Abstract:
A transducer for sub-ocean bottom imaging includes: a housing capable of withstanding hydrostatic pressure of about 9,000 pounds per square inch; a transmitting layer positioned within the housing to transmit two primary high frequency transmit beams that generate a low frequency signal whose frequency is an arithmetic difference between the two primary beams for high resolution sub-ocean bottom imaging while maintaining high spatial resolution or directivity; and a receiving layer collocated with the transmitting layer within the housing that is mechanically tuned to resonate at the difference frequency producing high receive sensitivity.

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 therefor. 
    
    
     CROSS REFERENCE TO OTHER PATENT APPLICATIONS 
     None. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention is directed generally to transducer and more specifically to a compact multi-layered transducer capable of withstanding hydrostatic pressures in the order of 9,000 pounds per square inch (psi) particularly for sub-ocean bottom imaging applications. 
     (2) Description of the Prior Art 
     Conventional sub-bottom profiling systems rely on relatively low frequency transducers in the order of 3.5 kHz transducers to penetrate and explore the oceans&#39; sediments. However, necessity dictates that these transducers need to be small enough to fit on their intended platforms (such as towed underwater vehicles, tethered bodies, and the like). As a result of their small size compared to the required acoustic wavelength, they provide little to no acoustic directionality, and their lateral spatial resolution is poor. 
     Background information in this field is provided in the following references, each of which is incorporated in their entirety: P. J. Westervelt, “Parametric Acoustic Array,” J. Acoust. Soc. Am. 35, p. 535 (1963); C. H. Sherman and J. L. Butler, “Transducers and Arrays for Underwater Sound” (Springer, New York, 2007) (See page 156); L. E. Kinsler and A. R. Frey, “Fundamentals of Acoustics” (John Wiley and Sons, New York, 1962) (See page 141); U.S. Pat. No. 6,255,761 to Benjamin, “Shaped piezoelectric composite transducer and method of making.” 
     In response to a need to explore the ocean sub-bottom from small autonomous vehicles and other platforms, there is a need for a low cost scalable piezocomposite-based multi-layer parametric mode transducer that overcomes the above and other shortcomings of the prior art. 
     SUMMARY OF THE INVENTION 
     Accordingly it is an object of the present invention to provide a simple, compact, multi-layered transducer configuration capable of withstanding hydrostatic pressures in the order of 9,000 psi for sub-ocean bottom imaging applications. 
     It is another object of the present invention to provide a transducer for use with sub-ocean bottom imaging having broadband parametric mode operation, typically over a decade in frequency bandwidth. 
     It is another object of the present invention to provide a transducer having a narrow low frequency transmit beam for high resolution sub-ocean bottom imaging applications. 
     It is another object of the present invention to provide a transducer having a high gain receiver tuned to the low transmitted difference frequency signal to provide increased system signal to noise ratio. 
     It is another object of the present invention to provide a transducer with transmit or receive apertures that could be subdivided for acoustic beam steering or variable spatial resolution. 
     It is yet another object of the present invention to provide a transducer having a scalable size and frequency range of operation. 
     Accordingly, there is provided a transducer for sub-ocean bottom imaging. The transducer includes a housing capable of withstanding hydrostatic pressure in the operating environment. A transmitting layer is positioned within the housing for generating two primary high frequency acoustic signals which interact parametrically to produce a low frequency, directive transmit beam for high resolution sub-ocean bottom imaging. The low frequency transmit beam&#39;s frequency is an arithmetic difference between the two primary frequencies. This beam maintains high spatial resolution or directivity. A receiving layer is collocated with the transmitting layer within the housing. The receiving layer is mechanically tuned by the other assembly layers to resonate at the low transmitted difference frequency. 
     The above and other objects and advantages of the present invention will become apparent from the following description, drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a perspective side view of a first embodiment of a transducer enclosed within a housing according to the principles of the invention; 
         FIG. 1B  is a perspective side view of the transducer of  FIG. 1A  with the housing removed; 
         FIG. 2  is a perspective side view illustrating two U-shaped counter bores located at the edge of the transmitting layer of the transducer of  FIG. 1B ; 
         FIG. 3  is a perspective top view of the steel composite layer of the transducer of  FIG. 1B ; 
         FIG. 4  is an exploded, perspective side view of a second embodiment of a transducer according to the principles of the invention; 
         FIG. 5  is a perspective side view of one layer of the transducer of  FIG. 4 ; and 
         FIG. 6  is a perspective side view of one layer of a second embodiment of a transducer such as that shown in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A simple compact multi-layered transducer configuration for sub-ocean bottom imaging applications is disclosed whereby an arrangement of materials and their mechanical combination renders the device capable of withstanding hydrostatic pressures of the order of 9,000 psi. 
     The ability to simultaneously achieve both high spatial resolution or directivity, and low frequency (for sub-ocean bottom penetration) is realized with parametric mode sound generation. Two intense high frequency primary signals are provided to a transmitter. These signals interfere in the environmental medium and produce a low frequency acoustic beam whose frequency is the arithmetic difference between the two primaries. For example, 200 kHz and 220 kHz primary frequencies would yield a difference signal of 20 kHz. Furthermore, this low difference frequency acoustic wave has the desired high directivity that is resident in the primary frequency waves. 
     The transducer consists of a parametric mode transmitter element and a high gain receiver element collocated within the same housing. The receiver&#39;s sensitivity response is mechanically tuned with the overlying layers to resonate within a parametric mode difference frequency range. Due to its layered packaging design the transducer&#39;s size and frequency range of operation are scalable. Additionally, the transducer can employ multiple drivers and time delay techniques to steer the parametric mode acoustic beam. 
     Although the transmitter element works well in the parametric mode for transmission by providing directional low frequency sound, the same transmitter element does not perform well in the receive mode. The element, which is typically made from a piezoceramic material, must be thin to have its resonance frequency lie in the primary signal range in order to achieve the intense primary sound fields on transmission. At the difference frequency, the relatively thin transmitter element has low sensitivity because the sensitivity of the element is directly proportional to the element thickness or distance between the two electrode surfaces; furthermore, the transmitter element is operating well below resonance in the hydrostatic mode. Below resonance, the thickness mode stresses are partially cancelled by the out of phase lateral stresses. To overcome this low sensitivity problem a separate receiving element, preferably collocated with the transmit array, is required. 
     Referring to  FIGS. 1A and 1B , a parametric mode transducer  10  includes six passive and active layers  10   a ,  10   b ,  10   c ,  10   d ,  10   e  and  10   f  that are adhesively bonded together and encapsulated within a housing  16 . Electrical cabling  22  (see  FIG. 2 ) is joined to the active transmit and receive layers and extends through an encapsulated region of the transducer  10  to a side of a water proof bulkhead  12 . The six layers  10   a - 10   f , beginning at a radiating surface  14  and working toward bulkhead  12 , are discussed below in greater detail. 
     A matching layer  10   a  is provided having a specific acoustic impedance between that of water and that of a transmitting layer  10   b . Preferably matching layer  10   a  is made from an epoxy material. More precisely the specific acoustic impedance of the matching layer  10   a  is determined as the geometric mean of the values of the specific acoustic impedance of water and the specific acoustic impedance of the transmitting layer  10   b . Also, the thickness of the matching layer  10   a  is chosen to be quarter-wave thick at the speed of sound through matching layer  10   a.    
     Transmitting layer  10   b  is shown in more detail in  FIG. 2 . A transducer material layer  18  is provided in electrical contact at  20  with conductors  22 . An electrode  24  is positioned on each face of transducer material layer  18 . The transducer material layer  18  is preferably made from a sheet of 1-3 piezoceramic polymer composite with conductive electrodes  24  thereon. Piezoceramic material used in this configuration would be poled in the direction indicated by arrow  26 . Electrodes  24  are preferably copper electrodes formed on the transducer material through an electroplating process. The volume fraction of the piezoceramic to polymer and the polymer&#39;s dynamic modulus define the material&#39;s specific acoustic impedance which in turn determines the matching layer  10   a  composition as noted above. The thickness of the transmitting layer  10   b  is chosen to support a half wave resonance effect in the primary signal range of about 200 kHz. (At 200 kHz with the intended use in seawater, this leads to a thickness of about 6.23 mm or 0.25 in. Of course this thickness could be varied for different operating parameters.) Shown in  FIG. 2  are two counter bores  20  located at the edge of the transmitting layer  10   b  that allow positive and negative electrical connections  22  to be flush with the major surface faces of the transducer material  18 . This is critical for a uniform adhesive bond line between adjacent layer surfaces. It should be realized that other transducer materials and electrical connection methods could be used. 
     Layers  10   c  and  10   d  are provided to reduce the back radiation from the transmit array on receive element layer  10   e  by creating a vibration null at the receive element layer  10   e . They also work in concert with the other upper layers  10   a  and  10   b  to mass load the receiving layer  10   e , thus providing the required mechanical tuning. Layer  10   c  is made from a glass reinforced epoxy. This can be the glass cloth reinforced epoxy known in the art as G-10 or FR-4 or the like. 
     Layer  10   d  provides an acoustic mass layer which is shown in more detail  FIG. 3 . Acoustic mass layer  10   d  includes rigid blocks  30  positioned in a backfill material  32 . In the preferred embodiment, blocks  30  are steel blocks  30  arranged to yield a 56 percent volume fraction of steel. The backfill material  32  is an unvoided polymer. For applications of high pressure that is greater than 4,000 psi, use of the steel composite configuration and unvoided polymer backfill material is critical. During the transducer&#39;s development it was determined that surviving high pressures requires that all layers uniformly, radially compress. Any gross mismatch between layers is likely to result in an interfacial delamination damaging the device. 
     The next layer along the transducer  10  is the receiving layer  10   e  which preferably includes piezocomposite material having electrodes on either major side as shown in  FIG. 2 . Receiving layer  10   e  should have a thickness that satisfies the harmonic spring mass resonance condition at the difference frequency of the primary transmitter signals. As with the transmitting layer  10   b , copper plating is used for the electrode surfaces. Counter bores allow electrical connections to the receiving layer  10   e  to be recessed for bonding purposes. Layer  10   e  features a thicker piezocomposite material substrate than transmitting layer  10   b . This thickness and mechanical tuning by the mass of the upper layers  10   b ,  10   c , and  10   d , results in high receiver sensitivity. 
     The last of the six layers is the backing layer  10   f . This material is a commercially available, particle loaded, syntactic foam composite. This material should be acoustically absorptive and capable of withstanding the 9,000 psi possible under operating conditions. Sound energy reaching the backing layer  10   f  is absorbed such that very little acoustic energy is reflected off the metallic bulkhead  12 . The amount of absorption that the backing layer  10   f  provides is proportional to frequency and thickness. 
     In the above-described embodiment the transmit layer  10   b  and receiving layer  10   e  both have fixed beam direction and fixed directivity (i.e., beamwidth). Other embodiments having different transmit layer  10   b  and receive layer  10   e  configurations will allow beam reconfiguration and steering. 
     An exploded perspective view of a second embodiment is shown in  FIG. 4 . The transducer  40  includes a matching layer  42   a , a transmitting layer  42   b , a back radiation reduction layer  42   c , a mass loading layer  42   d , a receiving layer  42   e , and a backing layer  42   f . In this embodiment, transmitting layer  42   b  is configured as a linear array  44  of n separate elements, shown typically as  46 . Through the use of n time delayed drive signals provided to each element, this embodiment provides a beam steering capability.  FIG. 5  shows layer  42   b  in detail. Layer  42   b  includes an array  44  having elements  46 . Electrical connections  48  are made to each side of the elements  46  by embedding the cabling within the transducer material, as described in U.S. Pat. No. 6,255,761, with the cable entering at the edge and contacting an electrode  50  formed on one of the major surfaces. The electrodes  50  are preferably created by electroplating the major planar face with copper. This plating can be patterned to form electrodes  50  that will make electrical contact with the connection  48 &#39;s cross section. 
     In yet a third embodiment as shown in  FIG. 6 , receiving layer  42   e  of the embodiment shown in  FIG. 4  is subdivided to form m smaller transducers  52   a  and  52   b  for either beam steering as noted above or for varying the receive beam&#39;s directivity.  FIG. 6  shows a dual aperture receiving element  42   e  where m is equal to 2. Transducer material  54  such as a piezoceramic composite is provided as the substrate of receiving layer  42   e . Transducer  52   a  has a connection  56   a  positioned within material  54  and contacting an electrode  58   a . Electrode  58   a  is positioned on the desired area of transducer material  54 . Likewise, transducer  52   b  is provided with a connection  56   b  positioned within material  54  and contacting an electrode  58   b . Area shading is realized through apodization of the electrodes  58   a  and  58   b.    
     The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description only. It is not intended to be exhaustive nor to limit the invention to the precise form disclosed; and obviously many modifications and variations are possible in light of the above teaching. Such modifications and variations that may be apparent to a person skilled in the art are intended to be included within the scope of this invention as defined by the accompanying claims.