The interpretation of hypersonic and ultrasonic 'velocities' in fluids

Abstract

It is suggested that the conventional view of Brillouin light scattering and ultrasonic excitation in fluids in terms of phonon phase velocity and attenuation can be misleading and leads to spurious anomalies. A simple comparison is made of the two types of experiments from a fundamental point of view. The analysis of an ultrasonic experiment from first principles is given and is illustrated by an explicit example, the classical hydrodynamic fluid. It is pointed out that an important parameter is usually overlooked in the conventional ultrasonic experiment.