Patent Grant
7177439
The present invention is directed to methods and apparatus for dissipating heat in a voice coil of a loudspeaker, which improves heat transfer from the voice coil to a heatsink.
Loudspeakers (commonly called “speakers”) are designed for the reproduction of audio signals having a frequency range of approximately 20 Hz to 20 kHz and a pressure range of approximately 10−5 to 50 pascals, or 10−9 to 7×10−3 lbf/in.2.
A loudspeaker system normally includes one or more drivers (a transducer mechanism without a structural radiation enclosure), a crossover network (ensuring that a received electrical drive signal is within an optimum frequency range), and an enclosure. Loudspeakers are used in many different consumer products, such as home and automobile stereos, television and radio receivers, electronic musical instruments, toys, etc. Loudspeakers are also used in any number of professional applications, such as in broadcast stations, recording studios, concert halls, etc.
Loudspeakers may be classified in accordance with several factors, including type of radiation, type of driving element, reproduction range, and diaphragm shape. The type of radiation may include direct radiation and horn-loaded radiation. The driving element may be a magnetic element, an electrostatic element, a piezoelectric element, an ionophone element, or an air-jet element. Magnetic driving elements include dynamic (moving-coil, ribbon, etc.), moving-armature, and magnetostrictive technologies. Reproduction ranges include low frequency (woofer and subwoofer) ranges, mid-frequency (midrange and squawker) ranges, high-frequency (tweeter and super-tweeter) ranges, and full-ranges. Diaphragm shapes include cone (e.g., straight, parabolic, flared, etc.), dome, and flat shapes.
A commonly used loudspeaker classification is the dynamic (moving-coil) direct-radiator loudspeaker. In this type of loudspeaker, a permanent magnet produces a high flux density in a narrow air gap in which a moving voice coil is located. The interaction of the flux of the permanent magnet and an alternating current flowing within the voice coil produces a force that moves a diaphragm to achieve a piston action. The movement of the diaphragm causes corresponding acoustic sound waves, which are preferably linearly related to the electrical driving signal in order to produce high fidelity sound. Further details concerning conventional loudspeaker technology may be found in McGraw-Hill, Encyclopedia of Electronics and Computers, pp. 512–518 (2nd ed., 1988).
A significant disadvantage associated with the dynamic (moving-coil) direct-radiator loudspeaker is that it has a relatively low radiation efficiency, i.e., a ratio of sound output power to electrical input power. Indeed, the radiation efficiency of this type of loudspeaker is on the order of 0.5 to 4 percent. This inefficiency generally results in a majority of the electrical input power being converted into heat.
The voice coil of the loudspeaker is the primary heat generating element. Conventional voice coil assemblies include a helical coil of electrical/magnet wire supported by a bobbin. The helical coil may be formed of a single layer or multiple layers of wire. The electrical/magnet wire may take on various shapes, such as IE, round, flat, etc. The bobbin typically consists of a single layer or multiple layers of sheet-like materials, for example, polyimide, aluminum, aromatic fiber, etc. The bobbin is shaped into a desired geometry around which the voice coil is wound. The bobbin supports the voice coil by way of adhesion between the voice coil and the bobbin. Such adhesion may be made to the inside, middle, outside, or a combination inside/outside of the voice coil. As the bobbin is typically used to provide a mechanical connection between the voice coil and the diaphragm (or speaker cone), a relatively high stiffness is desirable. In some instances, multiple layers of material are employed to increase the stiffness of the bobbin. Such layers may be placed in any number of locations along the bobbin to achieve such stiffness.
It is desirable that the bobbin exhibit stable thermal characteristics, particularly because the voice coil produces a significant amount of heat and operates at elevated temperatures. Conventional high-power loudspeakers may employ high-temperature materials in forming the bobbin such that it remains relatively stiff at elevated temperatures. Such materials include high glass transition point materials, i.e., TG and the like. Unfortunately, these high-temperature materials exhibit extremely poor thermal conductivity, which results in a thermal insulation layer between the voice coil and any fluids and/or structures proximate to the bobbin. For example, air, ferrofluids, etc. may occupy volumes within and/or around the bobbin; however, owing to the thermal insulation characteristics of the high-temperature materials utilized to produce the bobbin, relatively poor thermal conductivity is exhibited between the voice coil and such fluids. This disadvantageously increases thermal time constants between the voice coil and any nearby heat wicks (and/or other heatsink structures), and results in the elevation of voice coil temperatures.
Attempts at solving the above-described thermal management issue have been made, including forced air flow, metallic bobbin materials, impregnated bobbin materials, and inside/outside coil assemblies (e.g., a bobbin disposed between two voice coils). Each of these attempts were unsatisfactory. Forced air flow techniques require through-holes in the assembly or increasing the area around the voice coil to permit such air flow. These techniques, however, reduce the magnetic field and degrades performance. Although metallic bobbins exhibit good thermal conductivity, they cause back electro-motive force (BEMF), which further reduces the efficiency of the loudspeaker. Impregnated bobbin materials exhibit only marginal improvements in thermal conductivity, while exhibiting poor bonding strength and in some cases, BEMF. In inside/outside voice coil assemblies, the heat buildup between the voice coil and the bobbin (the bond line) is increased by a factor of two and the bond line exhibits poor thermal conductivity as compared with a single (inside or outside) design. This is so because the bond line is subjected to heat from both sides and any heat transfer out of one of the voice coils must traverse a heat source (the opposite voice coil) to reach ambient fluids.
Accordingly, there are needs in the art for new methods and apparatus for dissipating heat in a voice coil of a loudspeaker, which enjoy relatively high bobbin stiffness, bobbin thermal stability, and low thermal time constants between the voice coil and adjacent heat wicks.
A loudspeaker assembly in accordance with one or more aspects of the present invention includes: a voice coil; and a bobbin having a wall member operable to support the voice coil, the wall member including at least one aperture operable to provide thermal communication from the voice coil through the wall member. Preferably, the wall member is substantially cylindrical, although other shapes are also contemplated, such as oval, etc.
Preferably, the at least one aperture is shaped such that a reduction in a shear strength of the bobbin is substantially minimized. For example, the at least one aperture preferably has a shape that does not include sharp corners.
The total area defined by the respective sizes of the at least one aperture is preferably maximized.
Preferably, the voice coil includes an inner part defining an inner volume and an outer part; the loudspeaker further comprises a magnetic pole disposed at least partially within the inner volume of the voice coil and is operable to direct a magnetic flux therethrough; and the wall member of the bobbin includes an outer surface operable to support the voice coil and an inner surface defining an inner volume, the wall member including at least one aperture operable to provide thermal communication between the inner part of the voice coil and the magnetic pole.
The loudspeaker assembly preferably further includes a heatsink coupled to the magnetic pole and being operable to receive heat therefrom, wherein the aperture is sized, shaped, and located such that it is operable to provide thermal communication between the voice coil and the heatsink.
In accordance with one or more further aspects of the present invention, an apparatus includes a bobbin having a wall member (which is preferably cylindrical, oval, etc.) including an outer surface operable to support a voice coil of a loudspeaker and an inner surface defining an inner volume; and a heatsink coupled to the outer surface of the bobbin and being in thermal communication with the voice coil.
The heatsink preferably includes a plurality of fins extending radially away from the outer surface of the bobbin. The apparatus preferably further includes a diaphragm having an inner peripheral edge and an outer peripheral edge, the inner peripheral edge being operatively coupled to the outer surface of the bobbin, wherein the fins of the heatsink are operatively coupled to the diaphragm.
The diaphragm extends obliquely away from the outer surface of the bobbin to form a cone shape; and the fins of the heatsink each include a first edge extending along the outer surface of the bobbin and a second edge operatively coupled to the diaphragm thereby increasing a strength of the diaphragm.
The diaphragm includes a forward surface and a rearward surface, the forward surface defines an acute angle with respect to the outer surface of the bobbin; and the second edge of each fin defines a corresponding acute angle with respect to the first edge of the fin such that a substantial portion of the second edge of the fin is operable to couple to the forward surface of the diaphragm.
The apparatus preferably further includes a thermally conductive member being coupled to the outer surface of the bobbin and being in thermal communication with the voice coil and the heatsink. The thermally conductive member includes an inner surface and an outer surface; the inner surface of the thermally conductive member is coupled to the outer surface of the bobbin; and the inner wall of the heatsink is coupled to the outer surface of the thermally conductive member.
The voice coil defines an inner wall and an outer wall, the inner wall of the voice coil being coupled to the outer surface of the bobbin; and the inner surface of the thermally conductive member is in thermal communication with outer wall of the voice coil. Preferably, the voice coil defines an inner wall and an outer wall, the inner wall of the voice coil being coupled to the outer surface of the thermally conductive member.
In accordance with one or more further aspects of the invention, the speaker assembly may further include a heatsink coupled to the inner surface of the wall member of the bobbin such that it is in thermal communication with the voice coil. The heatsink includes a plurality of fins extending axially along and radially inward from the inner surface of the bobbin such that axial movement of the bobbin forces air within the inner volume of the bobbin to carry heat away from the second heatsink.
In accordance with one or more further aspects of the invention, the speaker assembly may further include: a pole disposed at least partially within the inner volume of the voice coil that is operable to direct a magnetic flux therethrough, the pole including an aperture extending therethrough that is in axial alignment with the bobbin and the voice coil; and a heatsink coupled to an inner surface of the aperture of the pole, wherein the heatsink includes a plurality of fins extending axially along and radially inward from the inner surface of the aperture such that axial movement of the bobbin forces air to carry heat away from the heatsink.
Other aspects, features, advantages, etc. will become apparent to one skilled in the art in view of the description herein taken in conjunction with the accompanying drawing.
For the purposes of illustrating the invention, there are shown in the drawings forms that are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.
With reference to the drawings, wherein like numerals indicate like elements, there is shown in
In general, the permanent magnet 22 induces a magnetic flux in the top plate 20, the back plate 24, and the magnetic pole 18 such that the magnetic flux is directed through the voice coil 14. An electrical drive signal is applied to the voice coil 14 in order to induce an alternating current in the voice coil 14, which creates a proportional electromagnetic flux. The electromagnetic flux of the voice coil 14 interacts with the magnetic flux produced by the permanent magnet 22, thereby creating a force on the voice coil 14 in the upward/downward direction. The force tends to move the voice coil 14 and the bobbin 12 because the voice coil 14 is mechanically coupled to the bobbin 12. As an inner peripheral edge 30 of the diaphragm 16 is mechanically coupled to the bobbin 12, the movement of the bobbin 12 in response to the electrical drive signal causes a corresponding movement of the diaphragm 16. The movement of the diaphragm 16 creates sound waves in proportion to the electronic drive signal.
The voice coil 14, which itself may be of conventional construction, may exhibit a real resistance of approximately 4, 8, or 16 Ohms. Other resistances are also contemplated. The current induced in the voice coil 14 by way of the electrical drive signal causes a temperature rise in the voice coil 14, which over time tends to reduce the useful life of the loudspeaker 10. This temperature rise also increases the resistance of the voice coil 14 and reduces the efficiency of the loudspeaker 10 (sometimes by 50%). So-called power compression may also occur. Power compression occurs when an operator increases the electrical drive signal (e.g., current) to the loudspeaker 10 in order to compensate for a lower acoustic output power resulting from the reduction in efficiency (caused by a temperature increase in the voice coil 14). The increased drive signal contributes to further increases in the temperature and resistance of the voice coil 14, and further reductions in efficiency and acoustic output power. This is an undesirable positive feedback scenario. In accordance with one or more aspects of the present invention, however, advantageous thermal management is employed, which tends to reduce the temperature elevation in the voice coil 14 resulting from the electrical drive signal.
With reference to
As shown in
The bobbin 12 (and any additional layers 12C) preferably include at least one aperture 32 through the wall member, which are operable to provide thermal communication between the inner part 14B of the voice coil 14 and an inner volume 34 defined by the inner surface 12B of the bobbin 12. Advantageously, the apertures 32 improve the thermal conductivity from the voice coil 14 to any structures and/or fluids within the volume 34, even in the presence of poor thermally conductive materials, such as a high temperature material layer 12 and 12C.
As best seen in
It is noted that in the embodiment illustrated in
Reference is now made to
As it is desirable that the bobbin 12 exhibit substantial stiffness and strength, even at elevated temperatures, it is preferred that the apertures 32 are sized and/or shaped such that any reductions in strength, such as sheer strength, of the bobbin 12 is substantially minimized. In this regard, it is preferred that the apertures 32 have shapes that do not include substantially sharp corners. Indeed, sharp corners represent relatively high energy sites that may develop cracks and/or otherwise weaken the bobbin 12, particularly in the presence of relatively severe accelerations. Thus, the curved shapes of the apertures 32 in
Turning again to
It is preferred that a total area defined by the respective sizes of the apertures 32 is maximized in order to maximize the thermal communication from the voice coil 14 through the apertures 32. While it is understood that the present invention is not limited by any theory of operation, it has been discovered that there is a combination of aperture size, shape, and/or location that can substantially minimize any reduction in strength of the bobbin 12 while substantially maximizing the sizes of the apertures 32 in order to achieve an advantageous balance between strength and thermal management goals.
Reference is now made to
In accordance with one or more aspects of the present invention, the apertures 32 are preferably sized, shaped, and/or located on the bobbin 12 in such a way that they provide thermal communication between the voice coil 14 and the heatsink 46 during at least some displacements. In this way, some of the heat generated by the voice coil 14 is communicated to the magnetic pole 18 through the apertures 32, while some of the heat is communicated to the heatsink 46 through the apertures 32. While the present invention is not intended to be limited by any theory of operation, the respective amounts of heat communicated to the magnetic pole 18 and the heatsink 46 through the apertures 32 is proportional to the integral of the time that the apertures 32 are located adjacent to the magnetic pole 18 and the heatsink 46.
Advantageously, the apertures 32 through the wall member of the bobbin 12 provide thermal communication from the voice coil 14 through the bobbin 12 to structures and/or fluids that assist in reducing the temperatures at which the voice coil 14, bobbin 12, etc. operate. This advantageously improves the performance and useful life of the loudspeaker 10.
Reference is now made to
The bobbin 12 includes a forward axial end 60 and a rearward axial end 62. The voice coil 14 may be disposed toward the rearward axial end 62, while the heatsink 50 may be disposed toward the forward axial end 60. As will be discussed in more detail below, this permits the heatsink 50 (in particular the fins 54 thereof) to be operatively coupled to the diaphragm 16. As it is desirable that the heatsink 50 be in thermal communication with the voice coil 14, it is preferred that a thermally conductive member 52 is coupled to the outer surface of the bobbin 12A such that it is in thermal communication with, and provides thermal communication between, the voice coil 14 and the heatsink 50. The thermally conductive member 52 preferably includes an inner surface 52A and an outer surface 52B. The inner surface 52A of the thermally conductive member 52 is preferably coupled to the outer surface 12A of the bobbin 12. The inner wall 56 of the heatsink 50 is preferably coupled to the outer surface 52B of the thermally conductive member 52. In this way, the heatsink 50 is advantageously in thermal communication with the voice coil 14. It is noted that the thermally conductive member 52 may be omitted when the bobbin 12 has sufficient thermal conductivity properties. It is also noted that the heatsink 50 and the thermally conductive member 52 may be integrally formed if desired.
As shown in
While any suitable material may be employed to form the thermally conductive member 52, such as copper, aluminum, etc., it is preferred that the materials are not metallic in order to avoid the creation of eddy currents (and BEMF), which oppose the desired movement of the bobbin 12/voice coil 14 assembly. For example, carbon pitch fiber may be utilized to form the thermally conductive member 52, which material exhibits satisfactory thermal conductivity without employing electrically conductive materials.
The inner peripheral edge 30 of the diaphragm 16 is preferably coupled to the outer surface 52B of the thermally conductive member 52, or to the outer surface 12A of the bobbin 12 (when a thermally conductive member 52 is not employed). The diaphragm 16 includes a forward surface 16A and a rearward surface 16B. The diaphragm 16 extends obliquely away from the outer surface 52B of the thermally conductive member 52 such that the forward surface 16A of the diaphragm 16 defines a cone. Stated another way, the forward surface 16 of the diaphragm 16 defines an acute angle with respect to the outer surface 52B of the thermally conductive member 52.
The heatsink 50 is preferably operatively coupled to the forward surface 16A of the diaphragm 16. In particular, the fins 54 are coupled to the diaphragm 16. Preferably, the fins 54 of the heatsink 50 each include a first edge 54A extending along the outer surface 52B of the thermally conductive member 52 (or the outer surface 12A of the bobbin 12). These fins 54 also preferably include a second edge 54B operatively coupled to the forward surface 16A of the diaphragm 16. Advantageously, the increased area of mechanical coupling between the diaphragm 16 and the thermally conductive member 52/bobbin 12 increases the strength of the diaphragm 16, thereby improving its useful life.
Viewing the heatsink 50 as an overall envelope (
Advantageously, the heatsink 50 provides an additional way in which the heat from the voice coil 14 may be dissipated. In addition, the structure of the heatsink 50, vis-à-vis its mechanical coupling to the diaphragm 16, increases the strength of the diaphragm 16 and improves the operation and useful life of the loudspeaker 10A.
Reference is now made to
Advantageously, the heatsink 80 provides further means by which heat from the voice coil 14 may be dissipated. In addition, the structure of the heatsink 80, vis-à-vis its mechanical coupling to the bobbin 12, increases the strength of the bobbin 12 and improves the operation and useful life of the loudspeaker 10.
In accordance with one or more further aspects of the present invention, the pole 18 includes an aperture 92 extending therethrough that is in axial alignment with the bobbin 12 and the voice coil 14. Preferably, the back plate 24 (
Reference is now to
Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.
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