Patent Publication Number: US-11051107-B2

Title: Miniature receiver

Description:
FIELD OF THE INVENTION 
     The present invention relates to a miniature receiver comprising at least first and second moveable diaphragms being acoustically connected via an intermediate volume having an acoustic compliance being smaller than the acoustic compliances of the respective first and second moveable diaphragms. 
     BACKGROUND OF THE INVENTION 
     The achievable sound pressure level (SPL) from receiver depends on a variety of parameters—one of them being the effective area of the moveable diaphragm of the receiver. A larger membrane area facilitates a larger SPL for a given membrane displacement. Thus, in order to enable large effective diaphragm areas, it can be useful to have multiple diaphragms in a receiver. These diaphragms are normally placed in parallel, both acoustically and electrically. 
     For a receiver with a substantially enclosed back volume, the acoustic back volume compliance can play a large role in optimizing a receiver for high SPL. A general rule is that the combined compliance of the motor and diaphragm should be similar to the acoustic back volume compliance. 
     For this reason, receivers with larger or multiple diaphragms need very high stiffness membranes or motors. This may however reduce the efficiency of driving the diaphragms. 
     In view of the above remarks it may be seen as an object of embodiments of the present invention to provide a miniature receiver being capable of generating a larger SPL. 
     It may be seen as a further object of embodiments of the present invention to provide a miniature receiver comprising a plurality of moveable diaphragms being acoustically coupled in series. 
     DESCRIPTION OF THE INVENTION 
     The above-mentioned object is complied with by providing, in a first aspect, a miniature receiver comprising 
     a first moveable diaphragm being acoustically connected to an intermediate volume, and 
     a second moveable diaphragm being acoustically connected to the intermediate volume and a rear volume wherein the acoustic compliance of the intermediate volume is smaller than the acoustic compliances of the respective first and second moveable diaphragms. 
     In the present context the term “miniature receiver” should be understood as a sound generating receiver having a size that allows it to be applied in ear pieces of for example hearing aids or hearables, such as a hearing device to be carried near or outside an ear, or at least partly inside an ear canal. 
     Moreover, the term “moveable diaphragm” should, in the present context, be understood as a moveable or deformable mechanical element, or a combination of a plurality of moveable and/or deformable elements, being acoustically coupled to air on both sides so that movements of a moveable diaphragm, or parts thereof, displaces the air in sections of an acoustical frequency band. 
     The low acoustic compliance of the intermediate volume relative to the acoustic compliances of the first and second moveable diaphragms ensures that movements of the first and second moveable diaphragms are coupled through a substantially stiff connection. A movement of one diaphragm in one direction will thus provide a force in the same direction to the other diaphragm. The intermediate volume thus acts as a stiff connection between the first and second moveable diaphragms thus transferring forces between them as well as ensuring that the first and second moveable diaphragms perform similar volume displacements in response to an applied electrical drive signal. 
     The miniature receiver of the present invention may further comprise a front volume, wherein 
     a first surface of the first moveable diaphragm is acoustically connected to the front volume, and wherein an opposing second surface of the first moveable diaphragm is acoustically connected to the intermediate volume, and wherein 
     a first surface of the second moveable diaphragm is acoustically connected to the intermediate volume, and wherein an opposing second surface of the second moveable diaphragm is acoustically connected to the rear volume. 
     The front volume may be acoustically connected to a sound outlet of the miniature receiver so that generated sound is allowed to leave the miniature receiver. 
     For typical miniature receivers the total volume may be in the range 10-400 mm 3 . For such miniature receivers the front volume, the rear volume, and the intermediate volume may be 2-20%, 2-20% and 25-80% of the total volume, respectively. 
     In contrast to the front volume the intermediate and rear volumes may constitute substantially closed volumes. 
     The first moveable diaphragm may form part of a first microelectromechanical system (MEMS) die, whereas the second moveable diaphragm may form part of a second MEMS die. The first and second MEMS dies may be arranged on opposing surfaces of a substrate at least partly separating the front and rear volumes of the miniature receiver. In particular, the first and second MEMS dies may be aligned with an opening in the substrate in a manner so that the first and second moveable diaphragms cover the opening in the substrate. 
     Alternatively, the first and second moveable diaphragms may form part of the same MEMS die. 
     The first and/or second moveable diaphragms may each comprise a substantially plane diaphragm. Moreover, the first and/or second moveable diaphragms may each comprise an integrated drive structure adapted to displace the first and/or second moveable diaphragms in response to one or more electrical drive signals applied to said integrated drive structures. The integrated drive structure of each of the first and/or second moveable diaphragms may comprise a piezoelectric material layer arranged between a first and a second electrode. Alternatively, the first and/or second moveable diaphragms may each comprise a substantially plane electrostatic diaphragm. 
     Alternatively, a separate drive structure, such as a separate piezoelectric driver or a balanced armature, may be applied to drive the first and second moveable diaphragms in response to one or more electrical drive signals applied to said separate drive structures. 
     The first and second moveable diaphragms may comprise respective first and second substantially plane diaphragms, said first and second substantially plane diaphragms being structurally arranged in a substantially parallel manner. Alternatively, the first and second moveable diaphragms may be arranged at an angle relative to each other. This angle may be up to 20 degrees. 
     The first and second electrodes of the respective first and second moveable diaphragms may electrically be coupled in parallel. With this arrangement the integrated drive structures of the first and second moveable diaphragms will receive the same electrical drive signal during operation. 
     Although the miniature receiver has being disclosed as having two moveable diaphragms it should be noted that the miniature receiver may further comprise additional moveable diaphragms being arranged in series with the first and second moveable diaphragms disclosed above. Also, moveable diaphragms in series may be combined with other moveable diaphragms via a parallel implementation, such as two moveable diaphragms in series being in parallel with a third moveable diaphragm. 
     In a second aspect the present invention relates to a personal device comprising a miniature receiver according to the first aspect, said personal device being selected from the group consisting of hearing aids, hearing devices, hearables, mobile communication devices and tablets. 
     In a third aspect the present invention relates to a method for operating a miniature receiver comprising a first moveable diaphragm being acoustically connected to an intermediate volume, and a second moveable diaphragm being acoustically connected to the intermediate volume and a rear volume, wherein the acoustic compliance of the intermediate volume is smaller than the acoustic compliances of the respective first and second moveable diaphragms, the method comprising the steps of operating the first and second moveable diaphragms in accordance with one or more electrical drive signals. 
     The miniature receiver may be implemented as discussed in connection with the first aspect of the present invention. Thus, a first surface of the first moveable diaphragm is acoustically connected to a front volume, and an opposing second surface of the first moveable diaphragm is acoustically connected to the intermediate volume. Moreover, a first surface of the second moveable diaphragm is acoustically connected to the intermediate volume, and an opposing second surface of the second moveable diaphragm is acoustically connected to the rear volume. 
     As discussed previously the first moveable diaphragm may form part of a first MEMS die, and the second moveable diaphragm may form part of a second MEMS die. Alternatively, the first and second moveable diaphragms may form part of the same MEMS die. 
     The first and second moveable diaphragms may each comprise a substantially plane diaphragm comprising an integrated drive structure. The integrated drive structure of each of the first and second moveable diaphragms may comprise a piezoelectric material layer arranged between a first and a second electrode. The first and second electrodes of the respective first and second moveable diaphragms may electrically be coupled in parallel. With this arrangement the integrated drive structures of the first and second moveable diaphragms will receive the same electrical drive signal during operation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be explained in further details with reference to the accompanying figures, wherein 
         FIG. 1  which shows the general concept of the present invention, 
         FIG. 2  shown a piezoelectric diaphragm, 
         FIG. 3  shows an electrostatic driven diaphragm, 
         FIG. 4  shows a single MEMS die, and a triple-stacked MEMS die, 
         FIG. 5  shows a double-stacked MEMS die, and a die-in-die MEMS die, 
         FIG. 6  shows flip-clip mounted MEMS dies, and a double-layer MEMS die, 
         FIG. 7  shows two double-stacked MEMS dies in a package, 
         FIG. 8  shows a miniature receiver applying two double-stacked MEMS dies, and 
         FIG. 9  shows a miniature receiver applying stacked MEMS dies. 
     
    
    
     While the invention is susceptible to various modifications and alternative forms specific embodiments have been shown by way of examples in the drawings and will be described in details herein. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In its most general aspect the present invention relates to a miniature receiver comprising first and second moveable diaphragms being acoustically connected via an intermediate volume having an acoustic compliance which is smaller than the respective acoustic compliances of the first and second moveable diaphragms. The smaller acoustic compliance of the intermediate volume relative to the acoustic compliances of the first and second moveable diaphragms ensure that the first and second moveable diaphragms are driven in the same direction and perform the same volume displacements in response to an applied electrical drive signal. 
     The miniature receiver of the present invention is advantageous in that it improves the SPL compared to conventional receivers having a substantially closed rear volume. In relation to the miniature receiver according to the present invention the compliance of the moveable diaphragm or diaphragms are of the same order of magnitude as an acoustic load which is dominated by the compliance of the rear volume. The miniature receiver of the present invention is thus advantageous for the following reasons: 
     1) Extra degrees of freedom to increase active diaphragm area, i.e. it is easier to find and allocate space for more diaphragm area when the moveable diaphragms are arranged in series. 
     2) Extra freedom in terms of optimization of the miniature receiver in that the ratio of receiver stiffness to the rear volume stiffness may be optimized which allows for more compliant diaphragm designs. 
     Referring now to  FIG. 1  a miniature receiver  100  according to the present invention is depicted. As seen in  FIG. 1  the miniature receiver  100  comprises a housing  104  and a sound outlet  112  arranged therein. The sound outlet  112  is acoustically connected to a front volume  101  which is acoustically sealed from a rear volume  102  via a substrate  107  and first and second MEMS dies  108 ,  109 . The MEMS dies  108 ,  109  are both aligned with an opening in the substrate  107  as well as secured to the substrate  107  via respective die attachments  110 ,  111 . 
     As seen in  FIG. 1  a first moveable diaphragm  105  forms part of the MEMS die  108 , whereas a second moveable diaphragm  106  forms part of the MEMS die  109 . The first and second moveable diaphragms  105 ,  106  are arranged in a substantially parallel manner. 
     As seen in  FIG. 1  an upper surface of the first moveable diaphragm  105  is acoustically connected to the front volume  101 , whereas the opposing lower surface of the first moveable diaphragm  105  is acoustically connected to the intermediate volume  103 . Similarly, an upper surface of the second moveable diaphragm  106  is acoustically connected to the intermediate volume  103 , whereas an opposing lower surface of the second moveable diaphragm  106  is acoustically connected to the rear volume  102 . 
     As previously addressed the intermediate volume  103  has an acoustic compliance which is smaller than the respective acoustic compliances of the first and second moveable diaphragms  105 ,  106 . The smaller acoustic compliance of the intermediate volume  103  relative to the acoustic compliances of the first and second moveable diaphragms  105 ,  106  ensure that the first and second moveable diaphragms are driven in the same direction and perform the same volume displacements in response to an applied electrical drive signal. 
     The first and second moveable diaphragms  105 ,  106  each comprises an integrated drive structure being adapted to displace the first and second moveable diaphragms  105 ,  106  in response to an applied electrical drive signal. Although not shown in  FIG. 1  the integrated drive structure of each of the first and second moveable diaphragms  105 ,  106  may comprise a piezoelectric material layer being arranged between a first and a second electrode. The first and second electrodes of the respective first and second moveable diaphragms are electrically coupled in parallel so that an electrical drive signal applied to the first moveable diaphragm  105  is also applied to the second moveable diaphragm  106 . 
     The piezoelectric arrangement for driving the first and second moveable diaphragms  105 ,  106  may be implemented as depicted in  FIG. 2 . Alternatively, the drive mechanism for driving the first and second moveable diaphragms  105 ,  106  may be implemented as an electrostatic arrangement each having an associated backplate as depicted in  FIG. 3 . 
     In the embodiment shown in  FIG. 2  piezoelectric levers  203  forming a moveably diaphragm are depicted. The moveable diaphragm may be any of the moveable diaphragms  105 ,  106  in  FIG. 1 . The piezoelectric levers  203  are secured to a MEMS bulk  201 . An opening or gap  202  is provided in the centre portion, cf.  FIG. 2 a   . The gap  202  between the levers  203  is so narrow that the acoustic leakage through the gap is not affecting the acoustic output in the audible frequency range. The piezoelectric levers  203  thus effectively behave as a sealed diaphragm. The acoustic leakage through the gap determines the low frequency roll-off of the acoustic output of the miniature receiver. 
       FIG. 2 b    shows an enlarged view of the encircled portion of  FIG. 2 a   . As depicted in  FIG. 2 b    the piezoelectric lever forms a layered structure comprising a piezoelectric material  207  arranged between two electrodes  206 ,  208 . The electrodes  206 ,  208  are adapted to be connected to a voltage source, cf.  FIG. 2 c   . An elastic layer  209  is secured to the electrode  208  and forms an integral part of the MEMS bulk  204  and define a volume  205  in combination therewith. The volume  205  forms part of either the front volume  101  or the rear volume  102 , cf.  FIG. 1 . 
       FIG. 2 c    shows the piezoelectric lever in a deflected position as indicated by the arrow  210 . The deflection of the piezoelectric levers is provided by applying a voltage to the electrodes  211 ,  212  whereby the levers deflect either up or down depending of the polarity of the applied voltage. Again, the volume  213  is provided below the levers. Since the gap between the levers is so narrow that the levers behave as a moveable diaphragm for the audible frequency range, a sound pressure can be generated when an appropriate drive signal/voltage applied to the electrodes  211 ,  212 . Alternatively, if a moveable diaphragm is secured to the piezoelectric lever and an appropriate drive signal/voltage applied to the electrodes  211 ,  212  sound pressure variations may be generated. Such a separate diaphragm may be a polymer diaphragm, a metal diaphragm or a composite, and can be comprised of rigid regions and compliant regions. 
       FIG. 3  shows an alternative drive mechanism for the first and second moveable diaphragms  105 ,  106  of  FIG. 1 . In  FIG. 3 a    an electrostatically actuated diaphragm having an associated backplate is depicted. With reference to  FIG. 3 a    an electrically conducting diaphragm  303 , a MEMS bulk  301  and a volume  302  are depicted. The volume  302  forms part of either the front volume  101  or the rear volume  102 , cf.  FIG. 1 .  FIG. 3 b    shows an enlarged version of  FIG. 3 a   . As seen in  FIG. 3 b    the diaphragm  304  is arranged on a spacer  305  so that a distance to a backplate  306  with perforations  307  is ensured. The MEMS bulk  309 , which supports the diaphragm  304  and the spacer  305 , defines in combination with the backplate  306 , the volume  308 . In  FIG. 3 c    a voltage source has been connected to the electrically conducting diaphragm  310  and the perforated backplate  311  above the volume  315 . As depicted in  FIG. 3 c    the applied voltage causes the diaphragm  310  to deflect in the direction of the backplate  311 . With an appropriate drive signal/voltage applied between the diaphragm  310  and the perforated backplate  311  sound pressure variations may be generated. As previously mentioned the diaphragm  310  is supported by the MEMS bulk  312  via the spacer  314 . 
     In relation to  FIG. 3  it should be noted that the electret based structures may be applied as well. In the following various embodiments of MEMS dies as well as combinations thereof are discussed. 
     Referring now to  FIG. 4 a    an embodiment in the form of a single MEMS die  401  comprising a moveable diaphragm  402  is depicted. The moveable diaphragm  402  may be of the type disclosed in connection with  FIG. 2  (piezoelectric),  FIG. 3  (electrostatic) or a completely different type of moveable diaphragm. Turning now to  FIG. 4 b    an embodiment comprising three stacked  403 ,  404 ,  405  MEMS dies  406 ,  408 ,  410  is depicted. Each of the MEMS dies  406 ,  408 ,  410  comprises respective moveable diaphragms  407 ,  409 ,  411  which are coupled in series. Intermediate volumes  412 ,  413  are provided between moveable diaphragms  407 ,  409  and between moveable diaphragms  409 ,  411 . The stacked MEMS dies  406 ,  408 ,  410  shown in  FIG. 4 b    are similar in size and may therefore be stacked directly onto each other. 
     As previously addressed a low acoustic compliance of the intermediate volumes  412 ,  413  relative to the acoustic compliances of the moveable diaphragms  407 ,  409 ,  411  ensures that movements of the moveable diaphragms  407 ,  409 ,  411  are locked through a substantially rigid connection. Thus, a movement of one diaphragm in one direction will provide a force in the same direction to the other diaphragms. The intermediate volumes thus act as a stiff connection between the moveable diaphragms  407 ,  409 ,  411  thus transferring forces between them as well as ensuring that the moveable diaphragms  407 ,  409 ,  411  perform similar volume displacements in response to an applied electrical drive signal. The drive structures of the moveable diaphragms  407 ,  409 ,  411  are electrically coupled in parallel so that a common electrical drive signal can be applied to the drive structures of the moveable diaphragms  407 ,  409 ,  411 . 
     Stacking of MEMS dies as depicted in  FIG. 4 a    is advantageous in that more diaphragm area may be easily provided when a plurality of diaphragms are arranged in series. 
     Referring now to  FIG. 5 a    an embodiment comprising two stacked MEMS dies  501 ,  503  is depicted. Each of the MEMS dies  501 ,  503  comprises respective moveable diaphragms  502 ,  504  which are arranged in series. An intermediate volume  506  is provided between moveable diaphragms  502 ,  504 . Contrary to the arrangement shown in  FIG. 4 b    the stacked MEMS dies shown in  FIG. 5 a    have different outer dimensions due to the enlarged support structure  505 . The intermediate volume  506  acts as discloses above, i.e. as a stiff connection between the moveable diaphragms  502 ,  504  thus transferring forces between them as well as ensuring that the moveable diaphragms  502 ,  504  perform similar volume displacements in response to an applied electrical drive signal. 
       FIG. 5 b    shows an embodiment where one MEMS die  509  is arranged in the hollow portion of another MEMS die  507 . Again, each of the MEMS dies  507 ,  509  comprises respective moveable diaphragms  508 ,  510  which are arranged in series. An intermediate volume  511  is provided between moveable diaphragms  508 ,  510 . The intermediate volume  511  acts as discloses above, i.e. as a stiff connection between the moveable diaphragms  508 ,  510 . An immediate advantage of the embodiment shown in  FIG. 5 b    is its limited height due to the die-in-die arrangement. 
     Referring now to  FIG. 6 a    an embodiment comprising two flip-chip mounted MEMS dies  601 ,  603  is depicted. Each of the MEMS dies  601 ,  602  comprises respective moveable diaphragms  602 ,  604  which are arranged in series. An intermediate volume  606  is provided between moveable diaphragms  602 ,  604 . The intermediate volume  606  acts as discloses above, i.e. as a stiff connection between the moveable diaphragms  602 ,  604 . The MEMS dies  601 ,  603  are attached to each other via die attachment  605 . In  FIG. 6 b    an embodiment comprising a MEMS die  607  having two moveable diaphragms  608 ,  609  separated by an intermediate volume  610  is depicted. Again, the intermediate volume  610  acts as a stiff connection between the moveable diaphragms  602 ,  604 . 
       FIG. 7  shows a miniature receiver  700  comprising a receiver housing  715  having a sound outlet  714  being acoustically connected to a common front volume  713 . Two MEMS assemblies each comprising two MEMS dies  701 ,  703  and  707 ,  709  are arranged within the receiver housing  715 . As seen in  FIG. 7  the upper MEMS assembly comprises two MEMS die  701 ,  703  which each comprises respective moveable diaphragms  702 ,  704  which are arranged in series. An intermediate volume  705  is provided between moveable diaphragms  702 ,  704 . The intermediate volume  705  acts as a stiff connection between the moveable diaphragms  702 ,  704 . A first rear volume  706  is provided behind the moveable diaphragm  702 . Similarly, the lower MEMS assembly comprises two MEMS die  707 ,  709  which each comprises respective moveable diaphragms  708 ,  710  which are arranged in series. Again, an intermediate volume  711  is provided between moveable diaphragms  708 ,  710 . The intermediate volume  711  acts as a stiff connection between the moveable diaphragms  708 ,  710 . A second rear volume  712  is provided behind the moveable diaphragm  702 . The drive structure of the four moveable diaphragms  702 ,  704 ,  708 ,  710  are adapted to be driven by the same drive signal. 
     Referring now to  FIG. 8 a    another embodiment  800  of the present invention is depicted. As seen in  FIG. 8 a    the miniature receiver  800  comprises a housing  811  and a sound outlet  812  arranged therein. The sound outlet  812  is acoustically connected to a front volume  801  which is acoustically sealed from two rear volumes  802 ,  803  via substrate portions  813 ,  818 ,  819  and first, second, third and fourth MEMS dies  814 ,  815 ,  816 ,  817 . The two rear volumes  802 ,  803  are acoustically separated from each other by the wall  810 . The MEMS dies  814 ,  815 ,  816 ,  817  are all aligned with openings in the substrate portions as well as secured to the substrate portions  813 ,  818 ,  819  via respective die attachments. 
     As seen in  FIG. 8 a    a first moveable diaphragm  806  forms part of the MEMS die  814 , whereas a second moveable diaphragm  807  forms part of the MEMS die  815 . The first and second moveable diaphragms  806 ,  807  are arranged in a substantially parallel manner. Similarly, a third moveable diaphragm  808  forms part of the MEMS die  816 , whereas a fourth moveable diaphragm  809  forms part of the MEMS die  817 . The third and fourth moveable diaphragms  808 ,  809  are arranged in a substantially parallel manner. 
     The upper surfaces of the first and third moveable diaphragms  806 ,  808  are acoustically connected to the front volume  801 , whereas the opposing lower surfaces of the first and third moveable diaphragms  806 ,  808  are acoustically connected to the intermediate volumes  804 ,  805 , respectively. Similarly, the upper surfaces of the second and fourth moveable diaphragms  807 ,  809  are acoustically connected to the respective intermediate volumes  804 ,  805 , whereas the opposing lower surfaces of the second and fourth moveable diaphragms  807 ,  809  are acoustically connected to respective rear volumes  803 ,  802 . 
     As mentioned above the intermediate volumes  804 ,  805  both have an acoustic compliance which is smaller than the respective acoustic compliances of the first, second, third and fourth moveable diaphragms  806 - 809 . The smaller acoustic compliance of the intermediate volumes  804 ,  805  relative to the acoustic compliances of the moveable diaphragms  806 - 809  ensure that the first and second moveable diaphragms  806 ,  807  are driven in the same direction and perform the same volume displacements in response to an applied electrical drive signal. The same applies to the third and fourth moveable diaphragms  808 ,  809 . 
     The moveable diaphragms  806 - 809  each comprises an integrated drive structure being adapted to displace the moveable diaphragms  806 - 809  in response to applied electrical drive signals. Although not shown in  FIG. 8 a    the integrated drive structure of each of the moveable diaphragms  806 - 809  may comprise a piezoelectric material layer being arranged between a first and a second electrode. The first and second electrodes of the respective moveable diaphragms  806 - 809  are electrically coupled in parallel so that an electrical drive signal applied to the first moveable diaphragm  806  is also applied to the second moveable diaphragm  807 . Similarly, an electrical drive signal applied to the third moveable diaphragm  808  is also applied to the fourth moveable diaphragm  809 . In fact the same electrical drive signal may be applied to all moveable diaphragms. 
     The piezoelectric arrangement for driving the moveable diaphragms  806 - 809  may be implemented as depicted in  FIG. 2 . Alternatively, the drive mechanism for driving the moveable diaphragms  806 - 809  may be implemented as an electrostatic arrangement each having an associated backplate as depicted in  FIG. 3 . 
     Referring now to the embodiment  820  depicted in  FIG. 8 b    an acoustical filter  821  has been inserted between the two rear volumes (reference numerals  802 ,  803  in  FIG. 8 a   ). The acoustical filter  821  may be implemented in various ways, including a mesh structure for attenuating sound pressure. Despite the acoustical filter  821  the embodiment shown in  FIG. 8 b    is identical to the embodiment shown in  FIG. 8   a.    
     Turning now to  FIG. 9  another embodiment  900  of the present invention is depicted. As seen in  FIG. 9  the miniature receiver  900  comprises a housing  908  and a sound outlet  909  arranged therein. The sound outlet  909  is acoustically connected to a front volume  901  which is acoustically sealed from two rear volumes  902 ,  903  via substrate portions  915 ,  916  and first, second, and third MEMS dies  911 - 913 . The two rear volumes  902 ,  903  are acoustically connected via the acoustical filter  910  which is arranged in the wall  914 . The MEMS dies  911 - 913  are all aligned with openings in the substrate portions  915 ,  916  as well as secured to the substrate portions  915 ,  916  via respective die attachments. 
     As seen in  FIG. 9  a first moveable diaphragm  905  forms part of the MEMS die  911 , whereas second and third moveable diaphragms  906 ,  907  form part of respective MEMS dies  912 ,  913 . The first, second and third moveable diaphragms  905 - 907  are arranged in a substantially parallel manner. 
     The upper surface of the first moveable diaphragm  905  is acoustically connected to the front volume  901 , whereas the opposing lower surface of the first moveable diaphragm  905  is acoustically connected to the intermediate volume  904 . Similarly, the upper surfaces of the second and third moveable diaphragms  906 ,  907  are acoustically connected to the intermediate volume  904 , whereas the opposing lower surfaces of the second and third moveable diaphragms  906 ,  907  are acoustically connected to respective rear volumes  903 ,  902 . 
     The intermediate volume  904  has an acoustic compliance which is smaller than the respective acoustic compliances of the first, second and third moveable diaphragms  905 - 907 . As previously addressed the smaller acoustic compliance of the intermediate volumes  904  relative to the acoustic compliances of the moveable diaphragms  905 - 907  ensure that the moveable diaphragms  905 - 907  are driven in the same direction and that the first moveable diaphragm  905  perform the same volume displacements as the second and third moveable diaphragms  906 ,  907  in combination in response to an applied electrical drive signal. 
     Similar to the previous embodiments the moveable diaphragms  905 - 907  each comprises an integrated drive structure being adapted to displace the moveable diaphragms  905 - 907  in response to applied electrical drive signals. Although not shown in  FIG. 9  the integrated drive structure of each of the moveable diaphragms  905 - 907  may comprise a piezoelectric material layer being arranged between a first and a second electrode. The first and second electrodes of the respective moveable diaphragms  905 - 907  are electrically coupled in parallel so that an electrical drive signal applied to the first moveable diaphragm  905  is also applied to the second and third moveable diaphragm  906 ,  907 . It should however be noted that other electrical connections may also be applicable. 
     The piezoelectric arrangement for driving the moveable diaphragms  905 - 907  may be implemented as depicted in  FIG. 2 . Alternatively, the drive mechanism for driving the moveable diaphragms  905 - 907  may be implemented as an electrostatic arrangement each having an associated backplate as depicted in  FIG. 3 . It should be noted that electret based structures may be applied as well.