PATENT DOCUMENT

Publication Number: US-11366552-B2
Application Number: US-201916268886-A
Country: US
Kind Code: B2

Title: Ultrasonic polarizer

Abstract:
A polarizer disposed between a transducer and a surface in which acoustic waves propagate can be used to filter out certain types of acoustic energy. For example, the polarizer can be used with a shear-polarized transducer to pass shear waves and filter out compressional waves that may interact with water, thereby improving water rejection. In some examples, the polarizer can include one or more layers of piezoelectric material with a poling direction different than (e.g., orthogonal to) the poling direction of the transducer. Energy of compressional waves may be extracted by one or more external electric circuits. In some examples, the polarizer can be a magneto-elastic polarizer. In some examples, the polarizer can be a mechanical polarizer.

Claims:
What is claimed is: 
     
       1. A polarizer for use with a shear-polarized transducer, the polarizer comprising:
 a plurality of layers including at least a first layer of a first type of material and a second layer of a second type of material different than the first type of material; 
 wherein a transmission coefficient of the polarizer for shear waves at one or more first frequencies in a first passband is greater than a first threshold and wherein a transmission coefficient of the polarizer for compressional waves at the one or more first frequencies in the first passband is less than a second threshold less than the first threshold. 
 
     
     
       2. The polarizer of  claim 1 , wherein the first layer has a first thickness and the second layer has a second thickness different than the first thickness. 
     
     
       3. The polarizer of  claim 1 , wherein the first type of material has a Young&#39;s modulus less than or equal to 5 GPa and the second type of material has a Young&#39;s modulus greater than or equal to 20 GPa. 
     
     
       4. The polarizer of  claim 1 , wherein the first type of material is an epoxy and the second type of material is a metal. 
     
     
       5. The polarizer of  claim 1 , the plurality of layers further including at least a third layer of a third type of material and a fourth layer of a fourth type of material different from the third type of material, wherein the fourth layer is disposed on the third layer, the third layer is disposed on the second layer, and the second layer is disposed on the first layer. 
     
     
       6. The polarizer of  claim 5 , wherein the third type of material is a same type of material as the first type of material and the fourth type of material is a same type of material as the second type of material. 
     
     
       7. The polarizer of  claim 5 , wherein the third type of material is a different type of material than the first type of material or the fourth type of material is a different type of material than the second type of material. 
     
     
       8. The polarizer of  claim 5 , wherein the third layer has a third thickness and the fourth layer has a fourth thickness different than the third thickness. 
     
     
       9. The polarizer of  claim 1 , wherein the plurality of layers comprises interleaved layers with a Young&#39;s modulus less than or equal to 5 GPa and layers with a Young&#39;s modulus greater than or equal to 20 GPa. 
     
     
       10. The polarizer of  claim 1 , wherein the transmission coefficient of the polarizer for the compressional waves at one or more second frequencies in a second passband is greater than a third threshold and wherein the transmission coefficient of the polarizer for the shear waves at the one or more second frequencies in the second passband is less than a fourth threshold less than the third threshold. 
     
     
       11. A polarizer for use with a shear-polarized transducer, the polarizer comprising:
 one or more layers of piezoelectric material, wherein each of the one or more layers of piezoelectric material has a poling direction different than a poling direction of the shear-polarized transducer; 
 one or more electrodes; and 
 one or more circuits coupled to the one or more layers of piezoelectric material via the one or more electrodes; 
 wherein the polarizer is configured to extract and dissipate energy of compressional waves and pass energy of shear waves. 
 
     
     
       12. The polarizer of  claim 11 , wherein extracting and dissipating energy of the compressional waves comprises attenuating compressional waves by a threshold amount within at least a first range of frequencies. 
     
     
       13. The polarizer of  claim 11 , wherein the poling direction of each of the one or more layers of piezoelectric material is orthogonal to the poling direction of the shear-polarized transducer. 
     
     
       14. The polarizer of  claim 11 , wherein the one or more electrodes comprises a first electrode on a first side of a first layer of piezoelectric material of the one or more layers of piezoelectric material and a second electrode on a second side, opposite the first side, of the first layer of the piezoelectric material. 
     
     
       15. The polarizer of  claim 11 , wherein the one or more circuits comprise a circuit comprising a resistor coupled between one of the one or more electrodes and a system ground. 
     
     
       16. The polarizer of  claim 11 , wherein the one or more circuits comprise a circuit comprising a resistor and an inductor coupled in series between one of the one or more electrodes and a system ground. 
     
     
       17. The polarizer of  claim 11 , wherein a first layer of piezoelectric material of the one or more layers of piezoelectric material has a first thickness and a second layer of piezoelectric material of the one or more layers of piezoelectric material has a second thickness different than the first thickness. 
     
     
       18. A polarizer for use with a shear-polarized transducer, the polarizer comprising:
 one or more layers of piezoelectric material; 
 one or more electrodes; and 
 one or more circuits coupled to the one or more layers of piezoelectric material via the one or more electrodes; 
 wherein the polarizer is configured to extract and dissipate energy of compressional waves and pass energy of shear waves, and wherein extracting and dissipating energy of the compressional waves comprises attenuating the compressional waves by a threshold amount within at least a first range of frequencies. 
 
     
     
       19. The polarizer of  claim 18 , wherein the threshold amount is at least 90% attenuation. 
     
     
       20. The polarizer of  claim 18 , wherein the first range of frequencies is wider than 100 kHz and includes frequencies greater than 500 kHz.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional Application No. 62/627,173, filed Feb. 6, 2018, and U.S. Provisional Application No. 62/627,174, filed Feb. 6, 2018, the entire disclosures of which are incorporated herein by reference for all purposes. 
    
    
     FIELD OF THE DISCLOSURE 
     This relates generally to acoustic touch sensing, and more particularly, to polarizers for transducers for acoustic touch sensing. 
     BACKGROUND OF THE DISCLOSURE 
     Many types of input devices are presently available for performing operations in a computing system, such as buttons or keys, mice, trackballs, joysticks, touch sensor panels, touch screens and the like. Touch screens are particularly popular because of their ease and versatility of operation as well as their declining price. Touch screens can include a touch sensor panel, which can be a clear panel with a touch-sensitive surface, and a display device such as a liquid crystal display (LCD) that can be positioned partially or fully behind the panel so that the touch-sensitive surface can cover at least a portion of the viewable area of the display device. Touch screens can allow a user to perform various functions by touching the touch sensor panel using a finger, stylus or other object at a location often dictated by a user interface (UI) being displayed by the display device. In general, touch screens can recognize a touch and the position of the touch on the touch sensor panel, and the computing system can then interpret the touch in accordance with the display appearing at the time of the touch, and thereafter can perform one or more actions based on the touch. In the case of some touch sensing systems, a physical touch on the display is not needed to detect a touch. For example, in some capacitive-type touch sensing systems, fringing electrical fields used to detect touch can extend beyond the surface of the display, and objects approaching near the surface may be detected near the surface without actually touching the surface. Capacitive-type touch sensing systems, however, can experience reduced performance due to floating objects (e.g., water droplets) in contact with the touch-sensitive surface. 
     SUMMARY 
     This relates to polarizers for use in an acoustic touch sensing system to improve performance of the acoustic touch sensing system. Acoustic touch sensing systems can utilize one or more transducers coupled to a surface of a device, such as piezoelectric transducers, to transmit ultrasonic waves along a surface and/or through the thickness of an electronic device. As the transmitted wave propagates along the surface, one or more objects (e.g., finger, stylus, etc.) in contact with the surface can interact with the transmitted wave causing a reflection of at least a portion of the transmitted wave, which can be received by the transducers. Portions of the transmitted wave energy after interaction with the one or more objects can be measured to determine the touch location(s) of the one or more objects on the surface of the device (e.g., using time-of-flight (TOF) techniques). In some examples, an acoustic touch sensing system can be configured to be insensitive to contact on the device surface by water, by using shear acoustic waves, for example. Thus, an acoustic touch sensing can be used for touch sensing in devices that are likely to become wet or fully submerged in water. A polarizer disposed between the transducer and the surface in which the shear acoustic waves propagate can be used to filter compressional waves that may interact with water, thereby improving water rejection by the acoustic touch sensing system. 
     In some examples, the polarizer can include one or more layers of piezoelectric material with a poling direction different than (e.g., orthogonal to) the poling direction of the transducer. Mechanical energy of compressional waves interacting with the one or more layers of piezoelectric material may be converted to electrical energy which may be extracted by one or more external electric circuits to dissipate the energy as heat (or to feed the energy back into the system at a different phase to cancel the incoming compressional wave). In some examples, the polarizer may be formed of a magnetic material that can generate eddy currents to dissipate undesired acoustic energy. 
     Additionally or alternatively, in some examples, the polarizer can be or include a multi-layer structure including at least a first layer of a first type of material of a first thickness and a second layer of a second type of material of second thickness. The types of materials, number of layers, and thicknesses of the layers can be tuned to filter out a first type of acoustic wave (e.g., a compressional wave) and pass a second type of acoustic wave (e.g., a shear wave). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-1E  illustrate exemplary electronic devices that can include an acoustic touch sensing system according to examples of the disclosure. 
         FIG. 2A  illustrates an exemplary block diagram of an electronic device including an acoustic touch sensing system according to examples of the disclosure. 
         FIG. 2B  illustrates an exemplary stack-up of an exemplary electronic device including an acoustic touch sensing system according to examples of the disclosure. 
         FIG. 3A  illustrates an exemplary method for acoustic touch sensing to determine a position of an object in contact with a surface according to examples of the disclosure. 
         FIG. 3B  illustrates an exemplary process for acoustic touch sensing of an object presence and contact position in various modes according to examples of the disclosure. 
         FIG. 4  illustrates an exemplary configuration of an acoustic touch sensing circuit according to examples of the disclosure. 
         FIGS. 5A-5F  illustrate exemplary system configurations and timing diagrams for acoustic touch sensing to determine position using a bounding box technique according to examples of the disclosure. 
         FIG. 6A  illustrates an exemplary stack-up of an exemplary multi-layer polarizer including two layers according to examples of the disclosure. 
         FIG. 6B  illustrates an exemplary stack-up of an exemplary multi-layer polarizer including more than two layers according to examples of the disclosure. 
         FIGS. 7A-7D  illustrate exemplary stack-ups including a surface, a transducer and a multi-layer polarizer according to examples of the disclosure. 
         FIGS. 8A-8B  illustrate exemplary plots of frequency dependent transmission coefficients through an exemplary polarizer for compressional and shear waves according to examples of the disclosure. 
         FIGS. 9A-9B  illustrate exemplary multi-dimensional polarizer structures according to examples of the disclosure. 
         FIG. 10A  illustrates an exemplary stack-up of an acoustic touch sensing system including a polarizer with a layer of piezoelectric material according to examples of the disclosure. 
         FIG. 10B  illustrates an exemplary stack-up of an acoustic touch sensing system including a polarizer with multiple layers of piezoelectric material according to examples of the disclosure. 
         FIGS. 11A and 11B  illustrate exemplary electric circuits for use with an exemplary polarizer according to examples of the disclosure. 
         FIG. 11C  illustrates an exemplary electric circuit representing multiple electric circuits for use with an exemplary multi-layer polarizer according to examples of the disclosure. 
         FIG. 12  illustrates exemplary performance of a polarizer according to examples of the disclosure. 
         FIG. 13  illustrates a stack-up of an exemplary magneto-elastic polarizer according to examples of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description of various examples, reference is made to the accompanying drawings which form a part hereof, and in which it is shown by way of illustration specific examples that can be practiced. It is to be understood that other examples can be used and structural changes can be made without departing from the scope of the various examples. 
     This relates to polarizers for use in an acoustic touch sensing system to improve performance of the acoustic touch sensing system. Acoustic touch sensing systems can utilize one or more transducers coupled to a surface of a device, such as piezoelectric transducers, to transmit ultrasonic waves along a surface and/or through the thickness of an electronic device. As the transmitted wave propagates along the surface, one or more objects (e.g., finger, stylus, etc.) in contact with the surface can interact with the transmitted wave causing a reflection of at least a portion of the transmitted wave, which can be received by the transducers. Portions of the transmitted wave energy after interaction with the one or more objects can be measured to determine the touch location(s) of the one or more objects on the surface of the device (e.g., using time-of-flight (TOF) techniques). In some examples, an acoustic touch sensing system can be configured to be insensitive to contact on the device surface by water, by using shear acoustic waves, for example. Thus, an acoustic touch sensing can be used for touch sensing in devices that are likely to become wet or fully submerged in water. A polarizer disposed between the transducer and the surface in which the shear acoustic waves propagate can be used to filter compressional waves that may interact with water, thereby improving water rejection by the acoustic touch sensing system. 
     In some examples, the polarizer can include one or more layers of piezoelectric material with a poling direction different than (e.g., orthogonal to) the poling direction of the transducer. Mechanical energy of compressional waves interacting with the one or more layers of piezoelectric material may be converted to electrical energy which may be extracted by one or more external electric circuits to dissipate the energy as heat (or to feed the energy back into the system at a different phase to cancel the incoming compressional wave). In some examples, the polarizer may be formed of a magnetic material that can generate eddy currents to dissipate undesired acoustic energy. 
     Additionally or alternatively, in some examples, the polarizer can be or include a multi-layer structure including at least a first layer of a first type of material of a first thickness and a second layer of a second type of material of second thickness. The types of materials, number of layers, and thicknesses of the layers can be tuned to filter out a first type of acoustic wave (e.g., a compressional wave) and pass a second type of acoustic wave (e.g., a shear wave). 
     Acoustic touch sensing can be used instead of, or in conjunction with, other touch sensing techniques, such as resistive and/or capacitive touch sensing. In some examples, the acoustic touch sensing techniques described herein can be used on a metal housing surface of a device, which may be unsuitable for capacitive or resistive touch sensing due to interference (e.g., of the housing with the capacitive or resistive sensors housed in the metal housing). In some examples, the acoustic touch sensing techniques described herein can be used on a glass or crystal surface of a display or touch screen. 
       FIGS. 1A-1E  illustrate examples of systems with touch screens that can include acoustic sensors for detecting contact between an object (e.g., a finger or stylus) and a surface of the system according to examples of the disclosure.  FIG. 1A  illustrates an exemplary mobile telephone  136  that includes a touch screen  124  and can include an acoustic touch sensing system according to examples of the disclosure.  FIG. 1B  illustrates an example digital media player  140  that includes a touch screen  126  and can include an acoustic touch sensing system according to examples of the disclosure.  FIG. 1C  illustrates an example personal computer  144  that includes a touch screen  128  and a track pad  146 , and can include an acoustic touch sensing system according to examples of the disclosure.  FIG. 1D  illustrates an example tablet computing device  148  that includes a touch screen  130  and can include an acoustic touch sensing system according to examples of the disclosure.  FIG. 1E  illustrates an example wearable device  150  (e.g., a watch) that includes a touch screen  152  and can include an acoustic touch sensing system according to examples of the disclosure. Wearable device  150  can be coupled to a user via strap  154  or any other suitable fastener. It should be understood that the example devices illustrated in  FIGS. 1A-1E  are provided by way of example, and other types of devices can include an acoustic touch sensing system for detecting contact between an object and a surface of the device. Additionally, although the devices illustrated in  FIGS. 1A-1E  include touch screens, in some examples, the devices may have a non-touch-sensitive display. 
     Acoustic sensors can be incorporated in the above described systems to add acoustic touch sensing capabilities to a surface of the system. For example, in some examples, a touch screen (e.g., capacitive, resistive, etc.) can be augmented with acoustic sensors to provide a touch sensing capability for use in wet environments or under conditions where the device may get wet (e.g., exercise, swimming, rain, washing hands). In some examples, an otherwise non-touch sensitive display screen can be augmented with acoustic sensors to provide a touch sensing capability. In such examples, a touch screen can be implemented without the stack-up required for a capacitive touch screen. In some examples, the acoustic sensors can be used to provide touch sensing capability for a non-display surface. For example, the acoustic sensors can be used to provide touch sensing capabilities for a track pad  146 , a button, a scroll wheel, part or all of the housing or any other surfaces of the device (e.g., on the front, rear or sides). 
       FIG. 2A  illustrates an exemplary block diagram of an electronic device including an acoustic touch sensing system according to examples of the disclosure. In some examples, housing  202  of device  200  (e.g., mobile telephone  136 , digital media player  140 , personal computer  144 , tablet computing device  148 , wearable device  150 ) can be coupled with one or more acoustic transducers  204 . In some examples, transducers  204  can be piezoelectric transducers, which can be made to vibrate by the application of electrical signals when acting as a transmitter, and generate electrical signals based on detected vibrations when acting as a receiver. In some examples, the transducers  204  can be formed from a piezoelectric ceramic material (e.g., PZT or KNN) or a piezoelectric plastic material (e.g., PVDF or PLLA). Similarly, transducers  204  can produce electrical energy as an output when vibrated. In some examples, the transducers  204  can be bonded to the housing  202  by a bonding agent (e.g., a thin layer of stiff epoxy). In some examples, the transducers  204  can be deposited on the surface (e.g., a cover glass or front crystal) through processes such as deposition, lithography, or the like. In some examples, the transducers  204  can be bonded to the surface using conductive or non-conductive bonding materials. When electrical energy is applied to the transducers  204  it can cause the transducers to vibrate, the surface material in contact with the transducers can also be caused to vibrate, and the vibrations of the molecules of the surface material can propagate as an acoustic wave through the surface material. In some examples, vibration of the transducers  204  can be used to produce ultrasonic acoustic waves at a selected frequency over a broad frequency range (e.g., 400 kHz−10 MHz) in the medium of the surface of the electronic device which can be metal, plastic, glass, wood, or the like. It should be understood that other frequencies outside of the exemplary range above can be used while remaining within the scope of the present disclosure. 
     In some examples, transducers  204  can also be partially or completely disposed on (or coupled to) a portion of a touch screen  208 . For example, the touch screen  208  (e.g., capacitive) may include a glass panel (cover glass), and a display region of the touch screen may be surrounded by a non-display region (e.g., a black border region surrounding the periphery of the display region of touch screen). In some examples, transducers  204  can be disposed partially or completely in the black mask region of the touch screen  208  glass panel (e.g., on the back side of the glass panel behind the black mask) such that the transducers are not visible (or are only partially visible) to a user. 
     Device  200  can further include acoustic touch sensing circuitry  206 , which can include circuitry for driving electrical signals to stimulate vibration of the transducers  204  (e.g., transmit circuitry), as well as circuitry for sensing electrical signals output by the transducers (e.g., receive circuitry) when the transducer is stimulated by received acoustic energy. In some examples, timing operations for the acoustic touch sensing circuitry  206  can optionally be provided by a separate acoustic touch sensing controller  210  that can control timing of acoustic touch sensing circuitry  206  operations. In some examples, touch sensing controller  210  can be coupled between acoustic touch sensing circuitry  206  and host processor  214 . In some examples, controller functions can be integrated with the acoustic touch sensing circuitry  206  (e.g., on a single integrated circuit). Output data from acoustic touch sensing circuitry  206  can be output to a host processor  214  for further processing to determine a location of an object contacting the device as will be described in more detail below. In some examples, the processing for determining location of a contacting object can be performed by the acoustic touch sensing circuitry  206 , controller  210  or a separate sub-processor of device  200  (not shown). 
     In some examples, a polarizer  220  can be disposed between a transducer  204  and the surface in which the acoustic waves propagate. In some examples, shear horizontal acoustic waves can be generated by transducer  204  so as to not interact with water on the surface. Discontinuous boundary conditions between the transducer  204  and the surface (in the absence of polarizer  220 ) can also cause the generation of compressional waves, such as Lamb waves, which may interact with water. The polarizer  220  can be designed to filter out compressional waves, such as Lamb waves, to transmit acoustic energy into the surface or receive acoustic energy reflected back from the surface primarily or only in shear modes. It should be understood that although examples described here focus on primarily on passing shear horizontal acoustic waves and stopping (e.g., absorbing or attenuating) compressional acoustic waves, the polarizer  220  can be designed to pass acoustic waves having a first displacement field direction and stopping acoustic waves having a second displacement field direction different from the first displacement field direction. 
     In addition to acoustic touch sensing, the device can include additional touch circuitry  212  and optionally a touch controller (not shown) that can be coupled to the touch screen  208 . In examples including a touch controller, the touch controller can be disposed between the touch circuitry  212  and the host processor  214 . The touch circuitry  212  can, for example, be capacitive or resistive touch sensing circuitry, and can be used to detect contact and/or hovering of objects (e.g., fingers, styli) in contact with and/or in proximity to the touch screen  208 , particularly in the display region of the touch screen. Thus, device  200  can include multiple types of sensing circuitry (e.g., touch circuitry  212  and acoustic touch sensing circuitry  206 ) for detecting objects (and their positions) in different regions of the device and/or for different purposes, as will be described in more detail below. Although described herein as including a touch screen, it should be understood that touch circuitry  212  can be omitted and touch screen  208  can be replaced by an otherwise non-touch-sensitive display (e.g., but-for the acoustic sensors). 
     Host processor  214  can receive acoustic or other touch outputs (e.g., capacitive) and perform actions based on the touch outputs. Host processor  214  can also be connected to program storage  216  and touch screen  208 . Host processor  214  can, for example, communicate with touch screen  208  to generate an image on touch screen  208 , such as an image of a user interface (UI), and can use touch sensing circuitry  212  and/or acoustic touch sensing circuitry  206  (and, in some examples, their respective controllers) to detect a touch on or near touch screen  208 , such as a touch input to the displayed UI. The touch input can be used by computer programs stored in program storage  216  to perform actions that can include, but are not limited to, moving an object such as a cursor or pointer, scrolling or panning, adjusting control settings, opening a file or document, viewing a menu, making a selection, executing instructions, operating a peripheral device connected to the host device, answering a telephone call, placing a telephone call, terminating a telephone call, changing the volume or audio settings, storing information related to telephone communications such as addresses, frequently dialed numbers, received calls, missed calls, logging onto a computer or a computer network, permitting authorized individuals access to restricted areas of the computer or computer network, loading a user profile associated with a user&#39;s preferred arrangement of the computer desktop, permitting access to web content, launching a particular program, encrypting or decoding a message, and/or the like. Host processor  214  can also perform additional functions that may not be related to touch processing. 
     Note that one or more of the functions described herein can be performed by firmware stored in memory and executed by the touch circuitry  212  and/or acoustic touch sensing circuitry  206  (or their respective controllers), or stored in program storage  216  and executed by host processor  214 . The firmware can also be stored and/or transported within any non-transitory computer-readable storage medium for use by or in connection with an instruction execution system, apparatus, or device, such as a computer-based system, processor-containing system, or other system that can fetch the instructions from the instruction execution system, apparatus, or device and execute the instructions. In the context of this document, a “non-transitory computer-readable storage medium” can be any medium (excluding a signal) that can contain or store the program for use by or in connection with the instruction execution system, apparatus, or device. The non-transitory computer readable medium storage can include, but is not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus or device, a portable computer diskette (magnetic), a random access memory (RAM) (magnetic), a read-only memory (ROM) (magnetic), an erasable programmable read-only memory (EPROM) (magnetic), a portable optical disc such a CD, CD-R, CD-RW, DVD, DVD-R, or DVD-RW, or flash memory such as compact flash cards, secured digital cards, USB memory devices, memory sticks, and the like. 
     The firmware can also be propagated within any transport medium for use by or in connection with an instruction execution system, apparatus, or device, such as a computer-based system, processor-containing system, or other system that can fetch the instructions from the instruction execution system, apparatus, or device and execute the instructions. In the context of this document, a “transport medium” can be any medium that can communicate, propagate or transport the program for use by or in connection with the instruction execution system, apparatus, or device. The transport readable medium can include, but is not limited to, an electronic, magnetic, optical, electromagnetic or infrared wired or wireless propagation medium. 
     It is to be understood that device  200  is not limited to the components and configuration of  FIG. 2A , but can include other or additional components in multiple configurations according to various examples. Additionally, the components of device  200  can be included within a single device, or can be distributed between multiple devices. Additionally, it should be understood that the connections between the components is exemplary and different unidirectional or bidirectional connections can be included between the components depending on the implementation, irrespective of the arrows shown in the configuration of  FIG. 2A . 
       FIG. 2B  illustrates an exemplary stack-up of an exemplary electronic device including an acoustic touch sensing system according to examples of the disclosure. The electronic device (e.g., mobile telephone  136 , digital media player  140 , personal computer  144 , tablet computing device  148 , wearable device  150 ) can include a stack-up  250  that includes a surface  252  in which acoustic waves can propagate, a transducer  254  (e.g., corresponding to one of transducers  204 ) and a polarizer  256  (e.g., corresponding to polarizer  220 ). In some examples, surface  252  can be a cover glass or front crystal of a touch screen (e.g., touch screen  208 ). In some examples, transducer  254  can be shear-polarized piezoelectric material primarily generating shear horizontal waves when stimulated that can propagate into surface  252  (e.g., in the z-direction) while its vibration or displacement can be in-plane with respect to surface  252  (e.g., in the x-y plane). The shear horizontal waves can be reflected due to a finger or other object touching surface  252 , but not when water or other liquids are in contact with surface  252  due to in-plane displacement of shear horizontal waves. As a result, an acoustic touch sensing system using shear horizontal waves can be water (or other liquid) agnostic. In some examples, transducer  254  can also generate parasitic waves (which can be reflected due to water on the surface), such as compressional waves or Lamb waves, at its corners due to discontinuous boundary conditions. Compressional waves can propagate into surface  252  (e.g., in the z-direction) while its vibration or displacement can be out-of-plane with respect to surface  252  (e.g., also in the z-direction) Polarizer  254  can be designed to filter out compressional waves or Lamb waves and pass shear horizontal waves. 
       FIG. 3A  illustrates an exemplary method  300  for acoustic touch sensing of an object contact position according to examples of the disclosure. At  302 , acoustic energy can be transmitted (e.g., by one or more transducers  204 ) along a surface of a device in the form of an ultrasonic wave, for example. In some examples, the wave can propagate as a compressive wave, a shear horizontal wave, a Rayleigh wave, a Lamb wave, a Love wave, a Stonely wave, or a surface acoustic wave. Other propagation modes for the transmitted acoustic energy can also exist based on the properties of the surface material and the manner of energy transmission from the transducers to the surface of the device. In some examples, the surface can be formed from glass or sapphire crystal (e.g., touch screen  208 ) or the surface can formed from metal, plastic, or wood (e.g., housing  202 ). Transmitted energy can propagate along the surface until a discontinuity in the surface is reached, which can cause a portion of the energy to reflect. In some examples, a discontinuity can be an irregularity in the shape of the surface (e.g., a groove or pattern etched into the surface). In some examples, a discontinuity can be a reflective material coupled to the surface (e.g., deposited). In some examples, an object in contact with the surface (e.g., a user&#39;s finger) can also be a discontinuity. In some examples, a discontinuity can occur at edges of the surface material (e.g., when the ultrasonic wave propagates to the edge of the surface opposite the transducer). When the transmitted energy reaches one of the discontinuities described above, some of the energy can be reflected, and a portion of the reflected energy can be directed to the one or more transducers  204 . In some examples, water or other fluids in contact with the surface of the device (e.g., device  200 ) will not act as a discontinuity to the acoustic waves (e.g., shear horizontal acoustic waves), and thus the acoustic touch sensing method can be effective for detecting the presence of an object (e.g., a user&#39;s finger) even in the presence of water drops (or other low-viscosity fluids) on the surface of the device or even while the device is fully submerged. 
     In some examples, the acoustic energy can be transmitted by one or more transducers  204  into the surface via a polarizer  220 . At  303 , the acoustic energy generated by the transducers  204  can be filtered by polarizer  220 . In some examples, the transducer can be shear-polarized and the acoustic energy generated by the transducer can thereby primarily include shear horizontal waves in order to prevent water or liquids on the surface from generating reflections and being identified as touches. However, as described herein, the acoustic energy generated by the transducer may also include compressional waves, Rayleigh waves, Lamb waves, Love waves, Stonely waves, or surface acoustic waves, some of which may be parasitic in that these waves may interact with water on the surface and be identified as touches. Polarizer  220  can provide a passband at frequencies corresponding to shear waves and a stopband at frequencies corresponding to parasitic modes such as compressional or Lamb waves, for example. 
     At  304 , returning acoustic energy can be received, and the acoustic energy can be converted to an electrical signal by one or more transducers  204 . In some examples, the acoustic energy can be received by the one or more transducers  204  from the surface via the polarizer  220 . At  305 , the acoustic energy received from the surface can be filtered by polarizer  220 . In some examples, polarizer  220  can provide a passband at frequencies corresponding to shear waves and a stopband at frequencies corresponding to parasitic modes such as compressional or Lamb waves, for example, so that the acoustic touch sensing system can avoid detecting water or other liquids as touches. At  306 , the acoustic touch sensing system can determine whether one or more objects (e.g., fingers) is contacting the surface of the device, and can further detect the position of one or more objects based on the received acoustic energy. In some examples, a distance of the object from the transmission source (e.g., transducers  204 ) can be determined from a time-of-flight between transmission and reception of reflected energy, and a propagation rate of the ultrasonic wave through the material of the surface (and accounting for the properties of the polarizer). In some examples, baseline reflected energy from one or more intentionally included discontinuities (e.g., barriers, ridges, grooves, etc.) can be compared to a measured value of reflected energy. The baseline reflected energy can be determined during a measurement when no object (e.g., finger) is in contact with the surface. Timing of measured deviations of the reflected energy from the baseline can be correlated with a location of the object. Although method  300 , as described above, generally refers to reflected waves received by the transducers that transmitted the waves, in some examples, the transmitter and receiver functions can be separated such that the transmission of acoustic energy at  302  and receiving acoustic energy at  304  may not occur at the same transducer. A polarizer can be included between the surface and both of or either of the transmitter and receiver transducers. Exemplary device configurations and measurement timing examples that can be used to implement method  300  will be described in further detail below. 
     In some examples, the acoustic touch sensing can be performed differently in different operating modes. For example, the acoustic touch sensing can include a low power mode (e.g., when objects are not detected, when display is turned off) and an active mode (e.g., when an object is detected, when the display is turned on).  FIG. 3B  illustrates an exemplary process  320  for acoustic touch sensing of an object presence and contact position in various modes (e.g., a low power mode and an active mode) according to examples of the disclosure. At  325 , the acoustic touch sensing system can perform a low power detection scan. In some examples, the low power detection scan can include sensing with fewer (in comparison to the active mode scan) of the transducers of the acoustic touch sensing system (e.g., four transducers may be used for the active mode detection scan as described below with respect to  FIG. 5A , and fewer than four transducers may be used for the low power detection scan). In some examples, the acoustic touch sensing system can use a single transducer to transmit acoustic waves and receive reflections to determine the presence of an object touching. Additionally or alternatively, in some examples, the low power detection scan can include sensing energy or waves received by one or more transducers for a shorter (in comparison to the active mode scan) period of time. For example, the low power scan can sense the energy or waves for the period of time corresponding to a reflection of an opposite edge of the touch sensing surface (rather than for a period that may include other reflections). Attenuation in the reflected energy or wave corresponding to the opposite edge compared with a no-touch baseline of reflected energy or wave corresponding to the opposite edge can be an indication that an object is touching the surface. Additionally or alternatively, low power detection scan can be performed at a reduced frame rate (e.g., 10 Hz−30 Hz for the low power detection scans rather than 30 Hz−120 Hz for active mode detection scans), thereby reducing the power consumption by the various ADC and DAC components. At  330 , the acoustic touch sensing system can process data from the low power detection scan and detect whether an object is or is not touching the surface. When no object is detected on the surface at  335 , the acoustic touch sensing system can remain in a low power mode, and continue to perform low power detection scans (in the same or in subsequent scan frames). When an object is detected on the surface at  335 , the acoustic touch sensing system can transition into an active mode and, at  340 , perform an active mode detection scan. At  345 , the data from the active mode detection scan can be processed to determine a location (e.g., centroid) of the object(s) contacting the surface (e.g., as described below with reference to  FIG. 5A ). 
     Although process  300  is described as a low power detection scan and an active mode detection scan, it should be understood that process  300  can generally provide a coarse detection scan (e.g., indicating the presence or absence of a touch) and a fine detection scan (e.g., indicating the location of the touch) without limiting the system to low power mode and/or active mode operation. 
       FIG. 4  illustrates an exemplary configuration of an acoustic touch sensing circuit  400  according to examples of the disclosure. Acoustic touch sensing circuit  400  can include acoustic touch sensing circuitry  402 - 404  and  408 - 420  (which can correspond to acoustic touch sensing circuitry  206  above) and control logic  422  (which can correspond to acoustic touch sensing controller  210  above). In some examples, acoustic touch sensing circuit  400  can also optionally include transducers  406  (which can correspond to transducers  204  above). In some examples, a transmitter  402  can generate an electrical signal for stimulating movement of one or more of a plurality of transducers  406 . In some examples, the transmitted signal can be a differential signal, and in some examples, the transmitted signal can be a single-ended signal. In some examples, transmitter  402  can be a simple buffer, and the transmitted signal can be a pulse (or burst of pulses at a particular frequency). In some examples, transmitter  402  can include a digital-to-analog converter (DAC)  402 A and an optional filter  402 B that can be optionally used to smooth a quantized output of DAC  402 A. In some examples, characteristics of the transducer itself can provide a filtering property and filter  402 B can be omitted. DAC  402 A can be used to generate an arbitrary transmit waveform. In some examples, the arbitrary waveform can pre-distort the transmit signal to equalize the channel. In some examples, the characteristics of each channel, such as the properties of the surface material coupled to transducers  406 , the discontinuities in the surface material, and the reflection characteristics of an edge of the device can be measured and stored. In some examples, the channel characteristics can be measured as a manufacturing step (or factory calibration step), and in other examples the characteristics can be measured as a periodic calibration step (i.e., once a month, once a year, etc. depending on how quickly the channel characteristics are expected to change). In some examples, the channel characteristics can be converted to a transfer function of the channel, and the arbitrary transmit waveform can be configured using the inverse of the channel transfer function such that the returning signal is equalized (e.g., returning signal can be detected as a pulse or a burst of pulses despite the transmitted waveform having a seemingly arbitrary waveform). In some examples, a single differential pulse can be used as a transmit waveform. For example, a bipolar square pulse (where the voltage applied to the transducer can be both positive and negative) can be used as the transmit waveform, and the bipolar square pulse can be implemented using a single-ended or differential implementation. 
     A pair of demultiplexers  404  (e.g., in a differential implementation) can be used to selectively couple transmitter  402  to one of transducers  406  that can be the active transducer for a particular measurement step in a measurement cycle. In some examples, demultiplexers  404  can have a ground connection, and the non-selected demultiplexer outputs can be shorted, open, or grounded. As described above, transducers  406  can also generate output electrical signals when motion is induced in the transducers by acoustic energy. A pair of multiplexers  408  (e.g., in a differential implementation) can be used to select a transducer  406  for coupling to a programmable gain amplifier  410  configured to amplify the received signals. In some examples, the same transducer  406  can be coupled to transmitter  402  by demultiplexers  404  during the drive mode and coupled to programmable gain amplifier  410  by multiplexers  408  during the receive mode. Thus, a single transducer  406  can be used both for transmitting and receiving acoustic energy. In some examples, a first transducer can be coupled to transmitter  402  by demultiplexers  404  and a second transducer can be coupled by multiplexers  408  to programmable gain amplifier  410 . For example, the transmitting transducer and the receiving transducer can be discrete piezoelectric elements, where the transmitting transducer can be designed for being driven by higher voltages (or currents) to produce sufficient motion in transducer  406  to generate an acoustic wave in the surface of a device (e.g., device  200  above), and the receiving transducer can be designed for receiving smaller amplitude reflected energy. In such an architecture, the transmit side circuitry (e.g.,  402  and  404 ) can be optionally implemented on a high voltage circuit, and the receive side circuitry (e.g.,  408 - 420 ) can be optionally implemented on a separate low voltage circuit. In some examples, multiplexers  408  can also be implemented on the high voltage circuit to properly isolate the remaining receive side circuitry (e.g.,  410 - 420 ) during transmission operations by transmit side circuitry. Additionally or alternatively, in some examples, the transmit circuit can include an energy recovery architecture that can be used to recover some of the energy required for charging and discharging the transducer. In some examples, the programmable gain amplifier output can be coupled to gain and offset correction circuit  412 . It should be understood that for a single-ended implementation, a single demultiplexer  404  and a single multiplexer  408  can be used, and transmitter  402 , programmable gain amplifier  410 , and the input to gain and offset correction circuit  412  can be single-ended as well. Differential implementations, however, can provide improved noise suppression over a single-ended implementation. 
     In some examples, the acoustic touch sensing circuit can be used in a system include multiple transmit transducers and one receive transducer. In such examples, demultiplexer  404  can be unnecessary and omitted from the acoustic touch sensing circuit. In some examples, the acoustic touch sensing circuit can be used in a system including multiple receive transducers and one transmit transducer. In such examples, multiplexer  408  can be unnecessary and omitted from the acoustic touch sensing circuit. 
     In some examples, the output of gain and offset correction circuit  412  can optionally be coupled to one or more analog processing circuits. In some examples, the output of gain and offset correction circuit  412  can be coupled to a demodulation circuit  414  configured to demodulate the received signals (e.g., by I/Q demodulation). In some examples, the output of the gain and offset correction circuit  412  can be coupled to an envelope detection circuit  415  configured to perform envelope detection on the received signals. In some examples, the output of gain and offset correction circuit  412  can be filtered at filter  416 . In some examples, these blocks can be placed in a different order. In some examples, the processing of these analog processing circuits can be performed in the digital domain. 
     The received signals, whether raw or processed by one or more of demodulation circuit  414 , envelope detection circuit  415  or filter  416  can be passed to an analog-to-digital converter (ADC)  418  for conversion to a digital signal. In some examples, an input/output (I/O) circuit  420  can be used to transmit received data for processing. In some examples, the output of I/O circuit  420  can be transferred to a host processor of the device, or to an auxiliary processor (sub-processor) separate from the host processor. For example, as illustrated, the output of I/O circuit  420  can be coupled to a processor system-on-chip (SoC)  430 , which can include one or more processors. In some examples, processor SoC  430  can include a host processor  432  (e.g., an active mode processor) and an auxiliary processor  434  (e.g., a low power processor). In some examples, some digital signal processing can be performed (e.g., by acoustic touch sensing circuit  400 ) before transmitting the data to other processors in the system (e.g., processor SoC  430 ). A control circuit  422  can be used to control timing and operations of the acoustic touch sensing circuitry  402 - 420 . In some examples, the I/O circuit is not only used for data transfer to processor SoC  430  (e.g., host processor  432 ), but also is used for writing the control registers and/or firmware download from processor SoC  430 . 
     It is to be understood that the configuration of  FIG. 4  is not limited to the components and configuration of  FIG. 4 , but can include other or additional components in multiple configurations according to various examples. Additionally, some or all of the components  402 - 404   404  and  408 - 420  can be included in a single circuit, or can be divided among multiple circuits while remaining within the scope of the examples of the disclosure. 
     As described herein, various acoustic sensing techniques can be used to determine position of an object in touching a surface. In some examples, one or more time-of-flight (TOF) measurements can be performed using one or more acoustic transducers to determine boundaries of the position that the object is touching.  FIGS. 5A-5F  illustrate exemplary system configurations and timing diagrams for acoustic touch sensing to determine position using a bounding box technique according to examples of the disclosure.  FIG. 5A  illustrates an exemplary acoustic touch sensing system configuration using four acoustic transducers  502 A-D mounted along (or otherwise coupled to) four edges of a surface  500  (e.g., cover glass). In some examples, transducers  502 A-D can be coupled to the four edges of surface  500  via corresponding polarizers (not shown). Transducers  502 A-D can be configured to generate acoustic waves (e.g., shear horizontal waves) and to receive the reflected acoustic waves. Propagation of shear horizontal waves can be unaffected by water on surface  500  because low viscosity fluids and gases (such as water and air) have a very low shear modulus, and therefore do not perturb the boundary conditions that affect wave propagation. Shear horizontal waves can be highly directional waves such that the active detection region (or active area)  504  can be effectively defined based on the position and dimensions of the acoustic transducers  502 A-D. It should be understood, however, that active area can change based on the directionality property of the acoustic waves and the size and placement of acoustic transducers  502 A-D. Additionally, it should be understood that although illustrated as transmit and receive transducers, in some examples, the transmit and receive functions can be divided (e.g., between two transducers in proximity to one another, rather than one transmit and receive transducer transducer). 
     The position of a touch  506  from an object in contact with surface  502  can be determined by calculating TOF measurements in a measurement cycle using each of acoustic transducers  502 A-D. For example, in a first measurement step of the measurement cycle, acoustic transducer  502 A can transmit an acoustic wave and receive reflections from the acoustic wave. When no object is present, the received reflection will be the reflection from the acoustic wave reaching the opposite edge of surface  500 . However, when an object is touching surface  500  (e.g., corresponding to touch  506 ), a reflection corresponding to the object can be received before receiving the reflection from the opposite edge. Based on the received reflection corresponding to the object received at transducer  502 A, the system can determine a distance to the edge (e.g., leading edge) of touch  506 , marked by boundary line  510 A. Similar measurements can be performed by transducers  502 B,  502 C and  502 D to determine a distance to the remaining edges of touch  506 , indicated by boundary lines  510 B,  510 C and  510 D. Taken together, the measured distances as represented by boundary lines  510 A- 510 D can form a bounding box  508 . In some examples, based on the bounding box, the acoustic touch sensing system can determine the area of the touch (e.g., the area of the bounding box). Based on the bounding box, the acoustic touch sensing system can determine position of touch  506  (e.g., based on a centroid and/or area of the bounding box). 
     The acoustic touch sensing scan described with reference to  FIG. 5A  can correspond to the active mode detection scan, described above with reference to  FIG. 3B , that can be used to determine the position/location of an object touching the surface. 
       FIG. 5B  illustrates an exemplary timing diagram  560  for an acoustic touch sensing scan described in  FIG. 5A  according to examples of the disclosure. As illustrated in  FIG. 5B , each of the transducers can transmit acoustic waves and then receive reflected waves in a series of measurement steps. For example, from t 0  to t 1  a first transducer (e.g., acoustic transducer  502 A) can be stimulated, and reflections at the first transducer can be received from t 1  to t 2 . From t 2  to t 3  a second transducer (e.g., acoustic transducer  502 B) can be stimulated, and reflections at the second transducer can be received from t 3  to t 4 . From t 4  to t 5  a third transducer (e.g., acoustic transducer  502 C) can be stimulated, and reflections at the third transducer can be received from t 5  to t 6 . From t 6  to t 7  a fourth transducer (e.g., acoustic transducer  502 D) can be stimulated, and reflections at the fourth transducer can be received from t 7  to t 8 . Although the transmit (Tx) and receive (Rx) functions are shown back-to-back in  FIG. 5B  for each transducer, in some examples, gaps can be included between Tx and Rx functions for a transducer (e.g., to minimize capturing portions of the transmitted wave at the receiver), and or between the Tx/Rx functions of two different transducers (such that acoustic energy and the transients caused by multiple reflections from a scan by one transducer does not impact a scan by a second transducer). In some examples, unused transducers can be grounded (e.g., by multiplexers/demultiplexers). 
     The distance between an object touching the surface and a transducer can be calculated based on TOF principles. The acoustic energy received by transducers can be used to determine a timing parameter indicative of a leading edge of a touch. The propagation rate of the acoustic wave through the material forming the surface (and the polarizer) can be a known relationship between distance and time. Taken together, the known relationship between distance and time and the timing parameter can be used to determine distance.  FIG. 5C  illustrates an exemplary timing diagram according to examples of the disclosure.  FIG. 5C  illustrates the transducer energy output versus time. Signal  550  can correspond to the acoustic energy at the transducer from the generation of the acoustic wave at a first edge of the surface. Signal  552  can correspond to the acoustic energy at the transducer received from the wave reflected off of a second edge opposite the first edge of the surface. Due to the known distance across the surface from the first edge to the opposite the second edge and the known or measured propagation rate of the acoustic signal, the reflection off of the opposite edge of the surface occurs at a known time. Additionally, one or more objects (e.g., fingers) touching the surface can cause reflections of energy in the time between the generation of the wave and the edge reflection (i.e., between signals  550  and  552 ). For example, signals  556  and  554  can correspond to reflections of two objects touching the surface (or a leading and trailing edge of one object). It should be understood that signals  550 - 556  are exemplary and the actual shape of the energy received can be different in practice. 
     In some examples, the timing parameter can be a moment in time that can be derived from the reflected energy. For example, the time can refer to that time at which a threshold amplitude of a packet of the reflected energy is detected. In some examples, rather than a threshold amplitude, a threshold energy of the packet of reflected energy can be detected, and the time can refer to that time at which a threshold energy of the packet is detected. The threshold amplitude or threshold energy can indicate the leading edge of the object in contact with the surface. In some examples, the timing parameter can be a time range rather than a point in time. To improve the resolution of a TOF-based sensing scheme, the frequency of the ultrasonic wave and sampling rate of the receivers can be increased (e.g., so that receipt of the reflected wave can be localized to a narrower peak that can be more accurately correlated with a moment in time). 
     In some examples, transducers  502 A-D can operate in a time multiplexed manner, such that each transducer transmits and receives an acoustic wave at a different time during a measurement cycle so that the waves from one transducer do not interfere with waves from another transducer. In other examples, the transducers can operate in parallel or partially in parallel in time. The signals from the respective transducers can then be distinguished based on different characteristics of the signals (e.g., different frequencies, phases and/or amplitudes). 
     Although four transducers are illustrated in  FIG. 5A , in some examples, fewer transducers can be used. For example, when using an input object with known dimensions, as few as two transducers can be used.  FIG. 5D  illustrates an exemplary acoustic touch sensing system configuration using two acoustic transducers  502 A and  50 B mounted along two perpendicular edges (e.g., one horizontal edge and one vertical edge) of a surface  500  (surface  500  is omitted for clarity of illustration). An object in contact within the active region  504  of the surface (represented by touch  516 ) can be an object with known dimensions. For example, a stylus tip can have a known size and shape (e.g., a diameter of 1-2 mm). As described above with respect to  FIG. 5A , a first distance illustrated by boundary line  520 A can be measured by the TOF of an acoustic wave transmitted and received by transducer  502 A, and a second distance illustrated by boundary line  520 B can be measured by the TOF of an acoustic wave transmitted and received by transducer  502 B. Based on the known dimensions of object, bounding box  518  can be formed (e.g., by adding the diameter of object to the first and second distances). Based on the bounding box, the acoustic touch sensing system can determine position of touch  516  (e.g., based on a centroid). In some examples, the position can be determined based on the two measured distances without requiring forming the bounding box (e.g., the position estimating algorithm can use the dimensions of the object and the two measured distances to calculate the centroid). 
     In some examples, a user&#39;s finger(s) can be characterized such that a two transducer scheme can be used to detect touches by one or more fingers. In some examples, user input can be primarily from an index finger. The user&#39;s index finger can be characterized (e.g., dimensions or size) and the bounding box scheme can be applied using two TOF measurements and the finger characteristics. In some examples, multiple fingers can be characterized. During operation, the finger(s) can be identified and then the characteristics of the identified finger(s) can be used with two TOF measurements to determine position. 
       FIGS. 5A and 5D  illustrate detection of a single object. In some examples, however, the acoustic touch sensing system can be configured to detect multiple touches.  FIG. 5E  illustrates an exemplary acoustic touch sensing system configuration configured to detect multiple touches. The acoustic touch sensing system can include four acoustic transducers  502 A- 502 D and an active area  504  as described above with respect to  FIG. 5A . Instead of one object touching within active area  504 , in  FIG. 5E  two objects can be touching within the active area  504 . The two objects, however, can create an ambiguity in the acoustic touch sensing system regarding the positions of the two objects. The two objects can correspond to either touches  526 A and  526 B or to touches  526 C and  526 D. Two of the touches can be actual touches and the other two of the touches can be phantom touches. 
     For example, TOF measurements can be performed by using transducers  502 A,  502 B,  502 C and  502 D to determine a distance to the two objects. For example, transducer  502 A can receive two packets of reflected acoustic energy corresponding to the two objects (e.g., as illustrated in  FIG. 5C , for example). A first TOF distance to the edge of either touch  526 A or touch  526 C can be marked by boundary line  530 A, and a second TOF distance to the edge of either touch  526 B or touch  526 D can be marked by boundary line  532 A. Likewise, transducer  502 B can be used to determine a boundary line  530 B corresponding to touch  526 A or touch  526 D, and a boundary line  532 B corresponding to touch  526 B or touch  526 C. Transducer  502 C can be used to determine a boundary line  530 C corresponding to touch  526 B or touch  526 C, and a boundary line  532 C corresponding to touch  526 A or touch  526 D. Transducer  502 D can be used to determine a boundary line  530 D corresponding to touch  526 B or touch  526 D, and a boundary line  532 D corresponding to touch  526 A or touch  526 C. Taken together, boundary lines  530 A-D and  532 A-D can form bounding boxes  538 A-D. For example, bounding box  538 A can be formed from boundary lines  530 A,  530 B,  532 C and  532 D. Similarly, bounding box  538 D can be formed from boundary lines  532 A,  530 B,  532 C and  530 D. 
     In some examples, the two actual touches can be disambiguated when they are sequential. The first touch can be registered and then the second sequential touch can be disambiguated based on the first touch. For example, in the example illustrated in  FIG. 5E , if touch  526 A is detected first, then in the subsequent measurement cycle the two touches can be determined to be touches  526 A and  526 B. In contrast, if touch  526 C is detected first, then in the subsequent measurement cycle the two touches can be determined to be touches  526 C and  526 D. As long as the touches remain far enough apart to be resolved into separate bounding boxes (and assuming the touch contact moves only small amounts between each measurement interval), the two touches can be tracked. In practice, the apparently simultaneous multi-touch by a user can be viewed as sequential touches if the acquisition time (measurement cycle) of the acoustic sensors is short enough to register the sequence. Thus, if the measurement cycle repeats frequently enough, the acoustic touch sensing system can disambiguate the multiple touches with four transducers. 
     In some examples, e.g., when multiple touches cannot be resolved, bounding box  528  can be used to determine the position of touch. Bounding box  528  can be formed from boundary lines  530 A-D. 
     The multi-touch capabilities described with reference to  FIG. 5E  can be limited based on the disambiguation requirements (e.g., sequential contact and tracking). In some examples, multi-touch capabilities can be provided by increasing the number of transducers in the system.  FIG. 5F  illustrates an exemplary acoustic touch sensing system configuration configured to detect multiple touches. The acoustic touch sensing system in  FIG. 5F  can include one or more transducers  542  and  544  arranged along edges of the surface and forming active area  504 . Each of the transducers  542  and  544  can transmit acoustic waves and measure the reflections to determine the presence and location of one or more objects. For example, as illustrated, bounding box  548 A can be formed around touch  546 A based on TOF measurements from eights transmitters, and bounding box  548 B can formed around touch  546 B based on TOF measurements from four of the transmitters. Multiple transducers can also be implemented in place of the two transducers illustrated in  FIG. 5D . 
     In some examples, the arrangement of multiple transducers illustrated in  FIG. 5F  can be implemented without the multi-touch capability described with respect to  FIG. 5F . Instead, the multiple transducers on each of the sides can be coupled together and can act as a single transducer on each of the four sides as described with reference to  FIGS. 5A and 5E  (or on two sides as described with reference to  FIG. 5D ). 
     TOF schemes described with reference to  FIGS. 5A-5F  can provide for touch sensing capability using a limited number of transducers, which can simplify the transmitting and receiving electronics (e.g., as compared with capacitive touch sensing, which may require a larger number of channels), and can reduce time and memory requirements for processing. Although  FIGS. 5A-5F  discuss using a bounding box based on TOF measurements to determine position of an object, in other examples, different methods can be used, including applying matched filtering to a known transmitted ultrasonic pulse shape, and using a center of mass calculation on the filtered output (e.g., instead of a centroid). 
     As described herein, a polarizer (e.g., polarizer  220 ,  256 ) can be disposed between a transducer and a surface in which the acoustic waves propagate. For a water-agnostic acoustic touch sensing system, the transducer can be shear-polarized to generate primarily shear horizontal waves with displacement within the surface parallel to the top and bottom of the surface (e.g., in-plane displacement). The polarizer can be designed to filter out other non-shear modes (e.g., compressional waves, Lamb waves, etc.), that may be generated due to discontinuous boundary conditions between the transducer and surface, and that may interact with water due to out-of-plane displacement. The polarizer can selectively absorb or reflect back a wave with specific displacement field direction while it is transparent to other type of waves having different displacement field direction. In some examples, the polarizer can be an electro-elastic piezoelectric polarizer with one or more layers of piezoelectric material. In some examples, the polarizer can be a magneto-elastic polarizer. In some examples, the polarizer can be a mechanical polarizer with multiple layers. Although each of the above polarizers is described herein separately, in some examples, an ultrasonic polarizer can be formed from combinations of elastic piezoelectric, magneto-elastic, and/or mechanical polarizer layers. 
     Mechanical Polarizer 
     In some examples, the polarizer can be a mechanical polarizer with multiple layers. Due to the differences between shear velocity (transverse velocity) and compressional velocity (longitudinal velocity) between materials, the frequency bandwidth shift in passband frequency of a multi-layer structure can be created between compressional and shear waves. For example, for a material such as steel or aluminum with a Poisson ratio of approximately 0.3, the resonant frequency for compressional waves can be approximately 1.6 times larger than the resonant frequency for shear waves. The resonant frequency for a polarizer layer can be calculated approximate by the expression 
                 f   resonance     =     v     2   ·   t         ,         
where f resonance  can represent the resonant frequency of the layer, v can represent the wave velocity (e.g., shear or compressional), and t can represent the thickness of the layer. At or near the resonance frequency of the layer for compressional waves, the polarizer can attenuate compressional waves, and thus this resonance frequency can be a starting point for selecting and designing some or all layers of the polarizer. The multi-layer polarizer, however, may not share the resonance of individual layers. For these multilayer stacks, the resonant frequency of each layer can depend on the material properties of that layer and its neighboring layer(s). Thus, for a multi-layer polarizer, the passband (or stopband) characteristics can be designed or derived using finite element analysis (FEA) stimulation or equations. Adjusting the material and thickness of each layer in the multi-layer polarizer, can provide for efficiently passing acoustic waves in some frequency bands and stopping (e.g., attenuating or damping) acoustic waves in other frequency bands.
 
     In some examples, a mechanical polarizer can be formed from two layers having different acoustic characteristics.  FIG. 6A  illustrates an exemplary stack-up of an exemplary multi-layer polarizer including two layers according to examples of the disclosure. Multi-layer polarizer  600  can include a first layer  602  and a second layer  604 . The first layer  602  can be coupled to a transducer (e.g., transducer  204 ,  254 ) and the second layer  604  can be coupled to a surface (e.g., surface  252 ). In some examples, the first layer can be formed from a material with a relatively low impedance characteristic and the second layer can be formed from a material with a relatively high impedance characteristic. For example, the first layer can be formed from a soft material such as silicone, epoxy or pressure sensitive adhesive, etc. and the second layer can be formed of a hard material such as steel, silicon, glass, aluminum, tungsten, alloys etc. As used herein, reference to a material as hard (stiff) or soft can refer to a materials Young&#39;s modulus or Shear modulus. The larger the Young&#39;s modulus and/or shear modulus (and these parameters often scale together) the harder a material can be and the smaller the Young&#39;s modulus and/or shear modulus the softer a material can be. As used herein materials with a Young&#39;s modulus greater than or equal to 20 GPa can be considered hard or stiff and materials with a Young&#39;s modulus less than 5 GPa can be considered soft. The first layer  602  can have a thickness, T 1 , in the z-direction and the second layer  604  can have a thickness, T 2 , in the z-direction, where T 2 &lt;T 1 . In some examples, T 1  can be between 100 μm and 250 μm (e.g., 130 μm) and T 2  can be between 25 μm and 100 μm (e.g., 70 μm). The dimensions of the polarizer in the x-y plane can be the same as (or within a threshold tolerance of) the dimensions of the transducer on which it is disposed. The stiffness (i.e., a characteristic of the type of material) and thickness of each layer can be selected to ensure separation of the passbands for compressional waves and for shear waves. 
     In some examples, a mechanical polarizer can be formed from more than two layers. For example, a polarizer can be formed from multiple polarizer cells, each cell including two layers.  FIG. 6B  illustrates an exemplary stack-up of an exemplary multi-layer polarizer including more than two layers according to examples of the disclosure. Multi-layer polarizer  610  can include multiple polarizer cells  620 ,  622  disposed on top of one another. Polarizer cell  620 , for example, can include a first layer  612  and a second layer  614 . The first layer  612  of polarizer cell  620  can be coupled to a transducer (e.g., transducer  204 ,  254 ). Polarizer cell  622 , for example, can include layer N-1  616  and layer N  618 . Layer N  618  can be coupled to a surface (e.g., surface  252 ). Each polarizer cell  620 ,  622  can include one layer formed from a material with a relatively low impedance characteristic (e.g., epoxy) and one layer formed from a material with a relatively high impedance characteristic (e.g., metal). Each layer can have a thickness, T 1 -T N , in the z direction. The x-y dimensions of each layer of the transducer can be the same as (or within a threshold tolerance of) the transducer on which the polarizer is disposed. 
     In some examples, each polarizer cell  620 ,  622  can use the same materials and corresponding thicknesses. For example, the multi-layer polarizer  610  can be constructed by alternating layers of a low-impedance material of a first thickness and a high-impedance material of a second thickness. In such a polarizer, the odd layers can be made of the same first material having the same first thickness and the even layers can be made of the same second material having the same second thickness.  FIG. 7A  illustrates an exemplary stack-up  700  including surface  702 , transducer  704  and multi-layer polarizer  706  according to examples of the disclosure. Polarizer  706  can be disposed between surface  702  (e.g., front crystal) and transducer  704 . Polarizer  706  can include three polarizer cells  708 ,  710  and  712 , with each polarizer cell including a first layer of a first material M 1  of a first thickness T 1  and a second layer of a second material M 2  of a second thickness T 2 . M 1  can be a soft, low-acoustic-impedance material and M 2  can be a hard, high-impedance material. Thicknesses T 1  and T 2  can be different thicknesses. In some examples, polarizer  706  can include alternating layers of the first material M 1  and second material M 2  (e.g., each of polarizer cells  708 ,  710  and  712  use M 1  and M 2 ), but the thicknesses of one or both layers M 1  and M 2  may be different between polarizer cells. 
     In some examples, polarizer cells  620 ,  622  can use different materials and/or corresponding thicknesses. For example, the multi-layer polarizer  610  can be constructed by alternating layers of different low-impedance materials and different high-impedance materials, and each of the layers can be a different thickness as well.  FIG. 7B  illustrates an exemplary stack-up  720  including surface  722 , transducer  724  and multi-layer polarizer  726  according to examples of the disclosure. Polarizer  726  can be disposed between surface  722  (e.g., front crystal) and transducer  724 . Polarizer  726  can include two polarizer cells  728  and  730 , with a first polarizer cell  728  including a first layer of a first material M 1  of a first thickness T 1  and a second layer of a second material M 2  of a second thickness T 2 , and with a second polarizer cell  730  including a third layer of a third material M 3  of a third thickness T 3  and a fourth layer of a fourth material M 4  of a fourth thickness T 4 . M 1  and M 3  can be different soft, low-acoustic-impedance materials (e.g., epoxy, silicone, etc.) and M 2  and M 4  can be different hard, high-impedance materials (aluminum, steel, etc.). Thicknesses T 1 , T 2 , T 3  and T 4  can be different thicknesses. 
     In some examples, some materials may be the same and some materials may be different between polarizer cells and some of the thicknesses may be the same and some of the thicknesses may be different between polarizer cells.  FIG. 7C  illustrates an exemplary stack-up  740  including surface  742 , transducer  744  and multi-layer polarizer  746  according to examples of the disclosure. Polarizer  746  can be disposed between surface  742  (e.g., front crystal) and transducer  744 . Polarizer  746  can include two polarizer cells  748  and  750 , with a first polarizer cell  748  including a first layer of a first material M 1  of a first thickness T 1  and a second layer of a second material M 2  of a second thickness T 2 , and with a second polarizer cell  750  including a third layer of the first material M 1  of a third thickness T 3  and a fourth layer of a third material M 3  of the second thickness T 2 . M 1  can be the same soft, low-acoustic-impedance material and M 2  and M 3  can be different hard, high-impedance materials. Thicknesses T 1 , T 2  and T 3  can be different thicknesses. 
     In some examples, a polarizer may include a plurality of polarizer cells and some of the polarizer cells may be the same (same materials and thicknesses) and other polarizer cells may be different (different material(s) and thickness(es)).  FIG. 7D  illustrates an exemplary stack-up  760  including surface  762 , transducer  764  and multi-layer polarizer  766  according to examples of the disclosure. Polarizer  766  can be disposed between surface  762  (e.g., front crystal) and transducer  764 . Polarizer  766  can include three polarizer cells  768 ,  770  and  772 . A first polarizer cell  768  and a third polarizer cell  772  can each include a first layer of a first material M 1  of a first thickness T 1  and a second layer of a second material M 2  of a second thickness T 2 . A second polarizer cell  770  can include a third layer of a third material M 3  of a third thickness T 3  and a fourth layer of a fourth material M 4  of a fourth thickness T 4 . M 1  and M 3  can be different soft, low-acoustic-impedance materials and M 2  and M 4  can be different hard, high-impedance materials. Thicknesses T 1 , T 2 , T 3  and T 4  can be different thicknesses. 
     Multi-layer polarizers (e.g., polarizers  600 ,  610 ,  706 ,  726 ,  746 ,  766 ) can provide wideband efficiency (e.g., on the order of a 100 kHz bandwidth or MHz bandwidth). For example, a bandwidth of a polarizer for use with a shear-polarized transducer polarizer can be defined by a range of frequencies for which the transmission efficiency for shear waves (or another wave of interest) is above a first threshold and the transmission efficiency for compressional waves (or another parasitic wave to the wave of interest) is below a second threshold. In some examples, the first threshold can be a transmission efficiency of 90% for shear waves and the second threshold can be a transmission efficiency of 10% for compressional waves. It should be understood that these thresholds are exemplary and addition thresholds are possible (e.g., first threshold of 60%, 70%, 80%, 90%; second threshold of 15%, 10%, 5%, 1%). Additionally, multi-layer polarizers described herein can be manufactured using conventional techniques and still provide for a passband with a high-frequency center frequency (e.g., greater than 500 kHz, greater than 1 MHz, greater than 5 MHz). 
       FIGS. 8A-8B  illustrate exemplary plots of frequency dependent transmission coefficients through an exemplary polarizer for compressional and shear waves according to examples of the disclosure.  FIG. 8A , for example, shows a passband of an exemplary polarizer with transmission coefficients greater than 70% in a frequency range between 8 and 9 MHz and a stop band with transmission coefficients less than 10% between 3 MHz and 7 MHz for compressional waves.  FIG. 8B , for example, shows a passband of an exemplary polarizer with transmission coefficients greater than 65% in a frequency range between 5 MHz and 6 MHz and a stop band with transmission coefficients less than 10% between 2 MHz and 4 MHz and between 6 MHz and 8 MHz for shear waves. Operating an acoustic touch sensing system between 5 MHz and 6 MHz can allow the polarizer to pass shear waves and reject compressional waves. 
       FIGS. 8A-8B  illustrate filter characteristics for one exemplary polarizer. The specific filter characteristics of the multi-layer polarizer can be optimized for an application by adjusting the material properties, thickness of layers and number of layers. The characteristics can include the center frequency of the passband for compressional and shear waves (which can be a function of the Young&#39;s modulus and/or shear modulus of the selected materials and thickness of the layers), the filter quality and the fractional bandwidth. For example, adding additional layers to the polarizer can be equivalent to increasing the order of a traditional filter, which generally improves the quality of the filter. A higher order filter can have a wider broadband response and a higher stopband attenuation for compressional waves. As a result, adding more layers may prevent the dual peaks in the high-frequency passbands for compressional and shear waves and widen the bandwidth of the passbands (as compared with  FIGS. 8A and 8B ), but the additional layers may tradeoff the peak transmission coefficient (which may be reduced in the passband) and the overall thickness to the polarizer (which may be limited by the space available in the application and the manufacturability of thin layers). Additionally, a higher impedance mismatch between layers (resulting from an optimization of the types of materials selected and their selected thicknesses) can improve filter selectivity. However, the choice of materials and thickness may be limited by manufacturability, reliability and cost. 
     Additionally, the selection of the material properties, thickness of layers and number of layers can be selected to ensure proper separation between the compressional and shear wave passbands. In some examples, for example as illustrated in  FIGS. 8A-8B , the passbands for both shear and compressional waves can be well-separated such that the transmission coefficient can be less than a threshold (e.g., 5%, 1%) for a threshold frequency range (e.g., 10 kHz, 100 kHz, 1 MHz) between the shear passband and the compressional passband. In some examples, the filter performance may be sufficient even if the passbands for shear and compressional waves can partially overlap so long as there is sufficient frequency bandwidth in which the shear waves can be passed (above a threshold transmission coefficient, e.g., 50%, 60%) and the compressional waves can be stopped (below a threshold transmission coefficient, e.g., 20%, 10%, 5%). 
     The multi-layer polarizer of  FIGS. 6A-6B and 7A-7D  corresponds to a one-dimensional filter structure which provides significant filter quality for plane waves having a propagation direction perpendicular to the plane of the polarizer. However, the filter quality can degrades for waves that are transmitted to the surface or reflect back from the surface with a different angle. The angular dependence of the polarizer can be overcome, in some examples, by using a multi-dimensional (e.g., two-dimensional) polarizer structure. The two-dimensional filter structure can be designed using photonic or phononic crystals having two-dimensional periodicity to provide the requisite filter quality with less or no angular dependence. 
       FIGS. 9A-9B  illustrate exemplary multi-dimensional polarizer structures according to examples of the disclosure. Polarizer  900  of  FIG. 9A  or polarizer  910  of  FIG. 9B  can be disposed between a surface and a transducer. Unlike a one-dimensional polarizer structure (e.g., illustrated in  FIG. 6A ), the two-dimensional structure of polarizer  900  can, for example, include strips of a hard material  902  (e.g., metal, glass, silicon, etc.) embedded in a soft material  904  (e.g., epoxy, PSA, rubber, etc.). For purposes of illustration, the outer layer of soft material  904  forming polarizer  900  is shown peeled away in the foreground to shown the hard material embedded therein. The strips of hard material  902  can be separated from one another in the y-direction and z-direction of the axes illustrated in  FIG. 9A . In some examples, rather than strips of hard material, polarizer  910  can include cubes of hard material  912  embedded in the soft material  914 . For purposes of illustration, the outer layer of soft material  914  forming polarizer  910  is shown peeled away in the foreground to shown the hard material embedded therein. The cubes of hard material  912  can be separated from one another in the x-direction, y-direction and z-direction by the soft material. Although described and illustrated as strips and evenly spaced hard materials having rectangular or square shapes, it should be understood that other shapes and patterns can be used for multi-dimensional polarizers. The proportions and geometry of these patterns could be approximated by hand calculations and verified by simulation, for example, to ensure the correct filter characteristic for the multi-dimensional polarizer. Additionally, the filter characteristics for multi-dimensional filters can dependent on the type materials selected. 
     Electro-elastic Piezoelectric Polarizer 
     In some examples, the polarizer can be an electro-elastic piezoelectric polarizer with one or more layers of piezoelectric material. One or more circuits coupled to the one or more layers of piezoelectric material can attenuate or damp compressional acoustic waves by dissipating the electrical energy extracted by the respective layer of piezoelectric material. 
     In some examples, a polarizer can be formed from a layer of piezoelectric material and a corresponding electric circuit.  FIG. 10A  illustrates an exemplary stack-up of an acoustic touch sensing system including a polarizer with a layer of piezoelectric material according to examples of the disclosure. Stack-up  1000  can include a polarizer  1004  disposed between surface  1002  and transducer  1006 . Transducer  1006  can be formed from a piezoelectric material (e.g., PZT, KNN, PVDF, PLLA, etc.) and can be shear-polarized (in the poling direction shown by the arrow in transducer  1006 ) such that transducer  1006  can generate, when stimulated, shear waves which propagate in the z-direction toward surface  1002 , but whose field displacement is orthogonal to the direction of propagation (e.g., in-plane). As described herein, transducer  1006  may also generate some compressional waves whose field displacement is in the same direction as the direction of propagation (in the z-direction). Polarizer  1004  can be designed to filter out these compressional waves, which may interact with water. 
     Polarizer  1004  can include a layer of piezoelectric material  1010  and a corresponding electric circuit  1008 . The layer of piezoelectric material  1010  can be polarized in a direction different than the polarization of transducer  1006 . In some examples, the poling direction of the layer of piezoelectric material  1010  (shown by the arrow in the layer of piezoelectric material  1010 ) can be orthogonal to the poling direction of the shear-polarized transducer  1006 . Compressional waves propagating from transducer  1006  into surface  1002  through polarizer  1004  can couple with the layer of piezoelectric material  1010  and the mechanical energy of the compressional wave can be converted to electrical energy (e.g., due to the orthogonal poling of the piezoelectric layer with respect to the mechanical vibration displacement of the compressional wave). The converted electrical energy can be transferred to electric circuit  1008  and can be dissipated (e.g., converted into heat). In some examples, the electric circuit  1008  can feed electrical energy back into the system at a different phase and can cancel out the incoming compressional wave. Shear waves propagating from transducer  1006  into surface  1002  through polarizer  1004  can pass through the layer of piezoelectric material  1010  without interacting and damping the shear waves. 
     Electrodes  1012  and  1014  illustrated in stack-up  1000  and can be used to couple electrical energy from the layer of piezoelectric material  1010  to electric circuit  1008  or to couple feedback energy from the electric circuit  1008  to the layer of piezoelectric material  1010 . Although  FIG. 10A  only illustrates electrodes for polarizer  1004 , it should be understood that stack-up  1000  can also include electrodes for transducer  1006  to stimulate and/or receive acoustic energy. In some examples transducer  1006  can include two electrodes on opposite sides of transducer  1006  in a similar manner that electrodes  1012  and  1014  are disposed on opposite sides of the layer of piezoelectric material  1010  of polarizer  1004 . In some examples, the adjacent respective electrodes for the polarizer  1004  (e.g., electrode  1014 ) and transducer  1006  (not-shown) can be isolated from one another. In some examples, electrode  1014  can be a shared electrode between polarizer  1004  and transducer  1006 . For example, electrode  1014  can be a ground terminal for both polarizer  1004  and transducer  1006 . 
       FIGS. 11A and 11B  illustrate exemplary electric circuits for use with an exemplary polarizer according to examples of the disclosure. The exemplary circuits  1100  and  1110  of  FIGS. 11A and 11B  can correspond to electric circuit  1008  of  FIG. 10A , for example. In some examples, exemplary circuit  1100 , including a resistor  1102 , can be used. A first terminal  1104  of circuit  1100  (corresponding to terminal  1016  in  FIG. 10A ) can be coupled to the layer of piezoelectric material via electrode  1012 . A second terminal  1106  of circuit  1100  (corresponding to terminal  1018  in  FIG. 10A ) can be coupled to the layer of piezoelectric material via electrode  1014 . Resistor  1102  can be coupled between terminals  1104  and  1106  to dissipate electrical energy generated by compressional waves interacting with the layer of piezoelectric material. In some examples, exemplary circuit  1110 , including a resistor  1112  and an inductor  1114 , can be used. A first terminal  1116  of circuit  1110  (corresponding to terminal  1016  in  FIG. 10A ) can be coupled to the layer of piezoelectric material via electrode  1012 . A second terminal  1118  of circuit  1100  (corresponding to terminal  1018  in  FIG. 10A ) can be coupled to the layer of piezoelectric material via electrode  1014 . Resistor  1112  and inductor  1114  can be coupled in series between terminals  1116  and  1118  to dissipate electrical energy generated by compressional waves interacting with the piezoelectric material and to shift the phase of some electrical energy and feed the phase-shifted electrical energy back into the layer of piezoelectric material to dampen the incoming compressional waves. Although  FIGS. 11A and 11B  include resistors to convert electrical energy to heat, other components can be used to convert electrical energy to heat (e.g., inductors, capacitors, transistors, diodes, active circuits, etc.). More generally, the electric circuit (e.g., electric circuit  1008  of  FIG. 10A ) can have an impedance Z to dissipate electrical energy. The electric circuit can include active electric components (e.g., transistors) and/or passive electric components (e.g., resistors) coupled in series between a polarizer electrode and a ground. For example, the electric circuit can include a resistor, an inductor and a capacitor (RLC circuit) in series between a polarizer electrode and ground. Although  FIG. 11B  illustrates an inductor to phase shift and feedback electrical energy, in other examples, different circuitry can be used. For example, a variable voltage or current source can be used to provide phase-shifted feedback to dampen or attenuate the incoming compressional waves in the polarizer. 
     The efficiency of the damping of single-cell polarizer  1004  can be characterized by a mechanical-to-electrical efficiency measuring the ability to convert mechanical energy of compressional acoustic waves into electrical energy in the layer of piezoelectric material  1010  and characterized by an electrical-to-heat efficiency measuring the ability to convert electrical energy of the compressional acoustic wave into heat in electric circuit  1008 . For example, a layer of PZT with a mechanical-to-electrical efficiency of 70% coupled to an electric circuit with an electrical-to-heat efficiency of 70% can attenuate compressional energy by 49%. Including an inductor can further attenuate compressional energy by canceling at least a portion of the incoming compressional wave. In some examples, a multi-layer polarizer can be used to further attenuate or dampen compressional energy. 
       FIG. 10B  illustrates an exemplary stack-up of an acoustic touch sensing system including a polarizer with multiple layers of piezoelectric material according to examples of the disclosure. Stack-up  1020  can include a polarizer  1024  disposed between surface  1022  and transducer  1026 . Transducer  1026  can be formed from a piezoelectric material (e.g., PZT, KNN, PVDF, PLLA, etc.) and can be shear-polarized such that transducer  1026  can generate, when stimulated, shear waves which propagate in the z-direction toward surface  1022 , but whose field displacement is orthogonal to the direction of propagation (e.g., in-plane). As described herein, transducer  1026  may also generate some compressional waves whose field displacement is in the same direction as the direction of propagation. Polarizer  1024  can be designed to filter out these compressional waves, which may interact with water. 
     Polarizer  1024  can include multiple polarizer cells  1028 ,  1030 , with each polarizer cell including a layer of piezoelectric material  1032 ,  1040  and a corresponding electric circuit  1034 ,  1042 . The layer of piezoelectric material  1032 ,  1040  of each respective polarizer cell  1028 ,  1030  can be polarized in a direction different than the polarization of transducer  1026 . In some examples, the poling direction of the layers of piezoelectric material  1032 ,  1040  can be orthogonal to the poling direction of the shear-polarized transducer  1026 . Compressional waves propagating from transducer  1026  into surface  1022  through polarizer  1024  can couple with the layers of piezoelectric material  1032 ,  1040  and the mechanical energy of the compressional wave can be converted to electrical energy (e.g., due to the orthogonal poling of the piezoelectric layers with respect to the displacement field of the compressional wave). The converted electrical energy can be transferred to electric circuits  1034 ,  1042  and can be dissipated (e.g., converted into heat). In some examples, the electric circuits  1034 ,  1042  can feed electrical energy back into the system at a different phase and can cancel out the incoming compressional wave. Shear waves propagating from transducer  1026  into surface  1022  through polarizer  1024  can pass through the layers of piezoelectric material  1032 ,  1040  without interacting and damping the shear waves. 
     Each of polarizer cells  1028 ,  1030  can include electrodes to couple a respective layer of piezoelectric material to a respective electric circuit. For example, electrodes  1036  and  1038  illustrated in stack-up  1020  can be used to couple electrical energy from the layer of piezoelectric material  1032  to electric circuit  1034  or to couple energy from the electric circuit  1034  to the layer of piezoelectric material  1032 . Likewise, electrodes  1044  and  1046  can be used to couple together the layer of piezoelectric material  1040  and electric circuit  1042 . Although  FIG. 10B  only illustrates electrodes for polarizer cells  1028 ,  1030 , it should be understood that stack-up  1020  can also include electrodes for transducer  1026  to stimulate and/or receive acoustic energy. Additionally, although two electrodes are shown for each polarizer cell, it should be understood that in some examples, an electrode could be shared between polarizer cells (and/or between a polarizer cell and transducer  1026 ). 
     The respective electric circuits  1034 ,  1042  can be implemented with circuits like those illustrated in  FIGS. 11A and 11B . In some examples, each polarizer cell in polarizer  1024  can use the same type of electric circuit. In some examples, different types of electric circuits can be used for different polarizer cells.  FIG. 11C  illustrates an exemplary electric circuit representing multiple electric circuits for use with an exemplary multi-layer polarizer according to examples of the disclosure. The exemplary circuit  1120  can correspond to the electric circuits  1034 ,  1042  of  FIG. 10B , for example. In some examples, exemplary circuit  1120  can include terminals  1126 ,  1128 ,  1130 ,  1132  that can be coupled to electrodes  1036 ,  1038 ,  1044 ,  1046 . For example, terminal  1132  can be coupled to electrode  1036 , terminal  1130  can be coupled to electrode  1038 , terminal  1128  can be coupled to electrode  1044  and terminal  1126  can be coupled to electrode  1046 .  FIG. 11C  illustrates four terminals for two piezoelectric layers, but additional terminals and circuitry can be included in electric circuit  1120  for additional piezoelectric layers. Circuit  1120  can include a resistor  1122  and inductor  1124  coupled in series for each polarizer cell to dissipate electrical energy generated by compressional waves interacting with the piezoelectric material and to shift the phase of some electrical energy and feed the phase-shifted electrical energy back into the layer of piezoelectric material to dampen the compressional waves. In some examples, the inductor can be omitted. Although  FIG. 11C  includes resistors and/or inductors to convert electrical energy to heat or to phase shift and feedback electrical energy, other components can be used to convert electrical energy to heat and/or to phase shift and feedback electrical energy. 
     The efficiency of the damping of multi-cell polarizer  1024  can be characterized by the mechanical-to-electrical efficiency and electrical-to-heat efficiency of each of the polarizer cells. For example, polarizer cell  1030  can include a layer of PZT with a mechanical-to-electrical efficiency of 70% coupled to an electric circuit (e.g., including a resistor) with an electrical-to-heat efficiency of 70% that can attenuate compressional energy by approximately 49%. Polarizer cell  1028  can be identical and can attenuate the compressional energy by approximately 49%, such that a two-cell polarizer can attenuate compressional energy by approximately 74%. Adding an additional polarizer cell of the same type could provide a three-cell polarizer with the ability to attenuate approximately 86% of the compressional energy. Including inductors can further attenuate compressional energy by canceling at least a portion of the incoming compressional wave. 
     In some examples, each polarizer cell can be the same (as described above). In some examples, polarizer cells can be different. For example, different polarizer cells can use the same or different materials of the same or different thicknesses along with an electric circuit using the same or different resistance and/or inductance values. The type of material, thickness, resistance and inductance can be used to determine the filtering characteristic of the polarizer cell. Whether the polarizer cells are the same or different, the type of material, thickness, resistance and inductance can be selected or optimized to produce the desired filter performance from the polarizer. 
       FIG. 12  illustrates exemplary performance of a polarizer according to examples of the disclosure. For example, plot  1200  illustrates the amplitude of compressional energy for a polarizer formed of multiple polarizer cells. Each polarizer cell can be formed of the same type of piezoelectric material of the same thickness, and each polarizer cell can have one electrode coupled to ground and one electrode coupled to a circuit including a series inductor (e.g., of 200 nH) and resistor (e.g., 10 ohms) to ground. As illustrated in plot  1200 , the compressional energy can be nearly zero in the bandwidth shown by the arrows. Operating the transducer to generate shear waves in this bandwidth can result in filtering out or otherwise suppressing compressional energy. In some examples, the bandwidth can be defined where the compressional energy output through the polarizer (transmission efficiency) is below a threshold (e.g., less than 10%, 5%, 1% of the compressional energy passing through the polarizer). The bandwidth can be designed to occur at relatively high frequencies (e.g., within a center frequency between 1 MHz and 10 MHz). The bandwidth, in some examples, can be greater than 50 kHz. In some examples, that bandwidth can be between 500 kHz−1 MHz. 
     In some examples, the thickness of the polarizer or the thickness of layers of piezoelectric material in multiple polarize cells can be selected such that the resonant frequency of the polarizer (or polarizer cell) matches the resonant frequency of the respective electric circuit. Matching the resonant frequency can improve the damping of the energy of the displacement field to be filtered out (e.g., compressional energy). 
     Magneto-elastic Piezoelectric Polarizer 
     In some examples, a magneto-elastic polarizer can be used.  FIG. 13  illustrates a stack-up  1300  of an exemplary magneto-elastic polarizer according to examples of the disclosure. For example, stack-up  1300  can include a polarizer  1304  disposed between surface  1302  and transducer  1306 . Transducer  1306  can be formed from a piezoelectric material (e.g., PZT, KNN, PVDF, PLLA, etc.) and can be shear-polarized (in the poling direction shown by the arrow in transducer  1306 ) such that transducer  1306  can generate, when stimulated, shear waves which propagate in the z-direction toward surface  1302 , but whose field displacement is orthogonal to the direction of propagation (e.g., in-plane). As described herein, transducer  1306  may also generate some compressional waves whose field displacement is in the same direction as the direction of propagation (in the z-direction). Polarizer  1304  can be designed to filter out these compressional waves, which may interact with water. 
     Polarizer  1304  can be formed of a magnetic or ferromagnetic material (e.g., neodymium, FINEMET, etc.) having a magnetic field in the direction indicated by the arrow in polarizer  1304 . When an elastic wave propagates in a conductive material, induced eddy currents can be generated according to the following equation 
               J   =     η   ⁡     (     E   +         ∂   u       ∂   t       ×     B   0         )         ,         
where J can correspond to the eddy current density, E can correspond to an applied electric field (zero in this case) and u can correspond to the displacement field vector and B can correspond to the magnetic field. This mathematical relation means that a first displacement field parallel to the magnetic field can induce zero current, whereas a second displacement field perpendicular to the magnetic field can introduce strong eddy currents that can dampen energy with the second displacement field. Thus, for example, shear-polarized transducer  1306  can generate shear waves with a displacement field parallel to the magnetic field of polarizer  1304  to allow shear waves to pass, whereas compressional waves with a displacement field perpendicular to the magnetic field of polarizer  1304  can generate eddy currents that can convert the mechanical compressional energy to heat through Joule heating. Joule heating, however, can have a relatively low efficiency when compared with the mechanical-to-electrical efficiency and electrical-to-heat efficiency of the electro-elastic polarizers of  FIGS. 10A, 10B .
 
     In some examples, the conductivity of the magnetic or ferromagnetic material can be adjusted for improved damping. If conductivity of the magnetic or ferromagnetic material is mismatch from the optimal conductivity (e.g., too high or too low), the damping effect can be reduced. The conductivity can be a function of frequency and material properties including, type of material and geometry. Calculations and/or simulations can be used such that the conductivity of the magneto-elastic polarizer can be optimized for improved damping of compressional waves. 
     Therefore, according to the above, some examples of the disclosure are directed to a polarizer for use with a shear-polarized transducer. The polarizer can comprise a plurality of layers including at least a first layer of a first type of material and a second layer of a second type of material different than the first type of material. A transmission coefficient of the polarizer for shear waves at one or more first frequencies in a first passband can be greater than a first threshold and a transmission coefficient of the polarizer for compressional waves at the one or more first frequencies in the first passband can be less than a second threshold less than the first threshold. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the first layer can have a first thickness and the second layer can have a second thickness different than the first thickness. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the first type of material can have a Young&#39;s modulus less than or equal to 5 GPa and the second type of material can have a Young&#39;s modulus greater than or equal to 20 GPa. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the first type of material can be an epoxy and the second type of material can be a metal. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the plurality of layers can further include at least a third layer of a third type of material and a fourth layer of a fourth type of material different from the third type of material. The fourth layer can be disposed on the third layer, the third layer can be disposed on the second layer, and the second layer can be disposed on the first layer. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the third type of material can be a same type of material as the first type of material and the fourth type of material can be a same type of material as the second type of material. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the third type of material can be a different type of material than the first type of material or the fourth type of material can be a different type of material than the second type of material. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the third layer can have a third thickness and the fourth layer can have a fourth thickness different than the third thickness. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the third thickness can be a same thickness as the first thickness and the fourth thickness can be a same thickness as the second thickness. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the third thickness can be a different thickness than the first thickness or the fourth thickness can be a different thickness than the second thickness. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the plurality of layers can comprise interleaved layers with a Young&#39;s modulus less than or equal to 5 GPa and layers with a Young&#39;s modulus greater than or equal to 20 GPa. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the first threshold can be greater than or equal to 50%. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the second threshold can be less than or equal to 10%. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the first passband can be wider than 100 kHz. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the first passband can begin at a frequency greater than 500 kHz. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the transmission coefficient of the polarizer for the compressional waves at one or more second frequencies in a second passband can be greater than a third threshold and the transmission coefficient of the polarizer for the shear waves at the one or more second frequencies in the second passband can be less than a fourth threshold less than the third threshold. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the first passband and the second passband can separated by a threshold amount. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the threshold amount can be at least 50 kHz. 
     Some examples of the disclosure are directed to an acoustic touch sensing system. The acoustic touch sensing system can comprise a surface, one or more shear-polarized transducers, and one or more polarizers. Each of the one or more polarizers can be disposed between a corresponding one of the one or more shear-polarized transducers and the surface. Each of the one or more polarizers can have a first passband for shear waves and a second passband for compressional waves. The one or more shear-polarized transducers can be configured to operate at a frequency within the first passband. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the one or more polarizers can comprise at least a one polarizer with a plurality of layers including at least a first layer of a first type of material and a second layer of a second type of material different than the first type of material. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the first layer can have a first thickness and the second layer can have a second thickness different than the first thickness. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the first type of material can have a Young&#39;s modulus less than or equal to 5 GPa and the second type of material can have a Young&#39;s modulus greater than or equal to 20 GPa. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the first type of material can be an epoxy and the second type of material can be a metal. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the plurality of layers can further include at least a third layer of a third type of material and a fourth layer of a fourth type of material different from the third type of material. The fourth layer can be disposed on the third layer, the third layer can be disposed on the second layer, and the second layer can be disposed on the first layer. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the third type of material can be a same type of material as the first type of material and the fourth type of material can be a same type of material as the second type of material. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the third type of material can be a different type of material than the first type of material or the fourth type of material can be a different type of material than the second type of material. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the third layer can have a third thickness and the fourth layer can have a fourth thickness different than the third thickness. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the third thickness can be a same thickness as the first thickness and the fourth thickness can be a same thickness as the second thickness. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the third thickness can be a different thickness than the first thickness or the fourth thickness can be a different thickness than the second thickness. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the plurality of layers can comprise interleaved layers with a Young&#39;s modulus less than or equal to 5 GPa and layers with a Young&#39;s modulus greater than or equal to 20 GPa. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the at least one polarizer can have a transmission coefficient for shear waves at one or more first frequencies in the first passband for shear waves greater than a first threshold and can have a transmission coefficient of for compressional waves at the one or more first frequencies in the first passband less than a second threshold less than the first threshold. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the first threshold can be greater than or equal to 50%. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the second threshold can be less than or equal to 10%. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the first passband can be wider than 100 kHz. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the first passband can begin at a frequency greater than 500 kHz. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the transmission coefficient of the polarizer for the compressional waves at one or more second frequencies in a second passband can be greater than a third threshold and the transmission coefficient of the polarizer for the shear waves at the one or more second frequencies in the second passband can be less than a fourth threshold less than the third threshold. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the first passband and the second passband can separated by a threshold amount. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the threshold amount can be at least 50 kHz. 
     Some examples of the disclosure are directed to a device. The device can comprise a housing, a crystal surface, one or more shear-polarized transducers, one or more polarizers, and a processor. Each of the one or more polarizers can be disposed between a corresponding one of the one or more shear-polarized transducers and the surface. Each of the one or more polarizers can have a first passband for shear waves and a second passband for compressional waves. The processor can be coupled to the one or more shear-polarized transducers and configured to stimulate the one or more shear-polarized transducers at one or more frequencies within the first passband and determine a location of an object based on reflected acoustic energy from the crystal surface. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the one or more polarizers can comprise at least a one polarizer with a plurality of layers including at least a first layer of a first type of material and a second layer of a second type of material different than the first type of material. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the first layer can have a first thickness and the second layer can have a second thickness different than the first thickness. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the first type of material can have a Young&#39;s modulus less than or equal to 5 GPa and the second type of material can have a Young&#39;s modulus greater than or equal to 20 GPa. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the first type of material can be an epoxy and the second type of material can be a metal. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the plurality of layers can further include at least a third layer of a third type of material and a fourth layer of a fourth type of material different from the third type of material. The fourth layer can be disposed on the third layer, the third layer can be disposed on the second layer, and the second layer can be disposed on the first layer. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the third type of material can be a same type of material as the first type of material and the fourth type of material can be a same type of material as the second type of material. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the third type of material can be a different type of material than the first type of material or the fourth type of material can be a different type of material than the second type of material. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the third layer can have a third thickness and the fourth layer can have a fourth thickness different than the third thickness. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the third thickness can be a same thickness as the first thickness and the fourth thickness can be a same thickness as the second thickness. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the third thickness can be a different thickness than the first thickness or the fourth thickness can be a different thickness than the second thickness. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the plurality of layers can comprise interleaved layers with a Young&#39;s modulus less than or equal to 5 GPa and layers with a Young&#39;s modulus greater than or equal to 20 GPa. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the at least one polarizer can have a transmission coefficient for shear waves at one or more first frequencies in the first passband for shear waves greater than a first threshold and can have a transmission coefficient of for compressional waves at the one or more first frequencies in the first passband less than a second threshold less than the first threshold. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the first threshold can be greater than or equal to 50%. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the second threshold can be less than or equal to 10%. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the first passband can be wider than 100 kHz. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the first passband can begin at a frequency greater than 500 kHz. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the transmission coefficient of the polarizer for the compressional waves at one or more second frequencies in a second passband can be greater than a third threshold and the transmission coefficient of the polarizer for the shear waves at the one or more second frequencies in the second passband can be less than a fourth threshold less than the third threshold. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the first passband and the second passband can separated by a threshold amount. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the threshold amount can be at least 50 kHz. 
     Some examples of the disclosure are directed to a multi-dimensional polarizer for use with a shear-polarized transducer. The multi-dimensional polarizer can comprise a multi-dimensional pattern of a first type of material embedded within a second type of material different than the first type of material. A transmission coefficient of the polarizer for shear waves at one or more first frequencies in a first passband can be greater than a first threshold and a transmission coefficient of the polarizer for compressional waves at the one or more first frequencies in the first passband can be less than a second threshold less than the first threshold. 
     Some examples of the disclosure are directed to a polarizer for use with a shear-polarized transducer. The polarizer can comprise one or more layers of piezoelectric material, one or more electrodes, and one or more circuits coupled to the one or more layers of piezoelectric material via the one or more electrodes. The polarizer can be configured to extract and dissipate energy of compressional waves and pass energy of shear waves. Additionally or alternatively to one or more of the examples disclosed above, in some examples, each of the one or more layers of piezoelectric material can have a poling direction different than a poling direction of the shear-polarized transducer. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the poling direction of each of the one or more layers of piezoelectric material can be orthogonal to the poling direction of the shear-polarized transducer. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the one or more electrodes can comprise a first electrode on a first side of a first layer of piezoelectric material of the one or more layers of piezoelectric material and a second electrode on a second side, opposite the first side, of the first layer of the piezoelectric material. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the one or more circuits can comprise a circuit comprising a resistor coupled between one of the one or more electrodes and a system ground. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the one or more circuits can comprise a circuit comprising a resistor and an inductor coupled in series between one of the one or more electrodes and a system ground. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the one or more circuits can comprise a circuit comprising one or more passive electric components or one or more active electric components coupled in series between one of the one or more electrodes and a system ground. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the one or more circuits can comprise a circuit comprising one or more passive electric components and one or more active electric components coupled in series between one of the one or more electrodes and a system ground. Additionally or alternatively to one or more of the examples disclosed above, in some examples, a first layer of piezoelectric material of the one or more layers of piezoelectric material can have a first thickness and a second layer of piezoelectric material of the one or more layers of piezoelectric material can have a second thickness different than the first thickness. Additionally or alternatively to one or more of the examples disclosed above, in some examples, a first layer of piezoelectric material of the one or more layers of piezoelectric material and a second layer of piezoelectric material of the one or more layers of piezoelectric material can be formed from a same type of material. Additionally or alternatively to one or more of the examples disclosed above, in some examples, a first layer of piezoelectric material of the one or more layers of piezoelectric material and a second layer of piezoelectric material of the one or more layers of piezoelectric material can be formed from different types of material. Additionally or alternatively to one or more of the examples disclosed above, in some examples, extracting and dissipating energy of the compressional waves can comprise attenuating compressional waves by a threshold amount within at least a first range of frequencies. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the threshold amount can be at least 90% attenuation. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the first range of frequencies can be wider than 100 kHz and includes frequencies greater than 500 kHz. 
     Some examples of the disclosure are directed to an acoustic touch sensing system. The acoustic touch sensing system can comprise a surface, one or more shear-polarized transducers, and one or more polarizers configured to extract and dissipate energy of compressional waves and pass energy of shear waves. Each of the one or more polarizers can be disposed between a corresponding one of the one or more shear-polarized transducers and the surface. At least one of the one or more polarizers can comprise: one or more layers of piezoelectric material, one or more electrodes, and one or more circuits coupled to the one or more layers of piezoelectric material via the one or more electrodes. Additionally or alternatively to one or more of the examples disclosed above, in some examples, each of the one or more layers of piezoelectric material of the at least one of the one or more polarizers can have a poling direction different than a poling direction of the corresponding one of the one or more shear-polarized transducer. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the poling direction of each of the one or more layers of piezoelectric material of the at least one of the one or more polarizers can be orthogonal to the poling direction of the shear-polarized transducer. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the one or more electrodes of the at least one of the one or more polarizers can comprise a first electrode on a first side of a first layer of piezoelectric material of the one or more layers of piezoelectric material and a second electrode on a second side, opposite the first side, of the first layer of the piezoelectric material. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the one or more circuits of the at least one of the one or more polarizers can comprise a circuit comprising a resistor coupled between one of the one or more electrodes and a system ground. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the one or more circuits of the at least one of the one or more polarizers can comprise a circuit comprising a resistor and an inductor coupled in series between one of the one or more electrodes and a system ground. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the one or more circuits of the at least one of the one or more polarizers can comprise a circuit comprising one or more passive electric components or one or more active electric components coupled in series between one of the one or more electrodes and a system ground. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the one or more circuits of the at least one of the one or more polarizers can comprise a circuit comprising one or more passive electric components and one or more active electric components coupled in series between one of the one or more electrodes and a system ground. Additionally or alternatively to one or more of the examples disclosed above, in some examples, a first layer of piezoelectric material of the one or more layers of piezoelectric material of the at least one of the one or more polarizers can have a first thickness and a second layer of piezoelectric material of the one or more layers of piezoelectric material of the at least one of the one or more polarizers can have a second thickness different than the first thickness. Additionally or alternatively to one or more of the examples disclosed above, in some examples, a first layer of piezoelectric material of the one or more layers of piezoelectric material of the at least one of the one or more polarizers and a second layer of piezoelectric material of the one or more layers of piezoelectric material of the at least one of the one or more polarizers can be formed from a same type of material. Additionally or alternatively to one or more of the examples disclosed above, in some examples, a first layer of piezoelectric material of the one or more layers of piezoelectric material of the at least one of the one or more polarizers and a second layer of piezoelectric material of the one or more layers of piezoelectric material of the at least one of the one or more polarizers can be formed from different types of material. Additionally or alternatively to one or more of the examples disclosed above, in some examples, extracting and dissipating energy of the compressional waves can comprise attenuating compressional waves by a threshold amount within at least a first range of frequencies. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the threshold amount can be at least 90% attenuation. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the first range of frequencies can be wider than 100 kHz and includes frequencies greater than 500 kHz. 
     Therefore, according to the above, some examples of the disclosure are directed to a device. The device can comprise a housing; a crystal surface; one or more shear-polarized transducers; one or more polarizers configured to extract and dissipate energy of compressional waves and pass energy of shear waves; and one or more processors. Each of the one or more polarizers can be disposed between a corresponding one of the one or more shear-polarized transducers and the surface. At least one of the one or more polarizers can comprises: one or more layers of piezoelectric material; one or more electrodes; and one or more circuits coupled to the one or more layers of piezoelectric material via the one or more electrodes. The processor can be coupled to the one or more shear-polarized transducers and can be configured to stimulate the one or more shear-polarized transducers at one or more frequencies within the first passband and determine a location of an object based on reflected acoustic energy from the crystal surface. Additionally or alternatively to one or more of the examples disclosed above, in some examples, each of the one or more layers of piezoelectric material of the at least one of the one or more polarizers can have a poling direction different than a poling direction of the corresponding one of the one or more shear-polarized transducer. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the poling direction of each of the one or more layers of piezoelectric material of the at least one of the one or more polarizers can be orthogonal to the poling direction of the shear-polarized transducer. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the one or more electrodes of the at least one of the one or more polarizers can comprise a first electrode on a first side of a first layer of piezoelectric material of the one or more layers of piezoelectric material and a second electrode on a second side, opposite the first side, of the first layer of the piezoelectric material. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the one or more circuits of the at least one of the one or more polarizers can comprise a circuit comprising a resistor coupled between one of the one or more electrodes and a system ground. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the one or more circuits of the at least one of the one or more polarizers can comprise a circuit comprising a resistor and an inductor coupled in series between one of the one or more electrodes and a system ground. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the one or more circuits of the at least one of the one or more polarizers can comprise a circuit comprising one or more passive electric components or one or more active electric components coupled in series between one of the one or more electrodes and a system ground. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the one or more circuits of the at least one of the one or more polarizers can comprise a circuit comprising one or more passive electric components and one or more active electric components coupled in series between one of the one or more electrodes and a system ground. Additionally or alternatively to one or more of the examples disclosed above, in some examples, a first layer of piezoelectric material of the one or more layers of piezoelectric material of the at least one of the one or more polarizers can have a first thickness and a second layer of piezoelectric material of the one or more layers of piezoelectric material of the at least one of the one or more polarizers can have a second thickness different than the first thickness. Additionally or alternatively to one or more of the examples disclosed above, in some examples, a first layer of piezoelectric material of the one or more layers of piezoelectric material of the at least one of the one or more polarizers and a second layer of piezoelectric material of the one or more layers of piezoelectric material of the at least one of the one or more polarizers can be formed from a same type of material. Additionally or alternatively to one or more of the examples disclosed above, in some examples, a first layer of piezoelectric material of the one or more layers of piezoelectric material of the at least one of the one or more polarizers and a second layer of piezoelectric material of the one or more layers of piezoelectric material of the at least one of the one or more polarizers can be formed from different types of material. Additionally or alternatively to one or more of the examples disclosed above, in some examples, extracting and dissipating energy of the compressional waves can comprise attenuating compressional waves by a threshold amount within at least a first range of frequencies. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the threshold amount can be at least 90% attenuation. Additionally or alternatively to one or more of the examples disclosed above, in some examples, the first range of frequencies can be wider than 100 kHz and includes frequencies greater than 500 kHz. 
     Although examples of this disclosure have been fully described with reference to the accompanying drawings, it is to be noted that various changes and modifications will become apparent to those skilled in the art. Such changes and modifications are to be understood as being included within the scope of examples of this disclosure as defined by the appended claims.

Metadata:
Filing Date: 20190206
Publication Date: 20220621
Grant Date: 20220621
Priority Date: 20180206
Inventors: KHAJEH, EHSAN
TUCKER, AARON SCOTT
KING, BRIAN MICHAEL
YIP, Marcus
Assignee: APPLE INC
CPC Classifications: [{"code": "G06F3/0416", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F3/0436", "inventive": true, "first": true, "tree": "[]"}, {"code": "G06F3/0436", "inventive": true, "first": true, "tree": "[]"}, {"code": "G06F3/0416", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F3/0436", "inventive": true, "first": true, "tree": "[]"}, {"code": "G06F3/0416", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 67476017