Low-frequency ultrasound and smectic dynamics

It is demonstrated that possible violation of the conventional hydrodynamics of smectic liquid crystals in the kilohertz frequency range is usually not observed in experiments. In ultrasonic experiments, deviations from the Stokes quadratic frequency dependence of the absorption coefficient are determined by the process of acoustic relaxation. This dependence is neither universal nor characteristic of only the smectic-A phase. The character of the frequency dependence can vary with the wave vector orientation relative to the direction of magnetic induction.