As any dolphin could tell us, sound travels well in water. While water absorbs electromagnetic radiation and light waves over relatively short distances, sound travels widely and even faster in water than in air. That’s why we use sonar (SOund NAvigation Ranging), the acoustic equivalent of radar, to detect and locate underwater objects.

Figure 1: Sonar uses transmitted and reflected sound waves to locate underwater objects.

Sonar system developers continually seek to increase performance while also reducing size, weight, and cost, but this is easier said than done. Factors that must be taken into account include the characteristics of acoustic-wave propagation in water of varying densities, the vibrations and elastic waves that come into play with respect to the sonar system’s materials and components, electrical considerations, and many others.

Building and testing a series of physical prototypes on a trial-and-error basis is one way to conduct system development, but this tedious approach is time-consuming, costly, and makes it difficult to achieve real-world performance close to the theoretical best case.

In contrast, the use of tightly coupled multiphysics modeling, simulation, and visualization capabilities, such as those provided by the COMSOL software environment, can speed up system development exponentially and lead to a better end result as well.

A case in point is the development of a new type of sonar acoustic projector, designed to provide improved performance at half the size and weight of existing projectors. Stephen Butler, an Acoustical Engineer and Principal Investigator at the U.S. Navy’s Naval Undersea Warfare Center Division Newport in Newport, RI (www.navsea.navy.mil/nuwc/newport/ default.aspx), used COMSOL Multiphysics with the Acoustics Module to accelerate the development of the new projector.

Figure 2: The images show (a) a conventional Class IV flextensional transducer, and (b) the new Class VII “dogbone” directional flextensional transducer.

The Naval Undersea Warfare Center Division Newport provides research, development, test and evaluation, engineering, analysis and assessment, and fleet support capabilities for submarines, autonomous underwater systems, and offensive and defensive undersea weapon systems. The Center also stewards existing and emerging technologies in support of undersea warfare.

Sonar Basics

A sonar system consists of a projector to transmit acoustic energy and an array of hydrophones to receive the reflected sound waves from underwater objects. Sound waves are not generally unidirectional, though, so one key design goal is to increase the strength of the acoustic beam in one direction and to null it in the other. This will result in a more precise acoustic beam, which increases the sonar system’s capability to detect objects (Figure 1).

Figure 3: These images of the Class VII dogbone flextensional prototype sonar transducer show (a) the shell and ceramic stack, and (b) the encapsulated unit.

Some acoustic projectors are based on flextensional transducer technology. These are rugged, high-power, and compact devices, with an actuator such as a piezoelectric ceramic stack positioned between curved metal shells or staves made from aluminum, steel, or fiberglass. The staves may be either convex or concave, but are usually convex in conventional devices (Figure 2). The actuator is fixed to each end of the curved shells so that as it expands and contracts with an applied electric field, that motion is converted into flexure of the shells with an approximately 3:1 amplification in motion. These flexures produce acoustic energy in a manner similar to that of an acoustic loudspeaker’s cone.

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