Normal Fluid Density, Second Sound, and Fourth Sound in an Anisotropic Superfluid

In a superfluid with an anisotropic quasiparticle energy, the normal-fluid density is a diagonal tensor. The second and fourth sound velocities are distinctly anisotropic, so that heat-pulse or fourth-sound studies with an applied magnetic field $\overrightarrow{\mathrm{H}}$ may permit identification of the direction of the gap axis with respect to $\overrightarrow{\mathrm{H}}$. High-frequency sound velocities may display a weaker anisotropy.