Implications of causality on acoustic propagation in highly dispersive bubbly media.

Abstract

Causality in nearly all physical systems has been a recurrent subject often causing apparent paradoxes since before the 20th century. Linear acoustic propagation through subsurface bubble clouds in the ocean offers an especially challenging physical system within which to investigate issues of causality, and has in the past had several competing fundamental theories. Signal travel times and absorption in such a system exhibit enormous variations depending on the acoustic signal frequency, bubble size distribution, void fraction, and other ambient physical parameters. In a recent work [Orris et al., J. Acoust. Soc. Am. 121, 3349–3362 (2007)] we presented a correction to some contemporary theories of acoustic propagation in bubbly media to bring them into compliance with the physical law of causality. We will discuss this theory within the context of complex analysis and some of its implications to acoustic signal propagation; also present experimental data, which commensurate with archival data, suggest that the theories are still far from offering a complete description of the physical phenomena surrounding acoustic propagation in bubbly media. Possible modifications will be explored that may lead to a complete self‐consistent causal theory of acoustic signal propagation in bubbly liquids. [Work supported by the Office of Naval Research.]