Second sound
A type of wave propagated in the superfluid phase of liquid helium (helium II) and in certain other substances under special conditions. The name is misleading since second sound is not in any sense a sound wave, but a temperature or entropy wave. In ordinary or first sound, pressure and density variations propagate with very small accompanying variations in temperature; in second sound, temperature variations propagate with no appreciable variation in density or pressure. See Liquid helium, Superfluidity
The two-fluid model of helium II provides further insight into the nature of second sound. In this model the liquid can be described as consisting of superfluid and normal components of densities ρs and ρn, respectively, such that the total density ρ = ρs + ρn. The superfluid component is frictionless and devoid of entropy; the normal component has a normal viscosity and contains the entropy and thermal energy of the system. In a temperature or second-sound wave, the normal and superfluid flows are oppositely directed so that ρs V s + ρn V n = 0, where V s and V n are the superfluid and normal flow velocities. Thus a variation in relative densities of the two components, and hence a temperature fluctuation, propagates with no change in total density or pressure. In a first-sound wave, the two components move in phase, that is, V n ≅ V s.
Theoretical predictions that second sound should exist in certain solid dielectric crystals under suitable conditions have been confirmed experimentally for solid helium single crystals at temperatures between 0.4 and 1.0 K (-459.0 and -457.9°F). See Dielectric materials
Another quite different class of materials can exhibit second sound. In smectic A liquid crystals, when the wave vector is oblique with respect to the layers of these ordered structures, a modulation of the interlayer spacing can propagate at nearly constant density.
Second Sound
weakly attenuated temperature waves that are propagated in superfluid liquid helium (He II) along with ordinary sound waves. Near a temperature of absolute zero the velocity v2 of second sound and the velocity v of ordinary sound (compression waves) are associated by the ratio 
At the point of phase transition from He II to He I (the A-point), v2 vanishes. Second sound is radiated from a heater with a variable temperature and is detected by a sensitive thermometer.
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