Patent Publication Number: US-9900700-B2

Title: Digital acoustic device with increased sound power

Description:
TECHNICAL FIELD AND PRIOR ART 
     The present invention relates to a digital acoustic device with increased sound power, for example a digital loudspeaker or a photoacoustic imaging device. 
     Loudspeakers can be found in a large number of equipments such as mobile phones, flat screens, etc. and the miniaturization thereof is desired. MEMS technologies make it possible to have ultrafine loudspeakers. 
     MEMS technology is particularly suited to create digital loudspeakers, for which the large diaphragm of the analog loudspeaker is replaced by several unit diaphragms or more, generally by several small sized ultrafine acoustic transducers, referred to as speaklets, enabling the sound to be reconstituted. 
     In the case of the digital loudspeaker, each speaklet is individually actuated according to the sound to be reconstructed, in a high position or a low position. 
     Nevertheless, the digital loudspeakers provide a low sound level. 
     Solutions have been provided to increase the sound pressure of a microphone. For example, US 2011/0075867 describes a microphone comprising a diaphragm fitted in its centre with a mass, the effect of which is to reduce the resonance frequency of the diaphragm and thus to increase the sound pressure. US 2011/0051985 describes a digital microphone comprising a diaphragm fitted with a piston fixed to the diaphragm. 
     DISCLOSURE OF THE INVENTION 
     As a result, a purpose of the invention is to provide a digital acoustic device, for example a loudspeaker with an increased sound power. 
     The above stated purpose is reached by a digital acoustic device comprising at least one suspended diaphragm, at least one actuator associated with said diaphragm to move it upwards or downwards, and means interrupting the movement of the diaphragm further to activating the actuator associated with the diaphragm. Interrupting means are sized so that the movement of the diaphragm is interrupted when it has a non-zero speed. 
     Preferably, the speed at which the upward or downward movement, due to the use of the associated actuator is stopped, is the maximum or substantially maximum speed that the diaphragm can have. The greater the deceleration of the diaphragm, the greater the sound pressure generated by the movement of the diaphragm. 
     In other words, in the digital acoustic device according to the invention, the movement of the diaphragm is deliberately interrupted, preferably when it has a high speed, or even a maximum speed to obtain a sharp deceleration of the diaphragm and thus generate a high sound pressure. These stop members are therefore sized to interrupt the movement of the diaphragm before it reaches the end of its stroke. 
     Advantageously, the means for stopping the diaphragm during its movement are carried by a substrate facing the diaphragm. They form an element or elements protruding from the substrate towards the diaphragm and are sized to contact the diaphragm when it has a non-zero speed, preferably a high speed and more preferably a maximum speed. Preferably, the distance between the free end of the stop member(s) and the diaphragm at rest is between 50% and 75% of the theoretical maximal stroke of the diaphragm. 
     Alternatively, the means for stopping the diaphragm during its movement are carried by the diaphragm, they form a protruding element or protruding elements and are sized to contact the substrate facing the diaphragm when it has a non-zero speed, preferably a high speed and more preferably a maximum speed. Preferably, the distance between the free end of the stop member(s) and the support when the diaphragm is at rest is between 50% and 75% of the theoretical maximal stroke of the diaphragm. 
     The digital acoustic device can be a digital loudspeaker or a photoacoustic imaging system. 
     The subject-matter of the present invention is therefore a digital acoustic device comprising at least one suspended diaphragm facing a support and at least one actuator associated with said diaphragm, said associated actuator being intended to move said diaphragm away from and/or closer to said support, said device also comprising stop means intended to interrupt the movement of said diaphragm further to activating said actuator when the diaphragm has a non-zero speed, the stop means being sized so as to interrupt the movement of the diaphragm when the movement of the diaphragm is greater than or equal to 50% of the theoretical maximum stroke of the diaphragm and lower than or equal to 75% of the theoretical maximum stroke of the diaphragm. 
     Preferably, the stop means are sized so as to interrupt the movement of the diaphragm when the movement of the diaphragm is between 50% and 60% of the theoretical maximum stroke of the diaphragm. 
     Advantageously, the stop means are sized so as to interrupt the movement of the diaphragm when it moves at its maximum speed or at a speed closer to its maximum speed, i.e. at a speed greater than or equal to 75% of its maximum speed. 
     The stop means can comprise at least one stop member protruding from the support towards the diaphragm and/or protruding from the diaphragm towards the support, and having a free end separated by a non-zero distance from the diaphragm and/or the support at rest, respectively. 
     The stop member can be located facing a central area of the diaphragm or can be fixed in a central area of the diaphragm. 
     Advantageously, the distance separating the free end from the stop member and the diaphragm or the free end from the stop member and the support is between 50% and 75% of the theoretical maximum stroke of the diaphragm. 
     Advantageously, the digital acoustic device comprises a plurality of stop members. 
     Preferably, the stop members are distributed on an area corresponding to a surface representing between 10% and 50% of the diaphragm surface. 
     In an exemplary embodiment, the digital acoustic device comprises a gaseous fluid between the diaphragm and the support, the device comprising at least one passageway in the support for the flow of the gaseous fluid so as to reduce the viscous damping. The passageway can be formed between two stop members. 
     For example, the stop member(s) are in the shape of a column with a circular, square, ellipsoidal or trapezoidal cross-section. 
     The stop member(s) can be integral with the support and/or the diaphragm. 
     According to a further feature, the stop member(s) are formed by one or more layers of materials applied on the substrate and/or the diaphragm. 
     The actuator can be carried by the diaphragm and is facing the free end of the stop member, said device comprising a protective layer deposited on the actuator so as to protect it from the contact with the free end of the stop member. 
     At least one actuator can be formed by a piezoelectric actuator. 
     The digital acoustic device can comprise a first actuator in contact with the diaphragm intended to exert a strain on the diaphragm along a first direction, a second actuator in contact with the diaphragm intended to exert a strain on the diaphragm along a second direction opposite to the first. 
     The first and second actuators can comprise a ferroelectric piezoelectric material, each of the first and second actuators being intended to deform the diaphragm in an opposite direction. 
     In an exemplary embodiment, the first actuator bounds the outer periphery of the diaphragm and the second actuator is substantially located in a central area of the diaphragm. 
     The digital acoustic device can comprise a second support facing the diaphragm opposite to the first support, said second support comprising stop means intended to interrupt the movement of said diaphragm further to activating said second actuator. 
     Preferably, the digital acoustic device comprises a plurality of diaphragms and actuators associated with each of the diaphragms. 
     Another subject-matter of the present invention is also a method for creating a digital acoustic device according the invention,
         a) creating the diaphragm and the actuator,   b) creating the stop means on the support and/or on the diaphragm,   c) assembling the diaphragm and the actuator with the support so that the stop means are facing the diaphragm and/or the support respectively, at a given distance when the diaphragm is at rest.       

     The stop member can also advantageously be created simultaneously to at least one electric connection of the actuator, between the support and the actuator. 
     In an exemplary embodiment, prior to creating the stop member and the electric connection, an electric line can be created, the electric connection being formed on said electric line, so that the height of the electric line connection assembly is greater than the one of the stop member. 
     In another exemplary embodiment, prior to creating the stop member, a recess can be created in an area of the support where the stop member is formed so that the height of the support and electric connection assembly is greater than the one of the support and stop member assembly. 
     The stop member is for example created in said substrate and/or the diaphragm by etching. 
     For example, assembling the support to the diaphragm is made by thermocompression and/or gluing, for example by molecular gluing. 
     Steps a) and b) are advantageously carried out by microelectronic techniques. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be better understood using the following description and the accompanying drawings in which: 
         FIG. 1A  is a top view of an exemplary diaphragm fitted with an actuator for a digital loudspeaker of the invention, 
         FIGS. 1B and 1C  are side views of the diaphragm of  FIG. 1A  in two different states, 
         FIG. 2  is a side view of an exemplary embodiment of the loudspeaker according to the invention, 
         FIG. 3  is a side view of another exemplary embodiment of the loudspeaker according to the invention, 
         FIG. 4  is a graphic representation of the acceleration of the diaphragm as a function of time t for a state of the art system and a loudspeaker according to the invention, 
         FIGS. 5A to 5R  are diagrammatic views of steps for creating a diaphragm and actuators of a loudspeaker according to an exemplary embodiment, 
         FIGS. 6A to 6D  are diagrammatic views of steps for creating a support fitted with stop means of the invention according to an exemplary embodiment, 
         FIG. 7  is a diagrammatic view of an alternative method of  FIGS. 6A to 6D , 
         FIGS. 8A to 8C  are diagrammatic views of steps for creating a support fitted with stop means of the invention according to another exemplary embodiment, 
         FIG. 9  is a diagrammatic view of a loudspeaker comprising stop means facing each of the diaphragm faces. 
     
    
    
     DETAILED DISCLOSURE OF PARTICULAR EMBODIMENTS 
     In the following description, the invention will be described while considering a digital loudspeaker but it will be understood that the invention also relates to a photoacoustic imaging system, and more generally to a digital acoustic device. 
     A digital loudspeaker comprises a plurality of acoustic transducers or individually controlled speaklets. The sound to be reproduced is reconstructed by the principle of additivity of the elementary sounds of the speaklets in air. In the following description, an elementary loudspeaker comprising one speaklet will be considered. 
     In  FIGS. 1A to 1C and 2 , a particularly advantageous exemplary elementary loudspeaker with a piezoelectric actuation can be seen. In  FIGS. 1A to 1C , only the diaphragm is represented with the actuators. In  FIG. 2 , the digital loudspeaker comprising a disk-shaped diaphragm  2  is suspended on a support  4 , a ring-shaped member of a piezoelectric material  6  located on an upper face of the diaphragm  2  and on the outer edge of the diaphragm  2  can be seen. 
     The outer periphery of the ring  6  is on the support  4  and the inner periphery is on the diaphragm  2 . The ring is connected to a voltage or current source  8  as schematized in  FIGS. 1C and 18  so as to form a first actuator able to set in motion the diaphragm  2 . To do so, an electrode is provided on the upper face and the lower face of the ring  6  to ensure its connection to the voltage source  8 . 
     In the example shown, the diaphragm device advantageously also comprises a second member of a piezoelectric material  10  as a disk in the example shown, and located in a central part of the upper face of the diaphragm  2 . The disk  10  is also connected to a voltage or current source  12  as schematized in  FIGS. 1C and 18  so as to form a second actuator able to set in motion the diaphragm  2 . An electrode is provided on each face of the ring to ensure its connection to the voltage source  8 . 
     In another alternative, the diaphragm can have a square or rectangular shape, in this case the actuator can have a shape similar to the one of the diaphragm but with a different surface. 
     It is to be noted that the second actuator does not have any part of its surface anchored to the support part. 
     The first and second actuators can be made with the same piezoelectric materials or with different piezoelectric materials. 
     In an exemplary embodiment, the actuators are made from ferroelectric piezoelectric materials such as PZT. The movements of the diaphragm obtained thanks to these actuators are those represented in  FIGS. 1B and 1C . 
     Indeed, whatever the sign of the applied voltage, if the latter is greater in absolute value than the coercive field of the ferroelectric piezoelectric material, then the latter material expands in thickness and radially contracts. Consequently, upward or downward movements of the diaphragm are caused by the shape and the position of the actuator on the diaphragm and not by the sign of the control voltage. 
     In the example shown, the application of a voltage on the first actuator  6  causes an upward movement of the diaphragm  2 , it then has a convex shape relative to the support  4 . The application of a voltage on the second actuator  10  causes a downward movement of the diaphragm  2  which then has a concave shape relative to the support  4 . 
     In another exemplary embodiment, the actuators are made from piezoelectric materials such as, for example, AlN, ZnO, etc. A positive voltage causes the expansion of the piezoelectric material whereas a negative voltage will induce its contraction. Thus, upward and downward movements can be obtained using a single actuator. 
     The amplitude of the movement of the diaphragm is proportional to the voltage applied to the terminals of the actuators. 
     The advantage of implementing two actuators is to be able to move the diaphragm upwards or downwards, which enables a loudspeaker providing a fine sound reproduction to be more easily made. 
     However, a loudspeaker comprising a single actuator does not depart from the scope of the present invention. Besides, the digital acoustic device according to the invention can comprise actuator types other than piezoelectric actuators, they can be electrostatic, magnetic, thermal etc. actuators, which are well-known of those skilled in the art. 
     According to an exemplary embodiment, and as can be more particularly seen in  FIG. 2 , protruding members are provided on a substrate facing the diaphragm and towards the diaphragm so as to form movement stop members of the diaphragm when it moves at a non-zero speed. The height of the stop members  14  is chosen so that their free end  14 . 1  contacts the substrate while the diaphragm has a non-zero speed. The distance between the diaphragm at rest and the free end  14 . 1  of the stop members is referred to as h. Preferably, the height of the stop members is such that the free end of the stop members contacts the diaphragm while the diaphragm has its maximum or substantially maximum speed. Alternatively, the protruding members can be located under the diaphragm in the view of  FIG. 2 . 
     Advantageously, the distance h is between 50% and 75% of the theoretical maximum stroke of the diaphragm. 
     Preferentially, the height of the stop members depends on the deformation of the diaphragm. 
     In the case of stop members all having the same height, the stop members located closest to the centre of the diaphragm will contact the facing diaphragm first. Due to the inertia produced by its movement, the diaphragm will continue to deform during a short time, and the more peripheral stops will then in turn come in contact. 
     Preferably, the stop members are located facing a central area of the diaphragm having the greatest deformation, it is also the area having the highest speed. The central part has for example half the diameter of the diaphragm. The stop members are distributed on part of the diaphragm surface so as not to risk damaging, or even breaking the diaphragm. 
     The distance between the free end of at least one stop member  14  and the diaphragm at rest in then lower than the theoretical maximum stroke of the diaphragm, so as to ensure a contact between the free end  14 . 1  of the stop member and the diaphragm before it has a zero speed. 
     Placing the stop members facing this area enables a more significant deceleration and therefore the generation of a high sound pressure to be caused. 
     From single stop member  14  up to several tens or even several hundreds stop members can be provided. 
     Preferably, the stop member(s) are distributed on an area having a surface corresponding to between 10% and 50% of the diaphragm surface. 
     The stop members can have any shape, for example a cylindrical shape with a circular, ellipsoidal cross-section, a shape of a straight block, etc. 
     The cross-section of the stop members is determined as a function of the number of stop members, and/or of the diaphragm surface and/or the diaphragm rigidity. The cross-section of the stop members can for example be comprised between a few tens of μm 2  and a few mm 2 . 
     The distance between the stop members is also chosen as a function of the diaphragm surface and/or the diaphragm rigidity. For example, the more flexible the diaphragm, the closer the stop members to each other in order to limit, or even avoid the parasitic deformations of the diaphragm. This distance is preferably between a few tens of μm and a few mm, this distance further enabling a gaseous damping which can be induced by the presence of these stop members to be reduced. 
     Advantageously, at least one passageway  16  is provided in the substrate facing the diaphragm in the substrate in order to limit the occurrence of a viscous damping when the diaphragm gets closer to the stop members, this passageway enabling air or any other gaseous fluid to flow and the movement of the diaphragm to be hardly or not dampened. In the example shown, passageways  16  are created between the stop members. 
     Advantageously, a protective layer is formed at least on the central actuator  10  so that the contact between the stop members and the actuator does not damage it. 
     In other actuator embodiments, the stop members are preferably provided relative to the actuator(s) so as not to contact the actuator. For example, in the configuration of the actuators  6  and  10  of  FIG. 1 , the stop members are disposed between the actuators  10  and  6 . Considering a diaphragm having a small surface, disposing the stop members only on a rim is sufficient to stop the diaphragm and not induce a too great deformation of the centre thereof. 
     The distance between the rest position of the diaphragm and the free end  14 . 1  of the stop member is determined as a function of the dimension of the diaphragm and of the materials forming it, in particular the mechanical properties thereof, especially defined by the Young modulus, the density and the Poisson coefficient, these parameters setting the maximum stroke of the diaphragm. 
     In order to determine the maximum stroke of the diaphragm, the movement of the diaphragm is calculated. The latter can be calculated by finite element calculation softwares such as the ANSYS, COVENTOR, etc. softwares, or analytically as in the following equation giving the dynamic movement of the diaphragm: 
     
       
         
           
             
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     g 31  is the piezoelectric coefficient which links the electric field applied out-of-plane (direction 3) and the stress in the plane (direction 1), V refers to the applied voltage. S 11   D  and S 12   D  are respectively the relative deformations in the plane (directions 1 and 2) obtained in response to a stress applied along the direction 1. The exponent D means “with a constant load”. b is the radius of the piezoelectric layer. This equation is for example explained in document Li, “ Theoretical modelling of a circular piezoelectric actuator for micro systems”, ICINA  2010. The distance between the diaphragm at rest and the free end  14 . 1  of the stop member  14  is later determined. 
     This distance is chosen sufficient to enable the diaphragm to reach a significant speed, preferably its maximum speed. Preferably, the contact between the diaphragm and the free end of the stop member occurs before the diaphragm decelerates. A distance between the diaphragm at rest and the free end of the stop member will be preferably chosen 50% and 75% of the maximum stroke of the diaphragm, preferably between 50% and 60%. 
     By way of example, for a diaphragm having a radius of 500 μm for example, the deflection of the diaphragm will be in the order of 3 μm. If h is chosen to be at 50% of the maximum stroke, h will then be 1.5 μm. 
     In  FIG. 3 , another exemplary embodiment can be seen, in which the stop members are carried by the diaphragm. In this case, the distance h′ between the free ends  14 . 1 ′ of the stop members  14 ′ and the substrate when the diaphragm is at rest is preferably set to be between 50% and 75% of the theoretical maximum stroke of the diaphragm. 
     The features of the stop members carried by the substrate, such as the cross-section, the spacing, etc. described for the stop members  14  also apply to the stop members  14 ′. 
     In  FIG. 4 , the variation in the acceleration a of the loudspeaker diaphragm of the invention (curve referred to as I) and the variation in the acceleration of a state of the art loudspeaker diaphragm without a stop member (curve referred to as II) as a function of time, are shown. 
     The surface areas of both curves are identical, but the peak referred to as p due to the deceleration of the diaphragm is sharper than the peak of the curve II and its amplitude is greater than the one of the peak of the curve II during the acceleration, A refers to the contact between the stop member(s) and the diaphragm in the embodiment of  FIG. 2 . 
     But the sound pressure caused by a speaklet and thus the capacity of the digital loudspeaker to reconstitute an audible sound of the desired sound level, depend on the speed (acceleration a) of the speaklet diaphragm and of its capacity not to cause parasite, that is its capacity not to have parasitic oscillations. 
     The sound pressure as a function of the acceleration is written as follows: 
     
       
         
           
             
               P 
               ⁡ 
               
                 ( 
                 
                   r 
                   , 
                   t 
                 
                 ) 
               
             
             = 
             
               
                 
                   
                     ρ 
                     0 
                   
                   · 
                   
                     a 
                     2 
                   
                 
                 r 
               
               ⁢ 
               
                 
                   dV 
                   ⁡ 
                   
                     ( 
                     t 
                     ) 
                   
                 
                 dt 
               
             
           
         
       
       
         
           or 
         
       
       
         
           
             
               P 
               ⁡ 
               
                 ( 
                 
                   r 
                   , 
                   t 
                 
                 ) 
               
             
             = 
             
               
                 
                   
                     ρ 
                     0 
                   
                   · 
                   
                     a 
                     2 
                   
                 
                 r 
               
               ⁢ 
               
                 Acc 
                 ⁡ 
                 
                   ( 
                   t 
                   ) 
                 
               
             
           
         
       
       
         
           or 
         
       
       
         
           
             
               P 
               ⁡ 
               
                 ( 
                 
                   r 
                   , 
                   t 
                 
                 ) 
               
             
             = 
             
               
                 
                   
                     ρ 
                     0 
                   
                   · 
                   S 
                 
                 
                   π 
                   · 
                   r 
                 
               
               ⁢ 
               
                 Acc 
                 ⁡ 
                 
                   ( 
                   t 
                   ) 
                 
               
             
           
         
       
     
     with 
     ρ0: per volume ratio of air 
     a: radius of the mobile member 
     S: surface of the mobile member 
     r: listening distance 
     V: speed of the mobile member 
     Acc: acceleration of the mobile member 
     If a increases, the sound pressure will increase. The above equation also applies with the deceleration. According to the present invention, the deceleration is increased. By increasing the latter, the sound pressure is also increased. 
     A particularly advantageous exemplary of a method for manufacturing a loudspeaker of the invention will now be described. 
     The steps are schematically shown in  FIGS. 5A to 5R . 
     For example, a silicon substrate  100  shown in  FIG. 5A  is used, having for example a thickness of 725 μm and a diameter of 200 mm. 
     During a first step, a thermal oxidation of the substrate is carried out so as to form an oxide layer  102  on all the surfaces of the substrate with for example a thickness of 2 μm. The member thus obtained is shown in  FIG. 5B . 
     Then, an oxide hard mask  104  is made on the back face of the substrate. This mask has for example a thickness of 7 μm. The mask is made by turning over the substrate; as a function of the chosen deposition composition, it is possible to deposit the mask only on this face. It can be, for example, PVD (Physical Vapour Deposition). The member thus obtained is shown in  FIG. 5C . 
     Then, a lithography at the back face is carried out so as to reach the silicon. The member thus obtained is shown in  FIG. 5D . 
     During a following step, the hard mask is etched at the back face, for example by a Reactive-Ion Etching (RIE), so as to reach the back face of the substrate  100 . The member thus obtained is shown in  FIG. 5E . 
     During a following step, the oxide layer is removed from the front face, for example by stripping or chemical etching. The member thus obtained is shown in  FIG. 5F . 
     During a following step, an oxide layer  106  is formed at the front face. Advantageously, a densification annealing occurs for example at a temperature in the order of 800° C. The member thus obtained is shown in  FIG. 5G . 
     During a following step, a layer  108  is formed at the front face intended to form the diaphragm  2 , and a layer  110  is formed at the back face. Preferably, these layers are for example of polysilicon, SiC or SiO 2 . The thickness of the layers  108 ,  110  is for example between a few hundreds of nm and several μm, or even several tens of μm. 
     The layers  108 ,  110  are for example made by a chemical vapour deposition (CVD) or by epitaxial growth. Preferably, the stresses of the layers  108 ,  110  are controlled. 
     The layers  108 ,  110  can be formed in several steps. For example, for a 4 μm thickness, two 1.5 μm thick layers and one 1 μm thick layer are successively created; 
     Advantageously, an annealing step then occurs. The member thus obtained is shown in  FIG. 5H . 
     During a following step, a layer  112  is formed on the layer  108 , for example of SiO 2  or SiN, having for example a thickness between a few hundreds of nm and several μm. The layer  112  is for example formed by thermal oxidation or CVD deposition. Advantageously, a densification annealing occurs for example at a temperature in the order of 800° C. 
     The member thus obtained is shown in  FIG. 5I . 
     During a following step, the first and second actuators are created. 
     To do so, a layer  114  intended to form the lower electrodes of the actuators is first made, for example of Pt, Mo. The layer  114  is for example made by deposition on the layer  112 . The layer  114  has for example a thickness between a few tens of nm and a few hundreds of nm. The member thus obtained is shown in  FIG. 5J . 
     A layer of piezoelectric material  116  is then formed on the layer  114 , in particular of PZT, AlN, ZnO, LNO the thickness of which is for example between a few hundreds nm and several μm. 
     The upper electrode is then made by forming a layer  118  on the piezoelectric material  116 , for example of Ru or Au, for example with a thickness between a few tens of nm and several hundreds of nm. The member thus obtained is shown in  FIG. 5K . 
     Then, the steps of etching occur. 
     First, the layer  118  is etched so as to delimit the annular actuator  8  and the disk-shaped actuator  10 . 
     Then, the piezoelectric material layer  116  is etched. 
     The member thus obtained is shown in  FIG. 5L . 
     Then, the remaining portions of the layer  118  are again etched so that they are recessed relative to the portions of the layer  116 . 
     The layer  114  as well as the oxide layer  112  are then etched. The member thus obtained is shown in  FIG. 5M . 
     Preferably, a stepped profile is made. The latter is obtained since all the layers are deposited and then etched, from the upper layer, by using different photolithography masks, the second mask being wider than the first, etc. This makes it possible to leave safety margins to avoid the overlapping of layers which could occur due to the positioning uncertainty of the masks. Any electric short circuit between the electrodes is thus avoided. The member thus obtained is shown in  FIG. 5N . 
     During the following steps, reconnection pads  120  are made. A layer  122  of dielectric material, for example of SiO 2 , is previously formed on the edges of the stacks formed by the lower and upper electrodes and by the piezoelectric material, this layer being etched so as to partially clear the lower and upper electrodes. The member thus obtained is shown in  FIG. 5O . 
     Then, a layer, for example of AlSi or TiAu, is formed and etched, thus forming contact pads at the areas where the electrodes have been cleared. The member thus obtained is shown in  FIG. 5P . 
     Advantageously, during a following step, a protective layer  124  is formed on the actuators, for example an oxide layer, in order to protect the actuators from the contact with the stop members. The thickness of this layer can be between a few hundreds of nm and a few μm, for example 500 nm. 
     During a following step, the layer  124  is etched to access the reconnections. 
     The member thus obtained is shown in  FIG. 5Q . 
     Preferably, during a following step, the actuators are protected, for example by depositing a dry film  126 . Then, the back face is etched in order to release the diaphragm  2 . 
     The diaphragm is released by deep etching of the substrate through the back face until the diaphragm is reached. 
     The member thus obtained is shown in  FIG. 5R . 
     The creation of the stop members will now be described, in this example, the latter are made on an “interposer”-type substrate, i.e. comprising electric connections and/or electronic circuits (control electronic circuits, sensors, etc.). In the example shown, the stop members are very advantageously made simultaneously to the electric connections  18 , also referred to as microbumps or copper pillars. The embodiment is thus quicker. The stop members have therefore a structure similar to the one of the electric connections. The electric connections are intended to route the signal from the speaklets to the pads at the periphery of the substrate  200  or to the electronics if it is made on the substrate  20 . In the described example, only one stop member is formed, but it will be understood that several stop members can be simultaneously formed. 
     To do so, let us take for example a silicon substrate  200  ( FIG. 6A ). 
     During a first step, lines intended to bring the signal from the speaklet to contact pads (not shown) are made at the periphery of the substrate  200 , they are for example copper lines. Then, on one of the faces, a layer, for example of TiCu  202 , is formed. Then, by means of a resin, areas intended to form thick copper layers are delimited. The copper layers  204  are then formed for example by growth. Then the resin is removed and the TiCu layer is etched. 
     The member thus obtained is shown in  FIG. 6B . The copper lines are only present vertically aligned with the contact pads connected to the electrodes. The area of the substrate  200  at least vertically aligned with the central area of the diaphragm does not yet comprise any layer. 
     During a following step, a TiCu layer  206  is again deposited at the locations where electric connections and stops are desired to be made, and then the Cu growth areas are delimited and the copper portions  208  are made to grow. During this step, microbumps are made on both copper lines and in the area of the substrate  200  in line with the central area of the diaphragm. 
     SnAg layers  210  are then formed on the three copper  208  and SnAg  210  portions. The resin is removed and the TiCu layer is etched. The portions  208  and  210  have a reduced cross-section relative to the one of the portions  204 . 
     Advantageously, during a following step, passageways  16  can be made in the substrate between the electric connections and the stop member  14  for example by etching; these passageways being intended to reduce the viscous damping as above described. 
     The member thus obtained is shown in  FIG. 6C . 
     Then, the diaphragm  2  and the actuators  6 ,  10  shown in  FIG. 5R  and the substrate fitted with the electric connections and the stop member  14  of  FIG. 6C  are assembled. The electric connections  18  are aligned with the contact pads, and then contacted with the contact pads. Assembling is for example made by thermocompression. The loudspeaker is shown in  FIG. 6D . In practice, the dry film is removed after assembling both substrates. 
     In this example, the height of the stop member is identical to the one of the electric connections, however the stop member being directly made on the substrate  200 , its free end  14 . 1  does not contact the diaphragm at rest. The distance between the free end  14 . 1  and the diaphragm referred to as h is thus determined in this exemplary embodiment by the thickness of the portions of the connection lines. In order to determine h, the diaphragm carrying the actuators is considered. In  FIG. 6D , h is the distance between the free end  14 . 1  and the actuator  6 . In the case of a loudspeaker comprising another type of actuator, the latter would not necessarily be taken into account in calculating the distance h. 
     The distance h between the free end  14 . 1  of the stop member and the diaphragm is chosen so as to be lower than the theoretical maximum stroke of the diaphragm, preferably h is comprised between 50% of the theoretical maximum stroke and 75% of the theoretical maximum stroke of the diaphragm. The maximum deformation of the diaphragm is determined from the dimensions of the diaphragm. 
     In an exemplary embodiment in which only one actuator is used, the method for creating the stop members and the connection(s) is similar to the one described above. 
     In  FIG. 7 , an alternative embodiment of the stop members can be seen, in the case where the control electronics is made on the substrate  200 . In this case, the copper traces are not required to set the height h of the stop members but only to route the signal up to pads or to the electronics. The electric connections can be directly made on the substrate by depositing a Ti/Cu layer and a thick copper growth. The distance h between the free end  14 . 1  of the stop member and the diaphragm is then obtained by creating a recess  20  in the electronic substrate  200  at the future location of the stop member(s) prior to making the electric connections and the stop member. This recess has a depth h. This recess is for example made by partially etching of the electronic substrate  200 . The stop member has the same height as the microbumps but, due to the presence of the recess having a depth h, the free end  14 . 1  of the substrate is at a distance h of the diaphragm at rest. 
     In  FIGS. 8A to 8C , another exemplary embodiment of the stop members can be seen. In this case, the substrate is of the “packaging”-type, i.e. the function of the substrate is to overlap the diaphragm in order to package it. 
     Let us take for example substrate  300  of silicon ( FIG. 8A ). 
     Then, a layer  302  for the sealing with the diaphragm is formed; it is for example a gold or oxide layer. 
     The sealing layer is then etched in order to leave this layer only on the periphery of the substrate. The substrate is then structured, for example by partially etching, in order to create the stop member  14 . The depth of the etching is chosen so as to obtain the desired distance h between the free end  14 . 1  of the stop member and the diaphragm. The depth of the etching takes into account the thickness induced by the assembly, for example by gold-gold gluing or molecular gluing. A thickness of the sealing layer has to be taken into account. 
     The member thus obtained is shown in  FIG. 8B . The stop member is integral with the “packaging”-type substrate. 
     This member is then sealed with the diaphragm of  FIG. 5Q , for example by gold-gold gluing, by molecular assembly ( FIG. 8C ). 
     It should be noted that the stop member could be made on the “packaging”-type substrate by inserting material as in the example of  FIG. 7 . Conversely, it could be considered, in the case of a interposer substrate, to make a stop member or stop members integral with the interposer substrate. 
     In  FIG. 9 , an alternative embodiment can be seen in which the loudspeaker comprises stop members of the diaphragm facing its two faces. This embodiment is particularly suited to the speaklet of  FIGS. 1A to 1C  which comprises two actuators able to move the diaphragm upwards and downwards in the view of  FIG. 8C . As indicated above, actuating the diaphragm in both directions enables the sound to be reproduced more finely. 
     The loudspeaker of  FIG. 9  is for example made as described in relation with  FIGS. 8A and 8B , but it is not limiting. 
     Then, a second substrate such as the one of  FIG. 8B  is made. 
     The substrate supporting the diaphragm is then thinned so as to make its thickness lower than a maximum stroke of the diaphragm. It is possible to totally suppress the supporting substrate. 
     Then, the substrate  300  is assembled facing the face of the diaphragm opposite the one carrying the actuators. 
     In the example shown, the stop members  14  on either side of the diaphragm  2  have the same height but they could have different heights, for example as a function of the thinning level of the supporting substrate of the diaphragm. 
     Furthermore, it could be provided to assemble on the sub-assembly of  FIG. 6C  and to assemble a substrate of  FIG. 8B . 
     In the embodiment where the stop members would be carried by the diaphragm, their creation would be for example obtained by depositing material after the step shown in  FIG. 5P  or the one shown in  FIG. 5Q . For example, the stop members would be made by keeping part of the original substrate  100 . The substrate would then be thinned to the desired thickness of the stop members, then the diaphragm would be released by etching the substrate, it could then be provided to leave the stop members. 
     The digital acoustic device according to the invention provides an increased sound power with a relatively simple structure. Furthermore, the creation method is hardly complexified relative to the creation of prior art digital acoustic devices, in particular in the case of the creation of microbumps. 
     It is particularly suited for making digital loudspeakers comprising a plurality of speaklets.