The visible interaction of low‐intensity ultrasonic waves with a nematic liquid crystal may be explained simply by the hypothesis of a torque tending to orient LC molecules perpendicular to the vibrations. It is shown how this torque can be related to anisotropic attenuation of ultrasound and minimization of propagation losses. First theoretical developments and optical measurements on a homotropic layer of MBBA are in good agreement and may predict a few simple general laws.