  We present VelCrys, a web-based interactive tool, that allows to perform
further post-processing of the elastic tensor in order to compute and plot the
group velocity of the acoustic waves for any crystal symmetry. We also
implemented the calculation of effective magnetic corrections to the elastic
tensor and corresponding fractional change in group velocity under a magnetic
field. We apply it to dry sandstone, cubic CoPt and hcp Co to show some of the
program features. In the analysis of magnetic corrections, we find complex
landscapes of fractional change in group velocity as a function of ray
direction, as well as a field dependence consistent with Simon effect.
