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Sound Perspectives
Megan S. Ballard
Applied Research Laboratories The University of Texas at Austin PO Box 8029 Austin, Texas 78713-8029 USA
Underwater Acoustics
The Technical Committee on Underwater Acoustics is concerned with sound-wave phe- nomena underwater and with the interaction of sound with the boundaries of the oceans.
In contrast to electromagnetic waves, which are highly attenuated in water, acous- tic waves can propagate long distances in underwater environments. For this rea- son, sound waves are used in water in much the same way that electromagnetic waves are used in the atmosphere to sense the environment and communicate. The Technical Committee (TC) on Underwater Acoustics (TCUW) is concerned with sound-wave phenomena underwater and with the interaction of sound with the boundaries of the ocean (the seabed and sea surface), with emphasis on the following topics: wave propagation, scattering and reverberation, ambient noise, sonar processing, and underwater acoustic instrumentation. The field of underwa- ter acoustics has a rich history that has been motivated by and asserted influence on world events. According to Goodman (2004), “The development of underwater acoustics in the Twentieth Century was closely related to, and for the most part, driven by the significant world events of the time, e.g., two world wars and the en- suing Cold War. No other field of acoustics was so affected by these events or had such importance in their outcome.” Today, work done by members of the TCUW represents a broad area of research, including topics with applications to fisher- ies, climate change, environmental remediation, and underwater communication. Here I describe some historical context as well as recent advances in each of the aforementioned research areas.
Propagation modeling has long been an essential area of study in underwater acoustics because it is necessary for understanding ambient noise, predicting de- tection thresholds, and designing acoustic arrays. The field of ocean acoustic mod- eling has reached a mature state of development, and a multitude of numerical techniques are available to solve the wave equation in the heterogeneous ocean waveguide (Jensen et al., 2011). Increasingly capable propagation models are con- tinually being developed, and some advancements include models that incorpo- rate elastic and poroelastic media. Another area of development is the inclusion of three-dimensional (3-D) propagation effects. Historically, sound propagation has been calculated using two-dimensional (2-D) models by assuming that the envi- ronment is symmetrical around the source. Although this approximation is suit- able for many underwater propagation environments, features in the water column (e.g., nonlinear internal waves) and bathymetry (e.g., the shelf break) can cause out-of-plane propagation effects. The development of 3-D propagation models has been spurred by these observations as well as by the availability of increasingly more capable computing resources.
Characterization of target scattering and environmental reverberation is necessary for the effective use of active sonar systems. The target-scattering problem involves the detection, identification, and acoustic signature (target strength as a function of angle and frequency) of an object that is compared with a library of templates (Burnett, 2015), including an accounting of the effect of propagation in the wave-
 ©2017 Acoustical Society of America. All rights reserved. volume 13, issue 4 | Winter 2017 | Acoustics Today | 63

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