Patent Publication Number: US-11388534-B2

Title: Electroacoustic convertor, audio device and audio method thereof

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional application No. 62/911,308, filed on Oct. 6, 2019, and U.S. provisional application No. 62/910,862, filed on Oct. 4, 2019, which are all incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electroacoustic convertor, an audio device and an audio method, and more particularly, to an electroacoustic convertor, an audio device and an audio method, which reduce the susceptibility to common mode noise during an acoustic signal to electrical signal conversion process. 
     2. Description of the Prior Art 
     During an acoustic signal to electrical signal conversion, minute sound pressure changes are converted to electrical signals of similarly faint level. Due to the very low signal level, these devices are susceptible to surrounding noise and signal quality are prone to contamination by the noise. Accordingly, efforts are being directed in developing innovative audio devices to guard against noise in different environments. 
     SUMMARY OF THE INVENTION 
     It is therefore a primary objective of the present invention to provide an electroacoustic convertor, an audio device and an audio method, which ensure superior sound quality when operating under noisy environments. 
     An embodiment of the present invention provides an audio device. The audio device includes an electroacoustic convertor and a differential amplifier. The electroacoustic convertor has a first output terminal and a second output terminal. A first polarity of a first capacitance variation corresponding to the first output terminal is opposite to a second polarity of a second capacitance variation corresponding to the second output terminal. The first capacitance variation and the second capacitance variation are associated with a magnitude of acoustic pressure. The differential amplifier has a first input terminal coupled to the first output terminal and a second input terminal coupled to the second output terminal. 
     Another embodiment of the present invention provides an audio method. The audio method for an audio device includes outputting a first signal corresponding to a first capacitance variation and outputting a second signal corresponding to a second capacitance variation, and amplifying a difference between the first signal and the second signal. A first polarity of the first capacitance variation is opposite to a second polarity of the second capacitance variation. The first capacitance variation and the second capacitance variation are associated with a magnitude of acoustic pressure. 
     Another embodiment of the present invention provides an electroacoustic convertor. The electroacoustic convertor includes an upper electrode, a lower electrode, and at least one spacer. The at least one spacer is configured to separate the upper electrode and the lower electrode. Each of the at least one spacer is disposed at a reentrant vertex or a tip vertex of the upper electrode. The upper electrode or a layer of the lower electrode has permanent electric charges. 
     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 audio device according to an embodiment of the present invention. 
         FIG. 2  is a schematic diagram illustrating a top view of an electroacoustic convertor according to embodiments of the present invention. 
         FIG. 3  is a cross-sectional view taken along a cross-sectional line in  FIG. 2 . 
         FIG. 4  is a cross-sectional view taken along a cross-sectional line in  FIG. 2 . 
         FIG. 5  is a schematic diagram illustrating a top view of another electroacoustic convertor according to embodiments of the present invention. 
         FIG. 6  is a schematic diagram illustrating a top view of another electroacoustic convertor according to embodiments of the present invention. 
         FIG. 7  is a cross-sectional view taken along a cross-sectional line in  FIG. 2 . 
         FIG. 8  is a schematic diagram of another audio device according to an embodiment of the present invention. 
         FIG. 9  is a schematic diagram of another audio device according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description and claims, the terms “include” and “comprise” are used in an open-ended fashion, and thus should be interpreted to mean “include, but not limited to”. Also, the term “couple” is intended to mean either an indirect or direct electrical connection. Use of ordinal terms such as “first” and “second” does not by itself connote any priority, precedence, or order of one element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one element having a certain name from another element having the same name. 
     Please refer to  FIG. 1 .  FIG. 1  is a schematic diagram of an audio device  10  according to an embodiment of the present invention. The audio device  10  may serve as a sound or pressure sensing apparatus or a microphone, but not limited thereto. The audio device  10  includes an electroacoustic convertor  110  and a differential amplifier  120 . The electroacoustic convertor  110  configured to convert acoustic signals to electrical signals has output terminals  110 T 1  and  110 T 2 . The differential amplifier  120  having input terminals  120 N 1  and  120 N 2  is configured to apply a gain to differences between electrical signals S 1  and S 2  at the input terminals  120 N 1  and  120 N 2  and to reject common mode components of the signals S 1  and S 2 , thereby ensuring noise reduction. When acoustic pressure is applied to the electroacoustic convertor  110 , signal S 1  (also referred to as the first signal) corresponding to a first capacitance variation is output from output terminal  110 T 1  (also referred to as the first output terminal), which is coupled to the input terminal  120 N 1  (also referred to as the first input terminal), and signal S 2  (also referred to as the second signal) corresponding to a second capacitance variation is output from output terminal  110 T 2  (also referred to as the second output terminal), which is coupled to the input terminal  120 N 2  (also referred to as the second input terminal). Both the first capacitance variation and the second capacitance variation are associated with the magnitude of the said acoustic pressure applied to the electroacoustic convertor  110 . The polarity (also referred to as the first polarity) of the first capacitance variation is however opposite to the polarity (also referred to as the second polarity) of the second capacitance variation; therefore, signal S 1  at output terminal  110 T 1  and signal S 2  at output terminal  110 T 2  are also of opposite polarities. 
     Differential amplifier  120  is configured to convert the difference between differential signals S 1  and S 2  into a single-ended signal S 3 , while rejecting the common mode signals. In other words, audio device  10  has common mode rejection (CMR) capability to reject common mode noise during the acoustic-to-electric conversion process involving electroacoustic convertor  110  and differential amplifier  120 . 
     Briefly, the relation between the magnitude of signal S 1  and the magnitude of the acoustic pressure may be similar to the relation between the magnitude of signal S 2  and the magnitude of the acoustic pressure; however, signal S 1  corresponds to a first capacitance variation with a reverse polarity of a second capacitance variation corresponding to signal S 2 . Thus, signals S 1  and S 2  form a pair of differential signals. The output signal S 3  of the differential amplifier  120  is ideally proportional to the difference between the signals S 1  and S 2 . 
     Specifically, please refer to  FIG. 2  to  FIG. 4 .  FIG. 2  is a schematic diagram illustrating a top view of an electroacoustic convertor  210  according to embodiments of the present invention where electroacoustic convertor  210  comprises an acoustic pressure sensing membrane made up of at least one capacitive sensing membrane  310  and at least one capacitive sensing membrane  410 .  FIG. 3  is a cross-sectional view of sensing membrane  310  taken along cross-sectional line A-A′ in  FIG. 2 .  FIG. 4  is a cross-sectional view of sensing membrane  410  taken along cross-sectional line B-B′ in  FIG. 2 . Electroacoustic convertor  210  shown in  FIG. 2  may serve as the electroacoustic convertor  110  shown in  FIG. 1 . 
     In some embodiments, the electroacoustic convertor  210  may include a plurality of sensing membranes  310  and equal number of sensing membranes  410 , each having same sensing area and abutting each other. In  FIG. 2 , there are two sensing membranes  310  and two sensing membranes  410 ; nevertheless, the number and the arrangements of the sensing membranes  310  and  410  (serving as sensing capacitors) may vary according different system design objectives. 
     As shown in  FIG. 3 , capacitive sensing membrane  310  (also referred to as the first sensing capacitor) may include a plurality of spacers  310 S (also referred to as the first spacers), an upper electrode  310 U (also referred to as the first upper electrode) and a lower electrode  310 L (also referred to as the first lower electrode). 
     The upper electrode  310 U comprises a thin membrane, typically 0.2-1 micrometers (μm) in thickness, having a cross-section profile of sawtooth contour as illustrated in  FIG. 3 . The lower electrode  310 L comprises a plate with a sawtooth profiled top surface, facing upper electrode  310 U, and a flat bottom surface; furthermore, as illustrated in  FIG. 3 , the contour of the top surface of lower electrode  310 L matches the contour of upper electrode  310 U. Electrodes  310 L and  310 U are separated from each other and bounded to each other by spacers  310 S, such that, during the neutral state of sensing membrane  310 , the spacing between electrodes  310 U and  310 L approximately equals the height H 1  of spacers  310 S which may be 100-400 nanometers (nm), but not limited thereto. Sensing membrane  310  is anchored to the frame  210 FR of electroacoustic convertor  210  at the left edge LD 3  and the right edge RD 3  of membrane  310  respectively. 
     As shown in  FIG. 3 , spacers  310 S are disposed at reentrant vertexes U 3 R (also referred to as the first reentrant vertexes) of the upper electrode  310 U and reentrant vertexes L 3 R of the lower electrode  310 L to electrically insulate the upper electrode  310 U from the lower electrode  310 L. A tip vertex U 3 T (also referred to as the first tip vertex) of the upper electrode  310 U and a tip vertex L 3 T of the lower electrode  310 L may be spaced apart by a distance D 1  (also referred to as the first distance) which substantially equals the height H 1  of spacers  310 S during the neutral state of sensing membrane  310 . Upon impingement of acoustic pressure waves, the surface of sensing membrane  310  deforms. That is, the center  310 C of membrane  310  moves up or down relative to left edge LD 3  and right edge RD 3  according to impinging pressure and such deformation of membrane  310  causes distance D 1  to change, which lead to the first capacitance variation associated with the distance D 1  between the upper electrode  310 U and the lower electrode  310 L. The upper electrode  310 U and the lower electrode  310 L are attached to terminals V 3 U and V 3 L. Either terminal V 3 U or V 3 L may serve as the output terminal  110 T 1 . 
     Similarly, as shown in  FIG. 4 , capacitive sensing membrane  410  (also referred to as the second sensing capacitor) may include a plurality of spacers  410 S (also referred to as the second spacers), an upper electrode  410 U (also referred to as the second upper electrode) and a lower electrode  410 L (also referred to as the second lower electrode). In some embodiments, materials of the spacers  410 S, the upper electrode and the lower electrode may be silicon or composite, but not limited thereto. 
     The construction of upper electrode  410 U, the lower electrode  410 L and the spacers  410 S are the same as that of upper electrode  310 U, lower electrode  310 L and spacers  310 S; therefore, their description will be omitted for brevity. Different from the construction of membrane  310 , spacers  410 S of membrane  410 , as shown in  FIG. 4 , are disposed at tip vertexes U 4 T (also referred to as the second tip vertexes) of the upper electrode  410 U and tip vertexes L 4 T of the lower electrode  410 L. As a result, a reentrant vertex U 4 R (also referred to as the second reentrant vertex) of the upper electrode  410 U and a reentrant vertex L 4 R of the lower electrode  410 L may be spaced apart by a distance D 2  (also referred to as the second distance) which substantially equals the height H 1  of spacers  410 S during the neutral state of composite membrane  410 . 
     Upon impingement of acoustic pressure waves, the surface of membrane  410  will deform accordingly and such deformation of membrane  410  causes the distance D 2  to change, which lead to the second capacitance variation associated with the distance D 2  between the upper electrode  410 U and the lower electrode  410 L. The upper electrode  410 U and the lower electrode  410 L are attached to terminals V 4 U and V 4 L. Either the terminal V 4 U or V 4 L serves as the output terminal  110 T 2 . 
     Refer to cross-sectional structure of membrane  310  (or  410 ), where the upper electrode  310 U (or  410 U) has a sawtooth cross-section profile along the direction of A-A′ (or B-B′) while the lower electrode  310 L (or  410 L) has a cross-section profile matching upper electrode  310 U on the side facing upper electrode  310 U and a flat surface on the other side. Due to the difference of the cross-sectional profile, in particular the flat surface side, lower electrode  310 L (or  410 L) is much stronger than upper electrode  310 U (or  410 U) against compressive and tensile force along the direction of A-A′ (or B-B′). 
     Upon the impingement of acoustic pressure wave, sensing membrane  310  (or  410 ), comprising upper electrode  310 U (or  410 U), lower electrode  310 L (or  410 L) and spacers  310 S (or  410 S), will deform concavely or convexly as one unit. Due to the much higher relative strength of lower electrode  310 L (or  410 L) along the direction of A-A′ (or B-B′), when composite membrane  310  (or  410 ) bends concavely or convexly, defined as center  310 C (or  410 C) moving down or up relative to its two anchored sides (left edge LD 3  and right edge RD 3 , or left edge LD 4  and right edge RD 4 ), the weaker upper electrode  310 U (or  410 U) will experience more deformation in response to either the compressing or the tensile force as a result of the deformation of membrane  310  (or  410 ) which cause upper electrode  310 U (or  410 U) to concave or convex more than the bottom electrode  310 L (or  410 L) and therefore cause the inter-electrode spacing, namely, distance D 1  (or D 2 ), to change according to the acoustic pressure impinging upon structure of membrane  310  (or  410 ). 
     By placing spacers  310 S at reentrant vertexes of lower electrode  310 L as shown in  FIG. 3 , while placing spacers  410 S at tipping vertexes of lower electrode  410 L as shown in  FIG. 4 , the changes of distances D 1  and D 2  due to the impingement of the same sound pressure upon membrane  310  and membrane  410  will be of substantially the same magnitude but opposite signs, that is, the changes of distances D 1  and D 2  will be differential, or, balanced. 
     Since the capacitance C of parallel-plate-capacitor is related to the distance D between the plates (namely, 
                 C   =       k   ·   A     D       )     ,         
when distances D 1  and D 2  change, the capacitance between terminals V 3 U and V 3 L and between terminals V 4 U and V 4 L also change accordingly. Therefore, when acoustic pressure wave impinges upon sensing membranes  310  and  410 , the capacitance between terminals V 3 U and V 3 L and the capacitance between terminals V 4 U and V 4 L also change in a differential manner.
 
     The parallel-plate capacitor construction of electroacoustic converter  210  is a type of condenser microphone. The operation of a condenser microphone requires an electric field (E-field) to be established between the plates of the sensing capacitor. This E-field can be established either explicitly by a low-noise, bias voltage source or implicitly by impregnating one of the plate electrodes with permanent charges. When such an E-field is established, for example by a voltage source V across terminals V 3 U and V 3 L, electrical charge Q=C·V will be established across membranes  310 U and  310 L. Since voltage V is a constant, altering the quantity of charge Q will change capacitance C. Therefore, when distance D 1  changes, the quantity of electrical charge Q stored between membranes  310 U and  310 L changes accordingly and such changing charge Q leads to current I 3  flowing between terminals V 3 U and V 3 L. Similarly, when distance D 2  changes, the amount of charge Q stored between membranes  410 U and  410 L changes accordingly and such changing charge Q leads to current I 4  flowing between terminals V 4 U and V 4 L. Due to the balanced nature of changes in distances D 1  and D 2 , current I 3  and current I 4  will also be balanced. 
     The balanced current pair I 3  and I 4  can be converted directly into single ended signal S 3  utilizing a differential amplification scheme such as the circuit shown in  FIG. 8 . However, the capacitor-and-resistor (C-R) configuration of the circuit in  FIG. 8  means its signal-to-noise (S/N) ratio will drop as frequency goes lower. Therefore, as an alternative, a current integration stage is utilized in  FIG. 9 , to first convert the current signals (namely, currents I 3  and I 4 ) into voltage signals respectively, before converting the pair of differential voltage signals into single end voltage signal. 
     With either embodiment of  FIG. 8  or  FIG. 9 , balanced signals S 1  and S 2  are converted into single end signal S 3  by a differential amplifier which will enhance signal symmetry, reduce even-order distortion and, most important of all, suppress common mode noise components of signals S 1  and S 2  and hence ensure high signal-to-noise ratio. 
     Except the placement difference between the spacers  310 S and  410 S, the structure of sensing membrane  310  is the same as that of the sensing membrane  410 . As viewed along the z-axis, each of the spacers  310 S at the reentrant vertexes U 3 R and L 3 R is interleaved with the spacers  410 S at the tip vertexes U 4 T and L 4 T. For the ease of manufacture, in some embodiments, there is uniformity between the sensing membranes  310  and  410  viewed from side. In some embodiments, portions of the upper electrode  310 U (or the lower electrode  310 L) are coplanar with portions of the upper electrode  410 U (or the lower electrode  410 L) respectively to reduce manufacture complexity. For example, a surface SF 3  of the upper electrode  310 U is coplanar with a surface SF 4  of the upper electrode  410 U. 
     In some embodiments, martials of the spacers  310 S,  410 S, the upper electrode  310 U,  410 U and the lower electrode  310 L,  410 L may be silicon or composite thereof, but not limited thereto. In some embodiments, the edges of electrode  310 U,  410 U,  310 L,  410 L of electroacoustic convertor  210  may be treated with chemicals along a frame  210 FR (thick solid line) to improve their resistance to water, humidity and dust. For instance, the electroacoustic convertor  210  may be sealed along its edges SDG to extend service life. 
     The cross section profile of the upper electrode  310 U,  410 U and the lower electrode  310 L,  410 L may have shapes other than the sawtooth pattern illustrated in  FIG. 3  and  FIG. 4  as long as the profile of upper electrode matches the profile of the opposing surface of lower electrode, so as to form a parallel plate capacitor between the upper electrode and the lower electrode. 
     In some embodiments, (the upper electrodes  310 U and  410 U of) the sensing membranes  310  and  410  viewed from top may be alternately disposed. In some embodiments, (the upper electrodes  310 U and  410 U of) the sensing membranes  310  and  410  viewed from top may be disposed with regularity and symmetry. That is to say, the area or shape (of one upper electrode  310 U) of one sensing membrane  310  may be substantially the same as the area or shape (of one upper electrode  410 U) of one sensing membrane  410 . 
     For example, as shown in  FIG. 2 , (the upper electrodes  310 U and  410 U of) the sensing membranes  310  and  410  viewed from top may be alternately disposed. In  FIG. 2 , the electroacoustic convertor  210  has two slits SLT 2  to separate the frame  210 FR, the sensing membranes  310  or  410 . Each of the slits SLT 2  viewed from top has a shape of a rectangle. For the best low frequency response, the width of slits SLT 2  should be 1 um or smaller. In an embodiment, the exposed edges of membranes  310  and  410  may be treated to improve resistance to water, dust and humidity. In some embodiments, left sides LD 3 , LD 4  and right sides RD 3 , RD 4  of the sensing membranes  310  and  410  are anchored to the frame  210 FR of the electroacoustic convertor  210 . In  FIG. 2 , all the sensing membranes  310  and  410  may be formed in one piece. 
     It is noteworthy that the electroacoustic convertor  210  shown in  FIG. 2  is an exemplary embodiment of the present invention, and those skilled in the art may readily make different alternations and modifications. For example, please refer to  FIG. 5 .  FIG. 5  is a schematic diagram illustrating a top view of an electroacoustic convertor  510  according to embodiments of the present invention, wherein a cross-sectional view taken along a cross-sectional line A-A′ and a cross-sectional view taken along a cross-sectional line B-B′ are shown in  FIG. 3  and  FIG. 4  respectively. Distinct from the electroacoustic convertor  210 , the electroacoustic convertor  510  has a slit pattern SLT 5  shaped like a rotated (or transverse) capital letter “H” to represent edges where the frame  510 FR of the electroacoustic convertor  510  is separated from the edges of sensing membranes  310 ,  410 . In other words, only the left side LD 3  (or LD 4 ) and the right side RD 3  (or RD 4 ) of the sensing membrane  310  (or  410 ) is anchored to the frame  510 FR, while the other side of the sensing membrane  310  (or  410 ) is unfixed or loosened due to slit SLT 5 . With one side being free, sensing membranes  310  and  410  of electroacoustic convertor  510  have more freedom of movement than the corresponding 2-side-anchored sensing membranes in electroacoustic convertor  210 , resulting in increased membrane deformation upon impingement of sound pressure which leads to increased changes of distances D 1  and D 2 , and thusly improves the sensitivity to acoustic pressure. 
     In summary, by changing from 2-side-anchored structure of electroacoustic convertor  210  to 1-side-anchored structure of electroacoustic convertor  510 , sensing membranes  310  and  410  can vibrate more freely, have larger displacement, produce higher current flow I 3  and I 4 , which result in stronger signals S 1 , S 2  at the terminals  120 N 1 ,  120 N 2 . 
     In  FIG. 5 , which is a view from the top, both on the left side and the right side of electroacoustic convertor  510 , (the upper electrodes  310 U and  410 U of) sensing membrane  310  and  410  are alternately disposed, abutting each other, and formed as one coplanar piece. 
       FIG. 6  is a top view of electroacoustic convertor  610  which is another embodiment according to the present invention, wherein a cross-sectional view taken along a cross-sectional line A-A′ and a cross-sectional view taken along a cross-sectional line B-B′ are shown in  FIG. 3  and  FIG. 4  respectively. In electroacoustic convertor  610  of  FIG. 6 , (the upper electrodes  310 U and  410 U of) the sensing membranes  310  and  410  are of triangle shape and are diagonally arranged. In addition, (the upper electrodes  310 U and  410 U of) sensing membranes  310  and  410  viewed from top may be alternately disposed with a slit pattern SLT 6  shaped like the capital letter “X” separating these four alternating sensing membranes from one another as shown in  FIG. 6 . 
     One of the left side LD 3  (or LD 4 ) or the right side RD 3  (or RD 4 ) of the sensing membrane  310  (or  410 ) is anchored to the frame  610 FR, but the opposite side of the sensing membrane  310  (or  410 ) is unfixed or free to move because of the (X-shaped) slit SLT 6 . Similar to the case of electroacoustic convertor  510 , sensing membranes  310  and  410  of electroacoustic convertor  610  will also be able to vibrate more freely than electroacoustic convertor  210 . Moreover, the stiffness of triangular shaped sensing membranes will be lower than rectangular shaped sensing membranes, allowing electroacoustic convertor  610  to have larger displacement, produce higher current flow I 3  and I 4  than electroacoustic convertor  510 . 
     In some embodiments, in order to establish the required E-field required for the operation of the condenser microphone, dielectric materials (such as electret) with permanent electric charges or dipole polarization may be introduced into upper electrode or lower electrode of sensing membrane  310  and  410 . For example, the upper electrode of the upper electrode  310 U or  410 U may have permanent electric charges or dipole polarization, which will produce a static electric field between upper electrode  310 U or  410 U and lower electrode  310 L or  410 L and, therefore, avoid the need for the low-noise bias voltage source. 
     Refer to  FIG. 7 , which may be a cross-sectional view taken along a cross-sectional line A-A′ in  FIG. 2 ,  FIG. 5 , or  FIG. 6 . A sensing membrane  710  shown in  FIG. 7  corresponds to the sensing membrane  310  shown in  FIG. 3 . Distinct from the sensing membrane  310 , lower electrode  710 L of the sensing membrane  710  further comprises a permanently charged layer  712 L, wherein the electrical charge imbued within layer  712 L establishes a dipole polarization between upper electrode  310 U and plate  712 L and therefore abolish the need for an explicit bias voltage source. 
     In some embodiments, the layer  712 L is made of dielectric materials (such as electret). In some embodiments, the lower electrode  710 L may be fabricated by semiconductor manufacturing processes or microelectromechanical system (MEMS) manufacturing processes. For example, ion implantation process may be employed to implant charged ions into a previously charge-neutral layer  712 L which situates on top of substrate  714 L of the lower electrode  710 L. In another example, a chemical vapor deposition (CVD) or sputtering process may be employed to deposit an electrically charged layer on top of layer  712 L. 
     To maintain the charges within layer  712 L, substrate  714 L and the spacers  310 S may be made of highly insulating materials. Alternatively, in some embodiment, barrier layers may be created at the interface of layer  712 L and substrate  714 L and the interface of layer  712 L and spacers  310 S such that charges within layer  712 L are repelled from the vicinity of barrier layers. 
     Similarly, by altering the placement of spacers  310 S in  FIG. 7  to the placements of spacers  410 S in  FIG. 4 , sensing membrane  710  will become the cross-sectional view corresponding to cross-sectional line B-B′ in  FIG. 2 ,  FIG. 5 , or  FIG. 6  and such sensing membranes can replace sensing membranes  410  while abolish the need for bias voltage source. 
     Please refer to  FIG. 8 .  FIG. 8  is a schematic diagram according to an embodiment of the present invention, where audio device  90  corresponds to audio device  10  of  FIG. 1 , electroacoustic convertor  910  corresponds to electroacoustic convertor  110  of  FIG. 1  and differential amplifier  920  corresponds to differential amplifier  120  of  FIG. 1 . 
     The electroacoustic convertor  910  may include sensing membranes  310 ,  410  and a common bias voltage source  910 B, connected to terminals V 3 L, V 4 L. Bias voltage source  910 B may be omitted if sensing membranes  310  and  410  are self-polarized as sensing membrane  710  in  FIG. 7  or any other suitably self-polarized sensing capacitors. 
     The bias voltage source  910 B is configured to bias sensing membranes  310  and  410  with a constant voltage. In some embodiments, voltage source  910 B may be a battery or other low-noise, stable voltage source. The capacitance of the sensing membrane  310  (or  410 ) is governed by the spacing between upper electrode  310 U (or  410 U) and lower electrode  310 L (or  410 L). With the presence of suitable bias voltage, when the capacitance of sensing membrane  310  (or  410 ) changes in response to the changes in upper-lower electrode distance D 1  (or D 2 ), the electrical charge Q between upper electrode  310 U (or  410 U) and lower electrode  310 L (or  410 L) also changes accordingly. Such changes in electrical charge Q gives rise to electric currents flowing through terminal  110 T 1  (or  110 T 2 ), where the rate of current flow reflects the rate of the electrical charge changes. 
     Note that, due to the placement of spacer  310 S and  410 S, the current produced by sensing membranes  310  and  410  will be similar in magnitude but opposite in direction, i.e. the currents produced by terminal V 3 U of sensing membrane  310  and terminal V 4 U of sensing membrane  410  form a pair of balanced current signal. As a result, the surface curvature changes upon the impingement of acoustic pressure are transformed into a pair of differential current signals I 3  and I 4  at terminals  110 T 1  and  110 T 2 . 
     In one embodiment as illustrated in  FIG. 8 , this pair of differential current signals I 3  and I 4 , which correspond to signals S 1  and S 2 , are transmitted to terminals  120 N 1  and  120 N 2  of differential amplifier  920  respectively where the differential signal pair S 1 , S 2  is converted to single-ended signal S 3 . 
     Note that current I 3  (or I 4 ) reflects the rate of the capacitance changes, 
     
       
         
           
             
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     This means the amplitude of signal I 3 , I 4  will decay as the frequency of sound goes lower. To circumvent such a shortcoming,  FIG. 9  employs two current integrators  1013  and  1014  to neutralize the time differential in the equation above, and convert the differential current signal pair I 3 , I 4  into differential voltage signal pair S 1 , S 2  before signals S 1 , S 2  are converted to single-ended signal S 3  by differential amplifier  920 . 
     Current integrators  1013 ,  1014  in  FIG. 9  only show the skeleton of current integrators. Proper optimizations may be added according to the characteristics of amplifiers  1013 M and  1014 M. Briefly, to function properly as a current integrator, amplifier  1013 M (or  1014 M) should have high input-impedance, high open-loop gain, low input-offset, low input-capacitance, and low-noise. 
     To sum up, since the placement of the first spacers of the first sensing capacitor is different from the placement of the second spacers of the second sensing capacitor, the distance between the first upper electrode and the first lower electrode of the first sensing capacitor may change oppositely to the second distance between the second upper electrode and the second lower electrode of the second sensing capacitor when the same acoustic pressure is applied to both the first sensing capacitor and the second sensing capacitor. This leads to the polarity of the first capacitance variation of the first sensing capacitor opposite to the polarity of the second capacitance variation of the second sensing capacitor, while the first capacitance variation and the second capacitance variation are associated with a magnitude of the acoustic pressure. The difference between the first signal corresponding to the first capacitance variation and the second signal corresponding to the second capacitance variation is subsequently amplified by the differential amplifier, thereby, suppressing the common mode noise, ensuring superior sound quality. The first sensing capacitor and the second sensing capacitor may be disposed with regularity and symmetry so as to reduce manufacture difficulty. The edge of electroacoustic convertor may be treated to enhance resistance to water, humidity and dust. 
     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.