  We consider a universe filled with perfect fluid with the constant equation
of state parameter $\omega$. In the theory of scalar perturbations, we study
the effect of peculiar velocities on the gravitational potential. For radiation
with $\omega=1/3$, we obtain the expression for the gravitational potential in
the integral form. Numerical calculation clearly demonstrates the modulation of
the gravitational potential by acoustic oscillations due to the presence of
peculiar velocities. We also show that peculiar velocities affect the
gravitational potential in the case of the frustrated network of cosmic strings
with $\omega =-1/3$.
