Kim C. Benjamin

Forward and backward projection of acoustic fields using FFT methods

The forward and backward propagation of harmonic acoustic fields using Fourier transform methods is presented. In particular, the forward propagation of a velocity distribution to obtain a pressure field and the backward propagation of a pressure field to obtain a velocity distribution are addressed. Numerical examples are presented to illustrate the nearfield behavior of the pressure field from complex planar vibrators, e.g,—an ultrasonic transducer or plate, with nonuniform velocity distributions. The numerical results, which were obtained via the use of FFT algorithms, are presented for vibrators which are operating above and below coincidence. These results illustrate the acoustic nearfield as a function of distance from the vibrator. Numerical results are also presented to illustrate the backward projection method. The pressure field of a 3×3 focused array is back projected to obtain the velocity distribution for several cases of interest. These results illustrate the utility of the transform method ...

The Development of a very Large, Dense Packed, Two-Dimensional, Ultrasonic Imaging Array

The design, construction, and measured results for a prototype module of probably the world’s largest dense‐packed, two‐dimensional, ultrasonic (1.5‐MHz) receive array is described. Containing over a quarter million diced elements, and a centrally located, constant beamwidth, spherical cap projector, the complete array was to consist of 25 modules, of which the central module is described. The individual receive element dimensions were (0.91×0.91×0.89) mm, with a center to center spacing of 1.07 mm. The ultrasonic spherical cap projector element had a 29‐mm radius of curvature and an active angular aperture of 40 deg. Both projector and receive elements were designed to achieve a minimum 3‐dB beamwidth of 30 deg at 1.5 MHz. In both cases the active material was piezoceramic. This presentation will discuss the various material tradeoffs associated with the selection of the: (1) active layer, (2) backing layer, (3) mechanical, and (4) electrical connections. Also the issues of dicing and back filling of the...

On the use of injection molded 1–3 piezocomposite for parametric mode sonar projectors

Recent experimental studies have indicated that injection molded 1–3 piezocomposite is suited for parametric mode sound generation. Using a new Navy calibration standard (F82), which relies on the 1–3 composite for the active layer, the authors were able to drive the projector at sufficient power levels and measure difference frequency sound fields comparable to conventional diced PZT‐based transducer designs. Although the particular 1–3 composite used in the experiment had a low primary mode efficiency, (10 μPa) were realized at a difference frequency of 50 kHz for very modest electric fields (1.5 V/mil). The subject presentation will describe the experimental results as well as include a discussion of modeling issues, and related predicted results. Suggestions will be made for improving the injection molded composite efficiency. [Work supported by NRaD Code D744.]

An affordable, acoustically transparent, two‐dimensional, high‐resolution, acoustic imaging array

The design, fabrication, and measured results for a prototype, two‐dimensional, ultrasonic imaging array will be discussed. The active sensor consists of virtually three layers: (1) a piezoelectric 1‐3 composite layer; (2) a flex‐circuit component; and (3) a special z‐axis conductive adhesive which joins them. The sensor construction utilizes electroplated, injection‐molded, 1‐3 piezoelectric composite as the active layer. Four acoustically thin, flex circuits provide the electrical connections required for addressing the 468 individual array elements. Of key importance to the fabrication process is the use of a composite, b‐stage, adhesive film layer, which combines both conductive and nonconductive regions in a pattern‐specific orientation within the plane comprising the bond line interface. The conductive regions of the adhesive film are registered with respect to the electroplated sections on both the flex circuit and the 1‐3 piezoelectric composite substrate. The transducer array is a reciprocal devi...

Experimental evaluation of dynamic fatigue in glass reinforced composite flextensional transducer shells

Results from a controlled experiment aimed at determining the fatigue life, critical flaw size, and critical flaw location for glass reinforced composite flextensional shells are presented. Four shells, two E‐Glass and two S‐Glass were built into active projectors and cycled in air for 109 cycles. Intentional interlaminar and surface type flaws were symmetrically located on a shell of each composition. Static and dynamic stress levels simulated the anticipated conditions in water at full dynamic displacement and maximum operational depth. The fatigue failure criterion was defined as a 15% change in shell stiffness. The stiffness in the shells was computed from the measured transducer admittance and strain gage readings. Other sensory data included those from accelerometers, thermistors, and pre‐ and postcycling color photographs of the shell. The results indicate that all four shells survived one billion cycles without any significant performance degradation. In addition to these fatigue data, creep measu...

Piezocomposite actuator designs for an active underwater acoustic control system coating

The design and development of piezocomposites for use as the actuating device in an underwater acoustic control system coating will be discussed. Both flexible piezoceramic and 1–3 piezocomposite designs are explored as actuator candidates in the thin‐walled polymer matrix encapsulant coating. The actuators will be presented in single layer form for the absorption of normally reflected acoustic pressure waves. A bilaminate design will be shown for the absorption of both the normally reflected and transmitted acoustic waves. Experimental measurements will be used for performance comparison of different composite and lamination designs.

Evaluation of the complex coefficients ŝ33H, d̂33, and μ̂33T of length expander magnetostrictive rods

An analytical model is presented for evaluating the effective complex material coefficients of length expander piezomagnetic rods with magnetic field in the direction of wave propagation. In particular, the model facilitates the evaluation of complex ŝ33H, k33, μ33, and d33 from the measured set of quantities [fRI,QMI, fRE,QME,Zex, fex], where the superscripts I and E denote the ac open‐circuit and short‐circuit conditions, respectively. Thus fRI and QMI are the measured ac open‐circuit resonance frequency and mechanical storage factor; likewise for fRE and QME. Here, Zex is the measured value of the electrical impedance at the frequency fex. An impact testing technique [J. Acoust. Soc. Am. Suppl. 1 67, S32 (1980)] is used to measure the resonance frequencies and mechanical storage factors of the free‐free magnetostrictive rod test specimen for the two electric boundary conditions. Experimental results are presented for ŝ33H, d33, and μ33T of grain‐oriented Terfenol‐D rods for several levels of magne...

Magnetostrictive metallic glassy ribbon gradient hydrophone

The development of a directional low‐frequency gradient hydrophone which utilizes the magnetostrictive Metglas alloy (2605SC) is presented. The Metglas ribbon was transversely annealed in a magnetic field which raises the magnetomechanical coupling factor above 0.90. The Metglas ribbon was laminated to a slightly thicker stainless steel strip, and the bilaminate was mounted within a neutrally buoyant housing in a cantilivered configuration. Three Metglas/stainless steel thicknesses were examined, 25.4/38.1, 50.8/76.2, and 101.6/127.0, where all dimensions are in micrometers. Measured results indicated that the hydrophone sensitivity is largely affected by the Metglas thickness, cross‐sectional area, and number of turns in the pickup coils. The hydrophone had a dipole directionality which was due to the ribbon geometry of the active element.