  The acoustic spacetime corresponding to perturbed
Friedman-Lemaitre-Robertson-Walker universe inherit the space isometries from
the original FLRW model, but essentially differs in dynamics. The scale factor
manifestly depends on the equation of state of the matter content. Despite the
higher complexity of the background evolution the perturbation equation in this
space is substantially simpler: the density perturbations obey d'Alembert
equation. Canonical formalism reconstructed in the acoustic spacetime enables
one to employ the Klein-Gordon scalar product. Consequently, the Fourier
decomposition of the perturbation field provide the time-independent Fourier
coefficients and the time-independent spectrum. The perturbation spectrum does
not depend of the choice of the Cauchy hypersurface from which the data are
collected. Noether constants associated with the six-parameter isometry group
define the components of the momentum, hyperbolic momentum and angular momentum
of sound.
