Abstract:
Disclosed are devices, systems, and methods for capturing acoustic data in an efficient manner. Some embodiments have piezoelectric sensing portions with polarization axes and conducting layers. In some embodiments, piezoelectric sensing portions can be positioned generally coplanar to each other and in partial electrical contact. In some embodiments, polarization axes of two piezoelectric sensing portions have a non-zero angle between them.

Description:
PRIORITY INFORMATION  
       [0001]     This application claims priority to U.S. Patent Provisional Application No. 60/692,515, titled “ACOUSTIC SENSOR,” filed Jun. 21, 2005, the entirety of which is hereby incorporated by reference and made part of this specification. 
     
    
     BACKGROUND OF THE INVENTIONS  
       [0002]     1. Field of the Inventions  
         [0003]     The inventions described herein relate generally to the field of transducers, and in particular acoustic transducers. For example, some embodiments relate to acoustic sensors that can detect biological sounds and generate accurate data for signal processing to determine biological characteristics relating to the source of those sounds.  
         [0004]     2. Description of the Related Art  
         [0005]     Transducers are the operative portion of many modem technologies. One useful class of transducers converts an analog signal, such as an acoustic vibration wave, into an electrical signal. In particular, microphones contain acoustic transducers and can detect and record signals that correspond to sounds. The human hear is itself an acoustic transducer.  
         [0006]     Designers of acoustic sensors are continually challenged by the problem of separating the desired signal from unwanted noise. This challenge applies to both the acoustic noise (or extraneous acoustic vibrations) as well as the electronic noise (or unwanted electrical signals). Acoustic noise can be distracting background chatter that would be detectable by a human ear, or minute, unheard vibrations caused by a distant truck driving down the street. This kind of noise can interfere with the input of an acoustic sensor. Electronic noise can be electromagnetic emissions that cause the electrons in an electrical device to vibrate or move. This kind of noise can interfere with the output of an acoustic sensor. Because a transducer changes one signal to another signal, it is subject to problems with noise for both types of signals.  
         [0007]     Another problem that occurs in current sensors is over-sensitivity to the direction of the signal. For example, in many cases, sensors are structurally capable of effectively detecting signals, but are too sensitive to the orientation of the sensor with respect to the signal. Even relatively small changes in the orientation of the sensor can significantly affect the strength of the received signal, or determine whether the signal is received at all. Thus, many sensors are inefficient because they depend too much on proper orientation. This can lead to repeated tests (if the error is perceived by the operator), or incorrect and unreliable readings.  
         [0008]     Another problem of existing sensors relates to the arrival of a signal at various portions of the sensor at different times. For example, in some sensors that have multiple sensing portions that are vertically stacked, one above another, signals arriving from below the stack reach one sensing portion at one time, but that same signal does not reach the other sensing portion until later. This time difference of arrival can create signal time incidence ambiguities in sensor output.  
         [0009]     Thus, there is a need for methods and devices for increasing the sensitivity of acoustic transducers, improving shielding, reducing unwanted noise, and enhancing signal to noise ratios. There is also a need for methods and devices for improving the ability of acoustic sensors to receive signals from various directions without requiring time-consuming and error-prone repositioning of the sensors. Moreover, a need exists for improving sensors to minimize problems with the time difference of arrival at various sensing elements and to minimize signal time incidence ambiguities in signal sensor outputs. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     Certain embodiments of the inventions will now be briefly described with reference to the drawings. These drawings are examples and the inventions are not limited to the subject matter shown or described.  
         [0011]      FIG. 1  is a schematic, perspective view of a sensing layer component of a sensor in accordance with one embodiment of the inventions.  
         [0012]      FIG. 2  is a schematic, cross-sectional side view of two sensing layer components, taken along the lines  2 - 2  of  FIG. 3 .  
         [0013]      FIG. 3  is a schematic plan view of the sensing layer components of  FIG. 2  with electrical leads and other components.  
         [0014]      FIG. 4  is a schematic, partial cross-sectional side view (taken along lines  4 - 4  of  FIG. 5 ) of a portion of a sensor in accordance with one embodiment of the inventions.  
         [0015]      FIG. 5  is a schematic perspective view of a sensor in accordance with one embodiment of the inventions.  
         [0016]      FIG. 6  is a schematic illustration of multiple sensors positioned on the surface of a patient&#39;s chest with electrical leads transmitting data to a processor.  
         [0017]      FIGS. 7A-7C  are schematic illustrations of certain concepts relating to piezoelectric polarity and electric charges induced by bending of pieozoelectric materials.  
         [0018]      FIGS. 8A-8B  are schematic illustrations of multi-dimensional bending of planar materials and corresponding vector principles.  
         [0019]      FIG. 9A  is a schematic, cross-sectional illustration of a cut-away side view of a sensor in accordance with one embodiment of the inventions positioned on the skin of a patient, and a point source emitting substantially spherical sound waves.  
         [0020]      FIG. 9B  is a schematic, cross-sectional illustration of the sensor, point source, and sound waves of  FIG. 9A  at a later instant in time.  
         [0021]      FIG. 9C  is a schematic, three-dimensional, elevational illustration of the sensor, point source, and sound waves of  FIG. 9B .  
         [0022]      FIG. 10  is a perspective view of one alternative embodiment of a sensor in accordance with the inventions.  
         [0023]      FIG. 11A  is a perspective view of another alternative embodiment of a sensor in accordance with the inventions.  
         [0024]      FIG. 11B  is a plan view of the sensor of  FIG. 11A .  
         [0025]      FIG. 11C  is a schematic, cross-sectional side view of the sensor of  FIG. 11A , taken along the lines  11 C- 11 C of  FIG. 11B .  
         [0026]      FIG. 12A  is a schematic electronic circuit diagram illustrating the effective electrical properties of one embodiment of a sensor with two sensing layers.  
         [0027]      FIG. 12B  is a schematic electronic circuit diagram illustrating electrical apparatus that can be attached to the circuit of  FIG. 12A  for testing and/or data processing.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]     With reference to  FIG. 1 , a sensing layer  100  comprises a piezoelectric central portion  114  with a top conductive layer  110  and a bottom conductive layer  112 . The piezoelectric material can be a piezoelectric co-polymer. In one preferred embodiment, the piezoelectric portion  114  is formed from polyvinylidene fluoride (PVDF). PVDF is an anisotropic piezoelectric polymer that produces surface charges of substantially equal magnitude and opposite polarity on opposite surfaces when a mechanical strain is imposed on the material. Other preferred materials that may form the sensing portions can comprise compounds that include vinylidene fluoride. One preferred material is a piezoelectric co-polymer that is 75% vinylidene fluoride by weight. Metallized PVDF can be obtained from Measurement Specialties, Inc., of Hampton Va. 23666. PVDF can be used to good effect due to its compliance and resilience, as well as its piezoelectric properties. Moreover, PVDF is inexpensive, is the most commonly used commercial piezoelectric polymer, and has properties which are relatively unaffected by synthesis conditions. Other piezoelectric materials can also be used, such as poly vinylidene cyanide and its copolymers; aromatic and aliphatic polyureas; poly vinyl chloride; aromatic polyamides (odd nylons); PVDF copolymers with trifluoroethylene (P[VDF-TrFE)), tetrafluoroethylene (P[VDF-TFE)), and hexafluoropropylene (P[VDF-HFP)); PVDF blends with poly methyl methacrylate (PMMA); poly vinyl fluoride; poly vinyl acetate; and ferroelectric liquid crystal polymers.  
         [0029]     As illustrated schematically, the piezoelectric portion  114  is not completely surrounded by the metallized portions (top conductive layer  110  and bottom conductive layer  112 ). Preferably, the top conductive layer  110  and the bottom conductive layer  112  are not in electrical contact with each other when the sensing layer is in the illustrated configuration. Neither the sensing layer  100 , nor its sub-layers (the top and bottom conductive layers  110  and  112  and the piezoelectric portion  114 ) are shown to scale in  FIG. 1 .  
         [0030]     The conducting layers  110  and  112  can comprise metallization layers that are adhered to the surfaces of the piezoelectric portion  114 . The conducting layers  110 ,  112  can adhere to the surface of the piezoelectric portion by any suitable process, such as a deposition process. Metallization of the surfaces of the piezoelectric portion  114  may be accomplished using any suitable material and any suitable technique known in the art. For example, thin layers of a metal, such as nickel, silver, copper or alloys thereof, can be deposited on the inner and outer surfaces of the sensing layer  114 . In other embodiments, the conductive layers  110  and  112  can comprise or be coated with a conducting ink.  
         [0031]     In a preferred embodiment, the piezoelectric portion  114  is preferably thicker than either of the two conducting layers  110 ,  112 . In some embodiments, the piezoelectric portion  114  has a thickness of about 100 microns or less. In certain preferred embodiments, the piezoelectric portion  114  has a thickness of less than 150 microns, and the top and bottom conducting layers  110  and  112  each have a thickness of less than 30 microns. In one preferred configuration, the sensing layer  100  comprises PVDF with a copper nickel alloy coating. The piezoelectric portion can have a thickness between approximately 150 μm and approximately 6 μm. In some preferred embodiments, the total thickness of the piezoelectric portion  114  and the two conducting layers  110  and  112  combined is approximately 28 μm.  
         [0032]     Typically, when a tensile strain is imposed on the piezoelectric portion  114 , one surface of the piezoelectric portion  114  acquires a positive charge relative to the other. The charge is typically transferred to one of the adjacent conductive layers  110  or  112 . The piezoelectric portion  114  is advantageously polarized such that the piezoelectric effect is greater when the piezoelectric portion is stretched in a particular direction. The polarization axis can also be referred to as the “stretch axis.” Although biaxial orientation (stretching in two in-plane dimensions) is possible, it produces piezoelectric films with lower bilaterally isotropic piezoelectric properties. Most commercially available PVDF is uniaxially drawn, providing a high level of piezoelectric response along the stretch axis, or axis of orientation.  
         [0033]     When the piezoelectric portion  114  is under strain, the oppositely polarized charge that accumulates on the opposite layers of the piezoelectric portion spreads out over the top and bottom conductive layers  110  and  112 , forming a capacitative effect between the two conductive layers  110  and  112 . Because of this configuration, the voltage as measured across the two conductive layers  110  and  112  is related through the capacitance equation: Q=CV, where Q is the amount of surface charge, C is the capacitance, and V is the voltage output. Q can be expressed in Coulombs, C can be expressed in Farads, and V can be expressed in Volts. Certain configurations of PVDF materials exhibit a predictable voltage output V in response to a specific applied force. Generally, the amount of surface charge Q is proportional to the strain on the piezoelectric material, and capacitance C is substantially constant for a given material and structure. Thus, both Q and V are generally proportional to the strain on the piezoelectric material. If the voltage or charge response function is known, a measurement of either parameter can provide information about the strength of the signal (e.g., acoustic vibration) causing the strain. Moreover, if the precise response function of the piezoelectric material for a given physical configuration is not known, the output voltage can still provide useful data because the responses at various times can be compared.  
         [0034]     Furthermore, if the piezoelectric portion is polarized, information relating to the direction of the acoustic energy can also be obtained. Alternatively, a combination of two piezoelectric portions that are polarized in different directions can be configured to provide accurate data regarding the magnitude of the sensed signal, independent of the signal direction upon arrival at the sensor.  
         [0035]     With reference to  FIG. 2 , two sensing layers  210  and  220  (each similar to the sensing layer  100  described above) are positioned in a generally parallel, partially displaced, and generally coplanar orientation. While the two sensing layers  210  and  220  are not precisely coplanar in the illustrated embodiment, they are only vertically shifted by the width “d” of a single sensing portion, which can be less than 100 microns, as discussed above. In  FIG. 2 , the two sensing layers  210  and  220  are schematically illustrated in cross section. The sensing layer  210  has a top conductive layer  212  and a bottom conductive layer  214 . The sensing layer  220  has a top conductive layer  222  and a bottom conductive layer  224 . In a preferred configuration, the conductive layers  214  and  222  are in electrical contact and are configured to receive charges of the same polarity from their respective piezoelectric portions (the layer  214  receives charge when the layer  216  is appropriately stressed, and the layer  222  receives charge from the layer  221  under the appropriate stress). For example, if the bottom conductive layer  214  accumulates negative charge when the piezoelectric portion  216  is under strain, the top conductive layer  222  accumulates negative charge when the piezoelectric portion  226  is under strain. Similarly, the outer two conductive layers,  212  and  224 , accumulate charge of the same polarity.  
         [0036]     The described configuration, where the sensing layers  210  and  220  are inverted with respect to each other (that is, configured to have charge of opposite polarity accumulate on the top and bottom layers of the two sensing layers, respectively), provides the advantage of allowing a single electrical lead to contact two conductive layers. (The electrical lead  244  is in contact with both the conductive layers  214  and  222 . See  FIG. 3 ).  
         [0037]     With reference to  FIG. 3 , a plan view of the two sensing layers  210  and  220  is illustrated schematically. In this figure, the polarization of the two sensing layers  210  and  220  is shown with the two-sided arrows and the letters “P.” Thus, the sensing layer  210  is polarized substantially orthogonally to the sensing layer  220 . The orthogonally polarized sensing portions  210  and  220  provide a multi-directional sensing capability. For example, a signal that bends the sensing portion  210  in such a way that little or no electrical response is produced in that portion will have a higher likelihood of bending the sensing portion  220  in such a way that an electrical response will be produced in that portion. Indeed, as explained further below, the two mechanically coupled but oppositely polarized sensing portions  210  and  220  can act together to sense any arriving signal. Moreover, those vector components that are less likely to be sensed by the sensing portion  210  are more likely to be sensed by the sensing portion  220 .  
         [0038]     Some embodiments have two polarized sensing portions where the polarization directions of the sensing portions are not orthogonal, but are non-parallel, having a relative angle of anywhere between zero and ninety degrees. Sensing portions that are not polarized parallel to each other can be used to sense incoming signals from multiple directions. Furthermore, the relative angle can be chosen to provide the sensor with direction-identification capabilities, or with more efficient magnitude sensing capabilities.  
         [0039]     With further reference to  FIG. 3 , the two sensing layers  210  and  220  overlap by an overlap distance  230 . In some embodiments, the overlap distance  230  is approximately 3 mm. The overlap between the sensing layers  210  and  220  provides an electrical connection, as discussed above, as well as a mechanical connection. The combination of the overlap distance  230  and the similar properties of the two sensing portions  210  and  220  can result in the coupled system approximating the behavior of a single plane. For example, the physical dimensions (width thickness, etc.) and characteristics (stiffness, elasticity, tensile strength, etc.) of the two sensing portions  210  and  220  are typically similar, because in some embodiments the two sensing portions have been cut from the same type of material. For example, the two sensing portions  210  and  220  can be cut using a template from a single sheet of stock PVDF. In some embodiments, the two sensing portions  210  and  220  have the same dimensions but are cut in different orientations with respect to the polarization of the stock PVDF. Furthermore, the thickness “d” of the two sensing portions  210  and  220  is typically small compared to the other dimensions of the sensing portions  210  and  220 , and the dimensions of the combined system are many times greater than either the thickness “d” or the overlap distance  230 . Some preferred embodiments have sensing portions, each having following dimensions: 14.2 mm×30 mm and a thickness “d” of approximately 28 μm.  
         [0040]     Preferably, the two sensing portions  210  and  220  have enough overlap  230  to remain mechanically coupled and electrically linked, but not so much overlap that the resilience of the planar system is significantly altered. Thus, the two sensing layers  210  and  220  can physically bend and respond much the same way a continuous plane of the same material would respond to an impinging acoustic signal. Some preferred embodiments have an overlap distance  230  of 3 mm. For example, when the sensing portions are 14.2 mm×30 mm, the overlap  230  can occur along the 30 mm length of the two sensing portions  210  and  220 . In this configuration, the total area of the sensor can be approximately 762 mm 2 .  
         [0041]     The illustrated configuration also has the advantage of allowing impinging acoustic signals to arrive at the two sensing layers  210  and  220  essentially in unison—that is, such that the time difference of arrival (TDOA) is minimal. Thus, in some embodiments, configurations described herein can be referred to as “iso surface optimal material adherent compliant,” or “ISOMAC” sensors. The two sensing layers with optimized areas can lie in the same plane, thus generally presenting an “iso surface,” or a surface at which various points lie generally at the same distance from the source of the impinging acoustic signal. Moreover, as described further below, the materials from which a sensor is constructed can be compliant to the skin surface, bending in response to an impinging acoustic signal, while at the same time adhering to the surface of the skin to allow efficient mechanical coupling.  
         [0042]     In the illustrated embodiment, the electrical lead  242  is in electrical contact with the conductive layer  212 . The electrical lead  244  is in contact with both the conductive layers  214  and  222 . The electrical lead  246  is in contact with conductive layer  224 . In some embodiments, the electrical leads  242 ,  244 , and  246  can comprise metal lugs, each having a 5 mm lip. Other ways of making electrical connections can also be used. As illustrated, the leads  242 ,  244 , and  246  each attach to a shielded pair of twisted wires  248 . Because each pair is similar in the illustrated embodiment, each pair of wires has been labeled  248  in  FIG. 1 . One way to connect the leads  242 ,  244 , and  246  to the wires  248  is by soldering or crimping. Electrical connections can also be formed using an EC adhesive.  
         [0043]     Electrical lead  242  corresponds to the A terminal, electrical lead  244  corresponds to the C terminal, and electrical lead  246  corresponds to the B terminal. A and B can be positive terminals, while C is a “common,” or ground terminal. Alternatively, A can be a positive terminal, while C is a ground terminal, and B is a negative terminal. Various electrical connections can be made to measure the voltage difference across the sensing layers  210  and  220 .  
         [0044]     The electrical leads  242 ,  244 , and  246  provide a way for the charge that accumulates on the conductive layers  212 ,  214 ,  222 , and  224  to be measured using an electrical circuit and connections that will be described further below. If the physical properties of the sensing portions  210  and  220  are known, the equations and physical relations described herein can allow for the calculation of the acoustic signal&#39;s magnitude, direction, etc. The charge that accumulates on the top and bottom surfaces of the piezoelectric portions encounters relatively little resistance in the conductive layers  212  and  214 ,  222 , and  224 . Thus, the charge present on any particular layer can be measured at and/or collected from any contact point on that layer.  
         [0045]     In a preferred embodiment, the sensing portions  210  and  220  can be modeled using the physical electrostatic equations for two-plate capacitors. For example, in the illustrated configuration, the two sensing portions  210  and  220  have equal area. This configuration is generally analogous to two displaced capacitors with opposite electrical orientations. Assuming that the conductive layers  214  and  222  that are in contact have a negative charge, the capacitance and voltage of the two elements individually can be described thus, after charge has accumulated: 
 
 C   j =( A   j  ε)/ d   j  . . . for j=1,2,   (1) 
 
 where C j  is the capacitance of the ‘j’th element, A j  is the area of the ‘j’th element, ε is the electrical permittivity of the medium (e.g., PVDF), and d j  is the separation between the conductive layers of the ‘j’th element (which corresponds to the widths of the piezoelectric portions  216  and  226 ), or the separation distance between the parallel plates of the capacitor. In a preferred embodiment, d 1 =d 2  =28 μm. Thus the corresponding voltage signal generated at the two plates of any element is 
 
 V   j   =Q   j   /C   j  . . . for j=1,2   (2) 
 
         [0046]     Assuming that the conductive layers  212 ,  214 ,  222 , and  224  each have equal area, (A 1 =A 2 ), that each piezoelectric portion  216  and  226  have the same thickness, (d is constant), and that the piezoelectric portions are formed from the same material (and thus are equal in electrical permittivity), (ε is constant), it follows from equations (1) and (2) above that 
 
C i =C 2 =C   (3) 
 
         [0047]     Thus, using the capacitance equation for charge ‘Q’ in coulombs given by 
 
Q=CV   (4) 
 
 it follows that 
 
V 1 =V 2 =V   (5) 
 
         [0048]     Thus, under the assumptions outlined above, one can optimize the physical configuration as needed. For example, multiple elements can be arranged or the surface area of the elements can be expanded to increase the voltage output of the sensor. That is, if  68  and d are known and held constant, A can be varied or optimized in order to optimize or maximize Q and V. Alternatively, A, d, j, and/or ε can be varied in order to achieve a desired Q or V. One of the characteristics of some of the embodiments described herein is a design where the sensor is sized to be placed on the skin over the intercostals muscles without significantly overlapping the ribs. The sensor can also be designed to fit within an adhesive envelope of a given size. One such envelope requires a sensor to be less than 1 inch×1 inch, for example. Thus, the sensor area may have a certain maximum value. Voltage output can also be engineered to fall within a certain range under any constraints of electronic hardware. For example, in some embodiments, preferred voltage output is between approximately 0 and approximately 5 volts. The desired gain, dynamic range, and other characteristics of the electronics into which the voltage signal will travel can all provide design parameters. Some embodiments achieve adequate signal strength under these parameters with a total sensor area of approximately 762 mm 2 , for example.  
         [0049]      FIG. 4  depicts the structure of  FIG. 2  after more materials have been added in layers. As shown, the sensing layers  210  and  220  are positioned between metallic shielding layers  410  and  413  (that can, for example, be formed from a metal such as aluminum). The layers  410  and  413  have been attached to the sensing portions  210  and  220  using layers of adhesive material  411  and  412 . The adhesive material that forms the layers  411  and  412  is preferably a flexible material that is not electrically conductive. An electrical connection can be made between both of the metallic shielding layers  410  and  413 , and terminal C (the common, or ground terminal). To make such a connection, a flexible, electrically conductive adhesive can be used. A flexible adhesive material  409  and  414  that is not electrically conductive can be used to attach two electrically insulating compliant membranes  415  and  417 , as illustrated. One material that can be used to form the compliant membranes  415  and  417  is silicone. A layer of biocompatible adhesive  416  can be placed on one of the compliant membranes  415  or  417 . In the illustrated embodiment, the biocompatible adhesive  416  has been placed on the compliant membrane  415 . One material that can be used to form the biocompatible adhesive is “Ludlow Hydrogel,” available from Ludlow, a division of Tyco Healthcare Group LP, Chicopee, Mass., 01022.  
         [0050]     The metallic shielding layers  410  and  413  can provide an electrical shielding effect to minimize unwanted electrical noise. Thus, they can form a continuous or substantially continuous conducting surface that prevents stray electrical charges from penetrating inside the shielding layers  410  and  413 . In some embodiments, the metallic shielding layers can be formed from discontinuous mesh that provides shielding. The shielding layers  410  and  413  preferably flex with the other layers, allowing the acoustic signal to freely deform the sensor  510  (see  FIG. 5 ). Thus, the shielding layers  410  and  413  preferably provide a faraday cage to electrically isolate the sensing portions  210  and  220 , while at the same time not unduly stiffening the sensor or having a decisive influence on its over-all mechanical impedance. However, the stiffness and resiliency of the shielding layers  410  and  413  can be selected to provide a portion of the mechanical impedance such that the overall impedance matches that of the surface of human skin, for example.  
         [0051]     The flexible, electrically non-conductive adhesive material that forms the layers  411  and  412  preferably provides a permanent connection between the sensing portions  210  and  220  and the shielding layers  410  and  413 . The layers  411  and  412  preferably flex readily when acoustic signals impinge on the sensor  510 , operating to mechanically couple the layers without contributing to the electrical response. The layers  411  and  412  also preferably insure that no charge passes from the sensing layers  210  and  220  to the shielding layers  410  and  413 . The layers  411  and  412  are preferably uniformly distributed, having very few irregularities or discontinuities. Furthermore, the layers  411  and  412  preferably adhere smoothly and evenly to the surfaces they contact.  
         [0052]     The compliant membranes  415  and  417  can provide a protective, water repellant layer that protects the electrical connections inside the compliant membranes  415  and  417  from unwanted moisture. In some embodiments, the compliant membranes  415  and  417  form a continuous outer layer that surrounds all other layers except the biocompatible adhesive  416 . The compliant membranes  415  and  417  can also have a mechanical impedance that corresponds to that of human skin, for example. The compliant membranes  415  and  417  can thus continuously conform to the changing contours of the surface of human skin as the skin responds to impinging acoustic energy. The compliant membranes  415  and  417  help keep the acoustical loss between the skin and the sensor at a minimum. The described configuration can provide for good sensor sensitivity by using a silicone compliant material to interface with the skin surface.  
         [0053]     The flexible, electrically non-conductive adhesive material that forms the layers  409  and  414  preferably provides a permanent connection between the shielding layers  410  and  413 , and the electrically insulating compliant membranes  415  and  417 . The layers  409  and  414  preferably flex readily when acoustic signals impinge on the sensor  510 , operating to mechanically couple the layers without contributing to the electrical response. The layers  409  and  414  also preferably help insure that no charge passes between the outside of the sensor  510  and the shielding layers  410  and  413 . The layers  409  and  414  are preferably uniformly distributed, having very few irregularities or discontinuities. Furthermore, the layers  409  and  414  preferably adhere smoothly and evenly to the surfaces they contact.  
         [0054]     In some embodiments, a biocompatible adhesive  416  is used to improve the mechanical and acoustic connection between the compliant membrane  415  and skin. The biocompatible adhesive  416  can be “Hydrogel,” (as previously described), which can be positioned at the skin-sensor interface to improve sensitivity and acoustic/mechanical coupling. In some embodiments, the biocompatible adhesive  416  is smeared onto the human skin surface where the sensor  510  will be placed, and the sensor  510  is pressed onto the same area of the skin. The adhesive  416  can also be placed on the sensor  510  before it is pressed into place. In some embodiments, the biocompatible adhesive  416  is located beneath a removable strip (not shown) on the sensor when the sensor  510  is packaged, and the user can remove the strip to reveal the biocompatible adhesive  416  underneath, immediately prior to using the sensor  510 .  
         [0055]     As schematically illustrated in  FIG. 5 , the layers described above can form a sensor  510  and leads that correspond to terminals A, C, and B, as described above. As illustrated,  FIG. 4  shows a schematic, cross-sectional view of the sensor  510  taken along lines “ 4 - 4 .” 
         [0056]     With reference to  FIG. 6 , the described sensors can be advantageously employed in a system for detecting and processing heart sounds, such as that described in U.S. patent application Ser. No. 10/830,719, filed Apr. 23, 2004, and published on Feb. 17, 2005, the entirety of which is hereby incorporated by reference and made part of this specification. The improved sensing abilities of the sensors  510  can help enable the accurate detection and localization of stenoses in portions of a human heart, for example. In some embodiments, multiple sensors can be employed to collect data from a plurality of locations on a human body surrounding a human heart, for example. In particular,  FIG. 6  schematically shows one possible configuration of multiple sensors  510  and their general placement on a human body. In some embodiments, four sensors can be positioned on the surface of the skin, generally on the external anatomy surrounding the human heart. The sensors  510  can collect acoustic data from sounds emanating from within the body (e.g., the coronary artery). For example, the sensors  510  can gather data for the same acoustic signal from multiple spatial points. The sensors can electronically communicate with a signal processing system  612 , conveying, for example, electrical signals that convey information relating to acoustic signals. An example of such a signal processing system is described in U.S. patent application Ser. No. 10/830,719. The described sensors can aid in the clinical benefits described by providing accurate acoustic data, whatever the arrival direction of the acoustic signal.  
         [0057]     With reference to  FIG. 7A , a polarized piezoelectric slab  710  is illustrated. The slab  710  undergoes a strain in the polarization direction, resulting in an accumulation of charge at the surfaces of the slab. The piezoelectric material at the top of the slab  710  undergoes tensile forces  711  in the polarization direction, which results in an accumulation of positive charge at the upper surface. In contrast, the piezoelectric material at the bottom of the slab  710  undergoes contractive forces  713  in the polarization direction, which results in an accumulation of negative charge at the lower surface.  
         [0058]      FIG. 7B  illustrates schematically how a polarized piezoelectric slab  720  that undergoes a strain in a direction orthogonal to its polarization does not accumulate significant charge at its surfaces. Thus, the single polarized piezoelectric slab  720  does not normally sense input effectively if that input does not cause a strain that is aligned with polarization of the slab  720 . The piezoelectric material at the top of the slab  720  is subjected to the same tensile forces as the slab  710  illustrated in  FIG. 7A , and the piezoelectric material at the bottom of the slab  720  undergoes the same contractive forces as the slab  710 . However, because the slab  720  is polarized orthogonally, no piezoelectric effect is produced in the slab  720 .  
         [0059]     In contrast,  FIG. 7C  illustrates how the same polarized piezoelectric slab  720  can accumulate charge, and thus effectively “sense” a signal that causes a differently oriented force on the slab  720 . In particular, the piezoelectric material at the top of the slab  720  undergoes tensile forces  721  in the same direction of polarization, which results in an accumulation of positive charge  722  at the upper surface. In addition, the piezoelectric material at the bottom of the slab  720  undergoes contractive forces  723  in the same direction of polarization, which results in an accumulation of negative charge  724  at the lower surface.  
         [0060]      FIG. 8A  illustrates a two dimensional vector  830 . The vector  830  has a magnitude (corresponding to its length) and a direction in the plane of the page. As illustrated, the vector  830  can be “resolved” into two vector components, the vertical component  832  and the horizontal component  834 . The combination of the two components  832  and  834  can provide the same information inherent in the original vector  830 . The vertical component  832  and the horizontal component  834  can also be characterized as a “basis set” onto which the vector  830  can be expanded.  
         [0061]     Just as this vector  830  can be resolved into components, an incoming signal can be represented as a vector quantity that can be resolved into two components in a Cartesian coordinate system (or another basis set). This concept can be employed to combine two orthogonally polarized (or non-parallel) sensing portions in a single sensor, and from the respective signals of the two sensing portions, directional components can be calculated and an approximation for the signal magnitude can be produced. Thus, if the polarized slabs  710  and  720  are physically combined such that one slab is polarized in a direction that is orthogonal (or non parallel) to the other slab, a device that senses signals coming from any direction can be constructed. Such a device preferably is configured to allow the impinging acoustic signals to arrive at the two slabs essentially in unison—that is, such that the time difference of arrival (TDOA) is minimal. This minimization of the TDOA can be achieved when the sensor comprises two portions of thin PVDF material that partially overlap as illustrated herein.  
         [0062]      FIG. 8B  illustrates a piezoelectric slab  810  that is undergoing strain that is not aligned with the slab&#39;s polarization axis. That is, the strain is in a direction “S” that is neither parallel to nor perpendicular to the polarization axis “P.” The strain “S” can be represented in terms of some orthogonal components  812  (components that are perpendicular to “P”) and some parallel components  814  (components that are parallel to “P”). Under the influence of these the various tensile and contractive forces that result from the strain “S,” the slab  810  does not sense the orthogonal components  812  of these forces as effectively as it senses the parallel components  814 . This kind of unaligned strain is typical of that generated by an acoustic signal that originates in the body and arrives at the surface of a patient&#39;s skin. Indeed, because of the difficulty in predicting the arrival direction of any acoustic signal to be measured (especially when such a direction may be part of the information sought from the test in the first place), it is unlikely that a sensor&#39;s polarity can be perfectly aligned with an incoming acoustic signal. Furthermore, if information is sought relating to a signal&#39;s magnitude, but the direction of the signal does not need to be known, a combined system can be sensitive to signals from any direction without a need for aiming the sensor. If the two slabs  710  and  720 , in the orientations illustrated in  FIG. 7 , are combined to form a mechanically coupled system that undergoes the strain illustrated in  FIG. 8B , the two sensing portions  710  and  720  can each detect vector components of the acoustic signal. Furthermore, if the two sensing portions  710  and  720  are located in generally the same plane, the TDOA will be minimized and the two sensing portions will be responding to essentially the same signal. Thus, the configuration of sensing portions  210  and  220  illustrated in  FIGS. 2-4  can provide multi-directional sensing capability and minimize sensing errors due to a large TDOA.  
         [0063]     With reference to  FIG. 9A , an acoustic source  910  is schematically illustrated, and a circular wavefront  914  is shown emanating from the source  910 . Another wavefront  918  is illustrated further away from the source  910 , as energy travels upward toward a surface  920 . In some embodiments, the source  910  can be located within a human body, and the surface  920  can be the human skin on the surface of the body. A sensor  510  is schematically illustrated, showing sensing portions  210  and  220  within the sensor  510 . Before the wavefront  918  arrives at the surface  920 , the sensor  510  is not under any significant strain and rests in an equilibrium, essentially zero-signal configuration.  
         [0064]     With reference to  FIG. 9B , the wavefront  918  has arrived at and deformed the surface  920 , which is protruding outward, causing a strain in the surface  920 . The sensor  510  is mechanically coupled to the surface  920  and undergoes a proportional strain when the wavefront  918  (corresponding to an acoustic signal) arrives at the surface  920 . The strain causes charge to accumulate on the surfaces of the sensing portions  210  and  220 , which can in turn cause a signal to be transmitted from the sensor  510  containing information about the magnitude and direction of the wavefront  918 .  
         [0065]      FIG. 9C  schematically illustrates a three-dimensional view of the arrival of the wavefront  918  at the surface  920  depicted in  FIG. 9B . As illustrated, the sensing portions  210  and  220  undergo strain in more than one dimension. Thus, the sensing portions  210  and  220  can be polarized in orthogonal directions and the system can gather effective directional information, as described above. This illustrates one advantage of the described sensor embodiments, namely their ability to detect signals from multiple directions. The described characteristics can enhance the sensors&#39; ability to convert acoustical to mechanical energy from acoustic waves that cause of the skin to bend. For example, when it is mounted on human skin, the sensor  510  can receive signals from any direction in the 360° field of view.  
         [0066]      FIG. 10  schematically illustrates an embodiment of two sensing portions  1010  and  1020 . The upper sensing portion  1010  has an upper conductive layer  1012  that contacts a lead A, and a lower sensing portion  1014  that is in electrical communication with a lead C. The upper conductive layer  1022  of the lower sensing portion  1020  is also in contact with the lead C, and the lower conductive layer  1024  of the lower sensing portion  1020  is in contact with a lead B. The upper sensing portion  1010  can be positioned to partially overlap the lower sensing portion  1020 , as shown. Preferably, the area of the upper and lower sensing portions of any given sensor embodiment are approximately equal. Equal areas simplify the capacitance calculations because the two areas can be cancelled out by dividing both sides of the equation by the area. Thus, in the embodiment illustrated in  FIG. 10 , the area of the upper sensing portion  1010  is approximately equal to the area of the lower sensing portion  1020 . Furthermore, the conductive layers  1012  and  1024  are preferably polarized in one direction, and the conductive layers  1014  and  1022  are preferably polarized in an orthogonal direction, as shown by the polarization arrows labeled “P” in  FIG. 10 .  
         [0067]      FIG. 11A  schematically illustrates a perspective view of an embodiment of two sensing portions, an outer ring  1110  and an inner ring  1120 . Preferably, the two rings have the same area, though they do not have the same radius. The outer ring  1110  has an upper conductive layer  1112  that electrically contacts the bottom conductive layer  1124  of the inner ring  1120  through a contact strip  1132 . This configuration allows the two sensing portions  1110  and  1120  to be in electrical contact without having any overlapping portions. Furthermore, in this embodiment, the two sensing portions  1110  and  1120  are approximately coplanar. A terminal A is shown to be in electrical contact with the upper conductive layer  1122  of the inner ring  1120 . A terminal B is shown to be in electrical contact with the lower conductive layer  1114  of the outer ring  1110 . A terminal C is shown to be in electrical contact with the upper conductive layer  1112  of the outer ring  1110 , and by extension with the lower conductive layer  1124  of the inner ring  1120 .  
         [0068]      FIG. 11B  schematically illustrates a plan view of the embodiment of  FIG. 11A .  FIG. 11C  schematically illustrates a cross-sectional view taken along the lines  11 C- 11 C of  FIG. 11B . The approximately coplanar nature of this embodiment is apparent in the cross-sectional view of  FIG. 11C . Furthermore, the conductive layers  1112  and  1124  are preferably polarized in one direction, and the conductive layers  1114  and  1122  are preferably polarized in an orthogonal direction, as shown by the polarization arrows labeled “P” in  FIG. 11 . The surface areas of the inner and outer rings are preferably equal, which can allow the sensor to achieve a constant gain or signal response when acoustical signals arrive from any direction. This configuration can provide especially efficient data when sensing a spherical wave front because of its general cylindrical symmetry.  
         [0069]      FIG. 12A  schematically illustrates an embodiment of an electrical circuit that can be used to represent the electrical response properties of the embodiments discussed above. A terminal A is connected to a variable voltage source V S2 . The variable voltage source V S2  can correspond to the conductive layers  212 ,  1012 , or  1122 , for example. A terminal B is connected to a variable voltage source V S1 . The variable voltage source V S1  can correspond to the conductive layers  224 ,  1024 , or  1114 , for example. The conductive layers have variable voltages that depend on the amount of strain on (and charge that accumulates on the surfaces of) the piezoelectric portions with which they are in electrical contact. A terminal C is connects the opposite sides of the two variable voltage sources—through resistances R S1  and RS 2 —to ground. In practical effect, the resistances R S1  and R S2  generally approach zero. The C terminal can correspond to the conductive layers  214  and  222 ,  1014  and  1022 , and  1112  and  1124 , for example. When these layers are connected to ground, they can draw (or deposit) as much charge as needed to balance out the charge that flows to terminals A and B as a result of the piezoelectric effect on the sensing portions. The voltage difference across terminals A and C is measured, and the voltage difference across terminals C and B is measured. In some embodiments, the A-C voltage can provide information relating to the signal corresponding to one vector component of the impinging acoustic signal, and the B-C voltage provides information relating to the other vector component.  
         [0070]      FIG. 12B  schematically illustrates an embodiment of an electrical circuit that can be used to test the circuit illustrated in  FIG. 12A  and/or analyze the data produced by the embodiments described above. The terminal a can be connected to A, the terminal c can be connected to C, and the terminal b can be connected to B. If the switch  1222  is closed, a multimeter  1220  can measure the current, resistance, and/or voltage drop across terminals A and C. If the switch  1224  is closed, the multimeter  1220  can measure the current, resistance, and/or voltage drop across terminals C and B. Similarly, if the switches  1226  and  1228  are both closed, the signal processor  612  can process the signals provided by the sensor to determine electrical and acoustic characteristics of the sensed system.  
         [0071]     The processes described herein can advantageously be adjusted for efficient manufacture. For example, electrical connections and circuits can be formed using chemical deposition and integrated circuit processes. Moreover, materials can be deposited, one onto another, in a form and using a deposition process that eliminates the need for the adhesive materials described herein.  
         [0072]     Although the present inventions have been described in terms of certain preferred embodiments, various features of separate embodiments can be combined to form additional embodiments not expressly described. Moreover, other embodiments apparent to those of ordinary skill in the art after reading this disclosure are also are within the scope of these inventions. Furthermore, not all of the features, aspects and advantages are necessarily required to practice the present inventions. Thus, while the above detailed description has shown, described, and pointed out novel features of the invention as applied to various embodiments, it will be understood that various omissions, substitutions, and changes in the form and details of the device or process illustrated may be made by those of ordinary skill in the technology without departing from the spirit of the invention. The inventions may be embodied in other specific forms not explicitly described herein. The embodiments described above are to be considered in all respects as illustrative only and not restrictive in any manner. Thus, scope of the invention is indicated by the following claims rather than by the foregoing description.