Abstract:
A pressure compensated sound transducer is provided wherein compensation and equalization of static ambient pressure variations occurring on the faces of at least one piezoelectric element is achieved by a mensural compensating duct communicating compensating fluid between two reservoirs, each located on an opposite face of a piezoelectric element. One reservoir is formed in the void of the support housing containing at least one piezoelectric element; the remaining reservoir is formed in the void between the housing and a sealed impermeable, compliant boot. The sealed boot prevents contamination of the compensating fluid from the external environment and loss of the compensating fluid to the outside environment.

Description:
BACKGROUND 
     1. Field of the Invention 
     The present invention concerns generally a sound transducer for either receiving or transmitting sound, including subsonic, audible, and ultrasonic sound. In particular, the present invention is directed to a pressure-compensated sound transducer incorporating at least one piezoelectric element fixed to a concomitant flexural metal plate for use in ambient environments wherein the pressure is greater than that normally expected at the earth&#39;s surface, especially in high-pressure hydrostatic or geostatic operating environments. For example, utilized either as a single element or combined in various two and three dimensional arrays, the present invention provides ambient pressure-compensation means for sound transducer systems used in either maritime or borehole environments by the petroleum industry for petroleum resource exploration, exploitation and monitoring. 
     2. Description of Related Art 
     The myriad benefits accrued from the sound transducer process of converting between mechanical and electrical energy has provided the impetus for many inventions, including the incorporation of dual piezoelectric elements in a single transducer assembly to gain certain signal conditioning advantages. 
     For example, FIG. 1, exploded perspective view, exemplifies a prior art uncompensated dual element piezoelectric transducer assembly. Uncompensated transducer assembly  1  comprises a ring support  3  that maintains a first flexural metal disk  7  and a second flexural disk  11  in a fixed relation to each other. First flexural disk  7  provides propagation medium coupling to first piezoelectric element  9  affixed thereto. In like manner, second piezoelectric element  13  is fixed to second flexural disk  11 , the disk providing mechanical coupling between the medium and the piezoelectric element. Electrically connected in either series or parallel configurations, the dual piezoelectric assembly provides numerous signal-conditioning advantages. 
     Typically, transducer assembly  1  has been hermetically sealed to keep out foreign fluids and substances, and ring support void  5  is filled with a gas. Unfortunately, this manufacturing process created problems whenever the transducer was operated in ambient pressure environments considerably above normal atmospheric pressure. Numerous schemes have been employed to compensate for this problem. Some remedies entail the use of absorbent substances in the void of the transducer ring support. For example, filling the interior of the ring support housing with an absorbent material saturated with a fluid has been used in conjunction with opening the ring interior to the surrounding medium. U.S. Pat. No. 4,996,675 “SIGNAL SENSOR INSENSITIVE TO STATIC PRESSURE VARIATIONS” is incorporated herein by reference for purposes of indicating the background of the present invention or illustrating the mature state of the art for use of an absorbent material to compensate for the effects of ambient static pressure on the sensor. 
     The continuing quest for new petroleum resources has led exploration teams from relatively benign sites to sites where the environment is more extreme, and perhaps hazardous. This geographic shift has heightened the desire for simple, reliable, economical sound transducers, employing a minimum of parts and no moving parts, which can be operated in high ambient pressure environments. 
     SUMMARY 
     It is an object of the present invention to provide a pressure compensated sound transducer incorporating at least one piezoelectric element and concomitant flexural metal plate, wherein the pressure compensation means is at least one mensurational duct, the duct having pre-determined duct mensural parameters derived from calculations, for communicating pressure compensating fluid between an inner reservoir and an outer reservoir, each piezoelectric element, each reservoir and the mensurational duct enclosed within a sealed, protective, compliant boot. It is an additional object of the present invention to provide a pressure compensated sound transducer incorporating at least one piezoelectric element and concomitant flexural metal disk, and at least one mensurational duct, wherein the transducer may be employed as either a sound receiving device, for example, a hydrophone, or a sound transmitting device, for example, a sound projector, and wherein the pressure compensation means is both passive and simple. It is a further objective of the present invention to provide a pressure compensated sound transducer incorporating at least one piezoelectric element and concomitant flexural metal disk, and at least one mensurational duct, wherein the pressure compensated sound transducer may be employed singly or assembled into an array, the array being a line array, a planar array, a volumetric array, or any combination thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING(S) 
     FIG. 1 is an exploded perspective view of an exemplary prior art dual piezoelectric element transducer. 
     FIG. 2 is an exemplary end view of an embodiment of the present invention with the end cap and sealing band removed, illustrating the protective compliant boot, viewed toward the boot singular opening, and four molded compliant ribs for receiving the support housing and maintaining position of the support housing within the inner wall of the boot. Lines  3 — 3 ,  4 — 4 , and  5 — 5  define three side sectional views to further expedite description of an embodiment of the present invention. Electrical circuitry connecting the two piezoelectric elements to each other and to a sound signal conditioning system is well known to those practicing the art; to provide maximum clarity to the illustration, this connecting circuitry is not shown. 
     FIG. 3 is an exemplary sectional side view of an embodiment of the present invention illustrating the pressure-compensated transducer incorporating two piezoelectric elements and their concomitant flexural metal disks. This exemplary sectional view reveals the outer reservoir, defined by the void formed by the inner cap surface, the inner wall of the boot, and the outer surface of the support housing. In this view, the inner reservoir is also revealed, defined by the void formed by the inner surface of the support housing and the placement of each of the piezoelectric elements and their concomitant flexural metal plates placed thereon. This sectional view is filled with pressure compensating fluid to emphasize the location and shape of the inner and outer reservoirs, and also to show the pressure-compensating mensurational duct with outer and inner orifices providing communicating fluid flow between the outer reservoir and the inner reservoir. This sectional view showing a fluid-filled outer reservoir obscures the view of the molded ribs. Electrical circuitry connecting the two piezoelectric elements to each other and to a sound signal conditioning system is well known to those practicing the art; to provide maximum clarity to the illustration, this connecting circuitry is not shown. 
     FIG. 4 is an exemplary sectional side view of an embodiment of the present invention illustrating the pressure-compensated transducer incorporating two piezoelectric elements and their concomitant flexural metal disks. This sectional view is empty of any pressure compensating fluid to emphasize the location and shape of the boot ribs that receive and maintain position of the support housing within the compliant boot. Electrical circuitry connecting the two piezoelectric elements to each other and to a sound signal conditioning system is well known to those practicing the art; to provide maximum clarity to the illustration, this connecting circuitry is not shown. 
     FIG. 5 is an exemplary sectional side view of an alternative embodiment of the present invention illustrating the pressure-compensated transducer incorporating a single piezoelectric element and a concomitant flexural metal disk, the second element and disk being replaced with a rigid, impermeable disk. This sectional view is filled with pressure compensating fluid to emphasize the location and shape of the inner and outer reservoirs, and also to show the pressure-compensating mensurational duct providing communicating fluid flow between the outer reservoir and the inner reservoir. Electrical circuitry connecting the single piezoelectric element to a sound signal conditioning system is well known to those practicing the art; to provide maximum clarity to the illustration, this connecting circuitry is not shown. 
     FIG. 6 is an exemplary block diagram of an alternative array embodiment of the present invention illustrating but one of a myriad possible combinations for connecting a replicated plurality of the present invention into a sound array, such as a line array, a planar array, or a volumetric array. A sound signal conditioning system, for transmitting or receiving a sound signal, normally connected to such an array embodiment is not shown. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 2 illustrates an exemplary end view of the general structure of the preferred embodiment of the present invention. In this view of pressure-compensated transducer  20 , end cap  60  and sealing band  65  (see FIG. 3) have been removed to reveal components normally totally enclosed within protective molded compliant impermeable boot  55 . Protective boot  55  is generally a hollow cylinder with one end closed and a singular opening in the opposing end. The singular opening is capable of being sealed by receiving a sealing end cap  60  (see FIG.  3 ). Boot  55  is molded and fabricated from an impermeable compliant material such as rubber, plastic, or the like. In addition to the singular opening, boot  55  has an boot inner wall  56 , an boot outer wall  57 , and a plurality of molded boot ribs  58  molded into wall  56 . The lengthwise axis of each of the ribs  58  is generally parallel to the lengthwise axis of boot  55 . The diameter of the void defined by inner wall  56  and the width, height, and length of ribs  58  are determined such as to receive and securely maintain position of hollow, cylindrical, impermeable housing  21  within inner wall  56 . The void within boot  55 , between the outer surface  22  of housing  21  and boot inner wall  56 , totally surrounding housing  21 , defines outer reservoir  75 . The extent of the void is limited by inner cap surface  61  of end cap  60  (see FIG.  3 ). 
     The view of FIG. 2 looks into the open end of boot  55 , revealing a first flexural metal plate  30 , affixed to housing  21 . Beneath plate  30 , illustrated with hidden lines, is an associated first piezoelectric element  28 , securely affixed to plate  30  by means of an electrically conducting bonding agent. Plate  30  and piezoelectric element  28  are commercially available and techniques for their assembly are well know to those skilled in the art. 
     The open-end view of FIG. 2 additionally reveals pressure-compensating mensurational duct  80 , shown by hidden lines, providing communication between outer reservoir  75  and the interior void of housing  21 . Pressure-compensating mensurational duct  80  is defined hereby to be a duct whose mensural parameters have been calculated and determined prior to fabrication thereof. With the exception of duct  80 , upon completion of manufacture the interior void of support housing  21  is sealed and impermeable. First piezoelectric element  28  is positioned within the interior void of housing  21 . First impermeable feedthrough electrical connector  38  provides means for electrical connectivity between piezoelectric element  28  and associated sound signal conditioning systems (not shown and not claimed in the present invention) external to housing  21 . Feedthrough  38  is readily and commercially available. 
     The preferred embodiment of the present invention is further described in exemplary FIG. 3, defined by section line  3 — 3  of FIG.  2 . FIG. 3 is a side sectional view of a complete transducer  20 , with end cap  60  inserted into the singular opening of protective boot  55  and securely fastened thereto by means of sealing band  65 , effectively sealing boot  55  from external environmental effects, such as foreign fluids and substances. To clarify description of the present invention, all voids within boot  55  are completely filled with a requisite pressure-compensating fluid  85 . The fluid  85  obscures viewing ribs  58  (not shown in FIG.  3 ). 
     The preferred embodiment of the present invention is further described in FIG. 4, defined by section line  4 — 4  of FIG.  2 . This view is devoid of pressure-compensating fluid  85  to more clearly describe the plurality of molded compliant boot ribs  58 . Boot  55  and ribs  58  are simultaneously molded from the same compliant material. 
     The preferred embodiment of the present invention, FIG. 3, provides a pressure compensated piezoelectric transducer  20  incorporating two piezoelectric elements. First piezoelectric element  28  is affixed to concomitant first flexural metal plate  30  with an electrically conducting bonding agent, and second piezoelectric element  45  is similarly affixed to concomitant second flexural metal plate  50  with an electrically conducting bonding agent, as is commonly practiced in the art. First plate  30  has a first peripheral edge  31 ; similarly, second plate  50  has a second peripheral edge  51 . The piezoelectric elements may be piezoelectric ceramic wafers, piezoelectric polymer film, such as poly vinylidene fluoride (PVDF), or like piezoelectric materials. Both piezoelectric elements and flexural metal plates are commercially available. 
     Support housing  21  is tubular, rigid, impermeable and preferably fabricated from an electrically conductive material. Housing  21  features an outer surface  22 , an inner surface  23 , a first rim  25  encompassing an open end of housing  21  and a second rim  26  encompassing the opposing open end of housing  21 . First peripheral edge  31  of plate  30  is affixed to rim  25  of housing  21  with an electrically conducting bonding agent. In like manner, second peripheral edge  51  of plate  50  is affixed to rim  26  of housing  21 , the aggregated fabrication of plates  30  and  50 , and inner surface  23  defining inner reservoir  70  in the void of housing  21 . 
     Piezoelectric elements  28  and  45  may be electrically connected and configured in various series and parallel circuits as a piezoelectric element configuration circuit (not shown and not claimed in the present invention) using techniques widely known to those skilled in the art. 
     Penetrating housing  21  is first impermeable feed-through electrical connector  38 , for connecting the piezoelectric element configuration circuit, via electrical connection leads  35 , to a sound signal conditioning system comprising receiving or transmitting circuitry (not shown). Such signal conditioning systems and circuits are well known to those practicing the art and are not claimed in the present invention. An alternative to first feed-through connector  38  is piercing housing  21 , feeding leads  35  through the resultant aperture, and finally sealing the aperture with an appropriate sealing compound, such as plastic resin, silicone putty, or synthetic rubber. 
     The nexus of pressure compensation for maritime and geophysical sound transducers is the relief of the static stress differential pressure across the opposite surfaces of a thin piezoelectric element. It can be seen from FIG. 3, that one surface of each of the two piezoelectric elements,  28  and  45 , faces inner reservoir  70 . Conversely, the opposite face of each of the two piezoelectric elements,  28  and  45  is bonded to a concomitant flexural plate,  30  and  50 , respectively, which has a surface exposed to outer reservoir  75 . Thus, any static stress differential pressure occurring across the opposite faces of each of the two piezoelectric elements,  28  and  45 , will be manifested as a difference in static pressure between the fluid in the inner reservoir  70  and the fluid in the outer reservoir  75 . 
     Pressure compensation between inner reservoir  70  and outer reservoir  75  is effected by piercing the impermeable structure of housing  21  with pressure compensating mensurational duct  80 , having inner orifice  82  and outer orifice  83 . The operational effectiveness of the pressure compensation technique embodied in the present invention is dependant upon the mensural parameters of duct  80 , the volume of inner reservoir  70  and the viscosity and bulk modulus of pressure compensating fluid  85 . 
     The mensural parameters of mensurational duct  80  include length and diameter. Judicious selection of these parameters will determine the effective operating sound bandwidth of the present invention. In the preferred embodiment, the mensural parameters of mensurational duct  80  are derived from calculations developed in “Hydrophone Development at Hudson Laboratories”, E. T. O&#39;Neil, Columbia University Hudson Labs, Technical Report No. 108, 1963. Technical Report No. 108 is incorporated by reference herein to demonstrate the state of the art in the calculated determination of mensural parameters of pressure-compensating ducts, conduits, and tubes. The mathematical expressions developed in O&#39;Neil&#39;s report are derived from the Hagen-Poiseuille law for laminar flow in circular pipes. O&#39;Neil&#39;s derivation determines ƒ 0  as the frequency at which the sound power is one-half (−3 dB) that of the sound power in the desired sound frequency band, i.e. the operating sound bandwidth of the transducer. ƒ 0 =½πT where T is the time constant of the pressure compensating system, from which O&#39;Neil derives,          f   0     =         D   4          B   e         256                 µ                 L                   V   r                                
     where, 
     D=diameter of pressure compensating mensurational duct  80  (inches) 
     L=length of pressure compensating mensurational duct  80  (inches) 
     B e =bulk modulus of pressure compensating fluid  85  (psi) 
     V r =volume of inner reservoir  70  (inches 3 ) 
     μ=dynamic viscosity of pressure compensating fluid  85  (lb-sec/inch 2 ) 
     Employment of the present invention includes operational use in adverse and hostile environments, such as, in geophysical boreholes, in the ocean depths, or buried beneath the seafloor. To protect housing  21  from potentially risky environments, boot  55 , having boot inner wall  56  and boot outer wall  57 , is sealed with endcap  60 , secured to boot  55  with sealing band  65 . 
     Endcap  65  features a high-pressure electrical feed-through connector  42  for passing a plurality of electrical connection leads  35  through the sealed boot  55 , connecting the piezoelectric element configuration circuit within housing  21  to an external sound signal conditioning system for either receiving or transmitting a sound signal. Feedthrough  42  is readily and commercially available. 
     Assembly of pressure compensated piezoelectric transducer  20  entails submerging all components beneath the surface of a pressure compensating fluid  85 . This technique is employed to purge any gas bubbles entrained within transducer  20 . Preferably, fluid  85  is a commercially available viscous, non-conducting fluid, such as castor oil (DB grade) or DC 200 silicone fluid, or the like. The key physical properties of fluid  85  affecting the choice of the pressure compensating fluid include bulk modulus and viscosity. Another important parameter influencing the choice of fluid  85  is the temperature range expected to be encountered in the operating environment. Upon purging all gases and foreign substances from within boot  55  and insuring both inner reservoir  70  and outer reservoir  75  are completely filled with compensating fluid  85 , housing  21  is received by and maintained in position by boot ribs  58 . Next, endcap  60  is inserted into boot  55 , snugly fitting against boot inner wall  56 . Sealing of boot  55  is completed by means of sealing band  65 . Such sealing prevents any environmental fluids or substances from contaminating the volume within inner wall  56  and additionally prevents any compensating fluid  85  from escaping into the external environment. 
     During assembly of pressure compensated transducer  20 , care is taken when inserting support housing  21  into boot  55  to insure outer orifice  83  is not obstructed or otherwise restricted by boot ribs  58 . 
     An alternative embodiment of the present invention is described in FIG. 5, defined by section line  5 — 5  of FIG.  2 . FIG. 5 illustrates a single piezoelectric element embodiment of the present invention, similar to FIG. 2, wherein rigid plate  90  is substituted for second piezoelectric element  45  and concomitant flexural disk  50 . 
     Pressure compensated transducer  20  can be utilized as a solitary source, for example, a sound projector, or a solitary receiver of sound wave propagation, for example, a hydrophone. Alternatively, a plurality of devices, each such device transducer  20 , can be configured and assembled into various line, surface, and volumetric arrays, or combinations thereof. FIG. 6 illustrates but one of a myriad possible series-parallel combinations of a plurality assemblage of the present invention. 
     Although only a few exemplary embodiments of the invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims. In the claims, means-plus-functions clauses are intended to cover the structures described herein as performing the recited functions and not only structural equivalents but also equivalent structures.