Anisotropic ultrasound propagation in a smectic-Aliquid crystal

Anisotropic ultrasound propagation was studied in a magnetically aligned monodomain smectic-$A$ liquid crystal (di-$n$-decyl azoxy methyl cinnamate). A large attenuation was observed near the isotropic-smectic-$A$ phase transition. The anisotropy in the sound velocity was found to be in agreement with the predictions of hydrodynamic theory, and no dispersion was found in the frequency range used (4-12 MHz). The temperature dependence of the physically meaningful elastic constants was determined. The anisotropy in the attenuation was also measured. Unlike the velocity anisotropy, no intermediate minimum was observed.