W. Jack Hughes

A class V flextensional transducer: the cymbal

The cymbal is a miniaturized class V flextensional transducer which was developed for potential use as a shallow water sound projector. In underwater tests, the experimental fixture was found to have a pronounced effect on the performance of the transducer through modifications of the mechanical boundary conditions imposed on the device. A less restrictive setup was devised for the underwater test of a cymbal transducer and very good agreement between the calculations and the experimentally measured transmitting voltage response was then obtained.

Capped ceramic underwater sound projector: The “cymbal” transducer

A new type of transducer has been developed for use as a shallow-water sound projector at frequencies below 50 kHz. Dubbed the “cymbal,” it is similar to the more commonly known “moonie” and class V ring/shell flextensional designs. Prototype cymbal arrays 2 mm thick with a radiating area of 11.4 cm2 have been developed and calibrated. Two mounting schemes have been examined: unpotted (oil-filled) and potted in a 5-mm thick layer of stiff polyurethane. In both cases, a transmitting response comparable to the more widely used Tonpilz transducer (with an equivalent radiating area) is attainable. When tested under hydrostatic pressures, a standard cymbal configuration has been shown to withstand exposures of 2.5 MPa (which corresponds to 250 m of water depth) before failure.

Cymbal array: a broad band sound projector.

A prototype 3 x 3 planar cymbal transducer array was built and tested. The array has a radiating area of 5.5 cm x 5.5 cm and a thickness of less than 8 mm. The measured transmitting voltage response was above 134 dB re 1 microPa/V @ 1 m and flat over the frequency range of 16 and 100 kHz. Array interaction was analyzed using an equivalent circuit model. The array interaction leads to variations in radiation resistance and velocity of the transducers in the array according to their surroundings in the array. The effect is enhanced overall efficiency and a flat response.

Design of and Fabrication Improvements to the Cymbal Transducer Aided by Finite Element Analysis

A miniature flextensional transducer, the “cymbal,” is an emerging underwater transducer technology for large area and restricted volume transmit and receive arrays. The performance of the device is being evaluated for a number of applications requiring a large number of elements. Simple design, low cost and the ability to tailor performance to the desired application are attractive features of the cymbal. Analysis of the fabrication and performance of the cymbal has revealed that the benefits of the cymbals flexible design also present production concerns. Asymmetry in the cavity depth or epoxy layer can result in unwanted resonances that can detract from the in-water performance. To avoid these spurious resonances, tolerances in cavity depth could be as low as 5 μm, resulting in low production yields. These and other fabrication concerns have been analyzed through experiment and finite element modeling software using the ATILA code. We were able to identify ways of improving manufacturing yields and minimizing variances to accommodate large array production. The origins of the spurious resonances are discussed along with the performance of a prototype array.

Tilted directional response patterns formed by amplitude weighting and a single 90° phase shift

It is shown that by appropriately processing the output signals of the elements in a symmetric array, a tilted sum pattern or difference pattern can be produced without numerous delay lines or phase‐shift networks. The tilted pattern is achieved by combining, in proper phase, two outputs of an array; the first output is that of a particular amplitude weighted phase‐symmetric array while the second is that of a particular amplitude weighted phase‐antisymmetric array. To achieve the proper phase relationship between these two outputs, one of them must be phase shifted by ±90°. Theoretical calculations and experimental results are presented for the case of a line array of nine elements that are equally spaced a half‐wavelength apart; both a sum pattern and a difference pattern tilted to 30° from the normal to the line array, by this method, are shown. The variation of the side‐lobe levels of these tilted patterns is examined as the ±90° phase shift is perturbed. The possibility of using this technique to con...

Grating lobe reduction in transducer arrays through structural filtering of supercritical plates

The effect of placing a structural acoustic filter between water and the transducer elements of an array to help reduce undesirable grating lobes is investigated. A supercritical plate is mounted to transducer elements with a thin decoupling polyurethane layer between the transducers and the plate. The plate acts as a radiation/incidence angle filter to pass energy at angles near normal incidence, but suppress energy at large incidence angles. Grating lobe reduction is achieved at the expense of limiting the available steering of the main lobe. Within this steer angle limitation, the main lobe can be steered as normal while the grating lobe level is reduced by the plates angular filtering. The insertion of a plate structural filter provides, in principal, an inexpensive and easily implemented approach to extend usable frequency bandwidth with reduced level grating lobes, without increasing the number of array elements. Even though the data match theory well, a practical material has yet to be found that possesses optimal material properties to make the proposed idea practical. This work represents the first attempt to advantageously utilize a plate above its critical frequency to provide angular dependent sound transmission filtering.

Analysis of resolution for an amplitude-steered array

In 1976, Hughes and Thompson introduced the idea of steering the maximum response of a linear array by amplitude weighting the output signals of the elements, thus eliminating the need for time delays or phase-shift networks. Currently that amplitude-steered array concept is being extended to a broadband two-dimensional array that can be used for real-time three-dimensional imaging. In shifting the use of the amplitude-steered array from underwater acoustic communications to imaging, we must consider different issues of the array’s performance such as lateral and axial resolution. For the linear amplitude-steered array, we show that both lateral and axial resolution are limited by the length of the array. The dependence of axial resolution on the length of the array is a unique feature of the amplitude-steered array, leading to an interesting tradeoff between lateral and axial resolution. A theoretical basis for the dependence is developed and simulation results are given.

Design, performance and modeling of piezoceramic hollow-sphere microprobe hydrophones

This paper describes the design and performance of a small, high-frequency, piezoelectric, underwater probe. The probes are fabricated from miniature, thin-wall, lead zirconate titanate ceramic hollow spheres with radii (r) from 0.5 mm to 2.5 mm and wall thickness (t) from 50 ?m to 250 ?m. The experimental results reported in this paper are focused on devices prepared from hollow spheres with an outer radius of 1.38 mm and a mean wall thickness of 75 ?m. As a hydrophone, these devices display a sensitivity of ?220.7 dB re 1 V ?Pa?1 at 250 kHz that stays flat to within ?3 dB over the frequency range from 10 kHz to 450 kHz with an omnidirectional response. Within the accessible range of sphere sizes (r = 0.5?2.5 mm, t = 50?250 ?m), this stable and flat sensitivity behavior could be extended to a range from ?212 to ?225 dB re 1 V ?Pa?1 and up to 1 MHz. Finite element analysis of this transducer using the ATILA? software is also included in this paper, comparing the computational results with the results from the experimental measurements. Based on the excellent agreement obtained, a parametric modeling study was also undertaken and its results are discussed in this paper. The miniature ceramic hollow-sphere transducers fill a gap in the frequency range of 200 kHz to 1 MHz with an omnidirectional response and much higher sensitivity than other ceramic and polymer piezoelectric probes.

On the steering of sound energy through a supercritical plate by a near-field transducer array

The ability to direct sound energy through the flexural vibrations of a submerged plate at various angles of incidence using a near-field transducer array is investigated. An alumina bar is placed in front of a one-dimensional, eight-element transducer array, between the array and the water. Operating in a receive mode, data were taken as a function of angle of incidence and compared to data taken without the presence of the alumina bar. The array was also operated in transmit mode and results were compared to corresponding receive mode data, showing that reciprocity holds. Results show that in fact sound energy can be steered through a plate, and that the measurement method used provides a convenient method of measuring the angular dependence of transmission through a plate, including measurements at frequencies above the plates critical frequency. Experimental results of sound transmission versus angle of incidence of finite sized plates agree qualitatively with theoretical results from an analysis of the transmission through an unbounded flexible partition.

Capped ceramic underwater sound projector

The cymbal‐type transducer developed at the Materials Research Laboratory at Penn State Univ., which is similar to the ring/shell flextensional transducer, is currently being investigated as a shallow‐water (<250 m) sound projector for the 10–75 kHz frequency range. The standard size cymbal transducer is 12.7 mm in diameter and approximately 2 mm in total thickness. It consists of a poled lead zirconate‐titanate (PZT) disk capped symmetrically on each face by identical thin metal caps. Each cap is shaped with a cavity on its inner surface so that an air space exists between the PZT disk and the top of the cap. The size of the air space, the thickness of the cap, and the type of cap material all influence the operating performance and allowable operating depth of the device. Parameters necessary to characterize an underwater projector such as resonance frequency, mechanical Q, electroacoustic efficiency, transmit voltage response (TVR), and source level will be presented. Experimental data will be compared...