Anomalous transport of ultrasound in a strongly scattering inhomogeneous medium

Abstract

Investigating the propagation of multiply-scattered waves in disordered materials allows the characterization of heterogeneous media and may reveal anomalous wave properties. We study wave transport in a deceptively simple system consisting of closed packed aluminum beads surrounded by low viscosity silicone oil, in which ultrasonic waves undergo many scattering events without incurring significant dissipative losses. Using ultrasonic pulsed transmission experiments, the small ballistic pulse at short observation times was separated from the much larger multiple scattering coda that extends over a wide range of arrival times, enabling the scattering strength to be assessed as a function of frequency and giving values of kls (wave vector times scattering mean free path) between 4 and 10. Based on kls, the propagation of the multiply scattered waves is expected to be well described by the diffusion approximation. However, this is not the case, and the observed behaviour suggests the presence of two coupled modes of propagation: a fast component traveling through the liquid and a slower component traveling through the bead network. The results are interpreted by developing a model for the coupled propagation of a diffusive component and a “renormalized” component that is described using the self-consistent theory of localization.