WO2024191334A1

WO2024191334A1 - A micro-electromechanical-system based micro speaker

Info

Publication number: WO2024191334A1
Application number: PCT/SE2024/050181
Authority: WO
Inventors: Josef HANSSON; Jonatan WÅRDH
Original assignee: Myvox AB
Current assignee: Myvox AB
Priority date: 2023-03-14
Filing date: 2024-02-27
Publication date: 2024-09-19
Anticipated expiration: 2025-09-14
Also published as: SE545958C2; SE2350283A1

Abstract

A micro-electromechanical-system (MEMS) based speaker inclu- des a support structure (110), and flexible membranes (221 222) connected to the support structure. The flexible membranes de- flect relative to the support structure (110) in response to a con- trol signals influencing piezoelectric actuators (231, 232) mecha- nically linked to the flexible membranes. The flexible membranes (221 222) contain at least one slit (340) forming at least one opening between the flexible membrane and the support struc- ture and/or at least one opening in the flexible membranes. At least one bridging member (350) mechanically interconnects the flexible membrane and the support structure that are separated by the opening there between formed by the at least one slit and/or at least two portions of the flexible membrane that are separated by the at least one opening therein formed by the at least one slit. The at least one bridging member (350) is arran- ged in a plane different from a plane in which the flexible mem- branes (221, 222) are arranged in relation to the support struc- ture (110).

Description

A Micro-Electromechanical-System based Micro Speaker

TECHNICAL FIELD

The present invention relates generally to miniature-sized sound generators. Especially, the invention relates to a micro-electro- mechanical-system (MEMS) based micro speaker according to the preamble of claim 1.

BACKGROUND

The vibration amplitude is a limiting factor for producing sound pressure from small membrane speakers. This is especially the case at lower frequencies. In general, a larger diaphragm diameter enables a given sound-pressure-level (SPL) at a smaller deflection amplitude. In other words, increased vibration amplitude allows for smaller speakers at the same level of performance.

As summarized in Wang, H. et al., Review of Recent Development of MEMS Speakers, Micromachines 2021 , 12, 1257. https: //doi.org/10.3390/mi12101257, MEMS based micro speakers are emerging as new technology. In this field, the piezoelectric MEMS micro speaker appear to be the most promising alternative. In its most basic configuration a piezoelectric MEMS micro speaker has a silicon membrane, which is obtained by etching a backside cavity from a silicon chip, and which is actuated by a piezoelectric layer on top of the membrane. The piezoelectric layer is capable to produce high forces. However, for this type of speaker, the vibration amplitude is limited by the tensile tension in the membrane. Moreover, silicon is a relatively stiff material, which also hampers the total amplitude.

For example, the deflection may be increased by creating slits in the membrane as suggested in the article Stoppel, A., Mann- chen, F. Niekiel, D. Beer, T. Giese and B. Wagner, New Integra- ted full-range MEMS speaker for in-ear applications, 2018 IEEE Micro Electro Mechanical Systems (MEMS), 2018, pp. 1068- 1071 , doi: 10.1109/MEMSYS.2018.8346744, which discloses a type of powerful and fully integrated piezoelectric MEMS speaker for in-ear applications. Measurements performed on first prototypes using an artificial ear simulator have revealed a remarkable acoustic performance with respect to SPL, reproduction range, total harmonic distortion (THD) and electroacoustic sensitivity. Due to the mechanically decoupled design without a closed membrane, high SPL values of about 110 dB are achieved from 20 Hz to 20 kHz, exceeding the reproduction range of typical electrodynamic and balanced armature speakers. At the same time, the MEMS speakers feature a very flat frequency response, which has been realized by means of electronic equalization. With respect to the reproduction quality, the speakers are capable of delivering low THD of less than 2 % for most frequencies. Moreover, electroacoustic sensitivity measurements have proven good energy efficiency with sensitivity values surpassing 110 dB/mW within almost the entire audible frequency range.

Nevertheless, providing slits in the membrane makes the speaker more different from a closed-membrane ditto. Instead, in such a slitted-membrane speaker, the speaker rather behaves like multiple cantilevers facing one another. Although here, the deflection is not limited by tensile stress in the membrane, however by the bending stiffness. Especially for larger deflections, the gaps between the cantilevers cause air to leak out. At least partially, this effect counteracts the sound pressure being produced.

One way to increase the deflection for a given speaker area is to decrease the restoring mechanical force of the system in which this area is included. The back-reaction of the air on a MEMS- based piezoelectric ultrasonic transducer is negligible. For a suspended membrane, the most important modes of deformation is bending and stretching of the membrane. Bending is the main mode of operation for deflections smaller than the thickness of the membrane. For larger deflections; however, the membrane area must stretch to accommodate the deflection. Stretching is generally a stiffer mode of operation than bending. Therefore, in practice, the stretching mode of operation limits the maximum amount of deflection achievable for a particular membrane.

One way to overcome stretching is to cut the membrane, i.e., to incorporate a slit in the design, as described above. However, this approach as also suggested by Cheng, H-H et al., On the design of piezoelectric MEMS microspeaker for the sound pressure level enhancement, Sensors and Actuators A 306 (2020) 111960, https://doi.Org/10.1016/j.sna.2020.111960 causes acoustical leakages, and, again, this phenomenon is more severe for lower frequencies.

Consequently the known MEMS based micro speaker designs leave room for further improvements.

SUMMARY

The object of the present invention is to offer an improved solution that enables higher sound pressures while maintaining the small physical size of the micro speaker and enabling high energy-efficiency.

According to the invention, the object is achieved by a MEMS- based micro speaker containing a support structure and a flexible membrane, which is connected to the support structure. The flexible membrane is configured to be deflected relative to the support structure in response to at least one control signal influencing at least one piezoelectric actuator mechanically linked to the flexible membrane. The flexible membrane has at least one slit that forms at least one opening between the flexible membrane and the support structure, and/or at least one opening in the flexible membrane itself. The speaker further contains at least one bridging member that mechanically interconnects the flexible membrane and the support structure being separated by the opening there between formed by the at least one slit, and/or at least two portions of the flexible membrane being separated by the at least one opening therein formed by the at least one slit. The at least one bridging member is arranged in a plane different from a plane in which the flexible membrane is arranged in relation to the support structure. Thus, for example, the at least one bridging member may form a general U-shaped crosssection profile together with first and second wall surfaces of the slit.

The above ME MS- based micro speaker is advantageous because the slit and the bridging member combine the advantages of a slitted membrane with an uninterrupted membrane in terms of high flexibility and air tightness. Namely, there is no acoustical leakage through the membrane while the membrane may be bent comparatively much.

Preferably, the at least one bridging member is configured to allow a bending of the flexible membrane along a line being parallel with the at least one slit while preventing fluid leakage through the at least one slit.

The above bridging member that mechanically interconnects the flexible membrane and the support structure is beneficial because it provides support to the flexible membrane, and at the same time, distributes the stress there from. Consequently, the speaker is less inclined to break.

According to one embodiment of the invention, the at least one slit specifically forms at least one division in the flexible membrane between at least two subdivisions thereof. This allows for substantial bending of the flexible membrane.

According to another embodiment of the invention, in the absence of the at least one control signal, a top surface of the flexible membrane is level with a first surface of the support structure, and a bottom surface of the at least one bridging member is level with a second surface of the support structure being parallel with and opposite to the first surface. In other words, an overall profile of the flexible membrane and the at least one bridging member has the same thickness as the support structure. Thereby, a very high degree of membrane bending is enabled.

According to yet another embodiment of the invention, in the absence of the at least one control signal, a top surface of the flexible membrane is level with a first surface of the support structure, and a bottom surface of the at least one bridging member is arranged at a level intermediate to the first surface and a second surface of the support structure being parallel with and opposite to the first surface. In other words, the overall profile of the flexible membrane and the at least one bridging member is thinner than the support structure. Thereby, somewhat less bending of membrane is enabled. However, less mass is added to the membrane, and less air needs to be compressed over the at least one bridging member than in the above-described embodiment.

According to still another embodiment of the invention, the ME MS- based micro speaker further contains at least one through slit that forms at least one opening between the flexible membrane and the support structure. The at least one through slit is configured to allow fluid to pass through the at least one opening. Thereby, the flexible membrane may for example be bendable in a first direction over a slit with a bridging member and in a second direction over a through slit. This allows for a large degree of flexibility when designing MEMS-based micro speakers.

According to further embodiments of the invention, the flexible membrane either has a general elliptical outline or a general polygonal outline in the plane in which the flexible membrane is arranged in relation to the support structure. Thus, for example, the flexible membrane may be round or rectangular-shaped, which is generally beneficial in terms of acoustic quality and spatial efficiency.

According to another embodiment of the invention, the at least one piezoelectric actuator covers the flexible membrane entirely and the at least one slit further forms at least one opening between the at least one piezoelectric actuator and the support structure and/or at least opening in the at least one piezoelectric actuator. This is beneficial with respect to controllability of the flexible membrane.

Alternatively, the flexible membrane comprises at least one uncovered area that is not covered by the at least one piezoelectric actuator. For instance, the at least one uncovered area may be encircled by at least one area of the flexible membrane being covered by the at least one piezoelectric actuator. Consequently, the movable mass of the design may be kept relatively low, which is beneficial with respect to energy-efficiency.

Further advantages, beneficial features and applications of the present invention will be apparent from the following description and the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now to be explained more closely by means of preferred embodiments, which are disclosed as examples, and with reference to the attached drawings.

Figures 1 a-d show a ME MS- based micro speaker according to a first embodiment of the invention;

Figures 2a-d show a MEMS-based micro speaker according to a second embodiment of the invention;

Figures 3a-c show a MEMS-based micro speaker according to a third embodiment of the invention;

Figure 4 shows a MEMS-based micro speaker according to a fourth embodiment of the invention;

Figure 5 shows a MEMS-based micro speaker according to a fifth embodiment of the invention;

Figures 6a-b show a MEMS-based micro speaker according to a sixth embodiment of the invention;

Figures 7a-b show a MEMS-based micro speaker according to a seventh embodiment of the invention;

Figures 8a-c show a MEMS-based micro speaker according to eighth and ninth embodiments of the invention; and

Figures 9a-b show a MEMS-based micro speaker according to a tenth embodiment of the invention.

DETAILED DESCRIPTION

In Figures 1 a to 1 d we see a MEMS-based micro speaker according to a first embodiment of the invention. Figures 1 a to 1 c show cross section views along a line AA in the top view of Figure 1 d. The MEMS-based micro speaker contains a support structure 110, a flexible membrane 120 and a piezoelectric actuator 130.

The flexible membrane 120 is connected to the support structure 110, and the flexible membrane 120 is configured to be deflected relative to the support structure 110 in response to a control signal CS influencing the piezoelectric actuator 130 that is mechanically linked to the flexible membrane 120. In this embodiment, the flexible membrane 120 includes a slit 140 forming an opening between the flexible membrane 120 and the support structure 110.

As is apparent in Figure 1 d, the flexible membrane 120 has a circular outline, and the slit 140 surrounds the flexible membrane 120 forming a circular-shaped trench between the support structure 110 and the flexible membrane 120. Specifically, a bridging member 150 mechanically interconnects the flexible membrane 120 and the support structure 110 being separated by an opening there between formed by the slit 140. The bridging member 150 is arranged in a plane different from a plane in which the flexible membrane 120 is arranged in relation to the support structure 110.

The bridging member 150 and respective first and second wall surfaces of the at least one slit 140 form a general U-shaped cross-section profile.

For example, in the absence of the control signal CS, a top surface of the flexible membrane 120 may be positioned in level with a first surface S1 of the support structure 110, and a bottom surface of the at least one bridging member 150 may be level with a second surface S2 of the support structure 110, which second surface S2 is parallel with and opposite to the first surface S1. Figure 1 a illustrates this situation, which thus represents an unbiased neutral position for the flexible membrane 120. Figures 1 b and 1 c illustrate how the flexible membrane 120 is deflected relative to the support structure 110 to attain positive and negative maximum positions in response to respective first and second extreme values of the control signal CS.

Alternatively, by including a constant bias component in the control signal CS, the neutral position for the flexible membrane 120 may be defined as any intermediate positioning of the flexible membrane 120 between the extreme positions illustrated in Figures 1 b and 1 c.

Figures 2a to 2d show a MEMS-based micro speaker according to a second embodiment of the invention. Figures 2a to 2c show cross section views along a line BB in Figure 2d. Here, the MEMS-based micro speaker contains a support structure 110 and a flexible membrane with two subdivisions 221 and 222 separated by a slit 240. In contrast to the embodiment above, the slit 140 thus forms an opening in the flexible membrane itself. Analogous to the embodiment illustrated in Figures 1 a to 1 d, the embodiment illustrated in Figures 2a to 2d, the flexible membrane has a circular outline; and as can be seen in Figure 2d, the slit 240 has a ring shape, which divides off the flexible membrane in an outer subdivision 221 being ring-shaped, and an inner subdivision 222 of circular shape. Therefore, the MEMS- based micro speaker further contains a first ring-shaped piezoelectric actuator 231 that is mechanically linked to the outer subdivision 221 of the flexible membrane and a second circular piezoelectric actuator 232 that is mechanically linked to the inner subdivision 222 of the flexible membrane. The piezoelectric actuators 231 and 232 are controllable by first and second control signals CS1 and CS2 respectively to cause the flexible membranes 221 and 222 respectively to deflect relative to the support structure 110.

A bridging member 250 mechanically interconnects the two portions 221 and 222 of the flexible membrane, which are separated by the opening therein formed by the slit 240. The bridging member 250 is arranged in a plane different from a plane in which the flexible membrane portions 221 and 222 respectively are arranged in relation to the support structure 110.

The bridging member 250 and respective first and second wall surfaces of the at least one slit 240 form a general U-shaped cross-section profile.

In the absence of the control signals CS1 and CS2, a top surface of the flexible membrane 120 may be positioned in level with a first surface of the support structure 110, and a bottom surface of the at least one bridging member 150 may be level with a second surface of the support structure 110, which second surface is parallel with and opposite to the first surface analogous to what is described above referring to Figures 1a to 1 c. Here, Figure 2a illustrates a situation, where the flexible membrane portions 221 and 222 are arranged in unbiased neutral positions. Figures 2b and 2c illustrate how the flexible membrane portions 221 and 222 are deflected relative to the support structure 110 to attain positive and negative maximum positions in response to respective first and second extreme values of the control signal CS. Of course, by including constant bias components in the control signals CS1 and CS2, the neutral position for the flexible mem- brane portions 221 and 222 may be defined as any intermediate positioning of the flexible membrane portions 221 and 222 between the extreme positions illustrated in Figures 2b and 2c.

Figures 3a to 3c and 2d show a ME MS- based micro speaker according to a third embodiment of the invention. Figures 3a to 3c show cross section views along a line BB in the top view of Figure 2d. In Figures 3a to 3c, all elements that also occur in Figures 2a to 2c designate the same elements as described above with reference to Figures 2a to 2c. In short, the difference between the second and third embodiments of the invention, is that in the third embodiment illustrated in Figures 3a to 3c and 2d, the bottom surface of the bridging member 350 is arranged at a level intermediate to the first surface S1 and the second surface S2 of the support structure 110 when the flexible membrane portions 221 and 222 are arranged in the neutral position of Figure 3a.

This may be advantageous compared to the second embodiment of Figures 2a to 2d because the general U-shaped cross-section profile of the bridging member 350 and the first and second wall surfaces of the slit 340 imply less added mass to the flexible membrane portions 221 and 222. Moreover, less air needs to be compressed over the bridging member 350 in the slit 340 when deflecting the flexible membrane portions 221 and 222 to the position shown in Figure 3c.

Figure 4 shows a top view of a MEMS-based micro speaker according to a fourth embodiment of the invention. Here, a circular flexible membrane is arranged in the support structure 110 and a ring-shaped piezoelectric actuator 430 is mechanically linked to the flexible membrane. The piezoelectric actuator 430 is configured to cause the flexible membrane to deflect relative to the support structure 110 in response to a control signal CS. A slit 440 forms an opening between the flexible membrane and the support structure 110 and a bridging member (not shown) mechanically interconnects the flexible membrane and the support structure 110 being separated by the opening there between formed by the slit 440. Thus, the overall design is similar to that of the first embodiment illustrated in Figures 1 a to 1 d. However, in the fourth embodiment of Figure 4, there are first and second uncovered areas 421 and 422 respectively of the flexible membrane, which first and second uncovered areas 421 and 422 are not covered by the piezoelectric actuator 430. Hence, in comparison to the first embodiment, the movable mass is somewhat lower, which is beneficial with respect to energy-efficiency.

Figure 5 shows a top view of a ME MS- based micro speaker according to a fifth embodiment of the invention. Here, a circular flexible membrane is arranged in the support structure 110. The flexible membrane is divided into two subdivisions being separated by a slit 540. A bridging member (not shown) mechanically interconnects the two subdivisions of flexible membrane. Two concentrically arranged ring-shaped piezoelectric actuators 531 and 532 respectively are mechanically linked to the respective subdivisions of the flexible membrane. The overall design is thus similar to that of the second embodiment illustrated in Figures 2a to 2d. However, analogous to the above, there is an uncovered area 520 of the inner subdivision of the flexible membrane, which uncovered area 520 is not covered by the piezoelectric actuator 532. Hence, in comparison to the second embodiment, the movable mass is somewhat lower, which is beneficial with respect to energy-efficiency.

Figures 6a and 6b show a MEMS-based micro speaker according to a sixth embodiment of the invention. Figure 6a shows a cross section view along a line CC in the top view of Figure 6b. Here, a rectangular-shaped flexible membrane 620 is connected to a support structure 110, and the flexible membrane 620 is configured to be deflected relative to the support structure 1 10 in response to a control signal CS that influences a piezoelectric actuator 630 mechanically linked to the flexible membrane 620. The flexible membrane 620 contains first and second slits 641 and 642 that form respective openings between the flexible membrane 620 and the support structure 110. First and second bridging members 651 and 652 mechanically interconnect the flexible membrane 620 and the support structure 110 being separated by the opening there between formed by the first and second slits 641 and 642. Each of the first and second bridging members 651 and 652 is arranged in a plane different from a plane in which the flexible membrane 620 is arranged in relation to the support structure 110.

Moreover, the MEMS-based micro speaker contains first and second through slits 661 and 662 that form first and second openings respectively between the flexible membrane 620 and the support structure 110. The first and second through slits 661 and 662 are configured to allow fluid, typically air, to pass through the first and second openings respectively. Such openings may enhance the controllability of the flexible membrane 620 in response to the control signal CS fed to the piezoelectric actuator 630.

Figures 7a and 7b show a MEMS-based micro speaker according to a seventh embodiment of the invention, where a rectangular-shaped flexible membrane is connected to a support structure 110. Figure 7a shows a cross section view along a line DD in the top view of Figure 7b. Here, the flexible membrane is divided off into three subdivisions 721 , 722 and 723 by first and second slits 741 and 742 respectively. Further, each subdivision 721 , 722 and 723 is configured to be deflected relative to the support structure 110 in response to a respective control signal CS1 , CS2 and CS3 to piezoelectric actuators 731 , 732 and 733 mechanically linked to the subdivision 721 , 722 and 723 respectively. Thus, here, first and second slits 741 and 742 form openings in the flexible membrane itself. Analogous to the above, first and second bridging members 751 and 752 mechanically interconnect the different portions of the flexible membrane separated by the openings therein formed the first and second slits 741 and 742. Specifically, the first bridging member 751 mechanically interconnect first and second subdivisions of the flexible membrane, and the second bridging member 752 mechanically interconnect second and third subdivisions of the flexible membrane. The first and second bridging members 751 and 752 are arranged in a plane different from a plane in which the subdivisions 721 , 722 and 723 of the flexible membrane are arranged in relation to the support structure 110.

Additionally, analogous to the sixth embodiment illustrated in Figures 6a and 6b, the ME MS- based micro speaker contains first and second through slits 761 and 762 that form first and second openings respectively between the flexible membrane 720 and the support structure 110. The first and second through slits 761 and 762 are configured to allow fluid, typically air, to pass through the first and second openings respectively. Such openings may enhance the controllability of the flexible membrane 720 in response to the control signal CS fed to the piezoelectric actuator 730.

Figures 8b and 8c show top views of a MEMS-based micro speaker according to eighth and ninth embodiments respectively of the invention, and Figure 8a shows a cross section view along lines EE and FF in Figures 8b and 8c respectively.

In both these embodiments, a flexible membrane 820 is connected to a support structure 110. The flexible membrane 820 is configured to be deflected relative to the support structure 110 in response to a control signal CS that influences a ring-shaped piezoelectric actuator 830 mechanically linked to the flexible membrane 820, which ring-shaped piezoelectric actuator 830 encircles a central uncovered area of the flexible membrane 820 similar to the fourth and fifth embodiments described above.

The flexible membrane here has four slits 841 , 842, 843 and 844, which form respective openings in the flexible membrane 820. Bridging members mechanically interconnect the portions of the flexible membrane 820 that are separated by the openings therein formed by the slits 841 , 842, 843 and 844. Figure 8a shows one of these bridging members 852 in the cross section view along a line EE in Figure 8b. Each of said bridging members is arranged in a plane different from a plane in which the flexible membrane 820 is arranged in relation to the support structure 110.

Figure 8c illustrates the ninth embodiment of the invention. Here, the innermost portions of the slits 841 , 842, 843 and 844 constitute through slits 860 forming openings in the flexible membrane 820 which through slits 860 interconnect the slits 841 , 842, 843 and 844 and are configured to allow fluid, e.g. air, to pass through the at least one opening. Nevertheless, at the outermost portions of the slits 841 , 842, 843 and 844, bridging members mechanically interconnect the portions of the flexible membrane 820 that are separated by the openings therein formed by the slits 841 , 842, 843 and 844 in an manner analogous to what is described above.

Figures 9a and 9b show a cross section view and a top view respectively of a ME MS- based micro speaker according to a tenth embodiment of the invention, where a flexible membrane 920 is connected to a support structure 1 10, which flexible membrane 920 is configured to be deflected relative to the support structure 110 in response to at least one control signal CS influencing a ring-shaped piezoelectric actuator 930 mechanically linked to the flexible membrane 920.

Similar to the above, the flexible membrane 920 contains slits 941 and 942 that form openings in the flexible membrane 920, and bridging members mechanically interconnect the portions of the flexible membrane 920 that are separated by the openings therein formed by the slits 941 and 942. Figure 9a shows one of these bridging members 952 in a cross section view along a line GG in Figure 9b. Further, each of the bridging members is arranged in a plane different from a plane in which the flexible membrane 920 is arranged in relation to the support structure 110. Hence, the bridging members mechanically interconnect said portions of the flexible membrane 920 being separated by the opening therein formed by the slits 941 and 942.

Although not being specifically illustrated in any drawings, the flexible membranes of the ME MS- based micro speaker according the embodiments of Figures 4 to 9 are controllable between first and second extreme positions in response to control signals as exemplified in Figures 1 a/1 b, 2a/2b and 3a/3b respectively.

Preferably, in all the above embodiments, each bridging member 150, 250, 350, 651 , 652, 751 , 752, 852; and 952 respectively is configured to allow a bending of the flexible membrane 120, 221 , 222, 420, 520, 620;721 , 722, 723, 820 and 920 respectively along a line being parallel with the slit in question 140, 240, 340, 440, 540, 641 , 642, 741 , 742, 841 , 842, 941 and 942 respectively while preventing fluid leakage through the slit.

Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

The term “comprises/comprising” when used in this specification is taken to specify the presence of stated features, integers, steps or components. The term does not preclude the presence or addition of one or more additional elements, features, integers, steps or components or groups thereof. The indefinite article "a" or "an" does not exclude a plurality. In the claims, the word “or” is not to be interpreted as an exclusive or (sometimes referred to as “XOR”). On the contrary, expressions such as “A or B” covers all the cases “A and not B”, “B and not A” and “A and B”, unless otherwise indicated. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

It is also to be noted that features from the various embodiments described herein may freely be combined, unless it is explicitly stated that such a combination would be unsuitable.

The invention is not restricted to the described embodiments in the figures, but may be varied freely within the scope of the claims.

Claims

1. A ME MS- based micro speaker comprising: a support structure (110), and a flexible membrane (120; 221 , 222; 420; 520; 620; 721 , 722, 723; 820; 920) connected to the support structure, which flexible membrane is configured to be deflected relative to the support structure in response to at least one control signal (CS, CS1 , CS2, CS3) influencing at least one piezoelectric actuator (130; 231 , 232; 430; 531 , 532; 630; 731 , 732, 733; 830; 930) mechanically linked to the flexible membrane, the flexible membrane comprising at least one slit (140; 240; 340; 440; 540; 641 , 642; 741 , 742; 841 , 842, 843, 844; 941 , 942) forming at least one of: at least one opening between the flexible membrane and the support structure, and at least one opening in the flexible membrane, characterized in that the speaker comprises at least one bridging member (150; 250; 350; 651 , 652; 751 , 752; 852, 952) mechanically interconnecting at least one of: the flexible membrane and the support structure separated by the opening there between formed by the at least one slit, and at least two portions of the flexible membrane separated by the at least one opening therein formed by the at least one slit, which at least one bridging member is arranged in a plane different from a plane in which the flexible membrane (120; 221 , 222; 420; 520; 620; 721 , 722, 723; 820; 920) is arranged in relation to the support structure (110).

2. The MEMS-based micro speaker according to claim 1 , wherein the least one slit (140; 240; 340; 440; 540; 641 , 642; 741 , 742) forms at least one division in the flexible membrane between at least two subdivisions (221 , 222; 721 , 722, 723) thereof.

3. The MEMS-based micro speaker according to any one of claims 1 or 2, wherein, in the absence of the at least one control signal (CS, CS1 , CS2, CS3): a top surface of the flexible membrane is level with a first surface (S1 ) of the support structure (110), and a bottom surface of the at least one bridging member (150; 250; 350; 651 , 652; 751 , 752) is level with a second surface (S2) of the support structure (110) being parallel with and opposite to the first surface (S1 ).

4. The ME MS- based micro speaker according to any one of claims 1 or 2, wherein, in the absence of the at least one control signal (CS, CS1 , CS2, CS3): a top surface of the flexible membrane is level with a first surface (S1 ) of the support structure (110), and a bottom surface of the at least one bridging member (350) is arranged at a level intermediate to the first surface (S1) and a second surface (S2) of the support structure (110) being parallel with and opposite to the first surface (S1 ).

5. The MEMS-based micro speaker according to any one of the preceding claims, wherein the at least one bridging member (150; 250; 350; 651 , 652; 751 , 752; 852; 952) is configured to allow a bending of the flexible membrane (120; 221 , 222; 420; 520; 620; 721 , 722, 723; 820; 920) along a line being parallel with the at least one slit while preventing fluid leakage through the at least one slit.

6. The MEMS-based micro speaker according to any one of the preceding claims, wherein the at least one bridging member (150; 250; 350; 651 , 652; 751 , 752; 852; 952) and respective first and second wall surfaces of the at least one slit (140; 240; 340; 440; 540; 641 , 642; 741 , 742; 841 , 842, 843, 844; 941 , 942) form a general U-shaped cross-section profile.

7. The MEMS-based micro speaker according to any one of the preceding claims, further comprising at least one through slit (661 , 662; 761 , 762) forming at least one opening between the flexible membrane (620; 721 , 722, 723) and the support struc- ture (110), which at least one through slit is configured to allow fluid to pass through the at least one opening.

8. The ME MS- based micro speaker according to any one of the preceding claims, wherein the flexible membrane (120; 221 , 222; 421 , 422; 520) has a general elliptical outline in the plane in which the flexible membrane is arranged in relation to the support structure (110).

9. The MEMS-based micro speaker according to any one of the claims 1 to 7, wherein the flexible membrane (620; 721 , 722, 723) has a general polygonal outline in the plane in which the flexible membrane is arranged in relation to the support structure (110).

10. The MEMS-based micro speaker according to claim 9, wherein the at least one piezoelectric actuator (130; 231 , 232; 630; 731 , 732, 733) covers the flexible membrane (120; 221 , 222; 620; 721 , 722, 723) entirely and the at least one slit (140; 240; 340; 440; 540; 641 , 642; 741 , 742) further forms at least one of: at least one opening between the at least one piezoelectric actuator and the support structure (110), and at least opening in the at least one piezoelectric actuator.

11. The MEMS-based micro speaker according to any one of the claims 1 to 9, wherein the flexible membrane comprises at least one uncovered area (421 , 422; 520; 820; 920) that is not covered by the at least one piezoelectric actuator (430; 531 , 532; 830; 930).

12. The MEMS-based micro speaker according to claim 11 , wherein the at least one uncovered area (422; 520; 820; 930) is encircled by at least one area of the flexible membrane being covered by the at least one piezoelectric actuator (430; 531 , 532; 830; 930).