  Superfluid $^3$He confined in aerogel offers a unique chance to study the
effects of a short mean free path on the properties of a well defined
superfluid Fermi liquid with anisotropic pairing. Transport coefficients and
collective excitations, e.g. longitudinal sound, are expected to react
sensitively to a short mean free path and to offer the possibility for testing
recently developed models for quasiparticle scattering at aerogel strands.
Sound experiments, together with a theoretical analysis based on Fermi liquid
theory for systems with short mean free paths, should give valuable insights
into the interaction between superfluid $^3$He and aerogel.
