Theory of sound propagation in superfluid 3He-A

The theory of sound propagation in pair-correlated Fermi liquids developed previously by Woelfle is applied to the Anderson-Brinkman-Morel state, believed to describe 3He-A. The effect of quasiparticle collisions is included. Expressions for the anisotropic sound absorption and velocity valid at arbitrary frequency, temperature, and pressure are derived in terms of averages in momentum space of certain simple functions. Numerical results are in good agreement with lower pressure data but deviate by 10-20% at the melting pressure. The discrepancies are attributed to strong coupling effects not included in the theory