Patent Publication Number: US-11039252-B2

Title: Membrane plate structure for generating sound waves

Description:
FIELD OF INVENTION 
     The present invention relates to a membrane plate structure for generating sound waves and to a loudspeaker comprising the membrane plate structure. 
     ART BACKGROUND 
     Loudspeaker, in particular in micro-speakers for portable devices (mobile phones), and more in particular receiver micro-speaker (also called ear-pieces, responsible for the voice sound-transmission), needs thin elements in order to reduce the overall size of the loudspeaker. In general, a loudspeaker comprises a diaphragm which is excited by a coil or another vibrating element. 
     In US 2013/0016874 A1 for example this function is represented by the element  121  of a diaphragm  12  which guarantees high break-up frequency and low weight. This element is often called membrane plate, to be distinguished from the surround (connecting area  123 ) which is often called membrane. The characteristics required by a membrane plate are:
         a. High material resonance frequency—to guarantee a linearity and the absence of acoustic peaks in the hearable region   b. Low weight—to reduce the moved mass and consequently increase the sound pressure level and the efficiency of the speaker   c. High temperature resistance—to guarantee the same mechanical stiffness at higher working temperatures       

     The resonance frequency of a material is directly proportional to its length and width and a figure of merit, here defined “Frequency Factor”. The frequency factor is defined as follow: 
             d   ⁢       B   ρ             
Where d, is the total thickness, B is the bending module, and ρ is the density of the membrane plate material. The square root is also the speed of sound of the material.
 
     The break-up frequencies of a (micro-) loudspeaker are dependent from the mechanical system formed by the coil and the membrane plate. Some break-up modes are partially dependent from the coil mechanical properties (here defined as coil modes), some other are dependent only from the membrane plate properties (here defined as plate modes). The membrane plate mechanical properties are strongly affecting also the coil modes. 
     In micro-speakers, due to very small available thickness, the membrane plates are generally having a total thickness lower than 500 μm. 
     For this applications, due to the low available thickness, in order to achieve high frequency factors, it is necessary to utilize high mechanical performance materials. Sandwich constructions represent in general the best solution for this application, since they offer the best ratio of bending module to weight (see also “An Introduction to Sandwich Construction”, Zenkert, D., 1995, Engineering Materials Advisory Services Ltd). 
     For these reasons, in micro-speaker applications, the actual state of the art is the use of a flat (or nearly flat) sandwich composite membrane plates, where the skin layers are aluminum foils between 8 and 20 μm, and the core layer is a very thin foam layer between 100 and 400 μm (disclosed for example in CN 204707266 U). The total weight of this sandwich oscillates normally between 80 and 160 g/m 2 . 
     The market is continuously looking for technical solutions which could improve the frequency factor at thicknesses lower than 500 μm and weight under 160 g/m 2 . 
     For some applications the market is looking for non-conductive materials. 
     Fiber reinforced composites are offering very high ratio of stiffness to weight among the all available materials. The characteristics of their unidirectional (UD) tape is to offer extremely high stiffness in the fiber direction, and very low stiffness in the perpendicular direction. To solve this problem, normally a multiple ply (0/90° or 30°/30°, etc) of UD tapes is formed, which has an improved anisotropy (in the direction of the plies), but its stiffness in both directions is lower since only one ply is contributing to the stiffness of its UD direction. 
     Example in table 1. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Young 
                   
                   
                   
               
               
                   
                 Young 
                 Modulus 
               
               
                   
                 Modulus 
                 perpendicular 
               
               
                   
                 in fiber 
                 to fiber 
                   
                   
                 Area 
               
               
                   
                 direction 
                 direction 
                 Density 
                 Thickness 
                 Density 
               
               
                 Material 
                 [GPa] 
                 [GPa] 
                 [g/cm 3 ] 
                 [μm] 
                 [g/m 2 ] 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 UD Aramid 
                 85 
                 4.5 
                 1.33 
                 150 
                 200 
               
               
                 0/90° 
                 45 
                 45 
                 1.33 
                 300 
                 400 
               
               
                 Aramid Ply 
               
               
                   
               
            
           
         
       
     
     Multiply fiber composites are very well known in the loudspeaker industry as diaphragm material thanks to their very high speed of sound. Their usual applications are as simple multi-ply (0/90°) or as skin layers of sandwich construction of a total thickness higher than 2 mm, like the construction indicated in U.S. Pat. No. 5,701,359A. 
     SUMMARY OF THE INVENTION 
     There may be a need to provide a component for a loudspeaker with very small space requirements (micro-speaker) 
     According to a first aspect of the present invention, a membrane plate structure comprising a membrane plate is attachable to a coil or another vibrating element for generating sound waves is presented. The membrane plate comprises at least one layer of thin UD (Uni Directional) fiber tape. In an exemplary embodiment, the fibers are oriented along the direction of the shorter size of the membrane plate geometry ( FIG. 2 ). 
     The fibers, i.e. the fiber tape, used for the membrane plate according to the present invention, may be formed of a polymer matrix reinforced by fibers. The membrane plate is made of plastic as a matrix material, in particular a thermoplastic plastic, a thermoset plastic or an elastomer plastic. 
     According to an exemplary embodiment of the present invention, the membrane plate has a different width with respect to its length (for example, the membrane plate has a rectangular form). The width is shorter than the length. The fibers of the UD layer are oriented along a fiber direction having an angle between approx. −30° and approx. +30°, in particular between approx. −15° and approx. +15°, more in particular approx. 5° and approx. +5°, with respect to the width (direction) of the membrane plate. Specifically, the fiber direction may be parallel to the width (direction) of the membrane plate. The membrane plate has a different width with respect to its length, wherein the width is shorter than the length. The width (direction) is defined as the shortest distance between opposing edges of the membrane plate. 
     In rectangular (micro-) loudspeakers according to the present invention, a thin UD tape displaced as membrane plate material with the fibers directed in the shorter (width) direction of the plate has a higher break-up mode than if directed toward the longer (length) direction of the plate. 
     This effect is shown both in simulations and in real measurements. 
     Main advantages of using a Fiber UD Tape along the shorter size of the membrane plate are:
         Possibility of creating membrane plate materials with speed of sound higher than aluminum (up to 20 times higher)   Possibility of creating low weight plate materials lower than 160 g/m 2      Possibility of creating sandwich materials with fiber UD tape as skin layers with total weight lower than 160 g/m 2      Possibility to increase the break-up frequency of a micro-speaker compared to a state of the art material (sandwich with aluminum as skin layers)   Possibility to reduce the thickness and/or the weight of the membrane plate obtaining the same break-up frequency of a state of the art material (sandwich with aluminum as skin layers).   Possibility of creating non-conductive high performance membrane plates.       

     Drawbacks of these materials are their high total mass, which is making them in general suitable only for woofer or sub-woofer, and their anisotropy outside the UD directions. 
     Unidirectional fiber-reinforced materials are not used in normal speakers due to their similar size of the length and width (mostly round) and their dimension (normally larger than 30 mm). 
     In micro-speaker application the utilization of a multi-ply is not effective since normally they are available only at masses over 200 g/m 2 . Moreover, even if they would be available, at the same mass their frequency factor would be worse than the one of aluminum sandwich (CIMERA ADR120-8H) (see table 2) 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 Bending 
                   
                 Area 
                 Frequency 
               
               
                   
                 Thickness 
                 Modulus 
                 Density 
                 density 
                 Factor 
               
               
                   
                 [μm] 
                 [Gpa] 
                 [kg/m 3 ] 
                 [g/m 2 ] 
                 [m 2 /s] 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 0°/90° Aramid 
                 59 
                 45 
                 1350 
                 80 
                 0.34 
               
               
                 Multiply 
               
               
                 0°/90° HM 
                 50 
                 135 
                 1500 
                 80 
                 0.46 
               
               
                 Carbon Multiply 
               
               
                 CIMERA 
                 120 
                 25 
                 800 
                 80 
                 0.67 
               
               
                 ADR120-8H 
               
               
                   
               
            
           
         
       
     
     A very important characteristic of micro-speakers is their rectangular form, which allows the best use of space. This form is causing also the utilization of rectangular membrane plates. 
     According to further embodiment of the present invention, the membrane plate material is constituted by two skin layers made of thin UD tape, and a core layer, constituting a sandwich structure. The UD skin layers are both parallel and directed along the shorter size of the plate. 
     A thin fiber UD tape is defined as a fiber reinforced plastic tape with an area density comprised between 5 and 100 g/m 2 . 
     According to a further embodiment of the present invention, the core layer of the sandwich structure is a material which is free of pores (e.g. free of pores having a size larger 1 μm) and act as binding elements between the two skin layers. 
     According to a further embodiment of the present invention, the core layer is a porous material, like a foam or a honeycomb. Usual structural foam can include polyester foams, polyurethane foams, polysulfonic foams, polyvinylchloride foams, PMI foams, etc. 
     According to a further embodiment of the present invention, the core layer is a fiber UD tape perpendicular to the direction of the fiber UD tape of the skin layers. 
     According to an exemplary embodiment, the plate material has a HDT (heat deflection temperature) higher than 80° C., in particular higher than 130° C., further in particular higher than 180° C. measured along the fiber direction. 
     According to an exemplary embodiment, the plate material maintains its geometrical dimensions (change in size lower than 5%) under temperatures higher than 130° C., higher than 180° C. and higher than 220° C. 
     According to an exemplary embodiment, the plate material is suitable as insert for an insert molding process. 
     According to an exemplary embodiment, the membrane plate material is characterized by having an area density lower than 200 g/m 2 , preferable lower than 160 g/m 2 , further in particular lower than 120 g/m 2 . 
     According to an exemplary embodiment, the membrane plate material is characterized by having a total thickness lower than 500 μm. 
     According to an exemplary embodiment, the fiber UD tape material is constituted by materials which are non-conductive. The non-conductive fibers can be constituted by polymer fibers such as LCPs (liquid crystal polymer), aramides, PBO (Zylon fibres), UHMWPE (Ultra-high-molecular-weight polyethylene) and/or ceramic fibers. The plastic which is reinforced by the fibers can be a thermoplastic plastic, a thermoset plastic or an elastomer plastic. 
     According to an exemplary embodiment, the fiber UD tape material is constituted by carbon based fibers. These fibers can be high strength, intermediate modulus, high modulus, ultra high modulus and pitch fibers (Young modulus higher than 600 GPa). 
     According to an exemplary embodiment, the UD fiber skin layer of the sandwich construction are characterized by an area density lower than 50 g/m 2 , better lower than 40 g/m 2 , at best lower than 30 g/m 2  for each skin layer. 
     According to an exemplary embodiment, the membrane plate structure extend within a plane. In other words, the membrane plate structure has a flat, uncurved shape extending along the plane. 
     According to an exemplary embodiment, the membrane plate structure comprises a curved, wavelike, or dished (trapezoid) like, or dome like or conus like structure and runs not within a plane. 
     According to an exemplary embodiment, the membrane plate structure form has a total depth of less than ⅕, in particular 1/10, further in particular 1/20, of a largest width of the stack. 
     According to an exemplary embodiment, the multi-layer material can be produced through a cold lamination process. 
     According to an exemplary embodiment, the multi-layer material can be produced through a lamination process of thermoplastic core between two skin layers, at a temperature higher than the melting point of the core layer and lower than then the melting point of the skin layer. 
     According to an exemplary embodiment, the multi-layer material can be produced with the application of a resin on one skin layer, the covering of the resin with second skin layer, and the curing of the resin. 
     It has to be noted that embodiments of the invention have been described with reference to different subject matters. In particular, some embodiments have been described with reference to apparatus type claims whereas other embodiments have been described with reference to method type claims. However, a person skilled in the art will gather from the above and the following description that, unless other notified, in addition to any combination of features belonging to one type of subject matter also any combination between features relating to different subject matters, in particular between features of the apparatus type claims and features of the method type claims is considered as to be disclosed with this application. 
     Examples and Comparison 
     Examples are shown in the table 3: 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                   
               
               
                   
                   
                 Bending 
                   
                 Area 
                 Frequency 
               
               
                   
                 Thickness 
                 Modulus 
                 Density 
                 density 
                 Factor 
               
               
                   
                 [μm] 
                 [Gpa] 
                 [kg/m 3 ] 
                 [g/m 2 ] 
                 [m 2 /s] 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 CIMERA 
                 220 
                 50 
                 680 
                 150 
                 1.89 
               
               
                 TDR220-35 (UD 
               
               
                 Aramid skin 
               
               
                 layers) 
               
               
                 CIMERA 
                 220 
                 80 
                 470 
                 103 
                 2.87 
               
               
                 CDR220-15 (UD 
               
               
                 HM Carbon 
               
               
                 skin layers) 
               
               
                 CIMERA 
                 220 
                 18 
                 540 
                 119 
                 1.27 
               
               
                 ADR220-12H 
               
               
                 (Aluminum skin 
               
               
                 layers) 
               
               
                   
               
            
           
         
       
     
     A sandwich construction with foam as core layer with UD fiber tapes as skin layers (CIMERA TDR or CDR) strongly outperforms the sandwich construction with aluminum skin layers (CIMERA ADR). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The aspects defined above and further aspects of the present invention are apparent from the examples of embodiment to be described hereinafter and are explained with reference to the examples of embodiment. The invention will be described in more detail hereinafter with reference to examples of embodiment but to which the invention is not limited. 
         FIG. 1  shows a schematic view of a loudspeaker comprising the membrane plate structure with aluminum as skin layer. 
         FIG. 2  shows the coil and membrane plate of a loudspeaker comprising the membrane plate structure according to an exemplary embodiment of the present invention, wherein the fibers are oriented along the shorter (width) size of the plate. 
         FIG. 3  shows the coil and membrane plate of a loudspeaker comprising the membrane plate structure according to an exemplary embodiment of the present invention, wherein the fibers UD skin layers are oriented along the shorter (width) size of the plate and the core layer is free of pores. 
         FIG. 4  shows the coil and membrane plate of a loudspeaker comprising the membrane plate structure according to an exemplary embodiment of the present invention, wherein the fibers UD skin layers are oriented along the shorter (width) size of the plate and the core layer is porous. 
         FIG. 5  shows a curved design of a membrane plate structure, according to an exemplary embodiment of the present invention. 
         FIG. 6  shows the break-up modes simulations of the system membrane plate and coil. 
         FIG. 7  shows a diagram illustrating sound pressure levels with respect to respective frequencies of three exemplary loudspeakers having different exemplary embodiments. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The illustrations in the drawings are schematic. It is noted that in different figures similar or identical elements are provided with the same reference signs. 
       FIG. 1  shows a schematic view of a loudspeaker comprising a membrane plate structure. The membrane plate structure comprises a carrier element  104 , a coil  105  which is coupled to the carrier element  104  and a membrane plate  100 . The membrane plate  100  is supported by the carrier element  104  such that the membrane plate  100  is excitable by the coil  105  for generating sound waves. 
     The membrane plate structure comprises a membrane plate  100  having a first skin layer  101 , a second skin layer  102  and a core layer  103  which is interposed between the first skin layer  101  and the second skin layer  102 . 
     The coil  105  may be electrically excited by a control unit (not shown). The membrane plate  100  is coupled to the coil  105  such that the excited coil  105  excites the membrane plate  100  as well. The membrane plate  100  vibrates in an excited state and thereby generates acoustic sound. 
     The first skin layer  101 , the second skin layer  102  and the core layer  103  form a stack extending within a plane. In other words, the membrane plate  100  has a flat, uncurved shape extending along the plane. More specifically, the first skin layer  101 , the second skin layer  102  and the core layer  103  extend along respective planes having parallel plane normals. In this specific example, the first skin layer  101  and the second skin layer  102  are made of aluminium. 
       FIG. 2  shows an exemplary embodiment of the present invention, wherein the membrane plate structure comprises a vibrating element  105  and a membrane plate  100 , which is coupleable to the vibrating element  105  for generating sound waves. The membrane plate  100  has a different width w with respect to its length, wherein the width w is shorter than the length. In particular, the width w is defined as the shortest distance between opposing edges of the membrane plate  100 . The membrane plate  100  comprises an UD layer made of fibers  107 , wherein the fibers of the UD layer  107  are oriented along the width w of the membrane plate  100  (indicated with fiber direction  106 ). The fibres may also be orientated along a further fiber direction  106 ′ which has an angle α with respect to the width direction w of the membrane plate  100 . The angle α may be between −30° and +30°. 
     The membrane plate  100  may consist of a matrix made of plastic or epoxy resin, in which fibers, in particular uni directional (UD) fibers  107  are integrated. UD fibres  107  extends along the fiber direction  106 . The fiber direction  106  is parallel to a width w direction of the membrane plate  100 . As can be taken from  FIG. 2 , the membrane plate  100  is formed rectangular, wherein the membrane plate  100  has a length and a with extension. The fibers  107  extends along the fiber direction  106  which is parallel to the width w direction of the membrane plate. 
     Furthermore, it is shown in  FIG. 2  that the coil  105  surrounds circumferentially the membrane plate  100 . Hence, a proper control and excitation of the membrane plate  100  is possible. 
       FIG. 3  shows a membrane structure according to an exemplary embodiment of the present invention, wherein the membrane plate  100  is formed in a sandwich design. The plate  100  comprises a first skin layer  107   a  and a second skin layer  107   b , wherein a core layer  103  is interposed between both skin layers  101 ,  102 . A young modulus of the core layer  103  may be lower than the young modulus of the first skin layer  101  and the second skin layer  102 . The first skin layer  107   a , the second skin layer  107   b  and/or the core layer  103  may be made of a fiber UD tape. 
       FIG. 4  shows a further exemplary embodiment of the present invention, wherein the membrane plate  100  comprises a sandwich design according to the embodiment shown in  FIG. 3 . Furthermore, the core layer  103  is made of a foam material. The foam material may be a plastic material comprising pores filled with gas, such as air, wherein the pore size is for example 5 μm to 300 μm (Micrometer), in particular 10 μm to 200 μm, more in particular 30 μm to 150 μm. 
       FIG. 5  shows an exemplary embodiment of a membrane plate structure wherein the membrane plate  100  is formed in a sandwich design. The plate  100  comprises a first skin layer  107   a  and a second skin layer  107   b , wherein a core layer  103  is interposed between both skin layers  107   a  and  107   b . In particular, the first skin layer  107   a , the second skin layer  107   b  and the core layer  103  form a stack having a curved, in particular wavelike, extension. In other words, the membrane plate structure  100  comprises a curved, wavelike structure and runs not within a plane. 
       FIG. 6  shows a simulation of a membrane plate  100  used in the simulation having a sandwich design with UD aramid fibers as skin layers  107   a ,  107   b  oriented along the longer (length) size of the membrane plate (S 1 ) and oriented along the shorter size (width w) of the membrane plate (S 2 ) according to the present invention. It is easy to understand that the first mode, i.e. the resonance frequency, in S 1  is happening earlier than in S 2 , showing the beneficial effect of orienting the fibers along the shorter size of the membrane plate  100 . 
       FIG. 7  shows a diagram illustrating sound pressure levels (SPL) with respect to respective frequencies of three exemplary loudspeakers. In the shown example in  FIG. 7 , three materials for a standard 11 mm×15 mm (millimeter) micro-speaker have been used. All the materials have a total thickness of 220 μm (Micrometer) to properly compare the frequency response. Exemplary values for the exemplary materials are shown in Table 4 below: 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                   
                 Bending 
                   
                 Area 
                 Frequency 
               
               
                   
                 Thickness 
                 Modulus 
                 Density 
                 density 
                 Factor 
               
               
                   
                 [μm] 
                 [Gpa] 
                 [kg/m 3 ] 
                 [g/m 2 ] 
                 [m 2 /s] 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 CIMERA 
                 220 
                 24* 
                 650 
                 143 
                 1.33* 
               
               
                 TDR220-30Y 
               
               
                 (UD Aramid 
               
               
                 skin layers) 
               
               
                 CIMERA 
                 220 
                 71* 
                 510 
                 112 
                 2.61* 
               
               
                 CER220-20H 
               
               
                 (UD HM Carbon 
               
               
                 skin layers) 
               
               
                 CIMERA 
                 220 
                 18  
                 620 
                 135 
                 1.21 
               
               
                 AXR220-12H 
               
               
                 (Aluminum 
               
               
                 skin layers) 
               
               
                   
               
               
                 *measured in fiber direction 
               
            
           
         
       
     
     Line  703  is indicative for a conventional loudspeaker made of a CIMERA AXR220-12H (AXR) material, wherein the loudspeaker comprises a sandwich material with 12 μm (Micrometer) of aluminum skin layer. 
     Line  701  is indicative for a loudspeaker according to the present invention made of CIMERA TDR220-30Y (TDR) material, wherein the loudspeaker comprises a sandwich material with 30 μm (Micrometer) aramid UD (Unidirectional) skin layers according to an exemplary embodiment of the present invention. 
     Line  702  is indicative for a loudspeaker according to the present invention made of CIMERA CER220-20H (CER), wherein the loudspeaker comprises a sandwich material with 20 μm (Micrometer) HM (High Modulus) Carbon UD (Unidirectional) skin according to an exemplary embodiment of the present invention. 
     A comparison of the mechanical properties of the three materials can be taken from table 4 above. As can be taken from the line  701 ,  702  presented in  FIG. 7 , TDR (CIMERA TDR220-30Y) in line  701  and AXR (CIMERA AXR220-12H) in line  703  presents very comparable mechanical and acoustic behavior, with the advantage that TDR is a non-conductive material. Instead, CER (CIMERA CER220-20H) in line  702  compared to AXR in line  703  is better performing in all the parameters with a higher break-up frequency and lower mass. 
     It should be noted that the term “comprising” does not exclude other elements or steps and “a” or “an” does not exclude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims. 
     LIST OF REFERENCE SIGNS 
     
         
           100  membrane plate 
           101  first skin layer 
           102  second skin layer 
           103  core layer 
           104  carrier element, membrane or surround 
           105  coil/vibrating element 
           106  fiber direction 
           107  fibers/UD fiber reinforced tape layer(s) 
           107   a  (top) skin layers layer 
           107   b  (bottom) skin layers layer 
           701  representative line for TDR 
           702  representative line for CER 
           703  representative line for AXR 
         w width 
         α angle