Observation of acoustic magneto-chiral anisotropy in $\alpha$-quartz

We report the experimental observation of magneto-chiral anisotropy in the longitudinal and transverse ultrasound propagation in $\alpha$-quartz. To perform such measurement, we have built an ultrasound spectrometer with unprecedented experimental resolution of the order of $\mathrm{\Delta }v\mathrm{/}v\sim {10}^{-8}$. We present a simple macroscopic Becquerel-like analytical model that accounts for the magnitude of the observed effect and its frequency dependence.