Thomas R. Howarth

Electroacoustic evaluations of 1-3 piezocomposite SonoPanel/sup TM/ materials

An advanced configuration 1-3 piezocomposite, designated by its manufacturer as SonoPanel/sup TM/, has been investigated for potential underwater acoustical applications. In-air electromechanical characteristics and in-water acoustical properties of the SonoPanel/sup TM/ were experimentally examined. The in-air impedance measurement results showed the existence of parasitic modes in the composite panel in addition to the expected thickness mode. This modal behavior is identified to be related to the piezocomposite structure. In-water acoustical properties of the new 1-3 piezocomposite panels were investigated as a function of temperature, hydrostatic pressure, and frequency. The effect of underwater explosive shock on the acoustic responses showed no detrimental effects in mechanical structure or acoustical performance of the piezocomposite panel. Linearity with electrical drive level and hydrostatic pressure stability of the 1-3 piezocomposites also were established. These results suggest that the SonoPanel/sup TM/ piezocomposite material is potentially useful for underwater acoustical applications, particularly in applications in which large area coverage is desired.

Development of a broadband underwater sound projector

An underwater sound projector has been developed for operation over a decade long frequency range. The output of the projector has been designed for 10 kHz to 100 kHz operation on an autonomous underwater vehicle (AUV). The transducer is resonant at 100 kHz but has been designed to deliver high sound pressure levels without impedance or phase instabilities. The transducer features the first successful stacking of 1-3 piezocomposite materials. The selection of the 1-3 piezocomposite materials has resulted in the mode-free sound output while the stacking arrangement permits acoustic operation with twice the sound output at half the resonance frequency of a single layer. The stacking is done mechanically in series and electrically in parallel with the center electrode as the positive plane. Furthermore, the center electrode has been segmented into four individual elements such that combinations of the sectors offer the ability to access nine different apertures.

A comparison of the underwater acoustic performance of single crystal vs. piezoelectric ceramic based cymbal projectors

The underwater acoustic performance of three same-design cymbal based electro-acoustic projectors are compared. Two of these projectors contain PMN-PT single crystal piezoelectric as the active material while the third utilizes conventional PZT-5H piezoelectric ceramic. The units containing the single crystal material show an improvement in the acoustic source level anywhere from 3 to 15 dB over the frequency band 1-25 kHz. The higher source level is a consequence of the higher drive voltage capability of the single crystal materials. Under appropriate drive conditions, a single crystal cymbal-based projector that is 64 mm thick with an acoustic aperture of 101 mm /spl times/ 101 mm can generate an acoustic source level of at least 175 dB (re: 1 /spl mu/Pa @ 1 m) from /spl sim/3 kHz to > 25 kHz.

Recent advances in thin, low frequency acoustic projectors

The Naval Research Laboratory (NRL) is currently developing low frequency acoustic projectors for use in shallow water environments. Its thin profile is intended for placement onto autonomous underwater vehicles (AUVs) and unmanned underwater vehicles (UUVs). The projectors are designed with a piston mode resonant frequency below 1 kHz. The transmitting output is then followed by a secondary resonance frequency in the 50 kHz range such that the overall output can be used for broadband operation from 1 kHz to 60 kHz.

Acoustic projectors for AUV and UUV applications in shallow-water regions

For acoustic identification of objects in a littoral environment, there are generally three frequency bands of interest; 1 kHz to 10 kHz, 10 kHz to 100 kHz and 100 kHz to >= 1 MHz, where the selection of these bands is dependent upon the specific Navy mission. This paper will discuss the progress of the Naval Research Laboratory in developing acoustic projector prototypes to address the lower two frequency bands for unmanned underwater vehicle (UUV) and/or autonomous underwater vehicle (AUV) applications. The band of 1 kHz to 10 kHz is currently being addressed sing cymbal flextensional vibrator elements sandwiched in to thin panels. In-air data has shown that high levels of acoustic displacement at low frequencies are possible with these devices while more recent in-water data has verified these expectations. This success has led to modeling and prototyping of similar devices for shallow water regions. The frequency range of 10 kHz to 100 kHz has been investigated for several years where the acoustic projector was originally reported during AeroSense 1998. The result of integrating the NRL broadband projector into the NSWC/Coastal Systems Station synthetic aperture sonar UUV will be presented. This system integration considers the projector as a constant source level over the 10 kHz to 100 kHz band by driving the 100 kHz resonant transducer with an inversely shaped transformed. The presentation will conclude with a discussion of the future development trends in shallow water transducers for AUV and UUV missions.

Thin, lightweight, low frequency acoustic projectors for shallow water environments

Miniature flextensional transducers, called cymbals, have been incorporated into thin, lightweight, large area panels for use a slow frequency acoustic projectors in shallow water. The prototype panels, measuring 100-mm by 100-mm by 6.35-mm thick exhibit a high acoustic output at a relatively low in-water resonance frequency. Furthermore a second resonance frequency that is over an order of magnitude higher suggests that the panel may be used to provide sound output over almost a two decade frequency band. The mass of the unplotted panel is less than 150 grams and the total thickness is 6.35 mm. The cymbal panels are believed to be excellent candidates as acoustic projectors on autonomous and/or unmanned underwater vehicle platforms as well as other shallow water platforms where low frequency, light weight and high acoustic output are desired.

Intelligent Sensor Systems for Underwater Acoustic Applications

Intelligent sensors for underwater acoustic applications are detection techniques that discriminate between the acoustic pressure wave field such that a manipulation of these waves can be performed. This is in contrast to smart vibration sensors that detect the modal or displacement characteristics of a structure. Intelligent sensors can characterize an acoustic pressure field into individual field components for signal processing purposes.

Piezocomposite Design, Manufacture, and Applications

Publisher Summary This chapter presents an overview of piezocomposite materials. It describes the phenomenon of piezoelectricity, which is the basis of all piezocomposites, and discusses the attributes of the primary piezoceramic materials. The design, manufacturing processes, and primary applications of two commercially available piezocomposite configurations are illustrated. Simple analytical models that are used to describe the properties of these two composite designs are also presented. The chapter then discusses the design issues and manufacturing processes, and describes some important applications for 0-3 and 1-3 type piezoelectric ceramicpolymer composites. The poled ferroelectric ceramics PT and PZT are the materials of choice as the active component in nearly all piezocomposite devices. This is because of their high piezoelectric response and stability. Piezocomposites offer the advantage of easy design flexibility for specific applications, especially in the case of 1-3 connectivity. Although shape apodization, increased band width, and constant beam width transducers can be accomplished with monolithic piezoceramics, it is at a high cost. With piezocomposites, these features can be obtained much more cheaply. In addition, the dice-and-fill and injection molding manufacturing processes have made these, as well as future, advances in product development possible.

Broadband acoustic projector for low-frequency synthetic aperture sonar application

Possibilities for increased mine detection and classification techniques have established a need for broadband, underwater acoustic projectors. An advanced version of a low frequency synthetic aperture sonar (SAS) for the mine reconnaissance hunter program has recently been developed. The transducer is resonant at 100 kHz but has been designed to deliver constant high sound pressure levels over an operating frequency range of 10 kHz to 100 kHz. This wide band operation is accomplished because of an absence of spurious modes within the operational frequency decade. The actual projector is constructed with a two layered 1 - 3 piezocomposite material stacked in mechanical series and electrically wired in parallel. This arrangement was selected in order to maximize the source level output. The center electrode of the monolithic 1 - 3 piezocomposite layers has been segmented to offer four individual elements such that combinations of the sectors offer the ability to access nine different apertures. A constant source level is maintained through the use of a preshaped transformer between the driver and the projector. The combination of the transformer design with the clean spectrum response of the composite material results in an acoustic projector with constant source level.