Abstract:
An electromagnetic transducer includes an electromagnetic dual-coil or multi-coil driver having at least one spacer member placed between at least two permanent magnets. The inclusion of at least one spacer member increases the axial dimension of the magnetic assembly of the driver so that the magnetic gaps in a dual-coil or multi-coil driver are moved farther apart than would occur with a corresponding electromagnetic driver using a permanent magnet instead of two permanent magnets separated by a spacer member.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This invention claims priority to U.S. Provisional Patent Application Ser. No. 60/787,054, filed on Mar. 28, 2006 and titled “Extended Multiple Gap Motors for Electromagnetic Transducers”, the provisional application of which is incorporated in its entirety into this application. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates generally to electromagnetic transducers of the type that may be employed as electro-acoustical drivers for loudspeakers. More particularly, the invention relates to electromagnetic transducers and loudspeakers adapted for extended motor excursion such as may be needed for high output or deep bass. 
         [0004]    2. Related Art 
         [0005]    An electro-acoustical transducer may be utilized as a loudspeaker or as a component in a loudspeaker system to transform electrical signals into acoustical signals. The basic designs and components of various types of electro-acoustical transducers are well-known and therefore need not be described in detail. An electro-acoustical transducer typically includes mechanical, electromechanical, and magnetic elements to effect the conversion of an electrical input into an acoustical output. For example, the transducer typically includes a magnetic assembly, a voice coil, and a diaphragm. The magnetic assembly and voice coil cooperatively function as an electromagnetic transducer (also referred to as a driver or motor). The magnetic assembly typically includes a magnet (typically a permanent magnet) and associated ferromagnetic components—such as pole pieces, plates, rings, and the like—arranged with cylindrical or annular symmetry about a central axis. By this configuration, the magnetic assembly establishes a magnetic circuit in which most of the magnetic flux is directed into an annular (circular or ring-shaped) air gap (or “magnetic gap”), with the lines of magnetic flux having a significant radial component relative to the axis of symmetry. The voice coil typically is formed by an electrically conductive wire cylindrically wound for a number of turns around a coil former. The coil former and the attached voice coil are inserted into the air gap of the magnetic assembly such that the voice coil is exposed to the static (fixed-polarity) magnetic field established by the magnetic assembly. The voice coil may be connected to an audio amplifier or other source of electrical signals that are to be converted into sound waves. The diaphragm includes a flexible or compliant material that is responsive to a vibrational input. The diaphragm is suspended by one or more supporting elements of the loudspeaker (e.g., a surround, spider, or the like) such that the flexible portion of the diaphragm is permitted to move. The diaphragm is mechanically referenced to the voice coil, typically by being connected directly to the coil former on which the voice coil is supported. 
         [0006]    In operation, electrical signals are transmitted as an alternating current (AC) through the voice coil in a direction substantially perpendicular to the direction of the lines of magnetic flux produced by the magnet. The alternating current produces a dynamic magnetic field, the polarity of which flips in accordance with the alternating waveform of the signals fed through the voice coil. Due to the Lorenz force acting on the coil material positioned in the permanent magnetic field, the alternating current corresponding to electrical signals conveying audio signals actuates the voice coil to reciprocate back and forth in the air gap and, correspondingly, move the diaphragm to which the coil (or coil former) is attached. Accordingly, the reciprocating voice coil actuates the diaphragm to likewise reciprocate and, consequently, produce acoustic signals that propagate as sound waves through a suitable fluid medium such as air. Pressure differences in the fluid medium associated with these waves are interpreted by a listener as sound. The sound waves may be characterized by their instantaneous spectrum and level, and are a function of the characteristics of the electrical signals supplied to the voice coil. 
         [0007]    The energy transmitted by a speaker to sound waves is a function of the amount of movement of the diaphragm. The movement of the diaphragm is a function of the frequency of sound being transmitted (how frequently the diaphragm changes directions of movement) and the electrical voltage applied to the coil. The range of movement of the diaphragm is a function of the axial movement of the voice coil. This axial movement is often called the excursion. 
         [0008]    For a loudspeaker to provide high output or deep bass, the loudspeaker may need a substantial excursion of the voice coil. In this context, an excursion is an axial movement of the voice coil from the position it assumes without electrical stimulus. Voice coils undergo excursions both towards and away from the diaphragm as the alternating electric current in the voice coil interacts with the magnetic field. 
         [0009]    Due to higher power handling, dual-coil/dual magnetic gap designs may result in greater motor excursion. While the use of dual-coil/dual magnetic gap designs is advantageous by providing increased power handling, the use of a dual-coil drive motor design with a pair of coil portions (upper and lower) operating in a pair of magnetic gaps can cause extreme distortion, if the coil excursion becomes too great. More specifically, if the coil excursion is too great the upper coil portion may actually travel down into the magnetic gap for the lower coil portion or the lower coil portion may actually travel up into the magnetic gap for the upper coil portion. Either situation leads to extreme distortion. 
         [0010]    The risks of such distortion are increased with the use of a thin magnet such as may be obtained with neodymium magnets as the use of a thin magnet reduces the distance between the upper and lower magnetic gaps and the corresponding spacing between the upper and lower coil portions. A need therefore exists for a dual-coil drive motor design (or other configurations using multiple coil portions) to allow such speakers to be used in applications calling for at least occasional performance of large excursions without extreme distortion. 
         [0011]    Further, due to high power dual-coil/dual magnetic gap designs, the dual-coil designs generate a large amount of resistive heat which can cause a loss of efficiency and may damage certain components in the loudspeaker. A need further exists for a dual-coil motor design that not only allows for large excursions without extreme distortion, but that also reduces some of the common problems that occur with the generation of resistive heat within a loudspeaker. 
       SUMMARY 
       [0012]    A dual-coil or multi-coil driver is provided that includes a magnet assembly having a spacer member to effectively axially elongate the permanent magnet. In one example of one implementation, the invention may be implemented as an electromagnetic transducer such as may be employed as electro-acoustical drivers for loudspeakers. The electromechanical driver may be configured to use a combination of a first permanent magnet and a second permanent magnet separated by at least one spacer member containing ferromagnetic material. This stack of two permanent magnets and a spacer may be longer in the axis of oscillation for the electro-acoustical driver than a single permanent magnet of equivalent strength. The elongation from adding the spacer may increase the separation between the first pole piece and the second pole piece and thus increases the distances between the magnetic gaps used to drive the voice coils. Increasing the distances between the magnetic gaps may increase the amount of excursion that a voice coil may undergo without entering into close proximity with a magnetic gap that is intended for use with a different voice coil. Further, by including a spacer between the magnets, the heat generated in the magnet structure is less concentrated, thereby minimizing problems associated with heat generated by the driver and allowing for the generate heat to be more easily dissipated. 
         [0013]    Other devices, apparatus, systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0014]    The invention may be better understood by referring to the following figures. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. In the figures, like reference numerals designate corresponding parts throughout the different views. 
           [0015]      FIG. 1  is a top perspective view of an example of a loudspeaker of the invention. 
           [0016]      FIG. 2  is a bottom perspective view of the loudspeaker illustrated in  FIG. 1 . 
           [0017]      FIG. 3  is a cross-sectional view of the loudspeaker illustrated in  FIGS. 1 and 2 , taken along line A-A, according to one example of an implementation of the invention. 
           [0018]      FIG. 4  is a cut away view of a portion of the loudspeaker illustrated in  FIG. 3 . 
           [0019]      FIG. 5  is a top plan view of a stacked arrangement of components of a magnetic assembly according to an example of an implementation of the invention. 
           [0020]      FIG. 6  is a side cross-sectional view of the stacked arrangement illustrated in  FIG. 6  taken along line B-B. 
           [0021]      FIG. 7  is a perspective cross-sectional view of the stacked arrangement illustrated in  FIG. 5  taken alone line C-C. 
           [0022]      FIG. 8  is a perspective, exploded view illustrating various components of the loudspeaker illustrated in  FIG. 1 , according to the example of the implementation illustrated in  FIG. 3 . 
           [0023]      FIG. 9  is a cross-sectional view of an example of an electromagnetic driver or motor according to another implementation. 
           [0024]      FIG. 10  is a top plan view of a stacked arrangement of components of a magnetic assembly according to another example of an implementation of the invention. 
           [0025]      FIG. 11  is a side cross-sectional view of the stacked arrangement illustrated in  FIG. 10  taken along line D-D. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]      FIGS. 1-11  describe various implementations of the present subject matter. For purposes of this application, in general, the term “communicate” (for example, a first component “communicates with” or “is in communication with” a second component) is used in the present disclosure to indicate a structural, functional, mechanical, electrical, optical, magnetic, ionic or fluidic relationship between two or more components (or elements, features, or the like). As such, the fact that one component is said to communicate with a second component is not intended to exclude the possibility that additional components may be present between, and/or operatively associated or engaged with, the first and second components. 
         [0027]    Turning now to  FIG. 1 ,  FIG. 1  is a perspective view of an example of an electro-acoustical transducer in which one or more implementations of the invention may be provided. By way of example, the electro-acoustical transducer may be provided as a loudspeaker  100  or as part of the loudspeaker  100  although in other examples the electro-acoustical transducer is not limited to loudspeaker-type implementations. For purposes of description, the loudspeaker  100  may be considered as being generally arranged or disposed about a central, longitudinal axis  104 . It will be understood, however, that the loudspeaker  100  is not limited to being completely symmetrical relative to such central axis  104 . Also for purposes of description, the loudspeaker  100  and its components and features generally have a front or upper side  108  and a rear or lower side  112 . It will be understood, however, that the use in this disclosure of terms such as “front,” “upper,” “rear” and “lower” is not intended to limit the loudspeaker  100  or any of its components and features to any particular orientation in space. 
         [0028]    The loudspeaker  100  may include a housing  116 . The housing  116  may be composed of any suitably stiff, anti-vibrational material such as, for example, a metal (e.g., aluminum, etc.). The utilization of aluminum or other thermally conductive material also enables the housing  116  to serve as a heat sink for the internal heat-generating components of the loudspeaker  100 . The outer periphery of the housing  116  is generally swept about the central axis  104 , such that the housing  116  may be considered as circumscribing or surrounding an interior space in which various components of the loudspeaker  100  are disposed. A housing  116  of this type may be referred to as a basket. Insofar as the housing  116  may constitute a combination of structural members and openings between structural members, the housing  116  may be considered as at least partially enclosing this interior space. The space external to the housing  116 , and more generally external to the loudspeaker  100 , will be referred to as the ambient environment. In other implementations, the housing  116  may be continuous so as to completely enclose the interior space in which the components of the loudspeaker  100  are disposed, but openings are considered useful for allowing air to flow to and from the confines of the housing  116  and thus for assisting in cooling the loudspeaker  100 . 
         [0029]    The loudspeaker  100  may also include a diaphragm  120  that spans the open front end of the housing  116 . The diaphragm  120  may be any device that may be attached to or suspended by the housing  116  or other portion of the loudspeaker  100  in a manner that secures the diaphragm  120  while permitting at least a portion of the diaphragm  120  to move axially—i.e., along the direction of the central axis  104 —in a reciprocating or oscillating manner. In the present example, the diaphragm  120  includes a generally cone-shaped member  124  (cone) that serves as an axially movable member, and a generally dome-shaped member  128  (dome) that may serve as a dust cover as well as an axially movable member. In other implementations, the movable portion of the diaphragm  120  may have a configuration other than conical, such as a dome or an annular ring. The cone  124  and dome  128  may be constructed from any suitably stiff, well-damped material such as paper. The cone  124  and dome  128  may be provided as a unitary or single-piece construction, or may be attached, connected, or adhered to each other by any suitable means. The cone  124  is attached to the housing  116  through one or more suspension members such as a surround  132  and a spider  136 , either or both of which may be annular. The surround  132  and spider  136  may be affixed to the housing  116  by any suitable means. The surround  132  and spider  136  may be any devices that provide a mechanical interconnection between the diaphragm  120  and the housing  116 , and allow the diaphragm  120  to move axially relative to the housing  116  while supporting the position of the diaphragm  120  radially relative to the housing  116 . For this purpose, the surround  132  and spider  136  may be constructed from flexible, fatigue-resistant materials such as, for example, urethane foam, butyl rubber, phenolic-impregnated cloth, etc. In the illustrated example, the surround  132  and spider  136  have corrugated or “half-roll” profiles to enhance their flexibility and compliance. The surround  132  and spider  136  may be considered with the cone  124  and dome  128  as being parts of the assembly of the diaphragm  120 , or may be considered as being components distinct from the diaphragm  120 . 
         [0030]    In the example illustrated in  FIG. 1 , the housing  116  generally includes an upper frame portion  140  and a lower frame portion  144 . The upper frame portion  140  surrounds the diaphragm  120 . The lower frame portion  144  surrounds several internal components of the loudspeaker  100 , including an electromagnetic transducer or driver described in detail below. 
         [0031]      FIG. 2  is another perspective view of the loudspeaker  100  illustrated in  FIG. 1 . From this perspective, it may be seen that the lower frame portion  144  is bent or folded inwardly at a rear-most end  202  of the housing  116 , and transitions to an inverted cup-shaped end frame portion or pedestal  206 . The end frame portion  206  is described further below. 
         [0032]    As a general matter, the loudspeaker  100  may be operated in any suitable listening environment such as, for example, the room of a home, a theater, or a large indoor or outdoor arena. Moreover, the loudspeaker  100  may be sized to process any desired range of the audio frequency band, such as the high-frequency range (generally 2 kHz-20 kHz) typically produced by tweeters, the midrange (generally 200 Hz-5 kHz) typically produced by midrange drivers, and the low-frequency range (generally 20 Hz-200 Hz) typically produced by woofers. In the examples provided in this description, the loudspeaker  100  may be considered as being of the direct-radiating type. However, in other alternative examples, the loudspeaker  100  may be considered as being of the compression driver type, the configuration of which is readily appreciated by persons skilled in the art. 
         [0033]      FIG. 3  is a cross-sectional view of a loudspeaker  100  that may have an external configuration as illustrated in  FIGS. 1 and 2 . In  FIG. 3 , the loudspeaker  100  may be considered as having a “dual-coil drive” or “dual-coil motor” configuration or, more generally, a multiple-coil configuration. As illustrated in  FIG. 3 , an electromagnetic driver or motor  302  (“driver”) is generally disposed in the lower frame portion  144  of the housing  116 . The driver  302  includes a magnetic assembly  304  and electrically conductive coils  306  (e.g., voice coils). As best seen in  FIG. 4  below, the coil  306  in this dual-coil drive has a pair of coil portions  348  and  350 . The magnetic assembly  304  may be any device suitable for providing a permanent magnetic field with which the coil  306  may be electro-dynamically coupled. 
         [0034]    In the illustrated example, the magnetic assembly  304  includes an inner magnetic portion  308  and an outer magnetic portion  310  (or gap sleeve). Generally, the terms “inner” and “outer” in this context refer to the radial positions of the two magnetic portions  308  and  310  relative to the central axis  104  and to each other. The outer magnetic portion  310  is generally coaxially disposed about the central axis  104  and may be in the form of a ring or annulus. The outer magnetic portion  310  may be referred to as, or considered as including, a gap sleeve or outer ring. The outer magnetic portion  310  is radially spaced from the inner magnetic portion  308  such that the inner magnetic portion  308  and outer magnetic portion  310  cooperatively define an annular air gap  312  (or magnetic gap) between these two components. In operation, the gap  312  is immersed in the permanent magnetic field established by the magnetic assembly  304 . 
         [0035]    The inner magnetic portion  308  includes a stacked arrangement of ferromagnetic components that may have any suitable configuration such as plates, disks, or the like. The prior art inner magnet portion consisted of a magnetic element (magnet) interposed between a first (upper or front) pole piece and a second (lower or rear) pole piece. As described in more detail, below, the implementation shown in  FIG. 3  utilizes a spacer and a pair of magnets in order to increase the axial dimension of the inner magnet portion. 
         [0036]    The magnetic assembly  304  may be secured within the housing  116  by any suitable means. In the example illustrated in  FIG. 3 , the outer magnetic portion  310  abuts an inside surface of the lower frame portion  144 . The lower or rear side of the inner magnetic portion  308  abuts another inside surface of the lower frame portion  144  and the upper or front side of the inner magnetic portion  308  abuts a centrally located support member  326 . More specifically in this example, the end frame portion or pedestal  206  of the housing  116  includes a base section  328  and a sidewall section  330  interconnecting the base section  328  at the rear-most end  202 . The base section  328  provides the inside surface to which the inner magnetic portion  308  abuts. This configuration provides large areas of surface contact between the outer magnetic portion  310  and the housing  116 , and between the inner magnetic portion  308  and the housing  116 , thus providing enhanced heat transfer from the magnetic assembly  304  to the housing  116 . Moreover, the dimensions of the lower frame portion  144  and end frame portion  206  relative to the coil  306  and magnetic assembly  304 , and the contiguous relation between the lower frame portion  144  and the pedestal  206 , result in a large, continuous solid mass that may serve well as a heat sink yet is relatively compact in design. 
         [0037]    As also illustrated in the example of  FIG. 3 , the base section  328  has a central bore  332  that is aligned with respective central bores of the components of inner magnetic portion  308 . By this configuration, the position of the inner magnetic portion  308  may be fixed by inserting the centrally located support member  326  through the respective central bores of the inner magnetic portion  308  and into the central bore  332  of the base section  328 . The centrally located support member  326  may include threads that mate with threads within the central bore  332  of the base section  328 , or the centrally located support member  326  may be coupled or attached to the base section  328  by any other suitable means. 
         [0038]    While use of a centrally positioned bore is one viable implementation, those of skill in the art will recognize that the inner magnetic portion  308  is fixed in place during use and does not need to rotate around the central bore. Thus, the bore does not need to be centrally located nor does there need to be just one bore. The bore does need to pass through the various components in the inner magnetic portion  308  and may allow for the inner magnetic portion to be fastened to the base section  328 . Thus, the component that passes through a bore may be thought of as an alignment rod or as a fastener depending on the implementation and the context. In this context, a rod may have a cross section that is not a circle but may be square or any other shape as long as the shape adequately corresponds to the relevant bore of the inner magnet portion  308 . 
         [0039]    The coil  306 , which may be referred to as a voice coil, may generally be any component that oscillates in response to electrical current while being subjected to the magnetic field established by the magnetic assembly  304 . In the illustrated example, the coil  306  is constructed from an elongated conductive element such as a wire that is wound about the central axis  104  in a generally cylindrical or helical manner. The coil  306  is mechanically referenced to, or communicates with, the diaphragm  120  by any suitable means that enables the oscillating coil  306  to consequently actuate or drive the diaphragm  120  in an oscillating manner, thus producing mechanical sound energy correlating to the electrical signals transmitted through the coil  306 . In the illustrated example, the coil  306  mechanically communicates with the diaphragm  120  through a coil support structure or member such as a coil former  344 . The coil former  344  may be cylindrical as illustrated by example in  FIG. 3 , and may be composed of a stiff, thermally resistant material such as, for example, a suitable plastic (e.g., polyamide, etc.). The coil former  344  also functions to support the coil  306 . The diameter of the coil former  344  is greater than the outside diameter of the inner magnetic portion  308  and less than the inside diameter of the outer magnetic portion  310 , enabling the coil former  344  in practice to extend into, and be free to move axially through, the gap  312  between the inner magnetic portion  308  and outer magnetic portion  310 . 
         [0040]    At least a portion of the coil  306  is wound or wrapped on the outer surface of the coil former  344  and may be securely attached to the coil former  344  such as by an adhesive. The coil  306  may be positioned on the coil former  344  such that at any given time during operation of the loudspeaker  100 , at least a portion of the coil  306  is disposed in the gap  312 . With this configuration, in operation the coil former  344  oscillates with the coil  306  and the oscillations are translated to the diaphragm  120 . 
         [0041]    The magnetic assembly  304  is axially spaced from the diaphragm  120 . The portion of the interior space of the loudspeaker  100  that generally separates the magnetic assembly  304  from the diaphragm  120  along the axial direction will be referred to as a medial interior region  346 . In the present example in which the coil former  344  is connected to the diaphragm  120  in the manner illustrated in  FIG. 3 , the coil  306  is likewise separated from the diaphragm  120  by the medial interior region  346 . 
         [0042]    As previously noted, the loudspeaker  100  may be considered as having “dual-coil drive” or “dual-coil motor” configuration. This configuration may be realized in the implementation illustrated in  FIG. 3  by forming the coil  306  so as to include a plurality of distinct coil portions ( 348  and  350  as shown in  FIG. 4 ), such that the coil  306  in effect constitutes a plurality of individual coils. In the implementation illustrated in  FIG. 3 , the wire of the coil  306  is wound around the coil former  344  for a desired number of turns to form a first (upper or front) coil portion  348 , then runs down the side of the coil former  344  for an axial distance, and then is wound around the coil former  344  for a desired number of turns to form a second (lower or rear) coil portion  350  that is axially spaced from the first coil portion  348 . The portion of the wire extending between the first coil portion  348  and the second coil portion  350  may be insulated to electrically isolate this portion of the wire from the two coil portions  348  and  350 . The two ends of the wire may be connected to any suitable circuitry (including, for example, an amplifier) for driving the loudspeaker  100 . The first coil portion  348  and the second coil portion  350  may be positioned on the coil former  344  such that at any given time during operation of the loudspeaker  100 , at least a portion of the first coil portion  348  and at least a portion of the second coil portion  350  are disposed in the gap  312 . Moreover, the first coil portion  348  may be positioned such that it is generally aligned with (i.e., adjacent to) the first pole piece  316 , and the second coil portion  350  may be positioned such that it is generally aligned with (i.e., adjacent to) the second pole piece  318 . By this configuration, the gap  312  may be considered as including a first (upper or front) gap  352  (or gap section) in which the first coil portion  348  extends between the first pole piece  316  and the outer magnetic portion  310 , and a second (lower or rear) gap  354  (or gap section) in which the second coil portion  350  extends between the second pole piece  318  and the outer magnetic portion  310 . 
         [0043]    In a case where the first coil portion  348  has the same number of turns (windings) as the second coil portion  350 , the number of turns for this dual-coil driver is doubled in comparison to a single-coil configuration having the same number of turns of either individual coil portion  348  or  350 . In addition, the surface area covered by the coil  306  having two coil portions  348  and  350  is also doubled. The wire forming the coil  306  may be run in a clockwise direction in one of the coil portions  348  or  350  and in a counterclockwise direction in the other coil portion  350  or  348 . By this configuration, the electrical current runs through one of the coil portions  348  or  350  in a direction opposite to the electrical current running through the other coil portion  350  or  348 . Because the magnetic flux lines established by the magnetic assembly  304  run in opposite directions in each of the first gap  352  and second gap  354  and the current in each coil portion  348  and  350  runs in opposite directions, Lorenz law holds that the force created by the current in each coil portion  348  and  350  runs in the same direction, thus doubling the force imparted to the coil former  344  and enabling the loudspeaker  100  to generate more power in comparison to a single-coil loudspeaker. 
         [0044]    Generally, in operation the loudspeaker  100  receives an input of electrical signals at an appropriate connection to the coil  306 , and converts the electrical signals into acoustic signals according to mechanisms briefly summarized above in this disclosure and readily appreciated by persons skilled in the art. The acoustic signals propagate or radiate from the vibrating diaphragm  120  to the ambient environment. 
         [0045]    While the specific example illustrated in  FIG. 3  provides two coil portions  348  and  350  and two corresponding gaps  352  and  354 , it will be understood that other implementations may provide more than two coil portions  348  and  350  and gaps  352  and  354 . 
         [0046]    Further, the magnets  372  and  374  may be composed of any permanent magnetic material such as, for example, a ceramic, alnico, or a magnetic rare earth metal, particularly neodymium (Nd) or a composition including neodymium such as a composition including neodymium, iron, and boron. In this context, a permanent magnet is a magnet that retains its magnetism after being removed from a magnetic field. 
         [0047]    The pole pieces  316  and  318  may be composed of any material capable of carrying magnetic flux such as, for example, steel or cast iron. In some implementations, one or more outer surface sections of the inner magnetic portion  308 , such as the outer surfaces of the pole pieces  316  and  318  and the inner surface of the outer magnetic portion  310 , may be covered with a sheathing, coating, or plating (not shown) composed of an electrically conductive material such as, for example, copper (Cu), aluminum (Al), or the like. Such sheathing may be employed to reduce distortion and inductance in the loudspeaker  100 . In one example, the sheathing has a thickness ranging from about 0.015 to 0.150 inch. 
         [0048]    One of ordinary skill in the art will recognize that a spacer member  376  may be constructed from a set of two or more spacers to achieve a desired amount of axial spacing. Allowing a spacer to be made of two or more components may allow the creation of a wide variety of spaces with different axial thicknesses. 
         [0049]    As shown in the implementation illustrated  FIG. 3 , the axial dimension of the spacer member  376  may be greater than that of the individual pole pieces  316  and  318  or that of the individual magnets  372  and  374 . The aggregate axial dimension of the spacer member  376  and the two magnets  372  and  374  may be significantly larger than the axial dimension of the equivalent single magnet replaced by the pair of magnets. The aggregate axial dimension may be in the range of double or triple or more of the axial dimension of the equivalent single magnet. In this context an equivalent single magnet is a magnet having a set of properties equivalent to a combination of the two magnets  372  and  374  including being composed of the same material and having the same magnetic characteristics. In summary, those skilled in the art may vary the size and relative size of the magnets  372  and  374  and spacer member  376  depending upon the desired application and may construct the magnets  372  and  374  and spacer member  376  from one or more components. 
         [0050]    Further, those skilled in the art will recognize that the spacer member  376  may be designed with different physical properties than those illustrated in the examples presented in this application. For example, the spacer member  376  may be designed with different physical dimensions, may have different magnetic properties and different thermal properties, whether uniform or non-uniform. 
         [0051]    The spacer member  376  may be composed of any suitable ferromagnetic material such as, for example, steel, cast iron, sintered ferromagnetic materials or a combination of other materials with steel, cast iron, or sintered ferromagnetic materials. The spacer member  376  may be composed of the same material as the pole pieces  316  and  318 , but is not required. The first magnet  372  is interposed between the first pole piece  316  and the spacer member  376 , and the second magnet  374  is interposed between the second pole piece  318  and the spacer member  376 . As a result, the overall axial dimension of the inner magnetic portion  308  may be greater than dual-coil drivers that lack the spacer member  376  and axially split magnets  372  and  374 . Accordingly, the axial dimension of the outer magnetic portion  310  may be increased as needed to accommodate the axially longer drivers  302 . One of ordinary skill in the art will recognize that the use of the term split magnets indicates the use of two or more magnets rather than a single permanent magnet. In context, the term split does not require that the split magnets come from the physical act of splitting a magnet into pieces. 
         [0052]    Although  FIG. 4  illustrates the inner magnetic portion  308  of the magnetic assembly  304  having the spacer member  376  positioned between two magnets  372  and  374 , alternately, and as illustrated in  FIGS. 10 &amp; 11  below, the outer magnetic portion  310  (or gap sleeve) may be designed to include two magnets and a spacer member  376  to elongate the gap sleeve  310 . Having magnets and a spacer member  376  positioned in the gap sleeve  310  may be in addition to, or opposed to, the spacer member  376  in the inner magnetic portion  308  of the magnetic assembly  304 . 
         [0053]      FIG. 4  is a cut-away view of a portion of the loudspeaker  100  illustrated in  FIG. 3 . In particular,  FIG. 4  illustrates the features of the driver  302  in more detail. It may again be seen that the axial separation of the first and second pole pieces  316  and  318 , first and second gaps  352  and  354 , and first and second coil portions  348  and  350  increases the axial excursion capability of the driver  302 . In the operation of the loudspeaker  100 , the increased excursion capability may result in the capacity for increased output, increased bass response, decreased distortion, and a lessening of the risk for damage due to excessive excursion. 
         [0054]    The spacer member  376  illustrated by way of example in  FIGS. 4 and 5  has a cylindrical geometry. In other implementations, the spacer member  376  may have other suitable geometries such as, for example, rings such as toroids, necked-down cylinders, and the like. Moreover, the spacer member  376  may have a hollow or solid cross-section. Additionally, the spacer member  376  may include features such as, for example, recesses, cavities, bores, grooves, channels, and the like. Furthermore, the spacer member  376  may provide for the provision of shorting rings and/or flux modulation rings for controlling flux modulation and distortion, as well as phase-change heat absorbers. 
         [0055]    While the specific example illustrated in  FIGS. 3 and 4  provides two coil portions  348  and  350  and two corresponding gaps  352  and  354 , it will be understood that other implementations may provide more than two coil portions  348  and  350  and gaps  352  and  354 . It will be also be understood that other implementations may provide more than one spacer member  376  or with more than two magnets  372  and  374 . 
         [0056]      FIG. 5  is a top plan view of an example of the inner magnetic portion  408  of the implementation illustrated in  FIGS. 3 and 4 . From this view, the first pole piece  316  is visible along with the central bore  332  that is aligned with respective central bores of the components of inner magnetic portion  308 . 
         [0057]      FIG. 6  is a cross section view of the inner magnetic portion  308  illustrated in  FIG. 5  taken along line A-A. First pole piece  316  visible in  FIG. 5  is adjacent to first magnet  372  that is adjacent to spacer member  376 . Spacer member  376  is adjacent to second magnet  374  that is adjacent to second pole piece  318 . A central bore portion  332  extends through the middle portion of the inner magnetic portion  308 . 
         [0058]      FIG. 7  is a perspective cross section view of the inner magnetic portion  308  illustrated in  FIG. 5  taken along line B-B. Like  FIG. 5 , the first pole piece  316  is adjacent to first magnet  372  that is adjacent to spacer member  376 . Spacer member  376  is adjacent to second magnet  374  that is adjacent to second pole piece  318 . The central bore portion  332  is also visible in  FIG. 7   
         [0059]      FIG. 8  is a perspective, exploded view illustrating various components of the loudspeaker  100  illustrated in  FIGS. 3-7  prior to assembly. Moving from the top of the drawing downward, the following components are identified: dome  128 , surround  132 , cone  124 , spider  136 , coil former  344 , outer magnetic portion  310 , first pole piece  316 , first magnet  372 , spacer member  376 , second magnet  374 , second pole piece  318 , and housing  116 . To avoid undue clutter, the centrally located support member  326  is not shown in this view. 
         [0060]    Although not shown, the loudspeaker  100  may be mounted by any suitable means to a structure such as a baffle plate, cabinet, wall, or the like. The structure may have an opening sized to receive the loudspeaker  100 . As appreciated by persons skilled in the art, the structure may include additional openings for mounting other loudspeakers, for outputting acoustic waves of the same frequency range as or different frequency ranges from the loudspeaker  100  described above. 
         [0061]    It may thus be seen that implementations provided in this disclosure may be useful in increasing the axial spacing of the magnetic gaps so that there is a greater tolerance of the dual-coil motor to excursions without the risk of distortion arising from a coil being driven out of its magnetic gap and into the magnetic gap of another coil (sometimes called a coil portion). 
         [0062]    The increased spacing of the coils may assist in the cooling of the two or more coils, magnets, and associated structures of an electromagnetic transducer such as the type utilized in or constituting a loudspeaker or other type of electro-acoustical transducer. 
         [0063]      FIG. 9  is a cross-sectional view of an example of an inner magnetic portion  908  of an electromagnetic driver or motor according to another implementation. In the example illustrated in  FIG. 9 , electrically conductive shorting rings  980  may be employed instead of sheathing to reduce distortion and inductance. The shorting rings may be composed of any suitable material (e.g., copper, aluminum, or the like), and may have thicknesses ranging from about 0.05 to 1 inch and axial lengths as much as the full length of the spacer. 
         [0064]    As shown in  FIG. 9 , in this alternative embodiment, the inner magnetic portion  908  of an electromagnetic driver or motor includes a first upper pole piece  916 , a second lower pole piece  918 , first upper magnet  972 , second lower magnet  974  and a spacer member  976 . Shorting rings  980  are disposed about the exterior of the spacer member  976 . 
         [0065]      FIG. 10  is a top plan view of a stacked arrangement of components of a magnetic assembly  1000  according to another example of an implementation of the invention. From this view, the magnetic assembly  1000 , including an inner magnetic portion  1002  and outer magnetic portion  1004  (or gap sleeve), may be seen. The first pole piece  1012  along with the central bore of inner magnetic portion  1002  and the first pole piece  1022  of the outer magnetic portion  1004  are illustrated. The coil  1018  is positioned in the gap between the inner magnetic portion  1002  and the outer magnetic portion  1004 . 
         [0066]      FIG. 11  is a side cross-sectional view of the stacked arrangement illustrated in  FIG. 10  taken along line D-D. In this example, the magnetic assembly  1000  includes an inner magnetic portion  1002  and an outer magnetic portion  1004  (or gap sleeve). The magnetic assembly in  FIGS. 10 &amp; 11  is similar to the magnetic assemblies illustrated in  FIGS. 1-9  except that the outer magnetic portion  1004 , rather than the inner magnetic portion  1002 , utilizes a pair magnets  1006  separated by a spacer  1008 . The inner magnetic portion  1002  consists of a magnetic element (dual pole piece)  1010  which forms both a first (upper or front) pole  1012  and a second (lower or rear) pole  1014 . The inner magnetic portion  1002  may abut a centrally located support member  1016 . The inner magnetic portion  1002  is shown as a single piece in this embodiment, but it may consist of a multiple of parts to provide distortion reduction, motor cooling, ease of assembly, or other functions as in the previous embodiments. 
         [0067]    An electrically conductive coil  1018  is positioned within the air gap between the inner magnetic portion  1002  and the outer magnetic portion  1004 . As previously described, the coil  1018 , which may be referred to as a voice coil, may generally be any component that oscillates in response to electrical current while being subjected to the magnetic field established by the magnetic assembly  1000 . In the illustrated example, the coil  1018  is constructed from an elongated conductive element such as a wire that is wound about the central axis in a generally cylindrical or helical manner. The coil  1018  is mechanically referenced to, or communicates with, the diaphragm  120  ( FIG. 1 ) by any suitable means that enables the oscillating coil  1018  to consequently actuate or drive the diaphragm  120  ( FIG. 1 ) in an oscillating manner, thus producing mechanical sound energy correlating to the electrical signals transmitted through the coil  1018 . In the illustrated example, the coil  1018  mechanically communicates with the diaphragm  120  (See  FIG. 1 ) through a coil support structure or member such as a coil former  1020 . As with the previously described embodiments, the coil  1018  may include a plurality of distinct coil portions, such that the coil  1018  constitutes a plurality of individual coils. 
         [0068]    As illustrated in  FIG. 11 , the outer magnetic portion  1004  of the magnetic assembly  1000  has a spacer member  1008  positioned between two magnets  1006 . Having magnets  1006  and a spacer member  1008  positioned in the outer magnetic portion or gap sleeve  1004  may be in addition to, or opposed to, the spacer member  376  ( FIG. 4 ) in the inner magnetic portion  308  ( FIG. 4 ) of the magnetic assembly  304  ( FIG. 4 ). The two magnets  1006  and spacer member  1008  are interposed between a first (upper or front) pole piece  1022  and a second (lower or rear) pole piece  1024 . The several pieces constituting the outer magnetic portion  1004  may be held together by a variety of means including but not limited to: adhesive between the various parts, adhesive between the various parts and the supporting speaker frame, press fit of the parts into the supporting speaker frame, mechanical fasteners positioned axially through the stack, mechanical fasteners between the parts and the speaker frame, retaining rings, clamps, or other suitable methods providing secure location of the parts in the overall assembly. 
         [0069]    As previously discussed, the magnets  1006  may be composed of any permanent magnetic material such as, for example, a ceramic, alnico, or a magnetic rare earth metal, particularly neodymium (Nd) or a composition including neodymium such as a composition including neodymium, iron, and boron. The spacer member  1008  may be composed of any suitable ferromagnetic material such as, for example, steel, cast iron, sintered ferromagnetic materials or a combination of other materials with steel, cast iron, or sintered ferromagnetic materials. Further, the pole pieces  1022  and  1024  may be composed of any material capable of carrying magnetic flux such as, for example, steel or cast iron. 
         [0070]    The spacer member  1008  may be constructed from a single spacer or a set of two or more spacers  1008  to achieve a desired amount of axial spacing. Allowing a spacer to be made of two or more components may allow the creation of a wide variety of spaces with different axial thicknesses. 
         [0071]    Further, the axial dimension of the spacer member  1008  may be greater than that of the individual pole pieces  1022  and  1024  or that of the individual magnets  1006 . The aggregate axial dimension of the spacer member  1008  and the two magnets  1006  may be significantly larger than the axial dimension of the equivalent single magnet replaced by the pair of magnets  1006 . In summary, those skilled in the art may vary the size and relative size of the magnets  1006  and the spacer member  1008  depending upon the desired application and may construct the magnets  1006  and spacer member  1008  from one or more components, and may vary the physical dimensions, magnetic properties and thermal properties, whether uniform or non-uniform, of the spacer member  1008 . 
         [0072]    The foregoing description of implementations has been presented for purposes of illustration and description. It is not exhaustive and does not limit the claimed inventions to the precise form disclosed. Modifications and variations are possible in light of the above description or may be acquired from practicing the invention. The claims and their equivalents define the scope of the invention.