  The cosmological electroweak phase transition can be strongly first order in
extended particle physics models. To accurately predict the speed and shape of
the bubble walls during such a transition, Boltzmann equations for the CP-even
fluid perturbations must be solved. We point out that the equations usually
adopted lead to unphysical behavior of the perturbations, for walls traveling
close to or above the speed of sound in the plasma. This is an artifact that
can be overcome by more carefully truncating the full Boltzmann equation. We
present an improved set of fluid equations, suitable for studying the dynamics
of both subsonic and supersonic walls, of interest for gravitational wave
production and electroweak baryogenesis.
