Patent Publication Number: US-7715584-B2

Title: Loudspeaker with air deflector

Description:
FIELD OF THE INVENTION 
   This invention relates to loudspeakers, and, more particularly, to an air deflector which is located with respect to the through bore in the pole piece of the motor of the speaker to direct cooling air, flowing in and out of the cavity located between the voice coil and dust cap, along a flow path in thermal communication with the inner surface of the voice coil of the speaker. 
   BACKGROUND OF THE INVENTION 
   Loudspeakers generally comprise a frame, a motor structure, a diaphragm, a lower suspension and an upper suspension. In one common type of speaker, the motor structure includes a permanent magnet mounted between a top plate and a back plate, a pole piece centrally mounted on the back plate and a voice coil axially movable with respect to the pole piece. The voice coil includes a hollow, cylindrical-shaped former having an outer surface which receives a winding of wire. 
   One end of the diaphragm is connected to the upper suspension, which, in turn, is mounted to the upper end of the frame. The lower suspension is connected at one end to the frame at a point between its upper and lower ends. The free ends of the diaphragm and lower suspension are mounted to the outer surface of the former of the voice coil and support it within the magnetic gap formed between the pole piece and top plate of the motor structure such that the former of the voice coil is concentrically disposed about the pole piece. In some speaker designs, a dust cap is mounted to the diaphragm in a position overlying the voice coil and pole piece to protect them from contaminants. This forms a dust cap cavity between the dust cap, diaphragm, the voice coil and pole piece. In alternative designs, the upper end of the voice coil is connected directly to the diaphragm, thus eliminating the need for a dust cap but nevertheless forming an internal or dust cap cavity in the area directly above the voice coil and pole piece. 
   In the course of operation of a speaker of the type described above, electrical energy is supplied to the voice coil causing it to axially move relative to the pole piece within the magnetic gap of the motor. The diaphragm, upper suspension and lower suspension all move as a unit with the excursion of the voice coil. A pervasive problem associated with speaker operation involves the build up of heat produced in the wire winding of the voice coil, and by its axial movement, which is radiated to surrounding surfaces, particularly the top plate. Both the voice coil and top plate become quite hot during speaker operation which can reduce the power handling of the speaker, and increase power compression, i.e. a reduction in acoustic output due to temperature-related voice coil resistance. 
   A variety of designs have been employed in the prior art to address the problems associated with heat build up in speakers. One approach has been to create a flow of cooling air in thermal communication with the voice coil, such as disclosed, for example, in U.S. Pat. No. 5,042,072 to Button, U.S. Pat. No. 5,357,586 to Nordschow et al. and U.S. Pat. No. 5,426,707 to Wijnker. Speaker designs of this type generally include a pole piece formed with passages which provide a flow path for the transfer of cooling air from outside of the speaker into and out of the dust cap cavity described above. An air flow through these passages is created in response to movement of the diaphragm with the excursion of the voice coil. When the diaphragm moves in one direction, air is drawn from outside of the speaker, along the passages in or along the pole piece, and then into the dust cap cavity. Movement of the diaphragm in the opposite direction creates a flow out of the dust cap cavity along the reverse flow path. 
   In the Button U.S. Pat. No. 5,042,072, the pole piece of the motor is formed with a series of circumferentially spaced, longitudinally extending grooves or channels. Each channel extends radially inwardly from the outer surface of the pole piece toward its center, and from the top end of the pole piece to its bottom end including in the area of the magnetic gap between the pole piece and top plate. The purpose of the radial channels in the pole piece is to direct a flow of air along the voice coil as the air passes in and out of the dust cap cavity. Although it is contemplated that at least some of the air flow contacts the voice coil in this design, because the radial channels in the pole piece are oriented parallel to the voice coil along the longitudinal axis of the pole piece a limited amount of the cooling air actually impinges directly against the voice coil. Additionally, the formation of a number of radial channels in the pole piece reduces its mass in the area of the magnetic gap. This increases the reluctance of the magnetic path between the pole piece and top plate resulting in a decrease in motor strength which can adversely impact the acoustic performance of the speaker. 
   U.S. Pat. No. 5,357,586 to Nordschow employs a pole piece including a central through bore forming an annular wall defining a hollow interior. An aerodynamically-shaped insert is mounted within the central bore of the pole piece by a series of fins or spacers, thus forming longitudinally extending channels between the insert and the wall. Additionally, the wall of the pole piece is formed with a number of transverse bores extending between its outer surface and the central bore. In response to movement of the voice coil and diaphragm in one direction, air from outside of the speaker is drawn into the central bore of the pole piece, through its transverse bores, along the exterior surface of the pole piece into the magnetic gap between the pole piece and top plate, and then through bores formed in the voice coil into the dust cap cavity. Movement of the diaphragm in the reverse direction causes a flow of air out of the cavity through the voice coil bores, and then predominantly through the central bore of the pole piece along the channels formed by the fins of the aerodynamically-shaped insert. 
   Although the intention in the &#39;586 patent is to cool the voice coil, it is unlikely that any effective cooling occurs with this design. The magnetic gap between the pole piece and top plate is exceedingly small, particularly considering that the voice coil is located therein, and no appreciable amount of air flow can be created through the magnetic gap without using a design such as described in the &#39;072 Button patent wherein longitudinal channels are formed in the pole piece to provide a flow path between the pole piece and the top plate. The &#39;586 patent does not include a pole piece with longitudinal channels along its exterior surface, but instead attempts to force a flow of air from the transverse bores in the pole piece through the magnetic gap, and, hence, along the outer surface of the voice coil. Additionally, the flow of air in the reverse direction noted above is for venting purposes only and does not result in the movement of cooling air along or adjacent to the wire winding of the voice coil. 
   The &#39;707 patent to Wijnker is similar to Nordschow et al. in that it includes, in one embodiment, a pole piece formed with a central bore and a number of transverse bores extending through the wall of the pole piece. The transverse bores in the Wijnker patent are employed to create a flow of air from outside of the speaker, into the central bore of the pole piece and then out the transverse bores to discharge ports formed in the back plate of the speaker. No cooling air passes from the transverse bores, along the voice coil and into and out of the dust cap cavity. Alternative embodiments of the Wijnker patent disclose a flow path into and out of the dust cap cavity, but employ a pole piece formed with a through bore and no transverse bores and wherein an attempt is made, as in Nordschow et al., to force air to flow within the magnetic gap between the top plate and pole piece. 
   The deficiencies of such prior designs have been addressed to some degree in U.S. Pat. No. 6,243,479 to Proni and U.S. Pat. No. 6,535,613 to Ssutu. Each of these patents discloses a speaker having a motor which employs a pole piece formed with a bore. Structure is provided for directing a flow of air entering and exiting the pole piece bore(s) into contact with the inner surface of the voice coil in an area opposite the wire winding located on the outer surface of the voice coil. In the Ssutu design, the pole piece is formed with a through bore and an air deflector is inserted within the through bore at the top end of the pole piece. Air entering and exiting the dust cap cavity of the speaker as a result of axial movement of the voice coil and diaphragm contacts the air deflector and is directed against the inside surface of the voice coil opposite the wire winding to enhance cooling of the wire winding and top plate. 
   The Proni patent discloses a number of alternative embodiments, some where the pole piece is formed with a through bore and others in which the pole piece has an axial bore extending from the top end of the pole piece toward its bottom end. In most versions, the top end of the bore in the pole piece is closed by an insert, and transverse vent holes are formed in the side wall of the pole piece which intersect the axial bore or the central through bore therein. The axial bore or through bore, and the vent bores, cause air entering and exiting the dust cap cavity to flow directly against at least a portion of the interior of the former of the voice coil for cooling. Neither the Ssutu nor Proni design attempts to force a flow of air through the magnetic gap of the speaker. Instead, the cooling air is directed along the inner surface of the former of the voice coil to assist in cooling the wire winding on its opposite surface. 
   SUMMARY OF THE INVENTION 
   This invention is directed to a loudspeaker having a motor structure including a voice coil and a pole piece formed with a thorough bore, in which an air deflector is mounted over the outer surface of the pole piece within the interior of the voice coil in position to deflect air entering and exiting the dust cap cavity of the speaker into contact with the inner surface of the voice coil opposite its wire winding. 
   In the presently preferred embodiment, the air deflector comprises a larger diameter lower portion joined to a smaller diameter upper portion. Both the upper and lower portions are hollow with the lower portion having an open bottom and the upper portion being closed at its top end. A number of bores are formed in the deflector in the area of the juncture of the upper and lower portions. 
   The lower portion of the air deflector is placed over the outer surface of the pole piece of the motor structure, within the interior of the voice coil, and may extend along at least a portion of the magnetic gap formed between the pole piece and top plate of the motor. The air deflector is formed of copper which helps to optimize the inductance and impedance characteristics of the speaker. Additionally, with the deflector mounted to the pole piece, its bores are located in a position to direct air entering and exiting the dust cap cavity of the speaker, via the through bore in the pole piece, against the inner surface of the voice coil opposite its wire winding. This assists in cooling the voice coil during operation of the speaker. 

   
     DESCRIPTION OF THE DRAWINGS 
     The structure, operation and advantages of the presently preferred embodiment of this invention will become further apparent upon consideration of the following description, taken in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is an elevational view, in partial cross section, of a speaker having one embodiment of the air deflector of this invention; 
       FIG. 2  is a perspective view of the air deflector depicted in  FIG. 1 ; 
       FIG. 3  is a further cross sectional view of the speaker shown in  FIG. 1 ; 
       FIG. 4  is view similar to  FIG. 1 , except of an alternative embodiment of the air deflector of this invention; 
       FIG. 5  is a perspective view of the air deflector shown in  FIG. 4 ; and 
       FIG. 6  is a further cross sectional view of the speaker of  FIG. 4 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to the FIGS., a loudspeaker  10  is illustrated which is identical in each of  FIGS. 1 ,  3 ,  4  and  6  except for the inclusion of a different air deflector, described in detail below. Although the detailed construction of the speaker  10  forms no part of this invention, for purposes of the present discussion it is briefly described as follows. 
   The speaker  10  generally comprises a motor structure  12 , a frame  14  mounted to the motor structure  12 , a diaphragm  16 , a lower suspension or spider  18  and an upper suspension or surround  20 . Conventionally, the motor structure  12  includes a top plate  22  and a back plate  24  which are spaced from one another and mount a permanent magnet  26  between them. A pole piece  30  is integrally formed with and extends upwardly from the back plate  24  into a central bore  28  formed in both the magnet  26  and top plate  22 . The pole piece  30  has a through bore  31  extending from its bottom end, which is exposed to ambient air, to its top end. A magnetic gap  33  is formed between the top plate  22  and the pole piece  30 , as best seen in  FIGS. 3 and 6 . A voice coil  32  is also provided which includes a hollow, cylindrical-shaped former  34  having an inner surface  35  and an outer surface  37  which mounts a wire winding  36 . The former  34  is concentrically disposed about the pole piece  30 , and the voice coil  32  is axially movable within the magnetic gap  33  during operation of the speaker  10 . 
   The voice coil  32  is held in place with respect to the pole piece  30  by the diaphragm  16 , spider  18  and surround  20 . One end of the diaphragm  16  is affixed to the former  34  by adhesive or the like, and its opposite end connects to the surround  20 . The surround  20 , in turn, is mounted to the upper end  38  of the frame  14  as shown. One end of the spider  18  connects to the former  34 , and its opposite end mounts to a seat  15  formed in the frame  14 . 
   A dust cap  44  is mounted to the diaphragm  16  in position to overlie the voice coil  32  and pole piece  30  in order to protect such elements from dirt, dust and other contaminants. A dust cap cavity  46  is therefore formed in the area defined by the lower portion of the diaphragm  16 , the dust cap  44 , the voice coil  32  and the pole piece  30 . In response to the input of electrical energy to the wire winding  36 , the voice coil  32  is moved axially with respect to the fixed motor structure  12 . Because the diaphragm  16 , spider  18 , surround  20  and dust cap  44  are operatively connected to the voice coil  32 , such elements also move with the excursion of the voice coil  32 . A “pumping” action results from the axial movement of the diaphragm  16  and dust cap  44 , which creates a flow of comparatively cool, ambient air from outside of the speaker  10  into and out of the cavity  46  via the through bore  31  in the pole piece  30 . 
   The air deflector of this invention functions to vent the cavity  46  thus preventing pressure build up within the speaker  10 , and also directs the flow of cooling air from outside of the speaker  10  against the inner surface  35  of the former  34 , and, indirectly, against the wire winding  36  carried on the opposite outer surface  37  of the former  34 . The structure of each embodiment of the air deflector of this invention is described first, followed by a discussion of its operation. 
   With reference initially to  FIGS. 1-3 , one air deflector  50  of this invention comprises a sleeve preferably formed of copper having a larger diameter lower portion  54  joined to a smaller diameter upper portion  56 . Both the lower portion  54  and upper portion  56  are hollow, with the bottom end  58  of the lower portion  54  being open and the top end  60  of the upper portion  56  being closed by a plate  62 . The lower portion  54  is inwardly tapered at its juncture with the upper portion  56  forming a shoulder  64  having a number of circumferentially spaced bores  66  which extend into the hollow interior of the air deflector  50 . 
   The air deflector  50  is mounted to the speaker  10  by sliding the lower portion  54  over the outer surface of the pole piece  30  until the shoulder  64  of the deflector  50  abuts the top end of the pole piece  30 . The diameter of the lower portion  54  is chosen to frictionally engage the outer surface of the pole piece  30  so that the deflector  50  remains in place during operation of the speaker  10 , while allowing the lower portion  54  to be readily slid along the pole piece  30  during assembly. As shown in  FIGS. 1 and 3 , when the deflector  50  is in position on the pole piece  30  it is located within the interior of the voice coil  32 . The bores  66  in the shoulder  64  of the deflector  50  are positioned in the area of the former  34  opposite at least a portion of the wire winding  36  of the voice coil  32 . 
   Referring now to  FIGS. 4-6 , a second air deflector  70  is depicted with the same loudspeaker  10  shown in  FIGS. 1-3  and described above. The deflector  70  is similar to deflector  50  and comprises a hollow lower portion  72  connected to an upper portion  74  at an inwardly tapering joint  76 . The lower portion  72  is open at its bottom end  78  and the top end  80  of upper portion  74  is closed by a plate  82 . A number of bores  84  are formed in the upper portion  74 , immediately above the joint  76 , and these bores  84  are preferably circumferentially spaced from one another and may form one or more rows extending from the joint  76  toward the top end  80  of top portion  74 . 
   The air deflector  70  is mounted to the speaker  10  in a manner similar to the deflector  50 . The lower portion  72  is slid over the outer surface of the pole piece  30  until the tapered joint  76  of the deflector  70  contacts the top end of the pole piece  30 . The diameter of the lower portion  72  is chosen to frictionally engage the outer surface of the pole piece  30  so that the deflector  70  remains in place during operation of the speaker  10 , while allowing the lower portion  72  to be readily slid along the pole piece  30  during assembly. As best seen in  FIGS. 4 and 6 , when the deflector  70  is in position on the pole piece  30  it is located within the interior of the voice coil  32 . The bores  84  in the top portion  74  of the deflector  70  are positioned in the area of the former  34  opposite at least a portion of the wire winding  36  of the voice coil  32 . 
   As shown in the FIGS., the lower portion  54  of deflector  50  and lower portion  72  of deflector  70  may, although do not necessarily, extend along the pole piece  30  throughout the length of the magnetic gap  33 . This construction, coupled with the fact that each of the deflectors  50  and  70  is formed of copper, assists in optimizing the inductance and impedance characteristics of the speaker  10 . 
   Speaker Operation 
   As described above, during operation of the speaker  10  the voice coil  32  is moved axially with respect to the fixed motor structure  12 . The diaphragm  16 , spider  18 , surround  20  and dust cap  44  move as a unit with the excursion of the voice coil  32  to create a “pumping” action causing comparatively cool air from outside of the speaker  10  to flow in and out of the dust cap cavity  46  inside of the speaker  10 . In response to movement of the voice coil  32  in one direction, air from outside of the speaker  10  enters the through bore  31  of the pole piece  30  and flows in a direction toward the dust cap cavity  46 . When the voice coil  32  reverses direction, air exits the dust cap cavity  46  and flows through bore  31  of the pole piece  30  out of the speaker  10 . 
   With an air deflector  50  or  70  in place as described above, the flow of air into and out of the dust cap cavity  46  is deflected and directed into contact with the inner surface  35  of the former  34  of the voice coil  32  in the area opposite the wire winding  36 . The relatively cool air entering the through bore  31  of the pole piece  30  from outside of the speaker  10  is blocked from directly flowing into the dust cap cavity  46  by the closed top end of deflector  50  or  70 . Instead it must pass through the bores  66  of deflector  50  or bores  84  of deflector  70 . These bores  66  or  84  direct the cooling air against the inner surface  35  of the voice coil former  34 , and then the air continues into the dust cap cavity  46 . When the air exits the dust cap cavity  46 , it must flow along the inner wall  35  of the former  34  opposite the wire winding  36  before entering the bores  66  or  84  leading to the through bore  31  of the pole piece  30 . This flow path created by the air deflectors  50  and  70  assists in cooling the voice coil  32 , and also provides venting for the dust cap cavity  46 . 
   The bores  66  in air deflector  50  and the bores  84  in air deflector  70  are shown in the FIGS. as generally aligning with at least some of the wraps of the wire winding  36  on the outer surface  37  of the voice coil former  34 . Particularly with high excursion speakers, such as subwoofers, the position of the wire winding  36  changes significantly with the movement of the voice coil  32  during speaker operation. Nevertheless, it is contemplated that a substantial portion of the inner surface  35  of the former  34  opposite the wire winding  36  will be impacted and cooled by the air flow from deflectors  50  and  70 . 
   Although the air deflectors  50  and  70  may be employed in subwoofers, it is contemplated that they may be especially useful in smaller, midrange speakers. Both air deflectors  50  and  70  are easily manufactured at relatively low cost, thus providing an efficient but modestly priced structure for enhancing cooling of the voice coil  32 . 
   While the invention has been described with reference to a preferred embodiment, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. 
   For example, the speaker  10  of this invention is illustrated with a dust cap  44  connected to the diaphragm  16  in position overlying the voice coil  32  and pole piece  30 . In this construction, the dust cap cavity  46  is formed by the diaphragm  16 , dust cap  44 , voice coil  32  and pole piece  30 . It is also contemplated that the dust cap  44  could be removed, and the diaphragm  16  directly connected to the top end of the voice coil  32  thus forming a cavity (not shown) in an area beneath the diaphragm  16 , overlying the voice coil  32  and pole piece  30 , without a dust cap  44 . The term “dust cap cavity” as used herein is therefore also intended to apply to such cavity where the dust cap  44  is removed. 
   Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.