A micro-electromechanical-system based sound producing device

Abstract

A MEMS-based sound producing device has a support structure (S) that surrounds a cavity. At least three flexible cantilevers are attached to the support structure (S) along a respective base side (b1, b2, b3, b4). Each cantilever has side edges (e11, e12; e21, e22; e31, e32; e41, e42) being movable relative to the support structure (S). The flexible cantilevers are arranged over the cavity such that each of them adjoins two neighboring flexible cantilevers along respective side edges. The flexible cantilevers together cover the cavity and each of them is configured to be deflected relative to the support structure (S) in response to a control signal influencing a respective piezoelectric actuator (A1, A2, A3, A4) mechanically linked to the flexible cantilever. A set of links (L1, L2, L3, L4) is arranged between the flexible cantilevers, such that the links (L1, L2, L3, L4) exclusively couple the side edges (e11, e21; e22, e32; e31, e41; e42, e12) of two neighboring flexible cantilevers mechanically to one another.

Description

[0001] A Micro-Electromechanical-System based Sound Producing Device

[0002] TECHNICAL FIELD

[0003] The present invention relates generally to miniature-sized sound generators. Especially, the invention relates to a micro-electro- mechanical-system (MEMS) based sound producing device according to the preamble of claim 1.

[0004] BACKGROUND

[0005] 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. Conversely, an increased vibration amplitude thus allows for smaller speakers at the same level of performance.

[0006] However, large vibration amplitudes are associated with various problems. Namely, to enable large vibration amplitudes, the diaphragm of a speaker for example in the form of a MEMS based sound producing device typically includes two or more separate elements, e.g. cantilevers that are mechanically connected to respective piezoelectric actuators. For larger vibration amplitudes, the gaps between the cantilevers become relatively large, which causes acoustic leakage that reduce the efficiency.

[0007] The acoustic leakage, as such, may be avoided by the strategy for obtaining high sound-pressure-level MEMS speakers via a rigid-flexible vibration coupling mechanism of unsealed piezoelectric cantilevers and a sealed Parylene C membrane described in the article Wang, Q., et al., Obtaining High SPL Piezoelectric MEMS Speaker via a Rigid-flexible Vibration Coupling Mechanism, Journal of Microelectromechanical Systems, Volume 30, No. 5, 2021 , DOI: 10.1 109 / JMEMS.2021 .3087718. Here, the speaker comprises six identical triangular vibration cantilevers elements arranged to form a regular hexagonal vibration membrane with a side length of 2 mm. The speaker has a PZT thin film layer and Pt layers as upper and lower electrodes; its elastic actuator layer is a SiO2 / Si / SiO2 / Si multilayer composite film, and its substrate is an SOI one. To form the rigid-flexible-coupling sealed vibration membrane, Parylene C is deposited on the vibration cantilever surfaces and the sidewalls and bottoms of the etched gaps before etching the back cavity. Compared with the SPL of the designed speaker with no deposited flexible Parylene C at a driving voltage of 2 V, the SPL produced by the speaker with the rigid-flexible-coupling sealed vibration membrane increased by 3 - 12.2 dB.

[0008] Nevertheless, irrespective of whether a flexible membrane is used, yet additional problems may arise, inter alia relating to synchronization issues. Although the same driving signal is fed to all the piezoelectric actuators, the cantilevers may behave slightly differently due to minor manufacturing differences between the cantilevers. As a result, the cantilever movements may not be perfectly synchronized with one another. One way to reduce this problem is to interconnect the cantilevers mechanically, for instance as shown in US 11 ,057,716. Here, a sound producing device includes a base and at least one chip disposed on the base. The chip includes at least one membrane and at least one actuator. The membrane includes a coupling plate and at least one spring structure connected to the coupling plate. The actuator is configured to receive a driving signal corresponding to an input audio signal to actuate the membrane, and the input audio signal and the driving signal have an input audio band which has an upper bound at a maximum frequency. The spring structure is situated between the coupling plate and the actuator. The membrane has a first resonance frequency higher than the maximum frequency.

[0009] WO 2018 / 035486 shows a piezoelectric, acoustic device compri- sing: a substrate; a plurality of plates, each plate comprising a first electrode layer, a piezoelectric layer and a second electrode layer, wherein the piezoelectric layer is in between the first electrode layer and the second electrode layer, and wherein, for each plate, a base of the plate is affixed to the substrate and a remaining portion of the plate is unaffixed to the substrate; and a plate coupling structure affixed to a first one of the plates and to a second one of the plates, wherein the plate coupling structure at least partly covers a gap between a first edge of the first one of the plates and a second edge of the second one of the plates, wherein the first edge faces the second edge; wherein the plate coupling structure comprises a mismatch reduction element and a stopping element, wherein the mismatch reduction element is configured to reduce an amount of mismatch in deflection of the first one of the plates and the second one of the plates, and wherein the stopping element is configured to enable the mismatch.

[0010] CN 209659620 II discloses a piezoelectric type MEMS microphone. The piezoelectric loudspeaker comprises a substrate with a back cavity and a piezoelectric vibrating diaphragm arranged on the substrate, the piezoelectric vibrating diaphragm is divided into a plurality of independent diaphragms; each diaphragm comprises a fixed end connected with the substrate and a free end connected with the fixed end and suspended above the back cavity; the free end comprises two side edges formed along a parting line; wherein the two side edges of the same diaphragm are intersected, the side edges of two adjacent diaphragms are in contact with each other or are arranged at intervals, the piezoelectric MEMS microphone further comprises a buckle structure used for connecting the side edges of the two adjacent diaphragms, and the side edges of at least two diaphragms are connected through one or more buckle structures. Compared with the prior art, the piezoelectric MEMS microphone provided by the utility model has the advantages that the free end of the diaphragm is limited in the same plane through the arrangement of the buckle structure, the uniformity of product performance is improved, and the anti-falling performance is better.

[0011] US 2020 / 0213770 teaches another piezoelectric microphone. Here, the microphone includes a substrate having a back cavity and a piezoelectric cantilever diaphragm fixed to the substrate. The piezoelectric cantilever diaphragm includes a plurality of diaphragm flaps, and each of the diaphragm flaps has one end fixed to the substrate and another end suspended above the back cavity. Every two adjacent ones of the diaphragm flaps are spaced apart to form a gap. The piezoelectric microphone further comprises an elastically stretchable member connecting the two adjacent diaphragm flaps, and the elastically stretchable member is provided between at least one set of two adjacent ones of the diaphragm flaps. The piezoelectric microphone of the present disclosure has better performance.

[0012] A piezoelectric microphone is a transducing device that is configured to convert sound into an electrical signal. Obviously, such a device is ill suited to produce sound based on an electrical signal. Namely, when designing a microphone accuracy is the most important parameter, i.e. that the induced diaphragm movements are as precise as possible in the sense that the diaphragms are influenced as little as possible by any other factors than the incoming sound waves. However, the magnitude of the diaphragm movements is essentially irrelevant. In a speaker / sound producing device essentially the opposite is true. Here, the magnitude of the induced movements is the most important parameter, whereas the accuracy of these movements is less critical.

[0013] Although for example the above-proposed spring structure and coupling plate may be beneficial in terms of maintaining synchronous actuator movements, the coupling plate introduces inertia in the system, which, in turn, leads to energy losses.

[0014] SUMMARY

[0015] The object of the present invention is therefore to offer an en- ergy-eff icient MEMS based sound producing device that allows a high total membrane deflection while mitigating the above problems.

[0016] According to the invention, the object is achieved by a MEMS- based sound producing device including a support structure, at least three flexible cantilevers and a set of links. The support structure surrounds a cavity, and each of the at least three flexible cantilevers has a respective base side along which the flexible cantilever is attached to the support structure. Each of the flexible cantilevers further has side edges that are movable relative to the support structure. The flexible cantilevers are arranged over the cavity, such that each of them adjoins two neighboring flexible cantilevers along respective side edges and the flexible cantilevers together cover the cavity. Each of the flexible cantilevers is configured to be deflected relative to the support structure in response to a control signal influencing a respective piezoelectric actuator that is mechanically linked to each of the flexible cantilevers. The piezoelectric actuator may cover either a part of or the entire upper and / or lower surface of the flexible cantilever to which it is mechanically linked. The links in the set of links are arranged between the flexible cantilevers such that exclusively the side edges of two neighboring flexible cantilevers of said flexible cantilevers are mechanically coupled to one another. Specifically, each of the flexible cantilevers is connected to two other ones of the flexible cantilevers via the links.

[0017] The above MEMS-based sound producing device is advantageous because it avoids the central coupling plate and enables larger displacements between the cantilevers while keeping their movements synchronized to one another.

[0018] According to one embodiment of the invention, at least a subset of the links in the set of links are constituted by respective unbroken sections of said flexible cantilevers between the respective flexible cantilevers being mechanically coupled to one another. For example, the slits between different flexible cantilevers may not continue all the way to the support structure such that two or more flexible cantilevers share a common base side. As a result, the flexible cantilevers become relatively firmly interconnected, especially if an extension of the unbroken section is large compared to a length of the slit between the flexible cantilevers.

[0019] According to another embodiment of the invention, each of the links between the flexible cantilevers includes a respective spring structure, which, in turn, is formed by one or more cut-outs in at least one of the flexible cantilevers between which the link is arranged. Such a spring structure design is advantageous because it is uncomplicated to implement. It is also straightforward to adjust the spring constant of the link by varying the number of cutouts and / or the respective lengths of the cut-outs.

[0020] Preferably, one or more of the cut-outs contains a stress-relief cut-out section at a terminating end to reduce the risk of mechanical damages in this region.

[0021] According to yet another embodiment of the invention, respective side edges of the flexible cantilevers form an opening between themselves, which opening is located in an area over the cavity. Such an opening is beneficial because it allows comparatively large cantilever deflections. To keep the acoustic leakage moderate, however, the opening should be relatively small.

[0022] Preferably, for synchronization reasons, said opening should be positioned centrally over the cavity.

[0023] According to still another embodiment of the invention, the cavity has a general polygonal outline in a plane covered by the cantilevers. Thus, the cavity may for example have a regular outline in the form of a triangle, a square or a hexagon. This is generally advantageous for manufacturing reasons and considering acoustic performance.

[0024] According to another embodiment of the invention, each of the cantilevers has such an outline that its width parallel to the base side generally tapers off from the base side towards an edge point on the respective cantilever, which edge point is furthest away from the base side. For example, the cantilever outline may be triangular. Such a cantilever outline is advantageous, since it harmonizes with the general polygonal outline of the cavity in a straightforward manner.

[0025] According to a further embodiment of the invention, each of the cantilevers instead has such an outline that its width parallel to the base side is wider than the base side at least at one position between the base side and an edge point of the cantilever, which edge point is furthest away from the base side. This type of cantilever outline is advantageous because it allows for comparatively large energy exchange between the cantilevers and the surrounding air.

[0026] According to one embodiment of the invention, the ME MS- based sound producing device further contains a flexible polymer membrane that covers the flexible cantilevers and a portion of the support structure adjoining the respective base sides of the flexible cantilevers. Thus, any acoustic leakages between the cantilevers are efficiently prevented.

[0027] According to another embodiment of the invention, the flexible polymer membrane is elastic and the flexible polymer membrane is arranged over the flexible cantilevers and the support structure to stretch in response to deflecting the flexible cantilevers relative to the support structure.

[0028] Alternatively, or additionally, the flexible polymer membrane is arranged such that in a first positioning of the flexible cantilevers in response to at least one first signal value of the control signal, the flexible polymer membrane is folded to form at least one fold between at least two of said flexible cantilevers. In a second positioning of the flexible cantilevers in response to at least one second signal value of the control signal, the at least one fold is unfolded due to a deflection of the flexible cantilevers relative to the support structure. Thereby, the flexible polymer membrane may be arranged to attain a good overall energy efficiency with respect to acoustic leakages and mechanical losses due to the interaction between the flexible cantilevers and the flexible polymer membrane.

[0029] According to further embodiments of the invention, respective side edges of each of the flexible cantilevers form an opening between themselves, which opening is located in an area over the cavity, where the flexible polymer membrane covers the opening, and the opening may or may not be positioned centrally over the cavity. This design is advantageous because it leaves room for additional optimizing of the MEMS based sound producing device being implemented.

[0030] Further advantages, beneficial features and applications of the present invention will be apparent from the following description and the dependent claims.

[0031] BRIEF DESCRIPTION OF THE DRAWINGS

[0032] 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.

[0033] Figures 1 a-b show top and side cross-section views respectively of a MEMS-based sound producing device according to one embodiment of the invention;

[0034] Figures 2-3 show top views of a MEMS-based sound producing device according to embodiments of the invention;

[0035] Figures 4a-e exemplify designs of spring structures mechanically interconnecting two neighboring flexible cantilevers according to different embodiments of the invention;

[0036] Figures 5-7 show top views of a MEMS-based sound produ- cing device according to embodiments of the in vention;

[0037] Figures 8a-b show side cross-section views of a MEMS-based sound producing device according to one embodiment of the invention, where a flexible polymer membrane covers the flexible cantilevers and a portion of the support structure;

[0038] Figures 9a-c show top and side cross-section views of a MEMS-based sound producing device according to one embodiment of the invention, where a flexible polymer membrane covers the flexible cantilevers, a portion of the support structure as well as a central opening formed between the flexible cantilevers;

[0039] Figures 10-13 show top views of a MEMS-based sound producing device according to embodiments of the invention, where the piezoelectric actuators cover the flexible cantilevers to various extent; and

[0040] Figures 14a-b illustrate stress-relief cut-out sections according to embodiments of the invention.

[0041] DETAILED DESCRIPTION

[0042] In Figure 1 a, we see a top view of a MEMS-based sound producing device according to one embodiment of the invention. Figure 1 b shows a side cross-section view of the MEMS-based sound producing device in Figure 1 a along AA.

[0043] The MEMS-based sound producing device contains a support structure S that surrounds a cavity V.

[0044] According to embodiments of the invention, the cavity V has a general polygonal outline in a plane covered by the cantilevers. Le. the cavity V may for example have a square outline as illustrated in Figure 1 a, or a general triangular outline as illustrated in Figures 10 and 11 . Of course, alternative outlines of the cavity V are likewise conceivable, such as hexagonal, octagonal or decagonal shapes. Although it is not technically necessary, for practical reasons, it is preferable that the general polygonal outline of the cavity V has a regular shape.

[0045] The ME MS- based sound producing device further contains at least three flexible cantilevers and a set of links L1 , L2, L3 and L4 arranged between the flexible cantilevers, wherein each link couples one of the flexible cantilevers mechanically to another one of said flexible cantilevers.

[0046] In the embodiment illustrated in Figures 1 a and 1 b, the MEMS- based sound producing device has four flexible cantilevers of which two, C1 and C3 respectively, are visible in Figure 1 b. Each of the flexible cantilevers has a respective base side b1 , b2, b3 and b4 along which the flexible cantilever is attached to the support structure S. Each of the flexible cantilevers further has side edges e11 , e12; e21 , e22; e31 , e32 and e41 , e42 respectively that are movable relative to the support structure S.

[0047] The flexible cantilevers are arranged over the cavity V, such that each of the flexible cantilevers adjoins two neighboring flexible cantilevers along respective side edges and said flexible cantilevers together cover the cavity V. Moreover, each of the flexible cantilevers is configured to be deflected relative to the support structure S in response to a control signal influencing a respective piezoelectric actuator that is mechanically linked to each of the flexible cantilevers. Figures 1 a and 1 b illustrate four such piezoelectric actuators in the form of A1 , A2, A3 and A4, which each is mechanically linked to one of the flexible cantilevers.

[0048] The links L1 , L2, L3 and L4 are arranged between the flexible cantilevers, wherein each link couples one of the flexible cantilevers mechanically to another one of the flexible cantilevers. Specifically, according to the invention, the links L1 , L2, L3 and L4 are arranged to exclusively couple the side edges e11 , e21 ; e22, e32; e31 , e41 and e42, e12 respectively of two neighboring flexible cantilevers mechanically to one another. In other words, in the four-cantilever embodiment illustrated in Figure 1 a, any given cantilever is only directly linked to the two cantilevers that have a respective side edge adjoining the cantilever in question, and not directly linked to the cantilever being directly opposite to the cantilever in question. In other words, each of the flexible cantilevers is connected to exactly two other flexible cantilevers via the links L1 , L2, L3 and L4.

[0049] Figure 1 b illustrates how the cantilevers C1 and C3, in response to the control signal to piezoelectric actuators A1 and A3 being mechanically linked to the cantilevers C1 and C3 respectively, may be deflected up and down relative to the support structure S above the cavity V, for example to produce sound. In Figures 1 a and 1 b, the piezoelectric actuators A1 and A3 are arranged on a top side of the cantilevers C1 and C3. However, according to embodiments of the invention, the piezoelectric actuators A1 and A3 may equally well be arranged on a bottom side of the cantilevers C1 and C3, or on both the top and bottom sides thereof. Moreover, in Figures 1 a and 1 b, the piezoelectric actuators A1 and A3 cover the entire top side surface of the cantilevers C1 and C3. As will be discussed below with reference to Figures 11 , 12 and 13, according to other embodiments of the invention, the piezoelectric actuators may cover only a portion of the respective cantilever surface.

[0050] In Figure 1 a, each of the cantilevers has such an outline that its width parallel to the base side b1 , b2, b3 and b4 respectively generally tapers off from the base side b1 , b2, b3 and b4 towards an edge point ep1 , ep2, ep3 and ep4 on the respective cantilever, which edge point ep1 , ep2, ep3 and ep4 is furthest away from the base side b1 , b2, b3 and b4 respectively. This means that the cantilevers may have triangular outlines. However, alternative outlines are also conceivable, e.g., where the side edges e11 , e12; e21 , e22; e31 , e32 and e41 , e42 are curved, and the side edges of neighboring flexible cantilevers may or may not be parallel to one another. Figure 2 shows a top view of a MEMS-based sound producing device according to one embodiment of the invention. In Figure 2, all reference signs that also occur in Figures 1 a and / or 1 b designate the same parts / components as described above with reference to these figures. In Figure 2, the links between the flexible cantilevers are constituted by respective unbroken sections L12, L22, L32 and L42 of the flexible cantilevers, which unbroken sections L12, L22, L32 and L42 are located between the respective flexible cantilevers that are mechanically coupled to one another. This design results in that the flexible cantilevers are relatively firmly linked to one another. In other words, the flexible cantilevers will be deflected in a highly synchronized manner in response to the control signal to the piezoelectric actuators A1 , A2, A3 and A4 respectively that are mechanically linked to the flexible cantilevers. This is especially the case if an extension of the unbroken sections L12, L22, L32 and L42 is large in comparison the lengths a set of cut-outs, which form the side edges e11 , e21 ; e22, e32; e31 , e41 and e42, e12 between the flexible cantilevers.

[0051] According to embodiments of the invention, only a subset of the links in the set of links may be constituted by the unbroken sections L12, L22, L32 and / or L42. The remaining links in the set of links may be constituted by one or more other types of links, for example as exemplified below with reference to Figures 3 and 4a to 4e. In any case, analogous to the above, each of the flexible cantilevers is connected to exactly two other flexible cantilevers via the links L12, L22, L32 and L42.

[0052] Figure 2 also shows an opening OP, which is located in an area Y over the cavity V and which opening OP is formed between the side edges e11 , e21 ; e22, e32; e31 , e41 and e42, e12 respectively of the flexible cantilevers. In particular, the opening OP is here positioned centrally over the cavity V. Such an opening OP is advantageous because it leaves room for additional optimizing of the MEMS based sound producing device. Further, the opening OP enables comparatively large deflections of the flexible cantilevers over the cavity V.

[0053] Figure 3 shows a top view of a MEMS-based sound producing device according to one embodiment of the invention. In Figure 3, all reference signs that also occur in Figures 1 a, 1 b and / or Figure 2 designate the same parts / components as described above with reference to these figures. In the embodiment of the invention shown in Figure 3, each of said links L1 , L2, L3 and L4 that couples the flexible cantilevers mechanically to another one includes a respective spring structure, which is formed by one or more cut-outs in at least one of the flexible cantilevers between which the link is arranged. More precisely, each of the flexible cantilevers is connected to exactly two other flexible cantilevers via the links L2, L2, L2 and L4 respectively.

[0054] Figures 4a, 4b, 4c, 4d and 4e exemplify different designs of links L forming such spring structures according to embodiments of the invention. Specifically, Figure 4a shows the spring-structure design of Figure 3 in somewhat further detail.

[0055] Here, a first cut-out s1 is merely a rectilinear section between the two cantilevers. A second cut-out s2 constitutes a continuation of the rectilinear section between the two cantilevers. However, near an end of the first cut-out s1 , the second cut-out s2 bifurcates into first and second branches extending parallel to the first cut-out s1 , where the first branch extends on a first side of the first cut-out s1 and the second branch extends on a second side of the first cut-out s1. The two cantilevers are thereby mechanically connected to one another via a spring structure formed in a zone where the first and second branches of the second cut-out s2 extend parallel to the first cut-out s1 . In simplified terms, a spring constant of the spring structure may be adjusted by varying a length of the zone where the first and second branches of the second cut-out s2 extend parallel to the first cut-out s1 , which zone is symbolized by F in Figure 4a.

[0056] Figure 4b shows a spring structure, where a first cut-out s3 in the form of a rectilinear section between the two cantilevers is terminated by an arrow-shaped cut-out, which meets a matching second cut-out s4 that also contains a continuation of rectilinear section between the two cantilevers. Here, the spring constant of the spring structure may be adjusted by varying the lengths F of the arrow-shaped portions of the first and second cut-outs s3 and s4.

[0057] Figure 4c shows a spring structure, where a first cut-out s5 in the form of a first rectilinear section between the two cantilevers is terminated by a first cut-out portion extending orthogonally to the first rectilinear section. A second cut-out s6 contains a second cut-out portion extending parallel to the first cut-out portion and also contains a continuation of rectilinear section between the two cantilevers. Here, the spring constant of the spring structure may be adjusted by varying the lengths F of the first and second cut-out portions.

[0058] Figure 4d shows a spring structure, where a first cut-out s7 in the form of a first rectilinear section between the two cantilevers meets a second cut-out s8 in the form of a second rectilinear section between the two cantilevers such that there is an uncut area between the first and second cut-outs s7 and s9. Third and fourth cut-outs s9 and s10 on each side of the uncut area extend parallel to the uncut area and respective end portions of the first and second cut-outs s7 and s8. Here, the spring constant of the spring structure may be adjusted by varying an extension F of the uncut area and the lengths of the third and fourth cut-outs s9 and s10.

[0059] Figure 4e shows a spring structure, where first and second rectilinear sections s11 and s12 respectively between the two cantilevers meet one another in a meander-shaped pattern. Here, the spring constant of the spring structure may be adjusted by varying different extensions F of the cut-outs forming the meandershaped pattern. Preferably, the spring structures described above further contain stress-relief cut-out sections. This namely reduces the risk of stress-related damages to the design. Figures 14a and 14b illustrate examples of how such stress-relief cut-out sections may be implemented according to embodiments of the invention.

[0060] In Figure 14a, we see a variant of the spring-structure design of Figure 4a, where a first stress-relief cut-out section r1 in the form of a circular slot is arranged at a terminating end of the first cut-out s1 , and second and third stress-relief cut-out sections r21 and r22 respectively in the form of circular slots are arranged at the terminating ends of the first and second branches that extend parallel to the first cut-out s1.

[0061] In Figure 14b, the first stress-relief cut-out section r1 is also arranged at the terminating end of the first cut-out s1. However, here, fourth and fifth stress-relief cut-out sections r23 and r24 respectively in the form of curved cut-outs are arranged at the terminating ends of the first and second branches that extend parallel to the first cut-out s1.

[0062] Figures 5, 6 and 7 show top views of a MEMS-based sound producing device according to embodiments of the invention. In Figures 5, 6 and 7, all reference signs that also occur in Figures 1 a, 1 b 2, 3 and / or 4a to 4e designate the same parts / components as described above with reference to these figures.

[0063] In Figure 5, the links L1 , L2, L3 and L4 that couple the flexible cantilevers mechanically to one another include spring structures of the type illustrated in Figure 4b.

[0064] In Figure 6, the links L1 , L2, L3 and L4 are of the same type as in Figure 5. However, in Figure 6, the links are arranged around the center of the cavity V. This means that all the arrow-matching second cut-outs s4 illustrated in Figure 4b meet in a common cross-shaped cut-out that includes the opening OP.

[0065] In the embodiments shown in Figures 5 and 6, each of the flexi- ble cantilevers is connected to exactly two other flexible cantilevers via the links L1 , L2, L3 and L4 respectively.

[0066] In Figure 7, the links of Figures 5 and 6 have been combined, such that each flexible cantilever is mechanically linked to two neighboring flexible cantilevers via respective pairs of links in the form of spring structures L1 ; L5, L2, L6; L3, L7 and L4, L8.

[0067] Thus, as exemplified in Figures 5 to 7, the mechanical interconnection between the flexible cantilevers may be varied substantially both with respect to the connection points and the spring constant.

[0068] Figures 8a and 8b show side cross-section views of a ME MS- based sound producing device according to one embodiment of the invention, where a flexible polymer membrane P covers the flexible cantilevers, here exemplified by C1 and C3. In Figures 8a and 8b, all reference signs that also occur in Figures 1 a, 1 b and / or 2 to 7 designate the same parts / components as described above with reference to these figures. The flexible polymer membrane P also covers a portion of the support structure S that adjoins the respective base sides b1 , b2, b3 and b4 of the flexible cantilevers C1 and C3. Hence, there will be no air / acoustic leakages between the flexible cantilevers C1 and C3 and / or between the flexible cantilevers C1 and C3 and the support structure S. This improves the general efficiency of the proposed MEMS-based sound producing device.

[0069] In Figure 8a, where, in response to at least one first signal value of the control signal, the flexible cantilevers C1 and C3 are in a first position, the flexible polymer membrane P is folded to form a fold FD between the flexible cantilevers C1 and C3.

[0070] In Figure 8b, the flexible cantilevers C1 and C3 are in a second position in response to at least one second signal value of the control signal. Here, the flexible cantilevers C1 and C3 are deflected in relation to the support structure S such that the fold FD in the flexible polymer membrane P is unfolded. Irrespective of whether the flexible polymer membrane P is arranged to be folded and unfolded as illustrated in Figures 8a and 8b, according to one embodiment of the invention, the flexible polymer membrane P may be elastic.

[0071] This is illustrated in Figures 9a, 9b and 9c, where the reference signs that also occur in the Figures discussed above designate the same parts / components as described above with reference to these figures.

[0072] Figure 9a shows a top view of a ME MS-based sound producing device according to one embodiment of the invention, where, analogous to the above, the flexible polymer membrane P covers the flexible cantilevers and a portion of the support structure S. Figures 9b and 9c show side cross-section views of the MEMS-based sound producing device in Figure 9a along BB.

[0073] Here, In contrast to the previously discussed embodiments, each of the flexible cantilevers has a respective side edge e13, e23, e33 and e43 that form the opening OP between themselves, where the opening OP is located in the area Y over the cavity V. The opening OP is thus larger than a mere crossing of the cut-outs formed by the side edges e11 , e21 ; e22, e32; e31 , e41 and e42, e12 between the different flexible cantilevers in Figures 1 a and 2 to 7. Nevertheless, each of the flexible cantilevers is connected to exactly two other flexible cantilevers via the links L1 , L2, L3 and L4.

[0074] The flexible polymer membrane P covers the opening OP. Preferably the opening OP is positioned centrally over the cavity V. Further, the flexible polymer membrane P may be elastic, such that it is stretched without forming a fold in any of a first or second positionings of the flexible cantilevers C1 and C3 in response to at least one first and second signal value of the control signal to the piezoelectric actuators A1 and A3 as illustrated in Figures 9b and 9c respectively. In Figure 10, the cavity V surrounded by the support structure S has a general triangular outline in the plane covered by the flexible cantilevers. The cavity V is here covered by three symmetric flexible cantilevers, which each has such an outline that its width parallel to the base side b1 , b2 and b3 respectively is wider than the base side b1 , b2 and b3 at least at one position between the base side b1 , b2 and b3 and an edge point of the cantilever, which edge point is furthest away from the base side b1 , b2 and b3. As an example, a flexible cantilever being mechanically linked to a piezoelectric actuator A2 has a base side b2 and side edges e21 , e22, e23 and e24, where the side edge e23 is furthest away from the base side b2, and the side edge e23 is also parallel to the base side b2. Together with corresponding edge sides of the other two flexible cantilevers, the side edge e23 also form the opening OP between themselves. Here, each of the flexible cantilevers is connected to exactly two other flexible cantilevers via the links L1 , L2 and L3 respectively.

[0075] The type of cantilever outline exemplified in Figure 10 is advantageous because it allows for comparatively large energy exchange between the cantilevers and the surrounding air.

[0076] Figures 11 , 12 and 13 show top views of a MEMS-based sound producing device according to embodiments of the invention, where, unlike the embodiments discussed so far, piezoelectric actuators A1 , A2 and A3 and A1 , A2, A3 and A4 respectively cover less than an entire top surface of the flexible cantilevers, in Figures 1 1 , 12 and 13 all reference signs that also occur in the figures discussed above designate the same parts / compo- nents as described above with reference to these figures

[0077] The embodiment of the invention illustrated in Figure 11 corresponds to the embodiment of Figure 10, however where the piezoelectric actuators A1 , A2 and A3 only cover a portion of the flexible cantilevers C1 , C2 and C3 respectively. Specifically, in this case, each piezoelectric actuator A1 , A2 and A3 covers a respective top-surface area of the flexible cantilevers C1 , C2 and C3 between the base side b1 , b2 and b3 and a line parallel thereto where the flexible cantilever is widest. Of course, according to the invention, however, any other partial coverage of the flexible cantilever by the piezoelectric actuator is conceivable. A couple of such additional examples are shown in Figures 12 and 13 respectively.

[0078] Figure 12 shows an embodiment of the invention corresponding to Figure 1 a, however where the piezoelectric actuators A1 , A2, A3 and A4 only cover a mid-portion of the flexible cantilevers C1 , C2, C3 and C4 respectively.

[0079] Figure 13 shows an embodiment of the invention corresponding to Figure 2, however where the piezoelectric actuators A1 , A2, A3 and A4 only cover the outer portions of the flexible cantilevers C1 , C2, C3 and C4 respectively, which outer portions adjoin the base sides b1 , b2, b3 and b4 respectively.

[0080] Also in the embodiments illustrated in Figures 12 and 13 , each of the flexible cantilevers is connected to exactly two other flexible cantilevers via the links L1 , L2, L3 and L4.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] The invention is not restricted to the described embodiments in the figures, but may be varied freely within the scope of the claims.

Claims

Claims1. A MEMS-based sound producing device comprising: a support structure (S) surrounding a cavity (V), at least three flexible cantilevers (C1 , C2, C3, C4) each having a respective base side (b1 , b2, b3, b4) along which the flexible cantilever is attached to the support structure (S), each of said flexible cantilevers further having side edges (e11 , e12; e21 , e22; e31 , e32; e41 , e42) being movable relative to the support structure (S), said flexible cantilevers being arranged over the cavity (V) such that each of said flexible cantilevers adjoins two neighboring flexible cantilevers along respective side edges and said flexible cantilevers together covering the cavity, and each of said flexible cantilevers being configured to be deflected relative to the support structure in response to a control signal influencing a respective piezoelectric actuator (A1 , A2, A3, A4) that is mechanically linked to each of said flexible cantilevers, and a set of links (L, L1 , L2, L3, L4, L12, L22, L32, L42) arranged between the flexible cantilevers (C1 , C3), wherein each link couples one of said flexible cantilevers mechanically to another one of said flexible cantilevers, characterized in that the links (L, L1 , L2, L3, L4, L12, L22, L32, L42) are arranged to exclusively couple the side edges (e1 1 , e21 ; e22, e32; e31 , e41 ; e42, e12) of two neighboring flexible cantilevers of said flexible cantilevers mechanically to one another, and each of the flexible cantilevers is connected to two other ones of the flexible cantilevers via the links.

2. The MEMS-based sound producing device according to claim 1 , wherein at least a subset of the links in the set of links are constituted by respective unbroken sections (L12, L22, L32, L42) of said flexible cantilevers between the respective flexible cantilevers being mechanically coupled to one another.

3. The MEMS-based sound producing device according to claim 1 , wherein each of said links (L, L1 , L2, L3, L4) comprises a respective spring structure being formed by one or more cutouts (s1 , s2, s3, s4, s5, s6, s7, s8, s9, s10, s11 , s12) in at leastone of the flexible cantilevers between which the link is arranged.

4. The MEMS-based sound producing device according claim 3, wherein at least one of said one or more cut-outs (s1 , s2) comprises a stress-relief cut-out section (r1 , r21 , r22, r23, r24) at a terminating end of cut-out in question (s1 , s2).

5. The MEMS-based sound producing device according to any of the preceding claims, wherein respective side edges (e11 , e21 ; e22, e32; e31 , e41 ; e42, e12; e13, e23, e33, e43) of said flexible cantilevers form an opening (OP) between themselves, which opening (OP) is located in an area (Y) over the cavity (V).

6. The MEMS-based sound producing device according to claim 5, wherein the opening (OP) is positioned centrally over the cavity (V).

7. The MEMS-based sound producing device according to any of the preceding claims, wherein the cavity (V) has a general polygonal outline in a plane covered by said cantilevers.

8. The MEMS-based sound producing device according to claim 7, wherein the general polygonal outline is regular.

9. The MEMS-based sound producing device according to any of the preceding claims, wherein each of said cantilevers has such an outline that its width parallel to the base side (b1 , b2, b3, b4) generally tapers off from the base side towards an edge point (ep1 , ep2, ep3, ep4) on the respective cantilever, which edge point is furthest away from the base side.

10. The MEMS-based sound producing device according to claim 9, wherein each of said cantilevers has a triangular outline.

11. The MEMS-based sound producing device according toany of claims 1 to 8, wherein each of said cantilevers has such an outline that its width parallel to the base side is wider than the base side at least at one position between the base side and an edge point of the cantilever, which edge point is furthest away from the base side.

12. The ME MS-based sound producing device according to any of the preceding claims, further comprising: a flexible polymer membrane (P) that covers said flexible cantilevers and a portion of the support structure (S) adjoining the respective base sides (b1 , b2, b3, b4) of said flexible cantilevers.

13. The MEMS-based sound producing device according to claim 12, wherein the flexible polymer membrane (P) is elastic, and the flexible polymer membrane is arranged over said flexible cantilevers and the support structure to stretch in response to deflecting said flexible cantilevers relative to the support structure (S).

14. The MEMS-based sound producing device according to any of claims 12 or 13, wherein the flexible polymer membrane (P) is arranged such that: in a first positioning of said flexible cantilevers in response to at least one first signal value of the control signal, the flexible polymer membrane (P) is folded to form at least one fold (FD) between at least two of said flexible cantilevers; and in a second positioning of said flexible cantilevers in response to at least one second signal value of the control signal, the at least one fold (FD) is unfolded due to a deflection of said flexible cantilevers relative to the support structure.

15. The MEMS-based sound producing device according to any of claims 12 to 14, wherein: a respective side edge (e13, e23, e33, e43) of each of said flexible cantilevers form an opening (OP) between themselves, which opening (OP) is located in an area (Y) over the cavity (V), andthe flexible polymer membrane (P) covers the opening (OP).

16. The ME MS-based sound producing device according to claim 15, wherein the opening (OP) is positioned centrally over the cavity (V).

17. The MEMS-based sound producing device according any one of the preceding claims, wherein each of respective piezoelectric actuators (A1 , A2, A3, A4) covers an entire upper and / or lower surface of the respective flexible cantilever (C1 , C3).

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