Patent Publication Number: US-6714656-B1

Title: Loudspeaker system with dust protection

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention generally relates to audio speaker systems and more specifically to dust barriers for loudspeakers used in such speaker systems. 
     2. Description of Related Art 
     A continuing effort is being applied to the development of loudspeakers for producing speaker systems that produce high-quality sound and that operate with maximum efficiency. This effort, in part, has been directed to developing new loudspeaker constructions, many of which are susceptible to damage from dust, particularly dust attracted by a magnetic field. 
     My U.S. Pat. No. 4,595,801 and U.S. patent application Ser. No. 09/251,815 filed Feb. 17, 1999 disclose dual cone loudspeakers with a primary speaker cone similar in function to a conventional dynamic loudspeaker mounted on a frame with a magnet structure. A secondary speaker cone mounts to a sub-frame on the back of the magnet structure and connects to the primary speaker cone through a rigid coupling device so the primary and secondary speaker cones move in unison. Sound waves from the secondary speaker cone travel through an orifice or throat through a center pole piece of the magnet structure and through an open center of the primary speaker cone radiating in the same direction as sound waves from the primary speaker cone. Consequently for a given excursion of the primary speaker cone my dual cone structure generates a sound having a greater sound volume than the primary cone alone by virtue of the simultaneous excursions of both the primary and secondary speaker cones that move a greater air volume for a given speaker cone displacement. 
     This dual cone speaker is one example of a speaker in which a single speaker cone or front speaker cone has an annular shape and a central orifice or throat. Dust can be attracted to the permanent magnet through the central orifice. If such particles accumulate in the air gap between a voice coil and the magnet, they can impede voice coil and speaker motion. When this occurs, the speaker&#39;s sound quality deteriorates. In more extreme situations, the accumulated attracted dust can cause permanent damage to the loudspeaker. What is needed is an apparatus or methodology for preventing the accumulation of attracted dust in the air gap, particularly in bass frequency loudspeakers in which a speaker cone undergoes a large linear displacement and requires high intensity magnetic fields in the air gap. 
     SUMMARY 
     Therefore it is an object of this invention to provide a dust barrier that minimizes dust accumulated in the air gap of a loudspeaker. 
     Another object of this invention is to provide a loudspeaker in which magnetically attracted dust particles are blocked physically from entering the air gap between a voice coil and speaker magnet. 
     In accordance with this invention, a loudspeaker with a frame that carries an annular speaker cone having a central opening driven by a voice coil on a bobbin in a magnetic field produced across an annular air gap in a magnetic structure that includes a throat aligned with the loudspeaker cone central opening. A ring is affixed to the speaker cone at the periphery of the central opening and an annular seal overlies a portion of the magnet structure within the throat. The ring operatively connects to the voice bobbin and the annular seal proximate the attachment of the speaker cone whereby the annular seal constitutes a barrier between the air gap and the throat thereby to block any dust that could be attracted into the air gap. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The appended claims particularly point out and distinctly claim the subject matter of this invention. The various objects, advantages and novel features of this invention will be more fully apparent from a reading of the following detailed description in conjunction with the accompanying drawings in which like reference numerals refer to like parts, and in which: 
     FIG. 1 is a cross section of a first embodiment of a loudspeaker constructed in accordance with this invention; 
     FIG. 2 is a detailed view of a portion of the loudspeaker shown in FIG. 1; 
     FIG. 3 is a cross section of another embodiment of a loudspeaker constructed in accordance with this invention; 
     FIG. 4 is a detailed cross section of an alternate version of the embodiment of the loudspeaker shown in FIG. 3; 
     FIG. 5 is a cross-section of a third embodiment of the loudspeaker constructed in accordance with this invention. 
    
    
     DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     FIG. 1 depicts, in a schematic view and for purposes of understanding this invention, a first embodiment of a dual-cone loudspeaker  100  as constructed in accordance with the aforementioned patent disclosures and this invention. For reference, the dual cone loudspeaker includes a rigid frame  1  to which a first speaker cone in the form of an annular primary speaker cone  2  is attached and a subframe  3  to which a second speaker cone in the form of a secondary speaker cone  4  is attached. Both frames  1  and  3  are mounted with a permanent magnet  5  to which pole pieces  6 A and  6 B are attached to form a magnetic field air gap  7  between the pole piece  6 A and a cylindrical extension  9  of the center pole piece  6 B into which a voice coil bobbin  8  with a voice coil is placed. The voice coil bobbin  8  attaches to the base of the primary speaker cone  2  that is resiliently suspended from the frame  1  by a flexible surround at its outer periphery and by a spider  10  at its bottom. The spider  10  mounts to the frame  1  and the voice coil bobbin  8  to prevent any magnetically attracted or other dust from migrating into the air gap  7  between the voice coil  8  and the magnet  5 . The spider also provide a centering force that maintains a coaxial relationship between the air gap  7  and the voice coil bobbin  8 . 
     A rigid link  11  mechanically connects the voice coil bobbin  8  to the secondary speaker cone  4  by a center attachment  13  that may comprise a separate fastener or an adhesive material that bonds the link  11  to the secondary speaker cone  4 . The secondary speaker cone  4  also attaches to the subframe  3  through a flexible surround and forms a second air piston that is pneumatically coupled to the primary speaker cone  2  through an orifice or aperture  14  through a center one of the pole pieces. This orifice  14  or throat is common to the closed sub-chamber formed by the secondary speaker cone  4  and subframe  3  and the open sub-chamber formed by the primary speaker cone  2 . 
     The voice coil bobbin  8  encircles and is closely spaced to the cylindrical extension  9 . As will be apparent, a transfer or migration path for magnetically attracted or other dust therefore exists from the exterior of the speaker  100  through the central orifice  14  into the air gap  7 , particularly an air gap portion  10  between the voice coil bobbin  8  and the cylindrical portion  9 . 
     FIGS. 1 and 2 depict one embodiment of a barrier  20  that can block this path. More specifically, the barrier  20  includes a main cylindrical portion  21  that has a slightly smaller outer diameter than the inner diameter of the cylindrical pole piece portion  9 . A transverse end portion  22  flares radially inwardly from the cylindrical portion  21  to form a closed support that attaches to the inside of the voice coil bobbin  8  by conventional attachment or by being formed integrally with the voice coil bobbin  8 . As a result, the voice coil bobbin  8  and the cylindrical portion  21  form a closed-end channel  23  that nests the cylindrical center pole piece portion  9 . As will be apparent, the depth of the channel  23  is selected to that the end portion  22  does not contact the open end of the cylindrical pole piece portion  9  during maximum excursions of the annual speaker cone  2 . 
     Given the distribution of magnet fields in the speaker  100 , any dust that passes through the orifice  14  is attracted to the barrier  20 , particularly at the end portion  22  that blocks any transfer into the air gap  10  and is most clearly shown in FIG.  2 . Rather the dust will coat an inner surface  24  of the barrier  20 . It does not migrate to cylindrical extension  9  because the maximum attraction occurs in proximity to the end portion  22 . Thus, the barrier  20  prevents magnetically attracted and other dust from migrating into the air gap  10 . 
     Such a barrier  20  can be formed of any of a plurality of lightweight materials that will not distort the main magnetic field and will not add any significant mass to the voice coil bobbin  8 . If the voice coil bobbin  8  is formed of a light-weight metal, the barrier  20  can be attached by welding, by adhesive or by other attachment procedures. If the voice coil bobbin  8  is formed of plastic, the barrier  20  can be integrally molded with the voice coil bobbin  8  or by attachment to the voice coil bobbin  8  by ultrasonic welding, adhesive or other attachment procedures. Although shown as a solid structure, the carrier  20  could also be formed as a stiff cloth or air permeable material that would block the passage of particles and yet allow air to pass. 
     FIGS. 3 and 4 depict another embodiment of this invention particularly useful in a dual cone loudspeaker  100  that includes magnetic poles  6 A and  6 B with the magnetic pole  6 B having an axially extending center pole portion  9  that defines the open orifice or throat  14  through the speaker. The throat continues through the annular primary cone  2 . A rigid link  11 ′ interconnects the primary cone  2  and the secondary cone  4 . The rigid link  11 ′ includes a center link  40  from the secondary cone and a center section  41  at the other end of the center link that carries equiangular spaced, radially extending spokes  42 . The spokes carry an annular, circumscribing ring structure  43 . The ring structure  43  supports the primary speaker cone  2  and the voice coil bobbin  8 . 
     As shown in FIG.  3  and more clearly in FIG. 4, the ring  43  includes a top portion  45  that flares outwardly from an inner or axially extending, cylindrical, bifurcated portion  46 . The bifurcation forms two spaced, axially extending fingers  47  and  48  that define an axially extending closed end slot  50 . 
     The apex of the portions  45  and  46  serve as a site for the attachment of the primary cone  2  at the periphery of the central opening through the cone as by applying an adhesive  51  or in other manner as known in the art. As a result, motion of the primary cone  2  produces corresponding motion of the secondary cone  4  as previously described. The axially extending, cylindrical slot  50  receives the voice coil bobbin  8  that extends through the magnetic air gap  7 . Only the bobbin  8  is shown in FIGS. 3 and 4 for simplicity. As will be apparent the bobbin also will carry a wound coil. 
     As previously indicated, with this structure the opening through the throat  14  and the space between angularly spaced spokes  42  constitute a path by which exterior dust can migrate into the air gap  7 , particularly the air gap portion  10 . In this embodiment, however, an annular seal  53  prevents such dust migration. The annular seal  53  includes a base portion  54  with an external bead  55 . The base portion  54  is relatively thick and stiff, but allows for some compressibility whereupon the seal  53  can be inserted in the throat until the bead  55  seats in a circumferential groove  56  formed in the center pole piece  9 . In this particular embodiment the groove  56  is formed in a seat for the thick body portion  54  thereby to minimize any interruption of air flow through the throat  14 . Alternate structures also could be used for fixing the seal  53  to the interior of the magnet structure. 
     While the base portion  54  has a constant thickness to provide necessary rigidity, a neck section  60  tapers to provide a transition from the thick, relatively rigid base portion  54  to a thin, relatively flexible, inverted U-shaped portion  61 . The portion  61  terminates proximate the axial ring portion  46  in an axially extending flange or collar  62  that fits into the slot  50 . 
     During manufacture a fixture positions the voice coil bobbin  8  and the flange  62  in the slot  50 . When positioned the slot  50  is filled with epoxy or another adhesive. When this process is finished, the ring  46  supports the seal  53 , the voice coil bobbin  8  and the primary cone  2 . As the voice coil  8  moves axially in response to electrical signals applied to the voice coil, the bobbin  8  drives the ring  43  that in turn drives the primary cone  2  and the secondary cone  4  through the central link  11 ′. During this motion the axially flexible thin portion  61  allows the unimpeded motion while providing an impermeable membrane between the throat  14  and the air gap portion  10 . 
     As shown particularly in FIG. 3, the spokes  42  are shaped with a vertical or axial offset at  63 . This offset allow full range of axial motion without any interference between the seal  53  and the spokes  42 . 
     In this embodiment the seal  53  serves two functions and may provide an optional third function. First, the seal  53  acts as a barrier to prevent dust from passing through the throat  14  to the air gap portion  10 . If the seal  53  is made of an impermeable material, the seal  53  prevents an air leakage from the throat  14  to the rear surface of the primary cone  2  that could have deleterious effect on performance. FIG. 3 shows the use of the seal  53  and the ring  43  without any attached spiders, such as the spider  10  in FIG.  1 . The seal  53  can also provide a centering function thereby to assure the proper positioning of the voice coil bobbin  8  within the air gap  7 . 
     FIG. 5 depicts still another embodiment of this invention with a seal or barrier  70  that comprises a resilient cylindrical element with a portion  71  folded back on itself to produce a U-shape, much like a common rolling rubber or foam cone edge surround. With this approach, as the rigid link  11  and voice coil  8  move longitudinally outward, the intermediate portion  71  expands so the amount of material within the intermediate portion reduces. As the voice coil  8  and rigid link  11  move in the opposite direction, the amount of material in the intermediate portion increases as the distance between the upper and lower portions  72  and  73  decreases. The configuration must be such that the folded portion  71  does not interfere mechanically with the full range of motions of the voice coil bobbin  8  and the rigid link  11 . FIG. 5 also depicts a loudspeaker  100  with a spider  10 . If the barrier  70  has sufficient lateral stability, that is, if the barrier  70  can maintain the coaxial relationship of the annular air gap  7  and the voice coil bobbin  8 , the spider  10  could be eliminated in these cases where centering is the primary function of the spider  10 . 
     Thus in accordance with this invention there have been disclosed a number of barriers that prevent the accumulation of magnetically attracted dust and other materials into critical air gaps of the loudspeaker through an annular speaker cone. This invention has been disclosed in terms of certain embodiments. It will be apparent that many modifications can be made to the disclosed apparatus without departing from the invention. Therefore, it is the intent of the appended claims to cover all such variations and modifications as come within the true spirit and scope of this invention.