Ultrasound in the isotropic phase of cholesteric liquid crystals

The acoustical properties of cholesteric liquid crystals in the isotropic phase including the vicinity of the phase transition point to the ordered phase are investigated. An interaction of fluctuating field of the tensor order parameter with the sound wave is considered. In Gaussian approximation the contribution of fluctuating modes to the sound velocity dispersion and attenuation is calculated. The obtained formulae are compared with ones for nematics in isotropic phase. It was found that the difference between these results rises in the vicinity of the phase transition point. This difference is connected, on the one hand, with the specific fluctuating spectrum and on the other hand with the closing of the instability temperatures of some fluctuating modes to the phase transition temperature T-c. The results are illustrated by the numerical calculations.