Abstract:
A transducer assembly for transmitting broadband sonar beams and receiving broadband sonar returned echoes with a low-cost transducer element mounted into a low-cost acoustic structure. By using a transducer element which is sufficiently thin, broadband can be achieved at a significant cost savings over existing methods and devices. Since the transducer element is sufficiently thin, a large portion of the signal energy is coupled transversely into the acoustic structure, resulting in a heavy acoustic load on the transducer element which in turn results in broadband operation. Broadband operation may be enhanced by at least partially enclosing the sufficiently thin transducer element within an aperture and/or a cap.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. patent application Ser. No. 14/211,049, entitled Cost Effective Broadband Transducer Assembly and Method of Use, filed Mar. 14, 2014, which claims priority to U.S. provisional patent application Ser. No. 61/788,469 which is entitled Cost Effective Broadband Sonar Transducer, filed Mar. 15, 2013, the entirety of which are incorporated herein by reference. 
     
    
     TECHNICAL FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to electroacoustic transducers and more particularly to ultrasonic broadband transducer assemblies used in marine applications. A method of using a broadband transducer assembly in marine environments also is provided. 
       SUMMARY OF THE INVENTION 
       [0003]    The present invention is directed to a broadband acoustic structure operable for the purpose of imaging in marine applications, the acoustic structure having a bandwidth. The acoustic structure comprises a sufficiently thin transducer element for transmitting and receiving acoustic signals, the sufficiently thin transducer element having a transverse resonant frequency, and a base to which the sufficiently thin transducer element is securable. The sufficiently thin transducer element produces transverse vibrations which result in loading of the transducer element and the loading of the transducer element results in broadening of the bandwidth of the acoustic structure, wherein the bandwidth of the acoustic structure includes the transverse resonant frequency of the sufficiently thin transducer element. 
         [0004]    The present invention further is directed to a broadband transducer assembly operable for the purpose of imaging in marine applications. The transducer assembly comprises an acoustic structure having a bandwidth, a base and a sufficiently thin transducer element having a transverse resonant frequency. The sufficiently thin transducer element is securable to the base of the acoustic structure, and the sufficiently thin transducer element produces transverse vibrations which result in loading of the transducer element and wherein the loading of the transducer element results in broadening of the bandwidth of the acoustic structure. The bandwidth of the acoustic structure includes the transverse resonant frequency of the sufficiently thin transducer element. 
         [0005]    Finally, the present invention is directed to a method of imaging marine environments. The method comprises the steps of transmitting an acoustic signal into a sufficiently thin transducer element having a bandwidth and producing transverse vibrations of the sufficiently thin transducer element and thereby loading the sufficiently thin transducer element to result in broadening of the bandwidth of the sufficiently thin transducer element. 
         [0006]    The foregoing and other objects, features, and advantages of the invention will appear more fully hereinafter from a consideration of the detailed description that follows. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  illustrates an exploded view of an exemplary configuration by which an embodiment of a transducer assembly of the present invention is mounted to the transom of a watercraft. 
           [0008]      FIG. 2  shows an isometric view of an exemplary broadband transducer assembly of the present invention. 
           [0009]      FIG. 3  is a side elevation view of an exemplary broadband transducer assembly of the present invention. 
           [0010]      FIG. 4  is a rear elevation view of an exemplary broadband transducer assembly of the present invention. 
           [0011]      FIG. 5  is a perspective view of an alternative housing embodiment of the broadband transducer assembly of the present invention, having a window over at least a portion of the acoustic element. 
           [0012]      FIG. 6  is a cross-sectional view of an exemplary broadband transducer assembly of present invention, taken along line  6 - 6  of  FIG. 4 . 
           [0013]      FIG. 7A  illustrates a cross-sectional view of an exemplary acoustic structure of the broadband transducer assembly of the present invention, the acoustic structure comprising a base and a transducer element. 
           [0014]      FIG. 7B  illustrates a cross-sectional view of an alternative exemplary acoustic structure wherein an aperture and cap at least partially enclose the transducer element. 
           [0015]      FIG. 7C  shows a perspective view of the bottom surface of an exemplary housing of the broadband transducer assembly of the present invention, wherein the base of the acoustic structure forms an aperture for receiving the transducer element. 
           [0016]      FIG. 8  is a graph illustrating favorable ranges of diameter-to-thickness ratios of transducer elements of the broadband transducer assembly of the present invention, both with and without an aperture and/or cap. 
           [0017]      FIG. 9  illustrates a perspective view of an exemplary acoustic structure of the broadband transducer assembly of the present invention, propagating ultrasonic waves longitudinally through water. 
           [0018]      FIG. 10  illustrates a conventional narrowband transducer element, without voltage applied. 
           [0019]      FIG. 11  illustrates a conventional narrowband transducer element in operation with voltage applied. 
           [0020]      FIG. 12  illustrates an exemplary transducer element of the broadband transducer assembly of the present invention, without voltage applied. 
           [0021]      FIG. 13  illustrates an exemplary transducer element of the broadband transducer assembly of the present invention, in operation with voltage applied. 
           [0022]      FIG. 14A  is a graph illustrating bandpass characteristics of an exemplary broadband transducer assembly of the present invention. 
           [0023]      FIG. 14B  is a graph illustrating bandpass characteristics of an alternative embodiment of an exemplary broadband transducer assembly of the present invention. 
           [0024]      FIG. 15  is a perspective view of an exemplary broadband array transducer assembly of the present invention containing an array of four acoustic structures in the bottom section of a transducer housing. 
           [0025]      FIG. 16  is a side elevation view of the exemplary broadband array transducer assembly of  FIG. 15 . 
           [0026]      FIG. 17  is a rear elevation view of an exemplary broadband array transducer assembly of  FIG. 15 . 
           [0027]      FIG. 18  is cross-sectional view of an exemplary broadband array transducer assembly of the present invention, taken along line  18 - 18  of  FIG. 17 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    Broadband transducers are electroacoustic devices used to increase sonar resolution and definition of products and have application, for example, in scanning sonar, three-dimensional sonar, echo sounders and sonar-GPS combinations. These devices can determine the depth of the marine floor, locate fish, identify other submerged targets, locate structure, show contours, avoid collisions and produce underwater images and the like. 
         [0029]    Conventional broadband transducers used in military and commercial applications are too expensive to incorporate into most fish finding systems. While broadband transducers offer new capabilities for these devices, a conventional broadband fishfinder must meet the requirements for broadband in each aspect of the device, including the transducer, transmitter, receiver, and signal-processing-software. Most broadband transducers are comprised of porous ceramic elements or composite ceramic elements, which are expensive and contribute to the high cost of broadband devices. These requisite materials and components make broadband fishfinders cost prohibitive for many commercial and recreational marine activities. 
         [0030]    Conventional narrowband fishfinders incorporate transducers that operate within a limited range of active frequencies. Lead zirconate titanate (Pb[Zr x Ti 1-x ]O 3  or “PZT”) is a piezoelectric ceramic material widely used in transducers. However, the range of active resonant frequencies of this PZT ceramic material are extremely narrow. Due to the discontinuity between the acoustic impedance of the piezoelectric ceramic material comprising the transducer and the surrounding environment, the bandwidth of conventional narrowband fishfinders typically have a Quality Factor (“Q Factor”) of about 15 and above. These conventional narrowband devices generally are useful in freshwater and some saltwater environments but are limited in capability as compared to broadband devices, which offer many advantages. 
         [0031]    Various tactics have been employed in attempts to create broadband transducers for use in marine applications, including the use of composite or porous piezoelectric ceramic materials. Composite PZT ceramic material (“composite PZT”) comprised of epoxy, plastic and rubber, are placed into a homogeneous mixture with small pieces of PZT ceramic to form a monolithic transducer. The composite PZT transducer will have an acoustic impedance between PZT and epoxy, moving the acoustic impedance closer to that of water and creating a broadband effect. Porous piezoelectric materials (“porous PZT”) are used in commercial and military sonar applications and medical electronics. To create a porous ceramic material, the PZT is mixed with select powders and is heated, leaving microscopic voids in the PZT. The voids reduce specific gravity of the PZT ceramic material, thereby moving the acoustic impedance of the device closer to that of water and achieving broadband results. Both porous PZT and composite PZT are extremely expensive due to material and manufacturing costs. Other methods of achieving broadband include the use of head and tail masses, also impedance matching layers are placed between the piezoelectric element and water. 
         [0032]    The present invention overcomes these problems of expense and complexity. The present invention comprises a cost-effective broadband transducer assembly that not only reduces the cost of existing broadband fishfinder systems but, due to the low cost of the transducer, will allow all fishfinding systems to operate with broadband. The present invention achieves broadband operation by using an internally-housed, low cost transducer element which is sufficiently thin, as described herein, thereby generating a relatively large amount of transverse vibration in the transducer element and increasing the load between the transducer element and an acoustic structure. Broadband operation may be enhanced by at least partially enclosing the transducer element with a cap or within an aperture sized to receive the transducer element, which has the effect of increasing the load between the transducer element and the acoustic structure. As used herein, the term “broadband” and the phrases “broadband operation” or “operates within broadband” and the like are used interchangeably to mean having a Q Factor of about 5 or less. 
         [0033]    Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are described herein. All references cited herein, including published or corresponding U.S. or foreign patent applications, issued U.S. or foreign patents, and any other references, are each incorporated by reference in their entireties, including all data, tables, figures, and text presented in the cited references. 
         [0034]    Turning now to the drawings in general and to  FIG. 1  in particular, there is shown therein an embodiment of the broadband transducer assembly  10  of the present invention mounted to the transom  12  of a watercraft  14  or other vessel for use in marine applications. As used herein, “marine” and “marine applications” are used interchangeably to refer to activities and/or applications involving or relating to bodies or accumulations water, whether fresh water or salt water, including, without limitation, oceans, seas, lakes, ponds, rivers, streams, springs, creeks, gulfs, sounds, harbors, coves, channels, lagoons and the like. The transducer assembly  10  may be affixed to the watercraft  14  via known methods, such as mounting bracket  13 , although it will be appreciated that other embodiments and other watercraft mounting methods are possible. For example, the transducer assembly  10  may be affixed via through-hull-mounting, in-hull-mounting, trolling-motor-mounting, pole-mounting, adhesives and the like. Additionally, the transducer assembly  10  may be used without affixation to any watercraft or other device and simply may be floated or suspended on or near the surface of the water  18  where it is to be employed, for example, in marine activities such as ice fishing or from a boathouse and other activities where physical connection with a vessel or watercraft is neither useful nor desirable. 
         [0035]    The broadband transducer assembly  10  optimally is used such that sonar beam  20  emitted from the transducer assembly is generally perpendicular to the water surface  18 . However, it will be appreciated that the present invention also may be used with the transducer assembly  10  in any orientation with respect to the water surface  18  so long as the sonar beam  20  is emitted from transducer assembly  10  in a direction that is within the water. For example, the broadband transducer assembly  10  may be positioned so that the sonar beam  20  is emitted at a 45 degree angle with respect to the surface of water  18  or even parallel with respect to surface  18 , so long as the sonar beam is emitted within the water. 
         [0036]    Turning now to  FIGS. 2 through 4 , the transducer assembly  10  comprises a housing  24 . In one embodiment of the invention, the housing  24  comprises a top surface  26 , a bottom surface  28  and a connector  30 . There are many types of brackets, clamps, struts, fixtures and other connectors  30  appropriate for use in mounting the transducer assembly  10  to the watercraft  14  or other vessel or device, depending upon the desired application. As aforementioned, the transducer assembly  10  need not be connected to another device or vessel to achieve optimal operation, so the connector  30  is optional. 
         [0037]    The housing  24  may be of any shape that adequately stores the interior components, yet to be described. In one embodiment of the invention, the housing  24  is comprised of interlocking top and bottom surfaces  26  and  28 , respectively, which are securely connected to protect the interior components of the broadband transducer assembly  10  from water, dust, contaminants and other foreign materials, particles or objects. It will be appreciated that the housing  24  may be constructed of multiple components or comprise a single, integrally-formed structure having a top surface  26  and bottom surface  28 . 
         [0038]    The housing  24  may be comprised of a variety of materials that preferably impart properties of impact resistance, toughness and water-resistance. Some such materials include plastics and metals. Examples of plastic materials suitable for construction of the housing  24  include polypropylene, delrin, polycarbonate, urethane, polyethylene, polystyrene, nylon, acrylic, polyvinylchloride and ultem. In one embodiment of the invention, the housing  24  of the broadband transducer assembly  10  is comprised of acrylonitrile butadiene styrene (“ABS”) plastic. The housing  24  also may be constructed of metals, such as bronze, brass, aluminum or steel. Alternatively, the housing  24  may be constructed from a combination of materials. The material comprising the housing  24  should be selected so as to yield the most desirable characteristics of acoustic performance, strength, durability and cost-effectiveness for the particular application. 
         [0039]    In an alternative embodiment, the housing  124  is generally tubular, as shown in  FIG. 5 , forming a top surface  126  and a bottom surface  128 , and may be constructed from metal. An acoustic structure  36  (not shown in  FIG. 5 ) is acoustically isolated within the housing  124 . Longitudinal waves or the sonar beam  20  from an acoustic structure, yet to be described, are coupled into water through an acoustic window  170 , made of urethane or similar material. 
         [0040]    Turning now to  FIGS. 6 and 7A  through  7 C, the broadband transducer assembly of the present invention further comprises an acoustic structure  36 .  FIG. 6  illustrates therein a cross-section of an embodiment of the broadband transducer assembly  10  taken long line  6 - 6  of  FIG. 4 , wherein an exemplary acoustic structure  36  of the present invention readily is seen. The acoustic structure  36  comprises a base  38  and a transducer element  40 . It will be appreciated that the base  38  of the acoustic structure  36  may take any form suitable for supporting the transducer element  40 . In one embodiment of the invention, shown in  FIG. 7A , the base  38  of acoustic structure  36  comprises a generally level support for the transducer element  40 . 
         [0041]    Alternatively, the base  38  of the acoustic structure  36  may form an aperture  42  for receiving the transducer element  40 , as illustrated in  FIGS. 6 ,  7 B and  7 C. The depth of the aperture  42  preferably approximates the thickness of the transducer element  40 . The aperture  42  need not be formed by a continuous sidewall  46  or sidewalls, depending upon the configuration, and may be formed, for example, with intermittent breaks in the sidewall. Consequently, the sidewall  46  of aperture  42  either may directly contact the transducer element  40  or any filler materials therebetween, such as an adhesive or potting compound. Typically, though not necessarily, a gap between the sidewall  46  of the aperture  42  and the transducer element  40  would be filled with an adhesive that is also used to secure the transducer element to the base  38 . 
         [0042]    The aperture  42  may be formed integrally with or from the base  38 . The aperture  42  may be formed from the same material as the base  38  or from another material. In one embodiment of the invention, the base  38  and the sidewall  46  forming the aperture  42  are formed as an integral unit from ABS. It will be appreciated that the aperture  42  may be formed from a separate component that is then connected to the base  38 . 
         [0043]    The base  38  and aperture  42  may comprise the same material or different materials, among which include ABS, nylon, polyethylene, polystyrene, polyvinylchloride, polypropylene, epoxy resin, vinyl ester resin, polyester resin, acrylic, delrin, polycarbonate, ultem and combinations thereof. 
         [0044]    The acoustic structure further may comprise a cap  44 , which may be employed in conjunction with the aperture  42  or without the aperture. The optional cap  44  serves a variety of purposes, one of which is to provide additional loading of the transducer element  40 . The cap  44  also reduces side lobes in the beam pattern. It will be appreciated that the broadband transducer assembly  10  of the present invention achieves broadband without the cap  44 . The cap  44  may be used in conjunction with the aperture  42  to completely enclose the transducer element  40 . The cap  44  may also be positioned atop of transducer element  40  without positioning the transducer element within the aperture  42 . The cap  44  may be formed as an integral part of the aperture  42  or as a separate component which is attached to or supported above or on the aperture  42 . It is not necessary that the cap  44  completely cover the transducer element  40  to constrict the transverse vibrations of the transducer element, and to that end the cap may only partially cover or enclose the transducer element. 
         [0045]    As shown in  FIG. 6 , the base  38  of the acoustic structure  36  may be integrally formed with bottom surface  28  of housing  24 . It will be appreciated, however, that the base  38  of acoustic structure  36  may comprise a discrete article separate from the bottom surface  28  of housing  24 . In one embodiment of the invention, the base  38  and optional cap  44  are made from ABS plastic, but other suitable materials such as nylon, polyethylene, polystyrene, polyvinylchloride, polypropylene, epoxy resin, vinyl ester resin, polyester resin, acrylic, delrin, polycarbonate, or ultem may be used to construct the base. In some embodiments, the base  38  or acoustic structure  36  may be constructed from the same material or materials as the housing  24 , ABS for example, or the housing and base may be constructed of different materials. Furthermore, the acoustic structure  36  may be constructed from multiple materials so as to yield the most desirable characteristics of acoustic performance, strength, durability, and cost effectiveness. When the base  38  and housing  24  are not integrally formed, the based is secured to the bottom surface  28  of the housing with adhesives or solvents, such as epoxy, vinyl ester, methyl ethyl ketone or acetone, which do not or only minimally acoustically impede vibrations from the acoustic structure  36  into the housing. 
         [0046]    While the acoustic structure  36  often is elliptical, the shape and design of the acoustic structure is not limited to an elliptical profile. Circular, rectangular, polygonal or free-form profiles could be used to tune the desired resonant modes of the transducer assembly  10 . The acoustic structure  36  could be constructed in any shape and dimension to achieve the desired tuning and minimize the effect of resonant characteristics of the components of the broadband transducer assembly  10 . 
         [0047]    With continuing reference to  FIGS. 6 and 7A  through  7 C, the acoustic structure  36  is at least partially surrounded by an isolation material  48  which serves to minimize radiation of acoustic signals from the acoustic structure into the housing  24 , excepting the bottom surface  28  of the housing. To this end, and with the exception of the bottom surface  28  of the housing  24 , the acoustic structure  36  is isolated from the other components of the transducer assembly  10  by an isolation material  48 . The isolation material  48  may be any material that creates discontinuity in acoustic impedance so that acoustic energy remains within the acoustic structure  36  or passes into and through the bottom surface  28  of housing  24 . Some materials suitable for this purpose include foam, cork, vacuum, air and the like. The isolation material  48  reduces coupling of acoustic signals from the acoustic structure  36  into other parts of the housing  24  except at the base  38  of the acoustic structure. 
         [0048]    Further, in order to impart rigidity and durability to the both the transducer assembly  10  and the housing  24 , the housing may be filled with a potting material  50 , such as epoxy or rigid foam, which at least partially surrounds the acoustic structure  36 . In some embodiments, the isolation material  48  and potting material  50  may be combined into a single item, such as a rigid cast-in-place foam which would furnish both the isolation and potting functions. 
         [0049]    To achieve optimal performance of the transducer assembly  10 , certain components of the transducer assembly must be designed properly for the application. One of these important characteristics includes the tuning of the acoustic structure  36 . The acoustic structure  36  is tuned through appropriate selection of the materials, shape, diameter, thickness and dimensions of the components of the acoustic structure. Nevertheless, while these characteristics are important to performance, they alone will not result in broadband operation. A sufficiently thin transducer element  40  is required to achieve broadband operation. 
         [0050]    With continuing reference to  FIGS. 6 and 7A  through  7 C, the transducer element  40  of the broadband transducer assembly  10  of the present invention now will be described. The transducer element  40  comprises a piezoelectric material or a magnetostrictive material. In one embodiment of the invention, the transducer element is a piezoelectric material selected from the group consisting of PZT (lead zirconium titanate, (Pb[Zr x Ti 1-x ]O 3 )) or barium titanate (BaTiO 3 ). In one embodiment of the invention, the piezoelectric material preferably comprises PZT. 
         [0051]    The transducer element  40  is of any shape to be accommodated within the acoustic structure  36  and the housing  24 . The transducer assembly  10  of the present invention achieves broadband by employing a sufficiently thin transducer element  40 , in operation with the base  38  of acoustic structure  36 . The extent to which the transducer element  40  is sufficiently thin can be expressed as the diameter-to-thickness (DTT) ratio of the transducer element. For a circular transducer element  40 , the diameter thereof is clearly identifiable. For a non-circular transducer element  40 , whether regular or irregular in shape, such as a rectangular, elliptical or polygonal, the characteristic length of the element is substituted for the diameter. As used herein, the term “DTT ratio” will be used to represent all scenarios. 
         [0052]    A range of DTT ratios achieve broadband operation in the present invention. For example, a transducer element  40  which has a DTT ratio of 9 (diameter is 9 times the thickness) or greater will result in broadband operation when included as part of a proper acoustic structure  36 . As the DTT ratio gets smaller, i.e. as the transducer element  40  gets thicker and/or smaller in diameter, the amount of transverse vibration in the transducer element decreases, causing less loading between the transducer element and the base  38  of acoustic structure  36 , thus narrowing the bandwidth. 
         [0053]    Typically, a transducer element  40  having a large DTT ratio of 75 or greater should exhibit broadband operation, although there is a practical upper limit to the DTT ratio for sonar and fishfinder applications. First, as the DTT ratio increases (as the element gets thinner with respect to the diameter) the amount of transmit power which can be input into the transducer element  40  without damaging it is reduced. If the transducer element  40  is too thin, it will not be able to handle the required transmit power to produce the desired results in a sonar or fishfinder application. This sets a practical lower limit to the thickness of the element. Second, as the DTT ratio increases, if a reasonable thickness is maintained, the diameter will also increase. Since the present invention operates the transducer element  40  in the transverse (or radial) mode, larger diameters will result in lower operational frequencies. Center operational frequencies below 20 kHz are not typically useful for sonar and fish-finder applications. This sets a practical upper limit to the diameter of the element. Having a practical lower limit to the element thickness and a practical upper limit to the element diameter (or characteristic length) effectively bounds the practical upper limit of DTT ratios. Preferably, the center frequency of the broadband operation of the broadband assembly of the present invention ranges from about 20 kHz to about 250 kHz. 
         [0054]      FIG. 8  illustrates some preferred DTT ratios of the transducer element  40  based on operational frequency of the broadband transducer assembly  10 . The graph in  FIG. 8  plots the DTT ratio of the transducer element  40  on the y axis versus the center frequency of broadband operation of the broadband transducer assembly  10  in operation, on the x axis. The expected useful DTT ratio of the transducer element  40  of the broadband transducer assembly  10  ranges from about 9 to about 55. A preferred range of DTT ratios exists based on the most favorable combination of transducer element  40  operational frequency, transducer element  40  bandwidth, transmit power capability, size and cost. Due to these factors, the most preferable DTT ratio range will be different based on the operational frequency of the transducer element. It will be appreciated that the transducer element  40  of the acoustic structure  36  is not limited to the DTT ratios shown in  FIG. 8 , although it is anticipated that a majority of viable DTT ratios of the transducer element will fall within this range. 
         [0055]    The acoustic structure  36  need not comprise an aperture  42  or a cap  44  for the broadband transducer assembly  10  to achieve broadband operation. It will be appreciated, however, that if an aperture  42  and/or a cap  44  are employed as part of or in connection with the acoustic structure  36 , the additional load imparted between the transducer element  40  and the base  38  due to inclusion of either of these components will allow smaller DTT ratios to achieve broadband operation. When an aperture  42  and/or a cap  44  are employed as part of or in connection with the acoustic structure  36 , useful DTT ratios range from about 4.5 to about 55, as shown in  FIG. 8 . 
         [0056]    Turning now to  FIG. 9 , and with continuing reference to  FIGS. 6 and 7A  through  7 C, the operation of the broadband transducer assembly  10  will be described.  FIG. 9  shows the transducer element  40  positioned within the acoustic structure  36 , which is transmitting a sonar beam  20  into the water  18 . The housing  24  is not shown for purposes of illustration. The transverse restraining forces on the transducer element  40  increase the load on the transducer element, thus broadening the bandwidth. 
         [0057]    The accentuated transverse vibrations from the sufficiently thin transducer element  40  enable broadband operation of the transducer assembly  10 . Additionally, the constriction of the transducer element  40  by the aperture  42  and cap  44  will constrict the transverse vibration of the transducer element, which causes loading between the transducer element and the acoustic structure, thus broadening the bandwidth of the acoustic structure. The amount of load created in these circumstances is dependent on a number of factors, including aperture  42  and cap  44  dimensions, construction materials and configuration. It will be appreciated that while the aperture  42  and/or cap  44  will load the transducer element  40 , the use of a sufficiently thin transducer element  40  in conjunction with both the aperture  42  and/or cap  44  will provide more load than use of only one of the components alone. Thus, in a number of exemplary embodiments of the present invention, both an aperture  42  and cap  44  will be utilized with a sufficiently thin transducer element  40  to achieve enhanced broadband performance. 
         [0058]    The transducer element  40  is connected with a sonar transmitter and a receiver (not shown) via a transducer cable  54 . When a sonar pulse is applied to the transducer cable  54 , the pulse, therefore, also is applied to the transducer element  40 . The transducer element  40  then vibrates longitudinally or axially, and because it is sufficiently thin, it vibrates aggressively in the transverse or radial direction. These aggressive transverse vibrations are coupled into the acoustic structure  36  and resonate within the structure. Transverse and longitudinal resonances within the acoustic structure  36  then produce longitudinal vibrations that are coupled into the water and longitudinally as the transmitted sonar beam  20  through water  18 . The longitudinal direction may also be referred to as the axial direction, while the transverse direction may also be referred to herein as the radial direction. 
         [0059]    With continuing reference to  FIG. 9 , transverse vibrations from the transducer element  40  will couple into the base  38  and cause associated longitudinal vibrations within the acoustic structure  36 . Different transverse and longitudinal resonance modes within the acoustic structure  36  are determined by the acoustic structure components, composition, shape, and dimensions. The composite result of these transverse and longitudinal resonances is realized longitudinally at the interface of the acoustic structure  36  with the bottom surface  28  of the housing  24  and, hence, to the water  18 . The transmit sonar beam  20  is then emitted from the acoustic structure  36  and propagates through water in a longitudinal fashion. 
         [0060]    Comparison of the transducer element of a conventional narrowband fishfinder transducer to the transducer element  40  of an embodiment of the present invention  10  demonstrates the following: 1) Longitudinal vibrations are the same in both; 2) transverse vibrations in the sufficiently thin transducer element  40  of the present invention are greatly accentuated over transverse vibrations of conventional narrowband fishfinder transducer elements. 
         [0061]      FIGS. 10 and 11  represent a transducer element of a conventional narrowband fishfinder transducer device.  FIGS. 12 and 13  illustrate a sufficiently thin transducer element  40  of an embodiment of the present invention. Both transducer elements are made of the same normal, low-cost, hard PZT. Both elements have the same piezoelectric characteristics and the same initial diameter D 0  in the unexcited state, illustrated in  FIG. 10  for the conventional transducer element and in  FIG. 12  for the transducer element  40  of the present invention  10 . However, with the application of an electric field, the transducer element  40  of the present invention in an excited state has a greater length change in the transverse direction than the conventional transducer element, as illustrated in  FIGS. 11  and  FIG. 13 . 
         [0062]    In general, the change in length of a transducer element due to an applied electric field is shown in EQ 1: 
         [0000]      Δ L=d   ij   ×E×L   0   EQ 1:
 
         [0063]    Where: L 0 =Initial length (m)
       d ij =piezoelectric charge constant (pm/V)   E=applied electric field strength (V/m)       
 
         [0066]    Since PZT has different piezoelectric charge constants based on orientation to the polarization vector, we arrive at EQ 2 and EQ 3 to find the change in diameter and thickness of a PZT element due to an applied electric field. 
         [0000]      Δ D=d   31   ×E×d   0   EQ 2:
 
         [0000]      Δ T=d   33   ×E×t   0   EQ 3:
 
         [0067]    Where: T 0 =Initial diameter (m)
       T 0 =initial thickness (m)   d 31 =piezoelectric charge constant orthogonal to the polarization   vector (pm/V) E=applied electric field strength (V/m)   d 33 =piezoelectric charge constant parallel to the polarization vector (pm/V)       
 
         [0072]    Since the electric field is applied over the initial thickness of the element, E is derived as follows: 
         [0000]        E=V/T   0   EQ. 4:
 
         [0073]    Based on established properties for hard PZT, using EQ. 2 and EQ 3. and applying a 600V electric field, the difference in the transverse length for the conventional transducer element and the sufficiently thin transducer element  40  of the present invention is calculated. 
         [0000]    
       
         
               
               
               
             
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Conventional Narrowband 
                 Present Invention Transducer 
               
               
                   
                 Transducer Element 
                 Element 
               
               
                   
                 Figures 10-11 
                 Figures 12-13 
               
               
                   
               
             
             
               
                 Unexcited 
                 25.4 mm 
                 25.4 mm 
               
               
                 Diameter (D 0 ) 
                   
                   
               
               
                 Unexcited 
                 11.2 mm 
                  2.0 mm 
               
               
                 Thickness (T 0 ) 
                   
                   
               
             
          
           
               
                 d 33   
                     5x10 −10  m/V 
               
               
                 d 31   
                 −2.3x10 −10  m/V 
               
               
                 Applied 
                 600 V 
               
             
          
           
               
                 Voltage (V) 
                   
                   
               
               
                 E 
                 V/t 0  = 600 V / .0112 m =  53571 V/m   
                 V/t 0  = 600 V / .002 m =  300000 V/m   
               
               
                   
               
               
                 ΔD 
                 
                   
                     
                       
                         
                           
                             - 
                             2.3 
                           
                            
                           
                             
                               x 
                                
                               10 
                             
                             
                               - 
                               10 
                             
                           
                            
                           
                               
                           
                            
                           
                             m 
                             V 
                           
                           * 
                           53571 
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         [0074]    As shown by the calculations in Table 1, both transducer elements in  FIGS. 11 and 13  have the same longitudinal length change due to an applied voltage, but the transducer element  40  of the present invention broadband transducer assembly  10  has 5.6 times greater length change in the transverse direction than the conventional narrowband transducer element. It will be appreciated that the calculations in Table 1 are a comparison of two specific transducer elements. As the DTT ratio of the sufficiently thin transducer element  40  is changed, so will the difference in transverse length change with respect to a conventional narrowband transducer element with a much smaller DTT ratio. 
         [0075]    A typical measurement of transducer performance is Q Factor, which is defined as follows: 
         [0000]    
       
      
       Q=f 
       c 
       /Δf  
      
     
         [0076]    Where: 
         [0077]    f c =Center frequency of the bandpass 
         [0078]    Δf=Bandwidth 
         [0079]    In general, transducer assemblies with a lower Q Factor are broader band. Table 2 contains a comparison of the Q Factor for a conventional narrowband transducer, typical low frequency broadband transducer, typical high frequency broadband transducer, with two embodiments of the broadband transducer assembly  10  of the present invention. While specific embodiments of the present invention will produce different performance, the measured bandpass of two embodiments substantially similar to that shown in  FIGS. 2 through 4  and  FIGS. 6 through 7C  is represented in  FIGS. 14A and 14B . Embodiment #1 has a usable bandwidth of 35 kHz with a center frequency of 82.5, while embodiment #2 has a usable bandwidth of 44 kHz with a center frequency of 85 kHz. These embodiments compare favorably with conventional narrowband, conventional high frequency broadband and conventional low frequency broadband transducers, as shown in Table 2. 
         [0000]                                                                      TABLE 2                           Conventional   Conventional   Present   Present           Conventional   High Frequency   Low Frequency   Invention   Invention           Narrowband   Broadband   Broadband   Broadband   Broadband           Transducer   Transducer   Transducer   Transducer   Transducer           Assembly   Assembly   Assembly   Assembly #1   Assembly #2                                    f c  (kHz)   200   200   53.5   82.5   85       Δf (kHz)   12.5   100   23.0   35   44       Q   16.0   2.0   2.3   2.4   1.9                    
As demonstrated in Table 2 and  FIGS. 14A and 14B , the present invention, though being significantly lower cost, is capable of broadband performance that is as good, if not better, than a typical higher cost broadband transducer assembly.
 
         [0080]    It will be appreciated that the present invention can be embodied in numerous ways. For example, the broadband transducer assembly may include a plurality of acoustic structures  36  with disk or plate-shaped transducer elements  40 , a single acoustic structure  36  with a plate transducer element  40  or any other arrangement of one or more acoustic structures  36  using transducer elements  40  which are a disk, plate, rectangular, ellipse, or other profile. 
         [0081]    Turning now to  FIGS. 15 through 18 , another embodiment of the present invention is illustrated therein. A broadband transducer assembly  210  comprises a housing  224  having a top surface  226  and bottom surface  228  and connector  230 . As shown in  FIG. 18 , the broadband transducer assembly  210  comprises a plurality of acoustic structures  236 A through  236 D contained in an array arrangement within housing  224 . Each of the plurality of acoustic structures  236 A- 236 D comprises the elements of the acoustic structure  36  heretofore described. Each of the plurality of acoustic structures  236 A through  236 D could be operated individually or in a group of two or more to provide multiple transducer cone angles for use in either shallow or deep water. Other embodiments of a broadband array transducer could include acoustic structures  36  and transducer elements  40  of various sizes to achieve multiple frequencies and combinations of cone angles within the same transducer housing. 
         [0082]    The present invention further comprises a method of using a broadband transducer assembly in a marine environment. The transducer element  40  is connected with a sonar transmitter and a receiver via a transducer cable  54  as heretofore described. When a sonar pulse is applied to the transducer cable  54 , the pulse is transmitted to the transducer element  40 , which then vibrates longitudinally, or axially. Because the transducer element  40  is sufficiently thin, it vibrates aggressively in the transverse, or radial, direction. These aggressive transverse vibrations are coupled into the acoustic structure  36  and resonate within the acoustic structure, producing longitudinal vibrations. The vibrations emitted from the acoustic structure  36  propagate longitudinally through the housing  24  and into the water  18 . 
         [0083]    When the aperture  42  and or cap  44  are incorporated, vibrations from the transducer element  40  cause loading between the transducer element and the other components of the acoustic structure  36 , broadening the band width of the transducer assembly  10 . 
         [0084]    The invention of this application has been described above both generically and with regard to specific embodiments. Although the invention has been set forth in what is believed to be preferred embodiments, a wide variety of alternatives known to those of skill in the art can be selected within the generic disclosure. Changes may be made in the combination and arrangement of the various parts, elements, steps and procedures described herein without departing from the spirit and scope of the invention as defined in the following claims.