Patent Publication Number: US-10327736-B1

Title: Ultrasound transducer arrays and associated systems and methods

Description:
BACKGROUND 
     Energy in the form of sonic, ultrasonic, or megasonic waves may be transmitted into liquid media for a variety of purposes. For example, an object may be cleaned or processed by immersing the object in liquid and subsequently transferring ultrasound to the liquid. As another example, liquids can be emulsified, homogenized, pasteurized, sterilized, or mixed by applying ultrasound thereto. As yet another example, organisms in a liquid can be inactivated by applying ultrasound to the liquid. 
     Ultrasound systems are commonly used to generate sound waves for transmitting into a liquid. Conventional ultrasound systems include an ultrasound transducer array constructed by bonding piezoelectric elements or Langevin assemblies to a tank for containing a liquid. A signal generator electrically drives the transducer array, and constituent transducers of the transducer array spatially oscillate in response thereto, thereby generating sound waves which are transmitted to the liquid. The sound waves and the liquid interact to produce cavitation, which may result in a cleaning effect and/or a processing effect. 
       FIG. 1  is an electrical schematic illustrating a conventional ultrasound system  100  including an ultrasound transducer array  102 , a signal generator  104 , and wiring  106 . Ultrasound transducer array  102  includes a plurality of ultrasound transducers  108  affixed to a radiating element (not shown), and each ultrasound transducer  108  includes one or more piezoelectric elements. Signal generator  104  includes an alternating current (AC) electrical power source  110  electrically coupled in series with a resonant inductor  112 . Wiring  106  electrically couples each ultrasound transducer  108  in parallel with signal generator  104 . Signal generator  104  generates an electrical signal to drive ultrasound transducers  108 . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an electrical schematic illustrating a conventional ultrasound system. 
         FIG. 2  is an electrical schematic illustrating an ultrasound system including an ultrasound transducer array which promotes low capacitance, according to an embodiment. 
         FIG. 3  is a side elevation view illustrating an example embodiment of an ultrasound transducer array of the  FIG. 2  ultrasound system. 
         FIG. 4  is a bottom plan view of the  FIG. 3  ultrasound transducer array. 
         FIG. 5  is a top plan view of a piezoelectric element, according to an embodiment. 
         FIG. 6  is a bottom plan view of the  FIG. 5  piezoelectric element. 
         FIG. 7  is a cross-sectional view of the  FIG. 5  piezoelectric element taken along line  7 A- 7 A of  FIG. 6 . 
         FIG. 8  is a top plan view of another piezoelectric element, according to an embodiment. 
         FIG. 9  is a bottom plan view of the  FIG. 8  piezoelectric element. 
         FIG. 10  is an elevation view of an end of the  FIG. 8  piezoelectric element. 
         FIG. 11  is a top plan view of yet another piezoelectric element, according to an embodiment. 
         FIG. 12  is a bottom plan view of the  FIG. 11  piezoelectric element. 
         FIG. 13  is an elevation view of an end of the  FIG. 11  piezoelectric element. 
         FIG. 14  is a bottom plan view of an embodiment of the  FIG. 3  ultrasound transducer array including the piezoelectric elements of  FIGS. 8-13 . 
         FIG. 15  is a side elevation view illustrating another example embodiment of an ultrasound transducer array of the  FIG. 2  ultrasound system. 
         FIG. 16  is a bottom plan view of the  FIG. 15  ultrasound transducer array. 
         FIG. 17  is a side elevation view illustrating yet another example embodiment of an ultrasound transducer array of the  FIG. 2  ultrasound system. 
         FIG. 18  is a side elevation view illustrating another example embodiment of an ultrasound transducer array of the  FIG. 2  ultrasound system. 
         FIG. 19  is a side elevation view illustrating another example embodiment of an ultrasound transducer array of the  FIG. 2  ultrasound system. 
         FIG. 20  is a side plan view of an ultrasound assembly including an instance of the  FIG. 3  ultrasound transducer array affixed to a tank for containing a liquid, according to an embodiment. 
         FIG. 21  is a side plan elevation of an ultrasound assembly including an instance of the  FIG. 3  ultrasound transducer array where a radiating element of the ultrasound transducer array is implemented by a bottom plate of a tank, according to an embodiment. 
         FIG. 22  is a flow chart illustrating a method for coupling sonic energy into a liquid, according to an embodiment. 
     
    
    
     DEFINITIONS 
     In this document, the following definitions apply: 
     The term “megasonic” refers sound energy with a fundamental frequency from about 350 kilohertz (kHz) to about 15 megahertz (MHz). 
     The term “ultrasonic” refers to sound energy with a fundamental frequency from about 18 kHz to about 350 kHz. 
     Each of the terms “sonic,” “sound waves,” and “sound energy” refers to the complete range of sound waves, including audible, ultrasonic, and megasonic frequencies, ranging from about 0.2 kHz to about 15 MHz. 
     The term “ultrasound” refers to both ultrasonic and megasonic sound energy, with a fundamental frequency ranging from about 18 kHz to about 15 MHz. 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Ultrasound transducers exhibit capacitances due to the capacitive nature of their constituent elements, and conventional ultrasound transducer arrays therefore have relatively large capacitance values. For example, conventional ultrasound transducer array  102  ( FIG. 1 ) has a capacitance value approximately equal to the sum of the respective capacitance values of each ultrasound transducer  108  in the array. 
     Applicant has determined that the relatively large capacitance values of conventional ultrasound transducer arrays can be problematic. In particular, resonant frequency (f o ) of an ultrasound transducer array is characterized by EQN. 1 below, where L is inductance of an electrical circuit including the ultrasound transducer array, and C is capacitance of the electrical circuit including the ultrasound transducer array. Capacitance C is typically dominated by capacitance of the ultrasound transducer array, while inductance L is typically attributed primarily to inductance of a resonant inductor electrically coupled to the circuit, e.g., resonant inductor  112  of  FIG. 1 . 
     
       
         
           
             
               
                 
                   
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     As evident from EQN. 1, small values of inductance L and/or capacitance C are required to achieve a high resonant frequency f o . Consequently, if the ultrasound transducer array has a large capacitance value, the inductance L value must be small to obtain a high resonant frequency f o . Such small inductance values are often impractical to achieve. For example, parasitic inductance of an electrical circuit including the ultrasound transducer array will often be greater that the inductance value L required to achieve a high resonant frequency f o  at a large value of capacitance C. Therefore, it can be difficult or even impossible to achieve high resonant frequencies with conventional ultrasound transducer arrays. 
     Furthermore, capacitance of an ultrasound transducer array is typically significantly temperature-dependent. As a result, capacitance C will vary with temperature, thereby causing resonant frequency f o  to vary with temperature, because resonant frequency is a function of capacitance C, as evident from EQN. 1 above. Temperature-induced variation of resonant frequency f o  is ordinarily undesirable, and such variation is more difficult to control with increasing values of capacitance C. 
     Applicant has developed new ultrasound transducer arrays for coupling sonic energy into a liquid. These new ultrasound transducer arrays promote low capacitance, thereby helping mitigate the problems discussed above. The new ultrasound transducer arrays include a plurality of transducer pairs, where each transducer pair includes an inverted ultrasound transducer and a non-inverted ultrasound transducer electrically coupled in series. In each transducer pair, the respective polarities of one or more piezoelectric elements of the inverted ultrasound transducer are reversed with respect to the polarities of corresponding piezoelectric elements of the non-inverted ultrasound transducer. The plurality of transducer pairs are electrically coupled in parallel such that like polarities of piezoelectric elements are connected together. The series connection of the inverted and non-inverted ultrasound transducers in each transducer pair causes the transducer pair to have a net capacitance value that is less than the respective capacitance value of each ultrasound transducer of the pair. For example, if the inverted and non-inverted ultrasound transducers have the same capacitance value, the net capacitance of the transducer pair will be half of the respective capacitance value of each ultrasound transducer of the pair. As a result, the new ultrasound transducer arrays have significantly lower capacitance values than conventional ultrasound transducer arrays with like number of ultrasound transducers of similar capacitance. 
     The relatively low capacitance values of the new ultrasound transducer arrays may advantageously enable the new ultrasound transducer arrays to operate at a higher resonant frequency than conventional ultrasound transducer arrays. Additionally, the relatively low capacitance values of the new ultrasound transducer arrays can help reduce temperature-induced resonant frequency variation, relative to conventional ultrasound transducer arrays. 
       FIG. 2  is an electrical schematic illustrating an ultrasound system  200  including an ultrasound transducer array  202  electrically coupled to a signal generator  204 . Ultrasound transducer array  202  is one embodiment of the new ultrasound transducer arrays developed by Applicant, and ultrasound transducer array  202  includes a plurality of transducer pairs  206  electrically coupled in parallel such that like polarities of piezoelectric elements are connected together to form the parallel configuration, i.e., positive poles from transducer pairs  206  are connected together, and negative poles from transducer pairs  206  are connected together. Although ultrasound transducer array  202  is depicted as including two transducer pairs  206 , ultrasound transducer array  202  could include additional transducer pairs  206  without departing from the scope hereof. Each transducer pair  206  includes an inverted ultrasound transducer  208  and a non-inverted ultrasound transducer  210  joined to a radiating element  212 , which is symbolically depicted by a dashed line in  FIG. 2 . Radiating element  212  is formed, for example, of at least one of one of quartz, sapphire, stainless steel, titanium, tantalum, boron nitride, silicon carbide, silicon nitride, aluminum and a ceramic material. In some embodiments, radiating element  212  is part, e.g., a wall or plate, of a tank for containing a liquid. In yet some other embodiments, radiating element  212  is omitted, such as in applications where ultrasound transducers  208  and non-inverted ultrasound transducers  210  are immersed in a liquid. 
     Each non-inverted ultrasound transducer  210  includes N piezoelectric elements (not shown), and each inverted ultrasound transducer  208  includes N corresponding piezoelectric elements (not shown), where N is an integer greater than or equal to one. In each transducer pair  206 , the one or more piezoelectric elements of inverted ultrasound transducer  208  are flipped relative to the orientation of each corresponding piezoelectric element of non-inverted ultrasound transducer  210 , with respect to radiating element  212 . For example, in one embodiment, each inverted ultrasound transducer  208  includes a piezoelectric element having a positive pole facing in a direction  220  away from radiating element  212 , and each non-inverted ultrasound transducer  210  includes a piezoelectric element having a positive pole facing in an opposite direction  222  toward radiating element  212 . As another example, in another embodiment, each inverted ultrasound transducer  208  includes a piezoelectric element having a positive pole facing in direction  222  toward radiating element  212 , and each non-inverted ultrasound transducer  210  includes a piezoelectric element having a positive pole facing in direction  220  away from radiating element  212 . As yet another example, in  FIG. 18  (discussed below), piezoelectric element  1530  is flipped relative to the orientation of corresponding piezoelectric element  1540 , and piezoelectric element  1832  is flipped relative to the orientation of corresponding piezoelectric element  1842 . 
     In each transducer pair  206 , inverted ultrasound transducer  208  is electrically coupled in series with non-inverted ultrasound transducer  210 . For example, in the depicted embodiment, the positive pole of each non-inverted ultrasound transducer  210  is electrically coupled to the negative pole of each inverted ultrasound transducer  208 , and in an alternate embodiment, the negative pole of each non-inverted ultrasound transducer  210  is electrically coupled to the positive pole of each inverted ultrasound transducer  208 . 
     Signal generator  204  includes an AC electrical power source  214  electrically coupled in series with a resonant inductor  216 . Wiring  218  electrically couples each transducer pair  206  to signal generator  204 . The configuration of signal generator  204  can vary without departing from the scope hereof. For example, in one alternate embodiment, resonant inductor  216  is electrically coupled in parallel with AC electrical power source  214 , instead of in series with AC electrical power source  214 . As another example, in another alternate embodiment, resonant inductor  216  is omitted, and ultrasound system  200  relies on parasitic inductance of an electrical circuit including ultrasound transducer array  202 , signal generator  204 , and wiring  218 , for resonant inductance. 
     Signal generator  204  drives each transducer pair  206  with an electrical signal. The fact that each transducer pair  206  includes an inverted ultrasound transducer  208  and a non-inverted ultrasound transducer  210  electrically coupled in series causes the one or more piezoelectric elements of inverted ultrasound transducers  208  and the one or more piezoelectric elements of non-inverted ultrasound transducers  210  to expand and contract together when driven by the electrical signal. For example, in one embodiment, the one or more piezoelectric elements of each of inverted ultrasound transducers  208  and non-inverted ultrasound transducers  210  contract together during a positive portion of the electrical signal from signal generator  204 , and the one or more piezoelectric elements of each of inverted ultrasound transducers  208  and non-inverted ultrasound transducers  210  expand together during a negative portion of the electrical signal from signal generator  204 . As another example, in another embodiment, the one or more piezoelectric elements of each of inverted ultrasound transducers  208  and non-inverted ultrasound transducers  210  expand together during a positive portion of the electrical signal from signal generator  204 , and the one or more piezoelectric elements of each of inverted ultrasound transducers  208  and non-inverted ultrasound transducers  210  contract together during a negative portion of the electrical signal from signal generator  204 . 
     Simultaneous expansion or contraction of all piezoelectric elements of ultrasound transducer array  202 , which is achieved by the configuration of transducer pairs  206 , results in sound generation. If transducer pairs  206  were modified to include two ultrasound transducers of the same type e.g., two non-inverted ultrasound transducers or two inverted ultrasound transducers, the two ultrasound transducers in each transducer pair  206  would operate in opposing manners. 
     In some embodiments, signal generator  204  drives transducer pairs  206  with an electrical signal at a single frequency, while in some other embodiments, signal generator  204  drives transducer pairs  206  with an electrical signal at two or more frequencies. For example, in some embodiments, signal generator  204  sweeps the frequency of the electrical signal with respect to a base frequency. In certain embodiments, the base frequency is a center frequency of a sweep frequency range, and signal generator  204  sweeps the frequency of the electrical signal within a predetermined percentage of the base frequency, e.g., within two percent, ten percent, or twenty percent, of the base frequency. In some other embodiments, signal generator  204  sweeps the frequency of the electrical signal in an asymmetric manner with respect to the base frequency, such that the base frequency is offset from the center frequency of the sweep frequency range. In particular embodiments, signal generator  204  sweeps the frequency of the electrical signal with respect to a base frequency according to a triangle function, a saw tooth function, a stair-step function, a dual-sweep function, or a random function. Applicant has found that sweeping the frequency of the electrical signal generated by signal generator  204  can achieve significant advantages in some applications. 
     The series electrical connection of inverted ultrasound transducer  208  and non-inverted ultrasound transducer  210  in each transducer pair  206  promotes low capacitance of ultrasound transducer array  202 . For example, consider an embodiment where each inverted ultrasound transducer  208  and each non-inverted ultrasound transducer  210  has a respective capacitance value C 1 . Ultrasound transducer array  202  will have a net capacitance C net   _   202  according to EQN. 2 below, in this embodiment. 
     
       
         
           
             
               
                 
                   
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     Thus, net capacitance C net   _   202  of this embodiment of ultrasound transducer array  202  is equal to the capacitance value of one ultrasound transducer instance. Now consider conventional ultrasound transducer array  102  of  FIG. 1 , assuming that each ultrasound transducer  108  also has a respective capacitance value C 1 . Ultrasound transducer array  102  will have a net capacitance C net   _   102  according to EQN. 3 below.
 
 C   net   _   102   =C   1   +C   1   +C   1   +C   1 =4 C   1   (EQN. 3)
 
     As evident when comparing EQNS. 2 and 3, the above-discussed embodiment of the new ultrasound transducer array  202  has one quarter of the capacitance of conventional ultrasound transducer array  102 , even though both transducer arrays have the same number of ultrasound transducers. Accordingly, ultrasound transducer array  202  achieves much lower capacitance values than conventional ultrasound transducer arrays having like numbers of ultrasound transducers of similar respective capacitance values. 
       FIG. 3  is a side elevation view of one embodiment of ultrasound transducer array  302 , and  FIG. 4  is a bottom plan view of ultrasound transducer array  302 . Ultrasound transducer array  302  is an example embodiment of ultrasound transducer array  202  ( FIG. 2 ), where ultrasound transducers are implemented by piezoelectric elements. Certain embodiments of ultrasound transducer array  302  are capable of operating at megasonic frequencies. Ultrasound transducer array  302  includes a plurality of transducer pairs  306 , a radiating element  312 , and wiring  318 , which are analogous to transducer pairs  206 , radiating element  212 , and wiring  218  of  FIG. 2 . The number of transducer pairs  306  could be increased without departing from the scope hereof. Wiring  318  electrically couples transducer pairs  306  in parallel, such that like polarities of piezoelectric elements are connected together. Wiring  318  is symbolically shown by dashed lines in  FIG. 3 , and the physical configuration of wiring  318  could accordingly vary from the physical configuration depicted in  FIG. 3  without departing from the scope hereof. Wiring  318  is not shown in the  FIG. 4  bottom plan view to promote illustrative clarity. 
     Each transducer pair  306  includes an inverted piezoelectric element  308  and a non-inverted piezoelectric element  310 , which are embodiments of inverted ultrasound transducer  208  and non-inverted ultrasound transducer  210 , respectively. Each inverted piezoelectric element  308  and each non-inverted piezoelectric element  310  is formed, for example, of a piezoelectric ceramic material. Inverted piezoelectric elements  308  are joined to an outer surface  324  of radiating element  312  such that their positive poles face a first direction, and non-inverted piezoelectric elements  310  are joined to outer surface  324  such that their positive poles face a second direction that is opposite of the first direction. For example, in the illustrated embodiment, the positive pole of each inverted piezoelectric element  308  faces a direction  320  away from radiating element  312 , and the positive pole of each non-inverted piezoelectric element  310  faces a direction  322  towards radiating element  312  (see  FIG. 3 ). In an alternate embodiment, the positive pole of each inverted piezoelectric elements  308  faces direction  322 , and the positive pole of each non-inverted piezoelectric elements  310  faces direction  320 . 
     Within each transducer pair  306 , inverted piezoelectric element  308  is electrically coupled to non-inverted piezoelectric element  310  in series by an electrical conductor  326 . For example, within each transducer pair  306  of the depicted embodiment, the positive pole of non-inverted piezoelectric element  310  is electrically coupled to the negative pole of inverted piezoelectric element  308 , and in an alternate embodiment, the negative pole of non-inverted piezoelectric element  310  is electrically coupled to the positive pole of inverted piezoelectric element  308 . Electrical conductors  326  are symbolically shown by dashed lines in  FIG. 3 , and the physical configuration of electrical conductors  326  could accordingly vary from the physical configuration depicted in  FIG. 3  without departing from the scope hereof. Electrical conductors  326  are not shown in the  FIG. 4  bottom plan view to promote illustrative clarity. 
     In certain embodiments, radiating element  312  forms at least a part of electrical conductors  326 . For example, in some embodiments, electrical conductors  326  are implemented by electrical conductors embedded in radiating element  312  or by electrical conductors disposed on outer surface  324  of radiating element  312 . In some other embodiments, electrical conductors  326  are separate from radiating element  312 . In yet other embodiments, radiating element  312  is electrically conductive and serves as electrical conductors  326 . 
     Discussed below with respect to  FIGS. 5-13  are several example embodiments of inverted piezoelectric element  308  and non-inverted piezoelectric element  310 . It should be realized, however, that inverted piezoelectric element  308  and non-inverted piezoelectric element  310  are not limited to these particular embodiments. 
       FIG. 5  is a top plan view and  FIG. 6  is a bottom plan view of a piezoelectric element  500 .  FIG. 7  is a cross-sectional view of piezoelectric element  500  taken along line  7 A- 7 A of  FIG. 6 . Piezoelectric element  500  can serve as either inverted piezoelectric element  308  or non-inverted piezoelectric element  310 , depending on how it is joined to radiating element  312 , as discussed below. Piezoelectric element  500  includes a body  502  formed of a piezoelectric material, e.g., a ceramic piezoelectric material, having a first outer surface  504  and an opposing second outer surface  506 . A first electrode  508  is disposed on first outer surface  504  adjacent to a positive pole of piezoelectric element  500 , and a second electrode  510  is disposed on second outer surface  506  adjacent to a negative pole of piezoelectric element  500 . First and second electrodes  508  and  510  provide electrical interface to the first and second poles, respectively. 
     Piezoelectric element  500  can serve as inverted piezoelectric element  308  when joined to radiating element  312  such that its negative pole faces radiating element  312 . Additionally, piezoelectric element  500  can serve as non-inverted piezoelectric element  310  when affixed to radiating element  312  such that its positive pole faces radiating element  312 . In an alternate embodiment, piezoelectric element  500  can serve as inverted piezoelectric element  308  when joined to radiating element  312  such that its positive pole faces radiating element  312 . Additionally, piezoelectric element  500  can serve as non-inverted piezoelectric element  310  when affixed to radiating element  312  such that its negative pole faces radiating element  312 . 
       FIG. 8  is a top plan view and  FIG. 9  is a bottom plan view of a piezoelectric element  800 .  FIG. 10  is an elevation view of an end  811  of piezoelectric element  800 . Piezoelectric element  800  can serve as non-inverted piezoelectric element  310 . Piezoelectric element  800  includes a body  802  formed of a piezoelectric material, e.g., a ceramic piezoelectric material, having a first outer surface  804  and an opposing second outer surface  806  separated from each other in a thickness  808  direction. A first electrode  810  is disposed on first outer surface  804  adjacent to a positive pole of piezoelectric element  800 , and first electrode  810  wraps around end  811  to second outer surface  806 . A second electrode  812  is disposed on second outer surface  806  adjacent to a negative pole of piezoelectric element  800 . First electrode  810  and second electrode  812  are separated from each other by an insulated region  814 . 
     First and second electrodes  810  and  812  provide electrical interface to the positive and negative poles, respectively. The fact that first and second electrodes  810  and  812  are accessible from a common side of piezoelectric element  800  enables all external electrical connections, e. g, connections to wiring  318  and electrical conductors  326 , to be on the common side of piezoelectric element  800 . The shape and location of first and second electrodes  810  and  812  could vary without departing from the scope hereof. 
       FIG. 11  is a top plan view and  FIG. 12  is a bottom plan view of a piezoelectric element  1100 .  FIG. 13  is an elevation view of an end  1111  of piezoelectric element  1100 . Piezoelectric element  1100  can serve as inverted piezoelectric element  308 . Piezoelectric element  1100  includes a body  1102  formed of a piezoelectric material, e.g., a ceramic piezoelectric material, having a first outer surface  1104  and an opposing second outer surface  1106  separated from each other in a thickness  1108  direction. A first electrode  1110  is disposed on first outer surface  1104  adjacent to a negative pole of piezoelectric element  1100 , and first electrode  1110  wraps around end  1111  to second outer surface  1106 . A second electrode  1112  is disposed on second outer surface  1106  adjacent to a positive pole of piezoelectric element  1100 . First electrode  1110  and second electrode  1112  are separated from each other by an insulated region  1114 . 
     First and second electrodes  1110  and  1112  provide electrical interface to the negative and positive poles, respectively. The fact that first and second electrodes  1110  and  1112  are accessible from a common side of piezoelectric element  1100  enables all external electrical connections, e. g, connections to wiring  318  and electrical conductors  326 , to be on the common side of piezoelectric element  1100 . The shape and location of first and second electrodes  1110  and  1112  could vary without departing from the scope hereof. 
       FIG. 14  is a bottom plan view of an ultrasound transducer array  1402 , which is an embodiment of ultrasound transducer array  302  where inverted and non-inverted piezoelectric elements  308  and  310  are embodied by piezoelectric elements  1100  and  800 , respectively. In certain embodiments of ultrasound transducer array  1402 , radiating element  312  is formed of quartz or another non-conductive material. 
       FIG. 15  is a side elevation view of an ultrasound transducer array  1502 , and  FIG. 16  is a bottom plan view of ultrasound transducer array  1502 . Ultrasound transducer array  1502  is an example embodiment of ultrasound transducer array  202  ( FIG. 2 ), where ultrasound transducers are implemented by Langevin assemblies. Ultrasound transducer array  1502  includes a plurality of transducer pairs  1506 , a radiating element  1512 , and wiring  1518 , which are analogous to transducer pairs  206 , radiating element  212 , and wiring  218  of  FIG. 2 . The number of transducer pairs  1506  could be increased without departing from the scope hereof. Wiring  1518  electrically couples transducer pairs  1506  in parallel, such that like polarities of piezoelectric elements are connected together. Wiring  1518  is symbolically shown by dashed lines in  FIG. 15 , and the physical configuration of wiring  1518  could accordingly differ from the physical configuration depicted in  FIG. 15  without departing from the scope hereof. Wiring  1518  is not shown in the  FIG. 16  bottom plan view to promote illustrative clarity. 
     Each transducer pair  1506  includes an inverted Langevin assembly  1508  and a non-inverted Langevin assembly  1510 , which are embodiments of inverted ultrasound transducer  208  and non-inverted ultrasound transducer  210 , respectively. Each inverted Langevin assembly  1508  and each non-inverted Langevin assembly  1510  is affixed to an outer surface  1524  of radiating member  1512 . Each inverted Langevin assembly  1508  includes a first mass  1528 , a piezoelectric element  1530 , an insulator  1532 , and a second mass  1534 , stacked in a thickness direction  1536 . Piezoelectric element  1530  is formed, for example, of a piezoelectric ceramic material, and piezoelectric element  1530  includes a positive pole and a negative pole. Piezoelectric element  1530  is disposed in inverted Langevin assembly  1508  such that its positive pole faces away from radiating element  1512  in a direction  1520  which is parallel to thickness direction  1536 . Piezoelectric element  1530  is disposed between first mass  1528  and insulator  1532  in thickness direction  1536 , and insulator  1532  is disposed between piezoelectric element  1530  and second mass  1534  in thickness direction  1536 . First mass  1528  is electrically coupled to second mass  1534 , e.g., by a bolt mechanically connecting first mass  1528  to second mass  1534 , as symbolically shown by a dashed line  1537  in  FIG. 15 . 
     Each non-inverted Langevin assembly  1510  includes a first mass  1534 , a piezoelectric element  1540 , an insulator  1542 , and a second mass  1544  stacked in thickness direction  1536 . Piezoelectric element  1540  is formed, for example, of a piezoelectric ceramic material, and piezoelectric element  1540  includes a positive pole and a negative pole. Piezoelectric element  1540  is disposed in non-inverted Langevin assembly  1510  such that its positive pole faces toward radiating element  1512  in a direction  1522  which is parallel to thickness direction  1536 . Piezoelectric element  1540  is disposed between first mass  1534  and insulator  1542  in thickness direction  1536 , and insulator  1542  is disposed between piezoelectric element  1540  and second mass  1544  in thickness direction  1536 . First mass  1534  is electrically coupled to second mass  1544 , e.g., by a bolt mechanically connecting first mass  1534  to second mass  1544 , as symbolically shown by a dashed line  1546  in  FIG. 15 . 
     Within each transducer pair  1506 , inverted Langevin assembly  1508  is electrically coupled to non-inverted inverted Langevin assembly  1510  in series by an electrical conductor  1526 , such that a positive pole of non-inverted Langevin assembly  1510  is electrically coupled to a negative pole of inverted Langevin assembly  1508 . In an alternate embodiment, the negative pole of non-inverted Langevin assembly  1510  is electrically coupled to the positive pole of inverted Langevin assembly  1508 . Electrical conductors  1526  are symbolically shown by dashed lines in  FIG. 15 , and the physical configuration of electrical conductors  1526  could accordingly vary from the physical configuration depicted in  FIG. 15  without departing from the scope hereof. For example, the radiating element  1512  can be used as the electrical conductors  1526 . Electrical conductors  1526  are not shown in the  FIG. 16  bottom plan view to promote illustrative clarity. 
     In an alternate embodiment, the positive pole of piezoelectric element  1530  in each inverted Langevin assembly  1508  faces direction  1522 , and the positive pole of piezoelectric element  1540  of each non-inverted Langevin assembly  1510  faces direction  1520 . Thus, the polarity of Langevin assemblies in ultrasonic transducer array  1502  can be generally described as follows: (a) the positive pole of piezoelectric element  1530  in each inverted Langevin assembly  1508  faces a first direction, and (b) the positive pole of piezoelectric element  1540  of each non-inverted Langevin assembly  1510  faces a second direction that is opposite of the first direction. 
       FIG. 17  is a side elevation view of an ultrasonic transducer array  1702 , which is like ultrasound transducer array  1500  of  FIGS. 15 and 16 , but with inverted Langevin assemblies  1508  and non-inverted Langevin assemblies  1510  replaced by inverted Langevin assemblies  1708  and non-inverted Langevin assemblies  1710 , respectively. Inverted Langevin assembly  1708  is like inverted Langevin assembly  1508  but with the positions of insulator  1532  and piezoelectric element  1530  swapped. Consequently, insulator  1532  is disposed between first mass  1528  and piezoelectric element  1530 , and piezoelectric element  1530  is disposed between insulator  1532  and second mass  1534 , in inverted Langevin assembly  1708 . Similarly, non-inverted Langevin assembly  1710  is like non-inverted Langevin assembly  1510  but with the positions of insulator  1542  and piezoelectric element  1540  swapped. Consequently, insulator  1542  is disposed between first mass  1534  and piezoelectric element  1540 , and piezoelectric element  1540  is disposed between insulator  1542  and second mass  1544 , in non-inverted Langevin assembly  1710 . 
       FIG. 18  is a side elevation view of an ultrasonic transducer array  1802 , which is like ultrasound transducer array  1500  of  FIGS. 15 and 16 , but with inverted Langevin assemblies  1508  and non-inverted Langevin assemblies  1510  replaced by inverted Langevin assemblies  1808  and non-inverted Langevin assemblies  1810 , respectively. Inverted Langevin assembly  1808  is like inverted Langevin assembly  1508  but with insulator  1532  replaced with a piezoelectric element  1832 . Consequently, piezoelectric element  1530  is disposed between first mass  1528  and piezoelectric element  1832 , and piezoelectric element  1832  is disposed between piezoelectric element  1530  and second mass  1534 , in inverted Langevin assembly  1808 . Piezoelectric element  1530  and piezoelectric element  1832  are electrically coupled in parallel with each other within inverted Langevin assembly  1808 . In inverted Langevin assembly  1808 , piezoelectric element  1530  and piezoelectric element  1832  have respective positive poles facing towards each other. Piezoelectric elements  1530  and  1832  expand and contract together when driven by an electrical signal, e.g., an electrical signal from signal generator  204 . 
     Similarly, non-inverted Langevin assembly  1810  is like non-inverted Langevin assembly  1510  but with insulator  1542  replaced with a piezoelectric element  1842 . Consequently, piezoelectric element  1540  is disposed between first mass  1534  and piezoelectric element  1842 , and piezoelectric element  1842  is disposed between piezoelectric element  1540  and second mass  1544 , in non-inverted Langevin assembly  1810 . Piezoelectric element  1540  and piezoelectric element  1842  are electrically coupled in parallel with each other within non-inverted Langevin assembly  1810 . In non-inverted Langevin assembly  1810 , piezoelectric element  1540  and piezoelectric element  1842  have respective positive poles facing away from each other. Piezoelectric elements  1540  and  1842  expand and contract together when driven by an electrical signal, e.g., an electrical signal from signal generator  204 . 
       FIG. 19  is a side elevation view of an ultrasonic transducer array  1902 , which is like ultrasound transducer array  1802  of  FIG. 18 , but with inverted Langevin assemblies  1808  and non-inverted Langevin assemblies  1810  replaced by inverted Langevin assemblies  1908  and non-inverted Langevin assemblies  1910 , respectively. Inverted Langevin assembly  1908  is like inverted Langevin assembly  1808  but with the positions of piezoelectric elements  1530  and  1832  swapped. Consequently, piezoelectric element  1832  is disposed between first mass  1528  and piezoelectric element  1530 , and piezoelectric element  1530  is disposed between piezoelectric element  1832  and second mass  1534 , in inverted Langevin assembly  1908 . Piezoelectric element  1530  and piezoelectric element  1832  are electrically coupled in parallel with each other within inverted Langevin assembly  1908 . In inverted Langevin assembly  1908 , piezoelectric element  1530  and piezoelectric element  1832  have respective positive poles facing away from each other. 
     Similarly, non-inverted Langevin assembly  1910  is like non-inverted Langevin assembly  1810  but with the positions of piezoelectric element  1842  and piezoelectric element  1540  swapped. Consequently, piezoelectric element  1842  is disposed between first mass  1534  and piezoelectric element  1540 , and piezoelectric element  1540  is disposed between piezoelectric element  1842  and second mass  1544 , in non-inverted Langevin assembly  1910 . Piezoelectric element  1540  and piezoelectric element  1842  are electrically coupled in parallel with each other within non-inverted Langevin assembly  1910 . In non-inverted Langevin assembly  1910 , piezoelectric element  1540  and piezoelectric element  1842  have respective positive poles facing towards each other. In this embodiment, first mass  1534  is insulated from first mass  1528 . 
     In any of the new ultrasound transducer arrays disclosed herein, the radiating element is optionally affixed to a tank for containing a liquid, for coupling sonic energy from the ultrasound transducer array into the liquid. For example,  FIG. 20  is a side plan view of an ultrasound assembly  2000  including an instance of ultrasound transducer array  302  affixed to a tank  2002  for containing a liquid. Ultrasound assembly  2000  is configured, for example, for cleaning an object in the liquid, processing the liquid, or inactivating organisms in the liquid. The other new ultrasound transducer arrays disclosed herein could be affixed to a tank in a similar manner. 
     Furthermore, in any of the new ultrasound transducer arrays disclosed herein, the radiating element is optionally a portion of a tank for containing a liquid, for coupling sonic energy from the ultrasound transducer array into the liquid. For example,  FIG. 21  is a side elevation view of an ultrasound assembly  2100  including an instance of ultrasound transducer array  302  where radiating element  312  is implemented by a bottom plate  2102  of a tank  2104  for containing a liquid. Ultrasound assembly  2100  is configured, for example, for cleaning an object in the liquid, processing the liquid, or inactivating organisms in the liquid. The other new ultrasound transducer arrays disclosed herein could be configured in a similar manner. 
       FIG. 22  is a flow chart illustrating a method  2200  for coupling sonic energy into a liquid. Method  2200  includes steps  2202  and  2204  which are executed in parallel. In step  2202 , a first transducer pair is driven with a first electrical signal, where the first transducer pair includes a first inverted ultrasound transducer and a first non-inverted ultrasound transducer electrically coupled in series. In one example of step  2202 , the left transducer pair  206  illustrated in  FIG. 2  is driven with an electrical signal from signal generator  204 . In step  2204 , a second transducer pair is driven with the first electrical signal, where the second transducer pair is electrically coupled in parallel with the first transducer pair, and where the second transducer pair includes a second inverted ultrasound transducer and a second non-inverted ultrasound transducer electrically coupled in series. In one example of step  2204 , the right transducer pair  206  illustrated in  FIG. 2  is driven with the electrical signal from signal generator  204 . Method  2200  optionally further includes step  2206  of generating the first electrical signal and step  2208  of sweeping a frequency of the first electrical signal with respect to a base frequency. Steps  2206  and  2208  are executed in parallel with steps  2202  and  2204 . In one example of steps  2206  and  2208 , signal generator  204  generates an electrical signal and sweeps a frequency of the electrical signal with respect to a base frequency. 
     Combinations of Features 
     Features described above may be combined in various ways without departing from the scope hereof. The following examples illustrate some possible combinations: 
     (A1) An ultrasound transducer array for coupling sonic energy into a liquid may include a plurality of transducer pairs, where each transducer pair includes an inverted ultrasound transducer and a non-inverted ultrasound transducer electrically coupled in series. 
     (A2) In the ultrasound transducer array denoted as (A1), the plurality of transducer pairs may be electrically coupled in parallel. 
     (A3) In any one of the ultrasound transducer arrays denoted as (A1) and (A2), in each of the plurality of transducer pairs, a negative pole of the inverted ultrasound transducer may be electrically coupled with a positive pole of the non-inverted ultrasound transducer. 
     (A4) In any one of the ultrasound transducer arrays denoted as (A1) and (A2), in each of the plurality of transducer pairs, a positive pole of the inverted ultrasound transducer may be electrically coupled with a negative pole of the non-inverted ultrasound transducer. 
     (A5) Any one of the ultrasound transducer arrays denoted as (A1) through (A4) may further include a radiating element, and each of the inverted and non-inverted ultrasound transducers of each of the plurality of transducer pairs may be joined to the radiating element. 
     (A6) In the ultrasound transducer array denoted as (A5), in each of the plurality of transducer pairs, the radiating element may form at least part of an electrical conductor electrically coupling the inverted and non-inverted ultrasound transducers of the transducer pair. 
     (A7) In any one of the ultrasound transducer arrays denoted as (A5) and (A6), the radiating element may be affixed to a tank for containing a liquid. 
     (A8) In any one of the ultrasound transducer arrays denoted as (A5) through (A7), the radiating element may be formed of at least one of quartz, sapphire, stainless steel, titanium, tantalum, boron nitride, silicon carbide, silicon nitride, aluminum, and a ceramic material. 
     (A9) In any one of the ultrasound transducer arrays denoted as (A1) through (A8), in each of the plurality of transducer pairs, the inverted ultrasound transducer may include a piezoelectric element having a positive pole facing a first direction, and the non-inverted ultrasound transducer may include a piezoelectric element having a positive pole facing a second direction that is opposite of the first direction. 
     (A10) In any one of the ultrasound transducer arrays denoted as (A1) through (A8), in each of the plurality of transducer pairs, the inverted ultrasound transducer may include a Langevin assembly having an inverted configuration, and the non-inverted ultrasound transducer may include a Langevin assembly having a non-inverted configuration. 
     (A11) In the ultrasound transducer array denoted as (A10), in each of the plurality of transducer pairs, the Langevin assembly having the inverted configuration may include a piezoelectric element having a positive pole facing toward a first direction, and the Langevin assembly having the non-inverted configuration may include a piezoelectric element having a positive pole facing toward a second direction opposite of the first direction. 
     (A12) In the ultrasound transducer array denoted as (A10), in each of the plurality of transducer pairs: (a) the Langevin assembly having the inverted configuration may include two piezoelectric elements electrically coupled in parallel and having respective positive poles facing towards each other, and (b) the Langevin assembly having the non-inverted configuration may include two piezoelectric elements electrically coupled in parallel and having respective positive poles facing away from each other. 
     (A13) In the ultrasound transducer array denoted as (A10), in each of the plurality of transducer pairs: (a) the Langevin assembly having the inverted configuration may include two piezoelectric elements electrically coupled in parallel and having respective positive poles facing away from each other, and (b) the Langevin assembly having the non-inverted configuration may include two piezoelectric elements electrically coupled in parallel and having respective positive poles facing towards each other. 
     (B1) An ultrasound transducer array for coupling sonic energy into a liquid may include a plurality of transducer pairs, where each transducer pair includes an inverted ultrasound transducer and a non-inverted ultrasound transducer electrically coupled in series and configured such that respective piezoelectric elements of the inverted and non-inverted ultrasound transducers expand and contract together when the transducer pair is driven by an electrical signal. 
     (B2) In the ultrasound transducer array denoted as (B1), the plurality of transducer pairs may be electrically coupled in parallel such that like polarities of piezoelectric elements of the plurality of transducer pairs are connected together. 
     (B3) Any one of the ultrasound transducer arrays denoted as (B1) and (B2) may further include a radiating element, where the inverted ultrasound transducer and the non-inverted ultrasound transducer of each transducer pair are joined to the radiating element. 
     (B4) In the ultrasound transducer array denoted as (B3), the radiating element may be affixed to a tank for containing a liquid. 
     (B5) In any one of the ultrasound transducer arrays denoted as (B3) and (B4), in each of the plurality of transducer pairs, the radiating element may form at least part of an electrical conductor electrically coupling the inverted and non-inverted ultrasound transducers of the transducer pair. 
     (B6) In any one of the ultrasound transducer arrays denoted as (B1) through (B5), in each of the plurality of transducer pairs, the inverted ultrasound transducer may include a piezoelectric element having a positive pole facing a first direction, and the non-inverted ultrasound transducer may include a piezoelectric element having a positive pole facing a second direction that is opposite of the first direction. 
     (B7) In any one of the ultrasound transducer arrays denoted as (B1) through (B5), in each of the plurality of transducer pairs, the inverted ultrasound transducer may include a Langevin assembly having an inverted configuration, and the non-inverted ultrasound transducer may include a Langevin assembly having a non-inverted configuration. 
     (C1) A method for coupling sonic energy into a liquid includes (a) driving a first transducer pair with a first electrical signal, the first transducer pair including a first inverted ultrasound transducer and a first non-inverted ultrasound transducer electrically coupled in series, and (b) driving a second transducer pair with the first electrical signal, the second transducer pair being electrically coupled in parallel with the first transducer pair, and the second transducer pair including a second inverted ultrasound transducer and a second non-inverted ultrasound transducer electrically coupled in series. 
     (C2) The method denoted as (C1) may further include sweeping a frequency of the first electrical signal with respect to a base frequency. 
     Changes may be made in the above methods, devices, and systems without departing from the scope hereof. For example, although the ultrasound transducers are discussed above as being either piezoelectric elements or Langevin assemblies, the ultrasound transducers could be other types of ultrasound transducers, including ultrasound transducers without piezoelectric elements, without departing from the scope hereof. It should thus be noted that the matter contained in the above description and shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense.