Application of fourth sound to study the superfluid helium-4 and the recently discovered superfluid helium-3

Fourth sound is propagated in a superfluid placed in the tiny pores of tightly packed fine powder. Since it is a type of usual sound (pressure-density) propagation, fourth sound may be excited and detected by condenser transducers. In superfluid helium-4 fourth sound has been used (1) to determine the superfluid component density by measuring the sound velocity, and (2) to study the nature of persistent currents using Doppler-shifted fourth sound. In superfluid helium-3 propagation of fourth sound provided conclusive evidence of superfluidity of liquid helium below 2.6 mK. Superfluid component density and some anisotropic properties of superfluid helium-3 have been investigated using fourth sound.