Patent Publication Number: US-11387747-B2

Title: System and method for a MEMS device

Description:
This application is a continuation of U.S. patent application Ser. No. 14/992,615, filed on Jan. 11, 2016, which application is hereby incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to transducers, and, in particular embodiments, to a system and method for a MEMS device. 
     BACKGROUND 
     Transducers convert signals from one domain to another and are often used as sensors. For example, acoustic transducers convert between acoustic signals and electrical signals. A microphone is one type of acoustic transducer that converts sound waves, i.e., acoustic signals, into electrical signals, and a speaker is one type of acoustic transducer that converts electrical signals into sound waves. 
     Microelectromechanical system (MEMS) based transducers include a family of transducers produced using micromachining techniques. Some MEMS transducers, such as a MEMS microphone, gather information from the environment by measuring the change of physical state in the transducer and transferring the signal to be processed by the electronics which are connected to the MEMS sensor. Other MEMS transducers, such as a MEMS microspeaker, convert electrical signals into a change in the physical state in the transducer. MEMS devices may be manufactured using micromachining fabrication techniques similar to those used for integrated circuits. 
     MEMS devices may be designed to function as oscillators, resonators, accelerometers, gyroscopes, pressure sensors, microphones, micro-mirrors, microspeakers, etc. Many MEMS devices use capacitive sensing or actuation techniques for transducing the physical phenomenon into electrical signals and vice versa. In such applications, the capacitance change in the transducer is converted to a voltage signal using interface circuits or a voltage signal is applied to the capacitive structure in the transducer in order to generate a force between elements of the capacitive structure. 
     For example, a capacitive MEMS microphone includes a backplate electrode and a membrane arranged in parallel with the backplate electrode. The backplate electrode and the membrane form a parallel plate capacitor. The backplate electrode and the membrane are supported by a support structure arranged on a substrate. 
     The capacitive MEMS microphone is able to transduce sound pressure waves, for example speech, at the membrane arranged in parallel with the backplate electrode. The backplate electrode is perforated such that sound pressure waves pass through the backplate while causing the membrane to vibrate due to a pressure difference formed across the foreside and backside of the membrane. Hence, the air gap between the membrane and the backplate electrode varies with vibrations of the membrane. The variation of the membrane in relation to the backplate electrode causes variation in the capacitance between the membrane and the backplate electrode. This variation in the capacitance is transformed into an output signal responsive to the movement of the membrane and forms a transduced signal. 
     Using a similar structure, a voltage signal may be applied between the membrane and the backplate in order to cause the membrane to vibrate and generate pressure pulses, such as sound pressure waves. Thus, a capacitive plate MEMS structure may operate as a microspeaker. 
     SUMMARY 
     According to an embodiment, According to an embodiment, a microelectromechanical systems MEMS device includes a first membrane attached to a support structure that a first plurality of acoustic vents; a second membrane attached to the support structure that includes a second plurality of acoustic vents, where the first plurality of acoustic vents and the second plurality of acoustic vents do not overlap; and a closing mechanism coupled to the first membrane and the second membrane. Other embodiments include corresponding systems and apparatus, each configured to perform various embodiment methods. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates a system block diagram of an embodiment variable flow transducer; 
         FIGS. 2A and 2B  illustrate waveform diagrams of illustrative acoustic signals; 
         FIGS. 3A, 3B, and 3C  illustrate side view cross-sections of an embodiment variable flow transducer; 
         FIGS. 4A, 4B, and 4C  illustrate an embodiment model variable flow transducer and a corresponding waveform diagram; 
         FIGS. 5A and 5B  illustrate side view cross-sections of additional embodiment variable flow transducer; 
         FIGS. 6A, 6B, and 6C  illustrate side view cross-sections of embodiment acoustic valves; 
         FIGS. 7A, 7B, 7C, and 7D  illustrate top views of further embodiment variable flow transducers; 
         FIGS. 8A and 8B  illustrate side view cross-sections of more embodiment variable flow transducers; 
         FIGS. 9A, 9B, and 9C  illustrate side view cross-sections and a top view of another embodiment variable flow transducer; 
         FIGS. 10A, 10B, and 10C  illustrate waveform diagrams of embodiment variable flow transducer operation; 
         FIG. 11  illustrates an additional waveform diagram of embodiment variable flow transducer operation; and 
         FIG. 12  illustrates a flowchart diagram of embodiment method of operation for a variable flow transducer. 
     
    
    
     Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the embodiments and are not necessarily drawn to scale. 
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     The making and using of various embodiments are discussed in detail below. It should be appreciated, however, that the various embodiments described herein are applicable in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use various embodiments, and should not be construed in a limited scope. 
     Description is made with respect to various embodiments in a specific context, namely acoustic transducers, and more particularly, MEMS microspeakers. Some of the various embodiments described herein include MEMS microspeakers, acoustic transducer systems, variable volume flow transducers, and variable volume flow MEMS microspeakers. In other embodiments, aspects may also be applied to other applications involving any type of transducer domain according to any fashion as known in the art. 
     Speakers are transducers that transduce electrical signals into acoustic signals. The acoustic signal is produced by the speaker structure generating pressure oscillations at a frequency. For example, the audible range of humans is about 20 Hz to 20 kHz, with some humans able to hear less than this range and some humans able to hear beyond this range. Thus, a speaker operating in order to produce audible acoustic signals transduces electrical signals into sound pressure oscillations with frequencies between 20 Hz and 20 kHz. A constant frequency signal is conveyed as a simple tone, similar to a note on a piano. Speech and other typical sounds such as, e.g., music, are composed of numerous acoustic signals with numerous frequencies at the same time. 
     Microspeakers operate according to the same principles as speakers, but are produced using micromachining or microfabrication techniques. Thus, audible microspeakers include small structures that are excited by electrical signals in order to generate pressure oscillations in the audible frequency range. 
     According to various embodiments, a speaker, or microspeaker, is configured to generate audible acoustic signals by oscillating at frequencies above the audible frequency range. In such embodiments, the speaker is configured to generate pressure oscillations at a frequency above the audible range and modify the volume flow of the pressure oscillations according to a lower frequency in the audible frequency range. In such embodiments, the human auditory system will recognize the envelope of the pressure oscillations and act like a low pass filer. In additional embodiments, the speaker may be configured to generate pressure oscillations at a frequency above the audible range and modify the volume flow of the pressure oscillations according to a lower frequency still outside the audible frequency range in order to operate as an ultrasound transducer. 
     In various embodiments, the speaker is referred to as a variable flow transducer. The frequency of the variable flow transducer may maintain operation outside the audible frequency range, while the volume flow alters the positive and negative sound pressures of the oscillations according to other frequencies that are inside the audible frequency range. In such embodiments, the variable flow transducer may include a deflectable membrane with multiple valve structures that are configured to adjust the acoustic impedance and alter the volume flow as the deflectable membrane oscillates above the audible frequency range. Various embodiments are further described herein below. 
       FIG. 1  illustrates a system block diagram of an embodiment variable flow transducer  100  including microspeaker  102 , application specific integrated circuit (ASIC)  104 , and audio processor  106 . According to various embodiments, microspeaker  102  generates acoustic signal  108 , which includes pressure oscillations at a frequency above the audible limit, e.g., 20 kHz, with adjustments of the positive and negative sound pressures during the oscillations. The positive and negative sound pressures may be adjusted by using embodiment valves to adjust the acoustic impedance of a membrane in microspeaker  102 . By adjusting the volume flow through control of the positive and negative sound pressures, low frequency sound pressure signals in the audible range may be generated from the membrane that oscillates at a frequency above the audible limit. Thus, microspeaker  102  generates acoustic signal  108  including an audible acoustic signal formed from an inaudible acoustic signal. In various embodiments, the pressure oscillations of acoustic signal  108  have a frequency that is at least twice the limit of the human auditory range, e.g., 40 kHz, in order to fulfill the Nyquist-Shannon sampling theorem. 
     In various embodiments, microspeaker  102  includes a deflectable membrane with valves. Various example embodiment structures are described further herein below. Microspeaker  102  is driven by drive signals provided from ASIC  104 . ASIC  104  may generate analog drive signals based on a digital input control signal. In some embodiments, ASIC  104  and microspeaker  102  are attached to a same circuit board. In other embodiments, ASIC  104  and microspeaker  102  are formed on a same semiconductor die. ASIC  104  may include biasing and supply circuits, an analog drive circuit, and a digital to analog converter (DAC). In further embodiments, microspeaker  102  may include a microphone, for example, and ASIC  104  may also include readout electronics such as an amplifier or analog to digital converter (ADC). 
     In some embodiments, the DAC in ASIC  104  receives a digital control signal at an input supplied by audio processor  106 . The digital control signal is a digital representation of the acoustic signal that microspeaker  102  produces. In various embodiments, audio processor  106  may be a dedicated audio processor, a general system processor, such as a central processing unit (CPU), a microprocessor, or a field programmable gate array (FPGA). In alternative embodiments, audio processor  106  may be formed of discrete logic blocks or other components. In various embodiments, audio processor  106  generates the digital representation of acoustic signal  108  and provides the digital representation of acoustic signal  108 . In other embodiments, audio processor  106  provides the digital representation of only the audible portion of acoustic signal  108  and ASIC  104  generates the driving signal for acoustic signal  108  with the higher inaudible frequency oscillations and the audible frequency oscillations based on variations in volume flow. 
     According to various additional embodiments, microspeaker  102  may also generate acoustic signal  108 , which includes pressure oscillations at a frequency above the audible limit, e.g., 20 kHz, with volume flow adjustments of the sound pressure oscillations that are adjusted at frequencies that are also above the audible range. For example, microspeaker  102  may operate as an ultrasound transducer for ultrasound imaging or for ultrasound near field detection. In such embodiments, microspeaker  102  operates with a higher frequency as a carrier signal that has positive and negative sound pressures adjusted according to a lower frequency of the generated target signal, such as an ultrasound signal for example. 
       FIGS. 2 a  and 2 b    illustrate waveform diagrams of illustrative acoustic signals.  FIG. 2 a    shows acoustic signal A SIG  that may be produced by a speaker, for example. Acoustic signal A SIG  has amplitude A amp  and frequency A freq , i.e., period A T =1÷A freq . Acoustic signal A SIG  may illustrate a sound wave produced by a speaker. During operation, the sound wave has frequency A freq  that is within the audible frequency range for a human, e.g., between about 20 Hz and 20 kHz.  FIG. 2 a    illustrates amplitude A amp  for acoustic signal A SIG  at an unspecified level. For a MEMS microspeaker, generating a large sound pressure level (SPL) may present challenges due to the small size of the membrane, especially at low frequency. For example, a MEMS microspeaker may include a decrease of 40 dB in SPL per decade as frequency decreases through the audible frequency range. Thus, it may be challenging to generate higher SPLs at frequencies below, for example, 1-10 kHz without increasing the size of the pumping structure, for example. 
       FIG. 2 b    shows modulated acoustic signal MA SIG  that may be produced by an embodiment variable flow transducer, such as a MEMS microspeaker. According to various embodiments, modulated acoustic signal MA SIG  has amplitude MA amp  and frequency MA freq , i.e., period MA T =1÷MA freq , and is formed of carrier signal C SIG , which has variable amplitude C amp  and frequency C freq , i.e., period C T =1÷C freq . As shown, frequency C freq  is much higher than frequency MA freq . Specifically, frequency C freq  is above the audible frequency range of a human, i.e., above 20 kHz, and frequency MA freq  is within the audible frequency range of a human, i.e., between about 20 Hz and 20 kHz. In such embodiments, amplitude C amp  is adjusted in order to form the rising and falling wave form of pumping acoustic signal PA SIG . Further, negative or positive sound pressures are removed or reduced for carrier signal C SIG  in order to form the rising and falling wave form of modulated acoustic signal MA SIG . The oscillations of a deflectable membrane generally include symmetric volume flow that includes equal positive and negative pressure. In various embodiments, carrier signal C SIG  includes only one type of sound pressure, e.g., positive sound pressure, for the first half-wave (MA T /2) of modulated acoustic signal MA SIG  and only a second type of sound pressure, e.g., negative sound pressure, for the second hale-wave (MA T /2) of modulated acoustic signal MA SIG . In such embodiments, carrier signal C SIG  shapes the positive sound pressure first half-wave of modulated acoustic signal MA SIG  by removing (or reducing) the negative sound pressure components and the negative sound pressure second half-wave of modulated acoustic signal MA SIG  by removing (or reducing) the positive sound pressure components. The variation of amplitude C amp  and direction of carrier signal C SIG , through the reducing or removing of positive or negative sound pressures, is performed at a specific frequency in order to form modulated acoustic signal MA SIG  with frequency MA freq , which is in the audible range, e.g., 20 Hz to 20 kHz. According to various embodiments, variable flow transducers adjust the acoustic impedance of a deflectable membrane in order to reduce or remove negative or positive sound pressures. 
     In particular embodiments, amplitude MA amp  of modulated acoustic signal MA SIG  may be larger than a traditional microspeaker that oscillates at an audible frequency. In specific embodiments, the oscillation of the pumping speaker remains at a higher frequency such that the SPL of modulated acoustic signal MA SIG  does not decrease much or at all when frequency MA freq  is below about 1-10 kHz and above about 10 Hz, for example. According to various embodiments, the produced sound or pressure pulses of modulated acoustic signal MA SIG  are equal to, or approximately equal to, the second derivative of the deflectable membrane position, which is the acceleration of the deflectable membrane. Thus, in various embodiments, the control of the pumping action, such as the control of the positive and negative sound pressures, may be based on the acceleration of the deflectable membrane. 
     In various embodiments, frequency C freq  may be held constant as amplitude C amp  and direction of carrier signal C SIG  are varied. In specific embodiments, frequency C freq  may be matched to the resonant frequency of the speaker or microspeaker in order to produce greater oscillations of the deflectable membrane. In other embodiments, frequency C freq  may be variable. In particular examples, frequency C freq  is between 40 kHz and 10 MHz. In more specific embodiments, frequency C freq  is between 100 kHz and 300 kHz. In such various embodiments, frequency MA freq  is below 20 kHz. Specifically, frequency MA freq  is in the audible frequency range of humans, i.e., between 20 Hz and 20 kHz, where this range may be expanded for some humans and narrowed for others. In alternative embodiments, frequency MA freq  may be above 20 kHz. In such embodiments, modulated acoustic signal MA SIG  may be, instead of an acoustic signal, an ultrasound signal used in an ultrasound transducer for ultrasound imaging or near field detection. 
     According to various embodiments, variable flow transducers, such as MEMS microspeakers, are operated as described in reference to  FIG. 2 b    by using a carrier signal above the audible frequency range to form a modulated acoustic signal within the audible frequency range. Various embodiment variable flow transducers are described herein below in order to illustrate some of the specific applications including capacitive plate structures and other pumping structures. Such embodiment variable volume flow transducers adjust the acoustic impedance of the deflectable membrane in order to reduce or remove negative or positive sound pressures. 
     Referring back to  FIG. 1  in view of  FIGS. 2 a  and 2 b   , ASIC  104  in variable flow transducer  100  is configured to determine the resonant frequency of microspeaker  102  in some embodiments. In such embodiments, ASIC  104  may excite microspeaker  102  at a plurality of frequencies and measure the response for each frequency. Based on the measured response, ASIC  104  determines the resonant frequency of microspeaker  102 . In such embodiments, ASIC  104  may set frequency C freq  for carrier signal C SIG  to the determined resonant frequency. In other alternative embodiments, ASIC  104  may control elements of microspeaker  102  in order to adjust the resonant frequency to match frequency C freq  for carrier signal C SIG . In one embodiment, controlling the elements includes adjusting mechanical components of microspeaker  102 . In an alternative embodiment, controlling the elements includes adjusting active or passive electrical components of microspeaker  102 . 
       FIGS. 3A, 3B, and 3C  illustrate side view cross-sections of an embodiment variable flow transducer  110 . According to various embodiments, variable flow transducer  110  adjusts the acoustic impedance during oscillations in order to regulate the generation of positive and negative sound pressures. In various embodiments, variable flow transducer  110  includes membrane  112 , acoustic valves  114 , and actuating structures  116 . In such embodiments, actuating structures  116  may include a piezoelectric layer or layers configured to generate a force on membrane  112  based on an applied voltage. Actuating structures  116  are formed on a surface of membrane  112  in actuation area  122   a . Actuating structures  116  may be formed on the top surface of membrane  112  in some embodiments, as illustrated, or may be formed on the bottom surface of membrane  112  in other embodiments. In further embodiments, actuating structures  116  may be formed on the top and bottom surfaces of membrane  112 . In such embodiments, the driving force is inversed between top and bottom actuating structures  116 . 
     In various embodiments, an electrical drive signal, such as a control voltage, is provided to actuating structures  116  in order to excite membrane  112  to oscillate at a first frequency above the audible range, i.e., above 20 kHz. For example, in some embodiments, membrane  112  is excited to oscillate at a resonant frequency, which may range from 75 kHz to 200 kHz. In such embodiments, the first frequency may correspond to frequency C freq  for carrier signal C SIG , as described hereinabove in reference to  FIG. 2B . Thus, membrane  112  oscillates with upward and downward movements as shown in  FIGS. 3B and 3C . In various embodiments, acoustic valves  114  are closed for movement in a first direction, such as displayed in  FIG. 3B  during a positive acceleration, and open during the negative acceleration as it occurs during braking of the membrane.  FIG. 3C  shows the second direction, where positive acceleration occurs in this direction and negative acceleration occurs in the inverse direction. 
     In various embodiments, membrane  112  has a first acoustic impedance when acoustic valves  114  are closed and a second acoustic impedance when acoustic valves  114  are open. The first impedance is much greater than the second impedance. In such embodiments, when the acoustic impedance is higher, i.e., when acoustic valves  114  are closed, the sound pressure generated by oscillations of membrane  112  are at a normal or large level. Conversely, when the acoustic impedance is lower, i.e., when acoustic valves  114  are open, the sound pressure generated by oscillations of membrane  112  are at a lower or reduced level. Thus, in various embodiments, variable flow transducer  110  is configured to adjust the acoustic impedance of membrane  112  by opening and closing acoustic valves  114  and generate normal or large sound pressure levels in a positive acceleration and lower or reduced sound pressure levels in a negative acceleration. 
     In various embodiments, the acoustic impedance of membrane  112  may be adjusted to be acoustically transparent for a certain percentage of the membrane area when acoustic valves  114  are open. For example, in some embodiments, the quality and the area of acoustic valves  114  cause membrane  112  to be 90% acoustically transparent in a particular embodiment. In another particular embodiment, the quality and the area of acoustic valves  114  cause membrane  112  to be 50% acoustically transparent. In other embodiments, the acoustic transparency of membrane  112  may range from 30% to 95%. 
     As described hereinabove in reference to  FIG. 2B , by adjusting the acoustic impedance of membrane  112  to be large during positive acceleration in upward movements and reduced during braking or negative acceleration for upward movements (as shown in  FIG. 3B ), variable flow transducer  110  may remove or reduce negative or positive sound pressures and form a first half-wave of an acoustic signal having a second frequency that is within the audible range. In such embodiments, the second frequency may correspond to frequency MA freq  for modulated acoustic signal MA SIG , as described hereinabove in reference to  FIG. 2B . Similarly, by adjusting the acoustic impedance of membrane  112  to be large during downward movements for positive accelerations and reduced during downward movements for negative acceleration (as shown in  FIG. 3C ), variable flow transducer  110  may remove or reduce negative sound pressures and form a second half-wave of the acoustic signal. Thus, by modulating the acoustic impedance to control the generate sound pressures, membrane  112  may oscillate at the first frequency, that is outside the audible range, and generate an acoustic signal at the second frequency, that is within the audible range. In such various embodiments, similar efforts or techniques referred to as digital sound reconstruction may be implemented. 
     In various embodiments, acoustic valves  114  include piezoelectric materials that open and close acoustic valves  114  based on electrical control signals. Acoustic valves  114  are formed throughout ventilation area  122   b  of membrane  112 . In various embodiments, membrane  112  is formed of structural layer  118  and isolation layer  120 . In some embodiments, structural layer  118  is a conductive layer, such as a semiconductor or metal, and isolation layer  120  is an electrically insulating layer, such as an oxide layer, a nitride layer, or an oxynitride layer. In other embodiments, structural layer  118  and isolation layer  120  may be combined into a single conductive or electrically insulating layer. As shown, membrane  112  may be anchored to a support structure at a periphery. Further structure details of various embodiments are described hereinafter in reference to the other Figures. In other embodiments, acoustic valves  114  or membrane  112  may be actuated electrostatically, instead of piezoelectrically as shown. 
       FIGS. 4A, 4B, and 4C  illustrate an embodiment model variable flow transducer and a corresponding waveform diagram. Specifically,  FIG. 4A  depicts annotated variable flow transducer  130 ,  FIG. 4B  depicts piston model  132 , and  FIG. 4C  depicts membrane displacement waveform  134  and membrane acceleration waveform  136 . According to various embodiments, when acoustic valves  114  are closed, membrane  112  has a high acoustic impedance, as illustrated by closed valve portion  138  of annotated variable flow transducer  130  and piston model  132 . Conversely, when acoustic valves  114  are open, membrane  112  has a low acoustic impedance, as illustrated by open valve portion  142  of annotated variable flow transducer  130  and piston model  132 . Transition between acoustic high impedance and acoustic low impedance is depicted by transition portion  140 . In such embodiments, oscillations of membrane  112  may be modeled with equal displacement of the entire membrane according to piston model  132 . When membrane  112  has a low acoustic impedance, the acoustic medium, such as air, is able to easily pass from one side of membrane  112  to the other. When membrane  112  has a high acoustic impedance, the acoustic medium, such as air, is unable to easily pass from one side of membrane  112  to the other. 
     According to various embodiments, transitioning from closed valve portion  138  to open valve portion  142  may be based on the acceleration of membrane  112 . As illustrated by membrane displacement waveform  134  and membrane acceleration waveform  136 , when acceleration of membrane  112  has a positive value, acoustic valves  114  are closed, and when acceleration of membrane  112  has a negative value, acoustic valves  114  are open. In such embodiments, the positive and negative sign of the acceleration may be switched based on the half-wave of the acoustic signal, positive or negative half-wave (see  FIG. 2B ), being generated. In various embodiments, the acoustic impedance may be adjusted based on the displacement or acceleration of membrane  112  in order to selectively generate positive or negative sound pressure waves for forming audible acoustic signals. 
     Further embodiment variable flow transducers are described hereinafter as illustrative embodiments. 
       FIGS. 5A and 5B  illustrate side view cross-sections of additional embodiment variable flow transducer  150  and embodiment variable flow transducer  151 . According to various embodiments, variable flow transducer  150  includes substrate  152 , membrane  154 , top backplate  156  or bottom backplate  158 , and acoustic valves  160 . Acoustic valves  160  are shown generically as dashed structures and may be implemented as piezoelectric or electrostatic controllable valves. Example embodiment acoustic valves are described further hereinafter in reference to  FIGS. 6A, 6B, and 6C . 
     In various embodiments, membrane  154  is a deflectable membrane that is actuated electrostatically by applying a voltage difference between membrane  154  and top backplate  156  or between membrane  154  and bottom backplate  158 . In some embodiments, variable flow transducer  150  is a dual backplate microspeaker that includes both top backplate  156  and bottom backplate  158 . In other embodiments, variable flow transducer  150  is a single backplate microspeaker that includes either top backplate  156  or bottom backplate  158 . In various embodiments, top backplate  156  and bottom backplate  158  include perforations  157  that allow fluidic transport from one side of top backplate  156  or bottom backplate  158  to the other side. In such embodiments, the fluidic transport allows acoustic signals to pass through top backplate  156  and bottom backplate  158 , which provide a low acoustic impedance. 
     In various embodiments, membrane  154  is electrostatically driven to oscillate at a frequency above the audible range. In specific embodiments, membrane  154  oscillates with a frequency ranging from 40 kHz to 300 kHz. During oscillations, acoustic valves  160  are controlled to regulate generation of positive or negative sound pressures from oscillations of membrane  154  and form modulated acoustic signals that have frequencies within the audible range, as described hereinabove in reference to  FIGS. 2A, 2B, 3A, 3B, 3C, 4A, 4B, and 4C . 
     In some embodiments, bypass route  166 , bypass structure  162 , and acoustic valves  160  in bypass structure  162  are included surrounding membrane  154 . In other embodiments, bypass route  166 , bypass structure  162 , and acoustic valves  160  in bypass structure  162  are omitted. In some embodiments including bypass route  166 , acoustic valves  160  on membrane  154  may be omitted. In other embodiments including bypass route  166 , acoustic valves  160  on membrane  154  are included. 
     In various embodiments, substrate  152  is formed of a semiconductor material. For example, substrate  152  may be silicon, such as polysilicon, gallium-arsenide (GaAs), indium phosphide (InP), or carbon in particular embodiments. In other embodiments, substrate  152  is formed of a dielectric material such as a glass. In still further embodiments, substrate is formed of a polymer, such as hexamethyldisilazane (HMDS) for example. In other alternative embodiments, substrate  152  is formed of a ceramic material. In various embodiments, membrane  154  is formed of a semiconductor or a metal, such as polysilicon, gold, aluminum, copper, or platinum. In other embodiments, membrane  154  formed of a non-conductive layer and a conductive layer. In various embodiments, top backplate  156  and bottom backplate  158  are formed of a semiconductor or a metal, such as polysilicon, gold, aluminum, copper, or platinum. In further embodiments, top backplate  156  and bottom backplate  158  are formed of multiple layers including conductive layers and non-conductive or electrically insulating layer. For example, in a particular embodiment, top backplate  156  and bottom backplate  158  are formed of polysilicon and silicon nitride. Substrate  152  includes cavity  164 , which may pass through the entirety of substrate  152 , such as through a wafer including substrate  152 . 
     According to various embodiments, variable flow transducer  151  includes substrate  152 , membrane  168 , and acoustic valves  160 . In such embodiments, membrane  168  is a deflectable membrane that is actuated piezoelectrically by applying a voltage signal to piezoelectric layer  170 . By applying a voltage signal to piezoelectric layer  170 , a deformation is generated in piezoelectric layer  170  that generates a force on membrane  168 . The excitation of membrane  168  is performed at a higher frequency above the audible range and acoustic valves  160  are controlled to form modulated acoustic signals that have frequencies within the audible range, as described hereinabove in reference to variable flow transducer  150  in  FIG. 5A . 
     In various embodiments, membrane  168  includes structural layer  172 , isolation layer  174 , and piezoelectric layer  170 . In some embodiments, structural layer  172  is a conductive layer, such as a semiconductor layer or a metal layer. Isolation layer  174  may be an electrically insulating layer, such as an oxide layer, a nitride layer, or an oxynitride layer. In various embodiments, piezoelectric layer  170  includes piezoelectric ceramics or piezoelectric crystals. In particular embodiments, piezoelectric layer  170  includes lead zirconate titanate (PZT) or barium titanate (BaTiO 3 ). In other particular embodiments, piezoelectric layer  170  includes zinc oxide (ZnO), aluminum nitride (AlN), or polyvinylidene fluoride (PVDF). 
     According to various embodiments, variable flow transducer  150  and variable flow transducer  151  are illustrated in  FIGS. 5A and 5B  in cross-section and may include any membrane shape when viewed from above. Specifically, membrane  154  and membrane  168  may be round, including circular or oval shapes, or rectangular in particular embodiments. In some embodiments, bypass route  166  is omitted and substrate  152  extends to and surrounds membrane  154  or membrane  168 . In other embodiments, bypass route  166  is included and substrate  152  includes a portion surrounding and supporting membrane  154  or membrane  168  that is connected to the main portion of substrate  152 . In such embodiments, portions of the perimeter of membrane  154  or membrane  168  include bypass route  166  and other portions of the perimeter of membrane  154  or membrane  168  include solid portions of substrate  152 . Various embodiment variable flow transducers are described hereinafter in reference to top views illustrated in  FIGS. 7A, 7B, 7C, and 7D . 
       FIGS. 6A, 6B, and 6C  illustrate side view cross-sections of embodiment acoustic valves  180 ,  181 , and  182 . According to various embodiments, acoustic valve  180 , acoustic valve  181 , or acoustic valve  182  may be used to implement any of the acoustic valves described herein, such as acoustic valve  114  or acoustic valve  160  as described hereinabove. 
     According to various embodiments, acoustic valve  180  includes structural layer  184 , isolation layer  186 , acoustic flap  188 , and piezoelectric layer  190 . In various embodiments, piezoelectric layer  190  may include any of the materials described hereinabove in reference to piezoelectric layer  170 . Piezoelectric layer  190  is disposed on acoustic flap  188 . In various embodiments, acoustic flap  188  has mechanical elasticity. In particular embodiments, acoustic flap  188  is single crystal silicon or polysilicon. In various further embodiments, acoustic flap  188  may be any type of electrically insulating material with suitable mechanical properties for actuation. In still further embodiments, acoustic flap  188  may include any type of electrically conductive material with an insulating layer. In specific embodiments, acoustic flap  188  is graphene with an insulating layer. In various embodiments, piezoelectric layer  190  extends over only part of the top surface of acoustic flap  188 , as shown. In alternative embodiments, piezoelectric layer  190  extends over the entire top surface of acoustic flap  188  (not shown). In alternative embodiments, piezoelectric layer  190  can be shaped in various ways to achieve different transient valve characteristics due to structural or mechanical interactions. For example, piezoelectric layer  190  may be shaped with a solid region, a comb region, a circular region, or another shape in order to adjust the transient valve characteristics. 
     In various embodiments, acoustic flap  188  seals opening  185  in structural layer  184  and isolation layer  186 . When an electrical drive signal, such as a control voltage, is applied to piezoelectric layer  190 , piezoelectric layer  190  begins to deform, causing a force on acoustic flap  188 . The force on acoustic flap  188  moves acoustic flap  188  to open and allow fluid transport through opening  185 . In some embodiments, a first control voltage is applied to piezoelectric layer  190  to close acoustic flap  188  and seal opening  185 , and a second control voltage is applied to piezoelectric layer  190  to open acoustic flap  188  and open opening  185 . 
     In various embodiments, isolation layer  186  is an electrically insulating material. In some embodiments, isolation layer  186  is an oxide, nitride, or oxynitride. In particular embodiments, isolation layer  186  is silicon nitride (SiN) or silicon oxide (SiO 2 ). According to various embodiments, structural layer  184  is an electrically conductive or semiconductive material. In some embodiments, structural layer  184  is a crystalline or amorous semiconductor element or compound. In particular embodiments, structural layer  184  is polysilicon. In other embodiments, structural layer  184  is a metal. In particular embodiments, structural layer  184  is aluminum, platinum, gold, or copper. In various embodiments, structural layer  184  may be a portion of a deflectable membrane, such as described herein in reference to the other figures. 
     According to various embodiments, acoustic valve  181  includes structural layer  184 , isolation layer  186 , acoustic flap  192 , and piezoelectric layer  194 . In such embodiments, acoustic flap  192  is a portion of structural layer  184 . Piezoelectric layer  194  may include any of the materials described hereinabove in reference to piezoelectric layer  190  in  FIG. 6A . Further, piezoelectric layer  190  may extend over only a portion of the top surface of acoustic flap  192 , as shown. In alternative embodiments, piezoelectric layer  190  extends over the entire top surface of acoustic flap  192  (not shown). 
     According to various embodiments, acoustic valve  182  includes structural layer  184 , isolation layer  186 , structural support  196 , and electrostatic seal layer  198 . In such embodiments, a control voltage is applied to electrostatic seal layer  198  in order to generate an electrostatic force that closes electrostatic seal layer  198  and seals opening  185 . In various embodiments, electrostatic seal layer  198  is a conductive or semiconductive material. In various particular embodiments, electrostatic seal layer  198  is polysilicon, gold, aluminum, cooper, or platinum. Structural support  196  is formed of an electrically insulating structural material. In some embodiments, structural support  196  is formed oxide, such as tetraethyl orthosilicate (TEOS) oxide. 
     In various embodiments, in order to generate an electrostatic force on electrostatic seal layer  198 , a voltage difference is applied between electrostatic seal layer  198  and structural layer  184 . When the voltage difference is applied, electrostatic seal layer  198  seals opening  185  and when no voltage difference is applied, electrostatic seal layer  198  moves away from opening  185  and allows fluid transport through opening  185 . 
       FIGS. 7A, 7B, 7C, and 7D  illustrate top views of further embodiment variable flow transducers  200   a ,  200   b ,  200   c , and  200   d .  FIG. 7A  illustrates variable flow transducer  200   a  including support structure  202 , membrane  204 , and acoustic valves  206 . According to various embodiments, membrane  204  is driven to oscillate above a higher first frequency and acoustic valves  206  are controlled to open and close in order to shape the positive and negative sound pressures that form acoustic signals with frequencies below a lower second frequency. In some embodiments, membrane  204  may oscillate with a frequency ranging from 40 kHz to 300 kHz and acoustic valves  206  may be opened and closed to form acoustic signals with frequencies ranging from 20 Hz to 20 kHz. 
     In such embodiments, acoustic valves  206  may be implemented as described hereinabove in reference to acoustic valves  114 ,  160 ,  180 ,  181 , or  182  in reference to the other figures. In particular embodiments, acoustic valves  206  correspond to acoustic valve  180  or acoustic valve  181  as described hereinabove in reference to  FIGS. 6A and 6B , respectively. In specific embodiments, acoustic valves  206  include acoustic flaps  208  and piezoelectric actuation layers  210  formed on a top surface of the acoustic flap  208 . 
     In various embodiments, support structure  202  may be a substrate, such as described hereinabove in reference to substrate  152  in  FIGS. 5A and 5B . In other embodiments, support structure  202  may be an oxide, such as a TEOS oxide, or a polymer. In such embodiments, support structure  202  may be formed on a substrate. Membrane  204  may include any of the structures and materials as described hereinabove in reference to membrane  154  or membrane  168  in  FIGS. 5A and 5B , respectively. In various embodiments a cavity is formed in the substrate below membrane  204 . 
       FIG. 7B  illustrates variable flow transducer  200   b  including support structure  202 , membrane  204 , and acoustic valves  212 . According to various embodiments, variable flow transducer  200   b  is similar to variable flow transducer  200   a , with the exception that acoustic valves  206 , which are piezoelectrically actuated, are replaced by acoustic valves  212 , which are electrostatically actuated. In such embodiments, acoustic valves  212  correspond to acoustic valve  182  as described hereinabove in reference to  FIG. 6C . Acoustic valves  212  include electrostatic seal layer  214 . 
       FIG. 7C  illustrates variable flow transducer  200   c  including support structure  202 , membrane  204 , and acoustic valves  216 . According to various embodiments, acoustic valves  216  are formed in support structure  202  around membrane  204 . In such embodiments, acoustic valves  216  correspond to bypass route  166 , bypass structure  162 , and acoustic valves  160  in bypass structure  162  as described hereinabove in reference to  FIGS. 5A and 5B . 
     In particular embodiments, acoustic valves  216  may be implemented as described hereinabove in reference to acoustic valves  114 ,  160 ,  180 ,  181 , or  182  in reference to the other figures. In such embodiments, acoustic valves  216  may include multiple separate acoustic valves, such as with square acoustic flaps or continuous curved acoustic valves surrounding the perimeter of membrane  204 . Acoustic valves  216  may be electrostatically or piezoelectrically actuated in different embodiments. In other embodiments, membrane  204  may also include acoustic valves (not shown), such as described hereinabove in reference to variable flow transducer  200   a  and variable flow transducer  200   b  in  FIGS. 7A and 7B , respectively. 
       FIG. 7D  illustrates variable flow transducer  200   d  including support structure  202 , membrane  204 , and acoustic flaps  220 . According to various embodiments, acoustic valves  218  are formed in membrane  204 . Membrane slits  222  in membrane  204  allow acoustic flaps  220  to deflect separately from membrane  204 . In such embodiments, piezoelectric actuation layers  224  are formed on a top surface of membrane  204  and cause acoustic flaps  220  to deflect when a control signal, such as an actuation voltage is applied to piezoelectric actuation layers  224 . In various embodiments, acoustic valves  218  correspond to acoustic valve  181  as described hereinabove in reference to  FIG. 6B . In other embodiments, variable flow transducer  200   d  and acoustic valves  218  may be modified to correspond to acoustic valve  180  as described hereinabove in reference to  FIG. 6A . 
     In various embodiments, variable flow transducers  200   a ,  200   b ,  200   c , and  200   d  include circular membranes, as shown. In other embodiments, variable flow transducers  200   a ,  200   b ,  200   c , and  200   d  may include oval or rectangular membranes (not shown). In still further embodiments, variable flow transducers  200   a ,  200   b ,  200   c , and  200   d  may include any shape of membrane, such as hexagonal or octagonal, for example. 
       FIGS. 8A and 8B  illustrate side view cross-sections of more embodiment variable flow transducers  111   a  and  111   b . Variable flow transducers  111   a  and  111   b  each include membrane  112 , acoustic valves  114 , and actuating structures  116  as described hereinabove in reference to variable flow transducer  110  in  FIGS. 3A, 3B, and 3C . According to various embodiments, acoustic valves  114  are included in various different numbers and configurations. Specifically, variable flow transducer  111   a  includes acoustic valves  114  are arranged in central region  123   a , as shown in  FIG. 8A . In such embodiments, peripheral region  123   b  is solid. In various embodiments, the deflection of membrane  112  is largest near the center and smallest near the anchor. 
     In other embodiments, variable flow transducer  111   b  includes acoustic valves  114  arranged in peripheral region  123   b , as shown in  FIG. 8B . In such embodiments, central region  123   a  is solid. In such embodiments, actuating structures  116  may be formed on the top surface of membrane  112  at the edge of central region  123   a . According to various embodiments, any number and arrangement of acoustic valves may be arranged in any portion of membrane. Further,  FIGS. 7A and 7B  illustrate only a single row of acoustic valves arranged in a circle on membrane  204 , but various other embodiments may include two, three, or more acoustic valves arranged in concentric circles on a membrane, such as illustrated for variable flow transducer  111   a  and variable flow transducer  111   b  in  FIGS. 8A and 8B , respectively. Those having skill in the art will readily appreciate various modifications of the number and configuration of embodiment acoustic valves for embodiment variable flow transducers. Such modifications are well within the scope of the embodiments described herein. 
       FIGS. 9A, 9B, and 9C  illustrate side view cross-sections and a top view of another embodiment variable flow transducer  250  including bottom membrane  252  and top membrane  254 . According to various embodiments, bottom membrane  252  includes acoustic vents  256  and top membrane  254  includes acoustic vents  258 . Acoustic vents  256  and acoustic vents  258  are offset so that the vents do not overlap. In such embodiments, when a voltage difference is applied between bottom membrane  252  and top membrane  254 , an electrostatic force attracts bottom membrane  252  and top membrane  254  together and seals acoustic vents  256  and acoustic vents  258 , as shown in  FIG. 9B . When no voltage difference or a small voltage difference is applied between bottom membrane  252  and top membrane  254 , the membranes stay separated and acoustic vents  256  and acoustic vents  258  are open, as shown in  FIG. 9A . 
     According to various embodiments, when acoustic vents  256  and acoustic vents  258  are sealed, bottom membrane  252  and top membrane  254  are acoustically solid, i.e., acoustically visible or acoustically opaque. When acoustic vents  256  and acoustic vents  258  are open, bottom membrane  252  and top membrane  254  are acoustically transparent. 
     In various embodiments, bottom membrane  252  and top membrane  254  are driven to oscillate above a higher first frequency and acoustic vents  256  and acoustic vents  258  are controlled to open and seal in order to shape the positive and negative sound pressures that form acoustic signals with frequencies below a lower second frequency. In some embodiments, bottom membrane  252  and top membrane  254  may oscillate with a frequency ranging from 40 kHz to 300 kHz and acoustic vents  256  and acoustic vents  258  may be opened and sealed to form acoustic signals with frequencies ranging from 20 Hz to 20 kHz. 
     An embodiment arrangement of acoustic vents  256  and acoustic vents  258  is shown in  FIG. 9C . In various embodiments, acoustic vents  256  and acoustic vents  258  may be arranged in any type of random arrangement or nonrandom pattern. 
     According to various embodiments, bottom membrane  252  and top membrane  254  are driven to oscillate either piezoelectrically or electrostatically. Specifically, bottom membrane  252  and top membrane  254  may be arranged with top or bottom perforated backplates or piezoelectric actuation layers, such as described hereinabove in reference to variable flow transducer  150  and variable flow transducer  151  in  FIGS. 5A and 5B , respectively. In such embodiments, bottom membrane  252  and top membrane  254  are driven together to oscillate at the higher frequency above the audible frequency range. 
     According to alternative embodiments, bottom membrane  252  and top membrane  254  may be actuated to open and seal acoustic vents  256  and acoustic vents  258  piezoelectrically. In such embodiments, optional piezoelectric actuation layers  255  are formed on bottom membrane  252  and top membrane  254  in order to provide forces to open and seal acoustic vents  256  and acoustic vents  258 . 
       FIGS. 10A, 10B, and 10C  illustrate waveform diagrams of embodiment variable flow transducer operation.  FIGS. 10A, 10B, and 10C  include waveform diagrams shown on a normalized vertical axis versus time.  FIG. 10A  illustrates membrane displacement waveform  270  and membrane acceleration waveform  272 , as similarly described hereinabove in reference to membrane displacement waveform  134  and membrane acceleration waveform  136  in  FIG. 4C . According to various embodiments, a membrane is driven, piezoelectrically or electrostatically, to oscillate at a frequency or frequencies above the audible range. For example, the membrane may be driven to oscillate at a resonant frequency of the membrane, such as, e.g., 100 kHz. 
     In various embodiments, the acoustic impedance of the membrane is adjusted during the oscillations in order to generate a modulated acoustic signal. In some embodiments, acoustic valves are opened when the membrane is decelerating, which may be referred to as braking.  FIG. 10B  illustrates braking waveform  276  and volume flow waveform  274 , which correspond to accelerations and decelerations of membrane acceleration waveform  272  in  FIG. 10A . 
     When the membrane is decelerating, i.e., when braking waveform  276  has a value of 1, the acoustic valves are open. In such embodiments, the membrane is acoustically transparent, e.g., the acoustic impedance is decreased, and the volume flow of the acoustic medium, e.g., air, is decreased as shown by volume flow waveform  274 . In some embodiments, during braking period  280 , when braking waveform  276  has a value of 1, the volume flow is half, as shown by volume flow waveform  274 . In such embodiments, the membrane is 50% acoustically transparent during braking period  280  when the acoustic valves are open. In other embodiments, the membrane may have other values for acoustic transparency. In various embodiments, the membrane is between 30% and 95% acoustically transparent when the acoustic valves are open. In specific embodiments, the membrane is between 50% and 80% acoustically transparent when the acoustic valves are open. In such various embodiments, the volume flow corresponds to the acoustic transparency. In some embodiments, acoustic transparence may also be referred to as an acoustic short circuit. 
     When the membrane is accelerating, i.e., when braking waveform  276  has a value of 0, the acoustic valves are closed. In such embodiments, the membrane is acoustically opaque, e.g., the acoustic impedance is increased or at a maximum, and the volume flow of the acoustic medium, e.g., air, is increased as shown by volume flow waveform  274 . In some embodiments, during accelerating period  278 , when braking waveform  276  has a value of 0, the volume flow is full, as shown by volume flow waveform  274 . 
       FIG. 10C  illustrates 100% volume flow waveform  282  and 50% volume flow waveform  284 , corresponding to braking waveform  276  and volume flow waveform  274  in  FIG. 10B . The volume flow for a membrane without embodiment acoustic valves, as described herein, may be equal for positive displacements (1,0) and negative displacements (0,−1) as shown by 100% volume flow waveform  282 . According to various embodiments, the volume flow for a membrane with embodiment acoustic valves, as described herein, may be controlled to have different values for positive displacements (1,0) and negative displacements (0,−1) as shown by 50% volume flow waveform  284 . In particular embodiments, the membrane is 50% acoustically transparent when the acoustic valves are open, such as during braking, which produces 50% of the volume flow (for negative values of 50% volume flow waveform  284 ). When the acoustic valves are closed, such as during accelerating, the membrane is acoustically opaque, which produces 100% of the volume flow (for positive values of 50% volume flow waveform  284 ). 
     According to various embodiments, the polarity of the acoustic valve control may be switched in order to shape both positive and negative half-waves of an audible acoustic signal. By opening and closing the acoustic valves strategically, positive and negative sound pressure levels may be shaped from higher frequency oscillations. In various embodiments, the quality of the acoustic transparency, which may be referred to as the acoustic impedance or acoustic short circuit, is related to the number, size, shape, distribution, and operation of the acoustic valves as described hereinabove in reference to the other figures. 
       FIG. 11  illustrates an additional waveform diagram of embodiment variable flow transducer operation including high frequency waveform  290 , high frequency waveform  292 , and modulated acoustic waveform  294 . According to various embodiments, high frequency waveform  290  and high frequency waveform  292  are carrier signals having frequencies above the audible frequency range, such as described hereinabove in reference to carrier signal C SIG  in  FIG. 2B . Modulated acoustic waveform  294  is a modulated signal formed from high frequency waveform  290  or high frequency waveform  292 , such as described hereinabove in reference to modulated acoustic signal MA SIG  in  FIG. 2B . 
     According to various embodiments, the quality of the acoustic valves, and the corresponding acoustic pathways or perforations in the membrane, affects the acoustic transparency of the membrane. In particular embodiments, high frequency waveform  290  corresponds to a membrane that is 50% acoustically visible (50% acoustically transparent) when the acoustic pathways or valves are open and high frequency waveform  292  corresponds to a membrane that is 10% acoustically visible (90% acoustically transparent) when the acoustic pathways or valves are open. In such embodiments, the membrane produces full volume flow in the positive acceleration state and reduced volume flow in the negative acceleration state due to the acoustic transparency during the first half-wave from 0 to 0.1 ms. Further, the membrane produces full volume flow in the negative acceleration state and reduced volume flow in the positive acceleration state due to the acoustic transparency during the second half-wave from 0.1 ms to 0.2 ms. The volume flow when the acoustic valves are open is not negligible for the 50% acoustically visible membrane, but is dominated by the larger amount of volume flow when the acoustic valves are closed. As shown by high frequency waveform  290  and high frequency waveform  292 , the volume flow when the acoustic valves are open is much greater for the membrane that is 50% acoustically visible than for the membrane that is 10% acoustically visible. 
     According to various embodiments, modulated acoustic waveform  294  is formed or shaped by high frequency waveform  290  or high frequency waveform  292 . In various embodiments, the amplitude of modulated acoustic waveform  294  may be dependent on the amplitude of high frequency waveform  290  or high frequency waveform  292  as well as the extent of the acoustic transparency of the membrane when the acoustic valves are open. 
       FIG. 12  illustrates a flowchart diagram of embodiment method of operation  300  for a variable flow transducer. According to various embodiments, a method of operation  300  is a method of operating a MEMS transducer, where the method includes steps  305 ,  310 , and  315 . In such embodiments, step  305  includes actuating a deflectable membrane to oscillate. The deflectable membrane may oscillate with a frequency or frequencies above the audible range. For example, in particular embodiments, the deflectable membrane oscillates with a frequency or frequencies ranging from 40 kHz to 300 kHz. 
     In various embodiments, step  310  includes controlling a plurality of controllable acoustic paths in the deflectable membrane to provide acoustic low impedance paths between a first volume and a second volume during a first mode. The acoustic paths may include controllable acoustic valves as described hereinabove in reference to the other figures. Providing the low impedance paths may include opening the acoustic valves in some embodiments. Step  315  includes controlling the plurality of controllable acoustic paths in the deflectable membrane to provide acoustic high impedance paths between the first volume and the second volume during a second mode. Providing the high impedance paths may include closing the acoustic valves in some embodiments. In such embodiments, the high impedance path may include a very large acoustic impedance. 
     According to an embodiment, a microelectromechanical systems MEMS transducer includes a deflectable membrane attached to a support structure, an acoustic valve structure configured to cause the deflectable membrane to be acoustically transparent in a first mode and acoustically visible in a second mode, and an actuating mechanism coupled to the deflectable membrane. Other embodiments include corresponding systems and apparatus, each configured to perform various embodiment methods. 
     In various embodiments, the actuating mechanism is configured to excite oscillations of the deflectable membrane, the oscillations having a frequency above 40 kHz. The MEMS transducer may further include a substrate, where the support structure is disposed on the substrate. In some embodiments, the acoustic valve structure includes a plurality of piezoelectric valves. In such embodiments, the plurality of piezoelectric valves may be formed on the deflectable membrane. 
     In various embodiments, the acoustic valve structure includes a plurality of electrostatic valves. In such embodiments, the plurality of electrostatic valves may be formed on the deflectable membrane. In some embodiments, the actuating mechanism includes a perforated backplate separated from the deflectable membrane by a separation distance. In other embodiments, the actuating mechanism includes a piezoelectric layer formed on the deflectable membrane. 
     According to an embodiment, a MEMS transducer includes a support structure disposed on a substrate, a deflectable membrane supported by the support structure and separating a first volume from a second volume, and an actuation structure coupled to the deflectable membrane. The deflectable membrane includes a plurality of controllable acoustic paths in the deflectable membrane, where each controllable acoustic path of the plurality of controllable acoustic paths is configured to provide an acoustic low impedance path between the first volume and the second volume during a first mode, and provide an acoustic high impedance path between the first volume and the second volume during a second mode. Other embodiments include corresponding systems and apparatus, each configured to perform various embodiment methods. 
     In various embodiments, the actuation structure is configured to excite the deflectable membrane to oscillate with a frequency above 40 kHz. In some embodiments, the MEMS transducer further includes a control circuit coupled to the actuation structure and configured to provide first control signals to the actuation structure. In such embodiments, the control circuit is may be further configured to provide second control signals to the plurality of controllable acoustic paths, and the second control signals are operable to switch the plurality of controllable acoustic paths between the first mode and the second mode in order to selectively generate positive and negative sound pressures forming audible acoustic signals with frequencies below 20 kHz while the deflectable membrane oscillates with the frequency above 40 kHz. 
     In various embodiments, the plurality of controllable acoustic paths includes a plurality of piezoelectric valves formed in the deflectable membrane. In some embodiments, the plurality of controllable acoustic paths includes a plurality of electrostatic valves formed in the deflectable membrane. 
     According to an embodiment, a method of operating a MEMS transducer includes actuating a deflectable membrane to oscillate, controlling a plurality of controllable acoustic paths in the deflectable membrane to provide acoustic low impedance paths between a first volume and a second volume during a first mode, and controlling the plurality of controllable acoustic paths in the deflectable membrane to provide acoustic high impedance paths between the first volume and the second volume during a second mode. Other embodiments include corresponding systems and apparatus, each configured to perform various embodiment methods. 
     In various embodiments, the deflectable membrane is actuated to oscillate with a frequency above 40 kHz. In some embodiments, the method further includes selectively generating positive and negative sound pressures by switching the plurality of controllable acoustic paths between the first mode and the second mode, the positive and negative sound pressures forming audible acoustic signals with frequencies below 20 kHz while the deflectable membrane oscillates with the frequency above 40 kHz. 
     In various embodiments, controlling the plurality of controllable acoustic paths in the deflectable membrane to provide acoustic low impedance paths may include piezoelectrically opening a plurality of piezoelectric acoustic valves, and controlling the plurality of controllable acoustic paths in the deflectable membrane to provide acoustic high impedance paths may include piezoelectrically closing a plurality of piezoelectric acoustic valves. In some embodiments, controlling the plurality of controllable acoustic paths in the deflectable membrane to provide acoustic low impedance paths includes electrostatically opening a plurality of electrostatic acoustic valves, and controlling the plurality of controllable acoustic paths in the deflectable membrane to provide acoustic high impedance paths includes electrostatically closing a plurality of electrostatic acoustic valves. 
     According to an embodiment, a MEMS transducer includes a first deflectable membrane attached to a support structure and including a first plurality of perforations, a second deflectable membrane attached to the support structure and including a second plurality of perforations, a closing mechanism coupled to the first deflectable membrane and the second deflectable membrane, and an actuating mechanism configured to excite oscillations of the first deflectable membrane and the second deflectable membrane. The second plurality of perforations are offset from the first plurality of perforations. The closing mechanism is configured to close an acoustic path through the first deflectable membrane and the second deflectable membrane by moving the first deflectable membrane and the second deflectable membrane into contact during a first mode and open the acoustic path by moving the first deflectable membrane and the second deflectable membrane out of contact during a second mode. In such embodiments, the first plurality of perforations are sealed to the second deflectable membrane and the second plurality of perforations are sealed to the first deflectable membrane when the acoustic path is closed. Other embodiments include corresponding systems and apparatus, each configured to perform various embodiment methods. 
     In various embodiments, the oscillations of the first deflectable membrane and the second deflectable membrane have a frequency above 40 kHz. In some embodiments, the closing mechanism includes an electrostatic structure configured to generate an electrostatic force between the first deflectable membrane and the second deflectable membrane during the first mode. In other embodiments, the closing mechanism includes a piezoelectric structure configured to generate a first force on the first deflectable membrane and a second force on the second deflectable membrane during the first mode, the first force and the second force configured to move the first deflectable membrane and the second deflectable membrane into contact. 
     In various embodiments, the actuating mechanism may include a perforated backplate attached to the support structure and configured to generate an electrostatic force between the perforated backplate and the first deflectable membrane and the second deflectable membrane. In other embodiments, the actuating mechanism includes a piezoelectric structure configured to generate a first force on the first deflectable membrane and a second force on the second deflectable membrane. 
     Advantages of various embodiments described herein may include high sound pressure level signals with low frequencies that are formed using higher frequency oscillations of a membrane. Other advantages of various embodiments described herein may include deflectable membranes with controllable acoustic impedance. Some advantages of various embodiments may include the ability to form positive sound pressures without, or with reduced, negative sound pressures or the ability to form negative sound pressures without, or with reduced, positive sound pressures. 
     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.