  An enhancement in high-frequency acoustic power is commonly observed in the
solar photosphere and chromosphere surrounding magnetic active regions. We
perform 3D linear forward wave modelling with a simple wavelet pulse acoustic
source to ascertain whether the formation of the acoustic halo is caused by MHD
mode conversion through regions of moderate and inclined magnetic fields. This
conversion type is most efficient when high frequency waves from below
intersect magnetic field lines at a large angle. We find a strong relationship
between halo formation and the equipartition surface at which the Alfv\'en
speed $a$ matches the sound speed $c$, lending support to the theory that
photospheric and chromospheric halo enhancement is due to the creation and
subsequent reflection of magnetically dominated fast waves from essentially
acoustic waves as they cross $a=c$. In simulations where we have capped $a$
such that waves are not permitted to refract after reaching the $a=c$ height,
halos are non-existent, which suggests that the power enhancement is wholly
dependent on returning fast waves. We also reproduce some of the observed halo
properties, such as a dual 6 and 8 mHz enhancement structure and a spatial
spreading of the halo with height.
