  Dust acoustic waves in the bulk of a dust cloud in complex plasma of
low-pressure gas discharge under microgravity conditions are considered. The
complex plasma is assumed to conform to the ionization equation of state (IEOS)
developed in our previous study. This equation implies the ionization
similarity of plasmas. We find singular points of IEOS that determine the
behavior of the sound velocity in different regions of the cloud. The fluid
approach is utilized to deduce the wave equation that includes the neutral drag
term. It is shown that the sound velocity is fully defined by the particle
compressibility, which is calculated on the basis of the used IEOS. The sound
velocities and damping rates calculated for different three-dimensional complex
plasmas both in ac and dc discharges demonstrate a good correlation with
experimental data that are within the limits of validity of the theory. The
theory provides interpretation for the observed independence of the sound
velocity on the coordinate and for a weak dependence on the particle diameter
and gas pressure. Predictive estimates are made for the ongoing PK-4
experiment.
