Patent Publication Number: US-9894453-B2

Title: Absolute sensitivity of a MEMS microphone with capacitive and piezoelectric electrodes

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 14/970,175, entitled “ABSOLUTE SENSITIVITY OF A MEMS MICROPHONE WITH CAPACITIVE AND PIEZOELECTRIC ELECTRODES” filed Dec. 15, 2015, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Embodiments of the disclosure relate to micro-electro-mechanical system (MEMS) microphones with both capacitive and piezoelectric electrodes. 
     The absolute sensitivity of an electrode in a MEMS microphone is the electrical response of the electrode&#39;s output to a given standard acoustic input. Allowable product variation of absolute sensitivities in MEMS microphones is, in general, decreasing. In addition, allowable testing time to determine the absolute sensitivities in MEMS microphones is also decreasing. 
     SUMMARY 
     Coupling a piezoelectric electrode to a capacitive electrode in a MEMS microphone adds a second reciprocal sensor which can be used to determine the absolute sensitivity. 
     One embodiment provides a microphone system. In one embodiment, the microphone system includes a speaker, a MEMS microphone, and a controller. The speaker is configured to generate an acoustic pressure. The MEMS microphone includes a capacitive electrode, a piezoelectric electrode, and a backplate. The capacitive electrode is configured to generate a first capacitive response based on the acoustic pressure. The capacitive electrode is also configured to generate a first mechanical pressure based on a capacitive control signal. The piezoelectric electrode is coupled to the capacitive electrode. The piezoelectric electrode is configured to generate a first piezoelectric response signal based on the acoustic pressure. The piezoelectric electrode is also configured to generate a second piezoelectric response signal based on the first mechanical pressure. The controller is configured to generate the capacitive control signal. The controller is also configured to determine an absolute sensitivity of the capacitive electrode based on the first capacitive response, the first piezoelectric response signal, and the second piezoelectric response signal. 
     Another embodiment provides a method of determining absolute sensitivities of a MEMS microphone. In one embodiment, the MEMS microphone includes a capacitive electrode, a piezoelectric electrode, and a backplate. The piezoelectric electrode is coupled to the capacitive electrode. The method includes generating acoustic pressure with a speaker. The method also includes generating a first capacitive response with the capacitive electrode based on the acoustic pressure. The method further includes generating a first piezoelectric response with the piezoelectric electrode based on the acoustic pressure. The method also includes generating a capacitive control signal with a controller. The method further includes generating a first mechanical pressure with the capacitive electrode based on the capacitive control signal. The method also includes generating a second piezoelectric response with the piezoelectric electrode based on the first mechanical pressure. The method further includes determining an absolute sensitivity of the capacitive electrode with the controller based in part on the first capacitive response, the first piezoelectric response, and the second piezoelectric response. 
     Yet another embodiment provides a microphone system. In one embodiment, the microphone system includes a speaker, a MEMS microphone, and a controller. The speaker is configured to generate an acoustic pressure. The MEMS microphone includes movable membrane and a backplate. The movable membrane includes a capacitive electrode and a piezoelectric electrode. The capacitive electrode is configured to generate a first capacitive response based on the acoustic pressure. The capacitive electrode is also configured to generate a first mechanical pressure based on a capacitive control signal. The piezoelectric electrode is configured to generate a first piezoelectric response signal based on the acoustic pressure. The piezoelectric electrode is also configured to generate a second piezoelectric response signal based on the first mechanical pressure. The backplate is positioned on the capacitive electrode. The controller is configured to generate the capacitive control signal. The controller is also configured to determine an absolute sensitivity of the capacitive electrode based on the first capacitive response, the first piezoelectric response signal, and the second piezoelectric response signal. 
     Still another embodiment provides a microphone system. In one embodiment, the microphone system includes a speaker, a MEMS microphone, and a controller. The speaker is configured to generate an acoustic pressure based on a speaker control signal. The MEMS microphone includes a capacitive electrode, a backplate, and a piezoelectric electrode. The capacitive electrode is configured such that the acoustic pressure causes a first movement of the capacitive electrode. The capacitive electrode is also configured to generate a first mechanical pressure based on a capacitive control signal. The backplate is positioned on a first side of the capacitive electrode. The piezoelectric electrode is coupled to the capacitive electrode. The piezoelectric electrode is configured to generate a first piezoelectric response signal based on the acoustic pressure. The piezoelectric electrode is further configured to generate a second piezoelectric response signal based on the first mechanical pressure. The controller is coupled to the speaker, the capacitive electrode, the backplate, and the piezoelectric electrode. The controller is configured to generate the speaker control signal. The controller is also configured to determine a first capacitive response based on the first movement of the capacitive electrode. The controller is further configured to generate the capacitive control signal. The controller is also configured to determine an absolute sensitivity of the capacitive electrode based on the first capacitive response, the first piezoelectric response signal, and the second piezoelectric response signal. 
     Another embodiment provides a method of determining absolute sensitivities of a MEMS microphone. In one embodiment, the MEMS microphone includes a capacitive electrode, a backplate, and a piezoelectric electrode. The piezoelectric electrode is coupled to the capacitive electrode. The method includes generating, by a speaker, an acoustic pressure based on a speaker control signal. The method further includes determining, by a controller, a first capacitive response of the capacitive electrode in response to the acoustic pressure. The method also includes determining, by the controller, a first piezoelectric response of the piezoelectric electrode in response to the acoustic pressure. The method further includes, generating, by the capacitive electrode, a first mechanical pressure based on a capacitive control signal. The method also includes determining, by the controller, a second piezoelectric response of the piezoelectric electrode in response to the first mechanical pressure. The method further includes determining, by the controller, an absolute sensitivity of the capacitive electrode based on the first capacitive response, the first piezoelectric response, and the second piezoelectric response. 
     Yet another embodiment provides a microphone system. In one embodiment, the microphone system includes a speaker, a MEMS microphone, and a controller. The speaker is configured to generate an acoustic pressure based on a speaker control signal. The MEMS microphone includes a movable membrane and a backplate. The movable membrane includes a piezoelectric electrode and a capacitive electrode. The capacitive electrode is configured such that the acoustic pressure causes a first movement of the capacitive electrode. The capacitive electrode is also configured to generate a first mechanical pressure based on a capacitive control signal. The piezoelectric electrode is configured to generate a first piezoelectric response signal based on the acoustic pressure. The piezoelectric electrode is further configured to generate a second piezoelectric response signal based on the first mechanical pressure. The backplate is positioned on the capacitive electrode. The controller is coupled to the speaker, the capacitive electrode, the backplate, and the piezoelectric electrode. The controller is configured to generate the speaker control signal. The controller is also configured to determine a first capacitive response based on the first movement of the capacitive electrode. The controller is further configured to generate the capacitive control signal. The controller is also configured to determine an absolute sensitivity of the capacitive electrode based on the first capacitive response, the first piezoelectric response signal, and the second piezoelectric response signal. 
     Other aspects of the disclosure will become apparent by consideration of the detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a MEMS microphone, in accordance with some embodiments. 
         FIG. 2  is a cross-sectional view of a MEMS microphone and a speaker, in accordance with some embodiments. 
         FIG. 3  is a cross-sectional view of a MEMS microphone, in accordance with some embodiments. 
         FIG. 4  is a cross-sectional view of a MEMS microphone, in accordance with some embodiments. 
         FIG. 5  is a schematic diagram of a microphone system, in accordance with some embodiments. 
         FIG. 6  is a flowchart of determining absolute sensitivities of a MEMS microphone, in accordance with some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Before any embodiments of the invention are explained in detail, it is to be understood that the disclosure is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The disclosure is capable of other embodiments and of being practiced or of being carried out in various ways. 
     Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The terms “mounted,” “connected” and “coupled” are used broadly and encompass both direct and indirect mounting, connecting and coupling. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings, and can include electrical connections or couplings, whether direct or indirect. Also, electronic communications and notifications may be performed using other known means including direct connections, wireless connections, etc. In addition, the terms “positive” and “negative” are used to distinguish one entity or action from another entity or action without necessarily requiring or implying any such attribute of the entity or action. 
     It should also be noted that a plurality of hardware and software based devices, as well as a plurality of other structural components may be utilized to implement the disclosure. Furthermore, and as described in subsequent paragraphs, the specific configurations illustrated in the drawings are intended to exemplify embodiments of the disclosure. Alternative configurations are possible. 
     In some embodiments, a MEMS microphone  100  includes, among other components, a movable membrane  103 . In the example illustrated, the movable membrane  103  includes a capacitive electrode  105  having a first side  107  and a second side  108 . The capacitive electrode  105  is also a movable membrane. The movable membrane  103  also includes a piezoelectric electrode  115 . A fixed member (i.e., a backplate  110 ) and a barrier  120  are provided in the MEMS microphone  100 . The second side  108  of the capacitive electrode  105  is opposite from the first side  107  of the capacitive electrode  105 . In some embodiments, the backplate  110  is positioned on the first side  107  of the capacitive electrode  105 , as illustrated in  FIGS. 1-4 . In other embodiments, the backplate  110  is positioned on the second side  108  of the capacitive electrode  105 . The barrier  120  isolates a first side  125  and a second side  130  of the MEMS microphone  100 . 
     In some embodiments, the capacitive electrode  105  is kept at a reference voltage and a bias voltage is applied to the backplate  110  to generate an electric sense field  135  between the capacitive electrode  105  and the backplate  110 . In other embodiments, the backplate  110  is kept at a reference voltage and a bias voltage is applied to the capacitive electrode  105  to generate the electric sense field  135  between the capacitive electrode  105  and the backplate  110 . In some embodiments, the reference voltage is a ground reference voltage (i.e., approximately 0 Volts). In other embodiments, the reference voltage is a non-zero voltage. The electric sense field  135  is illustrated in  FIGS. 1 and 2  as a plurality of diagonal lines. Deflection of the capacitive electrode  105  in the directions of arrow  145  and  150  modulates the electric sense field  135  between the capacitive electrode  105  and the backplate  110 . A voltage difference between the capacitive electrode  105  and the backplate  110  varies based on the electric sense field  135 . 
     As illustrated in  FIG. 2 , acoustic pressure  140  acting on the second side  108  of the capacitive electrode  105  causes a first movement (e.g., deflection) of the capacitive electrode  105  in the direction of arrow  150 . The acoustic pressure  140  is illustrated in  FIG. 2  as a plurality of wavy arrows in the direction of arrow  150 . The acoustic pressure  140  is generated by a transducer  155 . The transducer  155  may be a receiver, a speaker, and the like. Although one speaker is illustrated, more than one speaker may be used, depending on the application. The transducer  155  generates the acoustic pressure  140  based on a received speaker control signal. The first movement of the capacitive electrode  105  modulates the electric sense field  135  between the capacitive electrode  105  and the backplate  110 . A first voltage difference between the capacitive electrode  105  and the backplate  110  varies based on the first movement of the capacitive electrode  105 . 
     In some embodiments, a capacitive control signal is applied to the capacitive electrode  105 . The capacitive control signal causes the capacitive electrode  105  to generate a first mechanical pressure  160 , as illustrated in  FIG. 3 . The first mechanical pressure  160  is illustrated in  FIG. 3  as a plurality of straight arrows in the direction of arrow  145 . In some embodiments, the capacitive control signal is a current signal. 
     In one embodiment, the piezoelectric electrode  115  is a layer or material that uses the piezoelectric effect to measure changes in pressure or force by converting them to an electrical charge. In some embodiments, the piezoelectric electrode  115  includes aluminum nitride (AlN). In other embodiments, the piezoelectric electrode  115  includes zinc oxide (ZnO). In other embodiments, the piezoelectric electrode  115  includes lead zirconate titanate (PZT). The piezoelectric electrode  115  generates piezoelectric response signals in response to pressure (e.g., acoustic, mechanical) being applied to the piezoelectric electrode  115 . In some embodiments, the piezoelectric electrode  115  is formed on the capacitive electrode  105  by a suitable deposition technique (e.g., atomic layer deposition), and defines a fabricated piezoelectric membrane. 
     The piezoelectric electrode  115  is coupled to the capacitive electrode  105 . In some embodiments, the piezoelectric electrode  115  is coupled to the second side  108  of the capacitive electrode  105 , as illustrated in  FIGS. 1-4 . In other embodiments, the piezoelectric electrode  115  is coupled to the first side  107  of the capacitive electrode  105 . In some embodiments, the piezoelectric electrode  115  is formed on either side of the capacitive electrode  105  by a deposition technique. 
     The piezoelectric electrode  115  is configured to receive the acoustic pressure  140 . The piezoelectric electrode  115  generates a first piezoelectric response signal in response to the acoustic pressure  140 . The piezoelectric electrode  115  generates a second piezoelectric response signal in response to the first mechanical pressure  160  exerted by the capacitive electrode  105 . In some embodiments, the first and second piezoelectric response signals are voltage signals. 
     In some embodiments, a piezoelectric control signal is applied to the piezoelectric electrode  115 . The piezoelectric control signal causes a shape of the piezoelectric electrode  115  to change. The shape change results in the piezoelectric electrode  115  generating a second mechanical pressure  165 , as illustrated in  FIG. 4 . The second mechanical pressure  165  is illustrated in  FIG. 4  as a plurality of straight arrows in the direction of arrow  150 . In some embodiments, the piezoelectric control signal is a current signal. 
     The second mechanical pressure  165  generated by the shape change of the piezoelectric electrode  115  in turn causes a second movement of the capacitive electrode  105 . Similar to the first movement, the second movement of the capacitive electrode  105  modulates the electric sense field  135  between the capacitive electrode  105  and the backplate  110 . A second voltage difference between the capacitive electrode  105  and the backplate  110  varies based on the second movement of the capacitive electrode  105 . 
     In some embodiments, the piezoelectric material is deposited on the second side  108  of the movable membrane so as to form the piezoelectric electrode  115 . The first side  107  of the movable membrane defines the capacitive electrode  105 . The piezoelectric electrode  115  generates the first response signal in response to the acoustic pressure  140 . The piezoelectric electrode  115  generates the second piezoelectric signal in response to the first mechanical pressure  160  exerted by the capacitive electrode  105 . The second mechanical pressure  165  generated by the shape change of the piezoelectric electrode  115 , in turn, causes a second movement of the capacitive electrode  105 . Similar to the first movement, the second movement of the capacitive electrode  105  modulates the electric sense field  135  between the capacitive electrode  105  and the backplate  110 . A second voltage difference between the capacitive electrode  105  and the backplate  110  varies based on the second movement of the capacitive electrode  105 . 
     In some embodiments, a microphone system  200  includes, among other components, the MEMS microphone  100 , the transducer  155 , a controller  205 , and a power supply  210 , as illustrated in  FIG. 5 . 
     In some embodiments, the controller  205  includes a plurality of electrical and electronic components that provide power, operational control, and protection to the components and modules within the controller  205 , the MEMS microphone  100 , the transducer  155 , and/or the microphone system  200 . For example, the controller  205  includes, among other components, a processing unit  215  (e.g., a microprocessor, a microcontroller, or another suitable programmable device), a memory or computer readable media  220 , input interfaces  225 , and output interfaces  230 . The processing unit  215  includes, among other components, a control unit  235 , an arithmetic logic unit (ALU)  240 , and a plurality of registers  245  (shown as a group of registers in  FIG. 5 ), and is implemented using a known computer architecture, such as a modified Harvard architecture, a von Neumann architecture, etc. The processing unit  215 , the computer readable media  220 , the input interfaces  225 , and the output interfaces  230 , as well as the various modules connected to the controller  205  are connected by one or more control and/or data buses (e.g., common bus  250 ). The control and/or data buses are shown generally in  FIG. 5  for illustrative purposes. The use of one or more control and/or data buses for the interconnection between and communication among the various modules and components would be known to a person skilled in the art in view of the invention described herein. In some embodiments, the controller  205  is implemented partially or entirely on a semiconductor chip, is a field-programmable gate array (FPGA), is an application specific integrated circuit (ASIC), or is a similar device. 
     The computer readable media  220  includes, for example, a program storage area and a data storage area. The program storage area and the data storage area can include combinations of different types of memory, such as read-only memory (ROM), random access memory (RAM) (e.g., dynamic RAM [DRAM], synchronous DRAM [SDRAM], etc.), electrically erasable programmable read-only memory (EEPROM), flash memory, a hard disk, an SD card, or other suitable magnetic, optical, physical, or electronic memory devices or data structures. The processing unit  215  is connected to the computer readable media  220  and executes software instructions that are capable of being stored in a RAM of the computer readable media  220  (e.g., during execution), a ROM of the computer readable media  220  (e.g., on a generally permanent basis), or another non-transitory computer readable medium such as another memory or a disc. Software included in some embodiments of the microphone system  200  can be stored in the computer readable media  220  of the controller  205 . The software includes, for example, firmware, one or more applications, program data, filters, rules, one or more program modules, and other executable instructions. The controller  205  is configured to retrieve from memory and execute, among other things, instructions related to the control processes and methods described herein. In other constructions, the controller  205  includes additional, fewer, or different components. 
     The controller  205  is coupled to the capacitive electrode  105  and the backplate  110 . As described herein, the acoustic pressure  140  generated by the transducer  155  causes the first movement of the capacitive electrode  105 . The controller  205  determines a first capacitive response of the capacitive electrode  105  in response to the acoustic pressure  140  being applied. The first capacitive response is based on the first movement of the capacitive electrode  105 . In some embodiments, the controller  205  determines the first voltage difference between the capacitive electrode  105  and the backplate  110  caused by the first movement of the capacitive electrode  105 . Further, the controller  205  determines the first capacitive response based on the first voltage difference. 
     Also, as described herein, the second mechanical pressure  165 , generated by the piezoelectric electrode  115 , causes a second movement of the capacitive electrode  105 . The controller  205  determines a second capacitive response of the capacitive electrode  105  in response to the second mechanical pressure  165  being applied. The second capacitive response is based on the second movement of the capacitive electrode  105 . In some embodiments, the controller  205  determines the second voltage difference between the capacitive electrode  105  and the backplate  110  caused by the second movement of the capacitive electrode  105 . Further, the controller  205  determines the second capacitive response based on the second voltage difference. The controller  205  also generates and applies the capacitive control signal to the capacitive electrode  105 . 
     The controller  205  is also coupled to the piezoelectric electrode  115 . The controller  205  receives the first and second piezoelectric response signals generated by the piezoelectric electrode  115 . In some embodiments, the controller  205  generates and applies the piezoelectric control signal to the piezoelectric electrode  115 . 
     The controller  205  is further coupled to the transducer  155 . The controller  205  generates and applies the speaker control signal to the transducer  155 . 
     The power supply  210  supplies a nominal AC or DC voltage to the controller  205  and/or other components of the microphone system  200 . The power supply  210  is powered by one or more batteries or battery packs. The power supply  210  is also configured to supply lower voltages to operate circuits and components within the microphone system  200 . In some embodiments, the power supply  210  generates, among other things, the speaker control signal, the piezoelectric control signal, and the capacitive control signal. In some embodiments, the power supply  210  is powered by mains power having nominal line voltages between, for example, 100V and 240V AC and frequencies of approximately 50-60 Hz. 
     In one embodiment, the controller  205  determines absolute sensitivities of the capacitive electrode  105  and the piezoelectric electrode  115  using a reciprocity technique. The reciprocity technique includes a plurality of measurements. A first measurement includes the controller  205  applying the speaker control signal to the transducer  155  and determining the first capacitive response of the capacitive electrode  105 . A second measurement includes the controller  205  applying the speaker control signal to the transducer  155  and determining the first piezoelectric response (e.g., the first piezoelectric response signal) of the piezoelectric electrode  115 . A third measurement includes the controller  205  applying a capacitive control signal to the capacitive electrode  105  and determining the second piezoelectric response (e.g., the second piezoelectric response signal) of the piezoelectric electrode  115 . In some embodiments, a fourth measurement includes the controller  205  applying the piezoelectric control signal to the piezoelectric electrode  115  and determining the second capacitive response of the capacitive electrode  105 . 
     The first and second measurements can be used with the following equations:
 
 V   C1   =M   C   ×P   S   (1)
         wherein,
           V C1 =first capacitive response of the capacitive electrode  105 ,   M C =absolute sensitivity of the capacitive electrode  105 , and   P S =acoustic pressure  140  applied to the capacitive electrode  105  by the transducer  155  in response to the speaker control signal.
 
 V   P1   =M   P   ×P   S   (2)
   
           wherein,
           V P1 =first piezoelectric response of the piezoelectric electrode  115 ,   M P =absolute sensitivity of the piezoelectric electrode  115 , and   P S =acoustic pressure  140  applied to the piezoelectric electrode  115  by the transducer  155  in response to the speaker control signal.   
               

     The same amount of acoustic pressure  140  is applied by the transducer  155  to the capacitive electrode  105  and the piezoelectric electrode  115 . Therefore, equations 1 and 2 can be combined to form the follow equation:
 
 M   P   =M   C ×( V   P1   /V   C1 )  (3).
 
     The third measurement can be used with following equation:
 
 M   P   ×M   O =(1 /Z   M )×( V   P2   /I   C )  (4)
         wherein,
           Z M =mechanical transfer impedance,   V P2 =second piezoelectric response of the piezoelectric electrode  115 , and   I C =capacitive control signal.   
               

     The mechanical transfer impedance is a system variable that is determined based on the construction on the MEMS microphone  100 . In some embodiments, the mechanical transfer impedance is substantially equal to one. 
     Equations 3 and 4 can be combined to form the following equation to determine the absolute sensitivity of the capacitive electrode  105 :
 
( M   C ) 2 =( V   C1   /V   P1 )×(1 /Z   M )×( V   P2   /I   C )  (5).
 
     The fourth measurement can be used with the following equation:
 
 M   P   ×M   O =(1 /Z   M )×( V   C2   /I   P )  (6)
         wherein,
           V C2 =second capacitive response of the capacitive electrode  105 , and   I P =piezoelectric control signal.   
               

     Equations 3 and 6 can be combined to form the following equation to determine the absolute sensitivity of the piezoelectric electrode  115 :
 
( M   P ) 2 =( V   P1   /V   C1 )×(1 /Z   M )×( V   C2   /I   P )  (7).
 
       FIG. 6  illustrates a process  300  (or method) for determining the absolute sensitivities of the capacitive electrode  105  and the piezoelectric electrode  115 . Various steps described herein with respect to the process  300  are capable of being executed simultaneously, in parallel, or in an order that differs from the illustrated serial manner of execution. The process  300  may also be capable of being executed using fewer steps than are shown in the illustrated embodiment. As will be explained in greater detail, portions of the process  300  can be implemented in software executed by the controller  205 . 
     The process  300  begins with the generation of acoustic pressure  140  by the transducer  155  (step  305 ). In some embodiments, the transducer  155  generates the acoustic pressure  140  in response to receiving the speaker control signal from the controller  205 . The controller  205  determines the first capacitive response of the capacitive electrode  105  in response to the acoustic pressure  140  (step  310 ). The controller  205  also determines the first piezoelectric response of the piezoelectric electrode  115  in response to the acoustic pressure  140  (step  315 ). 
     Next, the capacitive electrode  105  generates the first mechanical pressure  160  (step  320 ). In some embodiments, the capacitive electrode  105  generates the first mechanical pressure  160  in response to receiving the capacitive control signal. The controller  205  determines the second piezoelectric response of the piezoelectric electrode  115  in response to the first mechanical pressure  160  (step  325 ). Next, the piezoelectric electrode  115  generates the second mechanical pressure  165  (step  330 ). In some embodiments, the piezoelectric electrode  115  generates the second mechanical pressure  165  in response to receiving the piezoelectric control signal. The controller  205  determines the second capacitive response of the capacitive electrode  105  in response to the second mechanical pressure  165  (step  335 ). 
     At step  340 , the controller  205  then determines the absolute sensitivity of the capacitive electrode  105 . In some embodiments, the controller  205  determines the absolute sensitivity of the capacitive electrode  105  based on the first capacitive response, the first piezoelectric response, and the second piezoelectric response. In some embodiments, the controller  205  determines the absolute sensitivity of the capacitive electrode  105  according to equation 5, described herein. At step  345 , the controller  205  determines the absolute sensitivity of the piezoelectric electrode  115 . In some embodiments, the controller  205  determines the absolute sensitivity of the piezoelectric electrode  115  based on the first capacitive response, the second capacitive response, and the first piezoelectric response. In some embodiments, the controller  205  determines the absolute sensitivity of the piezoelectric electrode  115  according to equation 7, described herein. 
     Thus, the disclosure provides, among other things, microphone systems and methods of determining absolute sensitivities on a MEMS microphone. Various features and advantages of the disclosure are set forth in the following claims.