Abstract:
A loudspeaker comprises a speaker frame and a diaphragm connected to the speaker frame for reciprocal movement relative thereto. A generally tubular former is connected to the diaphragm, and a voice coil is connected to the former at a location spaced from the diaphragm. The former is constructed of a thermally conductive material for conducting heat away from the voice coil. An airflow director is positioned at least partially in the former, with a first gap being formed between the airflow director and an inner surface of the former and a second gap being formed between the airflow director and the pole piece. The first and second gaps are in fluid communication with each other and the pole vent opening such that movement of the diaphragm causes airflow through the first and second gaps and the pole vent opening. With this construction, heat generated in the coil during operation of the loudspeaker is transferred to the former through conduction, and heat present in the former is transferred through the first and second gaps and the pole vent opening through convection to thereby cool the loudspeaker.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates in general to loudspeakers which produce sound in response to an audio signal, and more particularly to a loudspeaker with an improved air cooling system. 
     Conventional loudspeakers typically employ a diaphragm which is vibrated by an electromechanical driver. The driver generally comprises a permanent magnet and a voice coil through which an electrical signal is passed from an audio amplifier. Changing voltage in the audio frequency range is applied to the terminals of the voice coil causing a corresponding changing current to flow through the windings of the voice coil. The interaction between the current passing through the voice coil and the magnetic field produced by the permanent magnet causes the voice coil to oscillate in accordance with the electrical signal. Since the voice coil is rigidly attached to the a diaphragm, oscillation of the voice coil causes a corresponding oscillation in the diaphragm to produce acoustical output. 
     A substantial portion of the impedance associated with electromechanical drivers is caused by the wire that forms the voice coil due to the wire&#39;s DC resistance. Accordingly, most of the electrical power applied to the voice coil is converted into heat rather than sound. The ultimate power handling capacity of the voice coil, and thus the loudspeaker, is limited by the ability of the device to tolerate heat. Heat tolerance is generally determined by the lowest melting point of wire insulation and other components, as well as the heat capacity of the adhesive used to construct the voice coil. 
     The problems produced by heat generation are further compounded by temperature-induced resistance, commonly referred to as power compression. As the temperature of the voice coil increases, the DC resistance of the copper or aluminum conductors or wires used in the voice coil also increases, resulting in progressively decreasing efficiency. For example, a copper wire voice coil that has a DC resistance of eight ohms at 68° C. will have a DC resistance of 16 ohms at 270° C. At 270° C., the voice coil will draw less power from the voltage applied to its terminals, and a substantial portion of the power that it does draw will be converted into heat. Consequently, the loudspeaker, which is a relatively inefficient transducer at room temperature, will be further reduced in efficiency at high voice coil temperatures. This power compression increases as the voltage applied to the voice coil increases, and can reach a point where a further increase in applied voltage results in virtually no increase in acoustical output, only a further increase in heat. 
     It is therefore desirable to provide a loudspeaker with a voice coil that can be cooled during operation. Reducing voice coil temperature will increase both the efficiency and power capacity of a loudspeaker; as well as its reliability and service life. 
     The prior art offers different solutions to voice coil cooling. By way of example, U.S. Pat. No. 4,757,547 issued to Danley on Jul. 12, 1988, discloses an air-cooled loudspeaker that has a voice coil positioned in an annular gap formed by pole pieces of a permanent magnet. The voice coil is cooled by directing pressurized air through the gap and over the voice coil. Typically, the clearances between the voice coil and the boundaries of the gap are quite small, usually under 0.020 inch. In order to adequately cool the voice coil, air must be forced through these clearances at a relatively high air flow rate and pressure which, consequently, can cause undesirable noise and distortion in the loudspeaker. 
     U.S. Pat. No. 5,042,072 issued to Button on Aug. 20, 1991, discloses a self-cooled loudspeaker that has a voice coil positioned in an annular gap between a permanent magnet and a pole piece. Axially extending air channels are formed at particular locations around the circumference of the pole piece to cool portions of the voice coil. Although this structure does not require forcing pressurized air through a relatively small gap, there is a reduction of magnetic flux at the axial air passages since portions of the pole piece have been cut away. 
     U.S. Pat. No. 5,357,586 issued to Nordschow et al. on Oct. 18, 1994, discloses an air-cooled loudspeaker system having aerodynamically-shaped passages that primarily cool the magnetic structure through induced airflow from vibratory movement of a speaker cone. The only direct cooling of the voice coil results from air flowing in the narrow clearances between the voice coil and the boundaries of the magnetic gap. Because of the relatively low air pressure created by the induced airflow, relatively little air will actually flow over the voice coil to cool it. 
     SUMMARY OF THE INVENTION 
     According to the invention, a loudspeaker comprises a speaker frame and a diaphragm connected to the speaker frame for reciprocal movement relative thereto. A generally tubular former is connected to the diaphragm, and a voice coil is connected to the former at a location spaced from the diaphragm. The former is constructed of a thermally conductive material for conducting heat away from the voice coil. A permanent magnet has a central opening and a pole piece has a pole vent opening that is coincident with the central opening. The voice coil is located in a space formed between the permanent magnet and the pole piece. An airflow director is positioned at least partially in the former, with a first gap being formed between the airflow director and an inner surface of the former and a second gap being formed between the airflow director and the pole piece. The first and second gaps are in fluid communication with each other and the pole vent opening such that movement of the diaphragm causes airflow through the first and second gaps and the pole vent opening. 
     With this construction, heat generated in the coil during operation of the loudspeaker is transferred to the former through conduction, and heat present in the former is transferred through the first and second gaps and the pole vent opening through convection to thereby cool the loudspeaker. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The preferred embodiments of the present invention will hereinafter be described in conjunction with the appended drawings, wherein: 
     FIG. 1 is a sectional view of a prior art loudspeaker that employs a vented pole piece; 
     FIG. 2 is an enlarged sectional view of the prior art loudspeaker of FIG. 1 showing air flow through a vent opening in the pole piece; 
     FIG. 3 is a sectional view of a loudspeaker according to a first embodiment of the present invention; 
     FIG. 4 is a bottom plan view of an airflow director for use in the loudspeaker of FIG. 3; 
     FIG. 5 is sectional view of the airflow director taken along line  5 — 5  of FIG. 4 
     FIG. 6 is a sectional view of the airflow director taken along line  6 — 6  of FIG. 4; 
     FIG. 7 is a sectional view of a loudspeaker according to a second embodiment of the invention; 
     FIG. 8 is a sectional view of a loudspeaker according to a third embodiment of the invention; and 
     FIG. 9 is a sectional view of a loudspeaker according to a fourth embodiment of the invention. 
    
    
     The invention will now be described in greater detail with reference to the drawings, wherein like parts throughout the drawing figures are represented by like numerals. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, and to FIGS. 1 and 2 in particular, a prior art loudspeaker  10  is shown in cross section. The loudspeaker  10  comprises a diaphragm assembly  12  and a driver assembly  14  that operates the diaphragm assembly for producing acoustical output. 
     The diaphragm assembly  12  includes a cone  16  attached to a dome  18  through adhesive or the like to form a diaphragm  20 . The diaphragm  20  has a flexible upper suspension  22  that is connected to an upper end  24  of a rigid frame  26 . A lower end  28  of the cone  16  is connected to a former  30  which forms part of the driver assembly  14 . The former is in turn connected to the frame  26  through a flexible spider  32  that extends between the former  30  and a lower end  34  of the frame. With this arrangement, the diaphragm  20  is free to move in an axial direction but is restrained from movement in a radial direction with respect to the frame  26 . 
     The driver assembly  14  includes a voice coil  36  mounted on the former  30  and a permanent magnet assembly  40  that cooperates with the voice coil for driving the diaphragm. 
     The voice coil  36  is typically constructed of aluminum or copper wire and is attached to the former  30  through a conventional adhesive. The voice coil  36  is electrically connected to terminals  42  of the loudspeaker through wires  44 . 
     The permanent magnet assembly  40  is generally annular in shape and is centrally located with respect to a central axis of the diaphragm assembly  12 . The permanent magnet assembly  40  includes a permanent magnet  50  disposed between a top plate  52  and a back plate  54 . The top plate  52  is rigidly connected to the frame  26 . The top and back plates are constructed of a material capable of carrying magnetic flux, such as steel. A pole piece  56  of generally cylindrical shape is connected to the back plate  54  and extends generally toward the diaphragm  20 . The pole piece includes a pole vent  58  that is coincident with an opening  55  in the top plate  54 . A space or gap  60  is formed between the pole piece  56  and the top plate  52 , permanent magnet  50 , and back plate  54 . The voice coil  36  is positioned in the gap  60 . 
     In use, changing current is applied to the voice coil  36  through the terminals  42 . The voice coil  36  in turn produces a magnetic field which interacts with the magnetic field produced by the permanent magnet assembly  40 . The interaction of the magnetic fields causes the voice coil  26  to oscillate linearly in accordance with the applied changing current. Oscillation of the voice coil  26  in turn pumps the diaphragm  20  linearly to generate sound. Movement of the diaphragm causes a change in volume of the airspace between the diaphragm assembly  12  and driver assembly  14 . When the diaphragm  20  moves away from the pole piece  56 , air is drawn toward the diaphragm  20  through the vent opening  55  of the bottom plate  54  and the pole vent opening  58  of the pole piece  56 . Likewise, when the diaphragm  20  moves toward the pole piece  56 , air is pushed through the pole vent  58  and opening  55 , as represented by arrows  61  in FIG.  2 . Movement of air through the pole vent  58  in this manner provides some cooling for the pole piece  56 , but relatively little or no direct cooling of the voice coil  36 . 
     With reference now to FIG. 3, a sectional view of a loudspeaker  70  according to a preferred embodiment of the present invention is illustrated. The loudspeaker  70  includes a generally cup-shaped airflow director  72  that is preferably positioned on the top of the pole piece  56 . The airflow director  72  is preferably constructed of a relatively rigid material that exhibits stable material properties at the maximum operating temperature of the loudspeaker  70 . 
     With additional reference to FIGS. 4-6, the airflow director  72  preferably includes a continuous side wall  74  connected to a bottom wall  76  to form a hollow interior  78 . The bottom wall  76  is preferably concave and divided into sectors  80  with a support rib  82  extending between each sector. A raised rib portion  84  is preferably formed on an inner end of each rib  82 . Preferably, the support ribs  82  and raised rib portions  84  intersect at the center of the airflow director  72 . Each sector  80  is preferably concave in cross section as shown most clearly in FIG.  5 . 
     The ribs  82  of the airflow director  72  are preferably bonded to an upper surface  90  of the pole piece  56  with a suitable high temperature adhesive. The raised rib portions  84  are preferably dimensioned so as to extend into and fit snugly with the pole vent  58 . In this manner, the airflow director  72  can be quickly and easily aligned and installed on the pole piece  56  during assembly of the loudspeaker  70 . Although four ribs and four sectors are shown, it will be understood that more or less ribs and/or sectors can be provided. 
     The bottom wall  76 , including the ribs  82 , is preferably dimensioned and shaped to form a gap  92  between the upper surface  90  of the pole piece  56  and the bottom wall  76 . Preferably, the sectors  80  of the bottom wall are concave so that coaxial annular areas of the gap extending between the upper surface  90 , of pole piece  56  and the bottom wall  76  and transverse to the direction of air flow are substantially constant across substantially any diameter of the gap. As shown, the distance X1 between the pole piece  56  and the bottom wall  76  at their outer diameters is less than the distance X2 between the pole piece and the bottom wall at a smaller diameter. The constant area is maintained at each annular area in the gap due to the longer circumferential length associated with the distance X1 and the shorter circumferential length associated with the distance X2. Preferably, the substantially constant area of the gap is approximately equal to a cross sectional area of the pole vent  58 . The side wall  74  of the air flow director  72  is also preferably dimensioned and shaped to form a gap  94  between the former  30  and the side wall  74 . Preferably, the annular areas of the gap  94  are each approximately equal to the cross sectional area of the pole vent  58 . With this arrangement, air passing through the gaps  92 ,  94  and the pole vent  58  will be substantially unrestricted. 
     In use, air is pumped in and out of the pole vent  58  through the gap  92  adjacent the former  30  during movement of the diaphragm  20 . The former  30  is preferably constructed of a thermally conductive material, such as aluminum, so that heat generated by the voice coil  36  is conducted along the former  30  adjacent the gap  94 . Heat from the former  30 , and thus the voice coil  36 , can then be convectively removed from the loudspeaker  70  through air flow in the direction represented by arrows  96  during movement of the diaphragm  20  toward the pole piece  56 . Thus, the voice coil  36  can be cooled during operation of the loudspeaker  70  without forcing pressurized air through the relatively narrow gap  60  coincident with the voice coil  36 . In this manner, the loudspeaker  70  is capable of operation at higher temperature or electrical power and will have less noise and distortion than the prior art. 
     With reference now to FIG. 7, a sectional view of a loudspeaker  100  according to a further embodiment of the invention is illustrated. The loudspeaker  100  has a generally cup-shaped airflow director  102  that is preferably positioned on the top of the pole piece  56 . The airflow director  102  is similar in configuration to the airflow director  72 , with the exception that a generally inverted cup-shaped cap  104  is preferably positioned on a top edge of the continuous wall  74  to enclose the hollow interior  78 . As shown, the cap  104  includes an upper wall  106  a continuous side wall  108  that extends downwardly from the upper wall. The upper wall  106  preferably abuts the upper edge of the side wall  74 . The side wall  108  of the cap  104  has an inner diameter that is preferably greater than an outer diameter of the side wall  74  such that a gap  110  is formed between an outer surface of the former  30  and an inner surface of the side wall  108 . 
     In use, air that is pumped in and out of the pole vent  58  due to movement of the diaphragm  20  flows in the gaps  92 ,  94  and  110  adjacent the former  30 . Heat generated by the voice coil  36  is conducted along the former  30  adjacent the gaps  94  and  110 . Heat from the former  30 , and thus the voice coil  36 , can then be convectively removed out of the loudspeaker  70  by air flowing in the direction represented by arrows  112  and  96  during movement of the diaphragm  20  toward the pole piece  56 . The gap  110  is especially advantageous since the pumped air is directed across both the inner and outer surfaces of the former  30 , which functions as a cooling rib, to remove heat from the former  30  through convective heat transfer. 
     With reference now to FIG. 8, a sectional view of a loudspeaker  120  according to a further embodiment of the invention is illustrated. The loudspeaker  120  is similar in construction to the loudspeaker  100  previously described. In this embodiment vent holes  122  are preferably formed in the cone  16  below the dome  18 . A tube  125  is in fluid communication with the pole vent  58 . The tube  125  is in turn connected to a source of pressurized air (not shown). 
     In use, air under pressure from the pressurized air source enters the pole vent  58 , travels through the gaps  92 ,  94  and  100 , and exits through the vent holes  122  as shown by arrows  124 ,  126 , and  128 , respectively. The use of pressurized air in this manner increases the convective transfer of heat from the former  30  beyond that generated by the pumping action of the diaphragm alone, and thus increases the cooling of the voice coil  36 . 
     Turning now to FIG. 9, a sectional view of a loudspeaker  140  according to a further embodiment of the invention is illustrated. The loudspeaker  140  has a generally cup-shaped airflow director  142  that is preferably positioned on the top of the pole piece  56  in a manner similar to the airflow director  72  previously described. The airflow director  142  is preferably constructed of a heat conducting material and is similar in configuration to the airflow director  72 , with the exception that the raised rib portions  84  preferably extend along at least a substantial length of the pole vent  58 . The raised rib portions  84  may also extend through the opening  55 , as shown. The outer edge of each raised rib portion  84  is preferably bonded to the inner surface of the pole vent with a thermally conductive adhesive. Likewise, the ribs  82  of the airflow director  142  are preferably bonded to the upper surface  90  of the pole piece  56  with a thermally conductive adhesive. 
     In use, a portion of the heat generated by the voice coil  36  is transferred to the pole piece  56  through convection. This heat is then directly conducted to the raised rib portions  84  through conductive heat transfer. Movement of the diaphragm  20  toward the drive assembly  14  causes air to flow through the gaps  94 ,  92  and the pole vent  58  over the raised rib portions  84  to thereby transfer heat from the raised rib portions  84  and pole piece  56  out of the loudspeaker  140 , as well as heat from the former  30  as previously described. Although not shown, a generally inverted cup-shaped cap, as previously described, can be positioned on a top edge of the continuous wall  74  to form a gap to direct air around both sides of the former  30 . 
     Although the airflow director in each of the above embodiments has been shown and described as generally cylindrical, it will be understood that the airflow director can have different shapes. 
     It will be understood that the terms outer, inner, upper, lower and their respective derivatives and equivalent terms as may be used throughout the specification refer to relative, rather than absolute positions and/or orientations. 
     While the invention has been taught with specific reference to the above-described embodiments, those skilled in the art will recognize that changes can be made in form and detail without departing from the spirit and the scope of the invention. Thus, the described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.