Abstract:
An audio speaker with at least two electric coils on opposite sides of at least one ferro-magnetic plate, the coils and plate forming a radial electro-magnet. A radial electro-magnet can offer many advantages in stereo loudspeakers. The coils are electrically driven in opposite directions. Multiple sets of two coils and intervening ferro-magnetic plate may be provided, adjacent sets being separated by a non-magnetic plate.

Description:
A novel orientation of two electric coils is described that allows easy generation of a radial magnetic field, which can be used to build a radial electro-magnet. A radial electro-magnet can be used in stereo loudspeakers to offer many advantages. This method can also be used in the production of permanent radial magnets and other applications that require radial magnetic fields. 
     BACKGROUND 
     Many applications require a radial magnetic field (one pointing between the center and the circumference of a circle, along a radial line). One significant use of a radial magnetic field is in the design of a standard loudspeaker. A radial magnetic field creates a magnetic flux through the voice coil windings and generates a force in response to a current through the voice coil which moves the voice coil and the attached sound surface. Current loudspeakers use standard ring magnets (which generate an axial magnetic field) and channel the magnetic field into a radial direction using ferro-magnetic materials. This channeling weakens the magnetic field and reduces the efficiency of the loudspeaker. An alternative system uses wedge-shaped magnets that are glued together to create a radial magnetic field, but this is a complex process and has limited magnetic field potential. 
     Loudspeakers can require high power to drive them for several reasons, including the ability to move fast and long distances. Existing systems use a voice coil attached to the sound generator (cone), moving in the magnetic field of a fixed magnet-generated gap. The fixed magnet is of limited strength, so the bulk of the power is generated by passing a high current through the voice coil. This has several negative effects. The coil wires must pass high current, forcing them to be thicker. The larger wire puts less turns within the magnetic field, decreasing the force generated to move the voice coil. The increased wire thickness increases the mass of the voice coil, which increases the momentum and opposes changes in motion, requiring more force to move the coil. Finally, the high power causes heat, which forces design complications. Additionally, the requirement for a high power drive signal increases the complexity and cost of the amplifier that must provide this signal. 
     Loudspeakers typically use an overhung design, meaning the voice coil is longer than the magnetic field gap it moves through. This is because the length of travel that the voice coil has (its throw) is defined by the length of the overlap between the voice coil and the magnetic field gap it travels in. It is difficult to build long magnetic field gaps that are both strong and have a linear magnetic field through the gap, so the throw is increased by making the voice coil longer. This is inefficient because the voice coil has many wasted turns that are not within the magnetic field at any given time, not generating any force but increasing the coil resistance and the coil weight, both wasting power. There are also non-linearities in the magnetic fields around the ends of the voice coil and the magnetic field gap, both of which can cause distortion. To avoid this, the voice coil must be further lengthened to keep the end zones out of the throw of the voice coil. An underhung design, one where the magnetic gap is longer and the voice coil is short, is more efficient because it allows the voice coil to be lighter. The throw is defined by the length of the magnetic gap. 
     BRIEF DESCRIPTION OF INVENTION 
     The system invented uses a novel orientation of two electric coils to generate the radial magnetic field required to build a radial electro-magnet. The same coil assembly could be used to build permanent radial magnets by generating the appropriate magnetic field during production of the magnets. 
     An electro-magnet is generally built by winding a coil around a ferro-magnetic rod. This configuration creates an axial electro-magnet with the poles at each end of the rod. It is not currently possible to easily build a radial electro-magnet, since this would require wrapping the coil around and through a doughnut shaped core. We discovered that two coils can be placed one on top and one below the core and if the current is passed in the opposite direction through each coil, the proper magnetic field is generated to create a radial electric field. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a graphical representation of a magnetic field around a single wire. 
         FIG. 2  is a graphical representation of a magnetic field around a cross section of a coil. 
         FIG. 3  is a perspective representation of a magnetic field around an axial electro-magnet. 
         FIG. 4  is a perspective view of a prior art radial electro magnet. 
         FIG. 5  is a perspective view of a radial electro-magnet according to the present invention. 
         FIG. 6  is a sectional view of the radial electro-magnet of  FIG. 5 , showing a single coil. 
         FIG. 7  is a cross sectional view of an assembly of stacked radial electro magnets. 
         FIG. 8  is a cross sectional view of an assembly of stacked radial electro-magnets with an adjacent voice coil shown in a plurality of positions. 
         FIG. 9  is a cross sectional view of an assembly of stacked radial electro-magnets with shaped channels. 
         FIG. 10  is a cross sectional view of an assembly of stacked radial electro-magnets with a continuous outer layer. 
         FIG. 11  is a perspective view of an overhung voice coil assembly (prior art). 
         FIG. 12  is a perspective view of an underhung voice coil assembly (prior art). 
         FIG. 13  is a cross sectional view of an assembly of stacked radial electro-magnets with the assembly inside a magnetic gap. 
         FIG. 14  is a cross sectional view of an assembly of stacked radial electro-magnets with the assembly outside a magnetic gap. 
         FIG. 15  is a cross sectional view of a speaker configuration (prior art). 
         FIG. 16   a  is a cross sectional view of a speaker configuration with a radial electro-magnet assembly inside a voice coil assembly. 
         FIG. 16   b  is a cross sectional view of a speaker configuration with a radial electro-magnet assembly outside a voice coil assembly. 
         FIG. 17  is a cross sectional view of a speaker configuration with a radial electro-magnet assembly inside a voice coil assembly and a radial electro-magnet outside the voice coil assembly. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     A magnetic field  4  is generated around a wire  2  with a current passing through it ( FIG. 1 ). When several wires are placed near each other (as in a coil), the magnetic fields are added and strengthened ( FIG. 2 ), and if a ferro-magnetic core is added, the fields are channeled through the core and further strengthened.  FIG. 2  shows a magnetic field  8  around a coil  6 , in cross section. The circle and cross inside the coil  6  follow the convention that the current flows towards the circle or dot (the head of the arrow) and away from the cross or “x” (the tail of the arrow). This principle is used to create the typical axial electro-magnet. While there are magnetic flux fields all around the windings, the strongest ones lie along the highest concentration of windings and through the core ( FIG. 3 ).  FIG. 3  illustrates an axial electro-magnet  10 . The magnetic field  12  runs perpendicular to the coil windings  10 , thus going through the length of the core  14 , which strengthens the field. 
     An open air coil generates a magnetic field with flux lines running through the center (as would be used with an axial electro-magnet) but also radially (since the flux field is around the wires). The radial field is naturally weaker since it is not concentrated by the geometry of the coil. A radial magnet needs the magnetic field to run along a radial line from the center to the circumference. Applying the same principles used to build an axial electro-magnet would require many coils wrapped through the core ( FIG. 4 ).  FIG. 4  illustrates traditional logic applied to build a radial electro-magnet  18 . The doughnut-shaped core  18  is divided into axial segments with wire  22  wrapped around each segment. Top current  24  flows in one direction (for example, clockwise), while bottom current  26  flows in the opposite direction (for example, counter clockwise) in the bottom windings (not shown). This is obviously difficult or impossible to do practically. 
     We discovered that by placing two coils  30 ,  32  ( FIG. 6 ), with the current running in opposite directions, next to each other, a radial field is generated between the coils ( FIG. 5 ). By further adding a doughnut shaped core  34  in this plane, the radial magnetic field becomes the significant one, rather than the axial field used in rod electro-magnets ( FIG. 6 ).  FIG. 5  shows a radial electro-magnet  28  using coils  30 ,  32  placed above and below the core  34 . Top current  36  flows in one direction (for example, clockwise), while bottom current  38  flows in the opposite direction (for example, counter clockwise). 
     This radial electro-magnet assembly  28  has an inherently limited thickness, because the thicker the core  34 , the lower the flux density is within the core, and the efficiency decreases. This non-ferro-magnetic layer is required because an opposing radial magnetic field is be generated between the assemblies, and the non-ferromagnetic core will not reinforce this field.  FIG. 7  shows stacked radial electro magnets  40 . Ferro-magnetic cores  42  provide strong fields  41 , while non-ferro-magnetic spacers  44  do not reinforce fields  43 . The gap  46  between assemblies can also be longer to further reduce this field. If this is applied to a system with a moving coil in the magnetic field, as long as the coil length is a multiple of the length of one assembly, all non-linearities within the field, including the reverse field, are constant through the throw of the coil and the magnetic field appears linear ( FIG. 5 ). In this way, theoretically endless radial magnetic fields can be built. As shown in  FIG. 8 , the ferro-magnetic cores  42  may comprise flanges  48 . A magnetic assembly limit length, shown in  FIG. 8 , may be defined as the linear axial distance between two similar features, for example, the left-hand edge of a flange to the left-hand edge of an adjacent flange. In  FIG. 8 , voice coils  52  are shown in various positions. Notice that for all the various voice coil positions, the voice coil  52 E overlaps exactly one magnetic assembly unit. In this way, any non-linearity in the magnetic field within the assembly unit are integrated out. The magnetic core pieces of the electro-magnet can be shaped to improve the linearity of the magnetic field throughout the gap ( FIG. 9 ) and can even be designed to be continuous ( FIG. 10 ).  FIG. 9  shows stacked magnetic assemblies with shaped channels  54  to enhance field linearity. Beams can take a variety of shapes depending on the configuration, all intended to linearize the magnetic field.  FIG. 10  shows stacked magnetic assemblies with a continuous outer layer  56 . 
     This can be applied to several fields, but specifically to the stereo loudspeaker field the opportunities are numerous. 
     There is an inherent conflict within the design of the traditional voice-coil assembly. Loudspeakers sometimes require great force to move the sound generator (cone), especially within the lower frequency ranges where long throws are requited to generate the large sound pressures demanded for intense volume. The conflict is that in order to increase the force to move the voice coil, the current running through the voice coil must be increased. This is because the force generated to move the voice coil is defined by F=itB, where F is the moving force, i is the current through the coil, t is the number of turns within the magnetic field and B is the strength of the magnetic field. In order to increase F, i, t or B must be increased. B, the field strength, would be the logical choice, but it is currently generated by fixed magnets and channeling magnetics that are limited in power and efficiency. t can only be increased by lengthening the magnetic field gap (not practical with the current magnetics used) or by decreasing the thickness of the wire or wrapping multiple layers. The wire thickness cannot be decreased without increasing the resistance (opposing current, i, and decreasing the ultimate F) and reducing the current capacity of the wire (decreasing the maximum i that the wire can carry and again decreasing the ultimate F). Multiple layers dictate an increase in the width of the magnetic field gap, decreasing the strength of the field and, again, decreasing the force generated. This also increases the mass of the voice coil, yet again opposing the motion and requiring more force. Increasing i forces the wire to be thicker, increasing the weight (working against the moving F) and reducing t (once again decreasing the ultimate F). Increasing i also creates power dissipation and heating issues for the voice coil, as well as increasing demands on the amplifier driving the loudspeaker. Using the radial electro-magnet, the magnetic field can be strengthened allowing the voice coil to carry less current. Increasing the strength of the magnetic field simply requires a large DC current, which can be passed through very thick wires. The same formulae govern this power, and similar design constraints exist, but since this is not a moving part, the issues of weight and momentum are eliminated. Additionally, this signal is simply DC, since it is not the driving signal, so the fidelity requirements are much easier to address. The current through the voice coil can be decreased, allowing thinner wire which can both decrease the weight of the voice coil and increase the number of turns within the magnetic field (t), serving to further increase the force generated. The lower power also reduces the power dissipation issues (heating) for the voice coil as well as relieving the design requirements for high power amplifiers to drive the loudspeaker. 
     Since the magnetic field is generated electrically, its field strength can be varied, if desired. In fact, an interesting loudspeaker could be built where the driving current is fed through the magnetic field and the current through the voice coil is fixed. The current would have to be high, but the constraints are different since this coil does not have to move. It can use big, thick and heavy wire without the negative effects on the moving voice coil. In reality, a combination of the two would be useful, such as when large and fast movements are required of the vaicc coil, a combination of signals could be sent to both the voice coil and the electro-magnet coils to facilitate this movement. 
     With this new ability to build long radial magnetic fields, the benefits of a true underhung loudspeaker can finally be achieved. An overhung voice coil assembly is currently the most common configuration because generating long, consistent magnetic gaps is not possible or practical. The overhung design uses the length of the voice coil to drive the speaker&#39;s throw  66  ( FIG. 11 ). An overhung voice coil assembly comprises a voice coil former  60  supporting a voice coil  62  and surrounded by a relatively narrow circular magnet  64 . An underhung design  68  ( FIG. 12 ), is more efficient because it lets the voice coil  70  be short, reducing weight and coil resistance, both of which will increase speaker efficiency. The underhung design requires a long magnetic field gap, which is possible with this invention. The throw  72  is defined by the length of the circular magnet  74 . 
     The magnetic gap can be placed outside the electro-magnet  40   a  ( FIG. 13 ) or within the electro-magnet  40   b  ( FIG. 14 ).  FIG. 13  is an example of a magnetic gap  76  generated with the electro-magnet  40  inside the gap. The large arrows  78  represent induced magnetic fields, while the small arrows  80  show magnetic field channeling. A center rod  82  and a pole piece  84  are also shown.  FIG. 14  is an example of a magnetic gap  86  generated with the electro-magnet  40  outside the gap. The large arrows  88  represent induced magnetic fields, while the small arrows  90  show magnetic field channeling. An outer tube  92  and a pole piece  94  are also shown. In either case, the pole piece and the return path (center rod or outer tube) enhance the magnetic field through their proximity to the electro-magnet coils. The center rod forms several traditional axial rod magnets from the coils wrapped around them, and the outer tube strengthens the magnetic field in the same, although less obvious or common way. In this way, even the channeling magnetics are not entirely passive. 
     A traditional speaker assembly  94  uses an axial magnet  96  and channels the magnetic field to gap  98  ( FIG. 15 ). The speaker assembly has a movable voice coil assembly  100  comprising a voice coil former  102  with circumferential voice coil windings  104 . 
     Using this radial electro-magnet  40 , or permanent radial magnets, the assembly can use the radial magnets  40   a ,  40   b  either inside the voice coil  104   a  ( FIG. 16   a ), or outside the voice coil  104   b  ( FIG. 16   b ). Either assembly uses magnetic field channeling  106  to create one pole of the magnetic field gap  76 ,  86 . Placing the magnetics outside the voice coil assembly  100  allows the voice coil to be smaller diameter but requires a larger magnetic assembly  40   b . There will be advantages for either configuration. A stronger magnetic field can be generated by using two magnetic assemblies  40   a ,  40   b , one  40   a  inside and one  40   b  outside the voice coil assembly  100  ( FIG. 17 ). With this configuration, no magnetic field channel is required as the gap is generated between two magnets. The components  106  shown are only to hold the assembly together. 
     Key Ideas 
     A configuration to build a radial electro-magnet. 
     A coil configuration to build a permanent radial magnet. 
     A method to build a long radial electro-magnet (stacking assemblies). 
     A loudspeaker using radial magnets (electro or permanent, either inside the voice coil, outside the voice coil, or both). 
     A method to build a very strong magnetic field, allowing the voice coil power to be reduced. This provides several advantages over current systems: 
     The static magnetic field only requires high DC current, while the speaker drive signal can be low power. This simplifies amplifier design and reduces cost. High power through the non-moving electro-magnet can use thicker wire without deleterious effect. 
     The lower power through the voice coil allows thinner wire, reducing the weight of the voice coil and improving efficiency. 
     The lower power through the voice coil allows thinner wire, placing more windings within the magnetic field and increasing force and efficiency. 
     weight of the voice coil and improving efficiency. 
     A long-throw loudspeaker using a short and/or low power voice coil (for low weight and/or power) and an underhung design. 
     A loudspeaker using an electro-magnet to generate the magnetic field which allows the field to be varied with the drive signal as well as, or possibly instead of, the voice coil. Since this part does not move, some of the design constraints with high-power voice coils are eliminated. In practice, drive signals to both the voice coil and the electro-magnet coils is interesting.