result in a change in the size of the Brillouin zone and, assuming a fixed zone boundary energy, would give a corresponding shift in phonon velocity. This cannot explain our data for two reasons. First, the phonon anomaly is only observable in [[$\textrm{Mg}\textrm{V}_2\textrm{O}_4$]{}]{} over a range of intermediate wavevectors and no change in the $lim_{q\rightarrow 0}$ slope is observed using neutron scattering on the THz timescale. Second, the change in phonon energy is larger then would be expected from such an effect and is inconsistent with the thermal expansion data presented in Fig. \[fig:lattice\] and also from the higher resolution x-ray data for the unit cell volume presented in Ref. . Therefore, the anomaly observed here is not due to thermal changes in the lattice constants.

The phonon anomaly in [[$\textrm{Mg}\textrm{V}_2\textrm{O}_4$]{}]{} measured with neutron scattering differs from the response observed with ultrasound [@Ishikawa03]. At $T_{OO}$, Watanabe *et al.* [@Watanabe14] reported a temperature dependent change of the sound velocity of the TA2 mode $\Delta v \approx 25\,\%$. We find no observable change for momentum transfers near the zone centre, however measure a spoon-like anomaly for intermediate wavevectors. We speculate that the difference between the neutron low-$q$ data and the ultrasound data 