Abstract:
A compound electro-acoustic transducer for producing acoustic signals has a plurality of elements. Each element has a piezoelectric disk with electrically conductive plates fixed on the top and bottom sides of the piezoelectric disk. A stud is joined to an outer face of each plate. Conductors can be joined to each stud. The elements can be assembled on a resilient structure to form an array. Elements can be used in the array or individually accessed.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefore. 
    
    
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     This invention relates to acoustic projectors for sonar use and more particularly to a lightweight acoustic projector that can be used by itself or in an array. 
     (2) Description of the Prior Art 
     Low frequency transducers having resonances below about 10 kHz have numerous applications, one of which is as a low frequency sonar projector. This acoustic wavelength corresponding to these frequencies is on the order of the size of naval mines, and thus can hunt for and/or classify them, as well as objects of similar size. Also, wavelengths of this size permit sonar location of buried objects, a task of interest to a wide range of commercial and governmental concerns. Unfortunately, current underwater projectors at these frequencies are large and heavy, and are cumbersome to use on many underwater vehicles. 
     The U.S. Navy is particularly interested in detecting objects in littoral environments for which small, unmanned submersible vehicles are best-suited. Because of the size constraint of the vehicles, it is necessary to keep the dimensions of the associated acoustic projector systems small, particularly along the protrusion dimensions. Acoustically, the desire is for an acoustic source level greater than 180 dB, while geometrically the projector systems need to be thin (less than 60-65 mm) for installation onto the sides of underwater vehicles and tow sleds ranging in diameter from 15 cm to over 2.4 meters. Conventional transducer designs used to generate high power sound waves at frequencies under 30 kHz include free-flooded piezoelectric ceramic rings, electromagnetic and hydraulic drivers, tonpilz or piston transducers, and flextensional devices. However, because of their large size and weight, these technologies are not easily adaptable for mounting on advanced smaller underwater vehicle platforms. 
     There are also two other potential low frequency acoustic source candidates: 1-3 type piezocomposites and cymbal-based flat panels. Present state-of-the-art 1-3 piezocomposites have a thickness of 25.4 mm and although this meets the dimensional requirements, it also means that their acoustic source level at frequencies below 10 kHz is lower than desired. To form thicker 1-3 materials requires extensive electronic matching difficulties and impractical manufacturing and handling requirements. U.S. Pat. No. 6,438,242 to Howarth discloses a cymbal-based flat panel projector that meets the dimensional requirements. In this projector design, miniature flextensional electromechanical drivers that are known as ‘cymbals’ are used to drive a stiff radiating plate. In order to realize optimal acoustic output at low frequencies, an air gap between the radiating plates is required. The typical resonance frequencies for the thin panel projectors is less than 2 kHz. The flat panel design does not allow independent addressing of the projectors. Furthermore, the flat panel imposes an averaging affect on the signal received by each projector. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the invention is to reduce the cost of active electro-acoustic transducers by use of inherently inexpensive cymbal-type actuators. 
     Another object is to do the foregoing with a transducer that is inherently rugged. 
     Yet another object is to provide an acoustic projector that is small, lightweight, and has low vehicle volume occupation. 
     Still another object is to provide an acoustic projector that allows independent addressing of each projector element. 
     Accordingly, the invention provides a compound electroacoustic transducer for producing acoustic signals which has a plurality of elements. Each element has a piezoelectric disk with electrically conductive plates fixed on the top and bottom sides of the piezoelectric disk. A stud is joined to an outer face of each plate. Conductors can be joined to each stud. The elements can be assembled on a resilient structure to form an array. Elements can be used in the array or individually accessed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages of the present invention will be better understood in view of the following description of the invention taken together with the drawings wherein: 
     FIG. 1 is a cross-sectional view of a single cymbal driver in accordance with this invention; 
     FIG. 2A is a partially cross-sectional view of a single cymbal driver mounted on a support structure; and 
     FIG. 2B is a partially cross-sectional view of multiple cymbal drivers mounted on a support structure as an array; 
     FIG. 2C is a top view of multiple cymbal drivers mounted as an array; 
     FIG. 3A is a partially cross-sectional view of a single cymbal driver mounted on an alternative support structure; 
     FIG. 3B is a partially cross-sectional view of multiple cymbal drivers mounted on the alternative support structure as an array; and 
     FIG. 3C is a top view of multiple cymbal drivers mounted as an array on the alternative support structure; and 
     FIG. 4 is a top view of an alternate electrical connection structure for the array. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     This invention describes a thin, lightweight underwater electroacoustic projector with high acoustic output at frequencies from 0.5 kHz to approaching 1 MHz, with an initial resonance output below 10 kHz. In the design described herein, the preferred frequency band of operation is 2.5 kHz to 100 kHz. The device consists of miniature flextensional electro-mechanical drivers that are known as ‘cymbals’. FIG. 1 shows a cross-sectional rendering of the cymbal-type driver  10  used in this device. The active material in each driver  10  is a lead zirconate titanate (PZT) piezoelectric ceramic disk  12  poled in its thickness direction. An electrically conductive structural adhesive  14  is used to mechanically and electrically couple conductive endcaps  16 A and  16 B to the top and bottom faces of the piezoelectric ceramic disk  12 . The endcaps  16 A and  16 B are shaped such that a shallow air cavity  18  is formed between the cap  16 A and  16 B and the disk  12  after they are bonded together. Prior to bonding to the disk, threaded studs  20 A and  20 B are microwelded onto the apex of each of the endcaps  16 A and  16 B, respectively. For this purpose, each stud  20 A and  20 B can be provided with bosses  21  to provide a better mounting surface. The ceramic disk  12  and endcaps  16 A and  16 B can be sealed by applying a water proof coating  22  around the periphery of the assembly. 
     The studs  20 A and  20 B, in conjunction with the endcaps  16 A and  16 B, serve as the electrical conduit from the piezoelectric ceramic disk  12  to the electrical lead wires. When an electrical signal is applied to the piezoelectric ceramic disk  12 , it either expands or contracts in the radial direction. This expansion and contraction of the piezoelectric ceramic disk  12  causes the dome of the endcaps  16 A and  16 B to flex. The flexure of the endcaps  16 A and  16 B subsequently produces the low frequency sound waves that are transmitted into the surrounding medium. The magnitude of the acoustic output, its resonance frequency, and hydrostatic pressure tolerance of an individual cymbal element  10  are dependent upon its dimensions, the geometry of the endcaps, and the materials properties of the components. 
     In order to enhance acoustic output, lower the fundamental resonance frequency, and provide for directionality of the generated sound, the individual cymbal elements.  10  are incorporated into an array. For incorporating the cymbal elements  10  into an array, the individual elements  10  must be mounted in a way that does not transmit vibrations between the elements, yet acts to hold the elements in a predetermined configuration. 
     FIG. 2A shows one way to electrically interconnect the individual cymbal elements in a mounting  24 . In this case, metal ribbon  28 A is used to connect one side of all of the cymbal elements  10 . The other pole of cymbal element  10  is connected to metal ribbon  28 B. Together, this results in a parallel electrical connection of all of the elements. The ribbons  28 A and  28 B maintain mechanical and electrical contact with the respective studs  20 A and  20 B via nuts  30  and washers  32 . FIG. 2B shows a partially cutaway side views of an array of cymbal elements  10  held in the mounting  24 . FIG. 2C is a view looking from the top of the array. 
     FIGS. 3A,  3 B, and  3 C show an alternative mounting configuration for the cymbal elements  10 . FIG. 3A shows an array of cymbal elements  10  in a partially cut away side view, and FIG. 3B shows a top view of an array using this mounting. In this embodiment, the cymbal elements  10  are held in place around their outside rim with a rubber grommet  34  within a stiff grid  36 . Grommet  34  absorbs vibrations and prevents transfer of these vibrations to grid  36  or between elements  10 . Grommet  34  has an inner groove  38  receiving cymbal element  10  and an outer groove  40  contacting grid  36 . 
     The projector design taught in this invention allows for great flexibility in electrical wiring configurations. For instance, instead of electrically wiring in parallel such as in the device described above, each cymbal element  10  or groups of cymbal elements could be wired for individual addressing by individual wires or other conductors  42  which combine to form a wiring harness  44 . The bottom side can be configured in a similar fashion or it can use the conductive ribbons taught-in FIGS. 2C and 3C. This would allow for manipulation of electrical impedance, control of beam forming capability through variation of the radiating aperture, and multipurpose acoustic objectives because of this ability to form different apertures within the radiation profile. This means that this device design can have specific apertures for specific frequency bands and specific sonar operations within the same sonar wet-end packaging. Accordingly, this invention provides a projector element and array wherein the low frequency acoustic output from the projector primarily comes from the low frequency resonance associated with the flexure of the cymbal caps. This resonance can be manipulated via mass loading the individual cymbal elements by adding additional nuts and washers. As additional nuts (i.e., mass) are added to each individual cymbal driver, the projector resonance frequency is decreased with the caveat of reduced acoustic source level due to the larger volume velocity required as frequency is lowered. 
     This projector design is capable of wide frequency coverage because the lowest resonance frequency is controlled by the cymbal cap design, aperture, and mass loading conditions, whereas the upper frequency is determined by the diameter of the piezoelectric ceramic drive element. Consequently, within the same transducer volume package, a sonar capable of low frequency, weapons frequency, and imaging frequencies can be realized. Further manipulation of the operating frequency band can be achieved through the use of different size cymbal elements within the projector. 
     This projector design is also conducive to the formation of volumetric arrays. In volumetric arrays, two planes of transducers are separated by a given distance (typically a quarter wavelength) so that highly directional (cardioid) radiation beam responses can be realized. 
     Projectors that utilize this design exhibit hydrostatic pressure dependence at low frequencies. However, acoustic pressure vessel data show that the device can be used up to pressures of 2 MPa with little degradation in performance. In addition, when the device is exposed to very high pressures (e.g., 5.52 MPa) and then returned to a lower pressure (0.02 MPa), catastrophic failure was not experienced. For higher frequency operation (i.e., above 20 kHz), where the radial mode of the piezoelectric ceramic disk (−100 kHz in this device) is the primary contributor to acoustic source generation, hydrostatic pressure dependence is negligible. The utilization of this design should result in higher hydrostatic pressure tolerance at low frequency. This means that through proper design engineering, this projector design should be usable for all sonar applications.