Abstract:
A transducer utilizes a sound-producing member positioned in the area of magnetic flux concentration between magnetic poles of opposite polarity. The sound-producing member is variably vibratable in a magnetic structure between the poles to generate acoustic waves, and an acoustic conduit carries the acoustic waves through the magnetic poles e to a location outside the magnetic structure.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to the field of electro acoustic transducers. While the invention has applicability to a wide range of diverse applications, it will be specifically disclosed in connection with a class of electro acoustic transducers commonly referred to as “micro speakers” or “receivers” in the hearing aid industry. Transducers constructed in accordance with the principles of the invention also can be used in some applications to convert acoustic energy to electrical energy, i.e. as a microphone. 
     BACKGROUND 
     Balanced armature electro acoustic transducers have long been fundamental components of communications equipment ranging from telephones to hearing aids. Very early telephones utilized balanced armature transducers in their earpieces and such speakers took on the name of the entire hand piece and became known as “receivers.” In keeping with this commonly used terminology, the terms “speaker” and “receiver” will be used interchangeably in this specification. 
     In hearing aid applications, balanced armature devices have been used for both microphones and “receivers.” While other technologies, notably “electrets,” have largely supplanted the use of balanced armature transducers as microphones in the specific context of hearing aids, balanced armature devices continue to be the most commonly used technology for “receivers” in present day hearing aids. Most advantageously, balanced armature devices can produce extremely loud sounds with very little power and within a very small geometric volume and footprint. 
     A limitation to the performance of conventional balanced armature electro acoustic devices, whether used as speakers or microphones, is that their characteristic frequency spectra deviate from being perfectly flat, spectral flatness being one representation of a lack of distortion, a very desirable characteristic for acoustic (and most other) transducers. This spectral deviation or “signature” arises from the fundamental structural properties that are characteristic of all conventional balanced armature devices: the mass and springiness of: the armature itself, the sound producing diaphragm and its chamber(s), and of the connector element and its attachments that link the armature and the diaphragm. More particularly, the beam and connecting rod of the armature, the diaphragm, and even the air and ports into which the air exits all have associated masses and springiness, and the system has a characteristic resonance that reflects the energy exchange between such masses and springs. Numerous techniques have been developed to minimize the disadvantages of this inherent signature, including, for example, the use of so-called “ferro-fluids” for damping the system and improving the transducer&#39;s dynamic performance. 
     Notwithstanding the substantial enhancements to these general types of transducers, room remains for improving and simplifying the frequency signature and minimizing the frictional and other mechanical losses. Substantial room further exists for enhancing the relationship to the non-linear magnetic forces with a corresponding non-linear springiness of the armature/diaphragm. In many applications, it also is desirable to further reduce the size of the transducer. For example, when used in a hearing aid or earphone application, it is desirable to have a transducer that is small enough to comfortably fit within a human auditory canal. Similarly, when used as a component of a device, such as a cell phone, the small size of the transducer allows the size of the device to be minimized. 
     SUMMARY OF THE INVENTION 
     The present invention advantageously overcomes many of the disadvantages of the prior art by eliminating all of the individual elements comprising the sound producing/receiving diaphragm and the armature, effectively integrating these components into a single “balanced diaphragm” element. By integrating these multiple components into a single functional component, the frequency signature of these devices is greatly simplified. Furthermore, providing a sound conduction pathway through the magnetic structure in which the diaphragm is balanced, the sound producing or receiving balanced diaphragm element can be located entirely within the fluid (air or other) gap between the magnetic poles and still remain in substantially direct communication with fluid (air or other) in the environment. Particular choices for the spring, mass and damping characteristics of the balanced diaphragm and its containing chambers and conduits, (or multiple instances of the same) enable improved spectral control in this simplified, integrated system over the multi-element system it supersedes. A two-diaphragm version of the concept minimizes part vibration and allows for enhanced acoustic performance, for example, a micro-woofer micro-tweeter combination. 
     To achieve one or more of these objectives, one exemplary embodiment provides an electro-magnetic transducer that includes a magnetic structure with at least two magnetic poles of opposite polarity. The structure includes at least two magnetic poles of opposite polarity that create an area of magnetic flux concentration. A vibratable sound-producing member at least partially formed of magnetically permeable material and vibratable toward and away from the magnetic poles is disposed in the area of magnetic flux concentration. The sound-producing member vibrates toward and away from the magnetic poles to produce acoustic waves in the area of magnetic flux in response to electrical current passing through the coil. An acoustic conduit is provided for receiving sound waves generated by the sound-producing member and directing such waves from the area of magnetic flux concentration to a location outside the magnetic structure. 
     In at least one exemplary embodiment, the area of magnetic flux concentration is located between the magnetic poles of opposite polarity. 
     In one exemplary embodiment, the sound-producing member is generally positioned in a plane substantially equidistance between the magnetic poles. 
     In one exemplary embodiment, a support structure is provided for engaging and supporting the peripheral portions of the sound-producing member. 
     In one exemplary embodiment, the peripheral support structure for the sound-producing member is compliant. 
     In one exemplary embodiment, a flux concentrator, the transducer includes a flux concentrator, and the flux concentrator supports the coil about an axis. 
     In one exemplary embodiment, the flux concentrator supports the coil about an axis extending substantially perpendicular to the plane of the sound-producing member. 
     In one exemplary embodiment, the flux concentrator supports the coil about an axis extending substantially parallel to the plane of the sound-producing member. 
     In one exemplary embodiment, the sound-producing member includes a diaphragm. 
     In one exemplary embodiment, the sound-producing member is variably vibratable in response to varying electrical current passing through the coil. 
     In one exemplary embodiment, the acoustic conduit for receiving sound waves generated by the sound-producing member extends through the magnetic structure. 
     In one exemplary embodiment, the electro-magnetic transducer includes a case in which the magnetic structure is supported. The case includes at least one acoustic conduit aligned with the acoustic conduit extending through the magnetic structure. The acoustic conduit extending through the magnetic structure cooperates with the acoustic conduit of the case to joint form an acoustic pathway extending from the flux area to outside the case. 
     In one exemplary embodiment, at least one acoustic cavity is formed within the case. 
     In one exemplary embodiment, an electro-magnetic transducer includes a magnetic structure formed by an annular magnet; a first pole piece magnetically connected to the annular magnet and a second pole piece magnetically connected to the annular magnetic. The first and second pole pieces form magnetic poles of opposite polarity with an area of magnetic flux concentration being formed between the pole pieces. A sound producing structure is interposed in the area of magnetic flux concentration between the first and second pole pieces. The sound producing structure is at least partially formed of magnetically permeable material and is operable to produce acoustic waves in the area of magnetic flux concentration between the pole pieces. A coil is located in proximity to the sound producing structure with the sound producing structure being variably vibratable toward and away from the first and second pole pieces to produce acoustic waves in the area of magnetic flux concentration in response to variable electrical current passing through the coil. An acoustic conduit extends through one of the pole pieces for permitting the passage of an acoustic wave through the magnetic structure. The sound-producing surface is operative to generate sound waves in the flux area and to direct such waves through the acoustic path extending through the magnetic structure to an external sound environment. 
     In one exemplary embodiment, the magnetic structure is supported in the case, and the case includes at least one acoustic conduit aligned with the acoustic conduit extending through the magnetic structure. The acoustic conduit(s) extending through the magnetic structure and the acoustic conduit(s) of the case jointly form an acoustic pathway extending from the flux area to outside the case. 
     In one exemplary embodiment, an electro-magnetic transducer includes a magnetic structure that includes at least two magnetic flux fields between magnetic poles of opposite polarity. A sound producing structure is disposed in each of the two magnetic flux fields. Each of the sound producing structures is at least partially formed of magnetically permeable material and is located between magnetic poles of opposite polarity. A coil is located in proximity to each of the sound producing structures. Each of the sound producing structures are variably vibratable toward and away from the magnetic poles to produce acoustic waves in the flux areas in response to varying electrical current passing through the coil. A plurality of acoustic conduits extends through the magnetic structure to an external sound environment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention, and together with the description, they serve to explain the principles of the invention. In the drawings: 
         FIG. 1  is a cross-sectional view of a typical prior art balanced armature acoustic transducer in its application as either a microphone or a speaker; 
         FIG. 2  is a graphical representation comparing the frequency responses or spectra for prior art transducers to an ideal condition for a transducer used a speaker; 
         FIG. 3  is a perspective view showing the exterior of one exemplary embodiment illustrating some of the principles of the present invention in the form of a single diaphragm receiver; 
         FIG. 3   a  is a cross-sectional view of the exemplary embodiment of  FIG. 3 ; 
         FIG. 3   b  is an exploded view of the exemplary embodiment of  FIG. 3 ; 
         FIG. 3   c  is a perspective view of an integrated armature/diaphragm used in the exemplary embodiment of  FIG. 3 ; 
         FIG. 4  is a perspective view showing the exterior surface of another exemplary embodiment illustrating some of the principles of the present invention in the form of a “double bent armature” wherein the armature is doubled-back on itself; 
         FIG. 4   a  is a cross-sectional view of the exemplary embodiment of  FIG. 4 ; 
         FIG. 4   b  is an exploded view of the exemplary embodiment of  FIG. 4 ; 
         FIG. 4   c  is a perspective view of the integrated armature/diaphragm used in the exemplary embodiment of  FIG. 4 ; 
         FIG. 5  is an exploded view of illustrating the exterior view a further exemplary embodiment utilizing some of the principles of the present invention in the form of a “dual double bent armature having axial aligned sound ports;” 
         FIG. 5   a  is a perspective view of illustrating the exterior view a further exemplary embodiment utilizing some of the principles of the present invention in the form of a “dual double bent armature having axial aligned sound ports;” 
         FIG. 5   b  is a cross-sectional view of the exemplary embodiment of  FIG. 5   a  illustrating some of the principles of the present invention in the form of a dual diaphragm receiver wherein the armature elements are doubled-back on themselves and the central structure is common to both balanced diaphragm actions; 
         FIG. 5   c  is a perspective view of illustrating the exterior view a further exemplary embodiment utilizing some of the principles of the present invention in the form of a “dual double bent armature having radial aligned sound ports;” 
         FIG. 5   d  is a cross-sectional view of the exemplary embodiment of  FIG. 5   c  illustrating some of the principles of the present invention in the form of a dual diaphragm receiver wherein the armature elements are doubled-back on themselves and the central structure is common to both balanced diaphragm actions; and 
         FIG. 6  is an exploded view of another exemplary embodiment illustrating some of the principles of the present invention in the form of a “solenoidal armature” wherein the armature coil is perpendicular to the armature diaphragm. 
         FIG. 6   a  is a perspective view showing the exterior surface of a “solenoidal armature” wherein the armature coil is perpendicular to the armature diaphragm and the sound exit conduits are axially aligned. 
         FIG. 6   b  is a cross-sectional view of the exemplary embodiment of  FIG. 6   a  illustrating some of the principles of the present invention in the form of a “solenoidal armature” wherein the armature coil is perpendicular to the armature diaphragm and the sound exit conduits are axially aligned. 
         FIG. 6   c  is a perspective view showing the exterior surface of a “solenoidal armature” wherein the armature coil is perpendicular to the armature diaphragm and the sound exit conduits are radial aligned. 
         FIG. 6   d  is a cross-sectional view of the exemplary embodiment of  FIG. 6   c  illustrating some of the principles of the present invention in the form of a “solenoidal armature” wherein the armature coil is perpendicular to the armature diaphragm and the sound exit conduits are radial aligned. 
     
    
    
     Reference will now be made in detail to certain exemplary embodiments of the invention, examples of which are illustrated in the accompanying drawings. 
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The specifically illustrated exemplary embodiments relate to an acoustic transducer that minimizes frictional and other mechanical losses. When used in connection with a balanced armature type of transducer, these exemplary embodiments advantageously eliminate a connector element by integrating the armature and diaphragm. Acoustic conduits, specifically shown in the exemplary embodiments as holes in the poles of the magnets of the transducer, provide acoustic coupling between the integrated armature/diaphragm and the external sound environment. 
     Certain aspects of the illustrated exemplary embodiments are best appreciated by a comparison with conventional balanced armature type transducers of a type similar to these exemplary embodiments illustrated. Referring specifically now to the drawings,  FIG. 1  is a cross sectional depiction of a conventional state of the art balanced armature acoustic transducer  100 . This particular illustrated prior art transducer  100  includes a permanent magnet  114  with having a “north” pole  116  and a “south” pole  118  and an air gap  112  located between the poles  116  and  118 . The magnet  114  produces a magnetic field in an air gap  112 . In this conventional prior art transducer, a free end of a beam  120  extends into the air gap  112 . The beam  120  is made of magnetically permeable material and is supported in a cantilever fashion. A mechanical bond between the beam  120  and an internal surface of the housing  100  is provided at location  110  to secure a fixed end of the beam  120  such that the free end of the beam is centered between poles  116  and  118  in the air gap  112 . An electrical coil  130  created from turns of insulated conductor  129  is wound around a portion of beam  120  such that an electric solenoid is created whose beam  120  “core” is a dipole magnet. One end of a connecting rod  140  is connected to the free end of beam  120  through a joint  143 . The other end of the connecting rod  140  is connected to a sound-producing surface  150  through a joint  145 . The sound producing surface  150  has a compliant supporting peripheral portion or “surround”  152  at its outermost edge, and this outermost edge forms an acoustic seal along its periphery as it attaches to a supporting structure  151 , which supporting structure  151  extends inwardly from an interior surface of the structural housing  100  and forms a floor of an acoustic chamber structure  160 . The acoustic chamber structure  160  has an output port  165  to which a conduit or other acoustic conveyance (not shown) can be attached to direct sound energy to the external acoustic environment, typically a wearer&#39;s outer ear. 
       FIG. 2  depicts a comparative frequency response plot between representative of the acoustic output of a conventional state of the art balanced speaker, such as the speaker illustrated in  FIG. 1 , and the response of an ideal receiver in response to a constant input of electrical current. The abscissa of the plot depicted in  FIG. 2  is logarithmic frequency, and the ordinate representing decibels of sound pressure level, also a logarithmic form of measure. The solid line represents the spectral plot  200  for a typical existing state of the art balanced diaphragm receiver, such as illustrated in  FIG. 1 . This solid line is comprised of a relatively flat zone  210 , followed by a rising region  220 , resulting in a first peak  230  occurring at approximately 1100 Hz, followed by its declining region  240 , which reaches a trough  250  at approximately 1600 Hz, which is followed by a second peak at  260  at approximately 2200 Hz, and a continuum of repeated peaks and valleys in region  270  at the upper extent of the spectral plot. The frequently response of a conventional transducer is compared to that of an ideal receiver, which is depicted in the straight dashed spectral plot  280 . The spectral plot of line  280  represents the theoretically flat response of an ideal receiver whose output in response to a constant input energy as a function of frequency would be a constant and uniform acoustical output as a function of frequency. 
       FIGS. 3 ,  3   a  and  3   b  show a first exemplary embodiment of the present invention in a form utilizing a “straight armature” receiver. In this exemplary embodiment, a transducer is enclosed within a structural housing  300  that encloses the transducer. The structural housing  300  contains a magnet  340  (see  FIGS. 3   a  and  3   b ), which in this specifically illustrated embodiment has an annular configuration. A magnetic field is produced in an air gap or magnetic flux area  316  located between the opposite magnetic poles formed between an upper magnetic pole piece  380  and a lower pole piece  320 . Exemplary suitable permeable ferro-magnetic materials from which pole pieces  380  and  320  might be made include the iron-based “High mu 80” (Carpenter Steel Corporation). In the exemplary form illustrated, an acoustic conduit is formed in upper pole piece  380  by piercing through the upper pole piece to form holes  382 . The illustrated exemplary embodiment further includes correspondingly aligned holes  392  (see  FIG. 3   b ) in upper case portion  390 . These aligned holes form an acoustic path through which a fluid, such as air, maintains contiguous relationship with fluid present on the inside of pole piece  380  and the outside of upper case  390 . The magnetic structure, exemplarily illustrated as an annular magnet  340  may be a permanent magnet or it may be an electromagnet built using well-known principles of winding a coil around a magnetically permeable form. As those skilled in the art will readily appreciate, if an electromagnet is used, an electric current is supplied to the coil to form a magnetic field. 
     As best illustrated in  FIG. 3   c , this exemplary embodiment includes a vibratable sound-producing member, specifically illustrated in this drawing figure as an armature that is integrated with a diaphragm. The illustrated armature/diaphragm  350  includes at least a portion of magnetically permeable material  358 . The illustrated armature/diaphragm  350  also has a cantilevered geometry with a base that is rigidly affixed to a magnetic coil structure  360 . The diaphragm forming “free” end of the armature/diaphragm  350  is such that the magnetic forces in the air gap  316  just balance the supporting forces. A sound-producing surface  352  is intimately affixed to the magnetically permeable material  358  so as to be integral with the armature structure  350 . Compliance-producing surround  354  is also integrally disposed peripherally with sound producing surface  352  and is also continuously affixed to upper support ring  370  and lower support ring  330  on its flexible “surround” periphery  354 . An electrical to magnetic coil  360  is wound around a portion  356  of the armature  350  at a position starting near its fixed end. Acoustic cavities  326  and  386  (see  FIG. 3   a ) are formed within case structure  310  inside of lower pole  320  to as one form of acoustic tuning means. Case structure  310  further provides a structural support to the fixed end of the beam  320  as well as the annular magnet  340  and poles  320  and  380 . 
       FIGS. 4 ,  4   a  and  4   b  show a second exemplary embodiment of the present invention in the form of a “double bent armature” receiver  400 . In this exemplary embodiment, a magnetic field is produced in air gap  416  by an annular magnet  440 , an upper magnetic pole piece  480  and a lower pole piece  420 . Pole pieces  480  and  420  are made of a suitably permeable ferro-magnetic material such as “High mu 80” (Carpenter Steel Corporation), and upper pole piece  480  is configured with openings or holes  482  (see  FIG. 4   b ) through which a fluid such as air maintains contiguous relationship with fluid present on the inside of pole piece  480  and its outside boundary. Similarly, opening(s) or hole(s)  422  (see  FIG. 4   b ) in lower pole piece  420  provide a pathway through which fluid such as air maintains contiguous relationship with fluid below and above the pole piece  420 . The openings  422  so may be continued as shown by other openings, as illustrated by  412 , that extend through the bottom case  410 . As specifically illustrated, the exemplary embodiment of  FIG. 4  shows an annular magnet  440 , which may be a permanent magnet or it may be an electromagnet built using well-known principles of winding a coil around a magnetically permeable form and supplying said coil with an electric current to form a magnetic field. The armature  450  (shown in greater detail in  FIG. 4   c ) of this exemplary embodiment is comprised of at least a portion of magnetically permeable material  458 . The illustrated armature  450  has a cantilevered geometry with a base  456  that is rigidly affixed to lower body structure  410  at mounting block  465 . The armature  450  also includes a diaphragm forming “free” end configured and arranged so that the magnetic forces in the air gap  416  just balance the supporting forces. A sound-producing surface  452  is intimately affixed to the armature/diaphragm so as to be integral with the armature/diaphragm structure  450 . 
     A compliance-producing surround  454  is also integrally disposed peripherally with sound producing surface  452  and is also continuously affixed to upper support ring  470  and lower support ring  430  on its flexible “surround” periphery  454 . An electrical to magnetic coil  460  is wound around a portion  456  of the armature  450  at a position starting near its fixed end. Acoustic cavities shown as through gap  422  and hole(s)  424  are formed within case structure  410  inside of lower pole  420  to form acoustic tuning means in companion with which may, as shown by  412 , or may not entirely proceed from the inner portion of lower pole  420  and through lower case  410  to the external environment. Case structure  410  further provides a structural support to the fixed end of the bent beam  456  through mounting block  465  (see  FIG. 4   b ). Mounting block  465  provides support and concentric alignment for the annular magnet  440 , magnetic pole pieces  420  and  480  and the support rings  430  and  470 . 
       FIG. 5  illustrates, as an exploded view, a third exemplary embodiment of the present invention in the form of a “dual double bent armatures” receiver.  FIGS. 5   a  and  5   b  show a first variation  500 , and its cross-section  502  respectively, of the present embodiment having axially-aligned acoustic conduits  582  and  592  emerging from the top and bottom respectively of the device.  FIGS. 5   c  and  5   d  show a second variation  501 , and its cross-section  503  respectively, of the present embodiment having radial-aligned acoustic conduits  584  and  585  emerging from the side clamshell half  595  respectively of the device, and further combining into the single acoustic nosepiece conduit  599 . In general terms, this particular exemplary embodiment depicts two complete electro-mechanical-to-acoustic transducing sections, an upper transducing section  504  and a lower transducing section  505 , having similar, but not necessarily identical mechanical to acoustic elements. As most clearly seen in  FIG. 5 , these units share a common outer supporting structure comprised of two “clamshell style” halves,  595  and  596  respectively, and a common electrical winding in the form of an excitation coil  530 . Excitation coil  530  forms a continuous magnetic solenoid with upper diaphragm armature  550  and also with lower diaphragm armature  551  (See  FIG. 5 ). Both of these diaphragm armatures  550  and  551  in this exemplary embodiment are similar in composition to the single armature  450  in the exemplary embodiment illustrated in  FIG. 4 , and may be comprised of the same detail parts as delineated in connection with that earlier described exemplary embodiment. In the form of the invention represented by the present exemplary embodiment (of  FIGS. 5 ,  5   a ,  5   b ,  5   c , and  5   d ) the upper diaphragm armature  550  may or may not differ from lower diaphragm armature  551  as is depicted, depending upon the acoustical characteristics desired in any particular variation of the present embodiment. For instance, the upper diaphragm/armature  550  may be more stiffly supported and less massive than lower diaphragm armature  551 , and the diameters of the diaphragm armatures, their magnetic permeability, and material composition may be identical or different. In the general exploded representation of the embodiment of  FIG. 5 , the upper magnetic section of the receiver of this exemplary embodiment is comprised of an uppermost pole piece  580  of magnetically permeable material having aforementioned open acoustic conduits  582  traversing through its thickness, an upper magnetic source ring  540 , an upper outer side spacer support ring  572  and an upper inner side spacer ring  570  that each engage the surfaces on the periphery of the diaphragm portion of diaphragm armature  550 , and an innermost pole piece  520 , which has at least one pole gap  522 , a singular feature being required for the passage of diaphragm armature  550  on its way to excitation coil  530 , and, optionally, one or more auxiliary passages  524 . Similarly, the lower magnetic section of the receiver of this exemplary embodiment is comprised of a lowermost pole piece  581  of magnetically permeable material having aforementioned open acoustic conduits  583  traversing through its thickness, a lower magnetic source ring  541 , a lower inner side spacer support ring  571  and a lower outerside spacer ring  573  that each engage the surfaces on the periphery of the diaphragm portion of diaphragm armature  551 , and an innermost pole piece  521 , which has at least one pole gap  523 , a singular feature being required for the passage of diaphragm armature  551  on its way to common excitation coil  530  and, optionally, one or more auxiliary passages  525 . Clamshell halves  595  and  596 , when assembled as a continuous cylinder, provide physical encasement of the motor and sound producing parts in a stacked concentric fashion. Shelf detail  597  may have one or more conduits  598  as shown in the inner part of clamshell half  596 , and a similar structural element may or may not be present in the mating clamshell half  595 . 
       FIG. 6  illustrates, as an exploded view, a fourth exemplary embodiment of the present invention in the form of a “solenoid induction armature” receiver.  FIGS. 6   a  and  6   b  show a first variation  600 , and its cross-section  602  respectively, of the present embodiment having axially-aligned acoustic conduits  682  emerging from the device and one or more auxiliary secondary “tuning” acoustic conduits  624  emerging through other elements.  FIGS. 6   c  and  6   d  show a second variation  601 , and its cross-section  603  respectively, of the present embodiment a having radial-aligned acoustic conduit  684  emerging from the side of the device, and further continuing acoustic nosepiece conduit  699 . Appropriate gap features  671  and  673  are provided in this variation of the embodiment in companion with passages  644  and  645  that complete the unobstructed sound conduit connecting the sound-generating surface with the nosepiece conduit  699 . In general terms, this exemplary embodiment as most generally depicted in exploded view  600  shows the structure of a device which, while retaining the primary feature of a sound generating surface  650  contained within the static magnetic producing features (upper pole piece  680 , magnet  640 , and lower pole piece  620 ,) separates the magnetic flux concentration structure as core  680  with central pole  685  that supports the coil  680 , from the sound generating surface  650 . An alignment features such as step  628  on lower pole piece  620  is shown in alignment relation with the outer margin of pole piece  680 , and an outer step  626  is shown in alignment with magnet  640 . A magnetic air gap  625  may be provided between lower pole piece  620  and magnetic core  680 . Notwithstanding the geometric separation of these elements (the variable magnetic portion of the armature (coil  630 , core  680  and central plate  685  collectively) from the sound producing surface  650 , the elements together constitute a single physically (magnetically combined) armature/diaphragm structure. Diaphragm  650  is supported between support rings  670  and  672 . 
     The foregoing description of preferred embodiments of the invention has been presented for purpose of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments were chosen and described in order to best illustrate the principles of the invention and its practical applications to thereby enable one of ordinary skill in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.