Wiktionary
n. (context physics English) A wave of fluctuating temperature or entropy propagated in the superfluid phase of liquid helium
Wikipedia
Second sound is a quantum mechanical phenomenon in which heat transfer occurs by wave-like motion, rather than by the more usual mechanism of diffusion. Heat takes the place of pressure in normal sound waves. This leads to a very high thermal conductivity. It is known as "second sound" because the wave motion of heat is similar to the propagation of sound in air.
Normal sound waves are fluctuations in the density of molecules in a substance; second sound waves are fluctuations in the density of particle-like thermal excitations ( rotons and phonons). Second sound can be observed in any system in which most phonon-phonon collisions conserve momentum. This occurs in superfluids, and also in some dielectric crystals when Umklapp scattering is small. (Umklapp phonon-phonon scattering exchanges momentum with the crystal lattice, so phonon momentum is not conserved.)
Usage examples of "second sound".
The dumbing down of America is most evident in the slow decay of substantive content in the enormously influential media, the 30-second sound bites (now down to 10 seconds or less), lowest common denominator programming, credulous presentations on pseudoscience and superstition, but especially a kind of celebration of ignorance.
The second sound, clearly different from the first, sounded like nothing Ofelia had said.
His powers of sociopathic seduction were best exposited in five-second sound bites and best received by callow young women.
He paused a moment as he discerned a second sound, one that his keen hearing did not recognize.