Patent Publication Number: US-8121337-B2

Title: Free air magnetic circuit and speaker

Description:
FIELD OF THE INVENTION 
     The invention generally relates to speaker systems, and more particularly to a magnetic circuit for a speaker system in which no pole piece is included. 
     BACKGROUND OF THE INVENTION 
     Speakers convert electrical signals to sound via the use of drivers. A conventional driver  10  for a speaker is shown in  FIGS. 1 ,  2 , and  13 . Common drivers  10  include a basket or frame to which is attached a lightweight diaphragm  22 . Attached to the diaphragm  22  is a wire coil  20  wound round a pole piece  12  that sits at the center of a magnet ring  14  sandwiched between a top plate  16  and a back plate  18 . 
     The magnet ring  14  is usually polarized in the direction of the ring&#39;s thickness, e.g., with the north pole  80  adjacent the top plate  16  and the south pole  90  adjacent the back plate  18 . As shown in  FIG. 3 , the magnet ring  14  generates a magnetic flux  30  that is dense in the gap  24  between the top of the pole piece  12  and the top plate  16 . The wire coil  20  is located in this gap  24  between the pole piece  12  and the magnet layer  14  and metal plates  16 ,  18 . An electrical signal is applied to the wire coil  20 , which in turn makes the wire coil  20  an electromagnet and produces a magnetic field. The wire coil&#39;s magnetic field interacts with the magnet and metallic layers&#39; field so as to generate a mechanical force that causes the wire coil  20  to move up or down, depending on the signal sent to the wire coil  20 . The motion of the wire coil  20  causes the diaphragm  22  to move or vibrate, which vibration produces audible sounds. 
     The conventional driver  10  has its short comings. For example, conventional drivers  10  include a back surface that generally is integrated with the back plate  18  of the magnetic circuit. When the diaphragm  22  moves, the air and sound waves  70  formed behind the diaphragm  22  are trapped by the back surface  18 , as shown in  FIG. 13 . This causes distortion of the sound and inhibits the free motion of the diaphragm  22 . Further, speakers with conventional drivers  10  are generally thick, e.g., on the order of two to three inches. This makes these speakers less conducive to use in environments in which thinner speakers would be more desirable, such as when mounting speakers in walls or in automobiles. 
     Another large problem with conventional drivers  10  is the high level of inductance due to the use of the ferrous metal pole piece  12 . Any wire coil  20  with a current will create inductance, and the placement of a larger metal pole piece  12  inside the coil  20  creates an even larger amount of inductance. In the speaker, the higher the level of inductance, the greater the distortion in the sound quality. At higher frequencies, inductance is even more a problem. 
     The use of the ferrous metal pole piece  12  also leads to distortions in the loudspeaker&#39;s transducer for other reasons. For example, using a pole piece  12  causes dynamic non-linearity in a moving coil transducer, such as that of the wire coil  20  when in operation. Additionally, movement of the wire coil  20  along the pole piece  12  causes eddy currents and flux modulation. The eddy currents produce heat, which alters the resistance of the wire coil  20 . Accordingly, the inclusion of a ferrous metal pole piece  12  in the conventional driver  10  leads to much undesirable distortion in the produced sound. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present free air magnetic circuit and speaker provide for a driver that does not require a pole piece and does not utilize a backing behind the diaphragm. Accordingly, the speaker may be thinner and has reduced amounts of acoustic and electrical distortion due to the essential elimination of sound reflections behind the diaphragm and the reduction of eddy currents, inductance, and heat production. Thus, the sound of the speaker is less distorted than the sound produced by the conventional speaker. 
     In particular, the free air magnetic circuit and speaker includes a magnet layer sandwiched between two metal layers, i.e., between a top plate and a back plate. A gap is located in the center of the layers such that each layer surrounds the gap. The gap is designed to receive the wire coil of the speaker in the same manner that a wire coil is placed in the gap of a conventional magnetic circuit for a speaker. However, the gap of the free air magnetic circuit does not include a pole piece nor is there a backing behind the diaphragm that is attached to the wire voice coil. 
     It is preferred that the top plate and bottom plate include tapering portions that taper toward the gap such that each later is thinner in thickness closest to the gap than they are at their peripheral edges. This concentrates the magnetic flux in a narrower area within the gap, allowing for increased efficiency. In other embodiments, however, the top and bottom plates do not include the tapering portions such that they are of consistent thickness both near the gap and at their peripheral edges. 
     Further, it is preferred that the magnet layer be wider and extend past both the top and back plates. This discourages flux leakage between the top and back plates at the peripheral edges of each. In other embodiments, however, the magnet layer is no wider than the top and back plates. 
     Still further, it is preferred that the each of the top plate, magnet layer, and back plate are ring shaped. However, in other embodiments, any or each of the three layers are otherwise shaped, as in a rectangle, triangle, rhombus, parallelogram, oval, star, pentagon, hexagon, octagon, or other polygon. 
     Moreover, it is preferred that a unitary, solid magnet layer be utilized between the top and back plates. However, in other embodiments, a segmented magnet layer are utilized. The magnet segments comprising the segmented magnet layer may be wedge shaped, or shaped in any other shape between the top and back plates. Additionally, any number of magnet segments may be utilized to comprise the segmented magnet layer. 
     In any embodiment, because the free air magnetic circuit does not include the pole piece in the gap, the level of inductance, eddy currents, heat production, and therefore sound distortion, is decreased or eliminated. Additionally, because there is no backing behind the diaphragm, sound waves are not trapped behind the diaphragm, which also decreases the amount of acoustic sound distortion in the speaker containing this free air magnetic circuit. 
     The purpose of the foregoing Summary is to enable the public, and especially the scientists, engineers, and practitioners in the art who are not familiar with patent or legal terms or phraseology, to determine quickly from a cursory inspection, the nature and essence of the technical disclosure of the application. The Summary is neither intended to define the invention of the application, which is measured by the claims, nor is it intended to be limiting as to the scope of the invention in any way. 
     Still other features and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description describing preferred embodiments of the invention, simply by way of illustration of the best mode contemplated by carrying out my invention. As will be realized, the invention is capable of modification in various obvious respects all without departing from the invention. Accordingly, the drawings and description of the preferred embodiments are to be regarded as illustrative in nature, and not as restrictive in nature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top view of a conventional magnetic circuit for a conventional speaker without a wire coil in place in the gap. 
         FIG. 2  is an elevation sectional view of a conventional magnetic circuit for a conventional speaker with a wire coil in place in the gap. 
         FIG. 3  is a magnetic flux distribution map of the conventional magnetic circuit shown in  FIGS. 1 and 2 . 
         FIG. 4  is a top view of a free air magnetic circuit for a speaker according to a first embodiment. 
         FIG. 5A  is an elevation sectional view of a free air magnetic circuit for a speaker according to the first embodiment that depicts the use of a dome-shaped diaphragm. 
         FIG. 5B  is an elevation sectional view of a free air magnetic circuit for a speaker according to the first embodiment that depicts the use of a cone-shaped diaphragm. 
         FIG. 5C  is an elevation sectional view of a free air magnetic circuit for a speaker according to the first embodiment that depicts the use of a flat-shaped diaphragm. 
         FIG. 6  is a magnetic flux distribution map of a free air magnetic circuit for a speaker according to the first embodiment. 
         FIG. 7  is a side perspective view of a free air magnetic circuit for a speaker according to the first embodiment. 
         FIG. 8  is a side perspective view of a cross section of a free air magnetic circuit for a speaker according to the first embodiment. 
         FIG. 9  is a top view of a free air magnetic circuit for a speaker according to a second embodiment. 
         FIG. 10  is an elevation sectional view of a free air magnetic circuit for a speaker according to the second embodiment with a wire coil in place in the gap. 
         FIG. 11  is a top view of the magnet layer of a free air magnetic circuit for a speaker according to a third embodiment. 
         FIG. 12  is a top view of a free air magnetic circuit for a speaker according to a fourth embodiment. 
         FIG. 13  is an elevation sectional view of a conventional magnetic circuit for a conventional speaker depicting the sound waves under the diaphragm. 
         FIG. 14  is an elevation sectional view of a free air magnetic circuit for a speaker according to the first embodiment depicting the sound waves under the diaphragm. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     While the free air magnetic circuit and speaker is susceptible of various modifications and alternative constructions, certain illustrated embodiments thereof have been shown in the drawings and will be described below in detail. It should be understood, however, that there is no intention to limit the invention to the specific form disclosed, but, on the contrary, the invention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the invention as defined in the claims. 
     In the following description and in the figures, like elements are identified with like reference numerals. The use of “or” indicates a non-exclusive alternative without limitation unless otherwise noted. The use of “including” means “including, but not limited to,” unless otherwise noted. 
       FIGS. 4 through 8  depict a first embodiment of the free air magnetic circuit  50  for inclusion in a speaker. Like a conventional magnetic circuit  10 , the free air magnetic circuit  50  includes a top plate  16 , a back plate  18 , and a magnet layer  14  in between. In some embodiments, the top plate  16 , back plate  18 , and magnet layer  14  are all in the form of a ring such that each layer  16 ,  18 ,  14  has a peripheral edge forming the outer circumference of each ring and an inner edge forming the inner circumference of each ring. Other embodiments utilize layers of different shapes. 
     According to the first embodiment, between the inner edges of each layer  16 ,  18 ,  14  is a gap  24 . As with a conventional magnetic circuit  10 , the gap  24  of the free air magnetic circuit  50  is configured to receive a wire coil  20  that is attached to a diaphragm  22 . However, unlike a conventional magnetic circuit  10 , the gap  24  of the free air magnetic circuit  50  does not contain a pole piece  12 . Additionally, when the wire coil  20  is in place with its attached diaphragm  22 , there is no back plate behind the diaphragm  22 . The absence of the pole piece  12  reduces the level of inductance in the circuit  50 , and the absence of the backing eliminates the back trapping of sound and air waves  70 , as shown by  FIG. 14  in comparison with  FIG. 13 , which in turn reduces sound distortion as compared to that experienced in the conventional magnetic circuit  10 . 
     As shown in  FIGS. 5A ,  5 B, and  5 C, according to the first embodiment, the top plate  16  includes a top tapering portion  66  near the top plate&#39;s interior edge while the back plate  18  includes a bottom tapering portion  68  near the back plate&#39;s interior edge. This focuses the magnetic flux into a narrower area in the gap  24 . A magnetic circuit according to the first embodiment may be used with diaphragms of a number of shapes, including a dome-shaped diaphragm  22  (shown in  FIG. 5A ), a cone-shaped diaphragm  22 ′ (shown in  FIG. 5B ), or a flat-shaped diaphragm  22 ″ (shown in  FIG. 5C ). 
     According to the first embodiment shown in  FIGS. 4 through 8  the magnet layer  14  is wider than the top plate  16  and the back plate  18  so that the magnet layer  14  sticks out past the top and back plates  16 ,  18 . Accordingly, the magnet layer  14  is configured to discourage magnetic flux passing between the top plate  16  and the back plate  18 . 
     It is also preferred that the magnet layer  14  of the free air magnetic circuit  50  be a strong magnet so as to maximize the magnetic flux  30  passing through the gap  24 . This magnetic flux  30  is depicted in  FIGS. 8 and 6 .  FIG. 6 , in particular, maps an approximate distribution of the magnetic flux  30  of the free air magnetic circuit  50 . As shown, the magnetic flux  30  is dense in the gap  24 , particularly in an area near to the interior edge of each of the layers  16 ,  14 ,  18 . It is in this area of dense magnetic flux  30  that the wire coil  20  is to be placed in the speaker. 
       FIG. 7  and  FIG. 8  show the first embodiment of the free air magnetic circuit  50  from a perspective view. Again, it is preferred that the interior edges of the top plate  16  and back plate  18  taper toward the gap  24 . 
     According to the first embodiment, the magnet layer  14  be wider, i.e., configured so that, the circumference of its peripheral edge is greater than the circumference of the peripheral edge of both the top plate  16  and back plate  18 . 
     It is preferred that the top plate  16  and back plate  18  be made of a ferrous-containing steel. However, any material that is affected by magnetic forces may be alternatively used. 
     A second embodiment of the free air magnetic circuit  50  is shown in  FIGS. 9 and 10 . According to the second embodiment, the top plate  16  and back plate  18  do not contain tapering portions near in proximity of the gap  24 . In addition, the magnet layer  14  is not wider than either the top plate  16  or the back plate  18 . That is, the circumference of the peripheral edge of the magnet layer  14  is equal to the circumference of the peripheral edge of the top plate  16  and to the circumference of the peripheral edge of the back plate  18 . 
     The magnet layer  14  of a free air magnetic circuit  50  according to a third embodiment is shown in  FIG. 11 . According to the third embodiment, the magnet layer  14  is segmented, such that the magnet layer  14  is not of one solid, unitary layer as it is according to the first embodiment. As shown in  FIG. 11 , in some embodiments, the magnet layer  14  can be segmented into angular slices. In other embodiments, the magnet layer  14  may be segmented into other shapes. 
     A free air magnetic circuit  50  according to a fourth embodiment is shown in  FIG. 12 . According to the fourth embodiment, the top plate  16 , magnet layer  14 , and back plate  18  are each rectangular, rather than ring shaped. (While  FIG. 12  is a top view of the free air magnetic circuit  50  according to the fourth embodiment, the bottom view of the free air magnetic circuit  50  according to the fourth embodiment would look essentially the same in shape.) In other embodiments, the layers  16 ,  14 ,  18 , of the free air magnetic circuit  50  may be alternately shaped. 
     While there is shown and described the present preferred embodiment of the free air magnetic circuit for a speaker, it is to be distinctly understood that this invention is not limited thereto but may be variously embodied to practice within the scope of the following claims. For example, while the figures depict the magnet layer  14  being polarized so that the north pole  80  is adjacent the top plate  16  and so that the south pole  90  is adjacent the back plate  18 , the magnet layer  14  may be oppositely polarized. As another example, while the free air magnetic circuit  50  is configured so that the wire coil  20  is to be placed within the gap  24  that is essentially central to the top plate  16 , back plate  18 , and magnet layer  14 , the coil  20  may be alternatively configured to be placed externally to the peripheral edge of each of the layers  16 ,  14 ,  18  so that a gap  24  need not be defined internally to the layers  16 ,  14 ,  18 . Additionally, the top plate and bottom plate may include tapering portions proximate to the peripheral edge of each; the top plate, magnet layer, and bottom plate may be shaped differently, as in a triangle, parallelogram, pentagon, hexagon, or other polygon; the top plate and bottom plate may be wider than the magnet layer such that the circumference of the peripheral edge of each of the top plate and back plate is greater than the circumference of the peripheral edge of the magnet layer; the magnet layer may be segmented in any number of segments and in any shape of segments; the top or back plate or both may be segmented in any number of segments and in any shape of segments; the magnet layer may be comprised of any number of stacked magnet layers; and/or either or both of the top plate and back plate may be comprised of any number of stacked metal plates. In any regard, from the foregoing description, it will be apparent that various changes may be made without departing from the spirit and scope of the invention as defined by the following claims.