fourth sound
fourth sound
[¦fȯrth ′sau̇nd] (cryogenics)
A pressure wave which propagates in helium II contained in a porous material such as a tightly packed powder, and which results entirely from motion of the superfluid component, the normal component being immobilized by its viscosity.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.