FLUX TRACKED VOICE-COIL SPEAKER

Abstract

A loudspeaker includes a voice coil assembly and a diaphragm connected to the voice coil assembly, wherein the voice coil assembly is configured to electromagnetically drive the diaphragm. The voice coil assembly includes a voice coil and a voice coil former upon which the voice coil is wound. The voice coil former is formed at least partially or completely from, or comprises at least a portion comprising, a material having a high permeability property.

Claims

1. A loudspeaker comprising: a voice coil assembly, comprising: a voice coil; and a voice coil former upon which the voice coil is wound; and a diaphragm connected to the voice coil assembly, wherein the voice coil assembly is configured to electromagnetically drive the diaphragm, wherein the voice coil former is formed at least partially or completely from, or comprises at least a portion comprising, a material having a high permeability property.

2. The loudspeaker according to claim 1, wherein the material is one of mu-metal, supermalloy, supermumetal, nilomag, sanbold, molybdenum permalloy, Mo permalloy and Supermalloy.

3. The loudspeaker according to claim 1, further comprising a magnet assembly comprising: a permanent magnet; and at least one pole piece, wherein the magnet assembly is arranged concentrically with the voice coil assembly, and wherein the magnet assembly is configured to provide a gap to accommodate the voice coil assembly between poles of the magnet assembly.

4. The loudspeaker according to claim 3, wherein the magnet assembly comprises: a front plate and a back plate as pole pieces; and an annular permanent magnet sandwiched between the front plate and the back plate (5), wherein the back plate and the front plate are concentrically mounted about a central axis of the diaphragm, the magnet assembly and the loudspeaker.

5. The loudspeaker according to claim 4, wherein the front plate and the back plate are magnetically permeable.

6. The loudspeaker according to claim 4, wherein the back plate has an extending piece extending centrally from the back plate and the extending piece has a diameter smaller than the back plate.

7. The loudspeaker according to claim 6, wherein the front plate has an opening sized to fit the extending piece.

8. The loudspeaker according to claim 4, wherein the voice coil assembly is configured to move along the central axis.

9. The loudspeaker according to claim 1, wherein the voice call former formed from a first material, and wherein one or more collars of high permeability material formed on a surface of the voice call former.

10. The loudspeaker according to claim 9, wherein the voice coil former is made from one or more of aluminium or paper.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] FIG. 1 is a cross-sectional view illustrating a loudspeaker assembly according to the invention.

[0030] FIG. 2 is a cross-sectional view of an example of a loudspeaker assembly according to the present invention.

[0031] FIG. 3 illustrates magnetic field lines of the loudspeaker assembly according to the prior art.

[0032] FIG. 4 illustrates magnetic field lines of the loudspeaker assembly according to the present invention when the voice coil is at the maximum height.

[0033] FIG. 5 illustrates magnetic field lines of the loudspeaker assembly according to the present invention.

[0034] FIG. 6 shows a graph illustrating the force factor BL(x) versus displacement (x) of the present invention compared to the prior art.

[0035] FIG. 7 shows another embodiment of the present invention.

DETAILED DESCRIPTION

[0036] Referring to FIG. 1, a cross-sectional view of a loudspeaker 10 according to a non-limiting embodiment of the present invention is provided. The loudspeaker 10 comprises a frame 13 with a frusto-conical shape and a magnet assembly 3, 4, 5, which is mounted at the smaller end of the frame 13.

[0037] The loudspeaker 10 may comprise a diaphragm 12, usually having a membrane element that may generally have the shape of a truncated section of a cone or curved-sided cone, with a dust cap membrane 15 extending across a narrower part of the diaphragm 12. The diaphragm 12 is mounted in the frame 13, such that the wider end of the diaphragm is secured to the wider end of the frame, for example by means of an adhesive, welding or other known techniques.

[0038] A cylindrical voice coil former 2 extends from the narrower region of the diaphragm 12 in a direction towards the magnet assembly 3, 4, 5. The outer surface of the voice coil former 2 is concentric around an axis 6. This axis 6 is the central axis of the diaphragm 12, magnet assembly 3, 4, 5 and typically also of the loudspeaker 10.

[0039] Preferably, a spider 14 extends from the narrower end of the diaphragm to the inner sides of the frame 13 and is provided to secure the voice coil former 2 with respect to the frame 13. The spider 14 can be formed from a rubber-like material. The spider 14 is connected to the lower portion of the diaphragm 12 and prevents the voice coil former 2 from moving in a direction which is perpendicular to the axis 6.

[0040] A voice coil 1, such as a wire, is wound around the outer surface of the coil former 2 and has a pair of lead wires (not shown) that receive electrical signals from an external amplifier. The electrical signal is usually a varying signal and will be translated into sound output. The lead wires of the voice coil 1 are usually, although not always, mounted to the diaphragm 12, for example with glue, such that the lead wires do not interfere with the movement of the voice coil assembly. The voice coil 1 is usually made out of copper, silver or other known and appropriate conductors.

[0041] A magnet assembly 3, 4, 5 and the voice coil assembly is depicted in FIG. 2. The magnet assembly 3, 4, 5 has typically, an annular shaped permanent magnet 4. The permanent magnet 4 might also have a square or rectangular outer shape with a hole in the middle. The permanent magnet 4 is sandwiched between an annular front plate 3 and a back plate 5, which are made of magnetically permeable material, such as iron, steel, ferrous alloy, nickel or cobalt alloy.

[0042] The back plate 5, preferably circular in shape, is attached to one annular face of the annular permanent magnet 4 and has an outer diameter that is typically similar, or equal, to the outer diameter of the annular permanent magnet 4. In the present embodiment of FIG. 2, a T-shaped back plate 5 is used, although a U-shaped back plate 5 could be used to achieve the same effect in the present invention.

[0043] In the embodiment shown in FIG. 2, the back plate 5 has an extending pole piece 8, thus forming a T-piece. The extending pole piece 8 is an element of the back plate 5 extending generally towards the diaphragm 12 in a direction along the central axis 6 of the magnet assembly. The end of the extending pole piece 8, furthest away from the back plate 5, is positioned adjacent to the inner annular face of the top plate 3. A radial gap 7 is thus formed between the outer surface of the pole piece 8 and the inner annular face of the top plate 3 as well as the cylindrical inner surface of the annular magnet 4.

[0044] The top plate 3 is mounted to the other face of the annular permanent magnet 4 from the back plate 5. The top plate 3 has an annular form with an outer diameter that is typically similar to that of the back plate 5 and an inside diameter, which is usually smaller than the inner diameter of the annular permanent magnet 4. The annular permanent magnet 4, the back plate 5, and the top plate 3 are preferably concentrically positioned about the axis 6. The top plate 3 and back plate 5 are bonded, or otherwise fixed, to the upper and lower surfaces of the magnet 4, cither by welding or gluing with an adhesive. When the top plate 3 and back plate 5 are fixed to annular magnet 4, they combine to form a magnetic flux path which passes through the radial gap 7.

[0045] The voice coil assembly of the loudspeaker 10 is positioned in the radial gap 7 defined by the inner surface of the top plate 3 and the outer surface of the extending pole piece 8. Preferably, the top plate 3 and the back plate 5 are made of ferromagnetic material such as nickel or iron. The radial gap 7 is sized to accommodate the voice coil 1 and the lower end of the voice coil former 2.

[0046] The voice coil assembly comprises the voice coil 1 wound on the voice coil former 2. When an electrical signal is applied across the terminals of the voice coil 1, a current moving through the turns of the voice coil 1 produces a varying magnetic field. The magnetic field generated by the voice coil 1 interacts with the magnetic field of the magnet assembly 3, 4, 5. This interaction creates a force on the voice coil assembly and moves the voice coil assembly, generally in a direction parallel with the axis 6. The direction of this movement is determined by the polarity of the applied electrical signal. Since the voice coil assembly is attached to the diaphragm 12, the diaphragm 12 moves along with the voice coil former 2 either towards the magnet assembly 3, 4, 5, or away from the magnet assembly 3, 4, 5. The resulting back-and-forth motion causes the diaphragm 12 to vibrate and drives the air in front of the diaphragm 12, resulting in pressure differentials that travel away as sound waves.

[0047] It is desirable that the radial gap 7 be as small as possible such that the magnetic force exerted on the voice coil is maximized without interfering with the free movement of the voice coil 1. As such, a clearance between the extending pole piece 8 and the front plate 3 is maintained between 0.2 mm to 1 mm, preferably between 0.2 mm to 0.6 mm. Clearance between the moving voice-coil 1 and the top plate 3, as well as between the former 2 and the pole piece 8 is maintained between 0.05 mm and 0.75 mm, preferably between 0.15 mm and 0.5 mm, depending on the type of the driver and its intended use.

[0048] The density of magnetic flux passing through the voice coil 1 has a direct impact on the intensity of the movement of the voice coil 1. This is because the interaction between the magnetic field created by the applied electrical signal to the voice coil 1 and the magnetic field of the magnetic assembly, cause movement of the voice coil. In order to maximise the force exerted on the voice coil 1, it is desirable to concentrate the magnetic flux within the radial gap 7.

[0049] The radial gap 7 is typically filled with the ambient atmosphere in which the loudspeaker 10 is being used, thus meaning that the magnetic flux tends to spread out when traversing the radial gap 7 and is reduced in concentration when compared to the magnetic flux in the top plate 3 and back plate 5. In order to concentrate the magnetic flux within the radial gap 7, crucially without increasing the loudspeaker's size and cost or the power of the applied signal, the present invention uses materials with high permeability to form the voice coil former 2. In the context of the present invention, the high permeability materials are considered as materials with a relative permeability of 10,000 or more.

[0050] FIG. 3 shows a sectional view of magnetic field lines which represent the magnetic flux created around the magnet assembly 23, 24 and the coil assembly 21, 22 according to a conventional, prior art, loudspeaker. For comparison, FIGS. 4 and 5 show a sectional view of the magnetic field lines of the magnet assembly 3, 4, 5 and the voice coil assembly according to the present embodiment. The magnetic field lines represent the density of the magnetic field and hence the denser magnetic field lines shown in the radial gap 7 of FIGS. 4 and 5, as compared with FIG. 3, show the increased field density which is achieved by the present invention.

[0051] In FIG. 3, the magnetic field lines around the voice coil assembly are shown. In this conventional loudspeaker, the voice coil former 22 is made out of paper, KAPTON, NOMEX, aluminium or any other standard material. It can be seen that the magnetic field lines within the radial gap 27 are not influenced much, or even at all, by the voice coil former 22 and, thus, are not concentrated around the vicinity of the voice coil 21. The magnetic field formed in the radial gap 27 is generally that which would results from the ambient atmosphere present within the radial gap 27.

[0052] In FIGS. 4 and 5, magnetic field lines in the vicinity of the voice coil assembly are more concentrated around the voice coil former 2, when compared with the conventional case. The magnetic field is provided with a preferred path across the radial gap 7 by means of the voice coil former 2 constructed from a high permeability material, such a material being for example mu-metal. As is clear from FIGS. 4 and 5, the magnetic field lines are denser in the region of the radial gap 7 when compared with the conventional loudspeaker shown in FIG. 3. This increase in density of the magnetic field lines indicates a higher magnetic flux.

[0053] When the voice coil former 2 is formed of a material having a high permeability, such as mu-metal, the magnetic field lines are more heavily concentrated on the voice coil former 2. Consequently, the density of the magnetic field passing through the voice coil 1 is increased in the invention seen in FIGS. 4 and 5, in particular when compared with the conventional loudspeaker shown in FIG. 3. It can be seen from FIG. 3 that the magnetic field in the radial gap 27 is not modified by any material in the vicinity of the voice coil former 22. Conversely, the magnetic field is more concentrated in the region of the voice coil 1 in FIGS. 4 and 5.

[0054] Use of the high permeability material such as mu-metal, supermalloy, supermumetal, nilomag, sanbold, molybdenum permalloy, Mo permalloy, Supermalloy or other similar material, provides a low reluctance path for the magnetic field across the radial gap 7. Thus, using a high permeability material as the voice coil former 2, has the effect of increasing the magnetic field density through, and in the surrounding area of, the voice coil former 2. As a result of increasing the magnetic flux, the force exerted on the voice coil 1 is also increased such that the vibrational intensity of the voice coil 1 is enhanced. Improvement in the performance of the loudspeaker is thus obtained.

[0055] The high permeability material is preferably mu-metal, this is a soft magnetic alloy with high magnetic permeability having a higher saturation value when compared with other high permeability materials. Mu-metal has the further advantages of being ductile, malleable and workable, which allows it to be easily formed into thin sheets. In the present embodiment, mu-metal is used to produce the voice coil former 2, since mu-metal has a relative permeability value of 50,000/.sub.0 which is significantly higher than conventional materials (such as iron, steel, aluminium and paper) used to produce the voice coil formers in conventional loudspeakers.

[0056] The magnetic force which is applied onto the voice coil 1, is an important parameter for achieving a high mechanical force causing the diaphragm 12 to move back and forth. Due to the high permeability property of mu-metal, the magnetic field is concentrated on the voice coil former 2 and voice coil 1, hence providing improved flux density of the magnetic field in the radial gap 7. Even when an input signal from the external amplifier is of low power, the magnetic field strength acting on the voice coil 1 will be increased as a result of the voice coil former providing a preferred magnetic flux path, and the movement of the voice coil former 2 and diaphragm 12 will be assured. From the increased magnetic flux density, the movement of the voice coil 1 can still be enough to accurately reproduce the output sound even with a low power input; in this manner, a highly responsive loudspeaker is achieved.

[0057] FIG. 5 shows magnetic field lines of the loudspeaker 10 according to the present invention when the voice coil 1 is located above the radial gap 7. It is illustrated that, even when the voice coil is moved further away from and out of the radial gap 7, the magnetic field will still be concentrated on the voice coil 1.

[0058] As the voice coil former 2 comprises high permeability materials, the voice coil former 2 provides a low reluctance path for the magnetic field. As such, the voice coil former 2 provides a preferred path for the magnetic field thus improving the magnetic flux density and, thus, when the voice coil former 2 moving the magnetic field moves with the voice coil former 2 and the voice coil 1. In essence, the magnetic field tracks movement of the voice coil assembly and thus the magnetic field around the voice coil 1 is increased.

[0059] FIG. 6 illustrates the nonlinear force factor characteristic BL(x), which is a function of the voice coil 1 displacement x, of the present invention compared with conventional loudspeakers; in the graph, B is the magnetic flux and L is the length of the voice coil wire. FIG. 6 shows the BL curve when the voice coil 1 moves up and down along the axis 6. The force factor BL(x) is an important factor for the linearity, since it defines the maximum linear excursion the loudspeaker is capable of achieving, where the force=BLI is even. BL(x) is also a determinant factor in the efficiency and sensitivity of the driver. Looking at FIG. 6, the resulting force factor BL(x) of the present invention provides an improvement of around 20% over the force factor measured in a conventional loudspeaker. Thus, the performance of the loudspeaker of the present invention is improved, since the resulting efficiency of the driver is higher.

[0060] FIG. 7 shows another embodiment according to the present invention. Instead of forming the voice coil former entirely from a high permeability material, preferably mu-metal, collars 32 made out of mu-metal are added to the voice coil former 31. This structure still leads to the improved magnetic field density within the voice coil 1, as the collars 32 focus the magnetic field within the radial gap 7. Providing collars 32 assists in reducing the weight, size and cost of the voice coil former 31 over the embodiment above. In this case, the voice coil former (31) is made from one or more of aluminium, paper, KAPTON, NOMEX, or any other suitable material.

[0061] By providing a voice coil former 2 composed of a metal with high permeability as in the present invention, a significant improvement in diaphragm vibration and sound quality can be achieved for the same input signal and in comparison to loudspeakers constructed without such high permeability materials in the radial gap 7. In addition, the loudspeakers according to the present invention may be made smaller than conventional designs, yet still produce equivalent sound output. This is further achieved over the cited prior art documents, where mu-metal is used in the magnet assembly, as only using mu-metal in the voice coil former 2 saves weight and costs but still provides the improvement in loudspeaker 10 performance.