Abstract:
A first electro-acoustic transducer and a second electro-acoustic transducer are supported by a housing attached to a baffle to form an asymmetric acoustical system. An equalizer receives an input signal and generates an equalized signal which is transmitted to the second electro-acoustic transducer. The equalizer is configured to generate the equalized signal such that a net mechanical force acting on the baffle, generated by the first electro-acoustic transducer in response to the input signal and by the second electro-acoustic transducer in response to the equalized signal, is less than the net mechanical force that would be generated if the equalized signal were unchanged in magnitude and equal or opposite in phase to the input signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application is a continuation of and claims priority to U.S. patent application Ser. No. 12/423,572, filed Apr. 14, 2009, now U.S. Pat. No. 7,983,436, which is a continuation of U.S. patent application Ser. No. 10/999,419, filed Nov. 30, 2004, now U.S. Pat. No. 7,551,749, which is a continuation-in-part (CIP) of U.S. patent application Ser. No. 10/226,507, filed on Aug. 23, 2002, now U.S. Pat. No. 6,985,593, entitled Baffle Vibration Reducing, the entire disclosures of which are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    When an electroacoustic transducer, such as a loudspeaker driver, is mounted to a structure, such as a package shelf, the door of a vehicle, the wall of an enclosure, other wall or other baffle, where the attachment is usually on the periphery of the transducer frame, an energized transducer motor develops forces in response to an energizing electrical signal. The forces generated by the motor cause the diaphragm of the transducer to move relative to the transducer frame. These forces will also be transmitted through the frame to the structure through the attachment points of the frame. Package shelves and door panels of vehicles are often fabricated from thin material, such as thin sheet metal. Such structures typically have insufficient stiffness to resist vibration and are typically lightly damped. As a result, forces applied to the structure around modal resonance frequencies of the structure may result in excessive vibration of the structure, which can be acoustically perceived as unwanted buzzes and rattles, or degraded frequency response of the radiated sound. 
       SUMMARY 
       [0003]    According to one aspect of the invention, a first electroacoustical transducer incorporating a movable diaphragm is seated in and structurally coupled to a panel. The transducer is mechanically connected to a device containing a compensating moveable mass driven mechanically out of phase with the movement of the diaphragm of the first electroacoustical transducer, to significantly reduce the resultant force applied to the panel. Typically the device with compensating mass is a second electroacoustical transducer identical to the first transducer. According to another aspect of the invention, the acoustic output from the first side of the first transducer is acoustically coupled to a listening environment, such as a vehicle passenger compartment or living room. The acoustic output from the side of the second transducer facing away from the first transducer is also coupled to the listening environment through an acoustical element or elements such as compliant volume and/or port so that the acoustical output into the listening environment from the facing away side of the second transducer is effectively in phase with the output into the listening environment from the first side of the first transducer, over a desired frequency range. The acoustic elements are arranged such that the output from the away facing side of the second transducer is not acoustically coupled to the output from the second side of the first transducer or the output from the first side of the second transducer. Thus, the invention achieves both significant reduction in unwanted mechanical vibration of the supporting structure with enhanced acoustic output from the second transducer. 
         [0004]    In one aspect, the invention is embodied in an apparatus for reducing baffle vibration in a vehicle. The apparatus includes a baffle that is subject to vibration. A first transducer is seated in the baffle. The first transducer includes a first diaphragm having a first surface that is acoustically coupled to a listening area in the vehicle and a second surface that is acoustically coupled to a cavity in the vehicle. A first input signal is applied to the first transducer which causes the first diaphragm to move in a first direction, thereby generating an acoustic output. A second transducer is mechanically coupled to the first transducer. The second transducer includes a second diaphragm having a first surface that is acoustically coupled to one of the first and the second surfaces of the first diaphragm. A second surface is acoustically coupled to one of the listening area and the cavity in the vehicle. A second input signal is applied to the second transducer and causes the second diaphragm to move in a second direction that is substantially opposite to the first direction to reduce a vibration imparted to the baffle from the movement of the first diaphragm while substantially maintaining the acoustic output. The reduction in the vibration imparted to the baffle is generally observed over a large frequency range. However, there can be various frequencies where the reduction in the vibration imparted to the baffle is less pronounced. 
         [0005]    The first input signal and the second input signal can have opposite relative polarity. In another example, the first input signal and the second input signal are identical. 
         [0006]    The first surface of the first diaphragm can be a front surface of the first diaphragm and a second surface of the first diaphragm can be a rear surface of the first diaphragm. Alternatively, the first surface of the first diaphragm can be a rear surface of the first diaphragm and a second surface of the first diaphragm can be a front surface of the first diaphragm. 
         [0007]    The first surface of the second diaphragm can be a front surface of the second diaphragm and a second surface of the second diaphragm can be a rear surface of the second diaphragm. Alternatively, the first surface of the second diaphragm can be a rear surface of the second diaphragm and a second surface of the second diaphragm can be a front surface of the second diaphragm. 
         [0008]    In one embodiment, the second transducer is inverted relative to the first transducer. In one embodiment, at least one of the first and the second diaphragms is elliptically-shaped. The motor structure of the first transducer can be mechanically coupled to a motor structure of the second transducer. In one example, a frame of the first transducer is mechanically coupled to a frame of the second transducer. 
         [0009]    The second surface of the first diaphragm can be acoustically coupled to a passive radiator. The second surface of the second diaphragm can be acoustically coupled to the listening area in the vehicle through an acoustic conduit. The acoustic conduit can have a smoothly varying cross-sectional area. In one example, the second surface of the second diaphragm is acoustically coupled to the listening area in the vehicle through an acoustic port. 
         [0010]    The acoustic output from the second surface of the second diaphragm can be coupled to the listening area in the vehicle substantially in-phase with the acoustic output from the first surface of the first diaphragm. The cavity can be a trunk of the vehicle. 
         [0011]    In one embodiment, at least one of the first and the second transducers includes an inverted motor structure. A low pass filter can be coupled to at least one of the first and the second transducers. The low pass filter restricts spectral components of at least one of the first and the second input signals above a predetermined cutoff frequency. The low pass filter can be an electrical low pass filter or an acoustical low pass filter. 
         [0012]    The apparatus can also include a third transducer and a forth transducer that are mechanically coupled to the first and the second transducers. The first, the second, the third, and the fourth transducers can be aligned substantially in a column. 
         [0013]    In another aspect, the invention is embodied in an electro-acoustic transducer that includes a magnet assembly having a first magnetic flux gap and a second magnetic flux gap. A first voice coil is positioned in the first magnetic flux gap. A first diaphragm is mechanically coupled to the first voice coil and to a frame. The first voice coil moves the first diaphragm in a first direction in response to receiving a first input signal. A second voice coil is positioned in the second magnetic flux gap. A second diaphragm is mechanically coupled to the second voice coil and to the frame. The second voice coil moves the second diaphragm in a second direction that is substantially opposite to the first direction in response to receiving a second input signal. The movement of the second diaphragm reduces a vibration imparted to the frame by the movement of the first diaphragm. 
         [0014]    The first input signal and the second input signal can have opposite relative polarity. The first input signal and the second input signal can be identical. 
         [0015]    In one example, the second voice coil substantially surrounds the first voice coil. In one example, the first voice coil has substantially the same diameter as the second voice coil. The first magnetic flux gap can be substantially symmetrical to the second magnetic flux gap. The second magnetic flux gap can be concentrically positioned relative to the first magnetic flux gap. The magnet assembly can include a ring magnet, a donut magnet, or a slug magnet. 
         [0016]    The magnet assembly can also include a copper shorting ring that is positioned proximate to at least one of the first and the second magnetic flux gap. The magnet assembly can include a ring magnet that provides a static magnetic field to the first and the second magnetic flux gaps. 
         [0017]    The first input signal that is applied to the first voice coil generates a first magnetic field and the second input signal applied to the second voice coil generates a second magnetic field. The second magnetic field can have opposite polarity to the first magnetic field to reduce a modulation of magnetic flux in at least one of the first and the second magnetic flux gaps. 
         [0018]    A low pass filter can be electrically coupled to at least one of the first and the second voice coils. The first diaphragm can be inverted with respect to the second diaphragm. The first and/or the second diaphragm can be elliptically-shaped. The magnet assembly can include a motor structure that is inverted with respect to at least one of the first and the second diaphragms. 
         [0019]    In one embodiment, the frame of the electro-acoustic transducer is mechanically coupled to an infinite baffle in a vehicle. The frame of the electro-acoustic transducer can also be mechanically coupled to a wall. 
         [0020]    In another aspect, the invention is embodied in a loudspeaker system for a vehicle. The loudspeaker system includes an infinite baffle that is integrated with the vehicle. A first surface of the infinite baffle is coupled to a listening area in the vehicle and a second surface of the infinite baffle is coupled to a cavity. A first baffle is mechanically coupled to the infinite baffle and supports a first transducer that includes a first diaphragm. The first diaphragm has a first surface that is acoustically coupled to the listening area in the vehicle and a second surface that is acoustically coupled to the cavity. A second baffle is mechanically coupled the infinite baffle and supports a second transducer that includes a second diaphragm. The second diaphragm includes a first surface that is acoustically coupled to the listening area in the vehicle and a second surface that is acoustically coupled to the cavity. A rigid member mechanically couples the first baffle to the second baffle. The first and the second transducer are driven by a first and a second input signal, respectively, such that an acoustic output from the first surfaces of the first and the second diaphragms couples to the listening area substantially in phase and a vibration imparted to the infinite baffle from a movement of the first diaphragm is reduced by a movement of the second diaphragm. 
         [0021]    The first and the second signals can be identical. The first and the second baffles can be substantially perpendicular to the infinite baffle. The second baffle can be positioned substantially parallel to the first baffle. The first surface of the first diaphragm can be a front surface of the first diaphragm and a second surface of the first diaphragm can be a rear surface of the first diaphragm. Alternatively, the first surface of the first diaphragm can be a rear surface of the first diaphragm and a second surface of the first diaphragm can be a front surface of the first diaphragm. 
         [0022]    The first surface of the second diaphragm can be a front surface of the second diaphragm and a second surface of the second diaphragm can be a rear surface of the second diaphragm. Alternatively, the first surface of the second diaphragm can be a rear surface of the second diaphragm and a second surface of the second diaphragm can be a front surface of the second diaphragm. 
         [0023]    The second transducer can be inverted relative to the first transducer. The first and/or the second diaphragm can be elliptically-shaped. A motor structure of the first transducer can be mechanically coupled to a motor structure of the second transducer through the rigid member. A frame of the first transducer can be mechanically coupled to a frame of the second transducer. 
         [0024]    The loudspeaker system can include a passive radiator. A first surface of the passive radiator is acoustically coupled to the listening area and a second surface of the passive radiator is acoustically coupled to the cavity. An acoustic element can be used to couple acoustic energy from the cavity to the listening area. The acoustic element can be an acoustic port. The cavity can include a trunk of the vehicle. One or both of the first and the second transducer can include an inverted motor structure. 
         [0025]    A low pass filter can be coupled to at least one of the first and the second transducers. The low pass filter restricts spectral components of at least one of the first and the second input signals above a predetermined cutoff frequency. 
         [0026]    In one aspect, the invention is embodied in a method for reducing mechanical forces in a loudspeaker system in a vehicle. The method includes mounting a first baffle and a second baffle to an infinite baffle that is integrated with the vehicle. The method also includes mounting a first transducer having a first diaphragm to the first baffle such that a first surface of the first diaphragm is acoustically coupled to a listening area in the vehicle and a second surface of the first diaphragm is acoustically coupled to a cavity. The method further includes mounting a second transducer having a second diaphragm to the second baffle such that a first surface of the second diaphragm is acoustically coupled to the listening area in the vehicle and a second surface of the second diaphragm is acoustically coupled to the cavity. The method also includes mechanically coupling the first baffle to the second baffle with a rigid member. The method also includes driving the first and the second transducers with a first and a second input signal, respectively, such that an acoustic output from the first surfaces of the first and the second diaphragms couples to the listening area substantially in phase and a vibration imparted to the infinite baffle from a movement of the first diaphragm is reduced by a movement of the second diaphragm. 
         [0027]    The method can also include positioning the first baffle substantially parallel to the second baffle. The method can also include forming at least one of an acoustic port, an acoustic waveguide, an acoustic dampener, and a passive radiator in the cavity to couple acoustic energy from the cavity to the listening area. The cavity can include a trunk of the vehicle. 
         [0028]    In one example, driving the first and the second transducers with the first and the second input signals includes applying the first input signal to a first voice coil that is mechanically coupled to the first diaphragm and applying the second input signal to a second voice coil that is mechanically coupled to the second diaphragm. In one example, the movement of the first diaphragm is substantially opposite to the movement of the second diaphragm. In one example, the first diaphragm is inverted with respect to the second diaphragm. 
         [0029]    In another aspect, the invention is embodied in a loudspeaker system that includes an enclosure having a first acoustic volume and a first acoustic element. A combination of the first acoustic volume and the first acoustic element has a first resonant frequency. The first acoustic element couples acoustic energy from the first acoustic volume to outside the enclosure. A sub-enclosure is positioned at least partially inside the enclosure. The sub-enclosure includes a second acoustic volume and a second acoustic element. A combination of the second acoustic volume and the second acoustic element has a second resonant frequency. The second acoustic element couples acoustic energy from the second acoustic volume to outside the sub-enclosure. A first transducer is mounted to the sub-enclosure. The first transducer includes a first diaphragm having a first surface that is acoustically coupled to the first acoustic volume and a second surface that is acoustically coupled to the second acoustic volume. A first input signal is applied to the first transducer which causes the first diaphragm to move in a first direction. A second transducer is mounted to the sub-enclosure. The second transducer includes a second diaphragm having a first surface that is acoustically coupled to the first acoustic volume and a second surface that is acoustically coupled to the second acoustic volume. A second input signal is applied to the second transducer which causes the second diaphragm to move in a second direction that is substantially opposite to the first direction to reduce a vibration imparted to the sub-enclosure from the movement of the first diaphragm. 
         [0030]    The first resonant frequency and the second resonant frequency can be the same. Alternatively, the first resonant frequency can be lower than the second resonant frequency. The second acoustic element can couple acoustic energy from the second acoustic volume to outside the enclosure. For example, the second acoustic element can couple acoustic energy from the second acoustic volume to the first acoustic volume. The first and the second acoustic elements can include an acoustic port or a passive radiator. The sub-enclosure can be rigidly coupled to the enclosure. 
         [0031]    In one example, the movement of the second diaphragm reduces a vibration imparted to the enclosure from the movement of the first diaphragm. The first and/or the second transducer can include an inverted motor structure. The first transducer can be inverted relative to the second transducer. 
         [0032]    The loudspeaker system can also include an additional sub-enclosure that is positioned at least partially inside the enclosure. The loudspeaker system can also include an additional sub-enclosure that is positioned at least partially inside the sub-enclosure. 
         [0033]    In another aspect, the invention is embodied in a transducer assembly that includes a frame that is subject to vibration. A first pair of transducers are mechanically coupled to the frame. The first pair of transducers are oriented substantially in-line and include a first pair of diaphragms. Input signals applied to the first pair of transducers causing the first pair of diaphragms to move in substantially opposite directions relative to each other. A second pair of transducers are mechanically coupled to the frame. The second pair of transducers are oriented substantially in-line relative the first pair of transducers and include a second pair of diaphragms. Input signals applied to the second pair of transducers causing the second pair of diaphragms to move in substantially opposite directions relative to each other, thereby reducing a vibration in the frame. 
         [0034]    The movement of the second pair of diaphragms can be substantially in phase with the movement of the first pair of diaphragms. The transducer assembly can also include a baffle that acoustically separates a front surface of a transducer in the first pair of transducers from a back surface of the transducer. The transducer assembly can also include a baffle that acoustically separates a front surface of a transducer in the second pair of transducers from a back surface of the transducer. 
         [0035]    The transducer can further include an enclosure for housing at least a portion of the first and the second pairs of transducers. The diaphragms in the first pair of transducers can be inverted relative to the diaphragms in the second pair of transducers. One of the diaphragms can be inverted relative to the other diaphragm in at least one of the first and the second pairs of transducers. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0036]    The above and further advantages of this invention may be better understood by referring to the following description in conjunction with the accompanying drawings, in which like numerals indicate like structural elements and features in various figures. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
           [0037]      FIG. 1A-FIG .  1 C are diagrammatic representations of embodiments of the invention with the assemblies carried by an infinite baffle, such as a vehicle rear deck or door. 
           [0038]      FIG. 2  is a diagrammatic representation of an alternative embodiment of the invention with the assembly carried by an infinite baffle, such as a vehicle rear deck or door. 
           [0039]      FIG. 3  is a graphical representation showing the force exerted on the structure as a function of frequency for various systems. 
           [0040]      FIG. 4  is a diagrammatic representation of an alternative embodiment of the invention with the assembly carried by an infinite baffle, such as a vehicle rear deck or door, incorporating transducers with inverted motor structures. 
           [0041]      FIG. 5A-FIG .  5 E are diagrammatic representations of alternative embodiments of the invention with the assemblies mounted in enclosures. 
           [0042]      FIG. 5F  is a diagrammatic representation of an embodiment of the invention showing an unmounted assembly including four separate transducers. 
           [0043]      FIG. 6  is a perspective view of a typical loudspeaker assembly carried by an infinite baffle, such as a vehicle rear package shelf. 
           [0044]      FIG. 7  illustrates a perspective view of a loudspeaker system including a pair of electro-acoustic transducers according to one embodiment of the invention. 
           [0045]      FIG. 8  illustrates a perspective view of a loudspeaker system including a pair of electro-acoustic transducers and additional third and fourth electro-acoustic transducers according to one embodiment of the invention. 
           [0046]      FIG. 9A  illustrates a perspective view of a loudspeaker system including a first and a second electro-acoustic transducer that are mounted to an infinite baffle according to one embodiment of the invention. 
           [0047]      FIG. 9B  illustrates a side view of a loudspeaker system including a first and a second electro-acoustic transducer that are mounted to an infinite baffle according to another embodiment of the invention. 
           [0048]      FIG. 10  illustrates a perspective view of a loudspeaker system including a first pair and a second pair of electro-acoustic transducers that are mounted to an infinite baffle according to one embodiment of the invention. 
           [0049]      FIG. 11  illustrates a perspective view of a loudspeaker system including a pair of electro-acoustic transducers that are mounted to an infinite baffle which can be a seatback of a rear seat of a vehicle. 
           [0050]      FIG. 12  is a diagrammatic representation of an embodiment of the invention with a loudspeaker assembly mounted to a structural panel. 
           [0051]      FIG. 13  is a cross-sectional view of an electro-acoustic transducer according to one embodiment of the invention. 
           [0052]      FIG. 14  illustrates a cross-sectional view of a loudspeaker system including the electro-acoustic transducer of  FIG. 13  mounted to an infinite baffle according to one embodiment of the invention. 
           [0053]      FIG. 15  illustrates a cross-sectional view of a loudspeaker system including the electro-acoustic transducer of  FIG. 13  mounted to an enclosure according to one embodiment of the invention. 
           [0054]      FIG. 16  illustrates a cross-sectional view of a loudspeaker system including the electro-acoustic transducer of  FIG. 13  mounted to a structural panel according to one embodiment of the invention. 
           [0055]      FIG. 17A  is a cross-sectional view of an electro-acoustic transducer according to another embodiment of the invention. 
           [0056]      FIG. 17B  is a cross-sectional view of the motor structure in the electro-acoustic transducer of  FIG. 17A . 
           [0057]      FIG. 18  is a cross-sectional view of an electro-acoustic transducer according to another embodiment of the invention. 
           [0058]      FIG. 19  is a cross-sectional view of an electro-acoustic transducer according to another embodiment of the invention. 
           [0059]      FIG. 20  is a cross-sectional view of an electro-acoustic transducer according to another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0060]    With reference now to the drawings and more particularly  FIG. 1A  thereof, there is shown a diagrammatic representation of an embodiment of the invention with structure carried by infinite baffle  11 , typically a vehicle rear shelf or door panel carrying a first transducer  12 , such as a loudspeaker driver, mechanically connected to a second transducer  13 , such as a loudspeaker driver, preferably identical to the first transducer  12 , through a mechanical link  14 . The two transducers  12 ,  13  are ideally mounted in substantially parallel planes such that diaphragms  21 ,  22  of the two transducers  12 ,  13  move in the same axial direction. Although the infinite baffle  11  is described as a rear shelf or door panel in a vehicle, the infinite baffle  11  can be a structural panel, a wall, a ceiling, or a floor in a room, for example. 
         [0061]    A first surface  21   a  of the diaphragm  21  of the first transducer  12  is acoustically coupled to a listening area  18 . If the baffle  11  is the rear package shelf of a vehicle, the listening area  18  is the passenger compartment of the vehicle. A second surface  21   b  of the diaphragm  21  of the first transducer  12  is coupled to a volume or a cavity  30 , which would be the vehicle trunk if the baffle  11  is the rear package shelf. A second surface  22   b  of the diaphragm  22  of the second transducer  13  is coupled to the listening area  18  through an acoustic path consisting of one or more acoustic elements. Acoustic elements include lumped elements such as acoustic compliances (formed by cavities or volumes), acoustic resistances (elements that have losses proportional to acoustic volume velocity such as wire meshes, fiberglass, or other fibrous materials, foams, etc.), acoustic masses (formed by physical sections of tubes) or distributed elements such as waveguides or transmission lines, which can be described as conduits of constant or smoothly varying cross-section. In one embodiment, the second surface  22   b  of the diaphragm  22  of the second transducer  13  is coupled to the listening area  18  through a compliant volume  15  and a port tube  16 . A first surface  22   a  of the diaphragm  22  of the second transducer  13  is coupled to the cavity  30 . Instead of a rear package shelf of a vehicle, the baffle  11  can be a wall, a floor, or a ceiling of a room and the transducers  12 ,  13  can be positioned with the transducer  12  being located flush to the wall, floor or ceiling and the transducer  13  being located behind the wall, floor or ceiling. In another embodiment, the transducers  12 ,  13  can be recessed in the wall, floor or ceiling. Baffle  11  serves to separate acoustic output of first surface  21   a  and acoustic output from second surface  22   b , from acoustic output of second surface  21   b  and acoustic output of first surface  22   a.    
         [0062]    power amplifier  17  energizes the first transducer  12  and the second transducer  13  with the same signal but drives them in opposite polarity. The system is arranged such that when the diaphragm  21  of the first transducer  12  is moving in one direction, the diaphragm  22  of the second transducer  13  is moving in the substantially opposite direction, which significantly reduces a resultant force applied to the baffle  11 . This also reduces undesired resultant vibrations of the baffle  11 . By resultant force, we mean the vector sum of the applied forces (to a mechanical link or to a structure, such as the baffle, to which the transducer assembly is attached) applied by each transducer. For the case where equal and opposite forces are applied to each end of a mechanical link, the resultant force as defined is zero, even though the actual forces that place the mechanical link into tension or compression are in fact doubled. 
         [0063]    The reduction in the vibration imparted to the baffle  11  is generally observed over a large frequency range. However, there can be various frequencies where the reduction in the vibration imparted to the baffle  11  is less pronounced. 
         [0064]    The embodiments of the invention typically show a pair of diaphragms mounted in close proximity moving in mechanically opposite directions. We are assuming, for purposes of simplicity of describing the invention, that there is no mutual coupling present between the two diaphragms. In actual embodiments, while some amount of mutual coupling may be present, it will generally not be sufficient to substantially affect overall operation of the system. 
         [0065]    Additionally, throughout the following description, input signals are applied to the transducers in order to cause the movable elements in the transducers to move in desired directions. The desired direction of movement of the movable elements can be achieved through various methods. For example, the polarity of the input signal can be reversed prior to being applied to the transducer. This can be achieved by making a reverse connection at the terminals of the transducer. Alternatively, an inverting amplifier can operate on an input signal prior to it being applied to the transducer. In one embodiment, the geometry of one transducer is inverted with respect to the geometry of the other transducer. In one embodiment, the voice coil of each transducer is wound in opposite directions. In one embodiment, the permanent magnets in the motor structures of each transducer are magnetized in opposite directions. Any single technique or a combination of the above techniques can be used to control the direction of movement of the movable elements of each of the transducers. 
         [0066]    The movement of the second diaphragm  22  of the second transducer  13  generates acoustic output from the second surface  22   b  of the second transducer  13  that is coupled by the acoustic path to the listening area  18  for emission that is substantially in phase with the output from the first surface  21   a  of first transducer  12 . Thus, the input signal from the amplifier  17  that is applied to the second transducer  13  causes the second diaphragm  22  to move in an opposite direction from the direction of movement of the first diaphragm  21  to reduce a vibrating force imparted to the baffle  11  while maintaining the acoustic output. 
         [0067]    The output from the second surface  22   b  of the second driver  13  could also be coupled through a conduit of substantially constant or smoothly varying cross section to the listening area  18  without loss of generality. 
         [0068]    The second transducer  13  need not be identical to the first transducer  12 . All that is required for significant reduction in vibration is for the moving mass and generated motor force of the second transducer  13  to approximately equal the moving mass and generated motor force of the first transducer  12 . Such a component could be made at lower cost than the cost of a transducer that is identical to the first transducer  12 . It is generally desirable for a frame  25  of the first transducer  12  to be similar to a frame  26  of the second transducer  13  so that the second transducer  13  can be attached to the first transducer  12  at the same attachment points that are used to attach the first transducer  12  to the baffle  11 . An alternative means of assembly could be to rigidly attach the bottom of a motor structure  27  of the first transducer  12  to the top of a motor structure  28  of the second transducer  13 , using a rigid connecting member  19  such as a threaded metal rod. 
         [0069]      FIG. 1B  is a diagrammatic representation of an embodiment of the invention with the assembly carried by the infinite baffle  11 . The embodiment shown in  FIG. 1B  is substantially the same as the embodiment shown in  FIG. 1A  with the addition of an acoustic port  29  that is coupled to the cavity  30 . In some embodiments, the cavity  30  is substantially closed (other than through port tubes that may be present). In some embodiments, one or more ports, passive radiators, and/or other structures are used to acoustically couple the cavity  30  to some other element or physical space. The port  29  is an acoustic mass and the cavity  30  is an acoustic compliance. The dimensions of the port  29  are chosen to adjust the acoustic mass to set the port  29 /cavity  30  resonance at a desired frequency. The acoustic port  29  couples a portion of the acoustic energy from the cavity  30  to the listening area  18 . The acoustic energy is generated by the second surface  21   b  of the first diaphragm  21  and the first surface  22   a  of the second diaphragm  22 . The acoustic energy exiting from the acoustic port  29  reinforces the acoustic output that is coupled to the listening area  18  from the first surface  21   a  of the first diaphragm  21  and the second surface  22   b  of the second diaphragm  22 , over a limited, but useful frequency range. 
         [0070]    In some embodiments, a passive radiator (not shown) can be used instead of the acoustic port  29 . Like the acoustic port  29 , a passive radiator can be used to reinforce the acoustic energy entering the listening area  18 . The mechanical mass, area, and suspension compliance of the passive radiator are chosen so that the passive radiator resonates with the compliance volume  30  at a desired frequency. Sound waves from the second surface  21   b  of the first diaphragm  21  and the first surface  22   a  of the second diaphragm  22  strike and move the passive radiator. It in turn vibrates and creates its own sound waves from the front surface of its diaphragm. Although a passive radiator is a mechano-acoustic element, it is referred to as an acoustic element throughout the present specification. 
         [0071]      FIG. 1C  is a diagrammatic representation of an embodiment of the invention with the assembly carried by the infinite baffle  11 . The embodiment shown in  FIG. 1C  is substantially the same as the embodiment shown in  FIG. 1B  with the addition of an acoustic low-pass filter formed from the combination of a cavity  31  and the port  16 . Second surface  22   b  of the second diaphragm  22  is coupled to the cavity  31 , port tube  16  is coupled to cavity  31  and to listening area  18 . Therefore, the output from second surface  22   b  is filtered by the acoustic low pass filter. The dimensions of the enclosure  31  and the port  16  are determined based the desired cutoff frequency of the acoustic low pass filter, for example. The low-pass filter can be used to filter undesired frequencies in the acoustic energy before those frequencies can reach the listening area  18 . 
         [0072]    In one embodiment, the transducers  12 ,  13  are driven with input signals that include low frequencies and higher frequencies. The low-pass filter allows the low frequency acoustic waves generated by the second side  22   b  of the second diaphragm  22  to propagate to the listening area  18 , but prevents the higher frequency acoustic waves generated by the second side  22   b  of the second diaphragm  22  from propagating to the listening area  18 . This prevents potentially out-of-phase higher frequencies generated by both transducers  12 ,  13  from reaching the listening area  18  simultaneously, while only allowing higher frequencies from the first surface  21   a  of the first diaphragm  21  of the transducer  12  to reach the listening area  18 . 
         [0073]    Low-pass filters can be used to reduce a comb filter effect that occurs at higher frequencies due to acoustic path differences of the acoustic output from the transducers  12 ,  13 . By low-pass filtering one of the transducers  12 ,  13 , the comb filter effect is reduced. Skilled artisans will appreciate that variations in the low-pass filter can be made without departing from the invention. For example, the acoustic low-pass filter can include a passive radiator, an acoustic absorber, a Helmholtz resonator, and/or any other acoustic element or any combination of compliance, mass or resistive elements. Alternatively, electrical filtering can also be used. For example, an electrical low pass filter can be coupled to one or both transducer inputs. The enclosure  31  can also include sound absorbing material, such as fiberglass, polyester, batting, etc. 
         [0074]      FIG. 2  is a diagrammatic representation of an alternative embodiment of the invention with the assembly carried by an infinite baffle  11 , such as a vehicle rear deck or door. The second transducer  13  is now physically inverted with respect to the first transducer  12 .  FIG. 2  shows the rear of the motor structure  27  of the first transducer  12  being rigidly attached to the rear of the motor structure  28  of the second transducer  13  through a spacer  20 , although the spacer  20  is not required. Structural coupling of the two transducers  12 ,  13  could also be accomplished through an attachment around the periphery of the transducer frames  25 ,  26 , as shown in the system of  FIG. 1 . The arrangement of  FIG. 2  would work equally well if each of transducers  12 ,  13  were inverted as compared to what is shown. In this case, the structural connection would be accomplished through an attachment around the periphery of the transducer frames  25 ,  26 . In another embodiment, the structural connection can be made by attaching an optional connecting rod (not shown) to a pole piece in each of the motor structures  27 ,  28 . 
         [0075]    The orientation of the transducers  12 ,  13  relative to each other can be arbitrary, as long as the resultant force applied to a baffle  11  from the movement of the movable elements of one of the transducers  12 ,  13  is reduced by the movement of the movable elements of the other transducer  13 ,  12 . 
         [0076]    Since the transducers  12 ,  13  are physically inverted with respect to each other, cancellation of vibration will occur when electrical signals of the same relative polarity are applied to each transducer  12 ,  13 . Each transducer  12 ,  13  is attached to the output of the amplifier  17  such that when an electrical signal provided to the first transducer  12  causes the diaphragm  21  of the first transducer  12  to move in one direction, an electrical signal provided to the second transducer  13  causes the diaphragm  22  of the second transducer  13  to move in the substantially opposite direction relative to the motion of the diaphragm  21  of the first transducer  12 . 
         [0077]    Above a certain frequency, the output from the second transducer  13  will not be in phase with the output from the first transducer  12  at the listening area  18 . The frequency response of the combined system may exhibit a comb filter behavior with the first null occurring when the path length difference between the first surface  21   a  of the diaphragm  21  of the first transducer  12  and the listening area  18  and the second surface  22   b  of the diaphragm  22  of the second transducer  13  and the listening area  18  is a half-wavelength. 
         [0078]    One approach for reducing the effects of this comb filter behavior is by using a low-pass filter to restrict the spectral components delivered to both transducers  12 ,  13  to only spectral components that are below the first null and using other transducers (not shown) for reproducing higher frequency spectral components. The low-pass filters used could be identical for both transducers  12 ,  13 , or they can have different orders and/or corner frequencies. The low-pass filters can be acoustical filters or passive or active electrical filters. The output from one of the transducers  12 ,  13  could be restricted to being below a predetermined cutoff frequency while the other transducer  12 ,  13  is permitted to operate over a wider frequency range. Preferably, the first transducer  12  operates over a wider frequency range than the second transducer  13 . This result can be achieved by placing a low-pass filter in the signal path of the second transducer  13  only, or by having a low-pass filter in the signal path of the first transducer  12  with a higher corner frequency and/or lower order than a low-pass filter in the signal path of the second transducer  13 . The result can also be achieved either in combination with or solely by the appropriate design of the acoustic elements  15 ,  16  connecting the second transducer  13  to the listening area  18  such that the acoustic elements  15 ,  16 , in combination, form a low pass filter. It should be noted that acoustical or electrical filters can be used in any of the embodiments described herein and not simply the embodiment described with reference to  FIG. 2 . For example, in an asymmetric arrangement in which one of the transducers acoustically couples to the listening area while the other transducer couples to the listening area through an acoustic path, asymmetric filtering (such as using different filter cutoff frequencies, different orders, etc., in each transducer signal path) of the transducers can be used. 
         [0079]    It may also be advantageous to include a low-pass filter in the signal path of the second transducer  13  and a complementary all-pass filter in the signal path of the first transducer  12 . A complementary all-pass filter has the same phase response as a function of frequency as a corresponding low-pass filter. This feature can be accomplished, for example, by using a second-order critically damped low-pass filter in the signal path of the second transducer  13 , and a first-order all-pass filter in the signal path of the first transducer  12 , where the corner frequencies of the low-pass and all-pass filters are substantially identical. 
         [0080]    According to another embodiment, a fourth-order low-pass filter in the signal path of the second transducer  13  and a second-order all-pass filter in the signal path of the first transducer  12  may be used. Other examples of complementary all-pass filter/low-pass filter combinations will be evident to those skilled in the art. 
         [0081]    The use of complementary all-pass filters and low-pass filters as described above can be combined with other signal processing as disclosed in U.S. Pat. No. 5,023,914, incorporated by reference herein, to simultaneously achieve improved system frequency response and reduce vibration. 
         [0082]    Referring to  FIG. 3 , there is shown a graphical representation of force upon a baffle  11  ( FIG. 1 ) as a function of frequency for various structures. Curve  21  illustrates the resultant response of using two Bose® eight-inch Neodymium (Nd) transducers in an acoustic system having a low-pass filter. Curve  22  illustrates the applied force when using only a single eight-inch Nd transducer with a low-pass filter. Curve  23  shows the applied force when using just two eight-inch Nd transducers connected according to the invention without the low-pass filter. Curve  24  shows the applied force with just a single eight-inch Nd transducer. These graphical representations demonstrations the significant reduction in force applied to the baffle  11  using two eight-inch Nd transducers connected according to the invention without the low-pass filter and the advantage of incorporating the low-pass filter into the system. 
         [0083]    The embodiments shown in  FIGS. 1A-1C  and  FIG. 2  are described using an infinite baffle  11 . Although the arrangements are described showing the listening area  18 , this is not required. The invention can be adapted to emit sound equally well to either the volume  30  or the listening area  18  without loss of generality. Additionally, the infinite baffle  11  can be the ceiling, floor, wall, door, or any surface of a room. In this example, the room is either the volume  30  or the listening area  18 . 
         [0084]      FIG. 4  is a diagrammatic representation of an alternative embodiment of the invention with the assembly carried by an infinite baffle  11 , such as a vehicle rear shelf or door, incorporating transducers with inverted motor structures. Transducers  32  and  33  including diaphragms  41 ,  42  have motor structures  34 , that are inverted with respect to the motor structures  27 ,  28  of the transducers  12 ,  13  of  FIG. 2 .  FIG. 4  illustrates that the transducers  32 ,  33  are also physically inverted with respect to each other. The use of transducers  32 ,  33  having motor structures  34 ,  35  that are inverted is not limited to the orientation shown. Any of the previous arrangements described for non-inverted motor transducers is also applicable for transducers with inverted motor structures. Use of inverted motor structure transducers in the current invention can significantly reduce the overall thickness of the multiple transducer assembly, which can reduce intrusion into a vehicle trunk or allow a system to fit within a wall space where an arrangement using traditional transducers would not fit. Note also that mechanical links  36  can be made much shorter than the mechanical links  14  shown in  FIGS. 1A-1C  in the embodiment using transducers  12 ,  13  without inverted motor structures. Alternatively, an optional rigid connecting member  19  could be used to rigidly attach the motor structure  34  of the first transducer  32  to the motor structure  35  of the second transducer  33 . 
         [0085]      FIG. 5A  is a diagrammatic representation of an alternative embodiment of the invention with an assembly  50  mounted in an enclosure  52 . The assembly  50  includes the first  12  and the second transducers  13  that are mounted to a sub-enclosure  54 . This configuration illustrates a two-chamber band-pass enclosure. However configurations including any number of chambers can be used. For example, other multi-chamber configurations are described in more detail in U.S. Pat. No. 5,092,424, entitled “Electroacoustical Transducing with At Least Three Cascaded Subchambers” which is assigned to the assignee of the present application. The entire disclosure of U.S. Pat. No. 5,092,424 is incorporated herein by reference. The first  12  and the second transducers  13  are inverted with respect to each other and are mechanically coupled using optional mechanical links  14 . In one embodiment, an optional rigid connecting member  19  is coupled between the top of the motor structure  27  and the top of the motor structure  28  to increase the mechanical rigidity between the transducers  12 ,  13 . 
         [0086]    Alternatively, the transducers  12 ,  13  can be mechanically coupled to each other by using one or both of the mechanical links  14  and the rigid connecting member  19 . Also, the transducers  12 ,  13  can be arbitrarily oriented relative to each other as long as the moving parts of each of the transducers  12 ,  13  move in mechanical opposition in response to electrical signals applied to the transducers  12 ,  13  such that the resultant force applied to sub enclosure  54  to which transducers  12 ,  13  are mechanically coupled, and to the mechanical links  14  and/or the optional connecting member  19 , are reduced. 
         [0087]    The first surface  21   a  of the diaphragm  21  in the first transducer  12  and the first surface  22   a  of the diaphragm  22  in the second transducer  13  are acoustically coupled to a first acoustic volume of the sub-enclosure  54 . The first acoustic volume of the sub-enclosure  54  is acoustically coupled to a first acoustic element  56 , such as a first acoustic port. The first acoustic element  56  is acoustically coupled to the listening environment. The first acoustic element  56  couples acoustic energy from inside the sub-enclosure  54  to outside the sub-enclosure  54 . The acoustic compliance of the volume of the sub-enclosure  54  resonates with the acoustic mass of the first acoustic element  56 . The dimensions and volume of the sub-enclosure  54  and the first acoustic element  56  are determined based on the characteristics of the transducers  12 ,  13  and the desired frequency response of the system, for example. Adjustment of the volume of the sub-enclosure  54  and the dimensions of the first acoustic element  56  allows the resonant frequency of the sub-enclosure/port system to be tuned to a desired frequency. The first acoustic element  56  can be an acoustic port, an acoustic waveguide, a passive radiator, or any element that couples acoustic energy from inside the sub-enclosure  54  to outside the sub-enclosure  54 . 
         [0088]    The sub-enclosure  54  is mounted at least partially within the enclosure  52 . The enclosure  52  provides a second acoustic volume. The second surface  21   b  of the diaphragm  21  in the first transducer  12  and the second surface  22   b  of the diaphragm  22  in the second transducer  13  are acoustically coupled to the second acoustic volume. The enclosure  52  is acoustically coupled to a second acoustic element  58 , such as a second acoustic port. The second acoustic element  58  is acoustically coupled to the listening environment. The second acoustic element  58  couples acoustic energy from inside the enclosure  52  to outside the enclosure  52 . The volume of the enclosure  52  and the dimensions of the second acoustic element  58  are determined based on the characteristics of the transducers  12 ,  13  and the desired frequency response of the system, for example. Adjustment of the volume of the enclosure  52  and the dimensions of the second acoustic element  58  allows the resonant frequency of the enclosure/port system to be tuned to a desired frequency. The second acoustic element  58  can be an acoustic port, an acoustic waveguide, a passive radiator, or any element that couples acoustic energy from inside the enclosure  52  to outside the enclosure  52 . 
         [0089]    In one embodiment, the resonant frequency of the combination of the second volume and the second acoustic element  58  is the same as the resonant frequency of the combination of the first volume and the first acoustic element  56 . In another embodiment, the resonant frequency of the combination of the second volume and the second acoustic element  58  is lower than the resonant frequency of the combination of the first volume and the first acoustic element  56 . Additionally, although  FIG. 5A  illustrates a two-chamber band-pass enclosure, as previously described, other embodiments having additional chambers are also possible. 
         [0090]    The ported enclosure  52  including the ported sub-enclosure  54  can increase the low frequency output of the transducers  12 ,  13 . The resonances of the acoustic elements  56 ,  58  with the acoustic compliances (volumes) of enclosures  52 ,  54  serve to load the transducers  12 ,  13 . The loading reduces the excursion of the diaphragms  21 ,  22  near the resonant frequencies of each of the enclosure/port assemblies. The resonant frequencies of the enclosure/port assemblies are varied to alter the frequency response of the system. Typically, the resonant frequencies of the acoustic compliance of enclosure  52  with acoustic mass of the acoustic element  58  and the acoustic compliance of the enclosure  54  with acoustic mass of the acoustic element  56  are separated by a desired amount. In some embodiments, the acoustic elements  56 ,  58  include passive radiators (not shown) instead of acoustic ports. 
         [0091]    In operation, input signals are applied to the first transducer  12  and the second transducer  13 . The input signals cause the first diaphragm  21  in the first transducer  12  and the second diaphragm  22  in the second transducer  13  to move so that the motion of the first diaphragm  21  is substantially opposite to the motion of the second diaphragm  22 . The opposing motion of the diaphragms  21 ,  22  reduces a resultant mechanical force exerted on the structures to which the transducers are mechanically coupled, as well as to mechanical links  14 , compared to a motion of a single diaphragm from a single transducer for the same input signal. 
         [0092]      FIG. 5B  is a diagrammatic representation of an alternative embodiment of the invention with the assembly  50  mounted in the enclosure  52 . The assembly  50  includes the first  12  and the second transducers  13  that are mounted to a sub-enclosure  54 ′. The first  12  and the second transducers  13  are optionally mechanically coupled to each other using the mechanical links  14 . The system of  FIG. 5B  is similar to the system of  FIG. 5A  except that the acoustic element  56 ′ such as the acoustic port, couples acoustic energy from inside the sub-enclosure  54 ′ to inside the enclosure  52 . Adjustment of the dimensions of enclosure  52 , the second acoustic element  58 , the sub-enclosure  54 ′ and the acoustic element  56 ′ allows the resonant frequencies of the system to be tuned to desired frequencies. 
         [0093]      FIG. 5C  is a diagrammatic representation of an alternative embodiment of the invention with an assembly  60  mounted in an enclosure  62 . The assembly  60  includes the first  12  and the second transducers  13  that are mounted to a sub-enclosure  64 . The first  12  and the second transducers  13  are mechanically coupled to each other using the mechanical links  14 . The system of  FIG. 5C  is similar to the system of  FIG. 5A  except that the acoustic elements include passive radiators instead of acoustic ports. Any combination of passive radiators and/or acoustic ports can be used. 
         [0094]    The second surface  21   b  of the diaphragm  21  in the first transducer  12  and the first surface  22   a  of the diaphragm  22  in the second transducer  13  are acoustically coupled to the volume of the sub-enclosure  64 . The volume of the sub-enclosure  64  is acoustically coupled to a first passive radiator  66 . The dimensions and volume of the sub-enclosure  64  and the characteristics (such as area, mass, suspension compliance) of first passive radiator  66  are determined based on the characteristics of the transducers  12 ,  13  and the desired frequency response of the system, for example. The dimensions and volume of sub-enclosure  64  and characteristics of passive radiator  66  are chosen in order to locate the resonant frequency (of the acoustic compliance of the sub-enclosure with the acoustic mass of the passive radiator) at a desired frequency. The first passive radiator  66  can be replaced by an acoustic port, such as the acoustic port  56  of  FIG. 5A . 
         [0095]    The sub-enclosure  64  is mounted within the enclosure  62 . The first surface  21   a  of the diaphragm  21  in the first transducer  12  and the second surface  22   b  of the diaphragm  22  in the second transducer  13  are acoustically coupled to a volume of the enclosure  62 . The volume of the enclosure  62  is acoustically coupled to a second  68  and a third passive radiator  69 . The second  68  and third passive radiators  69  are located on opposing walls of the enclosure  62 . This arrangement allows the diaphragms of the passive radiators  68 ,  69  to move in substantially opposite directions when they are stimulated by the acoustic energy in the enclosure  62  from the movement of the diaphragms  21 ,  22 . The opposing motion of the diaphragms of the passive radiators  68 ,  69  reduce a resultant mechanical force on the enclosure  62  that would otherwise be applied if the diaphragms of the passive radiators  68 ,  69  moved in the same or in random directions. 
         [0096]    The volume of enclosure  62  and the dimensions of the passive radiators  68 ,  69  are determined based on the characteristics of the transducers  12 ,  13  and the desired frequency response of the system, for example. The volume of enclosure  62  and characteristics of passive radiators  68 ,  69  are chosen in order to locate the resonant frequency (of the acoustic compliance of the enclosure with the acoustic mass of the passive radiators) at a desired frequency. The number, shape, and size of passive radiators can be changed as long as the resultant force on the enclosure  62  from the movement of the diaphragms of the passive radiators is reduced. 
         [0097]    In one embodiment, the combination of the volume of the enclosure  62  and the passive radiators  68 ,  69  is tuned to a lower frequency than the combination of the volume of the sub-enclosure  64  and the passive radiator  66 . The lower frequency tuning can require passive radiators that have a higher mass. Thus, configuring the system with the high mass passive radiators  68 ,  69  located on opposite sides of the enclosure  62  reduces the resultant force applied to the enclosure  62  by the movement of the high mass passive radiators  68 ,  69 . It should be noted that the passive radiator  66  is not opposed. However, since it is tuned to a higher frequency, it typically has a lower moving mass than the high mass passive radiators  68 ,  69 . Thus, the force applied to the enclosure  62  from the movement of the passive radiator  66  is relatively small. 
         [0098]      FIG. 5D  is a diagrammatic representation of an alternative embodiment of the invention with an assembly  70  mounted in an enclosure  72 . The assembly  72  includes the first  12  and the second transducers  13  that are mounted to a sub-enclosure  74 . The first  12  and the second transducers  13  are mechanically coupled to each other using the mechanical links  14 . The first  12  and the second transducers  13  can alternatively be mechanically coupled through the connecting member  19 . In another embodiment, the first  12  and the second transducers  13  can be indirectly acoustically coupled through the walls of the enclosures  72 ,  74  without including the mechanical links  14  or the connecting member  19 . Also, it should be noted that the volumes  72   a  and  72   b  can be part of a contiguous acoustic volume, or could be physically separate volumes. 
         [0099]    The first surface  21   a  of the diaphragm  21  in the first transducer  12  and the first surface  22   a  of the diaphragm  22  in the second transducer  13  are acoustically coupled to the sub-enclosure  74 . The sub-enclosure  74  includes the first passive radiator  66  and a second passive radiator  76 . The first  66  and second passive radiators  76  are located on opposing walls of the enclosure  72 . This arrangement allows the diaphragms of the passive radiators  66 ,  76  to move in substantially opposite directions when they are stimulated by the acoustic energy in the sub-enclosure  74  from the movement of the diaphragms  21 ,  22 . The opposing motion of the diaphragms  66 ,  76  reduce a resultant mechanical force on the enclosure  72  compared to what would otherwise be applied if the diaphragms  66 ,  76  moved in the same or random directions. 
         [0100]    The dimensions and volume of the sub-enclosure  74  and the first  66  and the second passive radiators  76  are determined based on the characteristics of the transducers  12 ,  13  and the desired frequency response of the system, for example. The first  66  and the second passive radiators  76  can be replaced by one or more acoustic ports. 
         [0101]    The sub-enclosure  74  is mounted within the enclosure  72 . The second surface  21   b  of the diaphragm  21  in the first transducer  12  and the second surface  22   b  of the diaphragm  22  in the second transducer  13  are acoustically coupled to the enclosure  72 . The enclosure  72  includes a third  68  and a fourth passive radiator  69 . The third  68  and fourth passive radiators  69  are located on opposing walls of the enclosure  72 . This arrangement allows the diaphragms of the passive radiators  68 ,  69  to move in substantially opposite directions when they are stimulated by the acoustic energy in the enclosure  72  from the movement of the diaphragms  21 ,  22 . The opposing motion of the diaphragms  68 ,  69  reduces the resultant mechanical force on the enclosure  72  compared to what would otherwise be applied if the diaphragms  68 ,  69  moved in the same or random directions. 
         [0102]    The dimensions of the enclosure  72  and the dimensions of the passive radiators  68 ,  69  are determined based on the characteristics of the transducers  12 ,  13  and the desired frequency response of the system, for example. Skilled artisans will appreciate that the number, shape, and size of passive radiators can be changed as long as the resultant force on the enclosure  72  from the movement of the diaphragms of the passive radiators is reduced. 
         [0103]      FIG. 5E  is a diagrammatic representation of an alternative embodiment of the invention with an assembly  80  mounted in an enclosure  81 . The assembly  80  includes a first transducer  82  having an inverted motor structure and a first diaphragm  83 . The assembly  80  also includes a second  84 , a third  86 , and a fourth transducer  88 . The second transducer  84  includes an inverted motor structure and a second diaphragm  85 . The third transducer  86  includes an inverted motor structure and a third diaphragm  87 . The fourth transducer  88  includes an inverted motor structure and a fourth diaphragm  89 . The transducers  82 ,  84 ,  86 ,  88  are attached using rigid members  90 . Portions of the rigid members  90  are acoustically transparent such that acoustic energy can pass though them with substantially no attenuation. 
         [0104]    Input signals are applied to the transducers  82 ,  84 ,  86 ,  88  having the desired relative phase such that motions of the diaphragms  83 ,  85 ,  87 ,  89  reduce a resultant mechanical force on the rigid members  90  and on the enclosure  81  that would otherwise be applied if an input signal was applied to only one of the transducers  82 ,  84 ,  86 ,  88 . The input signals cause the first diaphragm  83  in the first transducer  82  and the third diaphragm  87  in the third transducer  86  to move in a first direction. The input signals cause the second diaphragm  85  in the second transducer  84  and the fourth diaphragm  89  in the fourth transducer  88  to move in a second direction that is substantially opposite to the first direction. 
         [0105]    Acoustic energy is coupled from a front surface of the first diaphragm  83  to the listening area  18 . Acoustic energy is indirectly coupled to the listening area  18  from the rear surface of the second diaphragm  85  and the front surface of the third diaphragm  87  through an acoustic path  91 . Acoustic energy is also indirectly coupled to the listening area  18  from the rear surface of the fourth diaphragm  89  through the acoustic path  91 . The directly-coupled acoustic energy from the first transducer  82  and the indirectly-coupled acoustic energy from the acoustic path  91  arrive at the listening area  18  substantially in-phase. 
         [0106]    Acoustic energy from the rear surface of the first diaphragm  83  and the front surface of the second diaphragm  85  as well as the rear surface of the third diaphragm  87  and the front surface of the fourth diaphragm  89  are acoustically coupled to the cavity  92  The multi-element assembly  80  can be used in any embodiment where a two element assembly is used. 
         [0107]    The dimensions of the enclosure  81  including the dimensions of the cavity  92  are determined based on the characteristics of the transducers  82 ,  84 ,  86 ,  88  and the desired frequency response of the system, for example. 
         [0108]      FIG. 5F  is a diagrammatic representation of an embodiment of the invention showing an unmounted assembly  80 ′ including four separate transducers  82 ′,  84 ′,  86 ′,  88 ′. The transducers  82 ′,  84 ′,  86 ′,  88 ′ are configured to be analogous to a single transducer with a front side  93  and a backside  95 . The pressure P 1  at the front side  93  (a first location) is out of phase with the pressure P 2  at the backside  95  (a second location). Thus, the assembly  80 ′ can be used to replace a single transducer. The assembly  80 ′ can also be mounted in any enclosure or baffle. The transducers  82 ′,  84 ′,  86 ′,  88 ′ are configured and oriented such that input signals applied to the first pair of transducers  82 ′,  84 ′ cause the movable elements in the transducers  82 ′,  84 ′ to move in substantially opposite directions relative to each other and input signals applied to the second pair of transducers  86 ′,  88 ′ cause the movable elements in the transducers  86 ′,  88 ′ to also move in substantially opposite directions relative to each other. Various other configurations can be used without departing from the invention. For example, additional transducers could also be used in various orientations as long as the pressure P 1  at the first location is out of phase with the pressure P 2  at the second location. 
         [0109]    In one embodiment, the transducers are oriented in a substantially columnar configuration. Additionally, an arbitrary number of transducers can be used including an odd number of transducers. A complex baffle can be used to maintain the phase relationship between the pressure P 1  at the first location and the pressure P 2  at the second location. 
         [0110]    In one embodiment, the first pair of diaphragms of the transducers  82 ′,  84 ′ and the second pair of diaphragms of the transducers  86 ′,  88 ′ are mechanically coupled to a frame  96 . The first pair of diaphragms and the second pair of diaphragms are oriented substantially in-line. As previously discussed, input signals applied to the motor structures of the transducers  82 ′,  84 ′ cause the first pair of diaphragms to move in substantially opposite directions relative to each other. Input signals applied to the motor structures of the transducers  86 ′,  88 ′ cause the second pair of diaphragms to move in substantially opposite directions relative to each other. 
         [0111]    In one embodiment, the movement of the second pair of diaphragms of the transducers  86 ′,  88 ′ is substantially in phase with the movement of the first pair of diaphragms of the transducers  82 ′,  84 ′. An acoustic shield  97  can be located proximate to the first pair of diaphragms. The acoustic shield  97  is configured to prevent acoustic energy from a first surface of a diaphragm in the first pair of transducers  82 ′,  84 ′ from destructively combining with acoustic energy from a second surface of the diaphragm. Another acoustic shield  98  can be located proximate to the second pair of diaphragms. The other acoustic shield  98  prevents acoustic energy from a first surface of a diaphragm in the second pair of transducers  86 ′,  88 ′ from destructively combining with acoustic energy from a second surface of the diaphragm. In one embodiment, a substantially contiguous baffle separates the front side pressure P 1  from the back side pressure P 2  in the transducer assembly  80 ′. 
         [0112]    The transducers  82 ′,  84 ′,  86 ′,  88 ′ can be mounted in an enclosure. Additionally, the diaphragms of the transducers  82 ′,  84 ′,  86 ′,  88 ′ can be arranged in various orientations, such as inverted or non-inverted relative to each other. Although the transducers  82 ′,  84 ′,  86 ′,  88 ′ are illustrated having standard motor structures, the transducers  82 ′,  84 ′,  86 ′,  88 ′ can include inverted motor structures. 
         [0113]      FIG. 6  is a perspective view of a typical loudspeaker assembly  100  carried by an infinite baffle  102 , such as a vehicle rear package shelf. The loudspeaker assembly  100  includes a first  104  and a second electro-acoustic transducer  106  that are seated into apertures in the infinite baffle  102 . The electro-acoustic transducers  104 ,  106  are rigidly mounted to the infinite baffle  102  using screws or other hardware. The first electro-acoustic transducer  104  includes a diaphragm  108  and a motor structure  110 . The second electro-acoustic transducer  106  also includes a diaphragm  112  and a motor structure  114 . 
         [0114]    A front surface of each of the diaphragms  108 ,  112  is acoustically coupled to a listening area  116  which can be the passenger compartment of a vehicle, for example. A rear surface of each of the diaphragms  108 ,  112  is acoustically coupled to a cavity  118  which can be the trunk of the vehicle. 
         [0115]    A power amplifier  17  energizes the motor structures  110 ,  114  in the first  104  and the second electro-acoustic transducers  106  with an input signal. In some systems, each of the first  104  and the second electro-acoustic transducers  106  are driven using different input signals, such as left or right stereo signals. The input signals applied to the motor structures  110 ,  114  cause the diaphragms  108 ,  112  to move, thereby creating acoustic energy that is coupled to the listening area  116 . 
         [0116]    The electro-acoustic transducers  104 ,  106  are rigidly mounted to the infinite baffle  102 , and therefore, the movement of the diaphragms  108 ,  112  generates mechanical forces on the infinite baffle  102 . These mechanical forces can degrade the sound emanating from the electro-acoustic transducers  108 ,  112  because the infinite baffle  102  is typically fabricated from thin material, such as thin sheet metal. Such thin materials typically have insufficient stiffness to resist vibration and are typically lightly dampened. As a result, the mechanical forces applied to the infinite baffle  102  around the modal resonance frequencies of the infinite baffle  102  can result in excessive vibration of the structure, which can be acoustically perceived as undesired buzzes and rattles, and/or degraded frequency response of the radiated sound. 
         [0117]      FIG. 7  illustrates a perspective view of a loudspeaker system  150  including the electro-acoustic transducers  104 ,  106  according to one embodiment of the invention. The first  104  and the second electro-acoustic transducers  106  can be arranged in a substantially parallel configuration so that the motion of the diaphragms  108 ,  112  is along the same axis. By substantially parallel, we mean that the transducers  104 ,  106  can be arranged to be exactly parallel to each other or can be arranged to be slightly non-parallel to each other while still providing an effective result. However, substantially non-parallel configurations can also be realized. For example, in an embodiment including an odd number of transducers, each of the transducers can be arranged to be non-parallel to the other transducers. In one embodiment, the first transducer  104  includes the first movable diaphragm  108 . The first movable diaphragm  108  includes a first surface  152  that is acoustically coupled to the listening area  116 . For example, the first surface  152  can be a front or rear surface of the first diaphragm  108 . A second surface  154  of the first diaphragm  108  is acoustically coupled to the cavity  118 . For example, the second surface  154  can be the front or rear surface of the first diaphragm  108 . The first transducer  104  is mounted to a first baffle  156 . 
         [0118]    In one embodiment, the second transducer  106  includes the second movable diaphragm  112 . The second movable diaphragm  112  includes a first surface  158  that is acoustically coupled to the listening area  116 . For example, the first surface  158  can be a front or rear surface of the second diaphragm  112 . A second surface  160  of the second diaphragm  112  is acoustically coupled to the cavity  118 . For example, the second surface  160  can be the front or rear surface of the second diaphragm  112 . The second transducer  106  is mounted to a second baffle  162 . The second baffle  162  can be positioned substantially parallel to the first baffle  156 . The first baffle  156  is structurally coupled to the second baffle  162 . A rigid member  164  can form part of an enclosure surrounding the cavity  118 . Other rigid members  180 ,  182  can also form part of the enclosure surrounding the cavity  118 . The rigid members  180 ,  182  are arranged to acoustically isolate the front surfaces  152 ,  158  of each diaphragm  108 ,  112  from the rear surfaces  154 ,  160 . The rigid members  180 ,  182  are shown without shading for clarity. 
         [0119]    The loudspeaker system  150  is coupled to an infinite baffle  166 . The first  156  and the second baffles  162  can be positioned to be substantially perpendicular to the infinite baffle  166 . The infinite baffle  166  can be a rear package shelf or a door panel in a vehicle, or a door, wall, floor, or ceiling of a room, for example. The loudspeaker system  150  can also be mounted behind a seatback, a dashboard, or in a headliner of a vehicle. The first  108  and the second diaphragms  112  of the first  104  and the second transducers  106  can be any shape or size depending on the requirements of the system. For example, the shape of the diaphragms  108 ,  112  can be round or elliptical, or any other suitable shape. Additionally, the diaphragms  108 ,  112  can be any diameter in the case of round diaphragms or any elliptical size such as 4×6, 5×7, or 6×9 in the case of elliptical diaphragms. Elliptical diaphragms have a different shape than round diaphragms, and thus, transducers having elliptical diaphragms can provide additional packaging flexibility as compared to similarly sized transducers having round diaphragms. 
         [0120]    In one embodiment, the frame of the first transducer  104  is molded into the first baffle  156  and the first diaphragm  108  is then attached to the molded frame. The frame of the second transducer  106  can also be molded into the second baffle  166  and the second diaphragm  112  can then be attached to the molded frame. Any transducer, regardless of the process of manufacturing the transducer, can be used. 
         [0121]    An input signal from a signal source (not shown) is coupled to an input port  168  of the power amplifier  17 . An output port  170  of the power amplifier  17  is electrically coupled to an input  172  of the first transducer  104  and an input  174  of the second transducer  106  through conductive paths  176 ,  178 . The power amplifier  17  energizes the first transducer  104  and the second transducer  106  with an input signal and drives the first  104  and the second transducer  106  substantially in phase to produce an acoustic output signal. The system  150  is arranged such that when the diaphragm  108  of the first transducer  104  is moving in one direction, the diaphragm  112  of the second transducer  106  moves in the substantially opposite direction, which significantly reduces resultant forces applied to the rigid members  164 ,  180 ,  182  while significantly maintaining the acoustic output signal. This also reduces undesired resultant vibrations in the baffles  156 ,  162  and the infinite baffle  166 . The resultant vibration in the baffle  166  due to the motion of the movable elements of both transducers  104 ,  106  being driven with an input signal is less than a vibration imparted to the baffle  166  due to the motion of a single transducer being driven alone with the same input signal. 
         [0122]    In one embodiment, the first  104  and the second transducer  106  are driven substantially in mechanical phase opposition by an input signal such that a resultant mechanical force applied to the rigid member  164  from the movement of the first  108  and the second movable diaphragms  112  is reduced, while the acoustic outputs from the first  104  and the second transducers  106  are combined acoustically in phase. In one embodiment, each transducer  104 ,  106  is identical and the force imparted to the baffle  166  due to the motion of the movable elements of the first transducer  104  is substantially equal and opposite to the force imparted to the baffle  166  due to the motion of the moveable elements of the second transducer  106 . The opposite motion of the movable elements of the transducers  104 ,  106  reduce a resultant force and a vibration imparted to the baffle  166 . 
         [0123]    In the embodiment shown in  FIG. 7 , the direction of movement of the movable elements of each of the transducers  104 ,  106  is substantially collinear. The transducers  104 ,  106  are shown being mounted in a symmetrical manner, but non-symmetrical embodiments can be realized by determining appropriate force vectors and mounting the transducers at suitable orientations with respect to each other. For example, in an embodiment having three transducers (not shown), each transducer can be oriented at a vertice of an isosceles triangle such that principle axis of radiation of each transducer intersect in the center of the triangle. The resultant forces imparted to the baffle by the movement of the moveable elements in the transducers is reduced when input signals having the appropriate polarity are applied to each transducer. 
         [0124]    The transducers  104 ,  106  can include low-pass, high-pass, or band-pass filters or crossover networks, for example. The filters can be acoustic or electrical filters. The electrical filters can be active or passive filters. The electrical filters can be analog, digital, or a combination of analog and digital filters. For example, a low-pass filter can restrict spectral components of the input signal above a predetermined cutoff frequency, whereas a high-pass filter can restrict spectral components of the input signal below a predetermined cutoff frequency. 
         [0125]    In one embodiment (not shown), the acoustic output from the second surfaces  154 ,  160  of the first  108  and the second diaphragms  112  can be coupled to the listening area  116  through an acoustic port (not shown) that is coupled to the cavity  118 . 
         [0126]    In one embodiment (not shown), the first transducer  104  and the second transducer  106  are arranged such that the first  108  and the second diaphragm  112  are positioned in the same orientation, such as is shown in  FIG. 1A . In this embodiment, the power amplifier  17  energizes the first transducer  104  and the second transducer  106  with signals having opposite polarity. The system is arranged such that when the diaphragm  108  of the first transducer  104  is moving in one direction, the diaphragm  112  of the second transducer  106  is moving in the substantially opposite direction, which significantly reduces the resultant force applied to the rigid members  164 ,  180 ,  182 . This also reduces undesired resultant vibrations in the baffle  166 . 
         [0127]    The second transducer  106  is not required to be identical to the first transducer  104 . All that is required for significant reduction in vibration is for the moving mass and generated motor force of the second transducer  106  to approximately equal the moving mass and generated motor force of the first transducer  104 . Such a component could be made at lower cost than the cost of a transducer that is identical to the first transducer  104 . 
         [0128]    In one embodiment, the first baffle  156 , the rigid members  164 ,  180 ,  182 , and the second baffle  162  are fabricated from a single sheet of material. The sheet of material can be formed from wood, metal, fiberglass, particle board, or any suitable material. In another embodiment, the first  156  and the second baffle  162  are rigidly mounted to the rigid member  164 . The assembly can then be mounted to the baffle  166 . As previously described, the baffle  166  can be a rear package shelf, a seatback, a floorboard, an interior door panel, a headliner or a dashboard of a vehicle. Alternatively, the baffle  166  can be a structural panel such as a wall, a ceiling, a floor, or a door in a residence, business, theater, stadium, or concert hall, for example. 
         [0129]    In one embodiment, the first baffle  156  and the second baffle  162  can be similarly sized so that the first  104  and the second transducers  106  can be arranged in a parallel configuration such that the first  108  and the second diaphragms  112  are aligned with each other. An optional means of assembly could be to rigidly attach the top of a motor structure of the first transducer  104  to the top of a motor structure of the second transducer  106  using a rigid connecting member  19 . In an embodiment in which the transducers  104 ,  106  are inverted relative to the illustrative embodiment of  FIG. 7 , the rigid connecting member  19  connects the rear of the motor structures of the transducers  104 ,  106 . 
         [0130]      FIG. 8  illustrates a perspective view of a loudspeaker system  200  including the electro-acoustic transducers  104 ,  106  and additional third  202  and fourth electro-acoustic transducers  204  according to one embodiment of the invention. The first  104  and the second electro-acoustic transducers  106  are arranged in a parallel configuration so that the motion of the diaphragms  108 ,  112  is in the same axial direction. In this embodiment, the first  104  and the second electro-acoustic transducers  106  are positioned in an inverted arrangement compared with the loudspeaker system  150  illustrated in  FIG. 7 , but can be arranged in the same configuration. 
         [0131]    The third  202  and the fourth electro-acoustic transducers  204  are also arranged in a parallel configuration so that the motion of diaphragms  206 ,  208  is in the same axial direction. In general, the direction of motion of the diaphragms  108 ,  112  of the electro-acoustic transducers  104 ,  106  is perpendicular to the direction of motion of the diaphragms  206 ,  208  of the electro-acoustic transducers  202 ,  204 . The first transducer  104  is mounted to the first baffle  156 . The second transducer  106  is mounted to the second baffle  162 . The second baffle  162  is positioned substantially parallel to the first baffle  156 . The rigid member  164  mechanically couples the first baffle  156  to the second baffle  162 . 
         [0132]    The third transducer  202  is mounted to a third baffle  210 . The fourth transducer  204  is mounted to the fourth baffle  212 . The third baffle  210  is positioned substantially parallel to the fourth baffle  212 . The rigid member  164  mechanically couples the third baffle  210  to the fourth baffle  212 . The rigid member  164  also forms part of an enclosure surrounding the cavity (not shown). The loudspeaker system  200  is coupled to the infinite baffle  166 . As previously discussed, the infinite baffle  166  can be a rear package shelf, a headliner, a floorboard, a seatback, or a door panel in a vehicle, or a structural panel, such as a wall, floor, or ceiling of a room, for example. 
         [0133]    The power amplifier  17  energizes the first transducer  104  and the second transducer  106  with electrical signals having the same polarity and drives the first  104  and the second transducer  106  mechanically in phase opposition. The system  200  is arranged such that when the diaphragm  108  of the first transducer  104  is moving in one direction, the diaphragm  112  of the second transducer  106  moves in the substantially opposite direction, which significantly reduces the forces applied to the rigid member  164 . This also reduces undesired resultant vibrations in the baffles  156 ,  162  and the infinite baffle  166 . 
         [0134]    Another power amplifier  214  (or the same power amplifier  17 ) energizes the third transducer  202  and the fourth transducer  204  with electrical signals having the same polarity and drives the third  202  and the fourth transducer  204  in mechanical phase opposition. The system  200  is arranged such that when the diaphragm  206  of the third transducer  202  is moving in one direction, the diaphragm  208  of the fourth transducer  204  moves in the substantially opposite direction, which significantly reduces the forces applied to the rigid member  164 . This also reduces undesired resultant vibrations in the baffles  210 ,  212  and the infinite baffle  166 . 
         [0135]    In one embodiment, one or more power amplifiers (not shown) energize the first  104 , the second  106 , the third  202 , and the fourth transducers  204  with electrical signals having appropriate polarity and drive the first pair of transducers  104 ,  106 , in mechanical phase opposition and the second pair of transducers  202 ,  204  in mechanical phase opposition. In this embodiment, the diaphragms  108 ,  112  of the transducers  104 ,  106  move in substantially opposite directions relative to each other and the diaphragms  206 ,  208  of the transducers  202 ,  204  move in substantially opposite directions relative to each other. 
         [0136]    In one embodiment (not shown), the first transducer  104 , the second transducer  106 , the third transducer  202 , and the fourth transducer  204  are arranged such that the diaphragms  108 ,  112 ,  206 ,  208  are positioned in various orientations, such as inverted relative to each another. In this embodiment, the power amplifiers  17 ,  214  energize the transducers  104 ,  106 ,  202 ,  204  with signals having specific phases that drive the pairs of diaphragms  108 ,  112  and  206 ,  208  in substantially opposite directions regardless of their orientation. The system is arranged such that when the diaphragm  108  of the first transducer  104  is moving in one direction, the diaphragm  112  of the second transducer  106  is moving in the substantially opposite direction. Similarly, when the diaphragm  206  of the third transducer  202  is moving in one direction, the diaphragm  208  of the fourth transducer  204  is moving in the substantially opposite direction. This substantially opposite movement significantly reduces the resultant forces applied to the rigid member  164 . This also reduces undesired resultant vibrations in the baffle  166 . 
         [0137]    In one embodiment, a common input signal is applied to the transducers  104 ,  106 ,  202 ,  204  such that the diaphragms  108 ,  112 ,  206 ,  208  of the transducers  104 ,  106 ,  202 ,  204  all move inward toward the center of the assembly, simultaneously, and move outward for the opposite polarity applied. In some embodiments, the transducers  104 ,  106 ,  202 ,  204  can include standard or inverted motor structures. 
         [0138]      FIG. 9A  illustrates a perspective view of a loudspeaker system  250  including a first  252  and a second electro-acoustic transducer  254  that are mounted to an infinite baffle  256  according to one embodiment of the invention. The first  252  and the second transducers  254  are positioned so that the front surfaces of diaphragms  258 ,  260  are substantially in the same orientation and facing each other. The distance (d)  262  between the first  252  and the second transducers  254  can be varied while still minimizing undesired vibrations in the baffle  256  that result from the movement the diaphragms  258 ,  260  of the first  252  and the second transducers  254 . The movement of the diaphragms  258 ,  260  is generated by the motor structures of the first  252  and the second transducers  254 . 
         [0139]    A front surface of each of the diaphragms  258 ,  260  of the transducers  252 ,  254  is acoustically coupled to the listening area  116  which can be the passenger compartment of a vehicle or a room, for example. A rear surface of each of the diaphragms  258 ,  260  is acoustically coupled to the infinite cavity  118  which can be the trunk of the vehicle or a volume of space between the walls or under a floor in a residence, for example. The transducers  252 ,  254  can be elliptically shaped. Elliptically shaped transducers can be oriented to minimize the depth intrusion into the cavity  118 . 
         [0140]    The amplifier  17  provides an input signal to the first  252  and the second transducer  254 . In the embodiment shown, each of the first  252  and the second transducer  254  receives the input signal having the same relative polarity. This causes the diaphragms  258 ,  260  of the transducers  252 ,  254  to move in substantially opposite directions. The movement of the diaphragms  258 ,  260  generates mechanical forces on the baffle  256 . The mechanical forces that are imparted to the baffle  256  can degrade the sound emanating from the transducers  252 ,  254  especially around the modal resonance frequencies of the baffle  256 . Excitation of these modal resonance frequencies can result in excessive vibration of the baffle  256 , which can be acoustically perceived as undesired buzzes and rattles, and/or degraded frequency response of the radiated sound. The substantially opposite movement of the diaphragms  258 ,  260  can effectively reduce these vibrations in the baffle  256 . 
         [0141]      FIG. 9B  illustrates a side view of a loudspeaker system  265  including a first  252  and a second electro-acoustic transducer  254  that are mounted to an infinite baffle  256  according to another embodiment of the invention. An optional rigid connecting member  19  can mechanically couple the motor structures  266 ,  267  of the transducers  252 ,  254 . The amplifier  17  provides input signals to the first  252  and the second transducer  254 . Each of the first  252  and the second transducer  254  receives the input signal having the same relative polarity. This causes the diaphragms of the transducers  252 ,  254  to move in substantially opposite directions, thereby reducing a resultant mechanical force imparted to the baffle  256 . In one embodiment, the amplifier  17  provides monophonic low frequency signals to the transducers  252 ,  254  such that the transducers move in substantially opposite directions in response to the low frequency signals. The amplifier  17  can also provide stereophonic higher frequency signals to the transducers  252 ,  254  since higher frequency signals generally create less forces on the baffle  256  than lower frequency signals. 
         [0142]      FIG. 10  illustrates a perspective view of a loudspeaker system  270  including a first pair  272 ,  274  and a second pair of electro-acoustic transducers  276 ,  278  that are mounted to an infinite baffle  280  according to one embodiment of the invention. A first plurality of rigid members  282 ,  283 ,  284  is mechanically coupled between the first pair of transducers  272 ,  274 . A second plurality of rigid members  285 ,  286 ,  287  is mechanically coupled between the second pair of transducers  276 ,  278 . 
         [0143]    A first amplifier  288  transmits an input signal to each of the first pair of electro-acoustic transducers  272 ,  274  through conductive paths  289 ,  290 . For example, the first amplifier  288  can be a first channel in a stereo or multichannel amplifier, such as a left channel amplifier in a stereo system. In one embodiment, the polarity of the input signal traveling through the conductive path  290  is modified before it reaches the transducer  272 . In another embodiment, an input signal is applied to the first pair of transducers  272 ,  274  and the motor structures of the first pair of transducers  272 ,  274  are inversely configured with respect to each other. For example, a magnet in the motor structure of the transducer  272  can have its poles inverted with respect to the magnet in the motor structure of the transducer  274 . This causes the diaphragms in the first pair of transducers  272 ,  274  to move in substantially opposite directions in response to being driven by the input signal. The opposing motion of the diaphragms in the first pair of transducers  272 ,  274  reduces a resultant mechanical force applied to the rigid members  282 ,  283 ,  284  that is generated from the movement of the diaphragms in the transducers  272 ,  274 . A front surface of a diaphragm in the transducer  272  and a rear surface of a diaphragm in the transducer  274  are acoustically coupled to the listening area  116 , which can be the passenger compartment of a vehicle. A rear surface of the diaphragm in the transducer  272  and a front surface of the diaphragm in the transducer  274  are acoustically coupled to the cavity  118 , which can be the trunk of the vehicle. 
         [0144]    A second amplifier  292  transmits an input signal to each of the second pair of electro-acoustic transducers  276 ,  278  through conductive paths  294 ,  296 . For example, the second amplifier  292  can be a second channel in a stereo or multichannel amplifier, such as a right channel amplifier in a stereo system. In one embodiment, the polarity of the input signal traveling through the conductive path  296  is modified before it reaches the transducer  278 . In another embodiment, an input signal is applied to the second pair of transducers  276 ,  278  and the motor structures of the second pair of transducers  276 ,  278  are inversely configured with respect to each other. This causes the diaphragms in the second pair of transducers  276 ,  278  to move in substantially opposite directions in response to being driven by an input signal. The opposing motion of the diaphragms in the second pair of transducers  276 ,  278  reduces a resultant mechanical force applied to the rigid members  285 ,  286 ,  287  that is generated from the movement of the diaphragms in the transducers  276 ,  278 . 
         [0145]    A front surface of a diaphragm in the transducer  278  and a rear surface of a diaphragm in the transducer  276  are acoustically coupled to the listening area  116 . A rear surface of the diaphragm in the transducer  278  and a front surface of the diaphragm in the transducer  276  are acoustically coupled to the cavity  118 . 
         [0146]    The first  272 ,  274  and second pairs of transducers  276 ,  278  effectively reduce the resultant forces on each of the rigid members  282 ,  283 ,  284 ,  285 ,  286 , and  287 . Thus, vibrations that can couple into the infinite baffle  280  from the first  272 ,  274  and second pairs of transducers  276 ,  278  are also reduced. 
         [0147]      FIG. 11  illustrates a perspective view of a loudspeaker system  300  including a pair of electro-acoustic transducers  302 ,  304  that are mounted to an infinite baffle  306  which can be a seatback  323  of a rear seat  308  of a vehicle. In this embodiment, the transducers  302 ,  304  are mounted to baffles  310 ,  312  such that the diaphragms of the transducers  302 ,  304  are positioned substantially parallel to each other. The baffles  310 ,  312  are coupled to each other through rigid members  314 ,  316 ,  318 . The rigid members  314 ,  316 ,  318  form part of the loudspeaker system  300  that intrudes into the cavity  116 . For example, the cavity  116  can be a trunk of a vehicle. 
         [0148]    Front surfaces of the diaphragms in the transducers  302 ,  304  are acoustically coupled to a listening area  118  through a pass-through  322  in the seatback  323  of the rear seat  308 . In one embodiment, an armrest  324  can fold into the pass-through  322  if desired. Rear surfaces of the diaphragms in the transducers  302 ,  304  are acoustically coupled to the cavity  116 . In some embodiments, acoustic ports, acoustic waveguides, passive radiators and/or acoustic dampening material can be added to the cavity  116  to improve the performance of the loudspeaker system  300 . 
         [0149]    The diaphragms in the transducers  302 ,  304  can be any desired shape including round or elliptical. Elliptically-shaped transducers have a different form factor than round transducers which can increase packaging options in the vehicle. For example, the a packaging option including elliptically-shaped transducers can be less intrusive in the cavity  116  then a packaging option using similarly sized round transducers. For example, the loudspeaker system  300  could include 4×6, 5×7, or 6×9 inch transducers. 
         [0150]    Although the loudspeaker system  300  is shown positioned behind the rear seat  308  of a vehicle, the loudspeaker system  300  could be positioned in other locations within the vehicle. For example, the loudspeaker system  300  could be positioned under the seat  308 , in the armrest  324 , in a door panel, under a dashboard, in a floor, in a headliner, or any other suitable location. The loudspeaker system  300  could also be implemented in walls, ceiling and/or floors in a residence, a business, a theater, a stadium, or a concert hall, for example. 
         [0151]    In operation, an input signal from a signal source (not shown) is coupled to a power amplifier (not shown). An output of the power amplifier is electrically coupled to the first transducer  302  and the second transducer  304 . The power amplifier energizes the first transducer  302  and the second transducer  304  with an input signal and drives the first  302  and the second transducer  304  substantially acoustically in phase and in mechanical phase opposition to produce an acoustic output signal that is coupled to the listening area  118 . The loudspeaker system  300  is arranged such that when the diaphragm of the first transducer  302  is moving in one direction, the diaphragm of the second transducer  304  moves in the substantially opposite direction, which significantly reduces a resultant mechanical force applied to the rigid members  314 ,  316 ,  318  while significantly maintaining the acoustic output signal. This also reduces undesired resultant vibrations in the rigid members  314 ,  316 ,  318 , the infinite baffle  306  and the seatback  323  of the rear seat  308 . In one embodiment, the seatback  323  of the rear seat  308  is the infinite baffle  306 . The resultant vibration in the infinite baffle  306  due to the motion of the movable elements of both transducers  302 ,  304  being driven with an input signal is less than a vibration imparted to the infinite baffle  306  due to the motion of a single transducer being driven alone with the same input signal. 
         [0152]      FIG. 12  is a cross-sectional view of a loudspeaker system  350  according to the invention that is mounted to a structural panel  352 . The arrangement of the system  350  is similar to the arrangement of the apparatus described with reference to  FIG. 1A . A first transducer  354  is mechanically coupled to the structural panel  352  (i.e., an infinite baffle). The first transducer  354  includes an inverted motor structure  356 . A second transducer  358  having an inverted motor structure  360  is mechanically coupled to the first transducer  354  through rigid members  362 . For example, the loudspeaker system  350  can be mounted into a wall, floor, or a ceiling in a residential or commercial building. 
         [0153]    A front surface of a diaphragm in the first transducer  354  is acoustically coupled to the listening area  318 . A rear surface of a diaphragm in the second transducer  358  is acoustically coupled to the listening area  318  through an acoustic path  366 . A rear surface of the diaphragm in the first transducer  354  and a front surface of the diaphragm in the second transducer  358  are acoustically coupled to a cavity  368 . The cavity  368  can be acoustically coupled to the listening area  318  through an acoustic element, such as an acoustic port, an acoustic waveguide, or a passive radiator (not shown). For example, as previously described, the acoustic path  366  can include one or more acoustic elements. 
         [0154]    The rigid members  362  mechanically couple the first  354  and the second transducers  358  to the structural panel  352  (i.e., infinite baffle). An input signal applied to the first  354  and the second transducers  358  causes the transducers  354 ,  358  to produce acoustic output into the listening area  318 . The transducers  354 ,  358  are driven so that mechanical vibrations imparted to the rigid members  362  and the infinite baffle  352  from the movement of the transducers  354 ,  358  are reduced. The system  350  can also include acoustic ports, acoustic waveguides, and/or passive radiators that acoustically couple acoustic energy from the cavity  368  to the listening area  318 . An acoustically transparent screen or grill  370  can be used to cover the system  350 . 
         [0155]      FIG. 13  illustrates a cross-sectional view of an electro-acoustic transducer  400  according to one embodiment of the invention. The electro-acoustic transducer  400  is a moving coil type transducer. However, a moving magnet type transducer could also be constructed. The electro-acoustic transducer  400  includes a magnet assembly  402  having two sets of motor structures  403 ,  404  each with a single magnetic flux gap. The transducer  400  also includes a first  406  and second diaphragm  408  mounted to a common frame or basket  410 . Other magnet assemblies having other motor structures geometries are also possible. For example,  FIG. 17A  illustrates a magnet assembly having a single motor structure with multiple magnetic flux gaps. 
         [0156]    The electro-acoustic transducer  400  of  FIG. 13  is shown generally circularly symmetric. However, circular symmetry is generally not required. For example, the first  406  and the second diaphragms  408  can be elliptical, oval, or any other desired shape. Additionally, the first  406  and the second diaphragms  408  can have the same or different surface areas. The motor structures  403 ,  404  can also be circularly symmetric or any other desired shape. 
         [0157]    The first diaphragm  406  is mechanically coupled to the common frame  410  through a first surround  412 . The first diaphragm  406  is also mechanically coupled to a first bobbin  414 . The first bobbin  414  can be fabricated from any suitable material including plastic, paper, cardboard, fiberglass, or Kapton, for example. A first voice coil  416  is mechanically coupled to the first bobbin  414 . For example, the first voice coil  416  can be wound around the first bobbin  414 . A first spider  418  couples the first bobbin  414  to a support structure  420 . The first bobbin  414  positions the first voice coil  416  in a first magnetic flux gap  422  of the first motor structure  403  in the magnet assembly  402 . 
         [0158]    The first motor structure  403  includes a top plate  424 , a permanent magnet  426 , and a back plate/pole assembly  428 . The permanent magnet  426  can be a slug magnet that is generally disk-shaped. The permanent magnet can be fabricated from a rare earth-based magnetic material such as Samarian Cobalt, Neodymium-Iron-Boron, and/or any other known magnetic material. 
         [0159]    In one embodiment, the permanent magnet  426  is magnetized in a direction that is normal to its flat surface. The north pole of the permanent magnet  426  can be located on its upper surface and the south pole can be located on its lower surface. However, the permanent magnet  426  can also be positioned with its poles reversed as long as the proper polarity is maintained to ensure that the first voice coil  416  moves in the desired direction. 
         [0160]    Magnetic flux emanates from the top of the permanent magnet  426  and is conducted through the top plate  424  which can be fabricated from any magnetically permeable material, such as steel. The magnetic flux then crosses through the first magnetic flux gap  422  and onto the back plate/pole assembly  428  and then to the permanent magnet  426 . Thus, a first magnetic circuit is formed with magnetic flux passing through the first magnetic flux gap  422 . The back plate/pole assembly  428  can also be fabricated from a magnetically permeable material, such as steel. 
         [0161]    A dust cap  432  can be positioned on the first diaphragm  406  to protect the first motor structure  403  from debris that can impact the operation of the transducer  400 . The dust cap is generally attached to the first diaphragm  406  with glue or tape. 
         [0162]    The first voice coil  416  includes wire leads  434  that are routed up the bobbin and lead through the dust cap and onto the first diaphragm  406 . The wire leads  434  are generally coupled to braided wires  436 , sometimes referred to as tinsel wires that are routed to the outer edge of the first diaphragm  406  and out to terminals  438  that are attached to the frame  410 . Skilled artisans will appreciate that there are various methods for attaching electrical terminals to the first voice coil  416 . For example, the braided wires  436  can be routed along the first spider  418 , down the support structure  420 , and out through the bottom of the frame  410 . 
         [0163]    The second diaphragm  408  is mechanically coupled to the common frame  410  through a second surround  442 . The second diaphragm  408  is also mechanically coupled to a second bobbin  444 . The second bobbin  444  can be fabricated from any suitable material including plastic, paper, cardboard, fiberglass, or Kapton, for example. A second voice coil  446  is mechanically coupled to the second bobbin  444 . For example, the second voice coil  446  can be wound around the second bobbin  444 . A second spider  448  couples the second bobbin  444  to the frame  410 . The second bobbin  444  positions the second voice coil  446  in a second magnetic flux gap  452  of the second motor structure  404  in the magnet assembly  402 . The second magnetic flux gap  452  substantially surrounds the first magnetic flux gap  422 . 
         [0164]    The second motor structure  404  substantially surrounds the first motor structure  403 . The second motor structure  404  includes a top plate  454 , a permanent magnet  456 , and a back plate/pole assembly  458 . The permanent magnet  456  can be a ring magnet. The permanent magnet  456  can be fabricated from a rare earth-based magnetic material or any other known magnetic material. 
         [0165]    In one embodiment, the permanent magnet  456  is magnetized in a direction that is normal to its flat surface. The north pole of the permanent magnet  456  can be located on its upper surface and the south pole can be located on its lower surface. However, the permanent magnet  456  can also be positioned with its poles reversed as long as the proper polarity is maintained to ensure that the second voice coil  446  moves in the desired direction. 
         [0166]    Magnetic flux emanates from the top of the permanent magnet  456  and is conducted through the top plate  454  which can be fabricated from any magnetically permeable material, such as steel. The magnetic flux then crosses through the second magnetic flux gap  452  and onto the back plate/pole assembly  458  and then to the permanent magnet  456 . Thus, a second magnetic circuit is formed with magnetic flux passing through the second magnetic flux gap  452 . 
         [0167]    The second voice coil  446  includes wire leads  464  that are routed up the bobbin and lead under the second diaphragm  408 . The wire leads  464  are generally coupled to braided wires  466  that are routed over the second spider  448  and out to terminals  468  that are attached to the frame  410 . Skilled artisans will appreciate that the braided wires can be routed to the terminals  468  in various ways. For example, the wires can be routed along the diaphragm  408  and out to terminals that are mounted to the frame  410 . 
         [0168]    The frame  410  can be formed using various techniques. For example, the frame  410  can be formed from a single piece of material or can be fabricated in multiple sections. The frame  410  includes vents  470  to allow acoustic energy from between the diaphragms  406 ,  408  to propagate substantially unattenuated. The frame  410  also includes vents  472  to allow acoustic energy from the rear surface of the second diaphragm  408  to propagate substantially unattenuated. The vents  470 ,  472  can be distributed around the circumference of the frame  410 . Acoustically transparent scrim cloth  474  can be used to cover the vents  470  to protect the second magnetic gap  452  from dust and debris. 
         [0169]    In operation, an external source (not shown) such as an amplifier includes output terminals that are electrically connected to the terminals  438 ,  468 . The terminals  438 ,  468  can be connected to the same output terminal on the amplifier as long as the diaphragms  406 ,  408  are properly configured to move in mechanically opposite directions when an input signal is applied to the terminals  438 ,  468 . This configuration can include inverting the magnetism of one of the magnets  426 ,  456  with respect to the other magnet  426 ,  456  while maintaining the proper relative polarity of the voice coils  416 ,  446 . For example, the windings in the voice coils  416 ,  446  can be inverted, the polarity of the terminals  438 ,  468  can be reversed, as previously described. 
         [0170]    An input signal applied to the terminals  438 ,  468  causes the first voice coil  416  to move in one direction and causes the second voice coil  446  to move in the substantially opposite direction. For example, a positive voltage applied to the terminal  438  causes the first voice coil  416  to move upward and a positive voltage applied to the terminal  468  causes the second voice coil  446  to move downward. This causes the first diaphragm  406  to move mechanically opposite to the second diaphragm  408 . The movement of the second diaphragm  408  reduces a resultant mechanical force applied to the frame  410  by the combined movement of the first  406  and the second diaphragms  408  as compared to a movement of the first diaphragm  406  alone. Thus, a reduction in the mechanical force applied to the frame  410  by the movement of the first diaphragm  406  is observed, even if the diaphragms  406 ,  408  and/or the motor structures  403 ,  404  are not identical. 
         [0171]    In addition, an input signal applied to the first voice coil  416  can be different than an input signal applied to the second voice  446 . For example, the input signals can be the same at low frequencies but can be different at higher frequencies. Also, the input signals can be modified to compensate for asymmetries in the motor structures  403 ,  404  and diaphragms  406 ,  408  to further reduce the resultant mechanical force applied to the frame  410  by the movement of the first  406  and the second diaphragms  408 . 
         [0172]    The motor structures  403 ,  404 , and diaphragms  406 ,  408  can be driven in mechanical phase opposition over at least a portion of the low frequency range of an input signal. The low frequency range of an input signal can create a significant amount of movement from the diaphragms  406 ,  408 . Some of that movement can create mechanical forces and/or vibrations in the frame  410 . The resultant mechanical forces/vibrations coupled into the frame  410  are significantly reduced in the transducer  400 , while the acoustic output from the transducer  400  is increased due to the combined acoustic output of both diaphragms  406 ,  408  compared to a similarly-sized transducer having a single diaphragm. It should be noted that various structures designed to couple acoustic output from the second diaphragm  408  to the listening area are shown in following figures. 
         [0173]    The motor structures  403 ,  404  and the diaphragms  406 ,  408  can be made similar in order to obtain a large reduction in mechanical forces applied to the frame  410 . However, even non-identical motor structures  403 ,  404  and the diaphragms  406 ,  408  can create a significant reduction in mechanical forces applied to the frame  410 . Skilled artisans will appreciate that it is generally desirable to make the resonant frequencies of the moving structures match each other closely. This can be accomplished by designing various assemblies to include approximately the same moving mass and total suspension stiffness. Moving masses can be adjusted by changing diaphragm materials or diaphragm geometry including thickness. Other methods can also be applied, such as by adding weights and/or modifying other moving structure components, such as voice coil windings, spiders, dustcaps, and surrounds. It can be also desirable to closely match the behavior of the motor structures  403 ,  404 . Since the magnetic gaps  422 ,  452  have substantially different diameters, the components in the motor structures  403 ,  404  can be designed such that the motor structures  403 ,  404  have similar characteristics. 
         [0174]    The fabrication of the transducer  400  can require certain design considerations due to the asymmetric nature of the transducer  400 . In one embodiment, the dynamics of the first motor structure  403 /diaphragm  406 /suspension combination and the second motor structure  404 /diaphragm  408 /suspension combination are designed to be substantially the same. For example, the diaphragms, spiders, dustcaps, and surrounds can be configured so that the moving masses are the same, the stiffness in each respective suspension system are the same, and the motor forces are the same. One way to characterize motor force is by using a quantity known as beta β. Beta β is a motor quality factor and is defined as: 
         [0000]    
       
         
           
             β 
             = 
             
               
                 
                   ( 
                   
                     b 
                      
                     
                         
                     
                      
                     l 
                   
                   ) 
                 
                 2 
               
               
                 r 
                 e 
               
             
           
         
       
     
         [0000]    where b is the flux density in the gap, I is the length of wire in the voice coil, and r e  is the DC resistance of the voice coil. Thus, by varying one or more of these parameters, the motor quality factor for each motor structure  403 ,  404  can be designed to be substantially the same. For example, the system can be designed so that a voltage applied to each voice coil  416 ,  466  results in a similar force output into a system having a similar moving mass and a similar stiffness. 
         [0175]    Thus, in one embodiment, the dynamics of the moving systems are designed to be substantially the same, and the force outputs of the motor structures are designed to be substantially the same. The moving system consists of the diaphragm, dust cap (if present), a portion of the surround, a portion of the spider, and the voice coil (including wire and bobbin). There can also be some mass associated with the air load on the diaphragm (if the areas of the diaphragms are similar, the mass of the air loads will be similar). The total moving mass for the transducer sub-assemblies should be approximately equal. The total suspension stiffness from the spider and the surround of each transducer sub-assembly should also be approximately equal. The total suspension stiffness can also be affected by an optional back enclosure (not shown). 
         [0176]    Each of the motor structures  403 ,  404  are formed from a magnet  426 ,  456 , a back plate/pole piece  428 ,  458 , a top plate  424 ,  454 , along with a voice coil  416 ,  446 . It can be desirable for the beta β of each transducer sub-assembly to be the same, as well as the DC resistance of the voice coils. If each of the parameters that define the characteristics of the transducer sub-assemblies are the same (e.g., beta, DCR, mass, compliance, etc.), the transducer sub-assemblies can have very similar dynamic behavior. 
         [0177]    However, due to possible asymmetry in some of the components, it can be necessary to choose actual components that are significantly different from each other so that the overall behavior of the transducer sub-assemblies is similar. For example, voice coils can have different diameters, with different numbers of windings, diaphragms can have different masses to make up for voice coil differences, magnetic gap dimensions can be made different, and/or different quantities of magnet material may be used, in order to improve the dynamic symmetry between the two transducer sub-assemblies. In one embodiment, the primary characteristics to match are the area of the diaphragms, the moving masses, the motor force constants (beta), and the stiffness of the suspensions of the transducer sub-assemblies. 
         [0178]    In one embodiment, the acoustic output from the second diaphragm  408  is not coupled to the listening area, and its movement is only used to reduce a resultant force on the frame  410  that is caused by the movement of the first diaphragm  406 . 
         [0179]      FIG. 14  illustrates a cross-sectional view of a loudspeaker system  500  including the electro-acoustic transducer  400  of  FIG. 13  mounted to an infinite baffle  11  according to one embodiment of the invention. The infinite baffle  11  is similar to the infinite baffle  11  described with reference to  FIG. 1A . The frame  410  of the electro-acoustic transducer  400  is mounted to the infinite baffle  11  using mounting hardware such as screws, clips, glue, or sealant, for example. An acoustic shield  501  can prevent acoustic energy from the cavity  30  from undesirably combining with acoustic energy in the acoustic path  15 ,  16 . 
         [0180]    The front surface of the first diaphragm  406  is acoustically coupled to the listening area  18 . If the baffle  11  is the rear package shelf of a vehicle, the listening area  18  is the passenger compartment of the vehicle. The rear surface of the first diaphragm  406  is acoustically coupled to the cavity  30 , which would be the vehicle trunk if the baffle  11  is the rear package shelf. 
         [0181]    The frame  410  in this embodiment includes additional vent holes  502  that allow acoustic energy from rear surface of the second diaphragm  408  to propagate to listening area  18  through the acoustic path  15 ,  16 . The rear surface of the first diaphragm  406  and the front surface of the second diaphragm  408  are acoustically coupled to the cavity  30 . Instead of a rear package shelf of a vehicle, the baffle  11  can be a wall, floor, or ceiling of a room and the electro-acoustic transducer  400  can be positioned behind or flush with the wall, below the floor, or above the ceiling. 
         [0182]    A power amplifier  17  energizes the motor structures  403 ,  404  of the electro-acoustic transducer  400  with the same signal but drives them in mechanical opposition. The system is arranged such that when the first diaphragm  406  is moving in one direction, the second diaphragm  408  is moving in the substantially opposite direction, which significantly reduces the resultant mechanical forces applied to the baffle  11  compared to forces applied to the baffle  11  from a movement of one of the diaphragms  406 ,  408 . This also reduces undesired resultant vibrations of the baffle  11 . Meanwhile, the acoustic output from the rear surface of the second diaphragm  408  is coupled through the acoustic path  16  to the listening area  18  for emission that is substantially in phase with the output from the front surface of the first diaphragm  406 . Thus, the input signal from the amplifier  17  that is applied to the second motor structure  404  causes the second diaphragm  408  to move in an opposite direction from the direction of movement of the first diaphragm  406  to reduce a resultant force imparted to the baffle  11  while maintaining the acoustic output. 
         [0183]    The acoustic output from the rear surface of the second diaphragm  408  could also be coupled through a conduit of substantially constant or smoothly varying cross section to the listening area  18  without loss of generality. In some embodiments (not shown), the cavity  30  can include one or more acoustic ports, acoustic waveguides, or passive radiators that can couple acoustic energy from the cavity  30  to the listening area  18 . As previously described, any acoustic element can be used. 
         [0184]    In one embodiment, separate amplifiers can be used to drive each voice coil  416 ,  446  ( FIG. 13 ) separately. This allows different signals to be applied to each voice coil  416 ,  446  in the transducer  400 . Equalization can be used to compensate for differences in the behavior of individual transducer sub-assemblies to achieve a larger reduction in the resultant force on the baffle  11  from the movement of the diaphragms  406 ,  408 . 
         [0185]      FIG. 15  illustrates a cross-sectional view of a loudspeaker system  520  including the electro-acoustic transducer  400  of  FIG. 13  mounted to an enclosure  522  according to one embodiment of the invention. The electro-acoustic transducer  400  is mounted to the enclosure  522  using mounting hardware. 
         [0186]    The front surface  406   a  of the first diaphragm  406  is acoustically coupled to a listening area  524 . The rear surface  406   b  of the first diaphragm  406  is acoustically coupled to a cavity  526  in the enclosure  522 . 
         [0187]    The frame  410  of the electro-acoustic transducer  400  includes vent holes  528  that allow acoustic energy from rear surface  408   b  of the second diaphragm  408  to propagate to listening area  524  through an acoustic path  530 . 
         [0188]    A front surface  408   a  of the second diaphragm  408  is also acoustically coupled to the cavity  526  through vent holes  532 . In some embodiments, the enclosure  522  can include one or more acoustic ports, acoustic waveguides, passive radiators, or other acoustic elements that can couple acoustic energy from the cavity  526  to the listening area  524 . 
         [0189]    In another embodiment, the second diaphragm  408  is merely utilized to reduce a mechanical force imparted to the enclosure  522  from the movement of the first diaphragm  406 . In this embodiment, the acoustic energy generated by the movement of the second diaphragm  408  is not used. In this embodiment, the path  530  that couples the rear surface of the diaphragm  408  to the listening area  524  can be omitted. 
         [0190]      FIG. 16  illustrates a cross-sectional view of a loudspeaker system  550  including the electro-acoustic transducer  400  of  FIG. 13  mounted to a structural panel  552  according to one embodiment of the invention. The structural panel  552  is similar to the infinite baffle  11  described with reference to  FIG. 14 . For example, the structural panel  552  can be a wall, floor, ceiling, door or other structure in a room. The frame  410  of the electro-acoustic transducer  400  is mounted to the structural panel  552  using mounting hardware such as screws, clips, glue, or sealant, for example. Mechanical structures  553 ,  554  can prevent acoustic energy emanating from between the diaphragms  406 ,  408  from undesirably combining with acoustic energy emanating from the rear surface of the second diaphragm  408 . A screen or grill  555  can be used to cover the electro-acoustic transducer  400 . 
         [0191]    The front surface of the first diaphragm  406  is acoustically coupled to a listening area  556 . The rear surface of the first diaphragm  406  is acoustically coupled to a cavity  558  through the vent holes  470  in the frame  410 . The cavity  558  is the space between the structural panel  552  and another structural panel  560 . The frame  410  in this embodiment also includes the vent holes  472  that allow acoustic energy from the rear surface of the second diaphragm  408  to propagate to the listening area  556  through an acoustic path  564 . A front surface of the second diaphragm  408  is coupled to the cavity  558 . 
         [0192]    An amplifier (not shown) energizes the motor structures  403 ,  404  ( FIG. 13 ) of the electro-acoustic transducer  400  with the same signal but drives them in mechanical opposition. The system is arranged such that when the first diaphragm  406  is moving in one direction, the second diaphragm  408  is moving in the substantially opposite direction, which significantly reduces the resultant force applied to the structural panel  552 . This also reduces undesired resultant vibrations of the structural panel  552 . Meanwhile, the acoustic output from the rear surface of the second diaphragm  408  is coupled by the acoustic path  564  to the listening area  556  for emission that is substantially in phase with the output from the front surface of the first diaphragm  406 . Thus, the input signal from the amplifier causes the second diaphragm  408  to move in an opposite direction from the direction of movement of the first diaphragm  406  to reduce the resultant force imparted to the structural panel  552  while maintaining the acoustic output. 
         [0193]    The acoustic output from the rear surface of the second diaphragm  408  could also be coupled through a conduit of substantially constant or smoothly varying cross section to the listening area  556  without loss of generality. In some embodiments (not shown), the cavity  558  can include one or more acoustic ports, acoustic waveguides, or passive radiators that can couple acoustic energy from the cavity  558  to the listening area  556 , as previously described. 
         [0194]      FIG. 17A  is a cross-sectional view of an electro-acoustic transducer  600  according to another embodiment of the invention. The embodiment shown in  FIG. 17A  is similar to the embodiment shown in  FIG. 13 , but includes a magnet assembly  602  having single motor structure  604  with a first  606  and a second magnetic gap  608 . The magnet assembly  602  includes a single ring magnet  610 . 
         [0195]    The electro-acoustic transducer  600  of  FIG. 17A  is shown generally circularly symmetric. However, circular symmetry is generally not required. For example, the first  406  and the second diaphragms  408  can be elliptical, oval, or any other desired shape. Additionally, the first  406  and the second diaphragms  408  can have the same or different surface areas. The motor structure  604  can also be circularly symmetric or any other desired shape. 
         [0196]    The first diaphragm  406  is mechanically coupled to the first voice coil  416  through the first bobbin  414 . The first spider  418  couples the first bobbin  414  to a support structure  420 . The first bobbin  414  positions the first voice coil  416  in the first magnetic flux gap  606  of the motor structure  604  in the magnet assembly  602 . The motor structure  604  includes a top plate  612 , the permanent magnet  610 , and a back plate assembly  614 . The permanent magnet  610  can be fabricated from a rare earth-based magnetic material such as Samarian Cobalt, Neodymium-Iron-Boron, and/or any other known magnetic material. 
         [0197]    In one embodiment, the permanent magnet  610  is magnetized in a direction that is normal to its flat surface. The north pole of the permanent magnet  610  can be located on its upper surface and the south pole can be located on its lower surface. However, the permanent magnet  610  can also be positioned with its poles reversed. 
         [0198]    Magnetic flux emanates from the top of the permanent magnet  610  and is conducted through the top plate  612  which can be fabricated from any magnetically permeable material, such as steel. The magnetic flux then crosses through the first magnetic flux gap  606  and onto the back plate assembly  614  and back to the permanent magnet  610 . Thus, a first magnetic circuit is formed with magnetic flux passing through the first magnetic flux gap  606 . 
         [0199]    The electrical leads that electrically couple an external amplifier to the first voice coil  416  are not shown. Skilled artisans will appreciate that there are various methods for attaching electrical terminals to the first voice coil  416 . 
         [0200]    The second diaphragm  408  is mechanically coupled to the second voice coil  446  through the second bobbin  444 . The second spider  448  couples the second bobbin  444  to the frame  410 . The second bobbin  444  positions the second voice coil  446  in the second magnetic flux gap  608  of the motor structure  604  in the magnet assembly  602 . The second magnetic flux gap  608  substantially surrounds the first magnetic flux gap  606 . The motor structure  604  also includes the back plate assembly  614  corresponding to the second magnetic flux gap  608 . 
         [0201]    Magnetic flux emanates from the top of the permanent magnet  610  and is conducted through the top plate  612 . The magnetic flux then crosses through the second magnetic flux gap  608  and onto the back plate assembly  614  and back to the permanent magnet  610 . Thus, a second magnetic circuit is formed with magnetic flux passing through the second magnetic flux gap  608 . 
         [0202]    The first and the second magnetic circuit can be designed such that the magnetic flux emanating from the magnet  610  is split in a desired manner. Thus, the dimensions of the air gaps  606 ,  608  can be designed in combination with the voice coils  416 ,  446  such that the desired beta β is observed for each motor structure. In one embodiment, the beta β is substantially equivalent for each motor structure. 
         [0203]    In operation, an external source (not shown) such as an amplifier includes output terminals that are electrically connected to the voice coils  416 ,  446 . The voice coils  416 ,  446  are configured having the appropriate polarity such that they move in substantially opposite directions when an input signal is applied to them. An input signal applied to the voice coils  416 ,  446  causes the first voice coil  416  to move in one direction and causes the second voice coil  446  to move in the substantially opposite direction. This causes the first diaphragm  406  to move in mechanical phase opposition to the second diaphragm  408 . The movement of the second diaphragm  408  reduces a mechanical force applied to the frame  410  by the movement of the first diaphragm  406 . Thus, a reduction in the mechanical force applied to the frame  410  by the movement of the first diaphragm  406  is observed, even if the diaphragms  406 ,  408  are not identical. 
         [0204]      FIG. 17B  is a cross-sectional view of the motor structure  604  in the electro-acoustic transducer  600  of  FIG. 17A . A line of symmetry  620  divides the motor structure  604  so that only the right side of the motor structure  604  is shown. The complete motor structure  604  can be illustrated by rotating  FIG. 17B  around the line of symmetry  620 . A static magnetic field having magnetic field lines  622 ,  622 ′ emanates from the top (north pole) of the ring magnet  610  and is conducted through the top plate  612 . The magnetic field lines  622 ,  622 ′ then cross through the first  606  and the second magnetic gaps  608 , respectively, and onto the back plate assembly  614 . The magnetic field lines  622 ,  622 ′ then return to the south pole of the ring magnet  610 . Thus, two magnetic circuits are formed having opposite relative polarity. 
         [0205]    An input signal having a current I 1  is applied to the first voice coil  416  in a first direction  624  that drives the first voice coil  416  with a first polarity. The current I 1  generates a first magnetic field having field lines  626 . The first magnetic field is generally an alternating-current (AC) magnetic field. The first magnetic field having field lines  626  can cause flux modulation distortion in the electro-acoustic transducer  600 . Flux modulation distortion is a phenomenon that results from the interaction of the first magnetic field having field lines  626  generated by the first voice coil  416  with the static magnetic field generated by the ring magnet  610 . Specifically, the strength of the static magnetic field is undesirably varied due to interference from the first magnetic field  626  generated by the first voice coil  416 . 
         [0206]    An input signal having a current I 2  is applied to the second voice coil  446  in the same direction  624  as the input signal applied to the first voice coil  416 . The second voice coil  446  is wound in the same direction as the first voice coil  416 . The current I 2  generates a second magnetic field having field lines  626 ′. Thus, the first voice coil  416  and the second voice coil  466  move in mechanical phase opposition relative to each other. This is due to the direction of the magnetic field lines  622  in the first magnet flux gap  606  being opposite to the direction the magnetic field lines  622 ′ in the second magnetic flux gap  608 . 
         [0207]    The flux modulation distortion can be reduced since the second magnetic field generated by the second voice coil  446  has opposite polarity to the first magnetic field generated by the first voice coil  416 . Thus, since the magnetic fields generated by the voice coils  416 ,  446  are in substantially opposite directions, the interaction between the static magnetic field from the magnet  610  and the magnetic fields generated by the voice coils  416 ,  446  can be reduced. 
         [0208]    The flux modulation distortion can also be suppressed by positioning one or more optional copper (or other conducting material) shorting rings  628  adjacent to magnetic gaps  606 ,  608  in the motor structure  604 . Shorting rings  628  can also minimize the change in inductance of a voice coil as a function of position in the gap. 
         [0209]      FIG. 18  is a cross-sectional view of an electro-acoustic transducer  630  according to another embodiment of the invention. The embodiment shown in  FIG. 18  is similar to the embodiment shown in  FIG. 17A , but includes a magnet assembly  632  having an alternative ring magnet  634  which is a radially magnetized ring magnet also known as a donut magnet. The magnet assembly  632  includes the ring magnet  634  as well as a magnetic isolator  636 . The magnetic isolator is configured to prevent a magnetic short circuit from occurring. The magnetic isolator  636  can be fabricated from a plastic or other non-magnetically permeable material. 
         [0210]    The ring magnet  634  is configured such that its magnetic poles are located on its vertical surfaces in a direction that is parallel to its flat surface. For example, the north magnetic pole can be located on the inside vertical surface of the ring magnet  634  and the south magnetic pole can be located on the outside vertical surface of the ring magnet  634 . However, the ring magnet  634  can also be magnetized with its poles reversed. 
         [0211]    The magnet assembly  632  includes a motor structure  638  having the ring magnet  634 , the magnetic isolator  636 , the back plate assembly  614 , a first side plate  640 , and a second side plate  642 . Magnetic flux emanates from the inside surface of the ring magnet  634 , propagates through the first side plate  640 , and crosses through the first magnetic flux gap  606  and onto the back plate assembly  614 . The magnetic flux then propagates through the second magnetic flux gap  608 , and then propagates through the second side plate  642  back to the outside surface of the ring magnet  634 . Thus, a magnetic circuit is formed having magnetic flux that passes through the first  606  and the second magnetic flux gap  608 . 
         [0212]    In operation, an external source (not shown) such as an amplifier includes output terminals that are electrically connected to the voice coils  416 ,  446 . The voice coils  416 ,  446  are configured having appropriate polarity such that they move in mechanical opposition when an input signal is applied to them. An input signal applied to the voice coils  416 ,  446  causes the first voice coil  416  to move in one direction and causes the second voice coil  446  to move in the substantially opposite direction. This causes the first diaphragm  406  to move in mechanical opposition to the second diaphragm  408 . The movement of the second diaphragm  408  reduces a mechanical force applied to the frame  410  by the movement of the first diaphragm  406 . Thus, a reduction in the resultant mechanical force applied to the frame  410  by the movement of the first diaphragm  406  is observed, even if the diaphragms  406 ,  408  are not identical. It should be noted that the input signals applied to the voice coils  416 ,  446  need not be identical as long as the proper polarity is maintained. In one embodiment, the input signals are modified individually prior to being applied to each of the voice coils  416 ,  446  to tune the movement of each voice coil  416 ,  446 . This tuning can further reduce the resultant mechanical force applied to the frame  410  from the movement of the diaphragms  406 ,  408 , as previously described with reference to  FIG. 13 . 
         [0213]      FIG. 19  is a cross-sectional view of an electro-acoustic transducer  650  according to another embodiment of the invention. The electro-acoustic transducer  650  is a moving coil type transducer. However, a moving magnet type transducer could also be constructed. The electro-acoustic transducer  650  includes a magnet assembly  652  having two motor structures  654 ,  656  each with a single magnetic flux gap  658 ,  660 . The two motor structures  654 ,  656  share a single ring magnet  662 . 
         [0214]    A first voice coil  664  is wound around a first bobbin  668 . The first voice coil  664  is positioned in the first magnetic flux gap  658 . A second voice coil  670  is wound around a second bobbin  672 . The second voice coil  670  is positioned in the second magnetic flux gap  660 . The first  664  and the second voice coils  670  can have substantially the same diameter. The two motor structures  654 ,  656  can have an over-hung or under-hung design. The first magnetic flux gap  658  is adjacent to the second magnetic flux gap  660 . In one embodiment, an air gap  674  separates the first magnetic flux gap  658  from the second magnetic flux gap  660 . A non-magnetically conducting isolator can be positioned in the air gap  674 . The first  658  and second magnetic flux gap  660  can have substantially the same diameter. 
         [0215]    The first motor structure  654  includes a first magnetically permeable plate  676  that is positioned adjacent to the magnet  662 . A first top plate  677  is mechanically coupled to the first magnetically permeable plate  676 . The first magnetically permeable plate  676  is also mechanically coupled to a first pole plate  678  through a first rigid support member  680 . The first rigid support member  680  is fabricated to be as thin as possible while still maintaining structural integrity. The first rigid support structure  680  can create a magnetic short circuit in the first motor structure  654 . It is desirable to minimize the magnitude of the magnetic short, and thus, the first rigid support structure  680  contains as little material as necessary to maintain the structural integrity of the first motor structure  654 . This is not limited to minimizing the thickness of the rigid support structure. Changing from a continuous disc to a series of radial spokes (not shown) can also aid in minimizing the magnitude of the magnetic short. In another embodiment (not shown), the first rigid support structure  680  is replaced by an isolator that increases structural integrity while preventing a magnetic short circuit in the first motor structure  654 . The isolator can be fabricated from a non-magnetically permeable material. 
         [0216]    The second motor structure  656  includes a second magnetically permeable plate  682  that is positioned adjacent to the magnet  662 . A second top plate  683  is mechanically coupled to the second magnetically permeable plate  682 . The second magnetically permeable plate  682  is also mechanically coupled to a second pole plate  684  through a second rigid support member  686 . Similar to the first rigid support structure  680 , the second rigid support member  686  is fabricated to be as thin as possible while still maintaining structural integrity of the second motor structure  656 . Other magnet assemblies having other motor structures geometries are also possible. For example,  FIG. 20  illustrates a magnet assembly having a first and a second motor structure each having a permanent magnet. 
         [0217]    The electro-acoustic transducer  650  also includes a first  690  and second diaphragm  692  mounted to a common frame or basket  694 . In order to prevent the common frame  694  from shorting the magnetic circuit at the outer diameter, the frame can be fabricated in sections and/or the frame can be fabricated from non-magnetically permeable material. The electro-acoustic transducer  650  is shown generally circularly symmetric. However, circular symmetry is generally not required. For example, the first  690  and the second diaphragms  692  can be elliptical, oval, or any other desired shape. Additionally, the first  690  and the second diaphragms  692  can have the same or different surface areas. The motor structures  654 ,  656  can also be circularly symmetric or any other desired shape. 
         [0218]    The first diaphragm  690  is mechanically coupled to the common frame  694  through a first surround  696 . The first diaphragm  690  is also mechanically coupled to the first bobbin  668 . A first spider  698  couples the first bobbin  668  to the common frame  694 . The first bobbin  668  positions the first voice coil  664  in the first magnetic flux gap  658  of the first motor structure  654  in the magnet assembly  652 . 
         [0219]    The second diaphragm  692  is mechanically coupled to the common frame  694  through a second surround  700 . The second diaphragm  692  is also mechanically coupled to the second bobbin  672 . A second spider  702  couples the second bobbin  672  to the common frame  694 . The second bobbin  672  positions the second voice coil  670  in the second magnetic flux gap  660  of the second motor structure  656  in the magnet assembly  652 . 
         [0220]    In one embodiment, the permanent magnet  662  is magnetized in a direction that is normal to its flat surface. The north pole of the permanent magnet  662  can be located on its upper surface and the south pole can be located on its lower surface. However, the permanent magnet  662  can also be positioned with its poles reversed as long as the proper polarity is maintained to ensure that the first  664  and the second voice coils  670  move in the desired directions. 
         [0221]    Magnetic flux emanates from the top surface of the permanent magnet  662  and is conducted through the first magnetically permeable plate  676  and through the first top plate  677 . The magnetic flux then crosses through the first magnetic flux gap  658  and onto the first pole plate  678 . A center plate  704  conducts the magnetic flux through the second pole plate  684 . The magnetic flux then crosses through the second magnetic flux gap  660  and onto the second top plate  683 . The magnetic flux then propagates through the second magnetically permeable plate  682  and back to the permanent magnet  662 . Thus, a magnetic circuit is formed with magnetic flux passing through the first  658  and the second magnetic flux gaps  660 . The center plate  704  can also be fabricated from a magnetically permeable material, such as steel. 
         [0222]    If the rigid support structures  680 ,  686  are magnetically permeable, magnetic flux will travel from the magnet  662  to the first magnetically permeable plate  676  through the rigid support structure  680 , to the first pole plate  678 , to the center plate  704  to the second pole plate  684  through the rigid support structure  686  to the second magnetically permeable plate  682  and back to the magnet  662 . As previously described, this effect is minimized by keeping the rigid support structures  680 ,  686  as thin as possible. 
         [0223]    A first  706  and a second dust cap  708  can be positioned on the first  690  and the second diaphragms  692  to protect the first  654  and the second motor structures  656  from debris that can impact the operation of the transducer  650 . The dust caps  706 ,  708  are generally attached to the diaphragms  690 ,  692  with glue. 
         [0224]    The frame  694  can be formed using various techniques. For example, the frame  694  can be formed from a single piece of material or can be fabricated in multiple sections. As previously described, the frame  694  should be fabricated so as not to short circuit the motor structures  654 ,  656 . The frame  694  can include a first  710  and second set of vents  712  to allow acoustic energy and air from between the diaphragms  690 ,  692  to radiate out from the from the frame  694 . The set of vents  710 ,  712  can be distributed around the circumference of the frame  694 . The vents  710 ,  712  are not drawn to scale and the vents can be located in other areas around the frame  694 . The vents  710 ,  712  should be designed so that the fluid velocity is kept low enough to prevent the vents  710 ,  712  from making noise when the transducer  650  is operating. 
         [0225]    The first  664  and the second voice coils  670  include wire leads (not shown) that are routed up the first  668  and the second bobbin  672  and eventually lead to terminals (not shown) that are attached to the frame  694 . For example, the wire leads can be routed through the dust caps  706 ,  708 , onto each of the diaphragms  690 ,  692 , and braided flexible wires can lead out to terminals attached to the frame  694 . Alternatively, the braided flexible wires can be routed along the spiders  698 ,  702  and out to terminals attached to the frame  694 . Skilled artisans will appreciate that there are various methods for attaching electrical terminals to the first  664  and the second voice coils  670 . 
         [0226]    In operation, an external source (not shown) such as an amplifier includes output terminals that are electrically connected to the terminals (not shown) that are attached to the frame  694 . The terminals attached to the frame  694  can be connected to the same output terminal on the amplifier as long as the motor structures  654 ,  656  are properly configured so that the moving elements move in substantially opposite directions when an input signal is applied to the terminals. 
         [0227]    An input signal applied to the terminals causes the first voice coil  664  to move in one direction and causes the second voice coil  670  to move in the substantially opposite direction. This causes the first diaphragm  690  to move in an opposite direction from the second diaphragm  692 . The movement of the second diaphragm  692  reduces a mechanical force applied to the frame  694  by the movement of the first diaphragm  690 . Thus, a reduction in the mechanical force applied to the frame  694  by the movement of the first diaphragm  690  is observed. 
         [0228]      FIG. 20  is a cross-sectional view of an electro-acoustic transducer  750  according to another embodiment of the invention. The electro-acoustic transducer  750  is similar to the electro-acoustic transducer  650  of  FIG. 19 , but includes an alternate magnet assembly  752 . The magnet assembly  752  includes a first motor structure  754  having a slug magnet  756  and a second motor structure  758  having a ring magnet  760 . The first motor structure  754  includes a first magnetic gap  762  and the second motor structure  758  includes a second magnetic gap  764 . 
         [0229]    A first voice coil  766  is wound around a first bobbin  768 . The first voice coil  766  is positioned in the first magnetic flux gap  762 . A second voice coil  770  is wound around a second bobbin  772 . The second voice coil  770  is positioned in the second magnetic flux gap  764 . The second voice coil  770  substantially surrounds the first voice coil  766 , and thus, the second magnetic flux gap  764  substantially surrounds the first magnetic flux gap  762 . The two motor structures  754 ,  758  can have an over-hung or under-hung design. An isolator  774  can be positioned between the two motor structures  754 ,  758 . The isolator  774  can be fabricated from plastic or any other suitable non-magnetically permeable material. 
         [0230]    The first motor structure  754  includes a first top plate  776  that is mechanically coupled to the magnet  756 . A first back plate assembly  778  is also mechanically coupled to the magnet  756 . The second motor structure  758  includes a second top plate  782  that is mechanically coupled to the ring magnet  760 . A second back plate assembly  784  is also mechanically coupled to the ring magnet  760 . 
         [0231]    The electro-acoustic transducer  750  also includes a first  788  and second diaphragm  790  mounted to a common frame or basket  792 . The first diaphragm  788  is mechanically coupled to the first voice coil  766  through the first bobbin  768 . The second diaphragm  790  is mechanically coupled to the second voice coil  770  through the second bobbin  772 . The first  788  and the second diaphragms  790  can be elliptical, oval, or any other desired shape. Additionally, the first  788  and the second diaphragms  790  can have the same or different surface areas. The motor structures  754 ,  758  can also be circularly symmetric or any other desired shape. 
         [0232]    An input signal applied to the first  766  and the second voice coils  770  causes the first voice coil  766  to move in one direction and causes the second voice coil  770  to move in the substantially opposite direction. This causes the first diaphragm  788  to move in an opposite direction from the second diaphragm  790 . The movement of the second diaphragm  790  reduces a mechanical force applied to the frame  792  by the movement of the first diaphragm  788 . Thus, a reduction in the mechanical force applied to the frame  792  by the movement of the first diaphragm  788  is observed. 
         [0233]    It is evident that those skilled in the art may now make numerous uses and modifications of and departures from the specific apparatus and techniques herein disclosed without departing from the inventive concepts. For example, in general inverted and non-inverted motor structures can be used. Transducer orientations can be the same or inverted relative to each other. As long as the motion from the diaphragms of the transducers is such that force vectors are oriented to destructively interfere (i.e., the resultant magnitude of the force is reduced). Consequently, the invention is to be construed as embracing each and every novel feature and novel combination of features present in or possessed by the apparatus and techniques herein disclosed and limited only by the spirit and scope of the appended claims.