Abstract:
An electrostatic transducer has two oppositely disposed counter-electrodes and a diaphragm which is disposed between the two counter-electrodes and which has a first weakly conductive coating. In its edge region the diaphragm has a second well-conductive coating on the first coating to protect the latter.

Description:
[0001]    The present application claims priority from German Patent Application No. 10 2015 122 698. 7 filed on Dec. 23, 2015, the disclosure of which is incorporated herein by reference in its entirety. 
       FIELD OF THE INVENTION 
       [0002]    It is noted that citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention. 
         [0003]    The present invention concerns an electrostatic headphone. 
         [0004]    An electrostatic headphone typically has two counter-electrodes and a diaphragm which is arranged therebetween and which has a thin metallic coating. The diaphragm which is actuated with an audio signal is driven in the electrostatic field of the counter-electrodes which are brought to a high electrical potential of differing polarity. The diaphragm is typically mechanically fixed in the edge region to a diaphragm or carrier ring, for example by gluing. The diaphragm side remote from the diaphragm ring is typically of a very weakly conductive configuration due to a very thin coating with metal. The coating can be on one or both sides. Electrical contacting of the diaphragm is typically effected in the edge region by applying a conductive contact ring which can be of the same shape as the diaphragm ring. The contact ring is typically made from the same material as the diaphragm ring. 
         [0005]    The diaphragm of the electrostatic transducer requires a low level of conductivity. This is typically made possible by a conductive coating on the diaphragm, which must be very thin or extremely thin. By virtue of the small thickness of the conductive coating on the diaphragm electrical interruptions within the coating can occur due to mechanical stressing of the diaphragm. 
         [0006]    On the German patent application from which priority is claimed the German Patent and Trade Mark Office searched the following documents: DE 41 15 221 A1, DE 43 29 991 A, US 2007/0189559 A1 and US 2009/0147972 A1. 
       SUMMARY OF THE INVENTION 
       [0007]    An object of the present invention is to provide an electrostatic transducer for example for headphones, which reduces or avoids electrical interruption in the coating on the diaphragm due to mechanical stresses. 
         [0008]    Thus there is provided an electrostatic transducer having two oppositely disposed counter-electrodes, a diaphragm between the two counter-electrodes and a contact ring for contacting the diaphragm (in its outer region). At least on one side the diaphragm has a first weakly conductive coating. In its edge region the diaphragm has a second well-conductive coating on the first coating. 
         [0009]    The second coating projects beyond the contact ring. 
         [0010]    According to an aspect of the present invention the second coating is in the form of a continuous annular coating or in the form of an annular coating with interruptions. 
         [0011]    By virtue of the fact that the second coating projects beyond the contact ring (that is to say is provided at the edge region of the diaphragm), damage to the first coating at the inner edge of the contact ring can be reduced or avoided. The diaphragm thus has a first region having a first electrically conductive coating and a second region having a second electrically conductive coating. The first coating is a poor conductor and corresponds to the normal coating on a diaphragm for an electrostatic transducer. The second coating is provided in the edge regions of the diaphragm and serves as protection for the first coating. In that case the second coating which is a good conductor is typically thicker or more robust. The purpose of the second coating is in particular to protect the first coating in the region of an inner edge of the contact ring of the diaphragm. It is at that location that in operation constant mechanical stressing occurs due to the vibrating diaphragm. The second coating can be made from another material, for example a less expensive material, than the first coating. 
         [0012]    The invention concerns the notion that slight mechanical stresses like stretching, bending, shearing and so forth or aging effects can lead to electrical interruptions in the coating on the diaphragm. In that respect in particular a transitional region at an inner sharp-edged edge of the contact ring can lead to problems. Very slight displacements of the contact ring under mechanical pressure can occur in particular upon assembly of the transducer. In addition, interruptions in the contact can also occur after prolonged operation. It should be noted that a movement of the diaphragm during operation of the transducer particularly stresses the coating. That also applies to the inner edge of the contact ring so that malfunctions can also occur after a prolonged period of operation. 
         [0013]    According to the invention there is therefore provided an electrostatic transducer which permits secure contacting in the edge region of the diaphragm. For that purpose an additional (further) well-conductive coating is applied in the edge region of the diaphragm, that covers over part of or the entire region of the diaphragm ring and a partial region of the adjoining exposed diaphragm. This can therefore provide for secure electrical contacting of the contact ring bearing thereagainst, even in the case of a weakly conductive diaphragm coating. The diaphragm thus has a first thin electrically conductive coating. In the edge region of the diaphragm the diaphragm at least portion-wise has a second well-conductive coating. That coating can be annular and continuous or can be of an only portion-wise configuration. 
         [0014]    It is thus possible to achieve in particular improved electrical contacting. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  shows a diagrammatic sectional view of an electrostatic transducer according to the state of the art. 
           [0016]      FIG. 2  shows a diagrammatic sectional view of an electrostatic transducer according to the invention. 
           [0017]      FIG. 3  shows a diagrammatic view of a diaphragm of the electrostatic transducer of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0018]    It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for purposes of clarity, many other elements which are conventional in this art. Those of ordinary skill in the art will recognize that other elements are desirable for implementing the present invention. However, because such elements are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements is not provided herein. 
         [0019]    The present invention will now be described in detail the basis of exemplary embodiments. 
         [0020]      FIG. 1  shows a diagrammatic sectional view of an electrostatic transducer according to the state of the art. The electrostatic transducer has two counter-electrodes  10  which are positioned in parallel and positively locking relationship at a spacing relative to each other. The spacing between the two counter-electrodes  10  can be afforded by two mutually superposed similar rings  20  and two insulating disks  30 . A diaphragm  40  is arranged between the two diaphragm rings  20  and can be fixed on one of the two diaphragm rings  20 . The second diaphragm  20  can be in the form of a contact ring. The counter-electrode  10 , the diaphragm  40  and the rings  20  and the insulating disks  30  can be for example glued, screwed or clamped together. The counter-electrodes  10  and the rings  20  can be made from a conductive material. The counter-electrodes  10  and the rings  20  can have electrical terminals  11 ,  13 ,  21  for electrical contacting. 
         [0021]    As an alternative thereto the counter-electrodes  10  can be made from an insulating material like for example ceramic, in which case the counter-electrodes  10  are then partially metalized. The counter-electrodes  10  can have a sound-transmissive perforation  12 . The diaphragm ring and the contact ring  20  are electrically connected together. The diaphragm  40  can be made for example from a non-conducting plastic and at at least one side can have a weakly conductive coating. The diaphragm  40  can be of a weakly electrically conductive nature in particular at the side not glued to the diaphragm ring. The diaphragm  40  can be charged lay way of a dc voltage in relation to the counter-electrodes  10 . The diaphragm together with the two counter-electrodes  10  can form an electrical capacitor which with the electrical resistance of the diaphragm forms a time constant for charge migration on the diaphragm. That time constant should be greater than the period duration of the lowest audio frequency to be transmitted. That is intended to apply the principle of constant charging which is a prerequisite for high linearity of the transducer. 
         [0022]    A high dc voltage can be applied between the two counter-electrodes, that is to say at the electrical terminals  11 ,  13  in order to set them to high different potentials and thereby to generate an electrostatic field. An audio signal is connected to the diaphragm  40  by way of the contact ring  20 . That signal is an electrical alternating signal which produces on the diaphragm an electrostatic force varying with the frequency of the audio signal. In that way the diaphragm is deflected in accordance with the signal voltage. 
         [0023]      FIG. 2  shows a diagrammatic sectional view of an electrostatic transducer according to the invention. The structure of the electrostatic transducer shown in  FIG. 2  is based on that of the electrostatic transducer of  FIG. 1 . In addition thereto the electrostatic transducer according to the invention, at the edge of the diaphragm  40 , has a second coating  50 ,  60  in the form of a reinforced diaphragm metallization which extends inwardly beyond the contact ring. 
         [0024]      FIG. 3  shows a plan view of a diaphragm  40  of the electrostatic transducer of  FIG. 2 . The diaphragm can be circular as shown in  FIG. 3 a   ) and can firstly have a diaphragm ring  20 . A weakly conductive, coated diaphragm  40  can be glued on the diaphragm ring  20 , as in  FIG. 3 b   ). That first coating  41  can extend similarly over the entire diaphragm surface, inclusive of the edge region  42  which covers the diaphragm ring. A second coating  50  can be provided at the edge region  42  of the diaphragm, as in  FIG. 3 c   ). That second coating  50  is a transitional coating which is a good conductor. That coating  50  can also be provided in an annular configuration in the edge region  42  of the diaphragm  40 , that covers over at least a partial region of the diaphragm ring  20  and projects inwardly beyond the contact ring  20  so that it is possible to ensure secure electrical contacting of the diaphragm coating with respect to the contact ring  20  bearing thereon. That is shown for example in  FIG. 3 d   ). This therefore provides a continuous annular second coating  50 . The diaphragm ring or contact ring  20  is separated from the counter-electrodes  10  by insulating rings  30 . 
         [0025]    Alternatively the arrangement comprising the diaphragm  40 , the diaphragm ring or contact ring  20 , the insulating rings  30 , the edge region  42  and the second coating  50  can also be oval. 
         [0026]    According to an aspect of the present invention which is shown in particular in  FIG. 3 e   ) the second coating  50  is not in the form of a continuous annular coating but in the form of an annular coating with radial interruptions  60 . Here too a part of the annular coating with radial interruptions  60  projects under the contact ring  20 , as shown in  FIG. 3 f   ). Those radial interruptions are advantageous if the second coating  60  is to be effected by vapor deposition or sputtering by means of a template. 
         [0027]    According to the invention therefore transmissive partial regions of the templates are connected together by limbs. The width of the limbs can be markedly less than the width of the coated segments. 
         [0028]    While this invention has been described in conjunction with the specific embodiments outlined above, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention as set forth above are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the inventions as defined in the following claims.