Title of Invention: ELECTRICAL AND THERMAL CONNECTIONS FOR ULTRASONIC TRANSDUCER Abstract: Disclosed is an ultrasonic surgical instrument comprising a transducer base plate, first and second piezoelectric elements positioned on opposite faces of the transducer base plate. The transducer base plate is coupled to a waveguide. The waveguide is electrically coupled to the first and second piezoelectric elements by a conductive adhesive. The first and second piezoelectric elements are electrically coupled to an ultrasonic signal generator through an electrode. A thermal conductor conducts thermal energy away from the first and second piezoelectric elements. Also disclosed is a method of fabricating such an ultrasonic surgical instrument. Detailed Description: PRIORITY This application claims the benefit of U.S. Provisional Application Ser. No. 62/379,550 filed Aug. 25, 2016, which is incorporated herein by reference in its entirety. BACKGROUND The present disclosure relates, in general, to ultrasonic surgical instruments and more particularly to ultrasonic transducers to drive ultrasonic waveguides. Ultrasonic instruments, including both hollow core and solid core instruments, are used for the safe and effective treatment of many medical conditions. Ultrasonic instruments, and particularly solid core ultrasonic instruments, are advantageous because they may be used to cut and/or coagulate organic tissue using energy in the form of mechanical vibrations transmitted to a surgical end effector at ultrasonic frequencies. Ultrasonic vibrations, when transmitted to organic tissue at suitable energy levels and using a suitable end effector, may be used to cut, dissect, elevate or cauterize tissue or to separate muscle tissue from bone. Ultrasonic instruments utilizing solid core technology are particularly advantageous because of the amount of ultrasonic energy that may be transmitted from the ultrasonic transducer, through a waveguide, and to the surgical end effector. Such instruments may be used for open procedures or minimally invasive procedures, such as endoscopic or laparoscopic procedures, wherein the end effector is passed through a trocar to reach the surgical site. Activating or exciting the end effector (e.g., cutting blade) of such instruments at ultrasonic frequencies induces longitudinal vibratory movement that generates localized heat within adjacent tissue. Because of the nature of ultrasonic instruments, a particular ultrasonically actuated end effector may be designed to perform numerous functions, including, for example, cutting and coagulation. Ultrasonic vibration is induced in the surgical end effector by electrically exciting a transducer, for example. The transducer may be constructed of one or more piezoelectric or magnetostrictive elements in the instrument hand piece. Vibrations generated by the transducer are transmitted to the surgical end effector via an ultrasonic waveguide extending from the transducer to the surgical end effector. The waveguide and end effector are designed to resonate at the same frequency as the transducer. Therefore, when an end effector is attached to a transducer, the overall system frequency is the same frequency as the transducer itself. The amplitude of the longitudinal ultrasonic vibration at the tip, d, of the end effector behaves as a simple sinusoid at the resonant frequency as given by: d=A sin(ωt) where: ω=the radian frequency which equals 27 times the cyclic frequency, f; and A=the zero-to-peak amplitude. The longitudinal excursion of the end effector tip is defined as the peak-to-peak (p-t-p) amplitude, which is just twice the amplitude of the sine wave or 2A. Often, the end effector can comprise a blade which, owing to the longitudinal excursion, can cut and/or coagulate tissue. U.S. Pat. No. 6,283,981, which issued on Sep. 4, 2001 and is entitled METHOD OF BALANCING ASYMMETRIC ULTRASONIC SURGICAL BLADES; U.S. Pat. No. 6,309,400, which issued on Oct. 30, 2001 and is entitled CURVED ULTRASONIC WAVEGUIDE HAVING A TRAPEZOIDAL CROSS SECTION; and U.S. Pat. No. 6,436,115, which issued on Aug. 20, 2002 and is entitled BALANCED ULTRASONIC WAVEGUIDE INCLUDING A PLURALITY OF BALANCE ASYMMETRIES, the entire disclosures of which are hereby incorporated by reference herein, disclose various ultrasonic surgical instruments. SUMMARY In one general aspect, various aspects are directed to an ultrasonic surgical instrument that comprises a transducer configured to produce vibrations along a longitudinal axis of a surgical tool at a predetermined frequency. In various aspects, the surgical tool may include an ultrasonic waveguide that extends along the longitudinal axis and is coupled to the transducer. In various aspects, the surgical tool includes a body having a proximal end and a distal end, wherein the distal end is movable relative to the longitudinal axis by the vibrations produced by the transducer, and the proximal end is mechanically coupled to the transducer. In one aspect, an ultrasonic surgical instrument is provided. The ultrasonic surgical instrument comprises a transducer base plate (e.g., a transducer mounting portion) comprising a first and second face; a first piezoelectric element positioned on the first face; a second piezoelectric element positioned on the second face, wherein the first and second piezoelectric element are each configured to operate in a D31 mode with respect to a longitudinal axis of a waveguide coupled to the transducer base plate, wherein the waveguide is electrically coupled to the first piezoelectric element and to the second piezoelectric element by a conductive adhesive; an electrode configured to electrically couple the first and second piezoelectric elements to an ultrasonic signal generator; and a thermal conductor configured to conduct thermal energy away from the first and second piezoelectric elements. In another aspect, a method of fabricating an ultrasonic surgical instrument is provided. The method comprises machining a transducer base plate from a portion of a flat metal stock, wherein the transducer base plate comprises a first and second face; positioning a first piezoelectric element on the first face; positioning a second piezoelectric element on the second face, wherein the first and second piezoelectric element are each configured to operate in a D31 mode with respect to a longitudinal axis of a waveguide coupled to the transducer base plate; coupling, by a first electrically conductive adhesive, the waveguide to the first piezoelectric element and the second piezoelectric element; compressing, by an electrode, against the first and second piezoelectric elements to electrically couple the first and second piezoelectric elements to an ultrasonic signal generator; and conducting, by a thermal conductor, heat away from the first and second piezoelectric elements. In another aspect, a transducer base plate is provided. The transducer base plate comprises a first and second face, wherein a first piezoelectric element is positioned on the first face and a second piezoelectric element is positioned on the second face, and wherein the first and second piezoelectric element are each configured to operate in a D31 mode with respect to a longitudinal axis of a waveguide coupled to the transducer base plate; a conductive adhesive to electrically couple the first and second piezoelectric elements to the waveguide; an electrode configured to electrically couple the first and second piezoelectric elements to an ultrasonic signal generator; and a thermal conductor configured to conduct heat away from the first and second piezoelectric elements. In another aspect, an ultrasonic transducer assembly is provided. The ultrasonic transducer assembly comprises a stack of a plurality of piezoelectric elements, wherein the stack is configured to operate in a D33 mode with respect to a longitudinal axis of a waveguide coupled to the transducer base plate; a compression plate to compress the stack of the plurality of piezoelectric elements to couple the stack of piezoelectric elements to a waveguide; and a compressive element for applying a compressive force against the compression plate. FIGURES The features of various aspects are set forth with particularity in the appended claims. The various aspects, however, both as to organization and methods of operation, together with further objects and advantages thereof, may best be understood by reference to the following description, taken in conjunction with the accompanying drawings as follows. FIG. 1 illustrates an ultrasonic surgical instrument system, according to one aspect of this disclosure. FIGS. 2A-2C illustrate a piezoelectric transducer, according to one aspect of this disclosure. FIG. 3 illustrates a D31 ultrasonic transducer architecture that includes an ultrasonic waveguide and one or more piezoelectric elements fixed to the ultrasonic waveguide, according to one aspect of this disclosure. FIG. 4 is a perspective view of a transducer assembly employing a wedge configuration and configured to operate in a D33 mode of operation, according to one aspect of this disclosure. FIG. 5A is a section view of the ultrasonic transducer assembly shown in FIG. 4 taken along section line 5A-5A shown in FIG. 4, according to one aspect of this disclosure. FIG. 5B is a section view of the ultrasonic transducer assembly shown in FIG. 4 taken along section line 5B-5B shown in FIG. 4, according to one aspect of this disclosure. FIGS. 5C-5D illustrate cross section views of the ultrasonic transducer assembly 6805 employing another wedge configuration taken along similar section lines as shown in FIGS. 5A and 5B, according to one aspect of this disclosure. FIG. 6 is perspective view of an ultrasonic transducer assembly employing a screw to compress against the compression plate to compress and acoustically couple piezoelectric elements to an ultrasonic waveguide, according to one aspect of this disclosure. FIG. 7 is a section view of the of the ultrasonic transducer assembly shown in FIG. 6 taken along section 7-7 as shown in FIG. 6, FIG. 8 is a perspective view of the screw, according to one aspect of this disclosure. FIG. 9 illustrates an ultrasonic surgical instrument configured to operate in a D31 mode of operation, according to one aspect of this disclosure. FIG. 10 is a section view of the transducer assembly comprising a first piezoelectric element positioned on a first side of the waveguide and a second piezoelectric element positioned on a second opposing side of the waveguide, according to one aspect of this disclosure. FIG. 11 is a section view of an ultrasonic surgical instrument configured to operate in a D31 mode of operation, according to one aspect of this disclosure. FIG. 12 is an exploded view of an ultrasonic surgical instrument configured to operate in a D31 mode of operation, according to one aspect of this disclosure. FIG. 13A is a side view of an ultrasonic surgical instrument configured to operate in a D31 mode of operation, according to one aspect of this disclosure. FIG. 13B is a cross sectional view of a side of the ultrasonic surgical instrument shown in FIG. 13A, according to one aspect of this disclosure. FIG. 14 is a top view of the ultrasonic surgical instrument shown in FIG. 13A, according to one aspect of this disclosure. FIG. 15 is a rear cross sectional view of the ultrasonic surgical instrument shown in FIG. 13A, according to one aspect of this disclosure. FIG. 16A is a plan view of an electrode shim in a flat configuration, according to one aspect of this disclosure. FIG. 16B is a perspective view of the electrode shim shown in FIG. 16A in a folded configuration, according to one aspect of this disclosure. FIGS. 17A-17B are views of a portion of the ultrasonic surgical instrument shown in FIG. 13A, with polyimide film material coating as an insulator positioned on the electrode shim, according to one aspect of this disclosure. FIG. 18 is a perspective view of an ultrasonic surgical instrument, which is configured to operate in a D31 mode of operation, according to one aspect of this disclosure. FIG. 19 is a perspective view of an electrode of the ultrasonic surgical instrument shown in FIG. 18, according to one aspect of this disclosure. FIG. 20 is a plan view of the transducer assembly of the ultrasonic surgical instrument shown in FIG. 18 including a recessed receiving portion configured to receive an electrode, according to one aspect of this disclosure. FIG. 21A is a longitudinal sectional view of the transducer assembly shown in FIG. 20, with a portion of the electrode, the molded interconnect pad, the receiving area, and a portion of the recessed receiving portion, according to one aspect of this disclosure. FIG. 21B is a lateral sectional view of the transducer assembly shown in FIG. 20, with a portion of the electrode, the molded interconnect pad, the receiving area, and the recessed receiving portion, according to one aspect of this disclosure. FIGS. 22A-22C illustrate an assembly process of an electrode of the ultrasonic surgical instrument shown in FIG. 18, into the recessed receiving portion, according to one aspect of this disclosure. FIGS. 23A-23B illustrate an assembly process of the transducer assembly shown in FIG. 20, with electrodes assembled from an initial uncompressed state to a final compressed state, according to one aspect of this disclosure. FIG. 24 shows aspects of an ultrasonic surgical instrument, which is configured to operate in a D31 mode of operation, according to one aspect of this disclosure. FIG. 25 is a sectional view of the ultrasonic surgical instrument shown in FIG. 24 taken along section line 25-25 as shown in FIG. 24, according to one aspect of this disclosure. FIG. 26 is a perspective view of an upper heat sink half of the ultrasonic surgical instrument shown in FIG. 24, according to one aspect of this disclosure. FIG. 27 is a perspective view of an ultrasonic surgical instrument, which is configured to operate in a D31 mode of operation, according to one aspect of this disclosure. FIG. 28 illustrates a D31 ultrasonic surgical instrument that includes piezoelectric elements attached on one side to an ultrasonic waveguide by a conductive adhesive and attached on another side to electrically conductive plates by a conductive adhesive, according to one aspect of this disclosure. FIG. 29 is a perspective view of an ultrasonic surgical instrument, which is configured to operate in a D31 mode of operation, according to one aspect of this disclosure. FIG. 30 is a sectional view of the ultrasonic surgical instrument shown in FIG. 29 with electrically conductive electrodes configured to compress the piezoelectric elements and the ultrasonic waveguide, according to one aspect of this disclosure. FIG. 31A illustrates an ultrasonic surgical instrument prior to assembly and poling, according to one aspect of this disclosure. FIG. 31B illustrates the ultrasonic surgical instrument of FIG. 31A prior to poling and the first and second unpoled piezoelectric elements secured to the ultrasonic waveguide in a D31 configuration, according to one aspect of this disclosure. FIG. 31C illustrates the ultrasonic instrument of FIG. 31B prior to poling and the first and second unpoled piezoelectric elements secured to the ultrasonic waveguide in a D31 configuration, according to one aspect of this disclosure. DESCRIPTION Applicant of the present application owns the following patent applications filed concurrently herewith and which are each herein incorporated by reference in their respective entireties: Attorney Docket No. END7919USNP/160045M, titled “Ultrasonic Transducer Techniques for Ultrasonic Surgical Instrument” by inventors Jeffrey Messerly et al. filed Aug. 17, 2017. Attorney Docket No. END7919USNP1/160045-1, titled “Ultrasonic Transducer For Surgical Instrument, by inventors Jeffrey Messerly et al. filed Aug. 17, 2017. Attorney Docket No. END7919USNP3/160045-3, entitled “Ultrasonic Transducer To Waveguide Acoustic Coupling, Connections, and Configurations” by inventors Jeffrey Messerly et al. filed Aug. 17, 2017. Attorney Docket No. END7919USNP4/160045-4, entitled “Ultrasonic Transducer to Waveguide Joining” by inventors Jeffrey Messerly et al. filed Aug. 17, 2017. Attorney Docket No. END7919USNP5/160045-5, entitled “Tissue Loading of a Surgical Instrument” by inventors Jeffrey Messerly et al. filed Aug. 17, 2017. Before explaining various aspects in detail, it should be noted that such aspects are not limited in their application or use to the details of construction and arrangement of parts illustrated in the accompanying drawings and description. The illustrative aspects may be implemented or incorporated in other aspects, variations and modifications, and may be practiced or carried out in various ways. For example, the surgical instruments disclosed below are illustrative only and not meant to limit the scope or application thereof. Furthermore, unless otherwise indicated, the terms and expressions employed herein have been chosen for the purpose of describing the illustrative aspects for the convenience of the reader and are not to limit the scope thereof. Certain aspects will now be described to provide an overall understanding of the principles of the structure, function, manufacture, and use of the devices and methods disclosed herein. One or more examples of these aspects are illustrated in the accompanying drawings. Those of ordinary skill in the art will understand that the devices and methods specifically described herein and illustrated in the accompanying drawings are non-limiting examples aspects and that the scope of the various aspects is defined solely by the claims. The features illustrated or described in connection with one aspect may be combined with the features of other aspects. Such modifications and variations are intended to be included within the scope of the claims. Various aspects described herein relate, in general, to ultrasonic surgical instruments and blades for use therewith. Examples of ultrasonic surgical instruments and blades are disclosed in U.S. Pat. Nos. 5,322,055; 5,954,736; 6,309,400; 6,278,218; 6,283,981; 6,325,811; and 8,319,400, wherein the entire disclosures of which are incorporated by reference herein. According to various aspects of the present disclosure, various methods of electrically coupling components of a surgical tool are disclosed. The surgical tool may be a component of an ultrasonic surgical instrument. FIG. 1 illustrates one aspect of an ultrasonic system 10. One aspect of the ultrasonic system 10 comprises an ultrasonic signal generator 12 coupled to an ultrasonic transducer 14, a hand piece assembly 60 comprising a hand piece housing 16, and an end effector 50. The ultrasonic transducer 14, which is known as a “Langevin stack,” generally includes a transduction portion 18, a first resonator or end-bell 20, and a second resonator or fore-bell 22, and ancillary components. In various aspects, the ultrasonic transducer 14 is preferably an integral number of one-half system wavelengths (nλ/2) in length as will be described in more detail below. An acoustic assembly 24 can include the ultrasonic transducer 14, a mount 26, a velocity transformer 28, and a surface 30. It will be appreciated that the terms “proximal” and “distal” are used herein with reference to a clinician gripping the hand piece assembly 60. Thus, the end effector 50 is distal with respect to the more proximal hand piece assembly 60. It will be further appreciated that, for convenience and clarity, spatial terms such as “top” and “bottom” also are used herein with respect to the clinician gripping the hand piece assembly 60. However, surgical instruments are used in many orientations and positions, and these terms are not intended to be limiting and absolute. The distal end of the end-bell 20 is connected to the proximal end of the transduction portion 18, and the proximal end of the fore-bell 22 is connected to the distal end of the transduction portion 18. The fore-bell 22 and the end-bell 20 have a length determined by a number of variables, including the thickness of the transduction portion 18, the density and modulus of elasticity of the material used to manufacture the end-bell 20 and the fore-bell 22, and the resonant frequency of the ultrasonic transducer 14. The fore-bell 22 may be tapered inwardly from its proximal end to its distal end to amplify the ultrasonic vibration amplitude of the velocity transformer 28, or, alternately, fore-bell 22 may have no amplification. Referring again to FIG. 1, end-bell 20 can include a threaded member extending therefrom which can be configured to be threadably engaged with a threaded aperture in fore-bell 22. In various aspects, piezoelectric elements, such as piezoelectric elements 32, for example, can be compressed between end-bell 20 and fore-bell 22 when end-bell 20 and fore-bell 22 are assembled together. Piezoelectric elements 32 may be fabricated from any suitable material, such as, for example, lead zirconate-titanate, lead meta-niobate, lead titanate, and/or any suitable piezoelectric crystal material, for example. In various aspects, as discussed in greater detail below, transducer 14 can further comprise electrodes, such as positive electrodes 34 and negative electrodes 36, for example, which can be configured to create a voltage potential across one or more piezoelectric elements 32. Each of the positive electrodes 34, negative electrodes 36, and the piezoelectric elements 32 can comprise a bore extending through the center which can be configured to receive the threaded member of end-bell 20. In various aspects, the positive and negative electrodes 34 and 36 are electrically coupled to wires 38 and 40, respectively, wherein the wires 38 and 40 can be encased within a cable 42 and electrically connectable to the ultrasonic signal generator 12 of the ultrasonic system 10. In various aspects, the ultrasonic transducer 14 of the acoustic assembly 24 converts the electrical signal from the ultrasonic signal generator 12 into mechanical energy that results in primarily longitudinal vibratory motion of the ultrasonic transducer 24 and the end effector 50 at ultrasonic frequencies. A suitable generator is available as model number GEN11, from Ethicon Endo-Surgery, Inc., Cincinnati, Ohio. When the acoustic assembly 24 is energized, a vibratory motion standing wave is generated through the acoustic assembly 24. A suitable vibrational frequency range may be about 20 Hz to 120 kHz and a well-suited vibrational frequency range may be about 30-70 kHz and one example operational vibrational frequency may be approximately 55.5 kHz. The amplitude of the vibratory motion at any point along the acoustic assembly 24 may depend upon the location along the acoustic assembly 24 at which the vibratory motion is measured. A minimum or zero crossing in the vibratory motion standing wave is generally referred to as a node (i.e., where motion is usually minimal), and an absolute value maximum or peak in the standing wave is generally referred to as an anti-node (i.e., where motion is usually maximal). The distance between an anti-node and its nearest node is one-quarter wavelength (λ/4). As outlined above, the wires 38 and 40 transmit an electrical signal from the ultrasonic signal generator 12 to the positive electrodes 34 and the negative electrodes 36. The piezoelectric elements 32 are energized by the electrical signal supplied from the ultrasonic signal generator 12 in response to a foot switch 44, for example, to produce an acoustic standing wave in the acoustic assembly 24. The electrical signal causes disturbances in the piezoelectric elements 32 in the form of repeated small displacements resulting in large compression forces within the material. The repeated small displacements cause the piezoelectric elements 32 to expand and contract in a continuous manner along the axis of the voltage gradient, producing longitudinal waves of ultrasonic energy. In various aspects, the ultrasonic energy produced by transducer 14 can be transmitted through the acoustic assembly 24 to the end effector 50 via an ultrasonic transmission waveguide 46. In order for the acoustic assembly 24 to deliver energy to the end effector 50, the components of the acoustic assembly 24 are acoustically coupled to the end effector 50. For example, the distal end of the ultrasonic transducer 14 may be acoustically coupled at the surface 30 to the proximal end of the ultrasonic transmission waveguide 46 by a threaded connection such as a stud 48. The components of the acoustic assembly 24 can be acoustically tuned such that the length of any assembly is an integral number of one-half wavelengths (nλ/2), where the wavelength λ is the wavelength of a pre-selected or operating longitudinal vibration drive frequency fd of the acoustic assembly 24, and where n is any positive integer. It is also contemplated that the acoustic assembly 24 may incorporate any suitable arrangement of acoustic elements. The ultrasonic end effector 50 may have a length substantially equal to an integral multiple of one-half system wavelengths (λ/2). A distal end 52 of the ultrasonic end effector 50 may be disposed at, or at least near, an antinode in order to provide the maximum, or at least nearly maximum, longitudinal excursion of the distal end. When the transducer assembly is energized, in various aspects, the distal end 52 of the ultrasonic end effector 50 may be configured to move in the range of, for example, approximately 10 to 500 microns peak-to-peak and preferably in the range of approximately 30 to 150 microns at a predetermined vibrational frequency. As outlined above, the ultrasonic end effector 50 may be coupled to the ultrasonic transmission waveguide 46. In various aspects, the ultrasonic end effector 50 and the ultrasonic transmission guide 46 as illustrated are formed as a single unit construction from a material suitable for transmission of ultrasonic energy such as, for example, Ti6Al4V (an alloy of titanium including aluminum and vanadium), aluminum, stainless steel, and/or any other suitable material. Alternately, the ultrasonic end effector 50 may be separable (and of differing composition) from the ultrasonic transmission waveguide 46, and coupled by, for example, a stud, weld, glue, quick connect, or other suitable known methods. The ultrasonic transmission waveguide 46 may have a length substantially equal to an integral number of one-half system wavelengths (λ/2), for example. The ultrasonic transmission waveguide 46 may be preferably fabricated from a solid core shaft constructed out of material that propagates ultrasonic energy efficiently, such as titanium alloy (i.e., Ti6Al4V) or an aluminum alloy, for example. In the aspect illustrated in FIG. 1, the ultrasonic transmission waveguide 46 comprises a plurality of stabilizing silicone rings or compliant supports 56 positioned at, or at least near, a plurality of nodes. The silicone rings 56 can dampen undesirable vibration and isolate the ultrasonic energy from a sheath 58 at least partially surrounding waveguide 46, thereby assuring the flow of ultrasonic energy in a longitudinal direction to the distal end 52 of the end effector 50 with maximum efficiency. As shown in FIG. 1, the sheath 58 can be coupled to the distal end of the handpiece assembly 60. The sheath 58 generally includes an adapter or nose cone 62 and an elongated tubular member 64. The tubular member 64 is attached to and/or extends from the adapter 62 and has an opening extending longitudinally therethrough. In various aspects, the sheath 58 may be threaded or snapped onto the distal end of the housing 16. In at least one aspect, the ultrasonic transmission waveguide 46 extends through the opening of the tubular member 64 and the silicone rings 56 can contact the sidewalls of the opening and isolate the ultrasonic transmission waveguide 46 therein. In various aspects, the adapter 62 of the sheath 58 is preferably constructed from Ultem®, for example, and the tubular member 64 is fabricated from stainless steel, for example. In at least one aspect, the ultrasonic transmission waveguide 46 may have polymeric material, for example, surrounding it in order to isolate it from outside contact. As described above, a voltage, or power source can be operably coupled with one or more of the piezoelectric elements of a transducer, wherein a voltage potential applied to each of the piezoelectric elements can cause the piezoelectric elements to expand and contract, or vibrate, in a longitudinal direction. As also described above, the voltage potential can be cyclical and, in various aspects, the voltage potential can be cycled at a frequency which is the same as, or nearly the same as, the resonant frequency of the system of components comprising transducer 14, wave guide 46, and end effector 50, for example. In various aspects, however, certain of the piezoelectric elements within the transducer may contribute more to the standing wave of longitudinal vibrations than other piezoelectric elements within the transducer. More particularly, a longitudinal strain profile may develop within a transducer wherein the strain profile may control, or limit, the longitudinal displacements that some of the piezoelectric elements can contribute to the standing wave of vibrations, especially when the system is being vibrated at or near its resonant frequency. It may be recognized, in reference to the ultrasonic surgical instrument system 10 of FIG. 1, that multiple components may be required to couple the mechanical vibrations from the piezoelectric elements 32 through the wave guide 46 to the end effector 50. The additional elements comprising the acoustic assembly 24 may add additional manufacturing costs, fabrication steps, and complexity to the system. Disclosed below are aspects of an ultrasonic medical device that may require fewer components, manufacturing steps, and costs than the equivalent device illustrated in FIG. 1 and as disclosed above. Again, referring to FIG. 1, the piezoelectric elements 32 are configured into a “Langevin” stack, in which the piezoelectric elements 32 and their activating electrodes 34 and 36 (together, transducer 14) are interleaved. The mechanical vibrations of the activated piezoelectric elements 32 propagate along the longitudinal axis of the transducer 14, and are coupled via the acoustic assembly 24 to the end of the waveguide 46. Such a mode of operation of a piezoelectric element is frequently described as the D33 mode of the element, especially for ceramic piezoelectric elements comprising, for example, lead zirconate-titanate, lead meta-niobate, or lead titanate. The D33 mode of a ceramic piezoelectric element is illustrated in FIGS. 2A-2C. FIG. 2A depicts a piezoelectric element 200 fabricated from a ceramic piezoelectric material. A piezoelectric ceramic material is a polycrystalline material comprising a plurality of individual microcrystalline domains. Each microcrystalline domain possesses a polarization axis along which the domain may expand or contract in response to an imposed electric field. However, in a native ceramic, the polarization axes of the microcrystalline domains are arranged randomly, so there is no net piezoelectric effect in the bulk ceramic. A net re-orientation of the polarization axes may be induced by subjecting the ceramic to a temperature above the Curie temperature of the material and placing the material in a strong electrical field. Once the temperature of the sample is dropped below the Curie temperature, a majority of the individual polarization axes will be re-oriented and fixed in a bulk polarization direction. FIG. 2A illustrates such a piezoelectric element 200 after being polarized along the inducing electric field axis P. While the un-polarized piezoelectric element 200 lacks any net piezoelectric axis, the polarized element 200 can be described as possessing a polarization axis, d3, parallel to the inducing field axis P direction. For completeness, an axis orthogonal to the d3 axis may be termed a d1 axis. The dimensions of the piezoelectric element 200 are labeled as length (L), width (W), and thickness (T). FIGS. 2B and 2C illustrate the mechanical deformations of a piezoelectric element 200 that may be induced by subjecting the piezoelectric element 200 to an actuating electrical field E oriented along the d3 (or P) axis. FIG. 2B illustrates the effect of an electric field E having the same direction as the polarization field P along the d3 axis on a piezoelectric element 205. As illustrated in FIG. 2B, the piezoelectric element 205 may deform by expanding along the d3 axis while compressing along the d1 axis. FIG. 2C illustrates the effect of an electric field E having the opposing direction to the polarization field P along the d3 axis on a piezoelectric element 210. As illustrated in FIG. 2C, the piezoelectric element 210 may deform by compressing along the d3 axis, while expanding along the d1 axis. Vibrational coupling along the d3 axis during the application of an electric field along the d3 axis may be termed D33 coupling or activation using a D33 mode of a piezoelectric element. The transducer 14 illustrated in FIG. 1 uses the D33 mode of the piezoelectric elements 32 for transmitting mechanical vibrations along the wave guide 46 to the end effector 50. Because the piezoelectric element also deforms along the d1 axis, vibrational coupling along the d1 axis during the application of an electric field along the d3 axis may also be an effective source of mechanical vibrations. Such coupling may be termed D31 coupling or activation using a D31 mode of a piezoelectric element. As illustrated by FIGS. 2A-2C, during operation in the D31 mode, transverse expansion of piezoelectric elements 200, 205, 210 may be mathematically modeled by the following equation: Δ   L L = Δ   W W = V d   31 T In the equation, L, W, and T refer to the length, width and thickness dimensions of a piezoelectric element, respectively. Vd31 denotes the voltage applied to a piezoelectric element operating in the D31 mode. The quantity of transverse expansion resulting from the D31 coupling described above is represented by ΔL (i.e., expansion of the piezoelectric element along the length dimension) and ΔW (i.e., expansion of the piezoelectric element along the width dimension). Additionally, the transverse expansion equation models the relationship between ΔL and ΔW and the applied voltage Vd31. Disclosed below are aspects of ultrasonic medical devices based on D31 activation by a piezoelectric element. In various aspects, as described below, a ultrasonic medical device can comprise a transducer configured to produce longitudinal vibrations, and a surgical tool having a transducer base plate operably coupled to the transducer, an end effector, and wave guide therebetween. In certain aspects, as also described below, the transducer can produce vibrations which can be transmitted to the end effector, wherein the vibrations can drive the transducer base plate, the wave guide, the end effector, and/or the other various components of the ultrasonic medical device at, or near, a resonant frequency. In resonance, a longitudinal strain pattern, or longitudinal stress pattern, can develop within the transducer, the wave guide, and/or the end effector, for example. The transducer base plate comprises flat faces on opposite sides to receive piezoelectric elements. In various aspects, such a longitudinal strain pattern, or longitudinal stress pattern, can cause the longitudinal strain, or longitudinal stress, to vary along the length of the transducer base plate, wave guide, and/or end effector, in a sinusoidal, or at least substantially sinusoidal, manner. In at least one aspect, for example, the longitudinal strain pattern can have maximum peaks and zero points, wherein the strain values can vary in a non-linear manner between such peaks and zero points. FIG. 3 illustrates an ultrasonic surgical instrument 250 that includes an ultrasonic waveguide 252 attached to an ultrasonic transducer 264 by a bonding material, where the ultrasonic surgical instrument 250 is configured to operate in a D31 mode, according to one aspect of this disclosure. The ultrasonic transducer 264 includes first and second piezoelectric elements 254a, 254b attached to the ultrasonic waveguide 252 by a bonding material. The piezoelectric elements 254a, 254b include electrically conductive plates 256a, 256b to electrically couple one pole of a voltage source suitable to drive the piezoelectric elements 254a, 254b (e.g., usually a high voltage). The opposite pole of the voltage source is electrically coupled to the ultrasonic waveguide 252 by electrically conductive joints 258a, 258b. In one aspect, the electrically conductive plates 256a, 256b are coupled to a positive pole of the voltage source and the electrically conductive joints 258a, 258b are electrically coupled to ground potential through the metal ultrasonic waveguide 252. In one aspect, the ultrasonic waveguide 252 is made of titanium or titanium alloy (i.e., Ti6Al4V) and the piezoelectric elements 254a, 254b are made of a lead zirconate titanate intermetallic inorganic compound with the chemical formula Pb[ZrxTi1-x]O3 (0≦x≦1). Also called PZT, it is a ceramic perovskite material that shows a marked piezoelectric effect, meaning that the compound changes shape when an electric field is applied. It is used in a number of practical applications such as ultrasonic transducers and piezoelectric resonators PZT. The poling axis (P) of the piezoelectric elements 254a, 254b is indicated by the direction arrow 260. The motion axis of the ultrasonic waveguide 252 in response to excitation of the piezoelectric elements 254a, 245b is shown by a motion arrow 262 at the distal end of the ultrasonic waveguide 252 generally referred to as the ultrasonic blade portion of the ultrasonic waveguide 252. The motion axis 262 is orthogonal to the poling axis (P) 260. In conventional D33 ultrasonic transducer architectures as shown in FIG. 1, the bolted piezoelectric elements 32 utilize electrodes 34, 36 to create electrical contact to both sizes of each piezoelectric element 34. The D31 architecture 250 according to one aspect of this disclosure, however, employs a different technique to create electrical contact to both sides of each piezoelectric element 254a, 254b. Various techniques for providing electrical contact to the piezoelectric elements 254a, 254b include bonding electrical conductive elements (e.g., wires) to the free surface of each piezoelectric element 254a, 254b for the high potential connection and bonding each piezoelectric element 254a, 254b the to the ultrasonic waveguide 252 for the ground connection using solder, conductive epoxy, or other techniques described herein. Compression can be used to maintain electrical contact to the acoustic train without making a permanent connection. This can cause an increase in device thickness and should be controlled to avoid damaging the piezoelectric elements 254a, 254b. Low compression can damage the piezoelectric element 254a, 254b by a spark gap and high compression can damage the piezoelectric elements 254a, 254b by local mechanical wear. In other techniques, metallic spring contacts may be employed to create electrical contact with the piezoelectric elements 254a, 254b. Other techniques may include foil-over-foam gaskets, conductive foam, solder. Electrical connection to both sides of the piezoelectric elements 254a, 254b the D31 acoustic train configuration. The electrical ground connection can be made to the metal ultrasonic waveguide 252, which is electrically conductive, if there is electrical contact between the piezoelectric elements 254a, 254b and the ultrasonic waveguide 252. In various aspects, as described below, an ultrasonic medical device may comprise a transducer configured to produce longitudinal vibrations, and a surgical instrument having a transducer base plate operably coupled to the transducer, an end effector, and wave guide therebetween. In certain aspects, as also described below, the transducer can produce vibrations which can be transmitted to the end effector, wherein the vibrations can drive the transducer base plate, the wave guide, the end effector, and/or the other various components of the ultrasonic medical device at, or near, a resonant frequency. In resonance, a longitudinal strain pattern, or longitudinal stress pattern, can develop within the transducer, the wave guide, and/or the end effector, for example. In various aspects, such a longitudinal strain pattern, or longitudinal stress pattern, can cause the longitudinal strain, or longitudinal stress, to vary along the length of the transducer base plate, wave guide, and/or end effector, in a sinusoidal, or at least substantially sinusoidal, manner. In at least one aspect, for example, the longitudinal strain pattern can have maximum peaks and zero points, wherein the strain values can vary in a non-linear manner between such peaks and zero points. In conventional D33 ultrasonic transducer architectures as shown in FIG. 1, a bolt provides compression that acoustically couples the piezoelectric elements rings to the ultrasonic waveguide. FIG. 4 is a perspective view of a transducer assembly 6800 configured to operate in a D33 mode of operation, according to one aspect of this disclosure. The transducer assembly 6800 comprises transducer 6818, piezoelectric elements 6808a-6808f, a horn shaped portion 6802, and a threaded connection such as a stud 6804 to connect to an ultrasonic waveguide. The transducer 6818 includes piezoelectric elements 6808a-6808f. As described above, the transducer 6800 is configurable as a “Langevin” stack (e.g., with a rectangular cross section), in which the piezoelectric elements 6808a-6808f and corresponding activating electrodes 6810a-c are interleaved. In one aspect, the activating electrodes 6809a-6809g can be thin sheets of an electrically conductive metal such as, for example, aluminum, silver, gold, copper, and/or alloys thereof. The activating electrodes 6809a-g are each electrically connected to an ultrasonic signal generator via the electrically conductive elements 6810a-6810f. The electrically conductive elements 6810a, 6810b, 6810c may form a path to a pole (e.g., the positive pole) of the generator. The electrically conductive elements 6810d, 6810e, 6810f, 6810g may form another path to an opposing pole (e.g., the negative pole) of the generator. The electrically conductive elements 6810a-6810g may be U-shaped shorting caps that appear similar to brackets. As shown in FIG. 4, shorting caps 6810a-6810c are positioned over the piezoelectric elements 6808a-f. In various aspects, rather than employing a bolt to compress and acoustically couple the piezoelectric elements 6808a-6808f to an ultrasonic waveguide, the transducer assembly 6800 instead employs wedges 6806a-6806b to compress against a compressing plate 6816. In turn, the compressing plate 6816 compresses and acoustically couples the piezoelectric elements 6808 to an ultrasonic waveguide. Such compression of the piezoelectric elements 6808a-6808f can be useful for operation of the transducer 6800 stack. The compressing plate 6816 may be made of an electrically conductive metal such as steel, for example. The wedges 6806a, 6806b also may be made of an electrically conductive metal such as steel, for example. In one aspect, the wedges 6806a (not shown in FIG. 4), 6806b can be machined and inserted by a suitable forming press. Additionally, the wedges 6806a, 6806b may be secured by a conductive bonding material or adhesive. In addition to the piezoelectric elements 6808, wedges 6806a, 6806b, electrodes 6809a-6809g, and shorting caps 6810a-6810g, the transducer assembly 6800 includes flanges 6812a, 6812b located on opposing sides of the transducer assembly 6800, such as a node location of the transducer assembly 6800. The flanges 6812a, 6812b may be configured to be received within a retainer of a housing (not shown) of an ultrasonic surgical instrument for secure attachment of the transducer assembly 6800 to the housing. Replacing the bolt with the wedges 6806a, 6806b can be advantageous because of easier machining and assembling of the transducer 6800 stack. Additional advantages of the wedge configuration include the ability to tune the acoustic assembly of the transducer assembly 6800 by modulating the tension in the transducer 6800 with the wedges 6806a, 6806b and the relatively inexpensive, simple geometric shape of the piezoelectric elements 6808a-6808f. FIGS. 5A-5B illustrate cross section views of the ultrasonic transducer assembly 6800 employing the wedge configuration, according to one aspect of this disclosure, where FIG. 5A is a section view taken along section line 5A-5A as shown in FIG. 4 and FIG. 5B is a section view taken along section line 5B-5B as shown in FIG. 4. In FIGS. 5A-5B, the wedges 6806a, 6806b each have an irregular quadrilateral or tetragon shape. If the individual shapes of the wedges 6806a, 6806b are laterally combined, the combination has a square shape. As described above, the wedges 6806a, 6806b are configured to compress and acoustically couple the piezoelectric elements 6808a-6808f to an ultrasonic waveguide through the stud 6804. The rightward facing sectional view of FIG. 5A is obtained by starting from the longitudinal axis passing through the center of the transducer assembly 6800 and continuing right in the direction of the corresponding arrow 5A, as depicted in FIG. 4. As can be seen in FIG. 5A, in one aspect, the first pair of shorting caps 6810a-6810c may be connected in an offset manner. Specifically, shorting cap 6810c may be connected to a pole of the ultrasonic signal generator on one end and to the activating electrode 6809b on the other end. Similarly, shorting cap 6810a may be connected to the activating electrode 6809b on one end and to the activating electrode 6809d on the other end. Shorting cap 6810b may be connected to the activating electrode 6809d on one end and to activating electrode 6809f on the other end. Accordingly, the shorting caps 6810a-6810c are configured to provide a safe means of closing the electrical circuit connecting the generator to the piezoelectric elements 6808a-6808f and electrodes 6809a-6809g. As can be seen in FIG. 5A, the shorting cap 6810a is positioned on the upper side of the transducer assembly 6800 while the shorting caps 6810b-6810c are positioned on the lower side of the transducer assembly 6800. The leftward facing sectional view of FIG. 5B is obtained by starting from the longitudinal axis passing through the center of the transducer assembly 6800 and continuing left in the direction of the corresponding arrow 5B, as depicted in FIG. 4. In another aspect, as can be seen in FIG. 5B, the second pair of shorting caps 6810a-6810c may be connected in an opposing offset manner. Specifically, shorting cap 6810g may be connected to a pole of the ultrasonic signal generator on one end (opposing the pole that shorting cap 6810c is connected to) and to activating electrode 6809a on the other end. Similarly, shorting cap 6810d may be connected to the activating electrode 6809a on one end and to activating electrode 6809c on the other end. Shorting cap 6810f may be connected to activating electrode 6809c on one end and to activating electrode 6809e on the other end. Shorting cap 6810e may be connected to activating electrode 6809e on one end and to activating electrode 6809g on the other end. Accordingly, the shorting caps 6810d-6810g also are configured to provide a safe means of closing the electrical circuit connecting the generator to the piezoelectric elements 6808a-6808f and electrodes 6809a-6809g. As can be seen in FIG. 5B, the shorting caps 6810g and 6810f are positioned on the lower side of the transducer assembly 6800 while the shorting caps 6810d-6810e are positioned on the upper side of the transducer assembly 6800. FIGS. 5C-5D illustrate cross section views of the ultrasonic transducer assembly 6805 employing another wedge configuration taken along similar section lines as shown in FIGS. 5A and 5B, according to one aspect of this disclosure. In FIGS. 5C-5D, the wedges 6807a, 6807b have a different irregular quadrilateral or tetragon shape than the shape of the wedges 6806a, 6806b in FIGS. 5A-5B. If the individual shapes of the wedges 6807a, 6807b are longitudinally combined, the combination has a square shape. As described above, the wedges 6807a, 6807b are configured to compress and acoustically couple the piezoelectric elements 6808a-6808f to an ultrasonic waveguide through the stud 6804. FIG. 5C depicts the same configuration of shorting caps 6810a-6810c as described in connection with FIG. 5A. Similarly, FIG. 5D depicts the same configuration of shorting caps 6810d-6810g as described in connection with FIG. 5B. FIG. 6 is a perspective view of an ultrasonic transducer assembly 6850 similar to the ultrasonic surgical instrument 6800 according to one aspect of this disclosure, except that the transducer assembly 6850 comprises a transducer 6819 employing a screw 6852 to compress against the compression plate 6816 to compress and acoustically couple the piezoelectric elements 6808a-6808f to an ultrasonic waveguide. FIG. 7 is a section view of the ultrasonic transducer assembly 6850 taken along section line 7-7 as shown in FIG. 6, according to one aspect of this disclosure. With reference now to FIGS. 6-7, in one aspect, no wedges are used in the ultrasonic surgical instrument 6850, as shown in the perspective view of FIG. 6. Instead, the screw 6852 may be fastened into the compression plate 6816 to apply a compressive force against the compression plate 6816, such as by threading the pointed point or tip 6854 (and a proximal portion of the thread) of the screw 6852 distally into the compression plate 6816. As can be seen in the sectional view of FIG. 7, the pointed tip 6854 of the screw 6852 is in contact with the most proximal of the piezoelectric elements 6808. A distal portion of the thread 6856 and the head 6858 of the screw 6852 extend distally from the compression plate 6816. Advantages of this screw configuration include avoiding the use of a central bolt for compression and coupling and the ability to tune the acoustic assembly of the transducer assembly 6850 by modulating the tension in the transducer 6819 with the screw 6852. Compared to a transducer assembly with a central bolt, the transducer assembly 6850 with the screw 6852 is easier to machine because the distal portion of the thread 6856 of the screw 6852 is simply fastened into the compression plate 6816. FIG. 8 is a perspective view of the screw 6852, according to one aspect of this disclosure. FIG. 9 illustrates an ultrasonic surgical instrument 6900 configured to operate in a D31 mode of operation, according to one aspect of this disclosure. The ultrasonic surgical instrument 6900 comprises a transducer assembly 6902 and an ultrasonic waveguide 6904. The transducer assembly 6902 includes a horn shaped portion 6908, piezoelectric elements 6906a, 6906b, and a threaded connection such as a stud 6804 to connect to the ultrasonic waveguide 6904. As illustrated in FIG. 10, the transducer assembly 6902 is configured to create a strong mechanical bond between each of the piezoelectric elements 6906a, 6906b and the waveguide 6904. FIG. 10 is a section view of the transducer assembly 6902 comprising a first piezoelectric element 6906a positioned on a first side of the waveguide 6904 and a second piezoelectric element 6906b positioned on a second opposing side (opposing the first side) of the waveguide 6904, according to one aspect of this disclosure. In one aspect, the waveguide 6904 includes a first and second interior recess 6908a, 6908b for insertion of the piezoelectric elements 6906a, 6906b, respectively. As illustrated in FIG. 10, the first interior recess 6908a may extend from an interior top surface into and the second interior recess 6908a may extend from an interior bottom surface into the waveguide 6904, respectively. The depths of the first and second recess 6908a, 6908b may each equal or substantially equal the heights of the piezoelectric elements 6906a, 6906b, respectively. Accordingly, when the piezoelectric elements 6906a, 6906b are inserted into the recesses
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