  The covariance of the d'Alembert equation for acoustic phenomena, described
by mechanical waves in one or three spatial dimensions, under Galilean
transformations, is demonstrated without the need to abandon the hypothesis
that time is absolute in Classical Mechanics. This is true only if and only if
the phase velocity of sound depends on the velocity of the observer. On the
other hand, it is also shown that the same d'Alembert equation is covariant
under Lorentz transformations if and only if the phase velocity of light does
not depend on the observer.
