Patent Publication Number: US-11049484-B2

Title: Miniature speaker with essentially no acoustical leakage

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of European Patent Application Serial No. 18248156.4, filed Dec. 28, 2018, which is incorporated herein by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to a miniature speaker comprising one or more piezoelectric cantilevers beams for generating sound pressure waves. The one or more cantilevers beams are arranged in a manner to that essentially no acoustical leakage exists between a front volume and a rear volume of the miniature speaker. 
     BACKGROUND OF THE INVENTION 
     It is well established that an acoustical leakage between a front volume and a rear volume of a miniature speaker significantly reduces the achievable sound pressure level (SPL) of such a speaker. Thus, in order to achieve a high SPL no acoustical leakage should ideally exist between the front volume and the rear volume of a speaker. 
     Known miniature speakers all seem to suffer from the disadvantages associated with acoustical leakage between front and rear volumes. 
     It may therefore be seen as an object of embodiments of the present invention to provide miniature speakers having enlarged SPL without increasing the overall volume of the miniature speaker. 
     It may be seen as a further object of embodiments of the present invention to increase the SPL of miniature speakers by improving the utilization of the miniature speaker area. 
     It may be seen as an even further object of embodiments of the present invention to increase the SPL of miniature speakers by reducing the acoustical leakage between a front and a rear volume of the miniature speaker. 
     DESCRIPTION OF THE INVENTION 
     The above-mentioned objects are complied with by providing, in a first aspect, a miniature speaker comprising
         a front and a rear volume, and   one or more moveable diaphragms each comprising one or more cantilever beams and associated one or more air gaps arranged between the front and rear volumes,
 
wherein the one or more cantilever beams are configured to bend or deflect in response to an applied drive signal, and wherein the one or more air gaps between the front and rear volumes remain essentially unaffected during bending or deflection of the one or more cantilever beams thus maintaining the acoustical leakage between the front and rear volumes at a minimum.
       

     The present invention thus relates to a miniature speaker comprising one or more moveable diaphragms each comprising one or more cantilever beams. The one or more cantilever beams may form an array of cantilever beams, such as a rectangular array of cantilever beams. The rectangular shape is advantageous in that it is highly applicable in relation to miniature speakers having a rectangular housing since a rectangular shaped moveable diaphragm may provide maximum SPL and minimum acoustical leakage. 
     Each of the one or more cantilever beams may comprise a piezoelectric material sandwiched between two electrodes configured to receive the applied drive signal. The applied drive signal either stretches or compresses the piezoelectric material causing the one or more cantilever beams to bend or deflect accordingly. Bending or deflection of one or more cantilever beams causes an associated moveable diaphragm to move accordingly and thus generate sound pressure waves. 
     The one or more cantilever beams may be secured to or form part of a MEMS die. The MEMS die may be arranged on a surface of a carrier substrate having a through-going opening arranged therein. The one or more cantilever beams of the MEMS die may be acoustically connected to said through-going opening. As it will be discussed in further details below the carrier substrate may form part of a separation between the front and rear volumes. 
     The carrier substrate may comprise a printed circuit board or a flex print, the printed circuit board or the flex print comprising electrically conducting paths configured to lead the drive signal to the one or more cantilever beams via the carrier substrate. 
     Each of the one or more cantilever beams may be pre-bended along a longitudinal direction. The degree of pre-bending may be selected in accordance with desired acoustical properties of the miniature speaker. Moreover, the degree of pre-bending may be set individually for each of the one or more cantilever beams. 
     An array of cantilever beams may comprise a plurality of cantilever beams, wherein a number of said cantilever beams may be mutually connected via one or more material layers. One or more air gaps may exist between neighboring cantilever beams, or between one or more cantilever beams and a frame structure of the array of cantilever beams. The one or more air gaps may be dimensioned in a manner so that they act as an acoustical low-pass filter having a predetermined acoustical cut-off frequency. The predetermined acoustical cut-off frequency may be between 1 kHz and 3 kHz, such as around 2 kHz. The width of the air gaps may typically be in the range between 0.5 μm and 5 μm. 
     In the miniature speaker according to the first aspect the front volume may be acoustically connected to a sound outlet of the miniature speaker. Moreover, one or more venting openings may be provided between the rear volume and an exterior volume of the miniature speaker. 
     In a second aspect the present invention relates to a receiver assembly for a hearing device, the receiver assembly comprising a miniature speaker according to the first aspect of the preceding claims. 
     In a third aspect the present invention relates to a hearing device, such as a receiver-in-canal hearing device, comprising a receiver assembly according to the second aspect. 
     In general the various aspects of the present invention may be combined and coupled in any way possible within the scope of the invention. These and other aspects, features and/or advantages of the present invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be explained in further details with reference to the accompanying figures, wherein 
         FIG. 1  shows various arrangements of cantilever beams, 
         FIG. 2  shows various arrangements of arrays of cantilever beams with essentially no acoustical leakage, 
         FIG. 3  shows further arrangements of arrays of cantilever beams with essentially no acoustical leakage, 
         FIG. 4  shows various top views of connected cantilever beams, 
         FIG. 5  shows various cross-sectional views of connected cantilever beams, 
         FIG. 6  shows a cross-sectional view of a pre-bended cantilever beam, and a top view of a row of pre-bended cantilever beams, 
         FIG. 7  shows a cross-sectional view of two opposing and pre-bended cantilever beams, and a top view of two rows of opposing and pre-bended cantilever beams, and 
         FIG. 8  shows two miniature speaker implementations. 
     
    
    
     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 a general aspect the present invention relates to miniature speakers having an increased SPL without increasing the overall volume of the miniature speaker. The increased SPL is provided via an improved utilization of the miniature speaker area, and a minimal acoustical leakage between front and rear volumes. The minimal acoustical leakage is achieved by ensuring that the dimensions of one or more air gaps between the front and rear volumes remain essentially unaffected during bending or deflection of one or more cantilever beams which are capable of generating sound pressure waves in response to applying a drive signal thereto. Thus, during generation of sound pressure waves, i.e. during operation of a miniature speaker according to the present invention, the dimensions of the one or more air gaps may slightly vary. However, these variations do not significantly affect the acoustical properties of the miniature speaker whereby an acoustical leakage in a desired frequency range is avoided. The widths of the air gaps are typically in the range between 0.5 μm and 5 μm. 
     The one or more cantilever beams may be arranged in various manners, such as a single row of cantilever beams or two opposing rows of cantilever beams. The one or more cantilever beams may thus be arranged in arrays which may be configured and/or optimized to form a moveable diaphragm having a rectangular shape. The rectangular shape is specifically useful and therefore advantageous in relation to miniature speakers having a rectangular housing in that a rectangular shaped diaphragm may provide maximum SPL and minimum acoustical leakage. 
     Moreover, selected cantilever beams may be connected in order to reduce acoustical leakage through arrays of cantilever beams. The one or more cantilever beams may be straight or they may be pre-bended along a longitudinal direction as explained in further details below. 
     Each of the one or more cantilever beams comprises an integrated drive mechanism, such as a piezoelectric material sandwiched between two electrodes to which electrodes the drive signal is applied. Upon applying a drive signal to the two electrodes the piezoelectric material will stretch or compress, and the one or more cantilever beams will bend or deflect accordingly. The typical drive signal has an RMS value of around 3 V, but it may, under certain circumstances, be as high as 50 V. 
     The overall volume of the miniature speaker is below 500 mm 3 , such as below 400 mm 3 , such as below 300 mm 3 , such as below 200 mm 3 , such as below 100 mm 3 , such as below 50 mm 3 , such as around 40 mm 3 . The typical dimensions of a miniature speaker are 7 mm×3.3 mm×2 mm (L×W×H). The miniature speaker of the present invention is advantageous in that it is capable of delivering a SPL larger than 90 dB, such as larger than 95 dB, although its overall volume is around 40 mm 3 . 
     Referring now to  FIG. 1 a    a cross-sectional view of two opposing cantilever beams  102 ,  103  is depicted. The cantilever beams  102 ,  103  are either secured to or integrated with a MEMS die  101  which thus forms a frame structure relative to the cantilever beams  102 ,  103 . As depicted in  FIG. 1 a    a small air gap  104  exists between the cantilever beams  102 ,  103 . In order to prevent, or at least reduce, acoustical leakage through the air gap  104 , the air gap  104  is dimensioned so that essentially no sound pressure waves above 2 kHz is capable of flowing through the air gap  104 . The air gap  104  thus functions as an acoustical low-pass filter. Now referring to  FIG. 1 b    a cross-sectional view of a pre-bended cantilever beam  105  is depicted. Again, the cantilever beam  105  is either secured to or integrated with a MEMS die  101  which thus forms a frame structure. As depicted in  FIG. 1 b    a small air gap  106  exists between the cantilever beam  105  and the MEMS die  101 . Again, in order to prevent, or at least reduce, acoustical leakage through the air gap  106 , the air gap  106  is dimensioned so that essentially no sound pressure waves above 2 kHz are capable of passing through the air gap  106  which thus functions as an acoustical low-pass filter. It should be noted that the dimensions of the air gaps  104 ,  106  remain essentially unaffected during bending or deflection of the cantilever beams  102 ,  103 ,  105  thus maintaining the acoustical leakage through the air gaps  104 ,  106  at a minimum. The widths of the air gaps  104 ,  106  are typically in the range between 0.5 μm and 5 μm. 
     Turning now to  FIG. 1 c    a top view of a rectangular array of cantilever beams  107  is depicted. Again, the cantilever beams  107  are either secured to or integrated with the MEMS die  101 . In order to prevent, or at least reduce, acoustical leakage through the regions to the right and left  108 ,  109  of cantilever beams  107  a number of moveable elements are arranged in these regions  108 ,  109 , i.e. between the array  110  of cantilever beams  107  and the MEMS die  101 . The moveable elements are adapted to follow the deflections of the cantilever beams  107  in order to prevent that an uncontrolled amount of air escapes through the regions  108 ,  109  containing the moveable elements. Thus, the array  110  of cantilever beams  107  and the moveable elements in the regions  108 ,  109  form in combination a moveable diaphragm configured to generate sound pressure waves. In order to prevent that air gaps are formed between the cantilever beams  111 , cf.  FIG. 1 d   , the cantilever beams  111  may be connected via one or more material layers  113  which are secured to each of the cantilever beams  111 . The one or more material layers  113  thus blocks the openings  112  between the cantilever beams  111 . The width of the opening  112  is typically in the range between 0.5 μm and 5 μm. 
     The cantilever beams  102 ,  103 ,  105 ,  107 ,  109  shown in  FIG. 1  may all be activated individually via an integrated drive mechanism, such as a piezoelectric material sandwiched between two electrodes. The integrated drive mechanism is also applicable in relation to the cantilever beams discussed in the following figures. 
     As it will be demonstrated in connection with  FIG. 2  arrays of cantilever beams may be implemented using various geometries. Starting with  FIG. 2 a    two opposing rows of cantilever beams  201  is depicted. Each row comprises five cantilever beams  201  arranged next to each other. Each cantilever beam  201  comprises a fixed end and an oppositely arranged moveable end. The moveable end of each cantilever beam  201  is the end in the middle portion of the array, whereas the fixed cantilever end is at the edge of the array. In order to prevent, or at least reduce, acoustical leakage through the array a total of eight moveable elements are arranged on both sides of the ten cantilever beams  201 . The eight moveable elements to the right of the ten cantilever beams  201  are encircled and denoted  202  in  FIG. 2 a   . The corresponding eight moveable elements to the left of the ten cantilever beams  201  are identical. The 16 moveable elements in  FIG. 2 a    are adapted to follow the deflections of the cantilever beams  201  in order to form a moveable diaphragm and to prevent that uncontrolled amounts of air escape through the two regions each containing eight moveable elements.  FIG. 2 b    shows a similar arrangement of cantilever beams  201 , i.e. ten cantilever beams arranged in two rows with the moveable ends of the cantilever beams facing each other in the middle portion of the array. Compared to  FIG. 2 a    the number of moveable elements in the region  203  has been reduced to four. Again, the ten cantilever beams  201  and the eight moveable elements form, in combination, a moveable diaphragm. In  FIGS. 2 c  and 2 d    the number of moveable elements in the region  204  has been further reduced to three. Moreover, in  FIG. 2 d    the number of cantilever beams  201 ,  205  has been reduced to six including four wide cantilever beams  205  and two narrow cantilever beams  201 . 
     Referring now to  FIGS. 3 a -3 c    arrays of cantilever beams each comprising 18 cantilever beams  301  arranged in two rows are depicted. The moveable end of each cantilever beam  301  is the end in the middle portion of the array. In  FIG. 3 a    triangular regions of moveable elements are provided to both the left and right of the 18 cantilever beams. The triangular region  302  to the right comprises two moveable elements which are adapted to follow the deflections of the cantilever beams  301  in order to form an air tight seal and thus prevent an acoustical leakage through this region  302 . This also applies to the region to the left of the 18 cantilever beams. In  FIG. 3 b    the triangular region  303  comprises four moveable elements which are also adapted to follow the deflections of the cantilever beams  301  in order to form an air tight seal and thus prevent an acoustical leakage through this region  303 . This also applies to the region to the left of the 18 cantilever beams in  FIG. 3 b   . In  FIG. 3 c    the semi-circular region  304  also comprises four moveable elements which are adapted to follow the deflections of the cantilever beams  301  in order to prevent an acoustical leakage. In  FIGS. 3 a -3 c    the cantilever beams  301  and the moveable elements form, in combination, a moveable diaphragm. 
     Turning now to  FIG. 4  various arrangements for connecting a plurality of cantilever beams are depicted via top views. Cantilever beams may be mutually connection in order to form an air tight seal and thus prevent acoustical leakages and/or they may be mutually connected in order to synchronise movements of a plurality of cantilever beams. 
     Referring now to  FIG. 4 a    a single row of seven cantilever beams  402  is depicted. These cantilever beams are either secured to or integrated with a MEMS die  401  which thus forms a frame structure. As depicted in  FIG. 4 a    air gaps  404 ,  405  exist between the cantilever beams  402  and the MEMS die  401 , i.e. next to the cantilever beams  402  (air gap  404 ) as well as at the ends of the cantilever beams  402  (air gap  405 ). As previously mentioned openings or gaps exist between the cantilever beams  402 . As depicted in  FIG. 4 a    a filling material in the form of one or more material layers  403  fill out the openings or gaps between the cantilever beams  402  and thus connect the cantilever beams  402 . The seven cantilever beams  402  thus form an integrated and moveable element. In  FIG. 4 b    two opposing rows of seven cantilever beams  402  are depicted. Again, the cantilever beams are either secured to or integrated with a MEMS die  401  which thus forms a frame structure. As depicted in  FIG. 4 b    air gaps  404 ,  405  exist between the cantilever beams  402  and the MEMS die  401 , i.e. next to the cantilever beams  402  (air gap  404 ), as well as between opposing ends of the cantilever beams  402  (air gap  405 ). A filling material in the form of one or more material layers  403  fill out the openings or gaps between the cantilever beams  402  and thus connect the cantilever beams  402 . The upper and lower rows of cantilever beams thus each form an integrated and moveable element. In  FIG. 4 c    a single row of seven cantilever beams  402  is depicted. Again, these cantilever beams are either secured to or integrated with a MEMS die  401  which thus forms a frame structure. As depicted in  FIG. 4 a    air gaps  404 ,  405 ,  406  exist between the cantilever beams  402  and the MEMS die  401 , i.e. next to the cantilever beams  402  (air gap  404 ), at the ends of the cantilever beams  402  (air gap  405 ) as well as between the third and fourth cantilever beams (air gap  406 ). As depicted in  FIG. 4 c    a filling material in the form of one or more material layers  403  fill out the openings or gaps between the first, second and third cantilever beams  402  (counted from the left) and between the fourth, fifth, sixth and seventh cantilever beams  403 . The seven cantilever beams  402  are thus grouped into two groups of cantilever beams. Referring now to  FIG. 4 d    a single row of seven cantilever beams  402  is depicted again. These cantilever beams are either secured to or integrated with a MEMS die  401  via a bridging element  407 . The MEMS die  401  forms a frame structure relative to the cantilever beams  402  which may be shorter compared to the implementations discussed previously. As depicted in  FIG. 4 d    air gaps  404 ,  405  exist between the cantilever beams  402  and the MEMS die  401 , i.e. next to the cantilever beams  402  (air gap  404 ) as well as at the ends of the cantilever beams  402  (air gap  405 ). Again, a filling material in the form of one or more material layers  403  fill out the openings or gaps between the cantilever beams  402  and thus connect the cantilever beams  402  so that they form an integrated and moveable element. Referring now to  FIG. 4 e    a single row of seven cantilever beams  402  is depicted. The cantilever beams are either secured to or integrated with a MEMS die  401  which thus forms a frame structure. A bridging element  408  connects the moveable ends of the cantilever beams. As depicted in  FIG. 4 e    air gaps  404  exist between the cantilever beams  402  and the MEMS die  401 , i.e. next to the cantilever beams  402  (air gap  404 ). Air gaps  405  also exist between the bridging element  408  and the MEMS die  401 . As previously mentioned openings or gaps exist between the individual cantilever beams  402 . A filling material in the form of one or more material layers  403  fill out these openings or gaps and thus connect the cantilever beams  402 . The seven cantilever beams  402  thus form an integrated and moveable element. 
     It should be noted that the dimensions of the air gaps  404 ,  405 ,  406  remain essentially unaffected during bending or deflection of the cantilever beams  402  thus maintaining the acoustical leakage through the air gaps  404 ,  405 ,  406  at a minimum. The widths of the air gaps  404 ,  405 ,  406  are, as previously addressed, typically in the range between 0.5 μm and 5 μm. 
     Referring now to  FIG. 5  various arrangements for connecting a plurality of cantilever beams are depicted via cross-sectional views. In  FIG. 5 a    four cantilever beams  501  are connected via one or more material layers  502  provided below the cantilever beams  501 . In  FIG. 5 b    four cantilever beams  501  are connected via one or more material layers  502  provided above the cantilever beams  501 . In  FIG. 5 c    four cantilever beams each comprising a piezoelectric material  503  sandwiched between two electrodes  504 ,  505  are connected via one or more material layers  502  provided below the cantilever beams. In  FIG. 5 d    four cantilever beams each comprising a piezoelectric material  503  sandwiched between two electrodes  504 ,  505  are connected via one or more material layers  502  provided below the cantilever beams. A carrier substrate  506  is provided below the one or more material layers  502 . In  FIG. 5 e    four cantilever beams each comprising a piezoelectric material  503  sandwiched between two electrodes  504 ,  505  are connected via one or more material layers  502  and a carrier substrate  506  provided below the cantilever beams. Four additional cantilever beams  501  are provided below the carrier substrate  506 . In  FIG. 5 f    four cantilever beams each comprising a piezoelectric material  503  sandwiched between two electrodes  504 ,  505  are connected via one or more material layers  502  and a carrier substrate  506  provided below the cantilever beams. Four additional cantilever beams each comprising a piezoelectric material  503  sandwiched between two electrodes  504 ,  505  are provided below the carrier substrate  506 . In  FIG. 5 g    four pairs of stacked cantilever beams, i.e. eight cantilever beams in total, where each cantilever beam comprises a piezoelectric material  503  sandwiched between two electrodes  504 ,  505 . The four pairs of cantilever beams are mutually connected via one or more material layers  502  and a carrier substrate  506  provided below the four pairs of cantilever beams. 
     Referring now to  FIG. 6  an implementation relying on a pre-bended cantilever beam  602  is depicted. With reference to the cross-sectional view in  FIG. 6 a    the pre-bended cantilever beam  602  is either secured to or integrated with the MEMS die  601  which thus forms a frame structure relative to the pre-bended cantilever beam  602 . As depicted in  FIG. 6 a    and as previously discussed a small air gap  603  exists between the cantilever beam  602  and the MEMS die  601 . In order to prevent, or at least reduce, acoustical leakage through the air gap  603 , it is dimensioned so that essentially no sound pressure waves above 2 kHz are capable of passing through the air gap  603  which thus functions as an acoustical low-pass filter. Referring now to  FIG. 6 b    a top view of a row of seven pre-bended cantilever beams  605  is depicted. Again, a MEMS die  604  to which the cantilever beams  605  are either secured or integrated with forms a frame structure. Various air gaps  606 ,  607 ,  608  exist between the cantilever beams  605  and the MEMS die  604 . Moreover, air gaps  609  exist between the individual cantilever beams. The widths of the air gaps  603 ,  606 ,  607 ,  608  are, as previously addressed, typically in the range between 0.5 μm and 5 μm. 
     As previously mentioned each of the cantilever beams  605  comprises an integrated drive mechanism in the form of a piezoelectric material sandwiched between two electrodes to which a drive signal may be applied in order to activate the cantilever beams. Moreover, one or more material layers may be provided to connect the seven cantilever beams in order to prevent, or at least reduce, acoustical leakage through the one-dimensional array of cantilever beams. 
       FIG. 7  also shows an implementation relying on pre-bended cantilever beams  702 ,  703 . With reference to the cross-sectional view in  FIG. 7 a    pre-bended cantilever beams  702 ,  703  are either secured to or integrated with the MEMS die  701  which thus forms a frame structure relative to the pre-bended cantilever beams  702 ,  703 . As depicted in  FIG. 7 a    a small air gap  704  exists between the respective ends of the cantilever beams  702 ,  703 . In order to prevent, or at least reduce, acoustical leakage through the air gap  704 , the air gap is dimensioned so that essentially no sound pressure waves above 2 kHz are capable of passing through the air gap  704  which thus functions as an acoustical low-pass filter. In  FIG. 7 b    a top view of two rows of seven pre-bended cantilever beams  706 ,  707  are depicted. Again, the MEMS die  705  to which the cantilever beams  706 ,  707  are either secured or integrated with forms a frame structure. Various air gaps  708 ,  709 ,  710  exist between the cantilever beams  706 ,  707  and the MEMS die  705 . Moreover, air gaps  710  exist between the individual cantilever beams  706 ,  707 . The widths of the air gaps  704 ,  708 ,  709 ,  711  are, as previously addressed, typically in the range between 0.5 μm and 5 μm. Each of the cantilever beams comprises an integrated drive mechanism in the form of a piezoelectric material sandwiched between two electrodes to which a drive signal may be applied in order to activate the cantilever beams. Moreover, one or more material layers may be provided to connect the seven cantilever beams of each row in order to prevent, or at least reduce, acoustical leakage through the two-dimensional array of cantilever beams. 
     In relation to  FIGS. 6 and 7  it should again be noted that the dimensions of the various air gaps remain essentially unaffected during bending or deflection of the cantilever beams thus maintaining the acoustical leakage through the various air gaps at a minimum. 
     Turning now to  FIG. 8  two implementations of miniature speakers are depicted. In  FIG. 8 a    the miniature speaker comprises a front volume  801  and a rear volume  802  being separated by a substrate  804  to which a MEMS die  805  comprising opposing cantilever beams  806  is secured using appropriate means. As depicted in  FIG. 8 a    a small air gap  807  (0.5-5 μm in width) exists between the respective ends of the opposing cantilever beams  806 . The air gap  807  is dimensioned so that essentially no sound pressure waves above 2 kHz are capable of passing through the air gap  807  which thus functions as an acoustical low-pass filter. A through-going opening  808  is provided in the substrate  804  in a manner so that it is acoustically connected to the cantilever beams  806 . Moreover, the front volume  801  is acoustically connected to a sound outlet  803 , and a venting opening  809  is provided between the rear volume  802  and the exterior of the miniature speaker. In  FIG. 8 b    the miniature speaker also comprises a front volume  801  and a rear volume  802  being separated by a substrate  804  to which a MEMS die  805  comprising opposing cantilever beams  806  is secured using appropriate means. Compared to  FIG. 8 a    the front and rear volumes  801 ,  802  have been swapped with the sound outlet now being denoted  811 . As the dimensions of the air gap  807  (0.5-5 μm in width) is essentially unaffected during bending or deflection of the cantilever beams the acoustical leakage between the front and rear volumes  801 ,  802  is maintained at a minimum level.