Thin Profile, Low-Frequency, Underwater Electroacoustic Projectors
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Motivation: The Physical Acoustics Branch at the Naval Research Laboratory has been investigating a structural acoustic approach to detect and identify underwater mines.1 This technique uses frequencies below 30 kHz to excite structural acoustic responses from the target. Through various algorithms, these structural clues can be converted into unique "fingerprints" that can be used to classify an unknown object as a mine. This low-frequency structural acoustic approach permits long-range detection as well as the ability to penetrate into sediment, which allows for the detection and identification of buried objects.
Because of their large size and weight, standard low-frequency source technologies are typically not adaptable for mounting on advanced underwater vehicles. An alternative technology, 1-3 piezoelectric ceramic-polymer composites, has the advantage of being thin and having low weight; however, its acoustic output at frequencies less than 10 kHz is lower than desired.
The Physical Acoustics Branch at the Naval Research Laboratory, in collaboration with the Naval Sea Systems Command, Divisions Crane and Newport, has designed and fabricated two projectors based on cymbal-type flextensional drivers. These projectors are thin enough for mounting on the side of an unmanned underwater vehicle and are capable of generating an adequate sound pressure level over the frequency band of interest.
Designs: Cymbal Drivers -- Both projector designs use 294 miniature Class V flextensional electromechanical drivers laid out in a 14 x 21 square matrix. These drivers, known as "cymbals," consist of a piezoelectric ceramic disk that is mechanically and electrically coupled to two specially shaped titanium caps. The cymbals measure 12.7 mm in diameter and are approximately 2 mm thick (excluding the stud length). Figure 9 shows various views of these cymbal drivers.
Panel Projector - In the panel projector design, the cymbal drivers are sandwiched between two rigid graphite-epoxy composite cover plates. The cover plates have holes in them that allow the studs to pass through, thus allowing the cover plates to rest on the apex of the cymbal caps. The plates are torqued onto the cymbals with nuts. The outer surface of each cover plate is electroplated with copper. The electrical connection is made to the piezoelectric ceramics via the electroplating, nuts, studs, and caps. A polyurethane gasket is stretched around the outside edge of the cover plates to maintain the interior air matrix after the projector is potted in polyurethane for waterproofing. The finished panel projector measures 350 x 248 x 15.9 mm and weighs 5 N in water.
Oil-filled Projector - In this design, the cymbal drivers are mounted within a molded sheet of 5-mmthick neoprene rubber. Recessed cavities within the neoprene sheet hold each cymbal in place on its flat rim around the circumference of the disk. The 294 cymbals are connected electrically in parallel using thin nickel ribbon that is held on the studs by nuts. A single Plexiglas sheet, containing holes that align over the cymbals, is bonded to the upper and lower surfaces of the neoprene sheets. The Plexiglas provides a means to secure the cymbal-loaded neoprene sheet to the projector housing while assuring that the cymbal drivers remain in the same plane. Its dimensions are 381 x 280 x 64 mm and it weighs 26 N in water. This particular design has the added feature of resonance-frequency tunability, from between 6 and 10 kHz, which is done by adding or subtracting mass (i.e., nuts) from the cymbals.
Performance: Three projectors (two Panel and one Oil-filled) were mounted next to each other on one side of a fiberglass I-beam platform (Fig. 10) and evaluated in water. The three projectors were electrically connected together in series to improve impedance matching to the amplifier. Figure 11 shows the sound pressure level (SPL) achieved by the projector assembly from 700 Hz to 30 kHz when 300 Vrms (at one percent duty cycle) is applied to the system. An SPL over 180 dB (re: 1 µPa @ 1 m) is generated across the frequency band from 3 to 30 kHz. By applying an appropriate voltage at frequencies below 3 kHz, it would be possible to obtain a higher source level in this band. An SPL of 180 dB has been shown to continue up to at least 100 kHz as the ceramic radial resonance mode dominates at these upper frequencies.2
No competing source technologies are currently capable of producing such a high acoustic output over this entire frequency band within such a thin package. This marks a significant advance in lowfrequency source technology, especially for use on future Navy vehicles.
Acknowledgments: The authors acknowledge the contributions of Bruce Johnson of ONR-321TS, Brian Houston of NRL, Joe Klunder of SFA, Inc., Kirk Robinson and Mel Jackaway of the Glendora Lake test facility, as well as Pat Arvin, Scott Small, and Phil Meadows of NSSC-Crane.
[Sponsored by ONR]References
1 T.J. Yoder, J.A. Bucaro, B.H. Houston, and H.J. Simpson, "Long Range Detection and Identification of Underwater Mines Using Very Low Frequencies (1-10 kHz)," in Detection and Remediation for Mines and Minelike Targets III, A.C. Dubey, J.F. Harvey, and J.T. Broach, eds. (SPIE, Bellingham, Washington, 1998), pp. 203-210.
2 T.R. Howarth, J.F. Tressler, and W.L. Carney, "Oil-filled Cymbal Panels for Acoustic Projection Applications," J. Acoust. Soc. Am. 110, 2753 (2001).