Causality Bubbles to the Surface: Investigating Acoustic Causality in Highly Dispersive Bubbly Liquids
NAVAL RESEARCH LAB WASHINGTON DC ACOUSTICS DIV
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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 in the past has 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. We have found a correction to some contemporary theories of acoustic propagation in bubbly media that has brought these theories into compliance with the physical law of causality. In doing so we have created a self-consistent theory that also matches higher-frequency data taken in the NRL Salt Water Tank Facility, as well as historical data. We have experimentally investigated this new theory and have observed some additional features that have significant implications upon acoustic signal propagation and suggest that we have only begun to scratch the surface of providing a comprehensive description of the physical phenomena surrounding acoustic propagation in the highly dispersive environment of bubbly media.
- Radiofrequency Wave Propagation