Patent Publication Number: US-11399228-B2

Title: Acoustic transducer, wearable sound device and manufacturing method of acoustic transducer

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefits of U.S. provisional application No. 63/050,763, filed on Jul. 11, 2020, U.S. provisional application No. 63/051,885, filed on Jul. 14, 2020, and U.S. provisional application No. 63/171,919, filed on Apr. 7, 2021, which are all incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present application relates to an acoustic transducer, a wearable sound device and a manufacturing method of an acoustic transducer, and more particularly, to an acoustic transducer capable of suppressing an occlusion effect, to a wearable sound device having an acoustic transducer and to a manufacturing method of an acoustic transducer. 
     2. Description of the Prior Art 
     Nowadays, wearable sound devices, such as in-ear (insert into ear canal) earbuds, on-ear or over-ear earphones, etc. are generally used for producing sound or receiving sound. Magnet and moving coil (MMC) based microspeaker have been developed for decades and widely used in many such devices. Recently, MEMS (Micro Electro Mechanical System) acoustic transducers which make use of a semiconductor fabrication process can be sound producing/receiving components in the wearable sound devices. 
     Occlusion effect is due to the sealed volume of ear canal causing loud perceived sound pressure by the listener. For example, the occlusion effect occurs while the listener does specific motion(s) generating a bone-conducted sound (such as walking, jogging, talking, eating, touching the acoustic transducer, etc.) and uses the wearable sound device (e.g., the wearable sound device is filled in his/her ear canal). The occlusion effect is particularly strong toward bass due to the difference of acceleration based SPL (sound pressure level) generation (SPL∝a=dD 2 /dt 2 ) and compression based SPL generation (SPL∝D). For instance, a displacement of merely 1 μm at 20 Hz will cause a SPL=1 μm/25 mm atm=106 dB in occluded ear canal (25 mm is average length of adult ear canals). Therefore, if the occlusion effect occurs, listener hears the occlusion noise, and the quality of listener experience is bad. 
     In the traditional technology, the wearable sound device has an airflow channel existing between the ear canal and the ambient external to the device, such that the pressure caused by the occlusion effect can be released from this airflow channel to suppress the occlusion effect. However, because the airflow channel always exists, in the frequency response, the SPL in the lower frequency (e.g., lower than 500 Hz) has a significant drop. For example, if the traditional wearable sound device uses a typical 115 dB speaker driver, the SPL in 20 Hz is much lower than 110 dB. In addition, if a size of a fixed vent configured to form the airflow channel is greater, the SPL drop will be greater, and the water and dust protection will become more difficult. 
     In some cases, the traditional wearable sound device may use a speaker driver stronger than the typical 115 dB speaker driver to compensate for the loss of SPL in lower frequency due to the existence of the airflow channel. For example, assuming the loss of SPL is 20 dB, then the required speaker driver to maintain the same 115 dB SPL in the presence of the airflow channel will be 135 dB SPL, were it to be used in a sealed ear canal. However, the 10× stronger bass output requires the speaker membrane travel to also increase by 10× which implies the heights of both the coil and the magnet flux gap of the speaker driver need to be increased by 10×. Thus, it is difficult to make the traditional wearable sound device having the strong speaker driver have the small size and light weight. 
     Therefore, it is necessary to improve the prior art, so as to suppress the occlusion effect. 
     SUMMARY OF THE INVENTION 
     It is therefore a primary objective of the present invention to provide an acoustic transducer capable of suppressing an occlusion effect, and to provide a wearable sound device having an acoustic transducer and a manufacturing method of an acoustic transducer. 
     An embodiment of the present invention provides an acoustic transducer configured to perform an acoustic transformation. The acoustic transducer is disposed within a wearable sound device or to be disposed within the wearable sound device. The acoustic transducer includes at least one anchor structure, a film structure and an actuator. The film structure is disposed within a first layer and anchored by the anchor structure disposed within a second layer. The actuator is disposed on the film structure, and the actuator is configured to actuate the film structure to form a vent temporarily. The film structure partitions a space into a first volume to be connected to an ear canal of a wearable sound device user and a second volume to be connected to an ambient of the wearable sound device. The ear canal and the ambient are to be connected via the vent temporarily opened when the film structure is actuated. 
     Another embodiment of the present invention provides a manufacturing method for an acoustic transducer. The manufacturing method includes: providing a wafer, wherein the wafer includes a first layer and a second layer; forming and patterning an actuating material formed on a first side of the wafer; patterning the first layer of the wafer, so as to form a trench line; and removing a first part of the second layer of the wafer. A second part of the second layer forms at least one anchor structure, and the patterned first layer forms a film structure anchored by the anchor structure. A slit is formed within and penetrates through the film structure because of the trench line. The film structure is configured to be actuated to form a vent temporarily, and the vent is formed because of the slit. The film structure partitions a space into a first volume to be connected to an ear canal and a second volume to be connected to an ambient of the wearable sound device. The ear canal and the ambient are to be connected via the vent temporarily opened. 
     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 a top view illustrating an acoustic transducer according to a first embodiment of the present invention. 
         FIG. 2  is a schematic diagram of a cross sectional view illustrating an acoustic transducer according to the first embodiment of the present invention. 
         FIG. 3  is a schematic diagram of a cross sectional view illustrating an acoustic transducer and a housing structure according to the first embodiment of the present invention. 
         FIG. 4  is a schematic diagram illustrating a first membrane in a first mode according to the first embodiment of the present invention. 
         FIG. 5  is a schematic diagram of a cross sectional view illustrating a first membrane in a second mode according to another embodiment of the present invention. 
         FIG. 6  is a schematic diagram illustrating multiple examples of relative position pairs on different sides of a slit according to the first embodiment of the present invention. 
         FIG. 7  is a schematic diagram illustrating frequency responses of multiple examples according to the first embodiment of the present invention. 
         FIG. 8  is a schematic diagram of a cross sectional view illustrating a first membrane in a first mode according to another embodiment of the present invention. 
         FIG. 9  is a schematic diagram illustrating a wearable sound device with an acoustic transducer according to an embodiment of the present invention. 
         FIG. 10  to  FIG. 12  are schematic diagrams of cross sectional views illustrating another type acoustic transducer according to an embodiment of the present invention. 
         FIG. 13  is a schematic diagram of a cross sectional view illustrating the acoustic transducer according to a second embodiment of the present invention. 
         FIG. 14  is a schematic diagram of a cross sectional view illustrating the acoustic transducer according to another second embodiment of the present invention. 
         FIG. 15  is a schematic diagram of a top view illustrating an acoustic transducer according to a third embodiment of the present invention. 
         FIG. 16  is a schematic diagram of a top view illustrating an acoustic transducer according to a fourth embodiment of the present invention. 
         FIG. 17  is a schematic diagram of a top view illustrating an acoustic transducer according to a fifth embodiment of the present invention. 
         FIG. 18  is a schematic diagram of a top view illustrating an acoustic transducer according to a sixth embodiment of the present invention. 
         FIG. 19  is a schematic diagram of a top view illustrating an acoustic transducer according to a seventh embodiment of the present invention. 
         FIG. 20  is an enlarge diagram illustrating a center part of  FIG. 19 . 
         FIG. 21  is a schematic diagram of a top view illustrating an acoustic transducer according to an eighth embodiment of the present invention. 
         FIG. 22  is a schematic diagram of a top view illustrating an acoustic transducer according to a ninth embodiment of the present invention. 
         FIG. 23  is a schematic diagram of a top view illustrating an acoustic transducer according to a tenth embodiment of the present invention. 
         FIG. 24  to  FIG. 30  are schematic diagrams illustrating structures at different stages of a manufacturing method of an acoustic transducer according to an embodiment of the present invention. 
         FIG. 31  is a schematic diagram illustrating a cross sectional view of an acoustic transducer according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     To provide a better understanding of the present invention to those skilled in the art, preferred embodiments and typical material or range parameters for key components will be detailed in the follow description. These preferred embodiments of the present invention are illustrated in the accompanying drawings with numbered elements to elaborate on the contents and effects to be achieved. It should be noted that the drawings are simplified schematics, and the material and parameter ranges of key components are illustrative based on the present day technology, and therefore show only the components and combinations associated with the present invention, so as to provide a clearer description for the basic structure, implementing or operation method of the present invention. The components would be more complex in reality and the ranges of parameters or material used may evolve as technology progresses in the future. In addition, for ease of explanation, the components shown in the drawings may not represent their actual number, shape, and dimensions; details may be adjusted according to design requirements. 
     In the following description and in the claims, the terms “include”, “comprise” and “have” are used in an open-ended fashion, and thus should be interpreted to mean “include, but not limited to . . . ”. Thus, when the terms “include”, “comprise” and/or “have” are used in the description of the present invention, the corresponding features, areas, steps, operations and/or components would be pointed to existence, but not limited to the existence of one or a plurality of the corresponding features, areas, steps, operations and/or components. 
     In the following description and in the claims, when “a A1 component is formed by/of B1”, B1 exist in the formation of A1 component or B1 is used in the formation of A1 component, and the existence and use of one or a plurality of other features, areas, steps, operations and/or components are not excluded in the formation of A1 component. 
     In the following description and in the claims, the term “substantially” generally means a small deviation may exist or not exist. For instance, the terms “substantially parallel” and “substantially along” means that an angle between two components may be less than or equal to a certain degree threshold, e.g., 10 degrees, 5 degrees, 3 degrees or 1 degree. For instance, the term “substantially aligned” means that a deviation between two components may be less than or equal to a certain difference threshold, e.g., 2 μm or 1 μm. For instance, the term “substantially the same” means that a deviation is within, e.g., 10% of a given value or range, or mean within 5%, 3%, 2%, 1%, or 0.5% of a given value or range. 
     Although terms such as first, second, third, etc., may be used to describe diverse constituent elements, such constituent elements are not limited by the terms. The terms are used only to discriminate a constituent element from other constituent elements in the specification, and the terms do not relate to the sequence of the manufacture if the specification do not describe. The claims may not use the same terms, but instead may use the terms first, second, third, etc. with respect to the order in which an element is claimed. Accordingly, in the following description, a first constituent element may be a second constituent element in a claim. 
     It should be noted that the technical features in different embodiments described in the following can be replaced, recombined, or mixed with one another to constitute another embodiment without departing from the spirit of the present invention. 
     In the present invention, the acoustic transducer may perform an acoustic transformation, wherein the acoustic transformation may convert signals (e.g. electric signals or signals with other suitable type) into an acoustic wave, or may convert an acoustic wave into signals with other suitable type (e.g. electric signals). In some embodiments, the acoustic transducer may be a sound producing device, a speaker, a micro speaker or other suitable device, so as to convert the electric signals into the acoustic wave, but not limited thereto. In some embodiments, the acoustic transducer may be a sound measuring device, a microphone or other suitable device, so as to convert the acoustic wave into the electric signals, but not limited thereto. 
     In the following, the acoustic transducer may be an exemplary sound producing device which configured to make those skilled in the art better understand the present invention, but not limited thereto. In the following, the acoustic transducer may be disposed within a wearable sound device (e.g., an in-ear device) for instance, but not limited thereto. Note that an operation of the acoustic transducer means that the acoustic transformation is performed by the acoustic transducer (e.g., the acoustic wave is produced by actuating the acoustic transducer with electrical driving signal). 
     Referring to  FIG. 1  to  FIG. 3 ,  FIG. 1  is a schematic diagram of a top view illustrating an acoustic transducer according to a first embodiment of the present invention,  FIG. 2  is a schematic diagram of a cross sectional view illustrating an acoustic transducer according to the first embodiment of the present invention, and  FIG. 3  is a schematic diagram of a cross sectional view illustrating an acoustic transducer and a housing structure according to the first embodiment of the present invention. As shown in  FIG. 1  and  FIG. 2 , the acoustic transducer  100  includes a base BS. The base BS may be hard or flexible, wherein the base BS may include silicon, germanium, glass, plastic, quartz, sapphire, metal, polymer (e.g., polyimide (PI), polyethylene terephthalate (PET)), any other suitable material or a combination thereof. As an example, the base BS may be a circuit board including a laminate (e.g. copper clad laminate, CCL), a land grid array (LGA) board or any other suitable board containing conductive material, but not limited thereto. 
     In  FIG. 1  and  FIG. 2 , the base BS has a horizontal surface SH parallel to a direction X and a direction Y, wherein the direction Y is not parallel to the direction X (e.g., the direction X may be perpendicular to the direction Y). Note that the direction X and the direction Y of the present invention may be considered as horizontal directions. 
     The acoustic transducer  100  includes a film structure FS and at least one anchor structure  140  disposed on the horizontal surface SH of the base BS, wherein the film structure FS is anchored by the anchor structure  140 . As shown in  FIG. 1 , the acoustic transducer  100  may include four anchor structures  140 , and the film structure FS includes a first membrane  110 . The anchor structure  140  is disposed outside the first membrane  110  and connected to at least one of outer edges  110   e  of the first membrane  110 , wherein the outer edges  110   e  of the first membrane  110  define a boundary of the first membrane  110 . For example, the anchor structures  140  may surround the first membrane  110  and be connected to all outer edges  110   e  of the first membrane  110 , but not limited thereto. 
     In the operation of the acoustic transducer  100 , the first membrane  110  can be actuated to have a movement. In this embodiment, the first membrane  110  may be actuated to move upwardly and downwardly, but not limited thereto. For example, in  FIG. 2 , when the first membrane  110  is actuated, the first membrane  110  may deform into a deformed type  110 Df, but not limited thereto. Note that, in the present invention, the terms “move upwardly” and “move downwardly” represent that the membrane moves substantially along a direction Z parallel to a normal direction of the first membrane  110  or parallel to a normal direction of the horizontal surface SH of the base BS (i.e., the direction Z may be perpendicular to the direction X and the direction Y). 
     During the operation of the acoustic transducer  100 , the anchor structure  140  may be immobilized. Namely, the anchor structure  140  may be a fixed end (or fixed edge) respecting the first membrane  110  during the operation of the acoustic transducer  100 . 
     The first membrane  110  (the film structure FS) and the anchor structure  140  may include any suitable material(s). In some embodiments, the first membrane  110  (the film structure FS) and the anchor structure  140  may individually include silicon (e.g., single crystalline silicon or poly-crystalline silicon), silicon compound (e.g., silicon carbide, silicon oxide), germanium, germanium compound (e.g., gallium nitride or gallium arsenide), gallium, gallium compound, stainless steel or a combination thereof, but not limited thereto. The first membrane  110  and the anchor structure  140  may have the same material or different materials. 
     In addition, owing to the existence of the first membrane  110  and the anchor structure  140 , a first chamber CB 1  may exist between the base BS and the first membrane  110 . In this embodiment, the base BS may further include a back vent BVT (e.g., the back vent BVT shown in  FIG. 3 ), and the first chamber CB 1  may be connected to the rear outside of the acoustic transducer  100  (i.e., a space back of the base BS) through the back vent BVT. 
     The acoustic transducer  100  includes a first actuator  120  disposed on the first membrane  110  (the film structure FS) and configured to actuate the first membrane  110  (the film structure FS). For instance, in  FIG. 1  and  FIG. 2 , the first actuator  120  may be in contact with the first membrane  110 , but not limited thereto. Furthermore, in this embodiment, as shown in  FIG. 1  and  FIG. 2 , the first actuator  120  may not totally overlap the first membrane  110 , as shown in the direction Z perspective of  FIG. 1 , but not limited thereto. Optionally, in  FIG. 2 , the first actuator  120  may be disposed on and overlap the anchor structure  140 , but not limited thereto. In another embodiment, the first actuator  120  may not overlap the anchor structure  140 , as shown in the direction Z perspective of  FIG. 1 , but not limited thereto. 
     The first actuator  120  has a monotonic electromechanical converting function with respect to the movement of the first membrane  110  along the direction Z. In some embodiments, the first actuator  120  may include a piezoelectric actuator, an electrostatic actuator, a nanoscopic-electrostatic-drive (NED) actuator, an electromagnetic actuator or any other suitable actuator, but not limited thereto. For example, in an embodiment, the first actuator  120  may include a piezoelectric actuator, the piezoelectric actuator may contain such as two electrodes and a piezoelectric material layer (e.g., lead zirconate titanate, PZT) disposed between the electrodes, wherein the piezoelectric material layer may actuate the first membrane  110  based on driving signals (e.g., driving voltages) received by the electrodes, but not limited thereto. For example, in another embodiment, the first actuator  120  may include an electromagnetic actuator (such as a planar coil), wherein the electromagnetic actuator may actuate the first membrane  110  based on a received driving signals (e.g., driving current) and a magnetic field (i.e. the first membrane  110  may be actuated by the electromagnetic force), but not limited thereto. For example, in still another embodiment, the first actuator  120  may include an electrostatic actuator (such as conducting plate) or a NED actuator, wherein the electrostatic actuator or the NED actuator may actuate the first membrane  110  based on a received driving signals (e.g., driving voltage) and an electrostatic field (i.e. the first membrane  110  may be actuated by the electrostatic force), but not limited thereto. 
     In this embodiment, the first membrane  110  and the first actuator  120  may be configured to perform an acoustic transformation. That is to say, the acoustic wave is produced due to the movement of the first membrane  110  actuated by the first actuator  120 , and the movement of the first membrane  110  is related to a sound pressure level (SPL) of the acoustic wave. 
     The first actuator  120  may actuate the first membrane  110  to produce the acoustic wave based on received driving signal(s). The acoustic wave is corresponding to an input audio signal, and the driving signal is corresponding to (related to) the input audio signal. 
     In some embodiments, the acoustic wave, the input audio signal and the driving signal have the same frequency, but not limited thereto. That is to say, the acoustic transducer  100  produces a sound at the frequency of sound (i.e., the acoustic transducer  100  generates the acoustic wave complying with the zero-mean-flow assumption of classic acoustic wave theorems), but not limited thereto. 
     As shown in  FIG. 1  to  FIG. 3 , the film structure FS of the acoustic transducer  100  includes at least one slit  130 , wherein the slit  130  may have a first sidewall S 1  and a second sidewall S 2  opposite to the first sidewall S 1 . In the present invention, an gap  130 P of the slit  130  exists between the first sidewall S 1  and the second sidewall S 2  in a plane parallel to the direction X and the direction Y (i.e., the gap  130 P of the slit  130  is parallel to the horizontal surface SH of the base BS), wherein the width of the gap  130 P of the slit  130  may be designed based on requirement(s) (e.g., the width may be, but not limited to, around 1 μm). In the present invention, based on the driving signal received by the first actuator  120 , the slit  130  may generate a vent  130 T between the first sidewall S 1  and the second sidewall S 2  temporarily (i.e., the film structure FS is configured to be actuated to form a vent  130 T temporarily), wherein the opening of vent  130 T is in the direction Z, such the opening of vent  130 T forms surfaces that are substantially perpendicular to the direction X and the direction Y. Note that, in the description and claims of the present application, “gap  130 P” is in a plane parallel to the direction X and the direction Y, and shall refer to a space widthwise along the slit  130  (i.e., the space between the first sidewall S 1  and the second sidewall S 2  in the plane parallel to the direction X and the direction Y); “vent  130 T” shall refer to a space between the first sidewall S 1  and the second sidewall S 2  in the direction Z (the normal direction of the horizontal surface SH of the base BS) perpendicular to the direction X and the direction Y. 
     The slit  130  may be any suitable type as long as it can generate a vent  130 T between the first sidewall S 1  and the second sidewall S 2  based on the driving signal received by the first actuator  120 . 
     The slit  130  may be disposed at any suitable position. In this embodiment, as shown in  FIG. 1 , the first membrane  110  may have the slit  130  (i.e., the slit  130  is a cut through the first membrane  110 , so as to be formed within the first membrane  110 ), such that the first membrane  110  may include the first sidewall S 1  and the second sidewall S 2  of the slit  130 , but not limited thereto. Namely, in this embodiment, the first membrane  110  performing the acoustic transformation may be configured to be actuated to form the vent  130 T, and the vent  130 T is formed because of the slit  130 . 
     In another embodiment (e.g.,  FIG. 10 ), the slit  130  may be a boundary of the first membrane  110 , such that the first membrane  110  may include the first sidewall S 1  of the slit  130  and not include the second sidewall S 2  of the slit  130 , and the first sidewall S 1  of the slit  130  may be one of the outer edges  110   e  of the first membrane  110 , but not limited thereto. 
     In the present invention, the number of the slit(s)  130  included in the acoustic transducer  100  may be adjusted based on requirement(s). For instance, as shown in  FIG. 1 , the acoustic transducer  100  may include four slits  130   a ,  130   b ,  130   c  and  130   d , such that the first membrane  110  may include four membrane portions  112   a ,  112   b ,  112   c  and  112   d  divided by the slits  130   a ,  130   b ,  130   c  and  130   d  (i.e., each slit  130  divides the first membrane  110  into two membrane portions), but not limited thereto. In  FIG. 1 , the membrane portion  112   a  is between the slits  130   a  and  130   d , the membrane portion  112   b  is between the slits  130   a  and  130   b , and so on and so forth. Correspondingly, the first actuator  120  includes four actuating portions  120   a ,  120   b ,  120   c  and  120   d  disposed on the membrane portions  112   a ,  112   b ,  112   c  and  112   d , respectively. 
     Therefore, the first sidewall S 1  and second sidewall S 2  of the slit  130  may respectively belong to different membrane portions of the first membrane  110 . Taking the slit  130   a  as an example, the slit  130   a  is formed between the membrane portions  112   a  and  112   b , such that the first sidewall S 1  and second sidewall S 2  of the slit  130   a  respectively belong to the membrane portions  112   a  and  112   b . In other words, the membrane portion  112   a  and the actuating portion  120   a  are at one side of the slit  130   a , and the membrane portion  112   b  and the actuating portion  120   b  are at another side of the slit  130   a . For instance, a point C is on the first sidewall S 1  of the slit  130   a , and a point D is on the second sidewall S 2  of the slit  130   a , such that the point C and the point D respectively belong to membrane portions  112   a  and  112   b  and form a pair of points separated by the gap  130 P of the slit  130   a.    
     In the present invention, the shape/pattern of the slit  130  is not limited. For example, the slit  130  may be a straight slit, a curved slit, a combination of straight slits, a combination of curved slits or a combination of straight slit(s) and curved slit(s). In this embodiment, as shown in  FIG. 1  and  FIG. 2 , the slit  130  may be a curved slit, but not limited thereto. In this embodiment, as shown in  FIG. 1  and  FIG. 2 , the slit  130  may extend toward a central portion of the first membrane  110  e.g., from a corner  110 R of the first membrane  110 . In this embodiment, a curvature of the slit  130  may increase as the slit  130  extending from the corner  110 R of the first membrane  110  toward the central portion of the first membrane  110 , such that the slit  130  may form as a hook pattern, but not limited thereto. Specifically, taking the slit  130   a  as an example, a first radius of curvature at a point A on the slit  130   a  is smaller than a second radius of curvature at a point B on the slit  130   a , where the point A is farther away from the corner  110 R compared to the point B (i.e., a first length along the slit  130   a  between the point A and the corner  110 R is larger than a second length along the slit  130   a  between the point B and the corner  110 R), but not limited thereto. Moreover, as shown in  FIG. 1 , the slits  130  may extend inward on the first membrane  110  and form a vortex pattern, but not limited thereto. 
     In another aspect, as illustrated in  FIG. 3 , the slit  130  may divide the first membrane  110  (the film structure FS) into two flaps opposite to each other. Namely, two membrane portions of the first membrane  110  divided by the slit  130  may be a first flap and a second flap respectively, such that the first sidewall S 1  may belong to the first flap, and the second sidewall S 2  may belong to the second flap. The first flap may include a first end and a second end (also referred as a free end), the first end may be anchored by one anchor structure  140 , and the second end (i.e., the free end) may be configured to perform a first up-and-down movement (i.e., the second end of the first flap may move upwardly and downwardly) to form the vent  130 T. The second flap may include a first end and a second end (also referred as a free end), the first end may be anchored by one anchor structure  140 , and the second end (i.e., the free end) may be configured to perform a second up-and-down movement (i.e., the second end of the second flap may move upwardly and downwardly) to form the vent  130 T. The movement of the free end of the second flap may be different from (e.g., in the embodiment of  FIG. 4 ) or opposite to (e.g., in the embodiment of  FIG. 8 ) the movement of the free end of the first flap. 
     Taking the slit  130   a  formed between the membrane portions  112   a  and  112   b  in  FIG. 1  as an example, the first sidewall S 1  of the slit  130   a  may be on the free end of the first flap (i.e., the point C may be on the second end of the first flap), and the second sidewall S 2  of the slit  130   a  may be on the free end of the second flap (i.e., the point D may be on the second end of the second flap), but not limited thereto. 
     Moreover, the slit  130  may release the residual stress of the first membrane  110 , wherein the residual stress is generated during the manufacturing process of the first membrane  110  or originally exist in the first membrane  110 . 
     As shown in  FIG. 1  and  FIG. 2 , because of the arrangement of the slits  130 , the first membrane  110  may optionally include a coupling plate  114  connected to the membrane portions  112   a ,  112   b ,  112   c  and  112   d . In this embodiment, all membrane portions  112   a ,  112   b ,  112   c  and  112   d  are connected to the coupling plate  114 , and the coupling plate  114  surrounded by the membrane portions  112   a ,  112   b ,  112   c  and  112   d  (i.e., the coupling plate  114  is the central portion of the first membrane  110 ) and/or the slits  130 , but not limited thereto. For instance, the coupling plate  114  is only connected to the membrane portions  112   a ,  112   b ,  112   c  and  112   d , but not limited thereto. For instance, in  FIG. 1 , the first actuator  120  may not overlap the coupling plate  114  in the direction Z (the normal direction of the horizontal surface SH of the base BS), but not limited thereto. In this embodiment, since the coupling plate  114  exists, even if the structural strength of the first membrane  110  is weakened due to the formation of the slit  130 , the breaking possibility of the first membrane  110  may be decreased and/or the break of the first membrane  110  may be prevented during the manufacture. In other words, the coupling plate  114  may maintain the structural strength of the first membrane  110  in a certain level. 
     Owing to the existence of the slit(s)  130 , it may be considered that the first membrane  110  includes a plurality of spring structures which are formed because of the slit(s)  130 . In  FIG. 1  and  FIG. 2 , the spring structure is considered to be connected between the coupling plate  114  and a part of the first membrane  110  overlapping the first actuator  120 . Because of the existence of the spring structure, the displacement of the first membrane  110  may be increased and/or the first membrane  110  may deform elastically during the operation of the acoustic transducer  100 . 
     In this embodiment, the acoustic transducer  100  may optionally include a chip disposed on the horizontal surface SH of the base BS, wherein the chip may include the film structure FS (including the first membrane  110  and the slit(s)  130 ), the anchor structure(s)  140  and the first actuator  120  at least. The manufacturing method of the chip is not limited. For example, in this embodiment, the chip may be formed by at least one semiconductor process to be a MEMS (Micro Electro Mechanical System) chip, but not limited thereto. 
     Note that the first membrane  110 , the slit(s)  130 , the first actuator  120  and the anchor structure  140  of the present invention may be considered as a first unit U 1 . 
     As shown in  FIG. 3 , the acoustic transducer  100  is disposed within a housing structure HSS inside the wearable sound device. In  FIG. 3 , the housing structure HSS may have a first housing opening HO 1  and a second housing opening HO 2 , wherein the first housing opening HO 1  may be connected to an ear canal of a wearable sound device user, the second housing opening HO 2  may be connected to an ambient of the wearable sound device, and the film structure FS is between the first housing opening HO 1  and the second housing opening HO 2 . Note that the ambient of the wearable sound device may not inside the ear canal (e.g., the ambient of the wearable sound device may be directly connected to the space outside the ear). Furthermore, in  FIG. 3 , since the first chamber CB 1  may exist between the base BS and the first membrane  110  (the film structure FS), the first chamber CB 1  may be connected to the ambient of the wearable sound device through the back vent BVT of the base BS and the second housing opening HO 2  of the housing structure HSS. 
     As shown in  FIG. 3 , the first membrane  110  (the film structure FS including the first flap and the second flap) may partition a space formed within the housing structure HSS into a first volume VL 1  to be connected to the ear canal of the wearable sound device user and a second volume VL 2  to be connected to the ambient of the wearable sound device. Thus, when the vent  130 T is temporarily formed between the first sidewall S 1  (i.e., the free/second end of the first flap) and the second sidewall S 2  (i.e., the free/second end of the second flap) of the slit  130  in the direction Z (the normal direction of the horizontal surface SH of the base BS) by the actuation of the first actuator  120 , the first volume VL 1  is to be connected to the second volume VL 2  through the vent  130 T, such that the ambient of the wearable sound device and the ear canal of the wearable sound device user are connected to each other. That is to say, the ambient of the wearable sound device and the ear canal are to be connected via the temporarily opened vent  130 T when the first membrane  110  is actuated. On the contrary, when the vent  130 T is not formed between the first sidewall S 1  (i.e., the free/second end of the first flap) and the second sidewall S 2  (i.e., the free/second end of the second flap) of the slit  130  in the direction Z, the first volume VL 1  is substantially disconnected from the second volume VL 2 , such that the ambient of the wearable sound device and the ear canal of the wearable sound device user are substantially separated from each other. That is to say, the ambient of the wearable sound device and the ear canal of the wearable sound device user are substantially separated (isolated) from each other when the vent  130 T is not formed and/or the vent  130 T is closed. 
     The condition “the vent  130 T is closed” means the first sidewall S 1  of the slit  130  in the  FIG. 3 , (i.e. the free/second end of the first flap) overlaps partially or fully with the second sidewall S 2  of the slit  130  in the  FIG. 3  (i.e. the free/second end of the second flap) in the horizontal direction, and the condition “the vent  130 T is opened”, or equivalently “the vent  130 T is formed”, means that the first sidewall S 1  of the slit  130  in the  FIG. 3 , (i.e. the free/second end of the first flap) does not overlap with the second sidewall S 2  of the slit  130  in the  FIG. 3  (i.e. the free/second end of the second flap) in the horizontal direction. Note that the heights of first sidewall S 1  and the second sidewall S 2  are defined by the thickness of the first membrane  110 . 
     In  FIG. 3 , the first volume VL 1  is connected to the first housing opening HO 1  of the housing structure HSS, and the second volume VL 2  is connected to the second housing opening HO 2  of the housing structure HSS. Thus, the first volume VL 1  is to be connected to the ear canal of the wearable sound device user through the first housing opening HO 1 , and the second volume VL 2  is to be connected to the ambient of the wearable sound device through the second housing opening HO 2 . Note that the first chamber CB 1  is a portion of the second volume VL 2 . 
     Further referring to  FIG. 4 ,  FIG. 4  is a schematic diagram illustrating a first membrane in a first mode according to the first embodiment of the present invention. As shown in  FIG. 2  and  FIG. 4 , when the first membrane  110  is actuated, the first membrane  110  deforms into a deformed type  110 Df. In the present invention, the acoustic transducer  100  may include a first mode and a second mode, wherein the first actuator  120  receives first driving signal(s) in the first mode to generate a vent  130 T formed between the first sidewall S 1  (i.e., the free/second end of the first flap) and the second sidewall S 2  (i.e., the free/second end of the second flap) of the slit  130  in the direction Z (the normal direction of the horizontal surface SH of the base BS), and the first actuator  120  receives second driving signal(s) in the second mode to not generate the vent  130 T between the first sidewall S 1  and the second sidewall S 2  of the slit  130  in the direction Z. 
     As shown in  FIG. 4 , in the first mode, the first sidewall S 1  and the second sidewall S 2  of the slit  130  may have different displacements, causing the overlapping across the gap  130 P of slit  103  between the first sidewall S 1  and the second sidewall S 2  to change. When the difference between these displacements in direction Z is greater than the thickness of the first membrane  110 , the first sidewall S 1  is no longer overlapped with the second sidewall S 2 , an opening between the first sidewall S 1  and the second sidewall S 2  is formed and the vent  130 T is said to be opened. Taking the points C and Don the two side of slit  130   a  of  FIG. 1  as an example, when the first membrane  110  is actuated in the first mode, point C of the first sidewall S 1  on the membrane portion  112   a  is actuated according to the first driving signal (e.g., a voltage) to have a first displacement Uz_a along the direction Z, point D on the second sidewall S 2  on the membrane portion  112   b  is actuated according to the first driving signal to have a second displacement Uz_b along the direction Z, and the first displacement Uz_a of point C is significantly larger than the second displacement Uz_b of pint D, such that the segment of the first sidewall S 1  near point C and the segment of the second sidewall S 2  near point D become non-overlapping and the vent  130 T is formed (or “opened”). The opening size U ZO  of the vent  130 T is determined by a membrane displacement difference ΔUz, between the first displacement Uz_a and the second displacement Uz_b, and the thickness of the first membrane  110 : U ZO =ΔUz−T 110 , where ΔUz=|Uz_a−Uz_b|, T 110  is the thickness of the first membrane  110  and T 110  may be 5-7 μm in practice, but not limited thereto. When the membrane displacement difference ΔUz is larger than the thickness T 110  of the first membrane  110  (the film structure FS) in the first mode, it is said that the vent  130 T will be “temporarily opened”. The larger is opening size U ZO  of the vent  130 T, the wider will the vent  130 T opens. 
     When the vent  130 T is temporarily opened, as illustrated in  FIG. 4 , the air may start to flow between the volumes (i.e., the first volume VL 1  and the second volume VL 2 ) due to the pressure difference between the two sides of the first membrane  110 , such that the pressure caused by the occlusion effect may be released (i.e., the pressure difference between the ear canal and the ambient of the wearable sound device may be released through the airflow flowing through the vent  130 T), so as to suppress the occlusion effect. 
     Rationale of forming the vent  130 T is described below. Refer to points C and D of the slit  130   a  illustrated in  FIG. 1 . The point C is located on the first sidewall S 1  on the membrane portion  112   a , the point D is located on the second sidewall S 2  on the membrane portion  112   b , and the point D is opposite to the point C, across the gap  130 P of the slit  130 . The displacement of the membrane portion  112   a  at the point C is driven by the actuating portion  120   a , and the displacement of the membrane portion  112   b  at the point D is driven by the actuating portion  120   b . A distance DC from the point C to an anchor edge of the membrane portion  112   a  is longer than a distance DD from the point D to an anchor edge of the membrane portion  112   b . Since less distance implies higher stiffness, deformation at the point D would be less than deformation of the point C, even applying the same driving force. In addition, the arrow DC overlaps with the region of the actuating portion while the arrow DD does not, which implies that the driving force applied by the actuating portion  120   a  at the point C is stronger than which applied by the actuating portion  120   b  at the point D. Combining those factors, the displacement of the membrane portion  112   a  at the point C, where driving force strength is stronger while stiffness is lower, would be larger than the displacement of the membrane portion  112   b  at the point D. 
     In the second mode, the membrane displacement difference is less than the thickness of the first membrane  110 , namely ΔUz≤T 110 , in other words, the sidewall at point C of the first sidewall S 1  and the sidewall at point D of the second sidewall S 2  may partially or fully overlap in the horizontal direction. For example, two membrane portions related to the slit  130  (i.e., the first flap and the second flap) in the second mode are shown in  FIG. 3 , these two membrane portions (two flaps) may be substantially parallel to each other and be substantially parallel to the horizontal surface SH of the base BS, but not limited thereto. In another example, two membrane portions related to the slit  130  (e.g., the first flap and the second flap) in the second mode are shown in  FIG. 5 , these two membrane portions (two flaps) may not be parallel to the horizontal surface SH of the base BS, the free/second end of the first flap (the first sidewall S 1 ) may be closer to the base BS than the anchored/first end of the first flap, and the free/second end of the second flap (the second sidewall S 2 ) may be closer to the base BS than the anchored/first end of the second flap, but not limited thereto, and ΔUz≤T 110 . Thus, in either case where the slit  130  and its associated membrane portions is in the second mode, namely ΔUz≤T 110 , the vent  130 T is not opened/generated, and/or the vent  130 T is closed. 
     The width of the gap  130 P of the slit  130  should be sufficiently small, e.g., 1 μm˜2 μm in practice. Airflow through narrow channels can be highly damped due to viscous forces/resistance along the walls of the airflow pathways, known as boundary layer effect within field of fluid mechanics. So, the airflow through the gap  130 P of the slit  130  in the second mode may be much smaller compared to the airflow through the vent  130 T of the slit  130  in the first mode (e.g., the airflow through the gap  130 P of the slit  130  in the second mode may be negligible or 10 times lower than the airflow through the vent  130 T of the slit  130  in the first mode). In other words, the width of the gap  130 P of the slit  130  is sufficiently small such that, the airflow/leakage through the gap  130 P of the slit  130  in the second mode is negligible compared to (e.g., less than 10% of) the airflow through the vent  130 T in the first mode. 
     According to the above, in the first mode and the second mode, the first sidewall S 1  serving as the free/second end of the first flap may perform the first up-and-down movement, and the second sidewall S 2  serving as the free/second end of the second flap may perform the second up-and-down movement. In particular, as shown in  FIG. 3  to  FIG. 5 , when the first sidewall S 1  (the free/second end of the first flap) performs the first up-and-down movement, the first sidewall S 1  makes no physical contact with any other component within the acoustic transducer  100 ; when the second sidewall S 2  (the free/second end of the second flap) performs the second up-and-down movement, the second sidewall S 2  makes no physical contact with any other component within the acoustic transducer  100 . 
     Referring to  FIG. 6  and  FIG. 7 ,  FIG. 6  is a schematic diagram illustrating multiple examples of relative position pairs on different sides of a slit according to the first embodiment of the present invention, and  FIG. 7  is a schematic diagram illustrating frequency responses of multiple examples according to the first embodiment of the present invention.  FIG. 6  illustrates six examples Ex1-Ex6 of relative position pairs of the point C (or a free/second end) on the membrane portion  112   a  (or a first flap) and the point D (or a free/second end) on the membrane portion  112   b  (or a second flap), corresponding to six progressively higher actuator driving voltage V1-V6, as labeled on the horizontal axis of  FIG. 6 . Vertical axis of  FIG. 6  represents displacements (Uz) of the point C and the point D in the direction Z. Note that the height of blocks representing the points C and D shown in  FIG. 6  corresponds to the thickness of the first membrane  110 .  FIG. 7  illustrates the frequency responses of the acoustic transducer  100  when the first membrane  110  actuated by the driving voltage V1-V6 (examples Ex1-Ex6) shown in  FIG. 6 . Note that, the numerical values shown in  FIG. 6  and  FIG. 7  are for illustrative purpose, practical applied voltage may be adjusted according to practical circumstance. 
     As shown in  FIG. 4  and  FIG. 6 , in this case (a first driving method), the point C of the first sidewall S 1  (i.e., the second end of the first flap) and the point D of the second sidewall S 2  (i.e. the second end of the second flap) of the slit  130  moves in the same direction, i.e., both the first sidewall S 1  and the second sidewall S 2  moves upward in the positive direction Z as the voltage applied to the first actuator  120  increases, and the voltage is raised above a threshold voltage, such as to voltage V5 or V6, to generate/open the vent  130 T; inversely, both the first sidewall S 1  and the second sidewall S 2  moves downward in the positive direction Z as the voltage applied to the first actuator  120  decrease, and the voltage is lowered below a threshold voltage, such as to V1˜V3, to close the vent  130 T. 
     As shown in  FIG. 6 , the point C is lower the point D when the voltage V1 (e.g., 1V) is applied on the first actuator  120 ; the point C is substantially aligned to the point D when the voltage V2 (e.g., 8V) is applied on the first actuator  120 ; the point C is higher than the point D by exactly the thickness of the first membrane  110  when the threshold voltage V4 (e.g., 22V) is applied on the first actuator  120 ; and the point C is higher than the point D by more than the thickness of the first membrane  110  when the voltages V5-V6 is applied on the first actuator  120 . Therefore, in  FIG. 6 , when the first actuator  120  receives the voltage higher than the threshold voltage V4, such as voltage V5˜V6, the vent  130 T is created, where the vent  130 T will be opened; and conversely, when the first actuator  120  receives the voltage lower than the threshold voltage V4, such as voltage V1˜V3, the vent  130 T will not be created, and the vent  130 T is said to be closed. 
     In other words, the membrane portion  112   a  at point C is partially below the membrane portion  112   b  at point D when the voltage V1 is applied on the first actuator  120 . The membrane portion  112   a  at point C is substantially aligned to the membrane portion  112   b  at point D, in the horizontal direction, when the voltage V2 is applied on the first actuator  120 . The membrane portion  112   a  at point C is partially above the membrane portion  112   b  at point D when the voltage V3 is applied on the first actuator  120 . The lower edge of the membrane portion  112   a  at point C is substantially aligned to the top edge of the membrane portion  112   b  at point D, in the horizontal direction, when the voltage V4 is applied on the first actuator  120 . The membrane portion  112   a  at point C is completely above the membrane portion  112   b  at point D, in the direction Z, when a voltage greater than the threshold voltage V4, such as the voltage V5 or V6, is applied on the first actuator  120 , such that the vent  130 T is generated and opened. 
     As shown in  FIG. 6 , in this embodiment, the voltage V5 or V6 is applied on the first actuator  120  in the first mode, and the voltage V1, V2 or V3 is applied on the first actuator  120  in the second mode. In other words, an absolute value of the first driving signal applied on the first actuator  120  in the first mode may be greater than or equal to a threshold value, and an absolute value of the second driving signal applied on the first actuator  120  in the second mode may be less than the threshold value, wherein the threshold value is illustrated as voltage V4 (22V) in  FIG. 6 , but not limited thereto. 
     According to the above, in the second mode, the membrane portion  112   a  may be partially below, partially above or substantially aligned to the membrane portion  112   b . That is to say, the first actuator  120  receives the second driving signal in the second mode to make the first sidewall S 1  be corresponding to (or overlapping with) the second sidewall S 2  in the horizontal direction parallel to the horizontal surface SH of the base BS (i.e., the vent  130 T is closed and/or is not generated). In this embodiment, the entire first sidewall S 1  is corresponding to the second sidewall S 2  in the horizontal direction in the second mode. 
     On the other hand, in the first mode, the first actuator  120  receives the first driving signal to make at least a part of the first sidewall S 1  be not corresponding to, or not overlapping with, the second sidewall S 2  in the horizontal direction, such that the vent  130 T is formed by the non-overlapping region between the first sidewall S 1  and the second sidewall S 2 . 
     As shown in  FIG. 7 , since the width of the gap  130 P of the slit  130  should be sufficiently small, in the frequency response of the acoustic transducer  100 , the low frequency roll-off (LFRO) corner frequency of the SPL in the second mode is low, typically 35 Hz or lower. Conversely, when the vent  130 T opens/exists in the first mode, the air will flow through the vent  130 T with an airflow impedance inversely proportional to the opening size of the vent  130 T, and therefore, in the frequency response of the acoustic transducer  100 , the LFRO corner frequency in the first mode will be significantly higher than the LFRO corner frequency in the second mode. For instance, the LFRO corner frequency in the first mode may fall between 80 to 400 Hz, depends on the opening size of the vent  130 T, but not limited thereto. 
     In the first driving method of the acoustic transducer  100 , when the occlusion effect occurs, the first driving signal may be applied on the first actuator  120  to make the acoustic transducer  100  in the first mode, such that the vent  130 T is generated/opened to allow the occlusion induced pressure to be released by the airflow through the vent  130 T, so as to suppress the occlusion effect. For example, in this embodiment, the first driving signal may include a vent generating signal (e.g., the voltage V5 or V6) and a common signal (e.g., the common signal plus the vent generating signal), but not limited thereto. When the occlusion effect does not occur, the second driving signal may be applied on the first actuator  120  to make the acoustic transducer  100  in the second mode, such that the vent  130 T is not generated. For example, in this embodiment, the second driving signal may include a vent restraining signal (e.g., the voltage V1, V2 or V3) and a common signal (e.g., the common signal plus the vent restraining signal), but not limited thereto. 
     The common signal may be designed based on requirement(s). In some embodiments, the common signal may include a constant (DC) bias voltage, an input audio (AC) signal or a combination thereof. For example, when the common signal includes the input audio signal, the common signal includes a signal corresponding to (related to) the value(s) of the input audio signal, such that the first membrane  110  may generate the acoustic wave while forming the vent  130 T in the first mode, or alternatively, the first membrane  110  may generate the acoustic wave while restraining (close) the vent  130 T. In an embodiment, the common signal may include a constant bias voltage, so as to maintain the first membrane  110  in a certain position. For example, the constant bias voltage, applied on the first actuator  120 , may cause the first membrane  110  (e.g., the first flap and the second flap) to be substantially parallel to the horizontal surface SH of the base BS. 
     Note that, the embodiments and examples shown in  FIG. 4  to  FIG. 7  belong to the first driving method which the first sidewall S 1  and the second sidewall S 2  of the slit  130  moves in the same direction for generating/opening and closing the vent  130 T. A second driving method for generating the vent  130 T may involve making the first sidewall S 1  and the second sidewall S 2  move in the different directions, and a third driving method for generating the vent  130 T may involve only the one of the sidewalls, such as the first sidewall S 1 , moves while the other sidewall, such as the second sidewall S 2 , is stationary. 
     Referring to  FIG. 8 ,  FIG. 8  is a schematic diagram of a cross sectional view illustrating a first membrane in a first mode according to another embodiment of the present invention, wherein  FIG. 8  shows that the first membrane  110  of the acoustic transducer  100  is actuated in the first mode according to the second driving method. As shown in  FIG. 8 , regarding one slit  130 , the first flap (one membrane portion containing the first sidewall S 1  of the slit  130 ) may be actuated to move toward a first direction, and the second flap (one membrane portion containing the second sidewall S 2  of the slit  130 ) may be actuated to move toward a second direction opposite to the first direction, such that the vent  130 T is formed. Namely, the first up-and-down movement of the first sidewall S 1  (the free/second end of the first flap) is opposite to the second up-and-down movement of the second sidewall S 2  (the free/second end of the second flap). For example, the first direction and the second direction may be substantially parallel to the direction Z, and in transition from a second, such as the one illustrated in  FIG. 3 , to a first mode, such as the one shown in  FIG. 8 , the free/second end of the first flap (the first sidewall S 1 ) may move upwards while the free/second end of the second flap (the second sidewall S 2 ) may move downwards. Conversely, in transition from the first mode as shown in  FIG. 8  back to the second mode as shown in  FIG. 3 , the free/second end of the first flap (the first sidewall S 1 ) may move downwards, and the free/second end of the second flap (the second sidewall S 2 ) may move upwards. In either transition discussed above, the first sidewall S 1  of the first flap and the second sidewall S 2  of the second flap move in opposite directions. 
     In addition, the free/second end of the first flap (the first sidewall S 1 ) may be actuated to have a first displacement Uz_a toward the first direction, and the free/second end of the second flap (the second sidewall S 2 ) may be actuated to have a second displacement Uz_b toward the second direction. In an embodiment, the first displacement of the first sidewall S 1  and the second displacement of the second sidewall S 2  may be of substantially equal in distance, but opposite in direction. 
     Furthermore, the first displacement of the first sidewall S 1  and the second displacement of the second sidewall S 2  may be temporarily symmetrical, i.e. the movements of the first sidewall S 1  and the second sidewall S 2  are substantially equal length wise, but opposite in direction over any period of time. When the movements of the first sidewall S 1  and the second sidewall S 2  of  FIG. 8  is temporarily symmetrical, regarding one slit  130 , a first air movement is produced because the first flap (one membrane portion containing the first sidewall S 1  of the slit  130 ) is actuated to move toward the first direction, a direction of the first air movement is related to the first direction, a second air movement is produced because the second flap (one membrane portion containing the second sidewall S 2  of the slit  130 ) is actuated to move toward the second direction opposite to the first direction, and a direction of the second air movement is related to the second direction. Since the first air movement and the second air movement may be respectively related to the opposite directions, at least a portion of the first air movement and at least a portion of the second air movement may cancel each other when the first flap (one membrane portion containing the first sidewall S 1  of the slit  130 ) and the second flap (one membrane portion containing the second sidewall S 2  of the slit  130 ) are simultaneously actuated to open/close the vent  130 T. 
     In some embodiments, the first air movement and the second air movement may substantially cancel each other when the first flap and the second flap are simultaneously actuated to open/close the vent  130 T (for example, the first displacement toward the first direction and the second displacement toward the second direction may be equal in distance but opposite in direction). Namely, a net air movement produced due to opening/closing the vent  130 T, which contains the first air movement and the second air movement, is substantially zero. As the result, since the net air movement is substantially zero during the opening and/or closing operation of the vent  130 T, the operations of the vent  130 T produces no acoustic disturbance perceivable to the user of the acoustic transducer  100 , and the opening and/or closing operation of the vent  130 T is said to be “concealed”. 
     In the embodiment related to  FIG. 1 ,  FIG. 2 ,  FIG. 4 ,  FIG. 6  and  FIG. 7 , one driving signal, refer to as the first driving method herein, is applied to the first actuator  120 . In a second driving method, such as the driving signal for embodiment of  FIG. 8 , the driving signal applied on the actuating portion of the first actuator  120  on the first flap (the portion containing the first sidewall S 1 ) may be different from the driving signal applied on the actuating portion of the first actuator  120  on the second flap (the portion containing the second sidewall S 2 ). In detail, the first actuator  120  disposed on the first flap (the membrane portion containing the first sidewall S 1 ) will receive the first signal, and the first actuator  120  disposed on the second flap (the membrane portion containing the second sidewall S 2 ) will receive the second signal. Thus, the first flap will move according to the first signal, and the second flap will move according to the second signal. 
     The first signal and the second signal may contain component signals designed to make the first flap (the membrane portion containing the first sidewall S 1 ) and the second flap (the membrane portion containing the second sidewall S 2 ) to move in the opposite directions respectively. For example, the first signal may include a common signal plus an incremental voltage, and the second signal may include the same common signal plus a decremental voltage, wherein the incremental voltage may toggle between 0V and a positive voltage, such as 0V⇔10V, and the decremental voltage may change between 0V and a negative voltage, such as 0V⇔−10V, but not limited thereto. Note that the common signal may include the constant bias voltage, the input audio signal or a combination thereof, but not limited thereto. 
     For example, in the first mode of the acoustic transducer  100  in  FIG. 8 , the incremental voltage may have a positive voltage, e.g., 10V, making the first signal 10V higher than the common signal, and the decremental voltage may have a negative voltage, e.g., −10V, making the second signal 10V lower than the common signal and the vent  130 T will be opened/formed when the delta displacement of the first membrane portion (containing the first sidewall S 1 ) and the second membrane portion (containing the second sidewall S 2 ) is greater than the thickness of the first membrane  110 . Conversely, in the second mode of the acoustic transducer  100 , both the incremental voltage of the first signal and the decremental voltage of the second signal may be approximately 0V, resulting in substantially the same driving signals being applied to the actuators on both portions of the first membrane  110 , leading to both membrane portions (one containing the first sidewall S 1 , the other containing the second sidewall S 2 ) producing approximately the same displacement and, as a result, the vent  130 T will not be formed/opened, or, will be closed. 
     Therefore, under certain circumstance, the incremental voltage and the decremental voltage may be of substantially the same magnitude, but not limited thereto; under certain circumstance, such as in the first mode where the vent  130 T is opened, the first signal may be higher than the second signal by a voltage level that is sufficient to cause delta displacement to be larger than the thickness of the membrane, but not limited thereto; under certain circumstances, such as in the second mode where the vent  130 T is closed, the incremental voltage and the decremental voltage may both be or be close to 0V, but not limited thereto. 
     According to the above, the slit  130  of the present invention may be driven by the first driving method or the second driving method to serve as a dynamic front vent of the acoustic transducer  100 , wherein the first volume VL 1  and the second volume VL 2  in the housing structure HSS are connected when the dynamic front vent is opened (i.e., the vent  130 T of the slit  130  is opened and/or formed), and the first volume VL 1  and the second volume VL 2  in the housing structure HSS are separated from each other when the dynamic front vent is closed (i.e., the vent  130 T of the slit  130  is closed and/or not formed). The wider is the vent  130 T, the greater will be the dynamic front vent. Thus, the size of the front vent can be changed by the driving signal(s) based on requirement(s). 
     Moreover, the acoustic transducer  100  of the present invention may have the better water protection and the better dust protection due to the dynamic front vent. 
     In the present invention, the acoustic transducer  100  may use any suitable driver. For instance, the acoustic transducer  100  may use small driver (e.g., a typical 115 dB driver), such that the acoustic transducer  100  of the present invention may be suitable for the small size device. 
     Referring to  FIG. 9 ,  FIG. 9  is a schematic diagram illustrating a wearable sound device with an acoustic transducer according to an embodiment of the present invention. As shown in  FIG. 9 , the wearable sound device WSD may further include a sensing device  150  and a driving circuit  160  electrically connected to the sensing device  150  and the actuator (e.g., the first actuator  120 ) of the acoustic transducer  100 . 
     The sensing device  150  may be configured to sense any required factor outside the wearable sound device WSD and corresponding to generate a sensing result. For example, the sensing device  150  may use an infrared (IR) sensing method, an optical sensing method, an ultrasonic sensing method, a capacitive sensing method or other suitable sensing method to sense any required factor, but not limited thereto. 
     In some embodiments, whether the vent  130 T is formed is determined according to the sensing result. The vent  130 T is opened (or formed) when a sensed quantity indicated by the sensing result crosses a certain threshold with a first polarity, and the vent  130 T is closed (or not formed) when the sensed quantity crosses the certain threshold with a second polarity opposite to the first polarity. For instance, the first polarity may be from low to high, and the second polarity may be from high to low, such that the vent  130 T is opened when the sensed quantity is changed from lower than the certain threshold to higher than the certain threshold, and the vent  130 T is closed when the sensed quantity is changed from higher than the certain threshold to lower than the certain threshold, but not limited thereto. 
     Moreover, in some embodiments, a degree of opening of the vent  130 T may be monotonically related to the sensed quantity indicated by the sensing result. Namely, the degree of opening of the vent  130 T increases or decreases as the sensed quantity increases or decreases. 
     In some embodiments, the sensing device  150  may optionally include a motion sensor configured to detect a body motion of the user and/or a motion of the wearable sound device WSD. For example, the sensing device  150  may detect the body motion causing the occlusion effect, such as walking, jogging, talking, eating, etc. In some embodiments, the sensed quantity indicated by the sensing result represents the body motion of the user and/or the motion of the wearable sound device WSD, and the degree of opening of the vent  130 T is correlated to the motion sensed. For instance, the degree of opening of the vent  130 T increases as the motion increases. 
     In some embodiments, the sensing device  150  may optionally include a proximity sensor configured to sense a distance between an object and the proximity sensor. In some embodiments, the sensed quantity indicated by the sensing result represents the distance between the object and the proximity sensor, and the degree of opening of the vent  130 T is correlated to the distance sensed. For instance, the vent  130 T is opened (or formed) when this distance smaller than a predetermined distance, and the degree of opening of the vent  130 T increases as this distance decreases. For instance, if the user wants to open (or form) the vent  130 T, the user can use any suitable object (e.g., the hand) to approach the wearable sound device WSD, so as to make the proximity sensor sense this object to correspondingly generate the sensing result, thereby open/form the vent  130 T. 
     In addition, the proximity sensor may further have a function for detecting that the user (predictably) taps or touches the wearable sound device WSD having the acoustic transducer  100  because these motions may also cause the occlusion effect. 
     In some embodiments, the sensing device  150  may optionally include a force sensor configured to sense the force applied on the force sensor of the wearable sound device WSD, the sensed quantity indicated by the sensing result represents the force pressing on the wearable sound device WSD, and the degree of opening of the vent  130 T is correlated to the force sensed. 
     In some embodiments, the sensing device  150  may optionally include a light sensor configured to sense an ambient light of the wearable sound device WSD, the sensed quantity indicated by the sensing result represents the luminance of the ambient light sensed by the light sensor, and the degree of opening of the vent  130 T is correlated to the luminance of the ambient light sensed. 
     The driving circuit  160  is configured to generate the driving signal(s) applied on the actuator (e.g., the first actuator  120 ), so as to actuate the first membrane  110 , wherein the driving signal(s) may be based on the sensing result of the sensing device  150  and the value of the input audio signal. In  FIG. 9 , the driving circuit  160  may be an integrated circuit, but not limited thereto. 
     For example, in the first driving method, the first driving signal and the second driving signal may be generated by the driving circuit  160 , and the vent generating signal of the first driving signal and the vent restraining signal of the second driving signal may be generated according to the sensing result, but not limited thereto. 
     For example, in the second driving method, the first signal and the second signal may be generated by the driving circuit  160 , and the incremental voltage of the first signal and the decremental voltage of the second signal may be generated according to the sensing result, but not limited thereto. 
     Similarly, since the degree of opening of the vent  130 T may be monotonically related to the sensed quantity indicated by the sensing result, the incremental voltage and/or the decremental voltage in the second driving method (or the vent generating signal in the first driving method) may have a monotonic relationship with the sensed quantity indicated by the sensing result. 
     Similarly, when the sensing device  150  includes the motion sensor, a magnitude of the incremental voltage and/or a magnitude of the decremental voltage in the second driving method (or the vent generating signal in the first driving method) may increase (or decrease) as the motion increases, but not limited thereto. Similarly, when the sensing device  150  includes the proximity sensor, a magnitude of the incremental voltage and/or a magnitude of the decremental voltage in the second driving method (or the vent generating signal in the first driving method) may increase (or decrease) as the distance decreases or decreases below a threshold, but not limited thereto. Similarly, when the sensing device  150  includes the force sensor, a magnitude of the incremental voltage and/or a magnitude of the decremental voltage in the second driving method (or the vent generating signal in the first driving method) may increase (or decrease) as the force increases, but not limited thereto. Similarly, when the sensing device  150  includes the light sensor, a magnitude of the incremental voltage and/or a magnitude of the decremental voltage in the second driving method (or the vent generating signal in the first driving method) may increase (or decrease) as the luminance of the ambient light decreases, but not limited thereto. 
     In addition, the driving circuit  160  may include any suitable component. For example, the driving circuit  160  may include an analog-to-digital converter (ADC)  162 , a digital signal processing (DSP) unit  164 , a digital-to-analog converter (DAC)  166 , any other suitable component (e.g., a microphone detecting the SPL of the environmental sound or the SPL of the occlusion noise) or a combination thereof. 
     In this embodiment, based on the sensing result generated by the sensing device, the driving circuit  160  may correspondingly apply the driving signal(s) on the first actuator  120 , so as to make the acoustic transducer  100  in the first mode or in the second mode. In the first mode, the acoustic transducer  100  forms the vent  130 T, so as to suppress the occlusion effect. Also, the acoustic transducer  100  in the first mode may optionally generate the acoustic wave. In second mode, the acoustic transducer  100  generates the acoustic wave. 
     Optionally, the driving circuit  160  may further include a frequency response equalizer configured to adjust the driving signal of the acoustic transducer  100  in a specific frequency range. As shown in  FIG. 7 , four different LFRO corner frequencies in the frequency response of the acoustic transducer  100  corresponding to four different vent  130 T conditions are shown. In an embodiment, a signal processing unit containing the frequency response equalizer may be configured to compensate for the differing LFRO corner frequency of the frequency response of the acoustic transducer  100  due to differing degree of opening of vent  130 T. For example, the frequency response equalizer may be enabled to compensate for the LFRO frequency response curve of the example Ex5 (or Ex6) when the driving voltage V5 (or V6) is applied to the first actuator  120  and the vent  130 T is opened as depicted in  FIG. 6 . In other words, the frequency response equalizer may be enabled in the first mode (the frequency response equalizer is enabled when the vent  130 T is opened), and the frequency response equalizer may be disabled in the second mode (the frequency response equalizer is disabled when the vent  130 T is closed). Furthermore, the amount of equalization generated by the frequency response equalizer may be adaptive, varying dynamically according to the opening size of the vent  130 T. As the result, the frequency response equalizer may compensate for the varying LFRO of the low-frequency response of the acoustic transducer  100  due to the vent  130 T being opened (i.e., the frequency response equalizer may compensate for the degradation of the low-frequency response of the acoustic transducer  100  in the first mode), such that the change in the frequency response of the acoustic transducer  100  may be equalized, the disruption of the sound production characteristics of the acoustic transducer  100  is minimized, and the listener&#39;s audio listening experience optimized. 
     The acoustic transducer of the present invention is not limited by the above embodiment(s). Other embodiments of the present invention are described below. For ease of comparison, same components will be labeled with the same symbol in the following. The following descriptions relate the differences between each of the embodiments, and repeated parts will not be redundantly described. 
     Referring to  FIG. 10  to  FIG. 12 ,  FIG. 10  to  FIG. 12  are schematic diagrams of cross sectional views illustrating another type acoustic transducer according to an embodiment of the present invention, wherein  FIG. 10  shows the second mode of the acoustic transducer  100 ′, and  FIG. 11  and  FIG. 12  show the first mode of the acoustic transducer  100 ′. As shown in  FIG. 10  to  FIG. 12 , a difference between this acoustic transducer  100 ′ and the acoustic transducer  100  is that the first membrane  110  of the acoustic transducer  100 ′ of this embodiment includes the first sidewall S 1  of the slit  130 , but the first membrane  110  does not include the second sidewall S 2  of the slit  130 . Namely, the slit  130  is a part of the boundary of the first membrane  110  (i.e., the first sidewall S 1  of the slit  130  may be one of the outer edges  110   e  of the first membrane  110 ). In  FIG. 10  to  FIG. 12 , the second sidewall S 2  of the slit  130  may be stationary/immobile during the operation of the acoustic transducer  100 ′. For example, the second sidewall S 2  of the slit  130  may belong to the anchor structure  140 , but not limited thereto. Because of the design of the slit  130  shown in  FIG. 10  to  FIG. 12 , the anchor structure  140  may be not connected to a portion of the outer edges  110   e  of the first membrane  110 , but not limited thereto. 
     In another aspect, as shown in  FIG. 10  to  FIG. 12 , the first membrane  110  only include the first flap and does not include the second flap, wherein the first end of the first flap is anchored by one anchor structure  140 , the second/free end of the first flap is configured to perform the first up-and-down movement (i.e., the second end of the first flap may move upwardly and downwardly) to form the vent  130 T (the vent  130 T is shown in  FIG. 11  and  FIG. 12 ), and the first sidewall S 1  of the slit  130  belongs to the second/free end of the first flap. 
     In this design, because the second sidewall S 2  is stationary/immobile during the operation of the acoustic transducer  100 ′, the vent  130 T may be formed by increasing the driving signal applied to first actuator  120  to cause the first sidewall S 1  to move upwards in the direction Z, as in the case of  FIG. 11 . For example, the voltage across the electrodes of the first actuator  120  is 30V, so as to make the first sidewall S 1  move upwards in the direction Z, but not limited thereto. Alternatively, in the case of  FIG. 12 , the first membrane  110  may have a negative initial displacement, i.e. the displacement of the first sidewall S 1  in the direction Z may be −18 μm, as an example, when voltage across the electrodes of the first actuator  120  is 0V. Assuming the membrane thickness is 5 μm, as an example, meaning the height of the first sidewall S 1  is 5 μm and status of the vent  130 T, when 0V is applied to the first actuator  120 , is “opened” with the opening size of the vent  130 T equals to 18-5=13 μm. As such, in this embodiment, the vent  130 T may be put in the second mode by applying a positive driving signal (e.g., 16V) to the first actuator  120  to cause the surface of the first membrane  110  to become substantially parallel to the horizontal surface SH, such as illustrated in  FIG. 10 ; and the vent  130 T may be put in the first mode by applying 0V to the first actuator  120 . 
     Referring to  FIG. 13 ,  FIG. 13  is a schematic diagram of a cross sectional view illustrating the acoustic transducer according to a second embodiment of the present invention. As shown in  FIG. 13 , a difference between this embodiment and the first embodiment is that the acoustic transducer  200  of this embodiment further includes a second membrane  210 , a second actuator  220  and an anchor structure  240  which are disposed on the horizontal surface SH of the base BS, wherein the second membrane  210  is anchored by the anchor structure  240 , the second actuator  220  is configured to actuate the second membrane  210 , and a second chamber CB 2  exists between the base BS and the second membrane  210 . In this embodiment, the film structure FS may include the first membrane  110  and the second membrane  210 , but not limited thereto. In this embodiment, the acoustic transducer  200  may optionally include a chip disposed on the horizontal surface SH of the base BS, and the chip may include the film structure FS (including the first membrane  110  and the second membrane  210 ), the first actuator  120 , the second actuator  220  and the anchor structures  140  and  240  at least (i.e., these structures are integrated in one chip), but not limited thereto. 
     The function provided from the first membrane  110  and the first actuator  120  is different from the function provided from the second membrane  210  and the second actuator  220 . In this embodiment, the first membrane  110  and the first actuator  120  may be configured to suppress the occlusion effect, and the second membrane  210  and the second actuator  220  may be configured to perform the acoustic transformation. That is to say, the first membrane  110  and the first actuator  120  do not perform the acoustic transformation. 
     In detail, in the first mode, the first actuator  120  may generate the vent  130 T formed between the first sidewall S 1  and the second sidewall S 2  of the slit  130  in the direction Z (the normal direction of the horizontal surface SH of the base BS). In the second mode, the first actuator  120  may not generate the vent  130 T between the first sidewall S 1  and the second sidewall S 2  of the slit  130  in the direction Z. Whether the acoustic transducer  200  is in the first mode or the second mode, the second actuator  220  may receive an acoustic driving signal corresponding to (related to) the value(s) of the input audio signal to generate the acoustic wave. Namely, the driving signal(s) applied on the first actuator  120  may not be corresponding to (related to) the value(s) of the input audio signal. For instance, in the first driving method, the first driving signal may include a vent generating signal (e.g., the 30V in discussion associated with  FIG. 11  or the 0V in discussion associated with  FIG. 12 ), and the second driving signal may include a vent restraining signal (e.g., the 16V in discussion associated with  FIG. 10 ), but not limited thereto. 
     The second membrane  210 , the second actuator  220  and the anchor structure  240  may be designed based on requirement(s), wherein the design of the second membrane  210 , the second actuator  220  and the anchor structure  240  needs to be suitable for generating the acoustic wave. For instance, in this embodiment, the top view of the second membrane  210 , the second actuator  220  and the anchor structure  240  may be similar to the first membrane  110 , the first actuator  120  and the anchor structure  140  of the first embodiment shown in  FIG. 1 , but not limited thereto. Note that the second membrane  210  may have at least one slit  230 , such that the displacement of the second membrane  210  may be increased and/or the second membrane  210  may deform elastically during the operation of the acoustic transducer  200 , but not limited thereto. 
     The material and the type of the second membrane  210  may be referred to the first membrane  110  described in the first embodiment, and thus, these will not be redundantly described. The material and the type of the second actuator  220  may be referred to the first actuator  120  described in the first embodiment, and thus, these will not be redundantly described. The material of the anchor structure  240  may be referred to the anchor structure  140  described in the first embodiment, and thus, this will not be redundantly described. 
     Note that the second membrane  210 , the slit(s)  230 , the second actuator  220  and the anchor structure  240  may be considered as a second unit U 2 . 
     The first unit U 1  may be designed based on requirement(s), wherein the design of the first membrane  110 , the first actuator  120  and the slit(s)  130  needs to be suitable for suppressing the occlusion effect. In this embodiment, the first membrane  110  of the first unit U 1  of this embodiment includes the first sidewall S 1  of the slit  130  but does not include the second sidewall S 2  of the slit  130  (i.e., the first membrane  110  only include the first flap and does not include the second flap). For example, as shown in  FIG. 13 , the first unit U 1  may be similar to the acoustic transducer  100 ′ shown in  FIG. 10 , but not limited thereto. 
     Moreover, the first chamber CB 1  may be connected to the second chamber CB 2 . In this embodiment, the base BS may include a plurality back vents BVT 1  and BVT 2 , the first chamber CB 1  may be connected to the rear outside of the acoustic transducer  200  (i.e., a space on the back of the base BS) through the back vent BVT 1 , the second chamber CB 2  may be connected to the rear outside of the acoustic transducer  200  (i.e., a space on the back of the base BS) through the back vent BVT 2 , and the first chamber CB 1  may be connected to the second chamber CB 2  through the back vent BVT 1 , the rear outside of the acoustic transducer  200  (i.e., a portion of the second volume VL 2 ) and the back vent BVT 2 , but not limited thereto. 
     In another embodiment, an air channel may exist between the first membrane  110  and the base BS, such that the first chamber CB 1  may be connected to the second chamber CB 2  through the air channel. For instance, the air channel may be a hole HL passing through the two opposite lateral sides of the anchor structure  140 / 240 , such that the first chamber CB 1  may be connected to the second chamber CB 2  through the hole HL, but not limited thereto. 
     During fabrication, as will be detailed later in the present disclosure, the first membrane  110  and the second membrane  210  may all be fabricated during one single planar thin film fabrication sequence; the first actuator  120  and the second actuator  220  may all be fabricated during another single planar thin film fabrication sequence; and the first chamber CB 1 , the second chamber CB 2  and the anchor structures  140 ,  240 ,  140 / 240  may be formed during one single bulk silicon etching sequence. 
     Referring to  FIG. 14 ,  FIG. 14  is a schematic diagram of a cross sectional view illustrating the acoustic transducer according to another second embodiment of the present invention. As shown in  FIG. 14 , compared with the acoustic transducer  200  in  FIG. 13 , the first membrane  110  of the first unit U 1  of the acoustic transducer  200 ′ includes the first sidewall S 1  and the second sidewall S 2  of the slit  130  (i.e., the first membrane  110  include the first flap and the second flap). For example, as shown in  FIG. 14 , the first unit U 1  may be similar to the acoustic transducer  100  shown in  FIG. 1 , but not limited thereto. 
     In some embodiment, such as illustrated in  FIG. 14 , the design of the first unit U 1  (the first membrane  110 , the first actuator  120  and the slit  130 ) may have the same cross-section, from a particular perspective, as the design of the second unit U 2  (the second membrane  210 , the second actuator  220 , the slit  230 ). 
     Referring to  FIG. 15 ,  FIG. 15  is a schematic diagram of a top view illustrating an acoustic transducer according to a third embodiment of the present invention. Note that the design of the membrane, the actuator, the slit(s) and the anchor structure of the acoustic transducer  300  of the third embodiment may be applied to the first unit U 1  and/or the second unit U 2 . 
     As shown in  FIG. 15 , a difference between the first embodiment and this embodiment is the arrangement of the slits  130  and the first actuator  120 . In this embodiment, the slit  130  may be a combination of straight slits and curved slits. In  FIG. 15 , the slit  130  of this embodiment may include a first portion e 1 , a second portion e 2  connected to the first portion e 1  and a third portion e 3  connected to the second portion e 2 , and the first portion e 1 , the second portion e 2  and the third portion e 3  are arranged in sequence from the outer edge  110   e  to the inner of the first membrane  110 . In the slit  130 , the first portion e 1  and the second portion e 2  may be straight slits extending different direction, and the third portion e 3  may be a curved slit, but not limited thereto. The third portion e 3  might have a hook-shaped curved end of the slit  130 , wherein the hook-shaped curved ends surround the coupling plate  114  of the first membrane  110 . The hook-shaped curved end implies that, a curvature at the curved end or at the third portion e 3  is larger than curvature(s) at the first portion e 1  or the second portion e 2 , from a top view perspective. In addition, the slit  130  with the hook shape extends toward the center of the first membrane  110 , or toward the coupling plate  114  within the first membrane  110 . The slit  130  may be carving out a fillet in the first membrane  110 . 
     The curved end of the third portion e 3  may be configured to minimize stress concentration near the end of the slit  130 . 
     Referring to  FIG. 16 ,  FIG. 16  is a schematic diagram of a top view illustrating an acoustic transducer according to a fourth embodiment of the present invention. Note that the design of the membrane, the actuator, the slit(s) and the anchor structure of the acoustic transducer  400  of the fourth embodiment may be applied to the first unit U 1  and/or the second unit U 2 . 
     As shown in  FIG. 16 , a difference between the third embodiment and this embodiment is the arrangement of the slits  130 . In this embodiment, some slits  130  may be shorter, and each shorter slit  130 _S is between two longer slits  130 _L, but not limited thereto. In  FIG. 16 , the shorter slit  130 _S may not be connected to the outer edge  110   e  of the first membrane  110 , but not limited thereto. 
     The shorter slit  130 _S may be a combination of straight slits and curved slits, and the pattern of the shorter slit  130 _S may be similar to the pattern of the longer slit  130 _L. Moreover, in  FIG. 16 , the shorter slit  130 _S may not be situated in the region on which the first actuator  120  is disposed, but not limited thereto. 
     Referring to  FIG. 17 ,  FIG. 17  is a schematic diagram of a top view illustrating an acoustic transducer according to a fifth embodiment of the present invention. Note that the design of the membrane, the actuator, the slit(s) and the anchor structure of the acoustic transducer  500  of the fifth embodiment may be applied to the first unit U 1  and/or the second unit U 2 . 
     As shown in  FIG. 17 , a difference between the first embodiment and this embodiment is the arrangement of the slits  130  and the first actuator  120 . In this embodiment, the longer slit  130 _L may be a combination of straight slits (e.g., three straight slits forming a Y-shape), but not limited thereto. In this embodiment, the shorter slit  130 _S may be between two longer slits  130 _L, and the shorter slit  130 _S may not be connected to the outer edge  110   e  of the first membrane  110 , but not limited thereto. In  FIG. 17 , the shorter slit  130 _S may be a straight slit, and the shorter slit  130 _S may be parallel to a portion of the longer slit  130 _L, but not limited thereto. 
     Referring to  FIG. 18 ,  FIG. 18  is a schematic diagram of a top view illustrating an acoustic transducer according to a sixth embodiment of the present invention. Note that the design of the membrane, the actuator, the slit(s) and the anchor structure of the acoustic transducer  600  of the sixth embodiment may be applied to the first unit U 1  and/or the second unit U 2 . 
     As shown in  FIG. 18 , a difference between the first embodiment and this embodiment is the arrangement of the slits  130  and the first actuator  120 . In this embodiment, the slit  130  may be a combination of straight slits and curved slits (e.g., two straight slits and a combined slit formed of one curved slit and one straight slit, and these slits forming a Y-shape), but not limited thereto. 
     Referring to the upper portion of  FIG. 18  which substantially shows a quarter of the first membrane  110 , a straight slit of one slit  130  and a straight slit of a combined slit of another slit  130  are parallel to each other and overlap along the direction Y, but not limited thereto. 
     Referring to  FIG. 19  and  FIG. 20 ,  FIG. 19  is a schematic diagram of a top view illustrating an acoustic transducer according to a seventh embodiment of the present invention, and  FIG. 20  is an enlarge diagram illustrating a center part of  FIG. 19 . Note that the design of the membrane, the actuator, the slit(s) and the anchor structure of the acoustic transducer  700  of the seventh embodiment may be applied to the first unit U 1  and/or the second unit U 2 . 
     As shown in  FIG. 19  and  FIG. 20 , a difference between the first embodiment and this embodiment is the arrangement of the slits  130  and the first actuator  120 . In this embodiment, the longer slit  130 _L may be a combination of straight slits (e.g., three straight slits), but not limited thereto. In this embodiment, the shorter slit  130 _S which is not connected to the outer edge  110   e  of the first membrane  110  may be a straight slit, wherein the shorter slit  130 _S may be parallel to a portion of the longer slit  130 _L, but not limited thereto. 
     Moreover, as shown in  FIG. 19  and  FIG. 20 , a ratio of the area of the coupling plate  114  to the area of the first membrane  110  may be much small, but not limited thereto. 
     Referring to  FIG. 21 ,  FIG. 21  is a schematic diagram of a top view illustrating an acoustic transducer according to an eighth embodiment of the present invention. Note that the design of the membrane, the actuator, the slit(s) and the anchor structure of the acoustic transducer  800  of the eighth embodiment may be applied to the first unit U 1  and/or the second unit U 2 . 
     As shown in  FIG. 21 , a difference between the first embodiment and this embodiment is the arrangement of the slits  130  and the first actuator  120 . In this embodiment, the outer slit  130 _T may be a combination of straight slits forming a Y-shape, but not limited thereto. In this embodiment, the inner slit  130 _N which is not connected to the outer edge  110   e  of the first membrane  110  may be a combination of straight slits forming a W-shape. In  FIG. 21 , a portion of the inner slit  130 _N is parallel to a portion of the outer slit  130 _T, but not limited thereto. 
     Moreover, in  FIG. 21 , a ratio of the area of the coupling plate  114  to the area of the first membrane  110  may be much small, but not limited thereto. 
     Note that, the arrangements of the slit(s)  130  described in the above embodiments are examples. Any suitable arrangement of the slit(s)  130  can be used in the present invention. 
     Referring to  FIG. 22 ,  FIG. 22  is a schematic diagram of a top view illustrating an acoustic transducer according to a ninth embodiment of the present invention. As shown in  FIG. 22 , the acoustic transducer  900  may include a plurality of units  902  (i.e., the first unit(s) U 1 , the second unit(s) U 2  or a combination thereof), so as to include a plurality of membranes. In  FIG. 22 , the acoustic transducer  900  includes four units  902  to form the 2×2 array, but not limited thereto. In the present invention, the acoustic transducer  900  may include one single chip including all units  902 , or the acoustic transducer  900  may include a plurality of chips (the chips may be the same or different) to achieve a plurality of units  902 . 
     Note that,  FIG. 22  is for illustrative purpose, which demonstrates a concept of the acoustic transducer  900  including multiple sound producing units  902 . Construct of each membrane is not limited, and the membranes are the same or different. 
     Because of the plurality of units  902  included in the acoustic transducer  900 , the acoustic wave may be generated by these units  902  with any suitable manner. In some embodiments, the units  902  may generate the acoustic wave at the same time, such that the SPL of the acoustic wave may be greater, but not limited thereto. 
     In some embodiments, the units  902  may generate the acoustic wave in a temporally interleaved manner. Regarding to the temporally interleaved manner, the sound producing units  902  are divided into a plurality of groups and generate air pulses, air pulses generated by different groups may be temporally interleaved, and these air pulses are combined to be the overall air pulses reproducing the acoustic wave. If the units  902  are divided into M groups, and the array of the air pulses generated by each group has the pulse rate PRG, the overall pulse rate of the overall air pulses is M·PRG. Namely, the pulse rate of the array of the air pulses generated by one group (i.e., one or some unit(s)) is less than the overall pulse rate of the overall air pulses generated by all group (i.e., all of the units  902 ) if the number of the group is greater than 1. 
     Referring to  FIG. 23 ,  FIG. 23  is a schematic diagram of a top view illustrating an acoustic transducer according to a tenth embodiment of the present invention. As shown in  FIG. 23 , a difference between the ninth embodiment and this embodiment is that the units  902  of the acoustic transducer  1000  of this embodiment may have different sizes, wherein the smaller unit  902  may be a high frequency sound unit (tweeter)  1002 , and the greater unit  902  may be a low frequency sound unit (woofer)  1004 . Note that the design of the high frequency sound unit  1002  may be the aforementioned first unit U 1 , the aforementioned second unit U 2  or a combination thereof, and the design of the low frequency sound unit  1004  may be the aforementioned first unit U 1 , the aforementioned second unit U 2  or a combination thereof. 
     In the operation of the acoustic transducer  1000 , the high frequency sound unit  1002  configured to the high frequency acoustic transformation, the low frequency sound unit  1004  configured to the low frequency acoustic transformation, but not limited thereto. The details of the high frequency sound unit  1002  and the low frequency sound unit  1004  may be referred to U.S. application Ser. No. 17/153,849 filed by Applicant, which is not narrated herein for brevity. 
     In the following, the details of a manufacturing method of the acoustic transducer will be further exemplarily explained. Note that the manufacturing method is not limited by the following embodiments which are exemplarily provided, and the manufacturing method may manufacture the acoustic transducer including the first unit(s) U 1  and/or the second unit(s) U 2 . Note that in the following manufacturing method, the actuator (e.g., the first actuator  120  and/or the second actuator  220 ) in the acoustic transducer may be a piezoelectric actuator for example, but not limited thereto. Any suitable type actuator can be used in the acoustic transducer. 
     In the following manufacturing method, the forming process may include atomic layer deposition (ALD), a chemical vapor deposition (CVD) and other suitable process(es) or a combination thereof. The patterning process may include such as a photolithography, an etching process, any other suitable process(es) or a combination thereof. 
     Referring to  FIG. 24  to  FIG. 30 ,  FIG. 24  to  FIG. 30  are schematic diagrams illustrating structures at different stages of a manufacturing method of an acoustic transducer according to an embodiment of the present invention. In this embodiment, the acoustic transducer may be manufactured by at least one semiconductor process, but not limited thereto. As shown in  FIG. 24 , a wafer WF is provided, wherein the wafer WF includes a first layer W 1 , an electrical insulating layer W 3  and a second layer W 2 , wherein the insulating layer W 3  is formed between the first layer W 1  and the second layer W 2 . 
     The first layer W 1 , the insulating layer W 3  and the second layer W 2  may individually include any suitable material, such that the wafer WF may be any suitable type. For instance, the first layer W 1  and the second layer W 2  may individually include silicon (e.g., single crystalline silicon or poly-crystalline silicon), silicon carbide, germanium, gallium nitride, gallium arsenide, stainless steel, and other suitable high stiffness material or a combination thereof. In some embodiments, the first layer W 1  may include single crystalline silicon, such that the wafer WF is a silicon on insulator (SOI) wafer, but not limited thereto. In some embodiments, the first layer W 1  may include poly-crystalline silicon, such that the wafer WF is a polysilicon on insulator (POI) wafer, but not limited thereto. For instance, the insulating layer W 3  may include oxide, such as silicon oxide (e.g., silicon dioxide), but not limited thereto. 
     The thicknesses of the first layer W 1 , the insulating layer W 3  and the second layer W 2  may be individually adjusted based on requirement(s). For example, the thickness of the first layer W 1  may be 5 μm, and the thickness of the second layer W 2  may be 350 μm, but not limited thereto. 
     In  FIG. 24 , a compensation oxide layer CPS may be optionally formed on a first side of the wafer WF, wherein the first side is upper than a top surface W 1   a  of the first layer W 1  opposite to the second layer W 2 , such that the first layer W 1  is between the compensation oxide layer CPS and the second layer W 2 . The material of oxide contained in the compensation oxide layer CPS and the thickness of the compensation oxide layer CPS may be designed based on requirement(s). 
     In  FIG. 24 , a first conductive layer CT 1  and an actuating material AM may be formed on the first side of the wafer WF (on the first layer W 1 ) in sequence, such that the first conductive layer CT 1  may be between the actuating material AM and the first layer W 1  (e.g., and/or between the actuating material AM and the compensation oxide layer CPS). In some embodiments, the first conductive layer CT 1  is in contact with the actuating material AM. 
     The first conductive layer CT 1  may include any suitable conductive material, and the actuating material AM may include any suitable material. In some embodiment, the first conductive layer CT 1  may include metal (such as platinum), and the actuating material AM may include a piezoelectric material, but not limited thereto. For example, the piezoelectric material may include such as a lead-zirconate-titanate (PZT) material, but not limited thereto. Moreover, the thicknesses of the first conductive layer CT 1  and the actuating material AM may be individually adjusted based on requirement(s). 
     As shown in  FIG. 25 , the actuating material AM, the first conductive layer CT 1  and the compensation oxide layer CPS may be patterned. In some embodiments, the actuating material AM, the first conductive layer CT 1  and the compensation oxide layer CPS may be patterned in sequence. 
     As shown in  FIG. 26 , a separating insulating layer SIL may be formed on the actuating material AM and be patterned. The thickness of the separating insulating layer SIL and the material of the separating insulating layer SIL may be designed based on requirement(s). For instance, the material of the separating insulating layer SIL may be oxide, but not limited thereto. 
     As shown in  FIG. 27 , a second conductive layer CT 2  may be formed on the actuating material AM and the separating insulating layer SIL, and then, the second conductive layer CT 2  may be patterned. The thickness of the second conductive layer CT 2  and the material of the second conductive layer CT 2  may be designed based on requirement(s). For instance, the second conductive layer CT 2  may include metal (such as aurum), but not limited thereto. 
     The patterned first conductive layer CT 1  functions as the first electrode EL 1  for the actuator, the patterned second conductive layer CT 2  functions as the second electrode EL 2  for the actuator, and the actuating material AM, the first electrode EL 1  and the second electrode EL 2  may be components in the actuator (e.g., the first actuator  120  and/or the second actuator  220 ) in the acoustic transducer, so as to make the actuator be a piezoelectric actuator. For example, the first electrode EL 1  and the second electrode EL 2  are in contact with the actuating material AM, but not limited thereto. 
     In  FIG. 27 , the separating insulating layer SIL may be configured to separate at least a portion of the first conductive layer CT 1  from at least a portion of the second conductive layer CT 2 . 
     As shown in  FIG. 28 , the first layer W 1  of the wafer WF may be patterned, so as to form a trench line WL. In  FIG. 28 , the trench line WL is a portion where the first layer W 1  is removed. That is to say, the trench line WL is between two parts of the first layer W 1 . 
     As shown in  FIG. 29 , a protection layer PL may be optionally formed on the second conductive layer CT 2 , so as to cover the wafer WF, the first conductive layer CT 1 , the actuating material AM, the separating insulating layer SIL and the second conductive layer CT 2 . The protection layer PL may include any suitable material, and may have suitable thickness. 
     In some embodiments, the protection layer PL may be configured to protect the actuator  120  from ambient exposure and to ensure the reliability/stability of the actuator  120 , but not limited thereto. As shown in  FIG. 29 , a portion of the protection layer PL may be disposed inside the trench line WL. 
     Optionally, in  FIG. 29 , the protection layer PL may be patterned for exposing a portion of the second conductive layer CT 2  and/or a portion of the first conductive layer CT 1 , so as to form a connecting pad CPD to be electrically connected to outer device. 
     As shown in  FIG. 30 , the second layer W 2  of the wafer WF may be patterned, so as to make the second layer W 2  form at least one anchor structure  140  (and/or  240 ) and to make the first layer W 1  form the film structure FS (e.g., including the first membrane  110  and/or the second membrane  210 ) anchored by the anchor structure(s)  140  (and/or  240 ), wherein the film structure FS includes the first membrane  110  and/or the second membrane  210 . In another aspect, the film structure FS includes the first flap (the first portion) and the second flap (the second portion). In detail, the second layer W 2  of the wafer WF may have a first part and a second part, the first part of the second layer W 2  may be removed, and the second part of the second layer W 2  may form the anchor structure  140  (and/or  240 ). Since the first part of the second layer W 2  is removed, the first layer W 1  forms the film structure FS. Namely, the components included in the film structure FS, such as the first membrane  110 , the second membrane  210 , the first flap and/or the second flap may be fabricated by the same process, where the same process represents the same sequence of steps illustrated in  FIGS. 24-30 . 
     Optionally, in  FIG. 30 , since the insulating layer W 3  of the wafer WF exists, after the second layer W 2  of the wafer WF is patterned, a part of the insulating layer W 3  corresponding to the first part of the second layer W 2  may be removed also, so as to make the first layer W 1  form the film structure FS, but not limited thereto. 
     In  FIG. 30 , since the first part of the second layer W 2  is removed to make the first layer W 1  form the film structure FS, the slit  130  is formed within and penetrates through the film structure FS because of the trench line WL. Since the slit  130  is formed because of the trench line WL, the width of the trench line WL may be designed based on the requirement of the slit  130 . For example, the width of the trench line WL may be less than or equal to 5 less than or equal to 3 μm, or less than or equal to 2 μm, so as to make the slit  130  have the gap  130 P with desire width, but not limited thereto. Moreover, since a portion of the protection layer PL may be disposed inside the trench line WL, the protection layer PL may make the width of the gap  130 P of the slit  130  less than the width of the trench line WL. 
       FIG. 31  is a schematic diagram illustrating a cross sectional view of an acoustic transducer according to another embodiment of the present invention. In another embodiment, compared with the structure shown in  FIG. 30 , the structure shown in  FIG. 31  does not have the insulating layer W 3  of the wafer WF. Namely, the first layer W 1  is directly formed on (in contact with) the second layer W 2 . As the result, the film structure FS is direct formed of the first layer W 1  of the wafer WF owing to patterning the second layer W 2  of the wafer WF. In this case, the first layer W 1  (i.e., the film structure FS) may include an insulation layer including oxide, such as silicon dioxide, but not limited thereto. 
     Then, a base BS is provided, and the structure shown in  FIG. 30  or the structure shown in  FIG. 31  may be disposed on the base BS, so as to complete the manufacture of the acoustic transducer. 
     In summary, because of the existence of the slit, the acoustic transducer may generate the acoustic wave and form the vent for suppressing the occlusion effect in the first mode, and the acoustic transducer may not form the vent in the second mode. That is to say, the slit serves as the dynamic front vent of the acoustic transducer. 
     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.