No abstract

The design of efficient low-frequency f/extensional sonar transducers is a present challenge which is made difficult by a partial lack of general accurate mathematical models. Thus the application of the finite-element method to this problem is a promising approach which has been worked recently. To test the ability of the finite-element code ATILA [Decarpigny et al., J. Acoust. Soc. Am. 78, 1499-1507 (1985) ] to predict the in-air and in-water dynamic behavior of such structures, an axisymmetrical thin-shell transducer was built, and its acoustical behavior was experimentally and numerically analyzed. This paper first presents the modal analysis of this projector, using different finite-element meshes as well as a mixed finiteelement-plane-wave model and the comparison of numerical displacement field values to holographic measurements. Second, it describes an in-water harmonic analysis in which the model of the infinite fluid domain is reduced to a portion of the acoustic nearfield, limited by a spherical boundary upon which special damping finite elements are attached. All farfield quantities are then calculated using an efficient extrapolation algorithm [R. Bossut and J. N. Decarpigny, J. Acoust. Soc. Am. 86, 1234-1244 (1989) ]. Finally, measured transmittifig voltage response and directivity patterns are compared to the finite-element predicted values. The comparison proves the ability of the finite-element approach of provide detailed and accurate insights in the behavior of transducers working with shell vibrations.

No abstract

Acoustic fields radiated by vibrating elastic bodies immersed in an infinite fluid domain are, in general, quite difficult to compute. This paper demonstrates in the two-dimensional (2-D) case that the radiated near field can be easily obtained using the finite element method if dipolar damping elements are attached to the mesh external circular boundary. These elements are specifically designed to absorb completely the first two components of the asymptotic expansion of the radiated field. Then, the paper provides a new extrapolation method to compute far-field pressures from near-field pressures, using the 2-D Helmholtz equation and its solution obeying the Sommerfeld radiation condition. These developments are valid for any radiation problem in 2D. Finally, two test examples are described, the oscillating cylinder of order m and a finite width planar source mounted in a rigid or a soft baffle. This approach is the generalization to 2-D problems of a previously described approach devoted to axisymmetrical and three-dimensional (3-D) problems [R. Bossut et al., J. Acoust. Soc. Am. 86, 1234–1244 (1989)]. It has been implemented in the ATILA code. It is well suited to the modeling of high-frequency transducers for imaging and nondestructive testing.

No abstract