Patent Publication Number: US-11039248-B2

Title: System and method for a pumping speaker

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 15/728,045 filed on Oct. 9, 2017, which is a divisional application of U.S. application Ser. No. 14/818,836 filed on Aug. 5, 2015, now issued as U.S. Pat. No. 9,843,862, all of which are hereby incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to speakers, and, in particular embodiments, to a system and method for a pumping speaker. 
     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 sensors include a family of transducers produced using micromachining techniques. Some MEMS, 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. Some MEMS, 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 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 sound pressure waves. Thus, a capacitive plate MEMS structure may operate as a microspeaker. 
     SUMMARY 
     According to an embodiment, a method of operating a speaker with an acoustic pump includes generating a carrier signal having a first frequency by exciting the acoustic pump at the first frequency and generating an acoustic signal having a second frequency by adjusting the carrier signal. In such embodiments, the first frequency is outside an audible frequency range and the second frequency is inside the audible frequency range. Adjusting the carrier signal includes performing adjustments to the carrier signal at the second frequency. Other embodiments include corresponding systems and apparatus, each configured to perform corresponding 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 pumping speaker system; 
         FIGS. 2 a  and 2 b    illustrate waveform diagrams of illustrative acoustic signals; 
         FIGS. 3 a  and 3 b    illustrate cross-sectional views of embodiment pumping speakers; 
         FIGS. 4 a , 4 b , 4 c , and 4 d    illustrate cross-sectional views of another embodiment pumping speaker; 
         FIGS. 5 a , 5 b , 5 c , and 5 d    illustrate cross-sectional views of a further embodiment pumping speaker; 
         FIGS. 6 a  and 6 b    illustrate cross-sectional views of still another embodiment pumping speaker; 
         FIGS. 7 a  and 7 b    illustrate a top view and a cross-sectional view of a still further embodiment pumping speaker; 
         FIGS. 8 a , 8 b , 8 c , 8 d , 8 e , and 8 f    illustrate cross-sectional views of valve systems for embodiment pumping speakers; 
         FIGS. 9 a  and 9 b    illustrate system diagrams of embodiment pumping speaker systems; 
         FIG. 10  illustrates a system diagram of another embodiment pumping speaker system; and 
         FIG. 11  illustrates a system block diagram of an embodiment method of operation for a pumping speaker. 
     
    
    
     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, pumping speakers, and pumping MEMS microspeakers. In other embodiments, aspects may also be applied to other applications involving any type of transducer converting a physical signal to another 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 22 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 pressure oscillations with frequencies between 20 Hz and 22 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. 
     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 direction and amplitude of the pressure oscillations according to a lower frequency in the audible frequency range. In additional embodiments, the speaker may be configured to generate pressure oscillations at a frequency above the audible range and modify the direction and amplitude 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 pumping speaker. The frequency of the pumping speaker may maintain operation outside the audible frequency range while the pumping action alters the amplitude and direction of the oscillations according to other frequencies inside the audible frequency range. In such embodiments, the pumping speaker may include a pump structure, or a micropump, which is configured to pump at a frequency above the audible frequency limit, vary the amplitude of pumping, and control the direction of pumping. Various embodiments are further described herein below. 
       FIG. 1  illustrates a system block diagram of an embodiment pumping speaker system  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., 22 kHz, with amplitude and direction adjustments of the pressure oscillations. The amplitude and direction of the pressure oscillations are adjusted at frequencies in the audible range. Thus, microspeaker  102  generates acoustic signal  108  including an audible acoustic signal formed from an inaudible acoustic signal. 
     In various embodiments, microspeaker  102  includes an acoustic pump or micropump. Various example embodiment micropumps 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 acoustic signal  108  with the higher inaudible frequency oscillations and the audible frequency oscillations based on amplitude and direction adjustments. 
     In other embodiments, microspeaker  102  may be implemented as any type of speaker fabricated using techniques known to those of skill in the art. 
     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., 22 kHz, with amplitude and direction adjustments of the 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 carried signal that has amplitude and direction 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 22 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 pumping acoustic signal PA SIG  that may be produced by an embodiment pumping speaker or microspeaker, such as a MEMS microspeaker. According to various embodiments, pumping acoustic signal PA SIG  has amplitude PA amp  and frequency PA freq , i.e., period PA T =1÷PA 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 P freq . Specifically, frequency C freq  is above the audible frequency range of a human, i.e., above 22 kHz, and frequency PA freq  is within the audible frequency range of a human, i.e., between about 20 Hz and 22 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, the direction of amplitude C amp  is also adjusted to allow for pumping in specific directions in order to form the rising and falling wave form of pumping acoustic signal PA SIG . The variation of amplitude C amp  and direction of carrier signal C SIG  is performed at a specific frequency in order to form pumping acoustic signal PA SIG  with frequency PA freq . 
     In particular embodiments, amplitude PA amp  of acoustic signal PA SIG  may be larger than a non-pumping speaker 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 pumping acoustic signal PA SIG  does not decrease much or at all when frequency PA freq  is below about 1-10 kHz and above about 10 Hz, for example. 
     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 membrane or pumping structure. In other embodiments, frequency C freq  may be variable. In particular examples, frequency C freq  is between 50 kHz and 10 MHz. In more specific embodiments, frequency C freq  is between 100 kHz and 300 kHz. In such various embodiments, frequency PA freq  is below 25 kHz. Specifically, frequency PA freq  is in the audible frequency range of humans, i.e., between 20 Hz and 22 kHz, where this range may be expanded for some humans and narrowed for others. In alternative embodiments, frequency PA freq  may be above 25 kHz. In such embodiments, pumping acoustic signal PA 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, speakers or microspeakers, 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 pumping acoustic signal within the audible frequency range. Various embodiment speakers are described herein below in order to illustrate some of the specific applications including capacitive plate structures and other pumping structures. 
     Referring back to  FIG. 1  in view of  FIGS. 2 a  and 2 b   , ASIC  104  in pumping speaker system  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. 3 a  and 3 b    illustrate cross-sectional views of embodiment pumping speakers  110  and  111 .  FIG. 3 a    shows single backplate pumping speaker  110  including substrate  112 , membrane  114 , lower backplate  116 , and structural material  120 . According to various embodiments, single backplate pumping speaker  110  operates as a capacitive plate transducer. A voltage applied through metallization  122  to membrane  114  and through metallization  124  to lower backplate  116  produces an attractive force between membrane  114  and lower backplate  116 . The attractive force between membrane  114  and lower backplate  116  causes membrane  114  to deflect. The voltage applied to these two plates can be applied at a frequency in order to cause the membrane to oscillate. As the membrane oscillates, pressure changes are produced by the membrane in the air, which causes acoustic signals, e.g., sound waves. The application of the voltage to membrane  114  and lower backplate  116  may be tuned to produce various frequencies of oscillations and, consequently, acoustic signals. In various embodiments, the voltage may be applied to membrane  114  and lower backplate  116  in order to cause membrane  114  to oscillate according to carrier signal C SIG  that produces pumping acoustic signal PA SIG  as described hereinabove in reference to  FIG. 2   b.    
     According to various embodiments, substrate  112  is a semiconductor wafer. Substrate  112  may be formed of silicon for example. In other embodiments, substrate  112  is formed of other semiconductor materials such as gallium-arsenide, indium-phosphide, or other semiconductors, for example. In further embodiments, substrate  112  is a polymer substrate. In alternative embodiments, substrate  112  is a metal substrate. In other embodiments, substrate  112  is glass. For example, in a particular embodiment, substrate  112  is silicon dioxide. In various embodiments, substrate  112  includes cavity  118 , which is formed in substrate  112  below the transducer plates that are formed by lower backplate  116  and membrane  114 . Cavity  118  may be formed with a Bosch etch from the backside of substrate  112 . 
     In various embodiments, structural material  120  is formed and patterned in multiple depositions to produce structural layers for supporting membrane  114  and lower backplate  116 . In a specific embodiment, structural material  120  is formed using a tetraethyl orthosilicate (TEOS) deposition in order to form layers of silicon oxide. In other embodiments, structural material  120  is formed of other materials or multiple materials. In such embodiments, structural material  120  is formed of materials including polymers, semiconductors, oxides, nitrides, or oxynitrides. 
     In various embodiments, membrane  114  and lower backplate  116  are formed of conductive materials. In specific embodiments, membrane  114  and lower backplate  116  are formed of polysilicon. In other embodiments, membrane  114  and lower backplate  116  may be formed of doped semiconductors or metals, such as aluminum, platinum, or gold, for example. Further, membrane  114  and lower backplate  116  may be formed of multiple layers of different materials. In some embodiments, membrane  114  is deflectable and lower backplate  116  is rigid. Lower backplate  116  is perforated in various embodiments. 
     In various embodiments, metallization  122  is formed in structural material  120  and electrically contacts membrane  114 , metallization  124  is formed in structural material  120  and electrically contacts lower backplate  116 , and metallization  126  is formed in structural material  120  and electrically contacts substrate  112 . 
     In various embodiments, membrane  114  is arranged over lower backplate  116  (as shown). In other embodiments, membrane  114  is arranged below lower backplate  116  (not shown). Similarly, a sound port may be included in packaging (not shown) around single backplate pumping speaker  110 . The sound port may be formed below, and acoustically coupled to, cavity  118 , such as in a circuit board attached to substrate  112 . In other embodiments, the sound port may be formed above single backplate pumping speaker  110 , such as in a package lid overlying single backplate pumping speaker  110 , for example. 
       FIG. 3 b    shows double backplate pumping speaker  111  including substrate  112 , membrane  114 , lower backplate  116 , upper backplate  117 , and structural material  120 . According to various embodiments, double backplate pumping speaker  111  includes elements as described hereinabove in reference to  FIG. 3 a   , with the addition of upper backplate  117  and metallization  128  formed in structural material  120  and electrically contacting upper backplate  117 . In various embodiments, upper backplate  117  may include materials and structures as similarly described hereinabove in reference to lower backplate  116  in  FIG. 3   a.    
     According to various embodiments, double backplate pumping speaker  111  operates as similarly described hereinabove in reference to single backplate pumping speaker  110 , with the addition that upper backplate  117  generates attractive forces on membrane  114 . In such embodiments, voltages may be applied between upper backplate  117  and membrane  114  or between lower backplate  116  and membrane  114  in order to generate attractive forces in either direction. Voltages are applied to membrane  114 , lower backplate  116 , and upper backplate  117  in order to cause membrane  114  to oscillate according to carrier signal C SIG  that produces pumping acoustic signal PA SIG  as described hereinabove in reference to  FIG. 2   b.    
     In various embodiments, amplitude C amp  and the direction of carrier signal C SIG  is adjusted in order to produce pumping acoustic signal PA SIG  as described hereinabove in reference to  FIG. 2 b   . Single backplate pumping speaker  110  and double backplate pumping speaker  111  may include asymmetric deflections, ventilation holes, or valves in order to control the direction of carrier signal C SIG . Various further embodiments are described herein below as illustrative embodiment pumping mechanisms. 
       FIGS. 4 a , 4 b , 4 c , and 4 d    illustrate a top view and cross-sectional views of another embodiment pumping speaker  130  including partitioned membrane  132 , upper backplate  134 , and lower backplate  136 . According to various embodiments, partitioned membrane  132  includes partitions  132   a ,  132   b ,  132   c , and  132   d  separated by slits  138  and able to move separately. Upper backplate  134  includes electrical partitions  134   a ,  134   b ,  134   c , and  134   d , which are able to generate different electric fields above partitions  132   a ,  132   b ,  132   c , and  132   d . For upper backplate  134 , electrode  140  is coupled to electrical partitions  134   b  and  134   d  and electrode  142  is coupled to electrical partitions  134   a  and  134   c . Similarly, lower backplate  136  includes electrical partitions  136   a ,  136   b ,  136   c , and  136   d , which are able to generate different electric fields below partitions  132   a ,  132   b ,  132   c , and  132   d . For lower backplate  136 , electrode  144  is coupled to electrical partitions  136   a  and  136   c  and electrode  146  is coupled to electrical partitions  136   b  and  136   d .  FIG. 4 a    shows a top view of partitioned membrane  132  and  FIGS. 4 b , 4 c , and 4 d    show cross-sectional views of pumping speaker  130  during different deflections of partitioned membrane  132  in order to illustrate a pumping action. 
     According to various embodiments,  FIG. 4 b    shows partitioned membrane  132 , with partitions  132   a ,  132   b ,  132   c , and  132   d , moving toward upper backplate  134  when a same voltage is applied to electrical partitions  134   a ,  134   b ,  134   c , and  134   d  through electrodes  140  and  142 . The same voltage applied to electrical partitions  134   a ,  134   b ,  134   c , and  134   d  of upper backplate  134  generates an attractive force on each of partitions  132   a ,  132   b ,  132   c , and  132   d , causing partitioned membrane  132  to deflect. In such embodiments, air moves through perforations in lower backplate  136  as shown in  FIG. 4 b   . The voltage applied to electrical partitions  136   a ,  136   b ,  136   c , and  136   d  of lower backplate  136  may be zero or small when partitioned membrane  132  is moving toward upper backplate  134 . 
       FIG. 4 c    shows partitions  132   b  and  132   d  of partitioned membrane  132  moving toward lower backplate  136  and partitions  132   a  and  132   c  remaining close to upper backplate  134 . In such embodiments, a voltage is applied to electrical partitions  134   a  and  134   c  through electrode  142  that generates an attractive force on partitions  132   a  and  132   c  toward upper backplate  134  and a voltage is applied to electrical partitions  136   b  and  136   d  through electrode  146  that generates an attractive force on partitions  132   b  and  132   d  toward lower backplate  136 . In such embodiments, air moves into the region behind partitions  132   b  and  132   d  as shown in  FIG. 4 c   . The voltage applied to electrical partitions  134   b  and  134   d  of upper backplate  134  and electrical partitions  136   a  and  136   c  of lower backplate  136  may be zero or small when partitioned membrane  132  is moving as shown in  FIG. 4   c.    
       FIG. 4 d    shows partitioned membrane  132 , with partitions  132   a ,  132   b ,  132   c , and  132   d , moving toward lower backplate  136  when a voltage is applied to electrical partitions  136   a ,  136   b ,  136   c , and  136   d  through electrodes  144  and  146 . As shown in  FIG. 4 c   , partitions  132   b  and  132   d  may already be near lower backplate  136  and may not be moving or moving very little. The voltage applied to electrical partitions  136   a ,  136   b ,  136   c , and  136   d  of lower backplate  136  generates an attractive force on each of partitions  132   a ,  132   b ,  132   c , and  132   d , causing partitioned membrane  132  to deflect. In such embodiments, air movement through perforations in upper backplate  134  may be small because of the air movement behind partitions  132   b  and  132   d  shown in  FIG. 4 c   . The voltage applied to electrical partitions  134   a ,  134   b ,  134   c , and  134   d  of upper backplate  134  may be zero or small when partitioned membrane  132  is moving toward lower backplate  136 . Further, the voltage applied to electrical partitions  134   a ,  134   b ,  134   c , and  134   d  of upper backplate  134  may be the same voltage or similar voltages for the different partitions. In still further embodiments, the voltage applied to electrical partitions  134   a ,  134   b ,  134   c , and  134   d  of upper backplate  134  may be different for the different partitions. 
     According to various embodiments, by splitting the movement of partitioned membrane  132  into sections in one direction and combining the movement of partitioned membrane  132  in the other direction, a pumping action may be performed. Thus, as shown in  FIGS. 4 b , 4 c , and 4 d   , the application of different voltages to electrodes  140 ,  142 ,  144 , and  146  produces pumping in an upward direction, i.e., through upper backplate  134  while reducing back pumping in a downward direction. The voltages applied to electrodes  140 ,  142 ,  144 , and  146  may be arranged to perform a pumping action in either direction by moving partitions  132   a ,  132   b ,  132   c , and  132   d  of partitioned membrane  132  together in the direction of pumping and separately in the other direction. Thus, in various embodiments, pumping speaker  130  may be controlled by voltages applied through electrodes  140 ,  142 ,  144 , and  146  in order to cause partitioned membrane  132  to oscillate according to carrier signal C SIG  that produces pumping acoustic signal PA SIG  as described hereinabove in reference to  FIG. 2 b   . In such embodiments, both amplitude C amp  and the direction of carrier signal C SIG  may be adjusted for partitioned membrane  132  in order to produce pumping acoustic signal PA SIG  as described hereinabove in reference to  FIG. 2 b   . Specifically, pumping speaker  130  is controlled to change the direction of pumping in accordance with producing pumping acoustic signal PA SIG . 
     According to various embodiments, partitioned membrane  132  is fixed to anchored structures, such as a structural material, on two edges as shown in  FIG. 4 a   . Further, the other two edges of partitioned membrane  132  may be free to move in some embodiments. In other embodiments, all the edges of partitioned membrane  132  may be fixed to anchored structures. In further embodiments, upper backplate  134  and lower backplate  136  may include additional electrical partitions or additional electrodes. 
       FIGS. 5 a  and 5 b    illustrate cross-sectional views of a further embodiment pumping speaker  150  including flexible membrane  152 , upper backplate  154 , and lower backplate  156 . According to various embodiments, flexible membrane  152  deflects significantly in both directions and is not stiff or rigid. During operation, flexible membrane  152  may deflect with a wavelike or serpentine deflection as shown in  FIGS. 5 a  and 5 b   . Similar to upper backplate  134  described hereinabove in reference to  FIGS. 4 a , 4 b , 4 c , and 4 d   , upper backplate  154  includes electrical partitions  154   a ,  154   b ,  154   c , and  154   d , which are able to generate different electric fields above flexible membrane  152 . For upper backplate  154 , electrode  160  is coupled to electrical partitions  154   b  and  154   d  and electrode  162  is coupled to electrical partitions  154   a  and  154   c . Similar to lower backplate  136  described hereinabove in reference to  FIGS. 4 a , 4 b , 4 c   , and  4   d , lower backplate  156  includes electrical partitions  156   a ,  156   b ,  156   c , and  156   d , which are able to generate different electric fields below flexible membrane  152 . For lower backplate  156 , electrode  164  is coupled to electrical partitions  156   a  and  156   c  and electrode  166  is coupled to electrical partitions  156   b  and  156   d .  FIGS. 5 a  and 5 b    show cross-sectional views of pumping speaker  150  during different deflections of flexible membrane  152  in order to illustrate a pumping action. 
     According to various embodiments, electrodes  160 ,  162 ,  164 , and  166  apply voltages to electrical partitions  154   a ,  154   b ,  154   c , and  154   d  of upper backplate  154  and to electrical partitions  156   a ,  156   b ,  156   c , and  156   d  of lower backplate  156  in order to generate a serpentine movement of flexible membrane  152  as shown in  FIGS. 5 a  and 5 b   . In such embodiments, the serpentine motion includes moving flexible membrane  152  upwards over perforated section  157  of lower backplate  156  in order to move air through perforated section  157  and into the space between upper backplate  154  and lower backplate  156  (as shown in  FIG. 5 a   ). The serpentine motion then includes moving flexible membrane  152  upwards under perforated section  155  of upper backplate  154  in order to move air from the space between upper backplate  154  and lower backplate  156  out through perforated section  155  (as shown in  FIG. 5 b   ). In such embodiments, flexible membrane  152  may include holes or slits (not shown) in the membrane. For example, membrane  152  may include holes or slits around the edge of flexible membrane  152  or in the center of flexible membrane  152 . In other particular embodiments, a support structures connected around the edge of the membrane includes holes of slits (not shown). Based on the holes or slits in flexible membrane  152 , air is able to pass through the holes during pumping of flexible membrane  152 . 
     In various embodiments, the sequence of voltages applied through electrodes  160 ,  162 ,  164 , and  166  may be applied in a reverse order in order to move air in the opposite direction. In various embodiments, pumping speaker  150  may be controlled by voltages applied through electrodes  160 ,  162 ,  164 , and  166  in order to cause flexible membrane  152  to oscillate according to carrier signal C SIG  that produces pumping acoustic signal PA SIG  as described hereinabove in reference to  FIG. 2 b   . In such embodiments, both amplitude C amp  and the direction of carrier signal C SIG  may be adjusted for flexible membrane  152  in order to produce pumping acoustic signal PA SIG  as described hereinabove in reference to  FIG. 2 b   . Specifically, pumping speaker  150  is controlled to change the direction of pumping in accordance with producing pumping acoustic signal PA SIG . In various embodiments, pumping speaker  150  may be referred to as a serpentine pump. 
     According to some embodiments, flexible membrane  152  is very flexible or soft. Thus, flexible membrane  152  may be formed of a thin layer of silicon or polysilicon. In some embodiments, flexible membrane  152  is less than 700 nm thick. In one particular embodiment, flexible membrane  152  is 660 nm thick. In other embodiments, flexible membrane  152  is less than 500 nm thick. In various other embodiments, flexible membrane  152  may be formed of a conductive material, such as a semiconductor material or a metal, for example. In some specific embodiments, flexible membrane  152  is formed of carbon or silicon nitride with a layer of polysilicon. 
     In some embodiments, additional electrodes may be included in order to couple electrical partitions  154   a ,  154   b ,  154   c , and  154   d  or  156   a ,  156   b ,  156   c , and  156   d  to independent electrodes. Further, upper backplate  154  and lower backplate  156  may include additional electrical partitions or additional electrodes. 
       FIGS. 5 c  and 5 d    illustrate cross-sectional views of embodiment pumping speaker  151 , which is a general version of pumping speaker  150 , including flexible membrane  153 , upper backplate  154 , and lower backplate  156 . According to various embodiments, flexible membrane  153  may include any of the features of flexible membrane  152  and may include holes or slits, for example. In such embodiments, flexible membrane  153  may exhibit any type of asymmetric motion that produces an asymmetric pumping action, resulting in directional pumping. In some embodiments, flexible membrane  153  may include ventilation holes or slits in the center or around the edge of flexible membrane  153 . In various embodiments, perforated section  155  and perforated section  157  may extend across any portion of upper backplate  154  and lower backplate  156 , respectively, depending on various embodiment applications. The asymmetric motion of flexible membrane  153  may be asymmetric in either direction to produce pumping in either direction through perforated section  155  and perforated section  157 . 
       FIGS. 6 a  and 6 b    illustrate cross-sectional views of still another embodiment pumping speaker  170  including membrane  172 , upper backplate  174 , and lower backplate  176 . According to various embodiments, membrane  172  includes valves  178  to control pumping direction. During operation, membrane  172  may deflect in both directions while valves  178  remain closed in one direction and open in the other direction in order to control the direction of pumping.  FIGS. 6 a  and 6 b    show cross-sectional views of pumping speaker  170  during different deflections of membrane  172  in order to illustrate a pumping action. 
     Similar to upper backplate  134  described hereinabove in reference to  FIGS. 4 a , 4 b , 4 c , and 4 d   , upper backplate  174  includes electrical partitions  174   a ,  174   b ,  174   c , and  174   d , which are able to generate different electric fields above membrane  172 . For upper backplate  174 , electrode  180  is coupled to electrical partitions  174   b  and  174   d  and electrode  182  is coupled to electrical partitions  174   a  and  174   c . Similar to lower backplate  136  described hereinabove in reference to  FIGS. 4 a , 4 b , 4 c , and 4 d   , lower backplate  176  includes electrical partitions  176   a ,  176   b ,  176   c , and  176   d , which are able to generate different electric fields below membrane  172 . For lower backplate  176 , electrode  184  is coupled to electrical partitions  176   a  and  176   c  and electrode  186  is coupled to electrical partitions  175   b  and  175   d.    
     According to various embodiments, electrodes  180 ,  182 ,  184 , and  186  apply voltages to electrical partitions  174   a ,  174   b ,  174   c , and  174   d  of upper backplate  174  and to electrical partitions  176   a ,  176   b ,  176   c , and  176   d  of lower backplate  176  in order to generate a movement of membrane  172  as shown in  FIGS. 6 a  and 6 b   . In such embodiments, the upward motion of membrane  172  generates pumping in an upward direction through perforations in upper backplate  174  when valves  178  remain closed. The following downward motion of membrane  172  does not generate pumping in a downward direction through perforations in lower backplate  176  because valves  178  are opened in order to allow air to move through valves  178 . In various different embodiments, valves  178  are configured to open or close during upward or downward motions in order to provide pumping through the movements of membrane  172  in either direction. In some such embodiments, valves  178  are configured to open only during downward motion of membrane  172 . In other such embodiments, valves  178  are configured to open only during upward motion of membrane  172 . In further embodiments, valves  178  are configured to open during upward or downward motion of membrane  172 . 
     In various embodiments, valves  178  may be controlled by applying voltages to open or close valves  178 . In other embodiments, valves  178  may be configured to open and close at a certain resonant frequency while membrane  172  oscillates at a different frequency. In such embodiments, the resonant frequency of membrane  172  may be different from the resonant frequency of valves  178  and the difference may be used to control the opening and close of valves  178  in relation to the oscillations of membrane  172 . 
     In various embodiments, pumping speaker  170  may be controlled by voltages applied through electrodes  180 ,  182 ,  184 , and  186  in order to cause membrane  172  to oscillate according to carrier signal C SIG  that produces pumping acoustic signal PA SIG  as described hereinabove in reference to  FIG. 2 b   . In such embodiments, both the amplitude C amp  and the direction of carrier signal C SIG  may be adjusted by controlling the oscillations of membrane  172  and the opening and closing of valves  178  in order to produce pumping acoustic signal PA SIG  as described hereinabove in reference to  FIG. 2 b   . Specifically, pumping speaker  170  is controlled to change the direction of pumping, by controlling valves  178 , in accordance with producing pumping acoustic signal PA SIG . 
     According to some embodiments, valves  178  may be included in upper backplate  174  or lower backplate  176 . In such embodiments, valves  178  may be omitted from membrane  172  or may be additionally included in membrane  172 . In some embodiments, additional electrodes may be included in order to couple electrical partitions  174   a ,  174   b ,  174   c , and  174   d  or  176   a ,  176   b ,  176   c , and  176   d  to independent electrodes. Further, upper backplate  174  and lower backplate  176  may include additional electrical partitions or additional electrodes. 
       FIGS. 7 a  and 7 b    illustrate a top view and a cross-sectional view of a still further embodiment pumping speaker  190  including rotor  192 , top stator  194 , and bottom stator  196 . According to various embodiments, rotor  192  includes multiple chambers and rotates based on applied voltages from top stator  194  and bottom stator  196 . As rotor  192  oscillates back and worth, valve  198  in top stator  194  and valve  199  in bottom stator  196  are opened and closed to control pumping direction of pumping speaker  19   o . During operation, rotor  192  may deflect in both directions while valve  198  and valve  199  alternatingly open and close in order to control the direction of pumping. According to various embodiments, pumping speaker  190  may be referred to as a rotor pump. 
     Similar to upper backplate  134  described hereinabove in reference to  FIGS. 4 a , 4 b , 4 c , and 4 d   , top stator  194  includes electrical partitions  194   a ,  194   b ,  194   c , and  194   d , which are able to generate different electric fields above rotor  192 . For top stator  194 , electrode  200  is coupled to electrical partitions  194   b  and  194   d  and electrode  202  is coupled to electrical partitions  194   a  and  194   c . Similar to lower backplate  136  described hereinabove in reference to  FIGS. 4 a , 4 b , 4 c , and 4 d   , bottom stator  196  includes electrical partitions  196   a ,  196   b ,  196   c , and  196   d , which are able to generate different electric fields below rotor  192 . For bottom stator  196 , electrode  204  is coupled to electrical partitions  196   a  and  196   c  and electrode  206  is coupled to electrical partitions  196   b  and  196   d.    
     According to various embodiments, electrodes  200 ,  202 ,  204 , and  206  apply voltages to electrical partitions  194   a ,  194   b ,  194   c , and  194   d  of top stator  194  and to electrical partitions  196   a ,  196   b ,  196   c , and  196   d  of bottom stator  196  in order to generate a movement of rotor  192  as shown in  FIGS. 7 a  and 7 b   . In such embodiments, the motion of rotor  192  generates pumping in either direction by opening and closing valve  198  or valve  199 . For example, an upward pumping may be generated by opening valve  198  while rotor  192  is rotating to force air movement through valve  198  and closing valve  198  while rotor  192  is rotating the other direction to prevent air from being pulled back through valve  198 . Similarly, a downward pumping may be generated by opening valve  199  while rotor  192  is rotating to force air movement through valve  199  and closing valve  199  while rotor  192  is rotating the other direction to prevent air from being pulled back through valve  199 . 
     In various different embodiments, valve  198  and valve  199  are configured to open or close during upward or downward motions in order to provide pumping through the movements of rotor  192  in either direction. In some such embodiments, valve  198  and valve  199  are configured to open only during clockwise motion of rotor  192 . In other such embodiments, valve  198  and valve  199  are configured to open only during counterclockwise motion of rotor  192 . In further embodiments, valve  198  and valve  199  are configured to open during clockwise or counterclockwise motion of rotor  192  and may be controlled accordingly. In various embodiments, valve  198  and valve  199  may be controlled by applying voltages to open or close valve  198  and valve  199 . In other embodiments, valve  198  and valve  199  may be configured to open only for air flow in one direction, i.e., valve  198  and valve  199  may be one way valves. 
     In various embodiments, pumping speaker  190  may be controlled by voltages applied through electrodes  200 ,  202 ,  204 , and  206  in order to cause rotor  192  to oscillate according to carrier signal C SIG  that produces pumping acoustic signal PA SIG  as described hereinabove in reference to  FIG. 2 b   . In such embodiments, both the amplitude C amp  and the direction of carrier signal C SIG  may be adjusted by controlling the oscillations of rotor  192  and the opening and closing of valve  198  and valve  199  in order to produce pumping acoustic signal PA SIG  as described hereinabove in reference to  FIG. 2 b   . Specifically, pumping speaker  190  is controlled to change the direction of pumping, by controlling valve  198  and valve  199 , in accordance with producing pumping acoustic signal PA SIG . In specific embodiments, rotor  192  is controlled to oscillate at a frequency above 50 kHz. 
     According to some embodiments, additional valves may be included in top stator  194  or bottom stator  196 . In some embodiments, additional electrodes may be included in order to couple electrical partitions  194   a ,  194   b ,  194   c , and  194   d  or electrical partitions  196   a ,  196   b ,  196   c , and  196   d  to independent electrodes. Further, top stator  194  and bottom stator  196  may include additional electrical partitions or additional electrodes. 
       FIGS. 8 a , 8 b , 8 c , 8 d , 8 e , and 8 f    illustrate cross-sectional views of valve systems  300 ,  301 , and  303  for embodiment pumping speakers.  FIGS. 8 a  and 8 b    illustrate self-closing valve system  300  including valve  302 . According to various embodiments, valve  302  closes automatically unless a large pressure difference exists between pressure P 1  and pressure P 2 . As shown in  FIG. 8 a   , valve  302  remains closed for pressure P 1  and P 2 . When pressure P 2  is much greater than pressure P 1 , valve  302  is forced open by the pressure difference as shown in  FIG. 8   b.    
       FIGS. 8 c  and 8 d    illustrate self-opening valve system  301  including valve  304 . According to various embodiments, valve  304  opens automatically unless a large pressure difference exists between pressure P 1  and pressure P 2 . As shown in  FIG. 8 c   , valve  304  remains open for pressure P 1  and P 2 . When pressure P 1  is much greater than pressure P 2 , valve  304  is forced closed by the pressure difference as shown in  FIG. 8   d.    
       FIGS. 8 e  and 8 f    illustrate voltage controlled valve system  303  including valve  306  and voltage supply  308  for controlling voltage V 1  applied to valve  306 . According to various embodiments, valve  306  is closed when voltage supply  308  is active to apply voltage V 1  across valve  306  as shown in  FIG. 8 e   . Valve  306  is opened when voltage supply  308  is inactive or disconnected and no voltage is applied across valve  306  as shown in  FIG. 8   f.    
     The materials and structures of various self-closing valves, self-opening valves, and voltage controlled valves are numerous and known by those of skill in the art. Such numerous material and structure implementations are included in various embodiments. 
       FIGS. 9 a  and 9 b    illustrate system diagrams of embodiment pumping speaker system  320  and embodiment pumping speaker system  321 . Pumping speaker system  320  includes back volume  322 , front volume  324 , filter membrane  326 , mono-directional pump  328 , valve  330 , and valve  332 . According to various embodiments, mono-directional pump  328 , valve  330 , and valve  332  operate as described herein above in reference to the other figures to generate carrier signal C SIG  that produces pumping acoustic signal PA SIG  as described hereinabove in reference to  FIG. 2 b   . In such embodiments, both the amplitude C amp  and the direction of carrier signal C SIG  may be adjusted by mono-directional pump  328 , valve  330 , and valve  332  in order to produce pumping acoustic signal PA SIG  as described hereinabove in reference to  FIG. 2 b   . In such embodiments, valve  330  and valve  332  are controlled in order to control the direction of pumping between back volume  322  and front volume  324 . By controlling valve  330  and valve  332 , pumping speaker system  320  is able to provide bidirectional pumping, and thus control the direction of pumping in order to generate pumping acoustic signal PA SIG , while using mono-directional pump  328 . 
     According to various embodiments, the direction and magnitude of pumping is adjusted, as described hereinabove, in order to produce pumping acoustic signal PA SIG  out of front volume  324 . In such embodiments, filter membrane  326  may be included at an interface or output of front volume  324  in order to provide low pass filtering of the generated signal and to provide additional dust and particulate protection for mono-directional pump  328 , valve  330 , and valve  332 . Filter membrane  326  passes frequencies in the audible frequency range and filters frequencies above the audible frequency range. In alternative embodiments, filter membrane  326  may also pass frequencies above the audible frequency range, for example in ultrasound or near field detection applications. Further, mono-directional pump  328 , valve  330 , and valve  332  may be sensitive to damage from particles or dust in the air and filter membrane  326  may provide additional protection from dust, dirt, or other particulates in the air. 
     Pumping speaker system  321  in  FIG. 9 b    includes back volume  322 , front volume  324 , filter membrane  326 , and bidirectional pump  334 . According to various embodiments, pumping speaker system  321  with bidirectional pump  334  operates as described in reference to pumping speaker system  320  and mono-directional pump  328  where valve  330  and valve  332  are omitted. In such embodiments, bidirectional pump  334  is able to provide bidirectional pumping between back volume  322  and front volume  324 , without valve  330  or valve  332 , and thus is able to control the direction of pumping in order to generate pumping acoustic signal PA SIG  as described hereinabove in reference to  FIGS. 2 b    and  9   a.    
     In various embodiments, back volume  322  and front volume  324  may be unsealed volumes, such as open volumes in a device package. In some embodiments, back volume  322  and front volume  324  may have designed shapes for different applications. For example, back volume  322  and front volume  324  may arranged to improve acoustic pumping efficiency, system cost, or system size. Thus, in various embodiments, back volume  322  and front volume  324  may have any type of shape. 
       FIG. 10  illustrates a system diagram of another embodiment pumping speaker system  350  with a microspeaker array including microspeakers  352 - 1 ,  352 - 2 ,  352 - 3 ,  352 - 4 ,  352 - 5 ,  352 - 6 ,  352 - 7 ,  352 - 8 ,  352 - 9 ,  352 - 10 ,  352 - 11 , and  352 - 12 . According to various embodiments, microspeakers  352 - 1 ,  352 - 2 ,  352 - 3 ,  352 - 4 ,  352 - 5 ,  352 - 6 ,  352 - 7 ,  352 - 8 ,  352 - 9 ,  352 - 10 ,  352 - 11 , and  352 - 12  may each include any of the various embodiment microspeakers and micropumps described herein. In some embodiments, each microspeaker in pumping speaker system  350  includes a same embodiment microspeaker. In other embodiments, pumping speaker system  350  may include multiple types of embodiment microspeakers. 
     Pumping speaker system  350  is illustrated with 12 microspeakers  352 - 1 ,  352 - 2 ,  352 - 3 ,  352 - 4 ,  352 - 5 ,  352 - 6 ,  352 - 7 ,  352 - 8 ,  352 - 9 ,  352 - 10 ,  352 - 11 , and  352 - 12 , but pumping speaker system  350  may include any number of microspeakers in an array in other embodiments. For example, pumping speaker system  350  may include between 2 and 24 microspeakers in some embodiments. In other embodiments, pumping speaker system  350  may include more than 24 microspeakers. In various embodiments, microspeakers  352 - 1 ,  352 - 2 ,  352 - 3 ,  352 - 4 ,  352 - 5 ,  352 - 6 ,  352 - 7 ,  352 - 8 ,  352 - 9 ,  352 - 10 ,  352 - 11  and  352 - 12  are formed in substrate  354 . In one embodiment, substrate  354  is a single semiconductor die. In another embodiment, substrate  354  is a printed circuit board (PCB). 
     According to various embodiments, a microspeaker array, such as included in pumping speaker system  350 , generates signals with higher combined amplitude compared to a single microspeaker. In such embodiments, the microspeakers formed in an array may together produce acoustic signals with higher SPLs. In particular embodiments, pumping speaker system  350  may include various microspeakers that are tuned to produce acoustic signals in different frequency ranges with better performance. For example, microspeakers  352 - 1 ,  352 - 2 ,  352 - 3 ,  352 - 4 ,  352 - 5 , and  352 - 6  may be tuned to produce frequencies between 20 Hz and 1 kHz with better performance and microspeakers  352 - 7 ,  352 - 8 ,  352 - 9 ,  352 - 10 ,  352 - 11 , and  352 - 12  may be tuned to produce frequencies between 1 kHz and 20 kHz with better performance. Thus, a microspeaker array may be tuned to operate with better performance and efficiency, in some embodiments, by using a heterogeneous selection of microspeakers instead of a homogeneous selection of microspeakers. 
       FIG. 11  illustrates a system block diagram of an embodiment method of operation  400  for a pumping speaker. According to various embodiments, method of operation  400  includes steps  402  and  404  and includes a method of operating a speaker that includes an acoustic pump. Step  402  includes generating a carrier signal having a first frequency by exciting the acoustic pump at the first frequency. The first frequency is outside an audible frequency range in such embodiments. Step  404  includes generating an acoustic signal having a second frequency by adjusting the carrier signal. The adjustments to the carrier signal are performed at the second frequency. In such embodiments, the second frequency is inside the audible frequency range. 
     According to some embodiments, generating the acoustic signal by adjusting the carrier signal in step  404  includes adjusting the magnitude of the carrier signal according to the second frequency and adjusting the direction of pumping for the acoustic pump according to the second frequency. Further steps may be included in method of operation  400  in various additional embodiments. 
     According to an embodiment, a method of operating a speaker with an acoustic pump includes generating a carrier signal having a first frequency by exciting the acoustic pump at the first frequency and generating an acoustic signal having a second frequency by adjusting the carrier signal. In such embodiments, the first frequency is outside an audible frequency range and the second frequency is inside the audible frequency range. Adjusting the carrier signal includes performing adjustments to the carrier signal at the second frequency. Other embodiments include corresponding systems and apparatus, each configured to perform corresponding embodiment methods. 
     Implementations may include one or more of the following features. In various embodiments, generating the acoustic signal by adjusting the carrier signal includes adjusting a magnitude of the carrier signal according to the second frequency and adjusting a direction of pumping for the acoustic pump according to the second frequency. In some embodiments, the second frequency includes a plurality of frequencies inside the audible frequency range and the acoustic signal includes a plurality of sounds having the plurality of frequencies inside the audible frequency range. Exciting the acoustic pump may include exciting a micropump structure. 
     In various embodiments, the first frequency is above 100 kHz and the second frequency is below 23 kHz. In some embodiments, the first frequency is selected to match a resonant frequency of the acoustic pump. In particular embodiments, the first frequency is held constant and the second frequency is varied. In further embodiments, the method further includes, before generating the carrier signal, exciting the acoustic pump at a plurality of frequencies, measuring a plurality of responses of the acoustic pump corresponding to the plurality of frequencies, and determining a resonant frequency of the acoustic pump based on measuring the plurality of responses. In still further embodiments, the method further includes, before generating the carrier signal, setting the first frequency to the resonant frequency. According to some embodiments, the method further includes, before generating the carrier signal, tuning the resonant frequency of the acoustic pump by adjusting mechanical components within the acoustic pump. 
     According to an embodiment, a microspeaker includes an acoustic micropump structure configured to pump at a first frequency above an upper audible frequency limit and generate an acoustic signal by adjusting a magnitude and a direction of the pumping according to a second frequency below the upper audible frequency limit. Other embodiments include corresponding systems and apparatus, each configured to perform corresponding embodiment methods. 
     Implementations may include one or more of the following features. In various embodiments, the microspeaker further includes an integrated circuit coupled to the acoustic micropump structure. The integrated circuit is configured to operate the acoustic micropump structure at a plurality of test frequencies, measure a plurality of frequency responses of the acoustic micropump structure corresponding to the plurality of test frequencies, determine a resonant frequency of the acoustic micropump structure based on measuring the plurality of frequency responses, and set the first frequency based on the resonant frequency. 
     In various embodiments, the acoustic micropump structure includes a deflectable membrane partitioned into a plurality of sections with slits separating the plurality of sections. In some embodiments, the acoustic micropump structure includes a serpentine pump. In further embodiments, the acoustic micropump structure includes a deflectable membrane having valves in the deflectable membrane. In such embodiments, the valves may include one way valves. In other such embodiments, the valves may include voltage controlled valves. 
     In various embodiments, the acoustic micropump structure includes a rotor pump. In some embodiments, the microspeaker further includes a back volume coupled to the acoustic micropump structure and a front volume coupled to the acoustic micropump structure and having an output configured to output the acoustic signal. In such embodiments, the acoustic micropump structure is further configured to pump between the back volume and the front volume. In some embodiments, the front volume includes a filter membrane on the output. In further embodiments, the acoustic micropump structure includes a plurality of acoustic micropump structures disposed in a same substrate and configured as a micropump array. 
     According to an embodiment, a speaker includes an acoustic pump configured to generate a carrier signal having a first frequency by exciting the acoustic pump at the first frequency and generate an acoustic signal having a second frequency by adjusting the carrier signal. The first frequency is outside an audible frequency range and the second frequency is inside the audible frequency range. In such embodiments, adjusting the carrier signal includes performing adjustments to the carrier signal at the second frequency. Other embodiments include corresponding systems and apparatus, each configured to perform corresponding embodiment methods. 
     Implementations may include one or more of the following features. In various embodiments, generating the acoustic signal by adjusting the carrier signal includes adjusting a magnitude of the carrier signal according to the second frequency and adjusting a direction of pumping for the acoustic pump according to the second frequency. In some embodiments, the second frequency includes a plurality of frequencies inside the audible frequency range and the acoustic signal includes a plurality of sounds having the plurality of frequencies inside the audible frequency range. 
     In various embodiments, the first frequency is selected to match a resonant frequency of the acoustic pump. In some embodiments, the first frequency is held constant and the second frequency is varied. In further embodiments, the speaker further includes an integrated circuit coupled to the acoustic pump and configured to excite the acoustic pump at a plurality of frequencies, measure a plurality of responses of the acoustic pump corresponding to the plurality of frequencies, and determine a resonant frequency of the acoustic pump based on measuring the plurality of responses. The integrated circuit may be further configured to set the first frequency to the resonant frequency. In a still further embodiment, the integrated circuit is further configured to tune the resonant frequency of the acoustic pump by adjusting mechanical components within the acoustic pump. 
     An advantage of various embodiments may include, for example, microspeakers capable of producing audible sounds with SPLs that diminish little or none at lower frequencies, e.g., below 100 Hz. Another advantage of various embodiments may include increased efficiency of operation for microspeakers. Further advantages of various embodiments may include microspeakers with large deflections based on resonant mode excitation and microspeakers capable of producing audible sounds with high SPLs. Still another advantage of various embodiments may include a microspeaker with a flat frequency curve. A yet further advantage of some embodiments may include a microspeaker capable of producing frequencies above the audible range for use in ultrasound or near field detection, for example. 
     Description is made herein primarily in reference to acoustic signals in air. However, in further embodiments, embodiment methods and structures may be applied to signals produced any medium. 
     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.