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 Alfven-to-sound speed ratio of about 0.1, a 5 per cent frequency increase of the f mode can be measured when k(x)H(p) = 3-4, where k(x) is the horizontal wavenumber and H-p is the pressure scaleheight at the surface. Since the solar radius is about 2000 times larger than H-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(x)H(p) approximate to 4, but decreased relative to its nonmagnetic value for k(x)H(p) approximate to 9.