Abstract:
A voice-coil transducer includes two radially concentric magnets, a voice-coil located in the gap between the inner and outer magnets, and a diaphragm coupled to the voice-coil. An audio loudspeaker includes the voice-coil transducer with two radially concentric magnets, a voice-coil located within the gap between the inner and outer magnets, a diaphragm coupled to the voice-coil in order to create sounds from the voice-coil, and a chassis to support the magnets, voice-coil, and diaphragm.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates to audio transducers. More particularly, this invention relates to lightweight, audio transducers. 
     2. Related Art 
     Electrodynamic loudspeakers include a diaphragm connected to a voice-coil. The voice-coil is positioned in an air gap between the poles of a magnet. The magnets produce magnetic flux in the air gap. These magnets are typically permanent magnets and are used in a magnetic circuit of ferromagnetic material to direct the flux produced by the permanent magnet into the air gap. 
     The voice-coil is placed in the air gap with its conductors wound substantially cylindrically so as to be placed perpendicular to the main component of the magnetic flux in the air gap. The coil is then connected mechanically to a loudspeaker diaphragm that is driven or vibrated by the axial motion of the voice-coil produced by the motor force on the voice-coil when it is connected to an audio amplifier. The coil is referred to the “voice” coil because, in loudspeakers or similar electromechanical transducers, the frequency range of interest is in the extended range of the human voice. 
     The voice-coil is normally connected to an audio amplifier of some type that produces a current in the voice-coil that is a function of the electrical signal to be transformed by the loudspeaker into an audible, sub-audible or ultrasonic pressure variation. The voice-coil is intended to carry a current in a direction that is substantially perpendicular to the direction of the lines of magnetic flux produced by the permanent magnet. The magnetic structure is often arranged to provide cylindrical symmetry with an annular air gap in which the magnet flux lines are directed radially with respect to the axis of cylindrical symmetry of the loudspeaker. 
     Permanent-magnet electro-dynamic loudspeakers employ a diaphragm that is vibrated by an electromechanical drive. The drive generally includes a motor structure comprised of one or more magnets plus ferrous material, and a voice-coil with an electrical signal passed through the voice-coil. The interaction between the current passing through the voice-coil and the magnetic field produced by the permanent magnet causes the voice-coil to oscillate in accordance with the electrical signal and, in turn, drives the diaphragm and produces sound. 
     In loudspeaker magnet systems, ferrous pole material is employed to create the gap and to guide the magnetic field, i.e., create the magnetic circuit. An axially magnetized magnet is positioned in a ferrous cylinder so that one pole of the magnet is in contact with bottom of the cylinder. The diameter of the magnet is less than that of the cylinder such that there is created an annular gap between the lateral sides of the magnet and interior walls of the cylinder. A second ferrous material, such as a disk that is roughly the same diameter as the magnet, is placed on top of the magnet so as to be in contact with the opposing pole of the magnet. The cylinder focuses the magnetic flux from the magnetic pole with which it is in contact and disk. One or multiple axially magnetized magnets may be included in such systems. 
     These ferrous materials may contribute a significant portion of the total mass of the system. Ferrous systems also may increase voice-coil inductance. Thus, as frequency increases, voice-coil inductance increases, resulting in reduced speaker output. Further, in operation, the resistance of the conductive material of the voice-coil causes the production of heat in the voice-coil or winding. The presence of ferromagnetic material may also contribute to an increased production of heat. 
     The problems produced by heat generation are further compounded by temperature-induced resistance, commonly referred to as power compression. As the temperature of the voice-coil increases, the DC resistance of copper or aluminum conductors or wires used in the voice-coil also increases. For example, a copper wire voice-coil that has a resistance of six ohms at room temperature has a resistance of twelve ohms at 270 degree C. (520 degree F.) At higher temperatures, power input is converted mostly into additional heat rather than sound, thereby seriously reducing loudspeaker efficiency. 
     Thus, heat production is a major determinant of loudspeaker maximum sound pressure output. Thus, devices may be limited in their maximum sound pressure because of the heat they generate. In a typical single voice-coil design using a ceramic magnet, the loudspeaker is very large and a heat sink is usually not employed. As such, because the driver must not overheat, the maximum allowable temperature limits the input power capacity of the loudspeaker. A common approach in the design of high power professional loudspeakers consists of simply making the motor structure large enough to dissipate the heat generated in the voice-coil. Producing a high power loudspeaker in this way results in a very large and heavy loudspeaker with a large motor structure. These large and heavy loudspeakers may not be feasible for use in vehicular applications due to weight and space limitations. 
     Thus, there is a need for loudspeaker systems that dissipate the heat generated by the voice-coil, thus, improving efficiency and producing greater power output. It may also be desirable to have a magnetic field system that is constant in a region and drops to a low value outside the region. Therefore, a need exists for a magnetic field system that can produce a desired magnetic field distribution without the use of any ferrous pole material. 
     SUMMARY 
     This invention provides a voice-coil transducer, which may include two radially concentric and radially polarized magnets, one magnet contained within the other. A voice-coil may be located within the gap between the inner and outer annular-shaped magnets. The voice-coil may be coupled to a diaphragm for generating sound through a loudspeaker. 
     An audio loudspeaker, which may include two radially concentric and radially polarized magnets, one magnet contained within the other also is provided A voice-coil including a former and windings may be located within the gap between the inner and outer annular-shaped magnets. The voice-coil may be coupled to a diaphragm for generating sound through a loudspeaker. The magnets, voice-coil, and diaphragm may be supported by a chassis which may also serve as a heat sink for the magnets. 
     The application presents an audio loudspeaker, which may include two radially concentric and radially polarized magnets, one magnet contained within the other. Alternatively, a number of voice-coils composed of a former and windings may be located within the gaps formed by the annular-shaped magnets, such as within the inner diameter of the inner magnet, or outside of the outer diameter of the outer magnet. The voice-coils may be coupled to a diaphragm for generating sound through a loudspeaker. The magnets, voice-coils, and diaphragm may be supported by a chassis which also serves as a heat sink for the magnets. 
     Other systems, methods, features and advantages of the invention will be, or will become, apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the following claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be better understood with reference to the following drawings and description. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like referenced numerals designate corresponding parts throughout the different views. 
         FIG. 1  is a cross-sectional view of a radial concentric magnet system for an audio loudspeaker. 
         FIG. 2  is a top-down view of the radial concentric magnets including a voice-coil. 
         FIG. 3  is the view of  FIG. 2  with a cut-line indicating a cross-sectional view. 
         FIG. 4  is a cross-sectional view of  FIG. 3  indicating the magnetic flux. 
         FIG. 5  is an audio loudspeaker with a double voice-coil design. 
         FIG. 6  is an audio loudspeaker with a triple voice-coil design. 
         FIG. 7  is a dual radial magnet design with a ferrous return path. 
         FIG. 8  is a chart comparing the performance of ring motor designs. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a cross-sectional diagram of a loudspeaker. The loudspeaker  100  includes a loudspeaker diaphragm  102 , a dome  104 , a voice-coil  106 , and magnet system  108 . The voice-coil  106  includes former  110  and windings  112 . The voice-coil windings  112  are wound cylindrically around former  110 . The loudspeaker diaphragm  102  is held within a chassis  114  by a suspension system provided by surround  116  and spider  118 . Magnet system  108  may include two generally circular or annular-shaped ring magnets arranged concentrically with regard to each other. The loudspeaker may be cylindrically symmetric about the axis of symmetry  180 . 
     As shown in  FIG. 2 , inner magnet  220  may be positioned within the interior of the former  210  and outer magnet  222  may be positioned exterior of the former  210  to form two concentric rings. Outer magnet  222  may be configured and adapted to at least partially encircle voice-coil former  210 , voice-coil  206 , and inner magnet  220 . Thus, outer magnet  222  may be a disk or annular-shaped ring having a central hole  224 . Inner magnet  220  may be configured to fit within the central hole  224  of outer magnet  222  and also may be disk or ring shaped. For example, outer magnet  222  and inner magnet  220  may be positioned as two concentric rings as shown in  FIG. 2 . The concentric design of the inner and outer magnets ( 220  and  222 ) increases the strength of the magnetic field in the vicinity of the voice-coil  206  approximately by a factor of two over a single magnet design, which results in higher output by the loudspeaker. 
     The interior edge  226  of the central hole  224  of outer magnet  222  may be positioned in close, but non-contacting, proximity to the voice-coil  206  and voice-coil former  210 . The outer edge  228  of inner magnet  220  may be positioned in close, but non-contacting, proximity to the interior surface  230  of former  210 . In this way, voice-coil  206  and former  210  are positioned in a gap  232  between the interior edge  226  of the central hole  224  of outer magnet  222  and the outer edge  228  of inner magnet  220 . The gap  232  may be from 1 mm to 10 mm in width. In one example, the gap may be from about 1.5 mm to about 5 mm in width. The outer diameter of the outer magnet  222  may be between about 25 cm and about 450 cm. In addition, the gap between inner magnet  220  and outer magnet  222  may be filled with a magnetic solution, such as a colloidal solution of oil and magnetic particles. 
     Alternatively, multiple voice-coils may be used with the concentric magnet design. For example, the loudspeaker may comprise a double voice-coil transducer as depicted in  FIG. 5 , where there is one voice-coil  505  located within the inner diameter of the inner magnet  520  along the axis of symmetry of the inner magnet  520 , and a second voice-coil  506  located in the gap between the inner and outer magnets ( 520  and  522 ), as described earlier. The system may also include a triple voice-coil transducer as depicted in  FIG. 6 , where there are two voice-coils ( 605  and  606 ) located as in the double voice-coil transducer depicted in  FIG. 5 , along with a third voice-coil  607  located outside the outer magnet  622 , where the concentric magnet system is wholly contained within the diameter of the third voice-coil  607 . 
       FIG. 4  depicts the cross-section of the inner and outer magnets ( 420  and  422 ) as represented by the cut-line A-A in  FIG. 3 . Inner magnet  420  and outer magnet  422  may be radially magnetized such that the interior edge  426  of outer magnet  422  and the interior edge  434  of inner magnet  420  may be of one polarity and the outer edges  428  and  436  are of the opposite polarity to the inner edges. In this way, when inner magnet  420  is positioned within central hole  424  of outer magnet  422 , the polarity of the outer edge  428  of inner magnet  420  is of the opposite polarity of the inner edge  426  of the outer magnet  422  as shown in  FIG. 4 . The inner and outer magnets ( 420  and  422 ) may be made of neodymium, ferrite, or other common magnetic materials known in the art. The inner and outer magnets ( 420  and  422 ) may also be composed of permanent magnetic materials. 
     The magnetic flux between the inner and outer magnets ( 420  and  422 ) may be directed approximately radially through the outer magnet  422 , radially through the air gap  432  to inner magnet  420 . The magnetic flux may be constant in a region including the gap  432  and dropping to a low value outside the region including the gap  432 . 
     Inner magnet  120  and outer magnet  122  may be held in place by the chassis  114 . The chassis  114  also may act as a heat sink for the loudspeaker by allowing heat to flow from the outer magnet into the chassis. The chassis  114  may be formed of any suitable material. For example, the chassis may be formed of aluminum, steel, plastic, or composite. 
     Former  210 , which may be attached to the diaphragm, may extend from the diaphragm into the gap  232 . The former may be constructed of a thermally conductive material for conducting heat away from the voice-coil. Airflow through the gap  232  removes heat from the former  210  through convective heat transfer. The former  210  may be made of any suitable material such as aluminum or copper, as well as plastics, paper, or composite. Former  210  may be a cylindrical tube having tube walls from about 0.05 mm to about 5.00 mm thick. Voice-coil  206  may be wound around the former  210  and extends in the gap  232 . Voice-coil  206  may be any suitable material, for example copper or aluminum wire and is attached to the former  210  through a conventional adhesive. Voice-coil  206  may be from about 3 mm to about 100 mm in length. The preferred number of times the voice-coil wire may be wound around the former depends upon the size of the loudspeaker. 
       FIG. 7  present a dual radial ring motor design with a ferrous return path  780 . There are three main functions of ferrous material in a magnetic circuit. One function is to focus the field (make it stronger in a smaller area). Permanent magnets operate at higher field levels when there is a high permeability path between their north and south poles. Another function for a ferrous return path  780  is to provide that path. The force on a moving current is perpendicular to both the direction of the current flow and the direction of the magnetic field. The cylindrical geometry of the voice-coil  206  requires a radial field to provide axial force. It has been easier to make magnets with an axial orientation. Ferrous materials are used to adjust the field into an axial orientation. 
     In  FIG. 7 , a dual radial voice-coil transducer  700  including an inner magnet  720 , an outer magnet  722 , voice-coil  706 , and a ferrous return path  780  are depicted. The ferrous return path  780  connects the first, inner magnet  720  to the second, outer magnet  722  in a region located exterior to the gap between the outer diameter  728  of the second, inner magnet  720  and the inner diameter  726  of the first, outer magnet  722 . The ferrous return path  780  may be composed of a magnetic material, such as steel, or permanent magnetic materials. The dual radial ring design may also be incorporated into voice-coil transducers with multiple voice-coils, such as those depicted in  FIGS. 5 and 6 . For automotive applications, a ferrous return path  780  is needed in a dual radial design because of the required magnetic field strength. 
     The loudspeaker diaphragm of the invention may be incorporated into any loudspeaker, including sub woofers, bass, and midrange loudspeakers. The diaphragms may also be suitable for use in loudspeakers for automobile applications. In automotive applications, the weight of a loudspeaker is an important design parameter. By avoiding the use of a steel pole piece in the magnet design, the concentric magnet design may reduce motor weight up to 60%. 
     The concentric radial magnet design also may enhance the linearity of the system performance by providing a region where the voice-coil-field interaction is approximately constant with no variation over the region. The absence of a steel pole piece in the concentric magnet design also may reduce the impedance of the system, as there is no ferrous metal to affect the voice-coil inductance. Ideally, a loudspeaker reproduces sound in proportion to the voltage supplied to it regardless of voltage amplitude and frequency. However, the presence of ferrous materials in the voice-coil will change this response by increasing the inductance, and therefore impedance, of the system. The concentric magnet design of the application removes this source of impedance. 
       FIG. 8  presents a chart depicting the magnetic field strength performance of a dual radial ring motor design compared to a dual axial ring motor design. The dual radial design provides a higher magnetic field strength at the center of the gap (indicated by 0.04085 along the x-axis of the graph) compared to the dual axial ring motor design. The weight characteristics of the dual radial ring design are higher than that of the dual axial ring design, which may present some design considerations. 
     In addition, the concentric magnet design may allow the system to run cooler than a system with a ferrous pole piece, because the concentric magnet system may be placed closer to a heat sink for heat dissipation. In a standard, non-concentric magnet system with a steel pole piece, the heat produced by the voice-coil  106  is dissipated through the steel. By avoiding the need for a magnetic material pole piece, a non-magnetic material with higher heat conduction capability may be used in the chassis  114 . For example, the frame may be composed of aluminum, which is five times more heat conductive than steel and lighter as well. 
     While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible within the scope of the invention. Accordingly, the invention is not to be restricted except in light of the attached claims and their equivalents.