Patent Publication Number: US-2022225031-A1

Title: Air-pulse Generating Device and Sound Producing Method Thereof

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional application No. 63/137,479 filed on Jan. 14, 2021, No. 63/138,449 filed on Jan. 17, 2021, No. 63/139,188 filed on Jan. 19, 2021, No. 63/142,627 filed on Jan. 28, 2021, No. 63/143,510 filed on Jan. 29, 2021, and No. 63/171,281 filed on Apr. 6, 2021, which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present application relates to an air-pulse generating device and a sound producing method thereof, and more particularly, to an air-pulse generating device and a sound producing method thereof capable of increasing overall air pulse rate, improving sound pressure level, and/or saving power. 
     2. Description of the Prior Art 
     Speaker driver and back enclosure are two major design challenges in the speaker industry. It is difficult for a conventional speaker to cover an entire audio frequency band, e.g., from 20 Hz to 20 KHz. To produce high fidelity sound with high enough sound pressure level (SPL), both the radiating/moving surface and volume/size of back enclosure for the conventional speaker are required to be sufficiently large. 
     Therefore, how to design a small sound producing device while overcoming the design challenges faced by conventional speakers is a significant objective in the field. 
     SUMMARY OF THE INVENTION 
     It is therefore a primary objective of the present application to provide an air-pulse generating device and a sound producing method thereof, to improve over disadvantages and/or restrictions of the prior art. 
     An embodiment of the present invention provides an air-pulse generating device, comprising a membrane structure and a valve structure; a cover structure, wherein a chamber is formed between the membrane structure, the valve structure and the cover structure; wherein an air wave vibrating at an operating frequency is formed within the chamber; wherein the valve structure is configured to be actuated to perform an open-and-close movement to form at least one opening, the at least one opening connects air inside the chamber with air outside the chamber; wherein the open-and-close movement is synchronous with the operating frequency. 
     Another embodiment of the present invention provides a sound producing method, applied in an air-pulse generating device, the method comprising forming an air wave within a chamber, wherein the air wave vibrates at an operating frequency, and the chamber is formed within the air-pulse generating device; and forming at least one opening on the air-pulse generating device at an opening frequency, wherein the at least one opening connects air inside the chamber with air outside the chamber; wherein the opening frequency is synchronous with the operating frequency. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an air-pulse generating device according to an embodiment of the present application. 
         FIG. 2  is a schematic diagram of a plurality of waveforms according to an embodiment of the present application. 
         FIG. 3  is a schematic diagram of a plurality of signals according to an embodiment of the present application. 
         FIG. 4  illustrates membrane driving signals according to an embodiment of the present application. 
         FIG. 5  is a schematic diagram illustrating a top view of the air-pulse generating device shown in  FIG. 1 . 
         FIG. 6  and  FIG. 7  are schematic diagrams of cross sectional views of air-pulse generating devices according to embodiments of the present application. 
         FIG. 8  and  FIG. 9  are schematic diagrams of cross sectional views of air-pulse generating devices according to embodiments of the present application. 
         FIG. 10  and  FIG. 11  are schematic diagrams of the air-pulse generating device shown in  FIG. 8  disposed within constructs according to embodiments of the present application 
         FIG. 12  is a schematic diagram of a mobile device according to an embodiment of the present application. 
         FIG. 13  to  FIG. 15  are schematic diagrams of cross sectional views of air-pulse generating devices according to embodiments of the present application. 
         FIG. 16  is a schematic diagram of valve movement according to an embodiment of the present application 
     
    
    
     DETAILED DESCRIPTION 
     U.S. Pat. No. 10,425,732 provides a sound producing device, or an air-pressure-pulse-speaker (APPS), comprising a plurality of air pulse generating elements which is capable of producing a plurality of PAM (pulse-amplitude modulation) air pulses at an ultrasonic pulse rate, higher than a maximum human audible frequency. U.S. Pat. No. 10,425,732 also discloses that the APPS may function as a fan, which may be disposed within an electronic device and help on heat dissipation of the electronic device. 
     U.S. Pat. No. 10,771,893 provides a SEAM (single ended amplitude modulation) driving signal for a sound producing device, or an APPS, capable of producing single-ended PAM air pulses at ultrasonic pulse rate, in order to further enhance the sound pressure level performance and low audio frequency response. The SEAM driving signal comprises a plurality of electrical pulses, where the plurality of electrical pulses has the same polarity compared to (or with respect to) a certain voltage. For SEAM driving signal, each electrical pulse cycle comprises a PAM (pulse, amplitude-modulated) phase and an RST (reset) phase, which will be illustrated later on. The SEAM driving signal may be a PAM signal within the PAM phase and return to a reset voltage within the RST phase. 
     U.S. application Ser. No. 16/802,569 provides a sound producing device, or an APPS, which produces air pulses via chamber compression/expansion excited by membrane movement and the air pulses are propagated via through pressure ejection orifices (PEOs) formed either on the membrane or on a plate of the sound producing device, in order to achieve significant air pressure with small size/dimension of the sound producing device. 
     U.S. Pat. No. 11,043,197 provides an air pulse generating element and an APPS which utilize membrane to perform compression/expansion of the air within a chamber, and utilizes slits formed on the membrane to form virtual valves which may open temporarily to provide air shunt, such that an air pressure balancing process between two sides of membrane is accelerated. 
     In an embodiment, the air-pulse generating device of the present application may be applied in an APPS application, which is configured to produce PAM air pulses at an ultrasonic pulse rate according to APPS sound production principle. In another embodiment, the air-pulse generating device of the present application may be applied in an air movement or fan application, which functions as a fan and is similar to U.S. Pat. No. 10,425,732. 
       FIG. 1  is a schematic diagram of a cross sectional view of an air-pulse generating device  890  according to an embodiment of the present application. The air-pulse generating device  890  may be applied within an APPS. The air-pulse generating device  890  comprises a membrane structure  12 , a valve structure  11  and a cover structure  804 . A chamber  105  is formed between the membrane structure  12 , the valve structure  11  and the cover structure  804 . The air-pulse generating device  890  produces its (air pressure) output at ports  707 L and  707 R.  FIG. 1  illustrates (solid outlines) the membrane structure  12  in a state in which the membrane structure  12  is (substantially) flat and parallel to XY-plane, and also illustrates (dashed outlines) the membrane structure  12  in an actuated state in which the membrane structure  12  is curved. 
     The membrane structure  12  and the valve structure  11  may have thin film structure, which may, e.g., be fabricated by MEMS (Micro-Electro-Mechanical System) fabrication process using SOI (silicon/Si of insulator) or POI (Poly-Si/polysilicon on insulator) wafers, but not limited thereto. In the embodiment shown in  FIG. 1 , the membrane structure  12  comprises a first membrane portion  102   a  and a second membrane portion  102   b . The valve structure  11  comprises a first valve portion  101  and a second valve portion  103 . The cover structure  804  comprises a top plate  804 T and side walls  804 L and  804 R. The chamber  105  is surrounded by/between the membrane portions  102   a  and  102   b , the valve portions  101  and  103 , the top plates  804 T, and the side walls  804 L and  804 R. Valve portion  101 / 103  is anchored to support structure  110 / 115  on one end and is free-moving on the other end, where the free-moving end is located close/next to side wall  804 L/ 804 R. 
     The membrane structure  12  is configured to be actuated, such that an air wave AW is produced. Furthermore, by carefully choosing driving signal(s) fed to the membrane structure  12 , the air wave AW may vibrate at an operating frequency f CY  and propagates along with a direction (e.g., X-direction) parallel to the membrane structure  12  within the chamber  105 . 
     In a perspective, air wave may be related that the mass of air molecules periodically moves in a back-and-forth direction (e.g., left-and-right in X-direction, in view of X-axis components movement) at a certain time period due to air pressure variation or variation of air-molecule density. Air wave vibrating at a certain frequency may be related to the operating frequency f CY  that the certain frequency is a reciprocal of the certain time period, and vice versa. 
     The valve structure  11  is configured to be actuated to perform an open-and-close movement, at an opening frequency, to form an at least one opening periodically, where the at least one opening connects the air inside the chamber  105  with the ambient/air outside the chamber  105 . Specifically, the valve portion  101  may be actuated to perform an up-and-down movement (in the Z direction) which cause an opening  112  to form-and-unform, and this is referred to as the open-and-close of valve  101 . Similarly, the valve portion  103  may be actuated to perform an up-down movement (in the Z direction) which cause an opening  114  to form-and-unform, and this is referred to as the open-and-close of valve  103 . The open-and-close movements of the valve structure  11 , including the valve (portions)  101  and  103 , (or the opening frequency) would be synchronous with the air wave AW, which is further synchronous with the operating frequency f CY . The open-and-close movements of the valve structure/portion being synchronous with the operating frequency f CY  means that, the open-and-close movements of the valve portion/structure is performed (preferably) at the operating frequency f CY , or at a frequency of (M/N)*f CY , wherein both M and N are integers. The open-and-close, up-and-down, form-and-unform movement will be elaborated later. In the following description, the valve  101 / 103  may be referred to the valve portion  101 / 103  for brevity. 
     The function of valve opening is similar to that of a variable resistor whose resistance to airflow, Z VALVE , is controlled by the degree of the valve opening. When the valve is closed, i.e. Z 101 &lt;Z O/C  or Z 103 &lt;Z O/C , the magnitude of Z VALVE  will be high (Hi-Z). When the valve is opened, i.e. Z 101 &gt;Z O/C  or Z 103 &gt;Z O/C , the magnitude of Z VALVE  will be inversely related to the degree of opening, or Z 101 −Z O/C  and Z 101 −Z O/C . The wider a valve is opened, the lower the value of Z VALVE  will be and the higher the airflow will be for any given chamber pressure. 
     Chamber Resonance 
     Note that, given the side walls  804 L and  804 R may serve as reflection walls, the air wave AW generated by the membrane structure  12  may comprise an incident wave and a reflected wave. In an embodiment, a width of the chamber  105 , denoted as W 105 , or a distance between the side walls  804 L and  804 R, may be designed such that, the incident wave and the reflected wave may be aggregated and form a standing wave within the chamber  105 . 
     In an embodiment, the distance between the side walls  804 L and  804 R or the width W 105  may equal to an integer multiple of a half wavelength (λ/2) corresponding to the operating frequency f CY  of the air wave AW, λ=C/f CY , where C is the speed of sound. 
     In an embodiment, the distance between the side walls  804 L and  804 R or the width W 105  may be designed such that, a 1 st  mode (or n=1 mode) resonance, also called fundamental mode resonance or 1 st  harmonic resonance, is formed within the chamber  105 . In this case, only 1 air-motion antinode (amplitude reaches peak) exists within the chamber  105  (which may be at a center of the chamber  105 ); only 2 air-motion nodes (amplitude near 0) locate at the side walls  804 L and  804 R; only 1 air-pressure node exists within the chamber  105  (which may be at the center of the chamber  105 ); only 2 air-pressure antinodes locate at the side walls  804 L and  804 R. 
     Herein, in chamber resonance or standing wave perspective, the air-motion antinode represents position at which amplitude of air-molecule velocity/displacement achieves maximum in air-motion over X-axis within the chamber; the air-motion node represents position at which amplitude of air-molecule velocity/displacement achieves minimum in air-motion over X-axis within the chamber (usually 0 movement); the air-pressure antinode represents position at which amplitude of air pressure variation achieves maximum in air pressure over X-axis within the chamber; the air-pressure node represents position at which amplitude of air pressure variation achieves minimum in air pressure over X-axis within the chamber. 
     In  FIG. 1 , curves U 102  schematically represent displacements of air particles distributed in the X-direction at different times, curves W 102  schematically represent pressure distribution within the chamber at different times. For example, dashed lines of the curves U 102  and W 102  are corresponding to a time t 0  and solid lines of the curves U 102  and W 102  are corresponding to a time t 1 . P 0  in  FIG. 1  may refer to an ambient pressure, which may be 1 atm. In an embodiment, to achieve 1 st  mode (or n=1 mode) resonance, the distance between the between the side walls  804 L and  804 R or the width W 105  may be one half wavelength (λ CY /2) corresponding to the operating frequency f CY  of the air wave AW. 
     Details of the valve movement of  101 / 103  are further illustrated in  FIG. 16 . At the time t 0  (or when t=t 0 ), the valve  101  is actuated to bend upward such that the opening  112  is opened or formed, and the valve  103  may be actuated to (substantially) seal the opening  114 , which means that the opening  114  is closed or unformed, as shown in the top of  FIG. 16 . On the other hand, at the time t 1  (or when t=t 1 ), the valve  101  may be actuated to (substantially) seal the opening  112 , which means that the opening  112  is closed or unformed, and the valve  103  is actuated to bend upward such that the opening  114  is opened or formed, as shown in the bottom of  FIG. 16 . In an embodiment, at some time t 2  (or when t=t 2 , where t 2 ≠t 0  and t 2 ≠t 1 ), the valves  101  and  103  are in a state where the openings  112  and  114  are barely opened or barely closed, as shown in the middle of  FIG. 16 , corresponding to Z 101 =Z O/C  and Z 103 =Z O/C  as shown in  FIG. 2  respectively. 
       FIG. 2  is a schematic diagram of a plurality of waveforms according to an embodiment of the present application. Waveform Z 101  schematically represents displacement in Z-direction of the free-moving end of valve portion  101 ; while waveform Z 103  schematically represents displacement in Z-direction of the free-moving end of valve portion  103 . Z O/C  represents a certain level of displacement, and the suffix O/C stands for a line separating the open-state from the close-state. When the displacement of the free-moving end of valve Z 101  is larger than (above) the displacement level Z O/C , the opening  112  is formed or the valve  101  is opened. When the displacement of the free-moving end of valve Z 103  is larger than the displacement level Z O/C , the opening  114  is formed or the valve  103  is opened. When the displacement of the free-moving end of valve Z 101  is less than (below) the displacement level Z O/C , the opening  112  is not formed or the valve  101  is closed. When the displacement of the free-moving end of valve Z 103  is less than the displacement level Z O/C , the opening  114  is not formed or the valve  103  is closed. 
     Waveform P 112  schematically represents air pressure at the opening  112  (within the chamber  105 ). Waveform P 114  schematically represents air pressure at the opening  114  (within the chamber  105 ). Waveform Z 102   a  represents displacement of the membrane portion  102   a , which may share similar waveform with P 112 . Waveform Z 102   b  represents displacement of the membrane portion  102   b , which may share similar waveform with P 114 . Waveform P 707 L schematically represents air pressure (or quantity analogous to air pressure) at the port  707 L (out of the chamber  105 ). Waveform P 707 R schematically represents air pressure (or quantity analogous to air pressure) at the port  707 R (out of the chamber  105 ). Waveform P 890  represents a sum/superposition of P 707 L and P 707 R, corresponding to an aggregated on-axis output acoustic pressure of the device  890 . Waveform Z 102   a /Z 102   b  whose unit is length, such as μM, generally has different amplitude from waveform P 112 /P 114  whose unit is pressure, such as Pa. However, since the purpose of  FIG. 2  is mainly to illustrate the timing relationship between different parts of the operation, these waveforms are merged in  FIG. 2  for brevity. 
       FIG. 3  is a schematic diagram of a plurality of signals according to an embodiment of the present application. S IN  represents an input audio signal. S 101 /S 103  represents a valve driving signal configured to drive the valve portion  101 / 103 . S 102   a /S 102   b  represents a membrane driving signal configured to drive the membrane portion  102   a / 102   b.    
     AM Modulation Waveform 
     As can be seen from the plots/waveforms P 112  and P 114  in  FIGS. 2 , P 112  and P 114  are/comprise amplitude-modulated waveforms, and amplitude-modulated waveform P 112 /P 114  may be expressed as a product of a carrier component and a modulation component, in general. The carrier component, usually expressed as cos(2πf CY  t), oscillates at the operating frequency f CY , where f CY =1/T CY , where T CY  denotes an operating cycle. The modulation component, may be expressed as m(t), is reflected by an envelope of the amplitude-modulated waveform (denoted by dotted envelope-curves in  FIG. 2  and  FIG. 3 ) which is corresponding to the input audio signal S IN . In an embodiment, the modulation component m(t) may be corresponding or proportional to the input audio signal S IN . 
     The amplitude-modulated waveform P 112 /P 114  may be achieved by driving the membrane structure  12  by pulse-amplitude modulated driving signal. For example, the membrane driving signal S 102   a /S 102   b  shown in  FIG. 3  driving the membrane portion  102   a / 102   b  are pulse-amplitude modulated signal, generated according to the input audio signal S IN . 
     Membrane Driving Signal 
     In other words, the membrane driving signal S 102   a  comprises a first pulse-amplitude modulated (PAM) signal comprising a plurality of first pulses with respect to a certain bias voltage V B . The first pulses are temporally distributed/arranged by the operating frequency f CY . Similarly, the membrane driving signal S 102   b  comprises a second PAM signal comprising a plurality of second pulses with respect to the bias voltage V B . The second pulses are temporally distributed/arranged by the operating frequency f CY . 
     In addition, the first pulses comprise first transition edges; while the second pulses comprise second transition edges. The first transition edges of the first pulses within the PAM signal S 102   a  coincide with the second transition edges of the second pulses within the PAM signal S 102   b . Furthermore, at a certain coincidence time of the first transition edge and the second transition edge, the first transition edge is corresponding to a first transition polarity, and the second transition edge is corresponding to a second transition polarity. The first transition polarity is opposite to the second transition polarity, at the certain coincidence time. Details of the coincidence of the first and second transition edges and the opposition of the first and second transition polarities may be referred to  FIG. 3  of the present application, or also be referred to U.S. Pat. No. 11,043,197 or No. U.S. Pat. No. 11,051,108, which are not narrated herein for brevity. 
     Note that, the membrane driving signal S 102   a /S 102   b  driving the membrane portion  102   a / 102   b  is bipolar (or double-ended) with respect to the bias voltage V B , which is not limited thereto. For example,  FIG. 4  illustrates a 2 nd  type of membrane driving signals S 102   a ′ and S 102   b ′. The membrane portions  102   a  and  102   b  may be driven by the membrane driving signals S 102   a ′ and S 102   b ′, respectively. Note that, the membrane driving signals S 102   a ′ and S 102   b ′ are SEAM driving signals, which are unipolar with respect to the bias voltage V B . Similar to the unipolar membrane driving signals S 102   a  and S 102   b , first pulses within the driving signal S 102   a ′ and second pulses within the driving signal S 102   b ′ are mutually interleaved, and have coincidence transition edges and opposite transition polarities, as shown in  FIG. 4 . Details of the unipolar SEAM driving signal may be referred to U.S. Pat. No. 10,771,893, which are not narrated herein for brevity. 
       FIG. 4  also illustrates a 3 rd  type of membrane driving signals S 102   a ″ (solid line in the bottom) and S 102   b ″ (dashed line in the bottom, together with S 102   a ″). In an embodiment, the membrane portion  102   a  may be driven by the membrane driving signal S 102   a ″ and the membrane portion  102   b  may be driven by the membrane driving signal S 102   b ″. The driving signal S 102   b ″ may be obtained from S 102   a ″ according to equations expressed as 
         S 102 b″=V   B   −S 102 a″   (eq. 1)
 
       or  S 102 b″=−S 102 a″   (eq. 2).
 
     In other words, a sum of the membrane driving signals S 102   a ″ and S 102   b ″ may be a constant. The constant may be the voltage level V B  (if eq. 1 is applied) or 0V (if eq. 2 is applied). Similar to the membrane driving signals S 102   a  and S 102   b , first pulses within the driving signal S 102   a ″ and second pulses within the driving signal S 102   b ″ have coincidence transition edges and opposite transition polarities, which may be observed from  FIG. 4 . 
     Pressure Gradient 
     In one perspective, during a first interval (which may be a first half of the operating cycle T CY ), by applying the membrane driving signal pair (S 102   a , S 102   b )/(S 102   a ′, S 102   b ′)/(S 102   a ″, S 102   b ″) to the membrane portions  102   a  and  102   b , the membrane portions  102   a  may be actuated to move toward a positive Z direction and the membrane portions  102   b  may be actuated to move toward a negative Z direction. Hence, during the first interval, the membrane portion  102   a  may be actuated to compress a first part/volume  105   a  (on top of the membrane portion  102   a ) within the chamber  105  and the membrane portions  102   b  may be actuated to expand a second part/volume  105   b  (on top of the membrane portion  102   b ) within the chamber  105 , such that a first air pressure gradient (indicated by the block arrow  116  in  FIG. 1 ) is formed from the first part/volume  105   a  toward the second part/volume  105   b.    
     Conversely, during a second interval (which may be a second half of the operating cycle T CY ), the membrane portions  102   b  may be actuated to move toward the positive Z direction and the membrane portions  102   a  may be actuated to move toward the negative Z direction. Hence, during the second interval, the membrane portion  102   b  may be actuated to compress the second part/volume  105   b  and the membrane portions  102   a  may be actuated to expand the first part/volume  105   a , such that a second air pressure gradient (opposite to  116 , not shown in  FIG. 1 ) is formed from the second part/volume  105   b  toward the first part/volume  105   a.    
     A pressure-gradient direction of the air pressure gradient (e.g.,  116  shown in  FIG. 1 ) generated by the membrane structure  12 , including the membrane portions  102   a  and  102   b , is parallel to the X-direction shown in  FIG. 1 . A propagation direction of the air wave AW propagating within the chamber  105  is also parallel to the X-direction. That is, the pressure-gradient direction is parallel to the air-wave propagation direction. In addition, the pressure-gradient direction, which is parallel to the X-direction, is perpendicular to a membrane displacement direction of the membrane structure  12 , largely in the Z-direction, wherein the membrane displacement direction refers to a direction which the membrane is actuated to move toward. Therefore, the pressure-gradient direction is parallel to the XY-plane, the plane of the membrane structure, and is orthogonal to the direction of the membrane displacements (Z). By taking the membrane structure being actuated or deformed into consideration, the pressure-gradient direction (generated by the membrane structure) may be regarded as being substantially parallel to the membrane structure and/or substantially perpendicular/orthogonal to the direction of the membrane displacements/movement. 
     Spatial Location of Valve Opening 
     When a standing wave is formed within chamber  105 , in order to enhance the acoustic output efficiency, the opening(s) is suggested to be located at or near the air-pressure antinode(s) of the standing wave. For the air-pulse generating device  890 , the opening may be formed spatially on a location where a peak of the air/standing wave is achieved, wherein the peak of the air/standing wave herein may be in terms of air pressure (for APPS application). 
     For APPS application, suppose that air pressure within the chamber may be expressed as a single-variable function p(x) or a two-variable function p(x, t), where x denotes variable in X-axis and t denotes variable in time-axis. The peak may be corresponding to a place where the 1 st  order (partial) derivative being zero, i.e., dp(x)/dx=0 or ∂p(x, t)/∂x=0 (to seek optimum spatial location of valve opening). In other words, (for some fixed time t 0 ) the peak may be interpreted as a local maximum or a local minimum of p(x)/p(x, t 0 ) over x-axis. 
     In this case, for the air-pulse generating APPS device  890 , the openings  112  and  114  are formed near the side walls  804 L and  804 R, since the air-pressure antinodes of standing wave will be located at the side walls  804 L and  804 R. 
     Temporal Alignment of Valve Opening 
     In another aspect, in order to enhance the air pulse generation efficiency, the timing of valve opening(s) is suggested to be formed during an interval in which a peak pressure of the air wave is achieved at the locations of the valve opening, such as illustrated by  112  and  114  of  FIG. 1 . The peak pressure timing herein may be corresponding to a time at which the 1 st  order (partial) time derivative is zero, i.e., dp(t)/dt=0 or ∂p(x, t)/∂t=0 (to seek optimum timing, i.e., temporal behavior, of valve opening), given that air pressure within the chamber may be expressed as a single-variable function p(t) or the two-variable function p(x, t). In other words, (for some fixed location x 0 , x 0  may be the location of valve opening  112  or  114 ) the peak may be interpreted as a local maximum or a local minimum of p(x)/p(x 0 , t) over t-axis. 
     For example, referring to  FIG. 2 , time intervals of the opening  112  being formed (i.e., the valve portion  101  being actuated to be opened or the valve  101  being opened) is illustrated as dotted regions in the plot Z 101 ; time intervals of the opening  114  being formed (i.e., the valve portion  103  being actuated to be opened or the valve  103  being opened) is illustrated as cross hatched regions in the plot Z 103 . The opening  112  is formed during a (first) interval T 1 ; while the opening  114  is formed during a (second) interval T 2 . Both intervals T 1  and T 2  may lie within the operating cycle T c y, meaning that T 1 ≤T c y, T 2 ≤T CY  and T 1 +T 2 ≤(1+d)×T CY , where T CY =1/f CY  and d&lt;0.5. 
     To enhance efficiency, the first opening  112  is formed within the first interval T 1  during which a first peak pressure pk 1  of the air wave AW at a first location (corresponding to the sidewall  804 L) is achieved; the second opening  114  is formed within the second interval T 2  during which a second peak pressure pk 2  of the air wave AW at a second location is achieved. 
     In one perspective, the opening frequency of the valves  101  and  103  equals the operating frequency f CY , in the embodiment shown in  FIG. 2 . 
     Note that, in the embodiment illustrated in  FIG. 2 , the first interval T 1  (representing the opening interval of the valve  101 ) covers one half of the operating cycle T c y, and the second interval T 2  (representing the opening interval of the valve  103 ) covers another half of the operating cycle T c y, meaning that T 1 =T 2 ≈T CY /2 (i.e., let a length of interval T y  be equal to half the length of the operating cycle T CY , then T y ≈T 1  or T y ≈T 2 ), which is not limited thereto. The interval T 1  or T 2  may be slightly shorter or longer than T CY /2 (for example, within ±10% or ±20%). As long as the opening interval of the valve  101  covers the first peak pk 1  and the opening interval of the valve  103  covers the second peak pk 2 , the requirements of present application are satisfied, which is within the scope of the present application. 
     Furthermore, the first interval T 1  (representing the opening interval of the valve  101 ) may cover a first over/under-pressure interval during which air pressure P 112 , produced by the membrane movement, is greater/smaller than a certain pressure P th , where the first over/under-pressure interval overlaps with T 1  in the embodiment illustrated in  FIG. 2 . Similarly, the second interval T 2  (representing the opening interval of the valve  103 ) may cover a second over/under-pressure interval during which air pressure P 114 , produced by the membrane movement, is greater/smaller than the certain pressure P th , where the second over/under-pressure interval overlaps with T 2  in the embodiment illustrated in  FIG. 2 . In this case, the air-pulse generating device  890  generate positive/negative air pulses during the valve opening intervals T 1  and T 2 , where the positive/negative air pulses herein may be propagated from the chamber  105  to ambient during the valve opening interval(s). 
     Note that, the AW pressure wave generated by driving waveform S 102   a ′/S 102   b ′ of  FIG. 4  will be simple AM while the AW pressure wave generated by driving waveform S 102   a /S 102   b  of  FIG. 3  or S 102   a ″/−S 102   a ″ of  FIG. 4  will be DSB-SC (double-sideband, suppress carrier). The timing relationship shown in  FIG. 2  corresponds to a simple AM modulated AW pressure wave and peaks pk 1 , pk 2  will not cross the line of P th . However, for DSB-SC modulated AW pressure wave, pk 1 , pk 2  will cross the line of P IE  whenever the polarity of S IN  changes, at which time over-pressure becomes under-pressure and vice versa. 
     Note that, the total pressure within the chamber may have two component pressures: one is produced by the membrane movement, the other is produced by the valve movement. Either of both components may be in the form of standing wave. The pressures P 112  and P 114  shown in  FIG. 2  only refer to component pressures produced by the membrane movements. 
     Synchronous Valve Opening 
     Furthermore, the valve portion  101  may form the opening  112  in/during a plurality of first valve opening intervals, and the air pressure P 112  may be greater than the certain pressure P th  in/during a plurality of first over-pressure intervals. In the embodiment shown in  FIG. 2 , the plurality of first valve opening intervals (of the valve  101 ) and the plurality of first over-pressure intervals (of pressure P 112 ) are temporally aligned or overlapped, where the first valve opening intervals (of the valve  101 ) and the first over-pressure intervals (of pressure P 112 ) are annotated as T 1  in  FIG. 2 . 
     Similarly, the valve portion  103  may form the opening  114  in/during a plurality of second valve opening intervals and the air pressure P 114  may be greater than the certain pressure P th  in a plurality of second over-pressure intervals. The plurality of second valve opening intervals (of the valve  103 ) and the plurality of second over-pressure intervals (of pressure P 114 ) may be also temporally aligned or overlapped, where the valve opening intervals (of the valve  103 ) and the over-pressure intervals (of pressure P 114 ) are annotated as T 2  as in  FIG. 2 . 
     In the present application, a plurality of first time intervals and a plurality of second time intervals being temporally aligned or overlapped may refer that, 1) the plurality of first time intervals and the plurality of second time intervals are temporally arranged (or temporally appear) at the same frequency; or 2) a first time interval and a second time interval with which the first time interval overlaps, forming an overlapped region, and a length of the overlapped region is at least 50% of a length of the first (or second) time interval. 
     By aligning the valve opening intervals and the over-pressure intervals, the air-pulse generating device  890  may produce a plurality of first air pulses AP 1  (shown as P 707 L in  FIG. 2 ) at the port  707 L via the opening  112 , and produce a plurality of second air pulses AP 2  (shown as P 707 R in  FIG. 2 ) at the port  707 R via the opening  114 . In addition, a time corresponding to the peak valve opening of Z 101 /Z 103  is preferably aligned to a time corresponding to the peak pressure of P 112 /P 114  produced by the membrane movement. 
     In different perspectives, T 1  in  FIG. 2  may denote, respectively: the first valve opening intervals of the valve  101  (in Z 101 &#39;s perspective); first membrane movement intervals of the membrane portions  102   a  (in Z 102   a &#39;s perspective) and  102   b  (in Z 102   b &#39;s perspective), creating a pressure gradient (vector) directing from volume  105   a , atop membrane portion  102   a , towards volume  105   b , atop membrane portion  102   b ; the first over-pressure intervals (in P 112 &#39;s perspective); and first duty periods of the first air pulses at port  707 L, AP 1 . Similarly, T 2  in  FIG. 2  may denote, respectively: the second valve opening intervals of the valve  103  (in Z 103 &#39;s perspective); second membrane movement intervals of the membrane portions  102   a  (in Z 102   a &#39;s perspective) and the membrane portion  102   b  (in Z 102   b &#39;s perspective), creating a pressure gradient (vector) directing from volume  105   b , atop membrane portion  102   b , towards volume  105   a , atop membrane portion  102   a ; the second over-pressure intervals (in P 114 &#39;s perspective), and second duty periods of the second air pulses at port  707 R, AP 2 . 
       FIG. 2  illustrates, the first valve opening intervals of the valve  101 , the first chamber pressure gradient intervals, the movements of membrane portions  102   a  and  102   b , the first over-pressure intervals and the first duty periods of the first air pressure pulses AP 1  are temporally aligned (peak-to-peak) and overlapped (period wise). Similarity, the second valve opening intervals of the valve  103 , the second chamber pressure gradient intervals, the movements of membrane portions  102   a  and  102   b , the second over-pressure intervals (in P 114 &#39;s perspective), and the second duty periods of the second air pressure pulses AP 2  are temporally aligned (peak-to-peak) and overlapped (period wise). 
     Combining Two Half-Wave Rectified Pulses into One Full-Wave Rectified Pulses 
     In a perspective, by comparing waveforms P 112  and P 707 L, P 707 L may be interpreted as a half-wave rectified version of P 112 , rectified by the timing varying impedance associated with valve  101  movement Z 101 . Also, by comparing waveforms P 114  and P 707 R, P 707 R may be interpreted as a half-wave rectified version of P 114 , rectified by the timing varying impedance associated with valve  103  movement Z 103 . The waveform P 890 , the summing the waveforms P 707 L and P 707 R and representing the on-axis output acoustic pressure of the device  890 , may be interpreted as a full-wave rectified version of P 112  or P 114 . 
     Referring to plot P 707 L, the plurality of first air pulses AP 1  are produced at a first (air) pulse rate APR 1  corresponding to the operating frequency f CY . Referring to plot P 707 R, the plurality of second air pulses AP 2  are produced at a second (air) pulse rate APR 2  corresponding to the operating frequency f CY . 
     Referring to plot P 890 , since the first plurality of air pulses AP 1  and the second plurality of air pulses AP 2  are temporally and mutually interleaved, it can be interpreted that the air-pulse generating device  890  produces a plurality of aggregated air pules AP. The plurality of aggregated air pules AP comprises the first air pulses AP 1  with the first pulse rate APR 1  and the second air pulses AP 2  with the second pulse rate APR 2 . The aggregated air pules AP is produced at an overall (air) pulse rate PRO. 
     Under a condition of APR 1 =APR 2 =f CY  as the embodiment illustrated in  FIG. 2 , the overall pulse rate PRO is twice of the pulse rate APR 1  (or APR 2 ). In other words, the overall pulse rate PRO is corresponding to twice of the operating frequency f CY , i.e., PRO=2*f CY , analogous to 60 Hz 110 VAC sine waveform will produce 120 Hz of half-sine waveform after being full-wave rectified. 
     Analogy to AM Radio Demodulation 
     In a perspective, the action of the membrane movement can be compared to the AM radio station which creates EM wave amplitude modulated by sound signal and radiates the AM EM wave into the air. Instead of EM wave, device  890  generates amplitude modulated ultrasound wave and transmits such AM ultrasound wave into chamber  105 . Such ultrasound wave is further amplified, at the location of the valve, by the standing wave construct of chamber  105 . The standing wave construct of chamber  105  is analogous to an EM waveguide where the signal strength is maximized by locating the port(s) at the node(s) and antinode(s) of the waveguide. The signal received at the location of the valve is then demodulated by the periodical operation of the valve(s), which is analogous to the synchronous local oscillator of an AM receiver, and the nonlinear characteristics of Z VALVE , which is analogous to the mixer of an AM receiver and generate the output, P 707 R/P 707 R, by dividing P 112 /P 114  by the impendence Z VALVE (t) of its corresponding valve. 
     As an example, supposed that the plots Z 101 , P 112 , Z 103  and P 114  are sinusoidal for simplicity, i.e., by virtue of interleaved driving signal S 101 , S 103  we have Z 101 ∝ sin(ωt), Z 103  ∝−sin(ωt); and in the example illustrated in  FIG. 1 , by virtue of n=1 standing wave, there will be a phase inversion between P 112  and P 114  and therefore we can express these two local pressure as ∝S IN ·sin(ωt), P 114 ∝−S IN ·sin(ωt), where the negative sign “−” represent the 180° phase difference, and ω=2πf CY . Assuming Z VALVE ∝1/(Z 101 −Z O/C ) when Z 101 &gt;Z O/C  and Z VALVE =∞ otherwise, then P 707 L may be expressed as P 707 L∝S IN ·sin 2 (ωt) when Z 101 &gt;Z O/C  and P 707 L=0 otherwise. Likewise, P 707 R may be expressed as P 707 R∝S IN ·sin 2 (ωt) when Z 103 &gt;Z O/C  and P 707 R=0 otherwise. The quantity P 890 , being P 707 L+P 707 R, representing an acoustic sound produced by the device  890 . After substituting P 707 L and P 707 R we get P 890 =P 707 L+P 707 R∝S IN ·sin 2 (ωt) for all time in which the device  890  operates. 
     Note that, when a DSB-SC AM radio waveform, which has a mathematical expression of S IN ·sin(ωt), is demodulated by a carrier signal sin(ωt), generated by a synchronous local oscillator, with a multiplier, the result can be expressed as S IN ·sin(ωt)·sin(ωt)=S IN  sin 2 (ωt), which is exactly the same mathematical expression for P 890  derived in the paragraph above. 
     As known by person having ordinary skill in the art, after multiplying the AM modulated signal/waveform S IN ·sin(ωt) by the demodulation signal sin(ωt), ⅔ of the energy of the resulting signal (i.e., S IN ·sin 2 (ωt)) is in the baseband and ⅓ of the energy of the resulting signal is on a frequency band centered at twice of the carrier frequency, i.e., 2·ω or 2·f CY . Illustratively, supposed that 
         P 890 ∝S   IN ·sin 2 (ω t )= S   IN ·(½−½ cos(2 ωt ))  (eq. 3).
 
     The 1 st  term in eq. 3, ½·S IN , represents demodulated component on the baseband; while the 2 nd  term in eq. 3, ½. S IN ·cos(2ωt), represents component in the ultrasonic band. As can be seen from eq. 3, a first energy of the 1 st  term within the baseband is twice of a second energy of the 2 nd  term. The baseband herein refers to a frequency band of the input audio signal S IN , and this baseband covers/overlaps with human audible frequency band. 
     In  FIG. 1  (or  FIG. 6 ), material of oxide substrate underneath the valves  101 ,  103 , the membrane portions  102   a ,  102   b  may be removed by photo lithography process/processes, and supports  110  and walls  111  may be formed. According to patterns of very fine lines, Si or POLY layer(s) may be etched to form openings/slits. Such slits create free moving ends on the valve  101 / 103  (e.g., these slits may form the opening  112 / 114  when the displacement of the free-moving ends of the valves exceed Z O/C ). Alternatively, slits can increase the compliance of the membrane portion  102   a / 102   b  (e.g., by forming slits  113   a ,  113   b  on the membrane portions  102   a ,  102   b ). 
       FIG. 5  is a schematic diagram illustrating a top view of the air-pulse generating device  890  shown in  FIG. 1 . The air-pulse generating device  890  may (optionally) include cross linked beams  871 ,  872  to break down the (long) valves  101 ,  103  or the (long) membrane portions  102   a ,  102   b  into shorter pieces and to reinforce the supports  110  and  891 . The air-pulse generating device  890  may (optionally) have slot(s)  873 , which may be created by widening one slit on a membrane portion to function as an airflow pathway to allow pressure to be release. Herein, a slit generally has a width corresponding to the etching resolution of a MEMS fabrication process, such as a width of 0.5˜1.8 μM over 3˜7 μM-thick Si membrane; a slot refers to a line geometry width that is not restricted to the limits of the MEMS fabrication process. 
     Higher Harmonics 
     Higher harmonic resonance may occur in an air-pulse generating device. For example,  FIG. 7  is a schematic diagram of a cross sectional view of an air-pulse generating device  850  according to an embodiment of the present application. In the air-pulse generating device  850 , the width W 105  between the side walls  804 L and  804 R may be one wavelength (λ) corresponding to the operating frequency f CY  to achieve 2 nd  mode (or n=2 mode) resonance. In 2 nd  mode resonance, 2 air-motion antinodes exist within the chamber  105  (for instance, at/near a quarter (¼) of the width W 105  from either side wall  804 L or side wall  804 R); 3 air-motion nodes locate at the center of the chamber  105  and near the side walls  804 L,  804 R; 2 air-pressure nodes exist within the chamber  105  (for instance, at/near a quarter (¼) of the width W 105  from either side wall  804 L or  804 R); 3 air-pressure antinodes locate at the center of the chamber  105  and the side walls  804 L,  804 R. The curve W 102  schematically representing pressure distribution within the chamber  105  over time may be caused by the movement of membrane portions  102   c  and  102   d  of the air-pulse generating device  830  and symmetrical relative to a center line  703 . As illustrated by W 102  in  FIG. 7 , when a n=2 mode standing wave is formed within chamber  105  of device  850 , by driving membranes  102   e  and  102   f  in synchronous with the one common waveform such as S 102   a ″, air-pressure waveform near sidewall  804 L and  804 R will be in-phase with each other and a phase inverted air-pressure waveform of similar amplitude will be produced at the center of chamber  105 . The valve opening  112  of the air-pulse generating device  850  may therefore be located at/near a center location between the side walls  804 L and  804 R, since an air-pressure antinode is located at the center of the chamber  105  (or the width W 105 ). In other words, for higher harmonic resonance (namely, n≥2), in addition next to side walls  804 L and  804 R, opening(s) of air-pulse generating orifice(s) may also be at/near any air-pressure antinode between the two side walls causing resonance. 
     The same description of the last paragraph is also applicable to device  830  of  FIG. 6 . 
     In the air-pulse generating device, such as device  830  of  FIG. 6 , device  850  of  FIG. 7  or device  890  of  FIG. 1 , the demodulation operation of the valves  101  and  103  will produce pulses of airflow which will accumulate across consecutive pulses, causing a long-tern net air mass change inside chamber  105  and increase/decrease the pressure P 0  within the chamber  105 . Since such back pressure will cause the output SPL to drop, it is therefore suggested to release such pressure. 
     In the case of air-pulse generating device  830  of  FIG. 6 , the slit opening  113   a */ 113   b * may be designed to be close to the air-pressure node located at W 105 /4 away from the side wall  804 L/ 804 R. Due to acoustic filter effect of air-pressure node of n=2 standing wave, as illustrated by the waveform W 102  crossing P 0 , enlarged slit  113   a */ 113   b * will have minimum impact on the operation of device  830  while releasing the pressure build up due to the demodulation operation of the valves  101  and  103 , illustrated by valve opening  112 . 
     In the case of air-pulse generating device  850  in  FIG. 7 , which also operate with at frequency f CY  corresponding to n=2 mode resonance across the width of the chamber W 105 , the membranes  102   e  and  102   f  each comprise of 1 single piece of thin flap, attached to their respective support  110 . As opposed to the situation in the device  830 , where the membranes  102   c ,  102   d  are each made of two sub-portions, separated by slits  113   a  and  113   b  respectively, in device  850 , since slits  112  and  114 , created to allow free movement of membranes  102   e  and  102   f , are located at the air-pressure anti-nodes within chamber  105  of device  850 , the width of these slits needs to be minimized to suppress the leakage of air-pressure. Therefore, one or multiple vent(s)  713 T can be created on the top cap, at the location(s) of the air-pressure node(s), for example, at a distance of W 105 /4 away from side walls  804 L and  804 R. Although theoretically speaking, one such vent may suffice for the back-pressure release purpose, however, for the consideration of optimal balancing of air pressure within chamber  105 , it is generally a good practice to have a pair of vents  713 T, positioned in a center-mirroring fashion, as illustrated in  FIG. 7 . 
     In the case of air-pulse generating device  890  in  FIG. 1 , pressure pulses of the acoustic sound (e.g., the acoustic sound P 890 ) out of the valves  112  and  113  have the same polarity, which combine together to increase/decrease the pressure P 0  within the chamber  105 . Therefore, vent openings  713 T on the top plates, located at or near the air-pressure node, as indicated by alignment to the position where air-pressure profile W 102  crosses P 0 , is created to allow airflow to pass through, releasing the pressure build up due to the demodulation operation of the valves  101  and  103 . 
     The length and width of the vent opening(s)  713 T may be adjusted to form a suitable acoustic low pass filter (LPF) with the volume of the chamber  105 . The location of the vent opening(s)  713 T may be at air-pressure node(s), relative to operating frequency f CY , where the amplitude of frequency components corresponding to the standing wave is nearly zero. As a result, an acoustic notch filter is formed and the pressure corresponding to the amplitude modulated standing wave may be suppressed near/at the vent opening(s) of  713 T inside the chamber  105 , and only the pressure change due to the demodulation operation may be present near/at the vent opening(s)  713 T. For devices operated in the 2 nd  mode resonance (e.g., the device  850 ), the vent opening  713 T of the air-pulse may be positioned approximately at a quarter of the width W 105  (W 105 /4) from either of the side walls  804 R and  804 L, which is different from the device operating in the 1 st  mode resonance (e.g., the device  890 ), where the vent opening  713 T (of the air-pulse generating device  890 ) may be near the midpoint between the two the side walls  804 R and  804 L. 
     The structure of an air-pulse generating device  850  may be altered according to different design consideration. For example, the membrane  102   e / 102   f  may have two membrane sub-portions, or 2-pieces, like membrane  102   a / 102   b  or  102   c / 102   d  does, but is not limited thereto. Note that the maximum Z-direction displacement of 1-piece membrane construct, such as  102   e / 102   f  in  FIG. 6 , needs to be significantly smaller than the thickness (a Z-direction value) of  102   e / 102   f  to avoid leakage of the air pressure inside chamber  105 . In comparison, in the 2-piece per membrane construct, since the two sub-portions always moves in tandem, such Z-direction membrane displacement limitation does not exist, meaning larger displacement may be possible and therefore lead to improved unit-device-area effectiveness (SPL per meter). 
     Furthermore, the valve portions  101  and  103  illustrated in  FIG. 7  may be considered as a virtual valve. In other words, a slit formed between the valve portions  101  and  103 , may become a temporarily formed/opened valve opening ( 112 ′) when the valve portions  101  and  103  is sufficiently actuated. In addition, the temporarily formed/opened valve opening is formed periodically. When the opening is opened, the chamber and ambient environment are connected via the opening ( 112 ′). When the opening is not opened, air flowing through the slit is negligible or less than a threshold. Details of virtual valve (temporarily formed opening) may be referred to U.S. Pat. No. 11,043,197, which is not narrated herein for brevity. 
     In addition, similar to the device  890  shown in  FIG. 1 , pressure gradients are also generated in the device  850  via membrane movement and the nature of standing wave. Different from the device  890 , the membrane portions  102   e  and  102   f  are actuated to move in an in-phase fashion, referring that at a certain time, both the membrane portions  102   e  and  102   f  are actuated to move upward (or downward). In this case, pressure gradients are also established by utilizing the nature of n=2 standing wave as well. Similar to description of  FIG. 1 , in  FIG. 7 , dashed lines of the curves U 102  and W 102  are corresponding to the time t 0  and solid lines of the curves U 102  and W 102  are corresponding to the time t 1 . At the time t 0 , the membrane portions  102   e  and  102   f  are actuated to move upward (in positive Z direction), pressure gradients are generated in inward direction (in X direction), as illustrated by the slope of dashed line of W 102 . At the time t 1 , the membrane portions  102   e  and  102   f  are actuated to move downward (in negative Z direction), pressure gradients are generated in outward direction (in X direction), as illustrated by the slope of solid line of W 102 . Similarly, the membrane movement directions are substantially perpendicular to the pressure gradient directions. 
     Air Movement or Fan Application 
     The structure/mechanism of device  890 / 830 / 850  may be reproduced/adapted for an air movement or fan application. Different from an acoustic wave traveling at the speed of sound, C, an air movement is the airflow related to the kinetic movement of air particles, as that of wind, and is produced by the displacement of membrane portion(s), corresponding to membrane portions  102   a ˜ 102   d / 102  of the air-pulse generating device  890 / 830 / 850 . In an air movement or fan application/mode of these devices, air particles within the device may be described mainly according to fluid dynamics or aerodynamics; in contrast, in an air-pulse (APPS) generating application/mode of these devices, the behavior of air within the device may be described mainly according to acoustics. 
     For air movement or fan application, valve opening(s), such as the openings  112  and  114  illustrated in device  890 / 830 / 850 , may be formed spatially on a location, and temporarily in time, such that the air motion is maximized, wherein the peak of the air motion may be in terms of the velocity of the air moved or in terms of the volume of air moved. 
     Driving signal(s) of the device for the air-flow generation or fan application differs from that of the APPS application. For example, in air movement or fan application, device  890  may actuate its two membranes ( 102   a  and  102   b ) to move synchronously, by applying the same driving signal to both membrane  102   a  and  102   b , to create a pressure difference between the volume inside chamber  105  and the ambient outside of device  890 . In comparison, in APPS application, device  890  would actuate its two membranes ( 102   a  and  102   b ) to move symmetrically, in opposite direction (along Z axis), by applying two interleaved (such as S 102   a , S 102   b ) or polarity inverted (such as S 102   a ″, −S 102   a ″) driving signals to membrane  102   a  and  102   b , to create pressure gradient (vector  116 ) within chamber  105 , atop the two membranes. 
     A key difference between these two modes of operation lies in the different relationship between the chamber dimension and the operating frequency of device. As described in association with device  890 / 830 / 850  for APPS application, the operating frequency may be selected to produce a standing wave of mode n within chamber. In other words, the operating frequency f CY  is related to chamber width W 105  by equation W 105 =n/2·λ CY , where λ CY =C/f CY  is the characteristic length or wavelength of f CY  and n is a small positive integer such as 1˜3. On the other hand, for the air movement or fan application of device  890 / 830 / 850 , the conversion rate of membrane movement into airflow generally increases as the ratio λ CY /W chamber  increases, where W chamber  is the chamber width of the device, corresponding to width of the chamber  105 , W 105 , of the air-pulse generating device  890 / 830 / 850 . In other words, the conversion rate of membrane movement into airflow typically increases when the pressure within the chamber of the air-flow generating device for the air movement or fan application (corresponding to the chamber  105  of the air-pulse generating device  890 / 830 / 850 ) becomes more uniform, exactly opposite to the desire to maximize the pressure gradient (or the nonuniformity of the pressure within chamber  105 ) of the air-pulse generating device  890 / 830 / 850 . 
     For example, in the air-pulse generating device  890 , W 105 =λ CY =3.6 mm at operating frequency of 96 KHz since the resonance frequency f of a cantilever beam may be related to its length L by f∝1/L 3 . On the other hand, by lowing the operating frequency of the air-pulse generating device for the air movement or fan application from 96 KHz down to 24 KHz and lowering the resonance frequency of both the membrane portion(s) and the valve portion(s) of the air-pulse generating device for the air movement or fan application also to 24 KHz, the width of the membrane portion may increase from 0.94 mm to 1.44 mm, the width of the valve portion may increase from 0.46 to 0.73 mm, and the resulting width of the chamber may be 2×(0.1+0.73+0.2)+1.44=3.5 mm, which is much shorter than the wavelength of 14.6 mm at frequency of 24 KHz, indicating a higher conversion rate of membrane movement into airflow. Therefore, despite almost identical cross-section view, an air-pulse generating device for the air movement or fan application with the resonance frequency of 24 KHz for both the membrane portion(s) and the valve portion(s) and driving both membrane portions of the air-pulse generating device for the air movement or fan application with the same waveform at 24 KHz may be suitable for air moving applications while the air-pulse generating device  890 , where the membrane portion  102   a  and  102   b  are driven by interleaved waveforms S 102   a ′, S 102   b ′ or symmetrical waveforms S 102   a ″, −S 102   a ″ to produce near-0 net air movement over each operating cycle T CY , may be optimized for sound production applications and not suitable as an air movement apparatus. 
     In a word, while symmetrical membrane displacements of the membrane portion  102   a / 102   b  or  102   c / 102   d  of device  890  may be used to maximize the in-chamber pressure gradient for APPS applications, synchronous/identical membrane displacement (by driving membrane portions with signal of the same polarity) may be adopted to maximize the conversion rate of membrane movement to airflow. In another perspective, for APPS applications, the chamber width (in X direction) W 105  may be equal or close to n/2×λ CY  (where n is a small positive integer) in order to maximize its acoustic output by leveraging chamber resonance (i.e. standing wave); on the other hand, for air movement applications, the chamber width (in X direction) of an air-pulse generating device for the air movement or fan application may be much smaller than λ CY /2 to maximize the conversion rate of membrane movement to airflow. 
     Different structural embodiments (air-pulse generating) device are described in the following paragraph. For example,  FIG. 8  is a schematic diagram of a cross sectional view of an air-pulse generating device  880  according to an embodiment of the present application. The membrane structure  12  of the air-pulse generating device  880  includes one membrane portion, which is divided into membrane subparts  102   e ′,  102   f ′ and  102   g . The membrane subparts  102   e ′ and  102   g  may be differentiated according to slits  113   e  and  113   f  on the membrane portion. The membrane structure  12  of the air-pulse generating device  880  with the membrane subparts  102   e ′ and  102   g  may serve/function as the membrane portions  102   a  and  102   b  of the air-pulse generating device  890  (or the membrane portions  102   c  and  102   d  of the air-pulse generating device  830 ). 
     For APPS applications, the membrane subparts  102   e ′ and  102   g  may be driven by a pair of membrane driving signals similar to the membrane driving signal pair (S 102   a , S 102   b )/(S 102   a ′, S 102   b ′)/(S 102   a ″, S 102   b ″), such that the membrane subparts  102   e ′ and  102   g  may move almost oppositely to have symmetrical membrane displacements. Similar to the membrane portion  102   a  bending downwards and the membrane portion  102   b  bending upwards, the membrane subparts  102   e ′ and  102   f ′ may be curved concavely to bend downwards while the membrane subparts  102   f ′ and  102   g  may be curved convexly to bend upwards, and vice versa. 
       FIG. 9  is a schematic diagram of a cross sectional view of an air-pulse generating device  800  according to an embodiment of the present application. The membrane structure  12  of the air-pulse generating device  800  includes membrane portions  102   g  and  102   h , which are anchored on the support  110  at the center of the air-pulse generating device  800 . The slits/tips of the membrane portions  102   g  and  102   h  are located close to the side wall  804 L and  804 R. 
     The valves  101  and  103  of the air-pulse generating device  890 / 830 / 850 / 880  are absent from the air-pulse generating device  800 . When the membrane portions  102   g  and  102   h  are driven by the pair of the membrane driving signals (S 102   a , S 102   b )/(S 102   a ′, S 102   b ′)/(S 102   a ″, S 102   b ″), the membrane portions  102   g  and  102   h  may provide the pressure regulation function of the valves  101 ,  103  of the air-pulse generating device  890  and the pressure generation function of the membrane portions  102   a ,  102   b  of the air-pulse generating device  890  by utilizing the slits between the membrane portions  102   g ,  102   h  and the walls  111  to perform the AM ultrasonic carrier rectification function of the openings  112 ,  114  of the valves  101 ,  103  of the air-pulse generating device  890 . 
     As a result, the membrane portion  102   g  may vibrate to form opening  112   g  functioned as the opening  112  of the valve  101  and meanwhile create the maximum/minimum change in pressure (e.g., the first peak pressure pk 1 ). The membrane portion  102   h  may vibrate to form the opening  114   h  functioned as the opening  114  of the valve  103  and meanwhile create the maximum/minimum change in pressure (e.g., the second peak pressure pk 2 ). 
     The air pressure waveform P 707 L may be expressed as P 707 L oc (S IN ·sin(ω·t)+Z 0AC ) 2  when Z 102   a &gt;Z O/C  and P 707 L=0 otherwise. The air pressure waveform P 707 R∝(S IN ·sin(−ω·t)+Z 0AC ) 2  when Z 102   b &gt;Z O/C  and P 707 R=0 otherwise. Herein, waveforms Z 102   a , Z 102   b  represent displacement of the membrane portions  102   g ,  102   h  respectively; waveform P 707 L, P 707 R represent air pressure at the ports  707 L,  707 R (out of the chamber  105 ) respectively. 
     A negative bias voltage may be applied to bottom electrode(s) of actuator(s) of the membrane portion  102   g / 102   h , such that the position of (the tip of) the membrane portion  102   g / 102   h  in the Z direction is lifted to be equal to or slightly above the displacement level Z O/C  when the input AC voltage is 0V. In other words, Z 0AC  may be positive. If the position of (the tip of) the membrane portion  102   g / 102   h  in the Z direction is below the displacement level Z O/C  when the input AC voltage is 0V, Z 0AC  may be negative, and a clipping phenomenon similar to class-B amplifiers may occur to low level input signal(s). In the clipping phenomenon, the membrane portion  102   g / 102   h  may not be fully opened. 
     When Z 0AC  is a positive number, an aggregated on-axis output acoustic pressure of the air-pulse generating device  800  (namely, P 800 =P 707 R+P 707 L) may be expressed as: 
         P 800∝( S   IN ·sin(ω· t )+ Z   0AC ) 2 +( S   IN ·sin(−ω· t )+ Z   0AC ) 2   =S   IN   2 ·(1−cos 2 (2ω· t ))+2· Z   0AC   2  when | S   IN ·sin(ω· t )|&lt; Z   0AC   (eq. 5a),
 
         P 800∝( S   IN ·sin(ω· t )+ Z   0AC ) 2 ≈½ S   IN   2 ·(1−cos 2 (2ω· t ))+2· S   IN ·sin(ω· t )· Z   0AC  when | S   IN ·sin(ω· t )|&gt;&gt; Z   0AC   (eq. 5b), and
 
         P 800∝( S   IN ·sin(ω· t )) 2 ≈½ S   IN   2 ·(1−cos 2 (2ω· t )) when  Z   0AC →0+  (eq. 5c).
 
     Z 0AC  is the membrane displacement relative to the displacement level Z O/C  when the input AC voltage is 0V. 
     In an embodiment, Z 0AC  may be set to a small positive value to reduce the second term 2·Z 0AC   2  in eq. 5a and the inaudible second term 2·S IN ·sin(ω·t)·Z 0AC  in eq. 5b. For example, Z 0AC  may range between 1%˜10% of the maximum membrane displacement. 
     In an embodiment, to compensate the nonlinearity of S IN   2  in eq. 5a to eq. 5c, linearity compensation may be performed by a DSP function block embedded within a host processor. 
     By setting Z 0AC  to a small positive value, the membrane portion  102   g / 102   h  may be slightly open when the input AC voltage is 0V. Given the symmetricity of the membrane driving signal (S 102   a , S 102   b )/(S 102   a ′, S 102   b ′)/(S 102   a ″, S 102   b ″), at least one of the openings  112   g ,  114   h  may be slightly open/formed at any time. Therefore, the pressure change inside the chamber  105  due to the rectification effect of the openings  112   g ,  114   h  may be balanced, and the vent opening(s)  713 T or the wider slit openings  113   a */ 113   b * may be absent from the air-pulse generating device  800 . 
     In the air-pulse generating device  800 , whether resonance occurs in the chamber  105  or not, the effect of full-wave rectification and synchronous demodulation may be produced by the air-pulse generating device  800 . Even without any standing wave to create the maximum acoustic pressure at or near the side walls  804 L and  804 R, such maximum acoustic pressure may occur simply as a result of the physical location of the openings  112   g ,  114   h  of the membrane portions  102   g ,  102   h  and the symmetrical membrane driving signals (S 102   a , S 102   b )/(S 102   a ′, S 102   b ′)/(S 102   a ″, S 102   b ″), which drive the actuators of the membrane portions  102   g ,  102   h  to cause the maximum displacements near the side walls  804 L and  804 R. For example, the membrane portion  102   g  may be actuated to compress the first part/volume  105   a  (on the top of the membrane portion  102   g ) within the chamber  105  to maximum the local pressure. The membrane portions  102   h  may be actuated to expand the second part/volume  105   b  (on the top of the membrane portion  102   h ) within the chamber  105  to minimum the local pressure. The pressure profile over time within the part/volumes  105   a  and  105   b  may be identical to that of a standing wave in the 1 st  mode resonance. In other words, the air-pulse generating device  800  may achieve full-wave rectification and synchronous demodulation without the resonance of the chamber  105 , thereby increasing flexibility in the design of an air-pulse generating device. 
     In the air-pulse generating device  800 , if resonance occurs, the output of the air-pulse generating device  800  may benefit from the standing wave of such resonance. For example, when the width W 105  of the chamber  105  of the air-pulse generating device  800  equals half of the wavelength (λ/2) corresponding to the operating frequency f CY , a pressure profile similar to that of a standing wave may be established by the movements of the membrane portions  102   g  and  102   h  and therefore enhance the output caused by the standing wave having already established within the chamber  105 . 
     Enclosure-Less 
     Since the air pulse generating device  890 / 850 / 830  do not generate a pair of out-of-phase baseband radiations, as produced by a conventional speaker (namely, a front radiation and a phase-inverted back radiation), the air-pulse generating device  890 / 850 / 830  do not require any back enclosure (whose purpose is to contain or transform to the back radiation and prevent the phase inverted back radiation from cancelling out the front radiation) as a conventional speaker does. Therefore, the air pulse generating device  890 / 850 / 830 , which produces sound, can be enclosure-less. 
     In the case of device  890 , by utilizing the 1 st  mode resonance of the chamber  105  and the interleaved timing of valve opening, the air-pulse generating device  890  produces two radiations that are in-phase instead of 180° out of phase. By proper timing alignment between open timing of valve  101 / 103  (denoted by Z 101 /Z 103  in  FIG. 2 ) and pressure wave P 112 /P 114 , the phase of acoustic energy is properly phase aligned and the ultrasonic radiation is transformed to double the baseband output SPL, increases the utilization rate of the total acoustic energy, achieve effective demodulation of ultrasonic AM signal while obliterate the need for an enclosure. 
     Acoustic Filter 
     An acoustic filter may be added in front of the air-pulse generating device. For example,  FIG. 10  is a schematic diagram of the air-pulse generating device  890  disposed within a construct A 00  according to an embodiment of the present application.  FIG. 11  is a schematic diagram of the air-pulse generating device  890  disposed within a construct A 30  according to an embodiment of the present application. The acoustic air pressure measured at the ports  707 L and  707 R of the air-pulse generating device  890  may include not only the demodulated AM ultrasonic waves P 707 L and P 707 R but also ultrasonic waves generated by the motion of the valves  101  and  103 . The symmetrical movements of the valves  101  and  103  may be characterized as a dipole. The superposition of the ultrasonic waves generated by the motion of the valves  101  and  103  may peak along the plane of the valves  101  and  103  and become null on the center plane between the side walls  804 L and  804 R. The construct A 00 /A 30  may be configured to minimize the ultrasonic waves generated by the motion of the valves  101  and  103  and thus served as an acoustic filter. 
     In  FIG. 10 , the construct A 00  may include a funnel structure A 05  configured to filter out the ultrasonic waves generated by the motion of the valves  101 / 103 . The funnel structure A 05  may have a wide opening on the inside of the construct A 00 , sloping sides, and a narrow tube near the outside of the construct A 00 . The wide opening of the funnel structure A 05  may be smaller than the width W 105  of the chamber  105 . The funnel structure A 05  may merge the output from the ports  707 L and  707 R, causing the ultrasonic waves produced by the symmetrical movement of the valves  101  and  103  to annihilate each other and leaving behind the wave P 890 , which is the sum/superposition of the waves P 770 L and P 770 R. 
     In  FIG. 11 , the construct A 30  may include an external chamber A 06  and a port A 07  serving as the output port for the construct A 30 . The width Wa 06  between side walls AO 6 T, A 06 B of the external chamber A 06  may equal the width W 105  of the chamber  105  (e.g., half of λ CY ), such that a standing wave may occur at both the frequency f CY  (for 1 4  mode resonance) and the frequency 2·f CY  (for 2 nd  mode resonance). The width Wa 07  of the port A 07  may be smaller than the width W 105  of the chamber  105 . The width Wa 07  of the port A 07  may be equal to half of the width W 105  of the chamber  105  or a quarter of λ CY . 
     The construct A 30  is configured to filter out the ultrasonic waves generated by the motion of the valves  101 / 103 . For the ultrasonic waves generated by the symmetrical movement of the valves  101  and  103 , which has the frequency f CY , the acoustic energy may reside in the 1 st  mode resonance of the external chamber A 06  with the air-pressure node at/near the midpoint between the side walls A 06 T and A 06 B, and the pressure of the standing wave may be merged to zero over the width Wa 07  of the port A 07 . For the acoustic wave P 890 , which has the pulse rate 2·f CY , the acoustic energy may reside in the 2 nd  mode of the external chamber A 06  with an air-pressure antinode at/near the midpoint between the side walls A 06 T and A 06 B, which is also the center of the port A 07 , and the maximum output pressure may be produced when the pressure of the standing wave is integrated over the width Wa 07  of the port A 07 . By utilizing two different resonance modes, the external chamber A 06  may remove the ultrasonic spectral component at the frequency f CY  by the 1 st  mode resonance and pass ultrasonic spectral component at the frequency 2·f CY  (namely, the wave P 890 ) by the 2 nd  mode resonance. 
     In  FIG. 11 , the construct A 30  may include a film A 08 , which may be made of aquaphobia material. The film A 08  may be place within the port A 07  to function both as a protective means (to prevent dust, vapors and moisture from entering) and as acoustic resistance (to attenuate the remaining ultrasonic spectral component at the frequency 2·f CY  by forming a low-pass filter with the volume of the external chamber A 06 ). 
       FIG. 12  is a schematic diagram of a mobile device A 60  according to an embodiment of the present application. Two air-pulse generating devices A 02  and A 03 , each of which may be any of the air-pulse generating devices  890 / 850 / 830 , are mounted onto an edge A 01  of the mobile device A 60  such as a smartphone or notepad. The ports  707 L and  707 R of the air-pulse generating devices A 02 , A 04  may face outward, and the ultrasonic acoustic wave produce by the air-pulse generating devices A 02 , A 03  may pass through orifice-arrays A 04 , A 05 . The mobile device A 60  may utilize the structure of the construct A 00  or A 30  to remove the ultrasonic spectral component at the frequency f CY  produced by the motion of the valves  101  and  103  while allowing the wave P 890  at the frequency 2·f CY  to pass through. The film A 08  of the construct A 30  may reduce the remaining ultrasonic spectral component around the frequency 2·f CY  further. 
       FIG. 13  is a schematic diagram of a cross sectional view of an air-pulse generating device  300  according to an embodiment of the present application. Similar to the air-pulse generating device  890 , when a standing wave is formed with the chamber  105  of the air-pulse generating device  300 , the movements of the membrane portions  102   c  and  102   d  of the air-pulse generating device  300  is symmetrical and may produce near 0 net air movement. Because of the near 0 net air movement over each operating cycle T CY , most of the energy exerted by the membrane portions  102   c / 102   d  becomes acoustic energy (in the form of air pressure gradient or a standing wave) and near zero energy becomes kinetic energy (in the form of air mass movement, i.e., wind). 
       FIG. 14  is a schematic diagram of a cross sectional view of an air-moving device  100  for moving air volume from one port of the device to another port, according to an embodiment of the present application. 
     Contrary to the air-pulse generating device  850 / 890 , the vibration frequency of the membrane  102  of the air-flow generating device  100  will produce a wavelength λ much greater than the width of chamber  105 , and the pressure inside the chamber  105  may be considered to be uniform. The interleaved valve driving signals S 101 , S 103  may be configured to open the valve portions  101 ,  103  in a time interleaved manner, or 180° out of phase, and produce air movement either from port  107  to port  108 , or from port  108  to port  107 . For example, if valve  101 / 103  is open and valve  103 / 101  is closed when membrane  102  moves in a positive Z direction (+Z direction) to compress the volume within chamber  105 , the air will flow out of chamber  105  via port  107 / 108 . Conversely, if valve  101 / 103  is opened and valve  103 / 101  is closed when membrane  102  moves in a negative Z direction (−Z direction) to expand the volume of chamber  105 , the air will flow into chamber  105  via port  107 / 108 . 
     The cap  104  of the air-moving device  100  may function as a heat dissipation plate/pad, making physical contact with heat generating components such as notebook central processing unit (CPU) or smartphone application processor(s) (AP), but is not limited thereto. The cap  104  may be made of heat conducting material such as aluminum or copper. To improve the heat transfer efficiency, fine fins (not shown) may be formed on the surface of the cap  104  inside the chamber  105 , but not limited thereto. 
     Notably, in the air-pulse generating device  850 / 890 , the cap  104  of the air device  100 / 300  is replaced by the top plate  804 T and the spacers  804 L,  804 R which also serve as side walls. The top plate  804 T may be a printed circuit board (PCB) or a land grid array (LGA) substrate and includes metal traces, vias and contact pads which may be otherwise presented on the substrate  109  or the plate  115 . The thicknesses may be 0.2˜0.3 mm for the top plate  804 T, 0.05˜0.15 mm for the side walls  804 L/ 804 R and 0.25˜0.35 mm for the wall  111 . The total thickness of an air-pulse generating device may be 0.6˜0.8 mm, but not limited thereto. 
     Furthermore, pulse interleaving concept disclosed in U.S. Pat. No. 10,536,770 may be also applied in the present application. In other words, while producing ultrasonic acoustic pulses for APPS, in order to improve the quality of sound, in an embodiment, multiple air-pulse generating devices (e.g., multiple air-pulse generating devices  100 ) may be cascaded together to form one single air-pulse generating device. The driving signals for the air-pulse generating devices  100  (e.g., the membrane driving signal S 102   a /S 102   b /S 102  or the valve driving signal S 101 /S 103 ) may be interleaved to form an interleaved group and raise the effective air pulse rate to a twice higher frequency as a result, away from human audible band. For example, pulses of the membrane driving signal of one air-pulse generating device  100  may be interleaved with pulses of the membrane driving signal of another air-pulse generating device  100 , such that the aggregated air pulses of one air-pulse generating device  100  may be interleaved with the aggregated air pulses of another air-pulse generating device  100  to increase the effective air pulse rate. Alternatively, each pulse of the membrane driving signal of one air-pulse generating device  100  may locate at/near a mid-point between two successive pulses of the membrane driving signal of the other air-pulse generating device  100 , such that each aggregated air pulse of one air-pulse generating device  100  locate at/near a mid-point between two successive aggregated air pulses of the other air-pulse generating device  100  to increase the effective air pulse rate. In an embodiment, two air-pulse generating devices  100 , each designed to operate at the operating frequency T CY  of 24 KHz, may be placed side-by-side or attached back-to-back and driven in interleaved manner, such that the effective air pulse rate becomes 48 KHz. 
     Illustratively,  FIG. 15  is a schematic diagram of an air-pulse generating device  400  according to an embodiment of the present application. The air-pulse generating device  400  may be regarded as two air-pulse generating devices  100  and  100 ′ stacked back-to-back. In the air-pulse generating device  400 , two chambers  105  and  105 ′ of the two air-pulse generating devices  100  and  100 ′ are connected together via an opening  116  to form a chamber  106  of the air-pulse generating device  400 . 
     The air-pulse generating device  400  may comprise a first valve portion  101 , a second valve portion  103 , a third valve portion  101 ′, and a fourth valve portion  103 ′. A first anchor where the valve portion  101  is anchored on the wall  111  and a second anchor where the valve portion  103  is anchored on the wall  111  are aligned to the X direction; on the other hand, the first anchor and a third anchor where the valve portion  101 ′ is anchored on the wall  111  are aligned to the Z direction. The valve portions  101  and  103  (or the valve portions  101 ′ and  103 ′) are symmetric with respect to the YZ plane; on the other hand, the (unactuated) valve portions  101  and  101 ′ (or the valve portions  103  and  103 ′) are symmetric with respect to a second plane (e.g., the XY plane) nonparallel to the YZ plane when the valve driving signal S 101  (or S 103 ) applied to the valve portions  101  and  101 ′ drops to zero. The valve portions  101  and  101 ′ (or the valve portion  103  and  103 ′) are noncoplanar, while the (unactuated) valve portion  101  and  103  (or the valve portion  101 ′ and  103 ′) may be coplanar when the valve driving signals S 101  and S 103 ) applied to the valve portions  101  and  103  drop to zero. 
     In an embodiment of APPS application, by interleaving the driving signals of the two air-pulse generating devices  100 , the displacement profile(s) of the membrane portion  102  (or the valve portions  101 ,  103 ) of the air-pulse generating device  400  may be mirror symmetric to the displacement profile(s) of membrane portion  102 ′ (or valve portions  101 ′,  103 ′) of the air-pulse generating device  400 . Alternatively, by interleaving or inverting the driving signals of the two air-pulse generating devices  100 , the displacement profile(s) of the membrane portion  102  (or the valve portions  101 ,  103 ) of the air-pulse generating device  400  may be the same as the displacement profile(s) of membrane portion  102 ′ and (or valve portions  101 ′,  103 ′) of the air-pulse generating device  400 , such that (the direction and the magnitude of) the displacement of the membrane portion  102  may equal (the direction and the magnitude of) the displacement of the membrane portion  102 ′, causing the pressure fluctuations in the chamber  106  to be cancelled. The membrane portion  102  may be parallel to (or be offset to match) the membrane portion  102 ′. 
     In an embodiment of air moving application, the characteristic length λ CY  is generally much longer than the dimension of the air-pulse generating device  400 . Since the displacement of the membrane portion  102  may equal the displacement of the membrane portion  102 ′, the air-pulse generating device  400  may include only one membrane portion, and one of the membrane portions  102 ,  102 ′ may be removed, thereby reducing power consumption and improving operation efficiency. 
     Power Saving 
     In another perspective, the output of an air-pulse generating device is related to A(t)·p(t), where A(t) is the area of the opening  112 / 114 , and p(t) represents air pressure with the chamber  105 . In other words, the opening  112 / 114  of the valve  101 / 103  is directly related/proportional to the intensity of the output of an air-pulse generating device. Specifically, the maximum SPL output is a combination of the maximum of the air pressure p(t) within the chamber  105 , produced by membrane movement, and the maximum of the area A(t) of the opening  112 / 114 , produced by valve movement. By properly modulating/manipulating the area A(t), the operating power of an air-pulse generating device may be reduced. 
     The area A(t) may not change at a rate audible to human hearing, but may be adjusted by changing the valve driving voltage S 101 /S 103  slowly according to the volume or the envelope of the sound being produced. For example, the valve driving voltage S 101 /S 103  may be controlled by an envelope detection with an attack time of 50 milliseconds and a release time of 5 seconds. When the sound produced by the air-pulse generating device is consistently of low volume, the valve driving voltage S 101 /S 103  may be gradually lowered with the (long) release time of 5 seconds. When high sound pressure is to be generated, the valve driving voltage S 101 /S 103  may be boosted with the (short) 50-millisecond attack time. 
     To sum up, an air-pulse generating device of the present invention may produce an acoustic pressure (or air movement) by first vibrating its membrane structure, subsequently opening/closing its valve structure to filter/reshape the acoustic pressure (or air movement) in response to the occurrence of the maximum/minimum of acoustic pressure (or air velocity), and finally outputting a sound wave (or airflow) under a full-wave rectification effect. Synchronous demodulation may be performed by opening/closing its valve structure in a phase-locked and time-aligned manner relative to the occurrence of the maximum/minimum of acoustic pressure (or air velocity) and/or by opening/closing valve portions of the valve structure in a temporarily interleaved manner. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.