PATENT DOCUMENT

Publication Number: US-9332351-B2
Application Number: US-201313764682-A
Country: US
Kind Code: B2

Title: Long-throw acoustic transducer

Abstract:
An acoustic transducer includes a housing, which may be a circular cylinder or may have a rectangular cross-section. Two permanent magnets that closely fit the inside of the housing are joined by a linkage having a high magnetic permeability to form a piston that is inserted into the housing. Two pole coils surround the housing with each coil adjacent one of the permanent magnets. The coils are arranged to cause the piston to oscillate within the housing and emit sound waves when coupled to an electrical signal. One end of the housing may be closed except for a barometric leak. A third permanent magnet or a spring may provide a restoring force that centers the piston between the coils when the piston is not subjected to other forces. One of the permanent magnets on the piston may include a vent passage.

Claims:
What is claimed is: 
     
       1. An acoustic transducer comprising:
 a housing; 
 a piston for displacing air in the housing to produce sound pressure waves, the piston including
 a linkage, 
 a first permanent magnet closely fit to the inside of the housing and coupled to a first end of the linkage to provide a first magnetic pole, and 
 a second permanent magnet closely fit to the inside of the housing and coupled to a second end of the linkage opposite the first end to provide a second magnetic pole; 
 
 a first pole coil surrounding the housing and the first permanent magnet; 
 a second pole coil surrounding the housing and the second permanent magnet; and 
 a third permanent magnet coupled to the inside of the housing adjacent a midpoint of the linkage with poles of the third permanent magnet arranged to repel the first permanent magnet and the second permanent magnet; 
 wherein the first pole coil and the second pole coil are arranged to cause the piston to oscillate within the housing when coupled to an electrical signal. 
 
     
     
       2. The acoustic transducer of  claim 1  wherein one end of the housing is closed except for a hole that provides a barometric leak. 
     
     
       3. The acoustic transducer of  claim 1  further comprising at least one pole piece magnetically coupling the first pole coil and the second pole coil. 
     
     
       4. The acoustic transducer of  claim 1  wherein one of the first and second permanent magnets includes a vent passage that couples a space between the first and second permanent magnets to a space external to the piston. 
     
     
       5. The acoustic transducer of  claim 1  wherein the housing has a substantially rectangular cross-section. 
     
     
       6. A method for constructing an acoustic transducer, the method comprising:
 assembling a piston by joining a first permanent magnet and a second permanent magnet to opposite ends of a linkage; 
 inserting the piston into a housing with the first permanent magnet and the second permanent magnet closely fit to the inside of the housing; 
 surrounding the housing and the first permanent magnet with a first pole coil; 
 surrounding the housing and the second permanent magnet with a second pole coil; and 
 attaching a third permanent magnet to the inside of the housing adjacent a midpoint of the linkage with poles of the third permanent magnet arranged to repel the first permanent magnet and the second permanent magnet; 
 wherein the first pole coil and the second pole coil are arranged to cause the piston to oscillate and displace air within the housing to produce sound pressure waves when coupled to an electrical signal. 
 
     
     
       7. The method of  claim 6  further comprising closing one end of the housing and providing a hole that creates a barometric leak. 
     
     
       8. The method of  claim 6  further comprising magnetically coupling the first pole coil and the second pole coil with at least one pole piece. 
     
     
       9. The method of  claim 6  further comprising providing a vent passage in at least one of the first and second permanent magnets to couple a space between the first and second permanent magnets to a space external to the piston. 
     
     
       10. The method of  claim 6  wherein the housing has a substantially rectangular cross-section. 
     
     
       11. An acoustic transducer comprising:
 a housing; 
 a first permanent magnet; 
 a second permanent magnet; 
 means for joining the first permanent magnet and the second permanent magnet to form a piston having opposite ends of different magnetic polarities; 
 means for causing the piston to oscillate and displace air within the housing to produce sound pressure waves when coupled to an electrical signal; and 
 means for providing a restoring force that moves the piston to substantially center the piston between the first and second pole coils when the piston is not subjected to any other forces. 
 
     
     
       12. The acoustic transducer of  claim 11  further comprising means for closing one end of the housing and providing a barometric leak in the closed end. 
     
     
       13. The acoustic transducer of  claim 11  wherein the means for causing the piston to oscillate and displace air includes a first pole coil and a second pole coil, the acoustic transducer further comprising means for magnetically coupling the first pole coil and the second pole coil. 
     
     
       14. The acoustic transducer of  claim 11  further comprising means for coupling a space between the first and second permanent magnets to a space external to the piston. 
     
     
       15. The acoustic transducer of  claim 11  wherein the housing has a substantially rectangular cross-section. 
     
     
       16. The acoustic transducer of  claim 1  wherein the close fit of the first permanent magnet and the second permanent magnet to the inside of the housing causes an audible sound to be emitted from the housing when the piston oscillates within the housing at an audible frequency. 
     
     
       17. The method of  claim 6  wherein the close fit of the first permanent magnet and the second permanent magnet to the inside of the housing causes an audible sound to be emitted from the housing when the piston oscillates within the housing at an audible frequency. 
     
     
       18. The method of  claim 6  further comprising closing one end of the housing and providing a small hole that creates a barometric leak. 
     
     
       19. An acoustic transducer comprising:
 a housing; 
 a piston for displacing air in the housing to produce sound pressure waves, the piston including
 a linkage, 
 a first permanent magnet closely fit to the inside of the housing and coupled to a first end of the linkage to provide a first magnetic pole, and 
 a second permanent magnet closely fit to the inside of the housing and coupled to a second end of the linkage opposite the first end to provide a second magnetic pole; 
 
 a first pole coil surrounding the housing and the first permanent magnet; 
 a second pole coil surrounding the housing and the second permanent magnet; and 
 a spring coupled to the piston to provide a restoring force that moves the piston to substantially center the piston between the first and second pole coils when the piston is not subjected to any other forces; 
 wherein the first pole coil and the second pole coil are arranged to cause the piston to oscillate within the housing when coupled to an electrical signal. 
 
     
     
       20. The acoustic transducer of  claim 19  wherein one end of the housing is closed except for a hole that provides a barometric leak. 
     
     
       21. The acoustic transducer of  claim 19  further comprising at least one pole piece magnetically coupling the first pole coil and the second pole coil. 
     
     
       22. The acoustic transducer of  claim 19  wherein one of the first and second permanent magnets includes a vent passage that couples a space between the first and second permanent magnets to a space external to the piston. 
     
     
       23. The acoustic transducer of  claim 19  wherein the housing has a substantially rectangular cross-section. 
     
     
       24. The acoustic transducer of  claim 19  wherein the close fit of the first permanent magnet and the second permanent magnet to the inside of the housing causes an audible sound to be emitted from the housing when the piston oscillates within the housing at an audible frequency. 
     
     
       25. A method for constructing an acoustic transducer, the method comprising:
 assembling a piston by joining a first permanent magnet and a second permanent magnet to opposite ends of a linkage; 
 inserting the piston into a housing with the first permanent magnet and the second permanent magnet closely fit to the inside of the housing; 
 surrounding the housing and the first permanent magnet with a first pole coil; 
 surrounding the housing and the second permanent magnet with a second pole coil; and 
 coupling one or more springs to the piston to provide a restoring force that moves the piston to substantially center the piston between the first and second pole coils when the piston is not subjected to any other forces; 
 wherein the first pole coil and the second pole coil are arranged to cause the piston to oscillate and displace air within the housing to produce sound pressure waves when coupled to an electrical signal. 
 
     
     
       26. The method of  claim 25  further comprising closing one end of the housing and providing a hole that creates a barometric leak. 
     
     
       27. The method of  claim 25  further comprising magnetically coupling the first pole coil and the second pole coil with at least one pole piece. 
     
     
       28. The method of  claim 25  further comprising providing a vent passage in at least one of the first and second permanent magnets to couple a space between the first and second permanent magnets to a space external to the piston. 
     
     
       29. The method of  claim 25  wherein the housing has a substantially rectangular cross-section. 
     
     
       30. The method of  claim 25  wherein the close fit of the first permanent magnet and the second permanent magnet to the inside of the housing causes an audible sound to be emitted from the housing when the piston oscillates within the housing at an audible frequency. 
     
     
       31. The method of  claim 25  further comprising closing one end of the housing and providing a small hole that creates a barometric leak.

Description:
BACKGROUND 
     1. Field 
     Embodiments of the invention relate to the field of audio speakers; and more specifically, to an audio speaker that uses a moving magnetic piston as the sound producing element. 
     2. Background 
     Audio speakers use electrical signals to produce air pressure waves which are perceived as sounds. Many audio speakers use a diaphragm that is movably suspended in a frame. The diaphragm is coupled to a voice coil that is suspended in a magnetic field. The electrical signals representing the sound flow through the voice coil and interact with the magnetic field. This causes the voice coil and the coupled diaphragm to oscillate in response to the electrical signal. The oscillation of the diaphragm produces air pressure waves. 
     It is necessary for the audio speakers to displace a volume of air to produce sound pressure waves that are perceptible to a listener. A speaker diaphragm is limited in the distance it can move, and this limit become smaller as the speaker is reduced in size. This limits the volume of sound that can be produced by a small speaker, particularly in lower frequency range. 
     It would be desirable to provide an audio speaker that can displace a larger volume of air from a more compact structure suitable for use in portable devices. 
     SUMMARY 
     An acoustic transducer includes a housing, which may be a circular cylinder or may have a rectangular cross-section. Two permanent magnets that closely fit the inside of the housing are joined by a linkage having a high magnetic permeability to form a piston that is inserted into the housing. Two pole coils surround the housing with each coil adjacent one of the permanent magnets. The coils are arranged to cause the piston to oscillate within the housing and emit sound waves when coupled to an electrical signal. One end of the housing may be closed except for a barometric leak. A third permanent magnet or a spring may provide a restoring force that centers the piston between the coils when the piston is not subjected to other forces. One of the permanent magnets on the piston may include a vent passage. 
     Other features and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description that follows below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may best be understood by referring to the following description and accompanying drawings that are used to illustrate embodiments of the invention by way of example and not limitation. In the drawings, in which like reference numerals indicate similar elements: 
         FIG. 1  is a pictorial view of an acoustic transducer with a front portion of a cylindrical housing cut away along a diameter. 
         FIG. 2  is a pictorial view of another acoustic transducer with a front portion of a cylindrical housing cut away along a diameter. 
         FIG. 3  is a pictorial view of still another acoustic transducer with a front portion of a cylindrical housing cut away along a diameter. 
         FIG. 4  is a pictorial view of yet another acoustic transducer with a front portion of a cylindrical housing cut away along a diameter. 
         FIG. 5  is a pictorial view of another acoustic transducer with a front portion of a rectangular housing cut away. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known circuits, structures and techniques have not been shown in detail in order not to obscure the understanding of this description. Explanations that duplicate one another may have been omitted. 
     The meaning of specific terms or words used in the specification and claims should not be limited to the literal or commonly employed sense, but may be different and should be construed in the context of the specification. The terms “first,” “second,” and the like, herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. The terms “a” and “an” herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item. 
       FIG. 1  is a pictorial view of an acoustic transducer  100 . The acoustic transducer shown includes a cylindrical housing  110  having a circular cross-section, which is shown with a front portion of the housing cut away along a diameter to allow the internal components to be seen. 
     The acoustic transducer  100  further includes a piston  120 . A first permanent magnet  122  and a second permanent magnet  126  are coupled to a linkage  124  having a high magnetic permeability to form the piston  120 . One of the permanent magnets provides a north magnetic pole for the piston structure  120  and the other permanent magnet provides a south magnetic pole. Each of the permanent magnets  122 ,  126  is closely fit to the inside of the housing  110 . A ferromagnetic liquid may be used between the first and second permanent magnets  122 ,  126  and the housing  110  to provide lubrication and a seal between the magnets and the housing. 
     A first pole coil  132  surrounds the housing  110  adjacent the first permanent magnet  122  and a second pole coil  136  surrounds the housing adjacent the second permanent magnet  126 . The pole coils  132 ,  136  are shown cut away along a diameter to allow the internal components to be seen. The first pole coil  132  and the second pole coil  136  are arranged to cause the piston  120  to oscillate within the housing  110  when the pole coils are coupled to an electrical signal. The two pole coils may be coupled to the electrical signal with a series or parallel connection. 
     Oscillations of the piston  120  at audible frequencies will cause an audible sound to be emitted from the two open ends  112 ,  114  of the housing  110 . The pressure waves coming out of each of the two open ends  112 ,  114  will be out-of-phase and could cancel in far field. However, if the acoustic transducer  100  is situated such that one end  114  is opened to the exterior of a device casing and the other end  112  radiates into the inside of the device casing, essentially putting it in a wraparound baffle, the concern of phase cancellation can be mitigated. 
     Oscillations of the piston  120  at lower frequencies, perhaps between 100 HZ and 250 Hz or perhaps even into sub-audible ranges, may produce a sufficiently strong vibration that the acoustic transducer  100  can be used to produce a tactile alert. For example, in a cellular telephone application it may be possible to use the acoustic transducer  100  as both a speaker and an alerting vibrator. By configuring the subsystem that delivers the electrical signal to the acoustic transducer  100  to provide either a signal suitable to produce an audio output or a signal suitable to produce tactile alert, which may also result in an audible audio output, the acoustic transducer may selectively perform the functions of both an audio speaker and an alerting vibrator. 
     The separation of the first and second permanent magnets  122 ,  126  and the first pole coil  132  and the second pole coil  136  allows the piston  120  to achieve larger displacements with a lower piston mass than would be possible with a single permanent magnet and/or a single coil. This in turn allow a larger volume of air to be displaced by the piston  120  which creates a louder sound. 
       FIG. 2  is a pictorial view of another acoustic transducer  200 . The acoustic transducer shown includes a cylindrical housing  210  having a circular cross-section, which is shown with a front portion of the housing cut away along a diameter to allow the internal components to be seen. One end  212  of the housing  210  is closed. Sound will be emitted from the open end  214  of the housing  210 . This prevents the emission of an out-of-phase pressure wave from the second, closed end  212  but the closed end may form an acoustic spring that dominates the effective mechanical stiffness of the piston  120 . 
     In some embodiments one or more pole pieces  238  having a high magnetic permeability may be placed adjacent the first pole coil  132  and the second pole coil  136  to enhance the magnetic flux density in the vicinity of the coils. In  FIG. 2  a single pole piece  238  is shown cut away along a diameter to allow the internal components to be seen. 
       FIG. 3  is a pictorial view of still another acoustic transducer  300 . The acoustic transducer shown includes a cylindrical housing  310  having a circular cross-section, which is shown with a front portion of the housing cut away along a diameter to allow the internal components to be seen. One end  312  of the housing  310  is closed. A small hole  316  is provided in the closed end to provide a barometric leak. The hole  316  is small enough that no appreciable amount of sound is emitted from the hole. However, changes in the ambient atmospheric pressure are transmitted through the small hole  316  so that the equilibrium of the piston  320  is not affected by the changes in atmospheric pressure. 
     In some embodiments the permanent magnet  326  adjacent the closed end  312  of the housing  310  includes one or more vent passages  328  to couple a space  342  between the first and second permanent magnets to a space  340  external to the piston. This increases the volume of the trapped air in the housing and reduces the stiffness of the acoustic spring formed by the closed end. The vent passages  328  may be arranged symmetrically to avoid creating unbalanced forces on the permanent magnet  326 . 
     In some embodiments multiple pole pieces  338   a ,  338   b ,  338   c  may be formed from two or more pieces of material having a high magnetic permeability, with each piece extending between the two pole coils  332 ,  336 . When multiple pole pieces are used, the pole pieces may or may not be contiguous around each of the coils. For example,  FIG. 3  shows pole pieces  338   a ,  338   b ,  338   c  that are not contiguous around each of the coils  332 ,  336 . 
     In some embodiments a third permanent magnet  350  is coupled to the inside of the housing  310  adjacent a midpoint of the linkage  324  that couples the first  322  and second  326  permanent magnets. The third permanent magnet  350  is arranged with its poles adjacent the like poles of the first  322  and second  326  permanent magnets. The third permanent magnet  350  thus repels both ends of the piston  320  and holds the piston at an equilibrium position when no power is applied to the two pole coils  332 ,  336 . 
       FIG. 4  is a pictorial view of yet another acoustic transducer  400 . The acoustic transducer shown includes a cylindrical housing  410  having a circular cross-section, which is shown with a front portion of the housing cut away along a diameter to allow the internal components to be seen. In some embodiments the acoustic transducer includes one or more springs coupled to the piston to provide a restoring force that moves the piston to substantially center the piston between the first and second pole coils when the piston is not subjected to any other forces such as the forces created by the pole coils when energized. For example,  FIG. 4  shows an acoustic transducer  400  that includes two compression springs  452 ,  456  that bear against their respective permanent magnets  422 ,  426  at a first end of the spring and the housing  410  at a second end of the spring. The springs  452 ,  456  hold the piston  420  at an equilibrium position when no power is applied to the two pole coils  432 ,  436 . 
       FIG. 5  is a pictorial view of another acoustic transducer  500 . The acoustic transducer shown includes a housing  510  having a substantially rectangular cross-section, which is shown with a front portion of the housing cut away to allow the internal components to be seen. In some embodiments the housing is a more generalized cylinder having a non-circular cross-section. For example, the acoustic transducer  500  shown in  FIG. 5  has a housing that is a cylinder with a cross-section of a rectangle with filleted corners. The use of a rectangular cylinder  510  for the housing allows the acoustic transducer  500  to have a larger cross-section and thus, a larger volume of air displacement than a circular cylindrical housing with a diameter equal to the smaller side of the rectangle. This may be advantageous for acoustic transducers that are used in “thin” devices where the height of the acoustic transducer must be small to fit within the device. As shown, a polygonal cylinder may have rounded corners on the polygon. 
     The acoustic transducer  500  shown in  FIG. 5  also includes features that have been previously described. High magnetic permeability pole pieces  538   a ,  538   b ,  538   c  that are not contiguous around each of the pole coils  532 ,  536  are placed adjacent the coils. A third permanent magnet, provided as two separate segments  550   a ,  558   b , holds the piston  520  at an equilibrium position when no power is applied to the two pole coils  532 ,  536 . The closed end  512  of the housing  510  includes a small hole  516  in the side wall of the housing to provide a barometric leak. The permanent magnet  526  adjacent the closed end  512  of the housing  510  includes two vent passages  528 . 
     While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive on the broad invention, and that this invention is not limited to the specific constructions and arrangements shown and described, since various other modifications may occur to those of ordinary skill in the art. The description is thus to be regarded as illustrative instead of limiting.

Metadata:
Filing Date: 20130211
Publication Date: 20160503
Grant Date: 20160503
Priority Date: 20130211
Inventors: PORTER SCOTT P.
WILK CHRISTOPHER
DAVE RUCHIR M.
Assignee: APPLE INC
CPC Classifications: [{"code": "H04R2209/041", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04R9/025", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04R2209/022", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04R2400/03", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04R9/066", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04R9/06", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04R11/02", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04R2209/041", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04R9/066", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04R11/02", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04R2209/022", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04R2400/03", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04R9/025", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04R9/06", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 51297446