Superluminal and negative group velocity of ultrasonic waves in microsphere suspensions

For acoustic propagation in a sufficiently dispersive medium, it is possible for the group velocity to exceed the vacuum speed of light and even become negative. In this talk a physically realizable suspension of elastic spheres in water is described that can support such abnormal (i.e., superluminal/negative) acoustic group velocities at moderate solids concentrations. The relative importance of single versus multiple scattering descriptions of these suspensions at volume fractions that can support abnormal velocities is discussed. The physical relevance of the group velocity concept to pulse propagation is also illustrated. Conditions under which superluminal/negative group velocities can be observed as the speed of a peak in the envelope of an acoustic pulse in the low‐megahertz ultrasound band are identified. Waveforms derived from propagation simulations are used to demonstrate how superluminal/negative group velocities could be directly observed experimentally.