  A simple perturbation description unique for all signs of curvature, and
based on the gauge-invariant formalisms is proposed to demonstrate that:
  (1) The density perturbations propagate in the flat radiation-dominated
universe in exactly the same way as electromagnetic or gravitational waves
propagate in the epoch of the matter domination.
  (2) In the open universe, sounds are dispersed by curvature. The space
curvature defines the minimal frequency $\omega_{\rm c}$ below which the
propagation of perturbations is forbidden.
  Gaussian acoustic fields are considered and the curvature imprint in the
perturbations spectrum is discussed.
