Abstract:
An electromagnetic transducer such as an audio speaker, having three or more voice coils disposed in one or more magnetic air gaps. The voice coils can be combined in any permutation of series/parallel connections, to select any one of a predetermined set of impedance values for the transducer. Optionally, the transducer is equipped with a set of different terminal connector plugs, each of which automatically performs the series/parallel connections for its designated impedance value. All voice coils may be selected as active, allowing for a constant maximum efficiency, regardless of which impedance option is selected.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Technical Field of the Invention  
         [0002]     This invention relates generally to electromagnetic transducers, and more specifically to voice coil configurations for them.  
         [0003]     2. Background Art  
         [0004]      FIG. 1  illustrates a conventional electromagnetic transducer  10  including a motor structure  12  and a diaphragm assembly  14  coupled to a frame or basket  16 . The motor structure may have an external magnet geometry, as shown, or it may have an internal magnet geometry. In the external magnet geometry, the motor structure includes a magnetically permeable (e.g. steel) pole plate  18  which includes a pole piece  20  and a back plate  21 . One or more axially polarized, annular, permanent magnets  22  are magnetically coupled to the back plate, and a magnetically permeable top plate  24  is magnetically coupled to the topmost magnet. The top plate defines a magnetic air gap  26  with the pole piece.  
         [0005]     The diaphragm assembly includes a cone or diaphragm  28  which is coupled at its outer perimeter to the frame by a flexible suspension component referred to as a surround  30 . A voice coil former or bobbin  32  is coupled to the diaphragm. A flexible suspension component referred to as a spider  34  couples the bobbin (or diaphragm) to the frame. An electrically conductive voice coil  36  is wound around the bobbin and is disposed within the magnetic air gap of the motor structure. Some transducers include only a single voice coil. Some, such as that shown, include a single layer of windings. The ends of the voice coil are connected (by wires or leads not shown) to a + terminal and a − terminal, respectively. The terminals may conveniently be located at a terminal block  38 .  
         [0006]     For ease of illustration, the particular routing of the wiring from the voice coil to the terminals has been omitted from the drawings, as it is well known in the art. In various transducers, the wires are routed up the outside of the bobbin, or the inside of the bobbin, along or in the spider from the bobbin to the terminal block, or hanging in the air between the bobbin and the terminal block, or along the diaphragm, and so forth.  
         [0007]      FIG. 2  illustrates a transducer  40  having two voice coils  42 ,  44 , one wound on top of the other. The four ends of the two voice coils are connected to a first + terminal, a first − terminal, a second + terminal, and a second − terminal, respectively, at a terminal block  46 . (Alternatively,  FIG. 2  may be interpreted as illustrating a dual-layer single voice coil. By winding the voice coil downward, then upward over the downward windings, both ends of the wire exit the voice coil at its upper end. This offers some advantages in manufacturing, and in routing the wires to the terminal block.) Many dual voice coil loudspeakers have been available, such as the Orion H2 subwoofer, which is available with dual 4Ω voice coils (or, optionally, dual 2Ω voice coils). It is known, depending upon the needs of the application at hand, such as the characteristics of the amplifier, that the dual voice coils can be wired either in series or in parallel. For example, two 4Ω voice coils can be wired in series, to present the amplifier with, in effect, a single 8Ω load, or in parallel, to present the amplifier with, in effect, a single 2Ω load. The series configuration has 4× the impedance of the parallel configuration; in other words, the series configuration has 300% more impedance than the parallel configuration.  
         [0008]     The system builder could, of course, wire only one of the voice coils, presenting a 4Ω load, which is only 2× the parallel configuration and ½× the series configuration. Unfortunately, with the other voice coil inactive, the efficiency of the transducer is remarkably reduced. Additionally, the inactive voice coil unnecessarily increases the moving mass, reducing the efficiency and limiting the high frequency range of the transducer as compared to a similar transducer which does not have the dead-weight coil.  
         [0009]     Unfortunately, each dual voice coil transducer offers only two selectable full efficiency loads which are at best 4× different if the coils are the same impedance, and one or two reduced efficiency loads. What is needed, then, is an improved electromagnetic transducer which gives the system builder a significantly greater number of selectable impedances in increments significantly tighter than 4×, allowing the system builder to more closely match a desired impedance.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  shows an electromagnetic transducer having a single voice coil, according to the prior art.  
         [0011]      FIG. 2  shows an electromagnetic transducer having dual voice coils (or, alternatively, a two-layer single voice coil), according to the prior art.  
         [0012]      FIG. 3  shows a three voice coil transducer according to one embodiment of this invention.  
         [0013]      FIGS. 4 and 5  are charts showing exemplary impedance sets of the  FIG. 3  transducer, according to the voice coil impedances shown in Table 1, on a linear scale and a logarithmic scale, respectively.  
         [0014]      FIG. 6  shows a four voice coil transducer according to another embodiment of this invention.  
         [0015]      FIGS. 7 and 8  are charts showing exemplary impedance sets of the  FIG. 6  transducer, according to the voice coil impedances shown in Table 2, on a linear scale and a logarithmic scale, respectively.  
         [0016]      FIG. 9  shows a dual gap transducer according to yet another embodiment of this invention, where the voice coils are not all disposed in a same, single magnetic air gap.  
         [0017]      FIGS. 10-15  show a terminal block and a ready-made set of five terminal block interface units which can be plugged into the terminal block, each selecting a predetermined one of the five voice coil series/parallel permutations taught in Table 1.  
         [0018]      FIG. 16  shows a dual magnetic air gap embodiment of this invention, using the voice coil “hand-off” technique to extend the Xmax of the motor. 
     
    
     DETAILED DESCRIPTION  
       [0019]     The invention will be understood more fully from the detailed description given below and from the accompanying drawings of embodiments of the invention which, however, should not be taken to limit the invention to the specific embodiments described, but are for explanation and understanding only.  
         [0020]      FIG. 3  illustrates an electromagnetic transducer  50  according to one embodiment of this invention. The diaphragm assembly includes three voice coils  52 ,  54 ,  56 . The wires extending from the two ends of the first voice coil are connected to a first + terminal and a first − terminal, respectively; the wires extending from the two ends of the second voice coil are connected to a second + terminal and a second − terminal, respectively; and the wires extending from the two ends of the third voice coil are connected to a third + terminal and a third − terminal, respectively. The terminals may advantageously be coupled to a terminal block  58 .  
         [0021]     Having three voice coils gives an increased number of options for the system builder, and the greater the number of different impedances there are among the three coils, the more options the system builder will have. Table 1 demonstrates the impedance choices available from each of six different sets of impedances for the three voice coils.  
                                                                           TABLE 1                           STEP Technologies Multi Voice Coil Impedance Sets - 3 Coils                Config 1   Config 2   Config 3   Config 4   Config 5   Config 6                        VC1   1.00Ω   1.00Ω   2.00Ω   0.80Ω   0.30Ω   0.20Ω       VC2   2.00Ω   1.30Ω   2.50Ω   1.90Ω   1.13Ω   1.30Ω       VC3   3.00Ω   1.70Ω   3.50Ω   3.40Ω   1.75Ω   2.00Ω       VC1||VC2||VC3   0.55Ω   0.42Ω   0.84Ω   0.48Ω   0.21Ω   0.16Ω       (VC1||VC2)--VC3   3.67Ω   2.27Ω   4.61Ω   3.96Ω   1.99Ω   2.17Ω       (VC1||VC3)--VC2   2.75Ω   1.93Ω   3.77Ω   2.55Ω   1.38Ω   1.48Ω       (VC2||VC3)--VC1   2.20Ω   1.74Ω   3.46Ω   2.02Ω   0.98Ω   0.99Ω       VC1--VC2--VC3   6.00Ω   4.00Ω   8.00Ω   6.10Ω   3.18Ω   3.50Ω       Rank       1   0.55Ω   0.42Ω   0.84Ω   0.48Ω   0.21Ω   0.16Ω       2   2.20Ω   1.74Ω   3.46Ω   2.02Ω   0.98Ω   0.99Ω       3   2.75Ω   1.93Ω   3.77Ω   2.55Ω   1.38Ω   1.48Ω       4   3.67Ω   2.27Ω   4.61Ω   3.96Ω   1.99Ω   2.17Ω       5   6.00Ω   4.00Ω   8.00Ω   6.10Ω   3.18Ω   3.50Ω       % more than next lower   303.33%   309.41%   310.06%   317.99%   372.07%   519.32%           25.00%   11.11%   9.09%   26.19%   40.24%   50.00%           33.33%   17.39%   22.22%   55.56%   43.86%   46.67%           63.64%   76.58%   73.49%   53.93%   59.80%   61.04%       Stdev of last 3   0.203   0.361   0.340   0.165   0.104   0.075                  
 
         [0022]     The impedances of the three voice coils can be selected in any combination suitable to achieve the transducer designer&#39;s goals. Six exemplary combinations are presented here, as “Config 1” through “Config6”. The three voice coils are identified as VC1, VC2, and VC3. There are five possible ways to wire three voice coils, using all three voice coils to maintain constant, maximum efficiency:  
         [0023]     1) all three in parallel, shown as VC1∥VC2∥VC3  
         [0024]     2) VC1 and VC2 in parallel, and VC3 in series with them, shown as (VC1∥VC2)—VC3  
         [0025]     3) VC1 and VC3 in parallel, and VC2 in series with them, shown as (VC1∥VC3)—VC2  
         [0026]     4) VC2 and VC3 in parallel, and VC1 in series with them, shown as (VC2∥VC3)—VC1  
         [0027]     5) all three in series, shown as VC1—VC2—VC3  
         [0028]     The “Rank” subtable sorts the five net impedances of each configuration into ascending order. The “X more than next lower” subtable indicates the increase of each of the last four impedances over the next lower one (no entry is present for the lowest impedance). The “Stdev of last 3” entry indicates the standard deviation of the last three entries in the “X more” subtable, and is a measure of the log scale linearity of the four highest impedance values for that voice coil configuration.  
         [0029]     The all-in-parallel combination will, by the mathematical relationship of parallel impedances, in many instances, be somewhat less usef uil to the system builder than the other four, and in some cases may be ignored or considered an “outlier”. For example, consider Config 6. Its four largest impedance possibilities present the system builder a log scale nearly linear progression of choices from 0.99Ω to 3.5Ω, which are very useable in e.g. car audio applications, in which amplifiers are commonly available in skus which are stable from 1Ω to 4Ω. The all-in-parallel impedance of 0.16Ω is significantly outside this range, and is not especially useable in car audio, home audio, etc. applications.  
         [0030]     It should be noted, however, that the system builder may wish to couple two or more transducers in series, in which case the all-in-parallel outlier may, in fact, be useful. In fact, it may be the case, depending upon the voice coil impedance values chosen, and the needs of the application at hand, that any of the combinations might be undesirable or substantially redundant.  
         [0031]      FIGS. 4 and 5  chart the “Rank” subtable on a linear scale and a logarithmic scale, respectively. The horizontal (X) axis denotes the five possible impedance values of a voice coil set. The charted lines all extend up and to the right, because the values have been sorted prior to charting; depending on the particular three voice coil impedances chosen, this may or may not correspond to the order in which the permutations were explained, four paragraphs above.  
         [0032]      FIG. 6  illustrates an electromagnetic transducer  60  which has four voice coils  62 ,  64 ,  66 ,  68 . The wires extending from the two ends of the first voice coil are connected to a first + terminal and a first − terminal, respectively; the wires extending from the two ends of the second voice coil are connected to a second + terminal and a second − terminal, respectively; the wires extending from the two ends of the third voice coil are connected to a third + terminal and a third − terminal, respectively; and the wires extending from the two ends of the fourth voice coil are connected to a fourth + terminal and a fourth − terminal, respectively. The terminals may advantageously be coupled to a terminal block  70 .  
         [0033]     It should be noted that, although  FIGS. 3 and 6 , for convenience and clarity, illustrate each voice coil as being a single layer, each voice coil may, of course, be constructed as a dual layer of windings or bifilar, trifilar, or quadfilar, etc.  
         [0034]     Having four voice coils gives an even greater number of options for the system builder. Table 2 demonstrates the impedance choices available from each of six different sets of impedances for the four voice coils. These are, of course, only exemplary values, and the designer will select values according to the needs of the transducer&#39;s target application.  
                                                                           TABLE 2                           STEP Technologies Multi Voice Coil Impedance Sets - 4 Coils                Config 1   Config 2   Config 3   Config 4   Config 5   Config 6                        VC1   1.00Ω   0.25Ω   0.50Ω   0.25Ω   0.30Ω   0.20Ω       VC2   2.00Ω   0.50Ω   0.88Ω   1.00Ω   1.30Ω   1.35Ω       VC3   3.00Ω   1.25Ω   1.13Ω   1.25Ω   1.70Ω   2.25Ω       VC4   4.00Ω   1.75Ω   1.50Ω   2.00Ω   2.00Ω   3.00Ω       VC1||VC2||VC3||VC4   0.48Ω   0.14Ω   0.21Ω   0.16Ω   0.19Ω   0.15Ω       (VC1||VC2||VC3)--VC4   4.55Ω   1.90Ω   1.75Ω   2.17Ω   2.21Ω   3.16Ω       (VC1||VC2||VC4)--VC3   3.57Ω   1.40Ω   1.39Ω   1.43Ω   1.92Ω   2.41Ω       (VC1||VC3||VC4)--VC2   2.63Ω   0.69Ω   1.16Ω   1.19Ω   1.53Ω   1.52Ω       (VC2||VC3||VC4)--VC1   1.92Ω   0.55Ω   0.87Ω   0.68Ω   0.84Ω   0.86Ω       (VC1||VC2)--(VC3||VC4)   2.38Ω   0.90Ω   0.96Ω   0.97Ω   1.16Ω   1.46Ω       (VC1||VC3)--(VC2||VC4)   2.08Ω   0.60Ω   0.90Ω   0.88Ω   1.04Ω   1.11Ω       (VC1||VC4)--(VC2||VC3)   2.00Ω   0.58Ω   0.87Ω   0.78Ω   1.00Ω   1.03Ω       (VC1||VC2)--VC3--VC4   7.67Ω   3.17Ω   2.94Ω   3.45Ω   3.94Ω   5.42Ω       (VC1||VC3)--VC2--VC4   6.75Ω   2.46Ω   2.72Ω   3.21Ω   3.56Ω   4.53Ω       (VC1||VC4)--VC2--VC3   5.80Ω   1.97Ω   2.38Ω   2.47Ω   3.26Ω   3.79Ω       (VC2||VC3)--VC1--VC4   6.20Ω   2.36Ω   2.49Ω   2.81Ω   3.04Ω   4.04Ω       (VC2||VC4)--VC1--VC3   5.33Ω   1.89Ω   2.18Ω   2.17Ω   2.79Ω   3.38Ω       (VC3||VC4)--VC1--VC2   4.71Ω   1.48Ω   2.02Ω   2.02Ω   2.52Ω   2.84Ω       VC1--VC2--VC3--VC4   10.00Ω   3.75Ω   4.00Ω   4.50Ω   5.30Ω   6.80Ω       rank        1   0.48Ω   0.14Ω   0.21Ω   0.16Ω   0.19Ω   0.15Ω        2   1.92Ω   0.55Ω   0.87Ω   0.68Ω   0.84Ω   0.86Ω        3   2.00Ω   0.58Ω   0.87Ω   0.78Ω   1.00Ω   1.03Ω        4   2.08Ω   0.60Ω   0.90Ω   0.88Ω   1.04Ω   1.11Ω        5   2.38Ω   0.69Ω   0.96Ω   0.97Ω   1.16Ω   1.46Ω        6   2.63Ω   0.90Ω   1.16Ω   1.19Ω   1.53Ω   1.52Ω        7   3.57Ω   1.40Ω   1.39Ω   1.43Ω   1.92Ω   2.41Ω        8   4.55Ω   1.48Ω   1.75Ω   2.02Ω   2.21Ω   2.84Ω        9   4.71Ω   1.89Ω   2.02Ω   2.17Ω   2.52Ω   3.16Ω       10   5.33Ω   1.90Ω   2.18Ω   2.17Ω   2.79Ω   3.38Ω       11   5.80Ω   1.97Ω   2.38Ω   2.47Ω   3.04Ω   3.79Ω       12   6.20Ω   2.36Ω   2.49Ω   2.81Ω   3.26Ω   4.04Ω       13   6.75Ω   2.46Ω   2.72Ω   3.21Ω   3.56Ω   4.53Ω       14   7.67Ω   3.17Ω   2.94Ω   3.45Ω   3.94Ω   5.42Ω       15   10.00Ω   3.75Ω   4.00Ω   4.50Ω   5.30Ω   6.80Ω       % more than next lower   300.64%   302.93%   307.44%   331.41%   335.18%   459.64%           4.00%   5.36%   0.39%   13.58%   18.99%   20.12%           4.17%   3.70%   3.24%   12.50%   4.55%   8.09%           14.29%   14.89%   6.93%   10.77%   11.49%   30.97%           10.53%   30.56%   20.31%   22.64%   31.26%   4.33%           35.71%   56.52%   20.00%   20.45%   25.63%   58.54%           27.27%   5.49%   25.98%   41.03%   15.43%   17.44%           3.71%   27.70%   15.44%   7.30%   13.81%   11.49%           13.13%   0.43%   7.92%   0.27%   10.68%   6.94%           8.75%   3.78%   9.06%   13.80%   8.92%   12.02%           6.90%   19.73%   4.93%   13.48%   7.38%   6.77%           8.87%   4.29%   9.19%   14.36%   9.02%   12.12%           13.58%   28.81%   8.16%   7.53%   10.94%   19.64%           30.43%   18.42%   35.91%   30.43%   34.39%   25.36%       Stdev of last 14   0.106   0.160   0.102   0.106   0.094   0.145                  
 
         [0035]     Similar notation is used in Table 2, as was explained above regarding Table 1. Six exemplary combinations are presented here, as “Config 1” through “Config 6”. The four voice coils are identified as VC1, VC2, VC3, and VC4. There are fifteen possible ways to wire four voice coils, using all four voice coils to maintain constant, maximum efficiency:  
         [0036]     1) VC1∥VC2∥VC3∥VC4  
         [0037]     2) (VC1∥VC2∥VC3)—VC4  
         [0038]     3) (VC1∥VC2∥VC4)—VC3  
         [0039]     4) (VC1∥VC3∥VC4)—VC2  
         [0040]     5) (VC2∥VC3∥VC4)—VC1  
         [0041]     6) (VC1∥VC2)—(VC3∥VC4)  
         [0042]     7) (VC1∥VC3)—(VC2∥VC4)  
         [0043]     8) (VC1∥VC4)—(VC2∥VC3)  
         [0044]     9) (VC1∥VC2)—VC3—VC4  
         [0045]     10) (VC1∥VC3)—VC2—VC4  
         [0046]     11) (VC1∥VC4)—VC2—VC3  
         [0047]     12) (VC2∥VC3)—VC1—VC4  
         [0048]     13) (VC2∥VC4)—VC1—VC3  
         [0049]     14) (VC3∥VC4)—VC1—VC2  
         [0050]     15) VC1—VC2—VC3—VC4  
         [0051]      FIGS. 7 and 8  chart the “Rank” subtable on a linear scale and a logarithmic scale, respectively. The horizontal (X) axis denotes the fifteen possible impedance values of a voice coil set, sorted.  
         [0052]      FIG. 9  illustrates an electromagnetic transducer  80  according to another embodiment of this invention. The transducer includes a motor structure  82  and a diaphragm assembly  84  coupled to a frame  86 . The motor structure (which could have an internal magnet geometry but is illustrated as having an external magnet geometry) includes a poleplate  88  with a pole piece  90 , as well as a lower top plate  92  and an upper top plate  94 , which define a lower magnetic air gap and an upper magnetic air gap, respectively.  
         [0053]     In one such embodiment, the motor structure uses a “push-push” dual gap geometry such as taught in U.S. Pat. No. 6,917,690, in which the magnetic flux flows in a same direction (e.g. radially inward) over both magnetic air gaps. In this case, the motor structure includes one or more permanent magnets  100  disposed between the back plate and the lower top plate. The motor structure includes a magnetically permeable member  102  disposed between the lower top plate and the upper top plate. The member  102  may be a permanent magnet, polarized in the same direction as the magnets  100 , as taught in the &#39;690 patent, or it may be a steel spacer, as taught in co-pending application Ser. No. 10/289,109, commonly assigned with the &#39;690 patent and the present application.  
         [0054]     In another such embodiment, the motor structure uses a “push-pull” geometry in which the magnetic flux flows in opposite directions over the two magnetic air gaps, that is, radially inward over one and radially outward over the other. In this case, elements  100  may be interpreted as aluminum spacers, and element  102  should be interpreted as a permanent magnet, with opposite polarity if elements  100  are permanent magnets rather than non-magnetic spacers.  
         [0055]     In either the push-push or the push-pull configuration, the bobbin  104  has wound about it at least three voice coils of more than one impedance, and the magnetic air gaps each has disposed within it some subset of the voice coils.  FIG. 9  illustrates four layers of voice coil windings  106 ,  108 ,  110 ,  112  disposed in the lower magnetic air gap, and four layers of voice coil windings  114 ,  116 ,  118 ,  120  disposed in the upper magnetic air gap. One magnetic air gap contains at least one voice coil, and the other magnetic air gap contains at least two voice coils. There are myriad possible ways in which the eight layers shown may be interpreted. Only a few will be explained, and the skilled reader will then be able to fully understand the possibilities taught by this disclosure.  
         [0056]     The lower magnetic air gap may contain a first two-layer voice coil  106 ,  108  and a second two-layer voice coil  110 ,  112 . The upper magnetic air gap may contain a third two-layer voice coil  114 ,  116  and a fourth two-layer voice coil  118 ,  120 . The eight wires extending from the eight ends of these four voice coils are coupled to respective + and − terminals at a terminal block  122 , and can then be connected in any of the fifteen combinations identified above in Table 2.  
         [0057]     Alternatively, the lower magnetic air gap may contain a four layer voice coil  106 ,  108 ,  110 ,  112 , and the upper magnetic air gap may contain both a three layer voice coil  114 ,  116 ,  118  and a single layer voice coil  120 . The six wires extending from the six ends of these three voice coils are coupled to respective + and − terminals at the terminal block, and the system builder can select any of five configurations identified above in Table 1.  
         [0058]     Alternatively, each magnetic air gap may contain four single layer voice coils, for a total of eight voice coils which can then be coupled in a very large number of series/parallel permutations.  
         [0059]     Alternatively, one or both of the magnetic air gaps may contain less than four layers of voice coil windings. The reduced number of layers can be created using wire of larger diameter, resulting in the same voice coil outer diameter.  
         [0060]     Alternatively, one or more voice coils (as determined by the wires available at the terminal block) may be present in both magnetic air gaps. For example, the layer  106  and the layer  114  may together comprise a single voice coil.  
         [0061]     Additionally, more than two magnetic air gaps may be present in the motor structure, as taught in the &#39;690 patent, and the three or more voice coils of the present invention may be distributed in them in any manner deemed appropriate by the transducer designer.  
         [0062]      FIG. 10  illustrates a terminal block (such as  58 ,  70 , or  122 ) coupled to three voice coils (such as  52 ,  54 ,  56 , or such as  62 / 64 ,  66 ,  68 , or such as  106 / 108 / 110 / 112 ,  114 / 116 ,  118 / 120 ). The terminal block may advantageously be coupled to the electromagnetic transducer (not shown), preferably but not necessarily to the frame, at an outward-facing position ideally near the outer perimeter, where it is easily reached by an system builder. The terminal block includes a first terminal connector VC1+ which is wired to the + end of the first voice coil, a second terminal connector VC 1− which is wired to the − end of the first voice coil, a third terminal connector VC2+ which is wired to the + end of the second voice coil, a fourth terminal connector VC2− which is wired to the − end of the second voice coil, a fifth terminal connector VC3+ which is wired to the + end of the third voice coil, and a sixth terminal connector VC3− which is wired to the − end of the third voice coil (It may optionally include additional terminal connectors, not shown, for wiring to the + and − ends of fourth etc. voice coils.) The terminal block has a predetermined form factor, such that the terminal connectors are in predetermined positions.  
         [0063]      FIGS. 11-15  illustrate the five different maximum efficiency terminal block interface units, or “plugs”, each of which has a form factor for mating with the terminal block, and each providing a unique one of the series/parallel combinations listed in Table 1. (They are maximum efficiency, in that they use all of the coils. There could also be additional, reduced efficiency plugs which use less than all of the coils.)  FIG. 11  illustrates Plug  1 , which includes a first input terminal T+ and a second input terminal T−, at which the + and − signals, respectively, are applied from the amplifier (not shown). Plug  1  includes six terminal connectors A+, A−, B+, B−, C+, and C−, each positioned to mate with a corresponding one of the terminal connectors VC1+, VC1−, VC2+, VC2−, VC3+, and VC3−, respectively, of the terminal block of  FIG. 10 . In one embodiment, the terminal connectors of the terminal block are male, and the terminal connectors of the plug are female. In another embodiment, the terminal connectors of the terminal block are female, and the terminal connectors of the plug are male. In another embodiment, there is a mixture of genders in the terminal block, with some being female and others being male, and the terminal connectors of the plug are appropriately opposite. In another embodiment, each is simply a contact, with one contact of each pair being optionally spring-loaded to maintain good connection.  
         [0064]     Any of a variety of fastening means can be used to secure the plug to the terminal block. For example, screws, bolts, elastic, hook-and-loop fasteners, clamps, clips, and so forth. Ideally, the plug is releasably secured to the terminal block, to enable the subsequent substitution of a different plug.  
         [0065]     Plug 1 achieves the all-in-parallel configuration via wires or traces (hereinafter simply “wires”, for convenience) connecting the T+ terminal to each of the A+, B+, and C+ terminal connectors, and wires connecting the T− terminal to each of the A−, B−, and C− terminal connectors. The + input signal is thus applied via the A+ etc. terminal connectors to the VC1+ etc. terminal connectors and thence to the + end of each of the voice coils, and thence via the VC 1− etc. terminal connectors to the A− etc. terminal connectors, and thence to the T− terminal and back to the amplifier to complete the circuit.  
         [0066]      FIG. 12  illustrates Plug  2 , which provides the (VC1∥ VC2)—VC3 configuration. Plug  2  has the same form factor and includes the same terminals and terminal connectors as Plug  1 . The difference between Plug  1  and Plug  2  (and the other plugs) lies in the wiring. Plug  2  includes wires connecting the T+ terminal to the A+ and B+ terminal connectors, wires connecting both the A− and B− terminal connectors to the C+ terminal connector, and the C− terminal connector to the T− terminal. With Plug  2  installed on the terminal block, the + amplifier signal is provided in parallel to both VC1 and VC2, and then in series to VC3, and then back to the amplifier.  
         [0067]      FIG. 13  illustrates Plug  3 , which is wired to provide the (VC1∥VC3)—VC2 configuration, as shown.  
         [0068]      FIG. 14  illustrates Plug  4 , which is wired to provide the (VC2∥VC3)—VC1 configuration, as shown.  
         [0069]      FIG. 15  illustrates Plug  5 , which is wired to provide the all-in-series configuration. It includes a wire from the T+ terminal to the A+ terminal connector, a wire from the A− terminal connector to the B+ terminal connector, a wire from the B− terminal connector to the C+ terminal connector, and a wire from the C− terminal connector to the T− terminal.  
         [0070]      FIG. 16  illustrates a dual magnetic air gap transducer  130  according to yet another embodiment of this invention. As taught in the &#39;690 patent, the transducer&#39;s motor structure includes a lower top plate  132  and an upper top plate  134 . One or more primary magnets  136  are magnetically coupled between the lower top plate and the back plate, and a balancing magnet  138  is magnetically coupled between the lower and upper top plates. The magnets are all polarized in the same axial direction, and the magnetic flux flows in the same radial direction over both magnetic air gaps.  
         [0071]     Three or more voice coils are disposed in the magnetic air gaps such that each coil extends substantially from the center of one gap to the center of the other gap, as shown. Minor variance may need to be made in the winding, with each next outermost layer of windings terminating slightly short of the layer over which it is wound. The layers of windings are shown as being of identical axial length, for convenience.  
         [0072]     In one embodiment, the transducer includes a first voice coil  142 , a second voice coil  144 , and a third voice coil  146 . In another embodiment, it further includes a fourth voice coil  148 . In some embodiments, each voice coil occupies its own, dedicated layer in the voice coil assembly.  
         [0073]     In other embodiments, a layer may include wire windings from two or more voice coils. For example, the first and second layers  142  and  144  may include alternating first and second voice coil wires such as a bifilar configuration simultaneously wound down the bobbin (layer  142 ) then back up the bobbin (layer  144 ) over the first layer; and the third and fourth layers  146  and  148  may include alternating third and fourth voice coil wires simultaneously wound down the bobbin (layer  146 ) and then back up the bobbin (layer  148 ).  
         [0074]     The transducer includes a terminal block  140  to which the ends of each voice coil are connected. By plugging in the right terminal block interface unit (not shown), the system builder can configure the transducer to have the desired impedance.  
       CONCLUSION  
       [0075]     When one component is said to be “adjacent” another component, it should not be interpreted to mean that there is absolutely nothing between the two components, only that they are in the order indicated.  
         [0076]     In its most simplistic configuration, a terminal connector may simply be the end of the voice coil wire, and the series/parallel connection is accomplished by joining wire ends together, e.g. by soldering them, or by fastening them with wire nuts, or the like.  
         [0077]     The term “series/parallel” is meant to include any of the possible permutations of voice coil connections, and should not be construed as requiring both a series connection and a parallel connection.  
         [0078]     In some embodiments, it may be acceptable to have e.g. four voice coils, with the connector plugs selecting series/parallel combinations of three or four of the voice coils. If some small number of the voice coils is unused in a configuration, the efficiency of the transducer is reduced. On the other hand, it will produce a different set of Thiele-Small small parameters (Qts will change) allowing for an alternate box tuning or different frequency response in a given box.  
         [0079]     One significant aspect of some embodiments of this invention is that, once the appropriate terminal block interface plug has been connected, the electromagnetic transducer has only a single pair of terminals which the installer needs to connect.  
         [0080]     The various features illustrated in the figures may be combined in many ways, and should not be interpreted as though limited to the specific embodiments in which they were explained and shown.  
         [0081]     Those skilled in the art, having the benefit of this disclosure, will appreciate that many other variations from the foregoing description and drawings may be made within the scope of the present invention. Indeed, the invention is not limited to the details described above. Rather, it is the following claims including any amendments thereto that define the scope of the invention.