| second sound [`sek·ǝnd `sau̇nd] ACOUSTICS A transverse sound wave which propagates in smectic liquid crystals, and whose behavior resembles mathematically that of second sound in superfluid helium. CRYOGENICS A type of wave propagated in the superfluid phase of liquid helium (helium II), in which temperature and entropy variations propagate with no appreciable variation in density or pressure. |