  The rapid exponential increase in the Alfv\'en wave speed with height above
the solar surface presents a serious challenge to physical modelling of the
effects of magnetic fields on solar oscillations, as it introduces a
significant CFL time-step constraint for explicit numerical codes. A common
approach adopted in computational helioseismology, where long simulations in
excess of 10 hours (hundreds of wave periods) are often required, is to cap the
Alfv\'en wave speed by artificially modifying the momentum equation when the
ratio between Lorentz and hydrodynamic forces becomes too large. However,
recent studies have demonstrated that the Alfv\'en wave speed plays a critical
role in the MHD mode conversion process, particularly in determining the
reflection height of the upward propagating helioseismic fast wave. Using
numerical simulations of helioseismic wave propagation in constant inclined
(relative to the vertical) magnetic fields we demonstrate that the imposition
of such artificial limiters significantly affects time-distance travel times
unless the Alfv\'en wave-speed cap is chosen comfortably in excess of the
horizontal phase speeds under investigation.
