Abstract:
Transducer for converting mechanical stress into electric signals, which transducer is composed of at least one electromechanical sheet ( 107,108 ) and is capable of converting mechanical stress into electric signals and in which transducer at least one of the electrodes required by the electromechanical sheet is disposed on the surface of one or more thin and flexible dielectric materials, said electrodes ( 109 ) forming electrically conductive surfaces of the transducer for connecting the transducer to a signal processing device, said electromechanical sheets being permanently charged cellular electret film and which transducer is constructed of a unitary, thin and flexible layered laminate structure.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   The present application is a continuation of U.S. application Ser. No. 09/851,493, filed May 8, 2001, now U.S. Pat. No. 6,689,948 which is a continuation-in-part of U.S. application Ser. No. 09/553,566, filed Apr. 21, 2000, now U.S. Pat. No. 6,242,683, which is a continuation-in-part of U.S. application Ser. No. 09/155,828, now U.S. Pat. No. 6,078,006. 

   FIELD OF THE INVENTION 
   The present invention relates to a transducer and, in particular, a flexible, unitary electret film transducer for converting mechanical stress, such as mechanical vibrations, into electric signals, and to a method for its fabrication. The transducer is especially applicable for use for example in musical instruments, such as stringed musical instruments (guitars etc.), and other applications. 
   PRIOR ART 
   Transducers for example pickups for acoustic guitars designed to transform string vibrations into electric signals, have a transducer part typically containing different layers of electromechanical transducer elements, dielectric material and electrically conductive electrode layers, and a connection cable part in which the signals are taken to a signal processing unit. Transducers may typically have a one or more transducer element layers. Transducers are typically positioned between a vibrating member and fixed part, for example in acoustic guitar between saddle (vibrating member) and saddle slot (fixed part). Contact transducers are also commonly used for measuring vibration, for example amplifying musical instruments sound, being positioned onto a vibrating member, like for example guitar top, attached by using some sort of adhesive. Typically they consist a piezo ceramic disk or piezoelectric film such as polarized pvdf film. 
   As electromechanical transducer elements, piezoelectric crystals, piezoelectric film (e.g. polyvinylidene fluoride PVDF) and piezoelectric cable are prior art. In the commonest transducer structures, the connecting cable part is implemented using screened coaxial cable, which is connected to the electrode layers of the transducer part by soldering. Such a transducer is presented e.g. in U.S. Pat. No. 5,319,153. One drawback with this type of structures is the difficulty of fabrication of the transducer and relatively high manufacturing costs, because much of the work has to be done manually. Moreover, the prior art structures typically have separate transducer part and connection cable which means the connections to the preamplifier either have to be made by soldering or by using a mini-plug soldered to the cable. This causes handwork and higher costs. 
   One major drawback of prior art transducers using crystalline piezoelectric materials is their certain characteristic to produce more of odd harmonic overtones and resonance peaks. In case of acoustic guitar that results to unpleasant sound that is not quite in keeping with the instruments own acoustic sound. Further, the prior art transducers structures comprising many different materials and being relatively thick, the transducers themselves affect to instruments own acoustics sound and thus also to amplified sound. 
   A dielectric cellular or porous electret film and manufacturing process for same, applicable for use as electromechanical material for a transducer, such as stringed musical instrument transducer, is described in U.S. Pat. No. 4,654,546, said dielectric film comprising permanently charged, biaxially oriented, foamed, usually homogenous film layer containing flat lens-like, shredded or cavitated gas bubbles which can also be called as voids or cells. The electret field, or the permanent electric charge, is achieved by injecting charges into dielectric material. The term “dielectric cellular electret film” is used here to refer to generally cellular type electromechanical films having a permanent electric charge injected into material. 
   WO-publication 96/06718 presents a procedure for pressure inflation of a pre-foamed plastic film, that makes it possible to manufacture strongly foamed film products, involving a high foaming degree and allowing the thickness of the product to be increased without increasing the amount of plastic material. The improvement in increased velocity of the gas voids from 30% up to 60 and even up to 70% of the thickness results up to 10 fold stronger electromechanical response. This means significantly better signal-to-noise ratio. The term “dielectric swelled cellular electret film” or “pressure inflated prefoamed cellular electret film” is used herein to refer to a foamed film-like plastic product as described in that WO-publication and which is permanently charged in strong electric field i.e. electric charge injected into material. 
   SUMMARY OF THE INVENTION 
   The object of the present invention is to eliminate the drawbacks of prior art and achieve an improved transducer of a completely new type, in which a dielectric swelled cellular electret film is used to transform the vibrations or other type mechanical stress into electric signals, instead of piezoelectric materials such as films or crystals. Flat lens-like gas bubbles in the electret film effectively limit the mobility of electret charges in the dielectric material, because the gases have an electric resistance five decades better than the best solid insulating materials have. Compared to hard structure of piezoelectric materials, they act as an elastic soft layer during the conversion of for example string vibrations into electric signals allowing pressure variations caused by vibrations to cause microscopic changes in its thickness. The change in thickness causes the opposite charges on the opposite sides of the voids to get closer or further which causes so called mirror charge over the electrodes arranged over the cellular electret film and thus an measurable electrical output voltage proportional to the force change. 
   Other object of the invention is to produce a new type of transducer which, due to its elastic charged cellular electret film, is capable of converting mechanical stress, such as string vibrations, into electric signals which, when processed or converted into sound, compared to prior art piezoelectric transducers, results to cleaner signal without resonance peaks based on transducer itself. Because of the elastic swelled cellular core, the young&#39;s modulus of the transducer is significantly lower and thus the impedance matching with for example wood is better than with hard piezoelectric materials. This results in cleaner signal which for example in case of acoustic instrument produces acoustic sound without any harshness or “quacking” as typically with piezoelectric materials or in case of analog to digital conversion, easier processor algorithms and more accurate reading. 
   Another object of the invention is to produce a transducer of a construction thin and flexible to conform both flat and curved surfaces depending on use and application. 
   Still another object of the invention is to produce a transducer having multiple areas each of them producing own electric signal. 
   A further object of the invention is to produce a transducer as simple as possible, having no separate transducer part and no separate conductor for connecting it to a signal processing device, but which has a unitary, flexible and laminated structure and connections for connecting it to a signal processing unit. 
   Also further object of the invention is to produce a new kind accelerometer type contact transducer. 
   The transducers of the innovation can be very economically fabricated for example by screen-printing the required electrodes with silver paste on sheets of dielectric film (e.g. polyester) and/or directly to electret film, placing several electrodes side by side on the same sheet. By laminating such sheets and dielectric cellular electret film, preferably swelled, on top of each other so that charged dielectric cellular electret film is only placed on a desired area at one end of the sheet while the other end is provided with a connector part with different electrode layers side by side, a laminate sheet is obtained from which the transducers can be cut out e.g. by punching. After that, it is only necessary to join a suitable connector to the electrodes at the connector end of the transducer by pressing mechanically. 
   With this method, it is possible to produce ultra thin and flexible transducers of desired length, design, shape and width, in which the electrodes in the transducer part are continuous extending from the transducer part to the connecting part and which are unitary, flexible and thin laminate in construction. Fabrication is faster and more economic than with conventional methods. The innovation thus allows an effective and economic production technique of transducers with charged cellular electret film as active material. 
   In one embodiment of the invention, no dielectric firm plastic layer, where the young&#39;s modulus value typically is significantly higher than with cellular electret film, to carry the conductive electrodes, would be needed in the transducer structure adjacent to instrument saddle. Thus the transducer becomes thinner and the acoustic properties become better because the firm plastic layers are not absorbing and dampening the vibration energy. Further, because of saved thickness exclusive firm plastic films, the amount of transducer elements can be increased, without adding too much thickness, and thus the output voltage and therefore the signal-to-noise ratio are further improved. Further, due possible increase in thickness of elastic soft dielectric cellular layers the structure becomes softer which, in case of acoustic guitar transducer, improves the string-to-string balance. Also in this embodiment the electrodes become more durable than screen-printed electrodes and the connectors in the preamplifier end can be easily connected to the transducer so that the there is no plastic layers in between and thus the electrical properties of connections become better and also more durable. Further, it is possible to simultaneously arrange the screening for the connection end and even soldering directly to the electrodes. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following, the invention is described in more detail by the aid of examples by referring to the attached drawings, in which 
       FIGS. 1   a – 1   d  present exploded perspective views illustrating the different components that comprise the transducer of the four different embodiments of the invention, 
       FIG. 1   e  presents top view of the embodiment of the invention presented in  FIG. 1   d,    
       FIG. 2   a  present the signal electrodes and  FIG. 2   b  ground electrodes, printed on a sheet of dielectric film, of the transducer of the embodiment in  FIGS. 1   a  and  1   b,    
       FIGS. 3   a – 3   d  present signal electrodes and ground electrodes printed on a sheet of dielectric film of two different embodiments of the invention, the two transducers having different electrodes at the connector end arranged side by side, 
       FIGS. 4   a – 4   b  present top view of the cutter blades of a punching unit of the transducer of the embodiment illustrated in  FIGS. 1   a ,  1   b ,  3   c ,  3   d,    
       FIG. 5  presents pattern for screen-printing the insulation over the signal and ground electrodes, of the transducer of the embodiment in  FIG. 3   c,    
       FIG. 6   a  presents an exploded perspective view illustrating the different components that comprise the transducer of the invention without extra dielectric layers carrying the electrodes at the transducer area, with connectors in preamplifier end arranged side by side, 
       FIG. 6   b  presents an exploded perspective view illustrating the different components that comprise the transducer of the invention without extra dielectric layers carrying the electrodes at the transducer area adjacent to saddle, with sequentially arranged connecting areas in preamplifier end, 
       FIG. 7  presents the signal electrodes of the transducer of the embodiment in  FIG. 6   a,    
       FIG. 8  presents one side ground electrodes of the transducer of the embodiment in  FIG. 6   a,    
       FIG. 9  presents an exploded perspective view illustrating the possible screening of the connector end, 
       FIG. 10  presents a microscope picture of dielectric cellular electret film. 
   

   DETAILED DESCRIPTION 
   In the embodiment of  FIG. 1   a  the transducer of the invention is composed of sheets  107  and  108  of dielectric film, which may be made e.g. of 0.1 mm thick polyester. On the underside of sheet  107 , a signal electrode  109  is screen-printed by using e.g. silver or graphite. Printed around the signal electrode  109  is a ground electrode  110 , which reduces electromagnetic interference noise in the signal. It is noted, however, that this ground electrode  110  is not essential to the invention but is also considered as innovative step compared to prior art transducers. Screen-printed on the top surface of film sheet  107  is a ground electrode  111 , which may also consist of aluminum foil or other electrically conductive foil suited for the purpose. Screen-printed on the top surface of sheet  108  is a ground electrode  112  and on the bottom surface also a ground electrode  113 . It should be noted that this ground electrode  113  is not essential for the structure in this and other embodiments of the invention, where the transducer is not a differential transducer. Sheet  108  may also consist of e.g. thin aluminum or brass foil or other electrically conductive foil suited for the purpose. It is noted that the ground electrodes  110 ,  111 ,  112 ,  113  are shorter at the end  114  pointing towards the preamplifier than the signal electrode  109 , whereas at the other end  117  the ground electrodes are somewhat longer than the signal electrode. Instead of being screen-printed, the electrodes may also be for example evaporated e.g. from aluminum onto dielectric films using a mask or etched from a metal/dielectric laminate such as copper/polyamide (for example Kapton®) laminate. 
   Between the sheets  107 ,  108  there is transducer element  118 . This element  118  is composed of three, preferably swelled, dielectric cellular electret films  119 ,  120 ,  121  having flat gas bubbles  301  inside the film material  300  ( FIG. 10 ). The underside of the topmost film  119  provides negative electric charge. The top side of the intermediate film  120  also has a negative electric charge, while a positive electric charge is provided on its underside. The top side of the bottommost film  121  has a positive electric charge. After being charged, the films have been glued together. The bottommost films  121  bottom side may also be provided with a metallic electrically conductive surface, e.g. evaporated aluminum, which is to be noted is not necessary when the ground electrode is arranged on separate dielectric layer. This electrically conductive surface is possible to have also on topside as well as on one or both sides of films  119  (on topside when ground electrode  110  is not printed) and  120  but it is not recommended. With the procedure described, a maximal electric charge density is achieved. Such an element responds only to the pressure generated and not to bending at all. From the point of view of operation, it is sufficient to have only the intermediate film  120  charged before or after gluing. The transducer element  118  may also consist of two dielectric cellular electret films, in which element  118  opposite polarities of the films  119 ,  121  are placed against to each other. Such a structure mainly responds to pressure only and very slightly to bending. By placing the films with same polarities facing each other, an element mainly responsive to bending is achieved. For many normal operations, it is sufficient that element  118  be composed of only one dielectric cellular electret film, preferably swelled. 
   In the embodiment of  FIG. 1   b  a transducer of the invention is fabricated in such manner that film  122  is continuous extending through areas  114 ,  115 ,  116 ,  117 . Screen-printed on both the top side and on the bottom side of the film  122  is a signal electrode  109  and around it ground electrode  110 , which ground electrode is again not essential to the structure. Screen-printed on both the top and bottom side of sheet  107  is a ground electrode  111 . Screen-printed on top side of sheet  108  is ground electrode  112  and on the underside another ground electrode  113 . Ground electrodes  111 ,  112 ,  113 , do not extend to area  114 . All ground electrodes are connected together by means of a connector  106 . Disposed in area  116  above and below sheet  122  are preferably swelled dielectric cellular electret films  119 ,  121 . Positive polarity is on the underside of sheet  119  and on the top side of sheet  121 . Negative polarity is on the top side of sheet  119  and on the underside of sheet  121  but it is not essential. By pressing a connector  124  on area  114 , the signal electrodes  109  are connected together. At the area  115  between the sheets  107 – 122  and  122 – 108  is a dielectrict film  127  to prevent short circuit between signal and ground electrodes. In this embodiment of the invention the dielectrict cellular electret films are connected in parallel. 
   In the embodiment of  FIG. 1   e  a differential transducer of the invention is implemented by screen-printing signal electrode  129  on the top side of sheet  130  and connecting this signal electrode  129  to the signal electrode  131  using electrically conductive glue between sheets  130  and  132 . This signal electrode  129  is made somewhat shorter than the sheet  130  itself. The signal electrode  133  screen-printed on the top side of sheet  134 , which is electrically connected to the underside of the bottommost sheet  121  of the element  118 , extends to the end of the sheet  134 . The ground electrode  135  printed on the top side of sheet  132  is somewhat shorter than the sheet  132 . At the transducer end  136 , the film sheet lengths are such that sheet  132  is the shortest one of the sheets. Sheet  130  is somewhat longer and sheet  134  is the longest one. At the other end  117  of the transducer is a connector  106  which connects ground electrodes  135 ,  137 ,  138 ,  139  together. It is to be noted again that ground electrodes  138 ,  139  are not essential to the structure. In this way, an arrangement is achieved in which all signal and ground electrodes of the differential transducer needed to connect to a signal processing device are located sequentially at one end  136  of the transducer and on the same side of it (ref.  FIG. 1   e ), enabling it to be connected to the circuit board of a signal processing device by pressing it onto the circuit board at a position provided with corresponding electrodes in sequence. By replacing the signal electrode  133  with an electrode which is printed in the shape of an ground electrode and has a length such that it is shorter at the transducer end  136  than sheet  130  and extends correspondingly to the other end  117  of the transducer, a non-differential transducer is obtained in which the electrodes for connecting the transducer to a signal processing device are on the same side in sequence at one end of the transducer. 
   Reference is now made to  FIGS. 3   a – 3   d . If desired, the signal and ground electrodes can also be printed so that they are placed side by side at the transducer end  114  as illustrated by  FIGS. 3   a – 3   c . In  FIG. 3   a  there is signal electrodes screen-printed on a dielectric sheet  139  of an embodiment of the invention in which there is a separate signal electrode  140 ,  141 ,  142 ,  143 ,  144 ,  145 , in this case for each string of a guitar, and particularly for an electric guitar. The vibration of each string of the instrument is transformed into electric signal by the means of having a separate saddle-like piece under each string against disposed signal electrode of the transducer, the charge-signal generated to each electrode being processed separately in the signal processing device. Part of the processing may also include automated gain control for each string. This type of hex-microphone is needed e.g. for making a stereo image or in midi equipment, where the electronics converts the tone pitch into a voltage value controlling a synthesizer. In this embodiment too, the dielectric cellular electret film is placed on the area  116 , an insulation is provided in the area  115  and metallic connectors  124  are mechanically pressed through the electrodes in the transducers end  114 . In  FIG. 3   b  there is the ground electrode  146  screen-printed on a dielectric sheet  138 , e.g. polyester of the embodiment described above. In  FIGS. 3   c ,  3   d  the pattern for printing the signal and ground electrodes of another embodiment of the invention where the transducer, in this case a differential transducer is obtained having the electrodes side by side at the connector end  114 . In that embodiment the pattern shown in  FIG. 3   c  shows signal electrodes  148  and around them ground electrodes  149 . This pattern is screen-printed say on top side of the dielectric sheet  147  and on bottom side is screen-printed the ground electrodes, as illustrated in  FIG. 3   d . The pattern for screen-printing the dielectric insulation  151  over the electrodes shown in  FIG. 3   c  is showed in  FIG. 5 . Still referring to  FIGS. 3   a – 3   b , when making above mentioned transducer containing several areas to pick up vibrations or stress, so called cross talking can be come a problem. To avoid such it is possible to first print the desired signal electrodes  140 ,  141 ,  142 ,  143 ,  144 ,  145 , then print a dielectric insulate where the desired areas to pick up vibration are left open and all the leads are covered with insulating material. Then to print a pattern to form a one more ground electrode having same shape as In  FIG. 3   b  the ground electrode  146 , with exception that holes with narrow space are left over the desired signal areas. In that way the leads from different signal areas to signal processing device will not be picking up undesired charge signals. This kind arrangement, whereby transducer has ground electrodes on both side, the other side having holes and signal electrodes are arranged according the holes, with insulate between ground electrode and signal electrodes, is beneficial in many other applications. In case there are very many signal areas and not enough space is allowable for signal leads, thru-holes can be used for carrying the signal electrode leads on the other side of the dielectric film carrying the signal electrodes. 
   Referring now to  FIGS. 1   a ,  1   c ,  2   a ,  2   b , The transducers of the two embodiments of the invention as shown  FIGS. 1   a ,  1   c  are fabricated by first applying suitable glue on the dielectric film  125  on the side where the signal and ground electrodes are screen-printed with silver or graphite paste as shown in  FIG. 2   a . To the other side of this film  125 , there are ground electrodes screen-printed as shown in  FIG. 2   b . After this, dielectric sheet cut to suitable size is glued into the area  117 . An element  118  size large enough, consisting a laminate of dielectric cellular electret films, preferably swelled, is glued on area  116  and sheet  122  on areas  114 ,  115 . Then glue is applied in the sheet  126  as shown  FIG. 2   b , where there is same ground electrode pattern screen-printed on the both sides of this sheet. The side with glue applied is then glued opposite to the above mentioned laminate, with the register marks  152  in corners in alignment. In this way, a laminate is obtained, from which the transducers can be punched off with a tool as shown in  FIG. 4   a . The transducers can also be cut out from the sheet using e.g. a laser or water jet or some other technique suited for the purpose. This procedure allows a considerably large number of thin and flexible transducers of desired length and width and having a continuous structure without joints than by conventional methods, to be fabricated by the same amount of work while the manufacturing costs remain low. 
   The transducers of invention in  FIGS. 6   a  and  6   b  consists of a connector part  114  including connectors connecting the transducer to a preamplifier, a connection part  115  corresponding to a connection cable in a conventional transducer and a transducer part  116  for converting vibrations into electric signals. As may be noted the transducers in  FIGS. 6   a  and  6   b  have no separate transducer part and no separate conductor for connecting it to a signal processing device, but are of a unitary, flexible and laminated structure extending from the end of transducer part  116  unitary as a connection part  115  up to the connector part  114  and in which the connections for connecting it to a preamplifier can be disposed in sequentially or side by side. 
   Referring now to  FIG. 6   a , signal electrode  209  is a thin metal film, for example tin-bronze-alloy or tinned copper or aluminium with thickness of preferably 0.035 mm. It is to be noted that many thin metal films and thickness are suitable for the application. On both sides of the signal electrode  209  there are swelled dielectric cellular electret films  119 ,  120 , and on the outer sides of the cellular electret films  119 ,  120 , ground electrodes  211 ,  212 . Signal electrode  209  has a form where the electrode is broad in the transducer part and narrow in the connection part. In the connector part the signal electrode has an area corresponding the connection area of the connector  124 . Ground electrodes  211 ,  212  each comprises of thin metal film. Both the ground electrodes  211 ,  212  are connected together with a connector  124  in the connector part  114 . 
   Cellular electret films  119 ,  120  in the transducer area may each comprise of several film layers. Each film  119 ,  120  is permanently. Preferably positive charges are injected onto the underside of sheet  119  and onto the top side of sheet  120 . Negative charges may be injected onto the top side of sheet  119  and onto the underside of sheet  120  but it is not essential. The films  127 ,  128  in the connection part are preferably uncharged operating thus as isolating film layers between the electrodes. It is also possible to extend the cellular electret films  119 ,  120  all the way to the connector part  114  but preferably use only partially charged film so that there are no charges in the connection part  115 , to avoid the connection part become microphonic and pick-up disturbing vibrations. The ground electrodes  211 ,  212  can also be sputtered, evaporated, chemically metallized or screen printed to the outer sides of the cellular electret film  119 ,  120 . It is also possible to arrange the signal electrode  209  directly on the face of cellular electret film  119  or  120  by for example chemical metallizing process or simply by screen-printing with silver paste. It is possible to use hybrid structure, with ground electrodes arranged on the surfaces of for example polyester film and signal electrode on the surface of the electret films  119 ,  120 , for example increase the output voltage. It is also possible to use two, or even more, signal electrodes  209  by using three or more transducer elements  119 – 120  and in between each said element having one signal electrode  209  and at the outermost faces of the outermost transducer elements having the ground electrodes  211 – 212 . Further, by using two signal electrodes, two ground electrodes and three transducer elements, and having the two signal electrodes in connection part arranged side-by-side, an differential transducer can be obtained. It is also possible to arrange the signal electrode in the transducer area to be for example round shape, or oval, or square, or multiple round areas in line, depending on the preferred embodiment. 
   The  FIG. 9  shows how the ground electrode  211  may have an extension  224  on the side to form shielding against electrical interference in the connector end  114 . Because the connector area in the signal electrode is open for electromagnetic interference, it must be shielded. Typically this is taken care by metal housing of the preamplifier circuitry, but by this way, a very small preamplifier circuitry can be integrated into the connector end. The components of the circuitry, for example one field-effect transistor (FET) and one resistor, are connected to the transducers electrodes  209 ,  211 ,  212  and the screening extension  224  is folded around the connector end  114  by using double sided tape  226 , which also forms the necessary insulating in between the components and extension  224 . It is also possible to integrate or one new USCP packed electret condenser microphone cartridge preamplifier like Maxim 9810 inside the structure. Referring back to  FIG. 1   a , it is even possible to arrange such small preamplifier circuit right after the active area corresponding to transducer element, on the dielectric layer  107 , connected to signal electrode  109  and ground electrode  110 . Depending on the circuit design, a lay-by maybe necessary and small bowl to the dielectric layer  108  facing cartridge preamplifier. Leads can be connected to the circuitry for taking the signals to the next device in signal processing chain. By having the preamplifier circuitry as close as possible to the transducer element area, the capacitance of the connection part is lowest possible and the signal-to-noise ratio becomes significantly better. This kind arrangement is especially suitable for hex-type transducers as described earlier, where several small signal electrodes facing the transducer element, are arranged, because the smaller transducer the smaller the relative capacitance and thus more important it becomes to have additional capacitances as small as possible, which is avoided by taking the preamplifier circuit as close to transducers as possible. 
   To make an contact transducer according to invention, simply the transducer area is arranged to be for example round 15 mm diameter disk-like, or multiple round areas in line, or one larger rectangular area, and an weigh, for example 0.1 mm thick copper plate of same shape is glued over the transducer area, on the opposite side of the side which attaches to the vibrating surface. The weigh, which can even be only the transducers polyester layer carrying the electrode, works as mass against which the instrument vibrates and which further causes signal output proportional to the vibration. The less mass, the higher is the transducers own resonance frequency, which is preferred to be above the necessary frequency response. 
   The transducers in  FIGS. 6   a  and  6   b  and  9  are fabricated as follows. Referring to  FIG. 7  signal electrodes  209  and ground electrodes  211 ,  212  are made of a thin metal film  231 ,  232 ,  233 . Firstly the thin metal film  231 ,  232 ,  233  is coated both sides with an insulating material in the areas to form the electrodes. Secondly the metal films  231 ,  232 ,  233  are taken into chemical corrode process where all metal except the areas coated with insulating material, is corroded away. Thirdly, the metal film is taken into next chemical process, where the insulating material is removed. After this, a metal film  231 ,  232 ,  233 , where the wanted electrodes are connected to each others and frame surrounding them with very narrow keepers  234 , is remained. In the corners of each metal film  231 ,  232 ,  233  there is a hole  235  to ease the assembly. It is to be noted that there is other ways too to make similar metal film  231 ,  232 ,  233 . One way is to laser cut the same pattern to the metal film, other way is die-cutting the metal film with suitable tool having the same pattern. Water cutting can also be used. By using laser or water cutting, several films can be manufactured simultaneously. 
   Cellular electret film elements  119 ,  120  size large enough, consisting typically a laminate of 1–3 dielectric cellular electret films, preferably swelled, and metal films  231 ,  232 ,  233  are glued together so that first against metal film  232  with ground electrodes, transducer element  119  and insulating layer  127  are glued, and next, on the other side of the transducer element  119  and insulating layer  127 , the metal film  231  with signal electrodes is glued, and next, to the other side of metal film  231 , second transducer element  120  and second insulating layer  128  are glued, and next, on the other sides of the transducer element  120  and insulating substrate  128 , metal film  233  with second ground layers is glued. In this way a laminate is obtained from which the transducers can be cut away by for example by die-cutting, laser cutting or water cutting. Further the connectors  124  are connected by pressing them to connector end  114 . 
   This procedure allows a considerably larger number of thin and flexible transducers of desired length and width and having a continuous structure without joints than by conventional methods to be fabricated by the same amount of work while the manufacturing costs remain low. Further, referred to the  FIGS. 6   a  and  6   b , the transducers can be manufactured very thin without any extra flexible firm insulating substrates to carry the electrodes. 
   It is also possible to arrange the electrodes  209 ,  211 ,  212  directly onto the cellular electret films  119 ,  120  by using for example screen-printing, evaporating, sputtering or chemical metallizing. 
   It is obvious to the person skilled in the art that different embodiments of the invention are not restricted to the examples described above, but that they can be varied within the scope of the claims presented below. The number of films and layers on top of each other can be chosen in accordance with the need in each case; there can be multiple transducer areas and area can also have a shape other than rectangular in top view. These transducers can be used in various applications such as musical instruments transducers.