  With the ultimate aim of using the fundamental or $f$-mode to study
helioseismic aspects of turbulence-generated magnetic flux concentrations, we
use randomly forced hydromagnetic simulations of a piecewise isothermal layer
in two dimensions with reflecting boundaries at top and bottom. We compute
numerically diagnostic wavenumber-frequency diagrams of the vertical velocity
at the interface between the denser gas below and the less dense gas above. For
an Alfv\'en-to-sound speed ratio of about 0.1, a 5% frequency increase of the
$f$-mode can be measured when $k_xH_{\rm p}=3$-$4$, where $k_x$ is the
horizontal wavenumber and $H_{\rm p}$ is the pressure scale height at the
surface. Since the solar radius is about 2000 times larger than $H_{\rm p}$,
the corresponding spherical harmonic degree would be 6000-8000. For weaker
fields, a $k_x$-dependent frequency decrease by the turbulent motions becomes
dominant. For vertical magnetic fields, the frequency is enhanced for
$k_xH_{\rm p}\approx4$, but decreased relative to its nonmagnetic value for
$k_xH_{\rm p}\approx9$.
