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Applied Physics Seminar: Revealing Hidden Information With Quadratic Products of Acoustic Field Amplitudes

Professor David R. Dowling
Wednesday, January 23, 2019
12:00-1:00 PM
335 West Hall Map
Acoustic waves are omnipresent in modern life and are well described by the linearized equations of fluid dynamics. Once generated, acoustic waves carry and collect information about their source and the environment through which they propagate, and this information may be retrieved by analyzing recordings of these waves. Because of this, acoustics is the primary means for imaging and remote sensing in otherwise opaque environments, such as the Earth's oceans and crust, and the interior of the human body. For these information-retrieval tasks, acoustic fields are nearly always interrogated within their recorded frequency range or bandwidth. However, this frequency-range restriction is not general; acoustic fields may also carry hidden information at frequencies outside their bandwidth that can be revealed by reintroducing a marquee trait of fluid dynamics: quadratic nonlinearity. In particular, the wave-triad interactions considered in turbulence motivate the use of a quadratic product of complex acoustic-field amplitudes at two different frequencies to obtain acoustic-field information at the difference and sum of these two frequencies. Despite some fundamental limitations, this reintroduction of nonlinearity to acoustics enables a variety of remote sensing applications that were long thought to be impossible. In particular, it allows the detrimental effects of sparse-array recordings, random scattering, and many other unknown source-to-receiver propagation effects to be suppressed when the recorded acoustic field has sufficient bandwidth. Examples and applications based on simulations, laboratory experiments, and ocean propagation measurements are provided that involve frequencies from a few Hertz to more than 100 kHz, and propagation distances from tens of centimeters to more than 300 kilometers. [Sponsored by ONR, NAVSEA, and NSF].
Building: West Hall
Event Type: Lecture / Discussion
Tags: Physics
Source: Happening @ Michigan from Applied Physics