Abstract:
Transducer for converting mechanical stress into electric signals, which transducer is composed of at least one electromechanical sheet 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 forming electrically conductive surfaces of the transducer for connecting the transducer to a signal processing device, and which transducer is constructed of a unitary, thin and flexible layered sheet structure and has the same width throughout its length.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]    The present application is a continuation-in-part of U.S. application Ser. No. 09/553,566, filed Apr. 21, 2000, 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  
         [0002]    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 in musical instruments, such as stringed musical instruments (guitars etc.), accelerometers and alike.  
         PRIOR ART  
         [0003]    Saddle transducers i.e. pickups for acoustic guitars, designed to transform string vibrations into electric signals, are mounted under the saddle of the guitar. They have a transducer part of a length corresponding to that of the saddle and 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 preamplifier inside the guitar via a small hole (diameter typically 3 mm) bored in the guitar&#39;s resonance box under the saddle. Saddle transducers may typically have a one or more transducer element layers. Contact pickups are also commonly used for amplifying musical instruments sound. Typically they consist a piezo ceramic disk. Contact pickups pick up the sound from vibrating surface to which it is attached by means of glue or putty.  
           [0004]    As electromechanical transducer elements, piezoelectric crystals or piezoelectric sheet (e.g. polyvinylidene fluoride PVDF) 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. A 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 connections to the preamplifier generally have to be made by soldering, because no connectors of sufficiently small size to go through the hole provided under the saddle are available for coaxial cables and because the connection between the transducer itself and the cable makes it impossible to mount the transducer from below. In addition, piezoelectric crystals and sheets are associated with a certain characteristic sound that is not quite in keeping with the guitar&#39;s own acoustic sound. Further, the prior art saddle transducers structures comprise many material types, which affects to the sound produced by the saddle transducer.  
           [0005]    The electret field, or the permanent electric charge, is achieved by injecting charges into dielectric material.  
           [0006]    A dielectric porous electret film and manufacturing process for same, applicable for use as electromechanical material for a 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 term “dielectric cellular electret film” is used here to refer to generally porous type electromechanical films having a permanent electric charge injected into material.  
           [0007]    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 term “dielectric swelled cellular electret film” is used herein to refer to a foamed film-like plastic product as described in that WO-publication and having a permanent electric charge injected into material.  
         SUMMARY OF THE INVENTION  
         [0008]    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 string vibrations into electric signals instead of piezoelectric 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. At the same time, 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 change in capacitance and produces an electrical output voltage in proportion to the sound source.  
           [0009]    A further object of the invention is to produce a new type of transducer which, due to its elastic cellular structure, is capable of converting mechanical stress, such as string vibrations, into electric signals which, when converted into sound, compared to prior art piezoelectric saddle transducers or contact pickups, better correspond to the instrument&#39;s own acoustic sound and allows playing at high volumes before feedback. Because of the elastic porous structure, the young&#39;s modulus of the material is significantly lower and thus the impedance matching with wood is better than with piezoelectric materials. This results in natural sound similar to instruments own acoustic sound without any harshness or “quacking” as typically with piezoelectric materials.  
           [0010]    Still another object of the invention is to produce a transducer which is of a construction thin enough to permit installation without changing any parts of the instrument, e.g. making the saddle lower, and which, when installed, does not affect the instrument&#39;s own acoustic sound, and is as easy to install as possible without soldering.  
           [0011]    Still another object of the invention is to produce a stringed musical instrument transducer capable of converting the vibration of each string separately into an electric signal.  
           [0012]    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 in which the connections for connecting it to a preamplifier can be disposed sequentially or side by side and which in itself is able to produce a balanced signal (differential transducer) according to the attached claims.  
           [0013]    A further object of the invention is to produce a new kind accelerometer type contact pickup.  
           [0014]    This kind of transducers 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.  
           [0015]    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 preamplifier and which are unitary, flexible and thin laminate in construction. Fabrication is faster and more economic than with conventional methods.  
           [0016]    The structure of the invention thus allows the application of an effective and economic production technique, not only for under saddle transducers, but also for contact transducers.  
           [0017]    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 excellent 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 improves the string-to-string balance. Even further, 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 excellent and also more durable. Further, it is possible to simultaneously arrange the screening for the connection end and even soldering directly to the electrodes.  
           [0018]    The structure of the invention thus allows the application of an effective and economic production technique with significantly improved sound and string-to-string balance properties. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    In the following, the invention is described in more detail by the aid of examples by referring to the attached drawings, in which  
         [0020]    [0020]FIG. 1 is a perspective view of a guitar, with a transducer as provided by the invention mounted on it,  
         [0021]    [0021]FIGS. 2 a - 2   c  present a cross-sectional view, top view and a longitudinal section of the saddle of a guitar with a transducer as provided by the invention mounted in conjunction with it,  
         [0022]    [0022]FIGS. 3 a - 3   d  present exploded perspective views illustrating the different components that comprise the transducer of the four different embodiments of the invention,  
         [0023]    [0023]FIG. 3 e  presents top view of the embodiment of the invention presented in FIG. 3 d,    
         [0024]    [0024]FIG. 4 a  present the signal electrodes and  
         [0025]    [0025]FIG. 4 b  ground electrodes, printed on a sheet of dielectric film, of the transducer of the embodiment in FIGS. 3 a  and  3   b,    
         [0026]    [0026]FIGS. 5 a - 5   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,  
         [0027]    [0027]FIGS. 6 a - 6   b  present top view of the cutter blades of a punching unit of the transducer of the embodiment illustrated in FIGS. 3 a ,  3   b ,  5   c ,  5   d,    
         [0028]    [0028]FIG. 7 presents pattern for screen-printing the insulation over the signal and ground electrodes, of the transducer of the embodiment in FIG. 5 c,    
         [0029]    [0029]FIG. 8 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 adjacent to saddle, with connectors in preamplifier end arranged side by side,  
         [0030]    [0030]FIG. 8 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,  
         [0031]    [0031]FIG. 9 presents the signal electrodes of the transducer of the embodiment in FIG. 8 a,    
         [0032]    [0032]FIG. 10 presents one side ground electrodes of the transducer of the embodiment in FIG. 8 a,    
         [0033]    [0033]FIG. 11 presents an exploded perspective view illustrating the possible screening of the connector end,  
         [0034]    [0034]FIG. 12 presents a microscope picture of dielectric cellular electret film. 
     
    
     DETAILED DESCRIPTION  
       [0035]    In FIGS. 1, 2 a ,  2   b ,  2   c , the cover  100  of the resonance box of an acoustic guitar is presented. Fitted on the resonance box is a transverse bridge for the six strings  103  of the guitar, consisting of a bridge body  101  placed against the resonance box  100  and a saddle  102 , whose upper edge is provided with notches for the strings  103 .  
         [0036]    Fitted under the saddle  102  is a transducer  104  as provided by the invention for transforming the vibrations of the strings  103  into electric signals.  
         [0037]    In the embodiment of FIG. 3 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 structure. 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.  
         [0038]    Between the sheets  107 ,  108  there is an 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. 12). Injected onto the underside of the topmost film  119  is a negative electric charge. Injected onto the top side of the intermediate film  120  is also a negative electric charge, while a positive electric charge is injected onto its underside. Injected onto the top side of the bottommost film  121  is 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. 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 charging procedure described, a maximal electric charge density is achieved. From the point of view of operation, it is sufficient to have only the surfaces of the intermediate film  120  charged. Such an element responds only to the pressure generated by the vibration of the strings, not to bending at all. The transducer element  118  may also consist of two dielectric cellular electret films, in which element  118  unlike charges of the films  119 ,  121  are placed opposite to each other. Such a structure mainly responds to pressure only and very slightly to bending and is thus applicable for converting the vibrations of the strings  103  into electric signals. By placing the films with like charges opposite to each other, an element mainly responsive to bending is achieved. For operation, it is sufficient that element  118  be composed of only one dielectric cellular electret film, preferably swelled.  
         [0039]    Between sheets  107  and  108  there is also a dielectric film  122 , which may be made e.g. of polyester, preferably of the same thickness as the film element  118 . This insulation prevents a short circuit between the signal electrode  109  and the ground electrode  112 . Instead of using a dielectric film  122 , it is possible to provide the bottom surface of film  107  at the area  115  or the top surface of film  108  at the area  115  with dielectric insulation screen-printed over the electrode(s) on the surface to prevent short circuit. Between the film sheets  107 ,  108  there is also a dielectric film  123  on the other side of the element  118  at the area  117 , preferably of the same thickness as film  122 . Another possibility is to extend the element  118  consisting of dielectric cellular electret films to the end of area  117 , in which case film  123  is not needed. Similarly, it is possible to extend the element  118  to the end of area  114  as well, in which case film  122  is not needed. At one end  117  of the transducer is a metallic connector  106  mechanically pressed through sheets  107 ,  123 ,  108 , shorting the ground electrodes  111 ,  110 ,  112 ,  113 . At the other end  114  is a metallic connector  124  mechanically pressed through sheets  107 ,  122 ,  108  to connect the signal electrode  109  to a signal processing device. The ground electrodes, which are all thus disposed on the outer surfaces of film sheets  107 ,  108 , are grounded e.g. by pressing them between the halves of the casing of the signal processing device. It is recommendable to use a soft, electrically conductive material in this area between the halves of the casing. The grounding can also be implemented by pressing one of the ground electrodes  111 ,  113  against the circuit board of the signal processing device at a point reserved for it, at which point it is also recommendable to use electrically conductive rubber as mentioned above. Reference is now made to the FIGS. 4 a - 4   b . Disposing the signal electrode and the ground electrodes in this way in sequence at the end of the transducer and grounding the transducer in the ways described above eliminates tension and also provides a transducer structure narrow enough to allow the transducers electrodes screen-printed closely side by side on the dielectric film sheets  125 ,  126 , e.g. polyester of thickness 0,1 mm, maximizes the amount of the transducers from material and work used. In addition (referring to FIGS. 2 a ,  2   b ,  2   c ), such a narrow transducer having the same width throughout its length is very easy to install, because the connector of an individual electrode is so narrow that, in all guitars commonly used, in which the saddle width is on the order of 3 mm, it can go from above through the two holes  105  made on the sides of the bridge body  101  under the saddle  102  through the resonance box cover  100  to the inside of the guitar to connect the transducer to a signal processing device.  
         [0040]    In the embodiment of FIG. 3 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 an 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 charges are injected onto the underside of sheet  119  and onto the top side of sheet  121 . Negative charges may be injected onto the top side of sheet  119  and onto 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.  
         [0041]    Reference is now made to FIG. 3 c . By making the length of area  115  so long that connector  128  reaches the signal processing device too, a transducer is obtained whose ground electrodes  111 ,  110 ,  112 ,  113  can be connected to the circuit board of a signal processing device by means of a connector  128 . Further, by using an arrangement where no ground electrode  110  is printed and on the top side of the sheet  108  to the areas  116 ,  117  is printed a signal electrode and by grounding both ground electrodes  111 ,  113  to the case of the signal processing device in the manner explained above, none of said ground electrodes  111 ,  113  extending to the connectors  124 ,  128 , a differential transducer is obtained.  
         [0042]    In the embodiment of FIG. 3 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. 3 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. If desired, grounding can also be effected via a connection between the halves of the casing as described above. 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.  
         [0043]    Reference is now made to FIGS. 5 a - 5   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. 5 a - 5   c . In FIG. 5 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  for each string of the guitar, in this case 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. 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. 5 b  there is the ground electrode  146  screen-printed on a dielectric sheet  138 , e.g. polyester of the embodiment described above. In FIGS. 5 c ,  5   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. 5 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. 5 d . The pattern for screen-printing the dielectric insulation  151  over the electrodes shown in FIG. 5 c  is showed in FIG. 7.  
         [0044]    Referring now to FIGS. 3 a ,  3   c ,  4   a ,  4   b , The transducers of the two embodiments of the invention as shown FIGS. 3 a ,  3   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. 4 a . To the other side of this film  125 , there is ground electrodes screen-printed as shown in FIG. 4 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. 4 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. 6 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 larger number of thin, flexible stringed musical instrument 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.  
         [0045]    The transducers of invention in FIGS. 8 a  and  8   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 the string vibrations into electric signals. As may be noted the transducers in FIGS. 8 a  and  8   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.  
         [0046]    Referring now to FIG. 8 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 .  
         [0047]    Cellular electret films  119 ,  120  in the transducer area may each comprise of several film layers. Each film  119 ,  120  is charged. 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 is no charges in the connection part  115 , to avoid the connection part become microphonic picking sounds from inside the instrument and handling noises. The ground electrodes  211 ,  212  can also be sputtered, evaporated, chemically metallized or screenprinted to the outer sides of the bubble films  119 ,  120 . It is also possible to arrange the signal electrode  209  directly on the face of bubble 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 . In this embodiment, to increase the output voltage and improve the string-to-string balance, 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 tranducer area to be for example round shape, or oval, or square, or multiple round areas in line, depending on the preferred embodiment. Multiple round areas in line, with small weigh over each round area, is very good design for contact pickup installed in the bridge plate inside the guitar, right under the saddle, working as an accelerometer.  
         [0048]    The outermost film layers  221 ,  223 , are uncharged cellular film layers, preferably less than 100 microns in thickness, which due their elastic structure even out the possible roughness and unevenness at the instruments saddle slot and saddle and therefore improve the string-to-string balance but do not change the instruments original acoustic sound. However, these film layers  221 ,  223  are not essential for the transducers operation. Rubber layers have been used to improve string-to-string balance, but using them effects more in instruments original acoustic sound and playing “touch”.  
         [0049]    The FIG. 11 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, an very small preamplifier circuitry can be integrated into the connector end. The components of the circuitry, preferably one field-effect FET) transistor 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 . Leads are connected to the circuitry for taking the signals to the amplifier and sound system. By having the preamplifier circuitry as close as possible to the transducer unit, the capacitance of the connection part is lowest possible and the signal-to-noise ratio becomes significantly better.  
         [0050]    To make an contact pickup 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, for example 5, and an separate weigh, for example 0,5 mm thick copper plate of same shape is glued over the round transducer area, on the opposite side of the side which attaches to instrument. The weigh works as mass against which the instrument vibrates and which further causes signal output proportional to sound-source. The transducer end with weigh can further be encapsulated to prevent the transducer to pickup up air-movements which can cause unclear sound, but only vibrations from the surface it is attached to. With this type use, we have noticed that the signal output increases proportional to frequency due the acceleration effect. Therefore a low-pass filter is needed at the preamplifier part, preferably between 100 Hz and 1000 Hz, whether fixed or with adjustable control. In a double bass, for example, interesting features can be obtained, by dividing the contact transducers signal into two channels in the preamplifier, and having in one channel the low-pass filter at high frequency, for example at 5 KHz, and having the low-pass filter at other channel for example at 200 Hz, a and having a switch to change the sound between each channel.  
         [0051]    The transducers in FIGS. 8 a  and  8   b  and  11  are fabricated as follows:  
         [0052]    Referring to FIG. 9 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.  
         [0053]    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 .  
         [0054]    This procedure allows a considerably larger number of thin, flexible stringed musical instrument 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. 8 a  and  8   b , the transducers can be manufactured very thin without any extra flexible firm insulating substrates to carry the electrodes. Because there is thickness saved due no extra firm insulating substrates, there can be more of active layers, easily 4 layers, which further improves the output voltage and thus also the signal-to-noise ratio.  
         [0055]    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 metallising. Further, cellular film strips  221 ,  223  may be arranged to the outer faces or ground electrodes  211 ,  212 , to even out the possible roughness of saddle and saddle slot and thus improve the string-to-string balance.  
         [0056]    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. The transducer can also be used not only with most string instruments, like guitar, violin, bass, mandolin and so on, but also for example with wind instruments.