M. Krisch, G. Monaco, G. Ruocco and F.   Sette