coils were aligned such that the focusing condition for the TA2 branch was fulfilled. The data was corrected for resolution effects reducing polarisation [@Habicht03; @Habicht04]. These are caused by the sample mosaic and the slope of the phonon dispersion.

 \[sec:fields\] Field dependence of phonon energy 
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We finally present the magnetic field dependence of the transverse acoustic phonon lifetimes and energies in the same wavevector region where temperature dependent anomalies are observed. While a magnetic field response has been reported for polar phonons in spinels based on optical data [@RudolfA07], magnetic field effects on the acoustic phonon response have been reported in superconductors and also in metals where the Fermi surface topology is relatively flat in momentum space [@Pynn74]. Attempts on semiconducting materials have failed to observe any effect. [@Comes81]

The field dependence of $\Gamma_p$ of the TA2 phonon at $q=\left(\overline{0.4}\ 0.4\ 0 \right)$ was determined for [[$\textrm{Mg}\textrm{V}_2\textrm{O}_4$]{}]{} by application of a magnetic field $B_{[001]}$ along the $[001]$-direction (Fig. \[fig:field\_001\]). The phonon energy $E$ is independent of $B_{[001]}$ in the cubic phase while $E$ decreases subtly at $T = 40$ K in the tetragonal phase above 7 T by $\approx 0.2$ meV up to the highest magnetic field achievable in 