No abstract

A requirement for underwater acoustic sources with higher source level and wider bandwidth at lower frequencies has, in the past 5 years, lead to interest in flextensional transducers. A low-frequency prototype has been developed, manufactured, and tested at ARE(Portland); the technology is the subject of a license agreement with industry. In this paper, finite element (FE) analyses performed in support of the prototype development using PAFEC software are described. PAFEC is a commercially available FE package with the facility to model piezoelectric materials, and exterior fluid loading by the doubly asymptotic approximation. In-air and in-water resonant frequencies are well predicted, as is the effect of hydrostatic pressure on stack prestress. The transducer is subjected to large stresses during assembly, and predictions for these have also been obtained. Problems remaining in the FE modeling, and improvements in the capabilities of PAFEC under development to overcome them, are discussed. An equivalent circuit model for an array of these flextensional transducers is described, and a comparison of admittances and farfield predictions with measurements presented.

A fast technique for deconvolving signals in a dispersive multipath shallow-water environment using inverse filters is compared with the more commonly used deconvolution technique of time reversal (also known as phase conjugation). The objective of such techniques is to improve the accuracy of sound transmission from a source array to some receiving space. Time reversal provides benefits in this regard but here the additional performance that can be gained from inverse filters is examined. Several strategies for obtaining a set of inverse filters are discussed, each aimed at improving the accuracy of the reconstruction of the desired time signals through inverse techniques. It will be shown that an “optimal” inversion (in the sense of achieving a flat system response in the frequency domain) does not necessarily achieve a realizable time domain filter. A fast field model (using OASES) of the Giglio Basin shallow-water test facility is used as the basis for evaluating the various focusing strategies for single receiver locations. It will be seen that inverse filter arrays provide enhanced temporal and spatial focusing when compared to time reversal arrays. In addition, inverse filtering allows multiple receivers to be used, thereby increasing bandwidth or improving redundancy.

The ambient noise model CANARY calculates noise coherence and array noise response by treating the noise sources as surface distributions rather than points. This assumption leads to simplification of the propagation, even in range-dependent environments, and by allowing variations in the source density one can represent distant storms or groups of ships. Included is a description of the numerical algorithms used to calculate coherence. Some recently developed analytical solutions for uniform source distributions and uniformly sloping seabed3 are used as test cases for CANARY. Some additional examples demonstrate CANARY's performance in more realistic environments and conditions including wind and shipping sources, and comparisons are made with the noise model RANDI.

A requirement for underwater acoustic sources with higher source level and wider bandwidth at lower frequencies has, in the past 5 years, lead to interest in flextensional transducers. A low-frequency prototype, which consists of a GRP shell and convention ceramic stacks, has been developed, manufactured, and tested at ARE(Portland); the technology is the subject of a license agreement with industry. In this paper, finite element (FE) analyses performed in support of the prototype development using PAFEC software are described. PAFEC is a commercially available FE package with the facility to model piezoelectric materials, and exterior fluid loading by the doubly asymptotic approximation. In-air and in-water resonant frequencies are well predicted, as is the effect of hydrostatic pressure on stack prestress. The shell is subjected to large interlaminar shear stresses during stack insertion, and predictions for these have also been obtained. Work is in hand at PAFEC to add the capability to couple a Helmholtz integral equation representation for the exterior fluid to the existing finite element package for the structure. This will enable source level and bandwidth calculations to be made, which, at present, cannot be accurately predicted.