Magnetic assembly for a loudspeaker

Abstract

 A magnetic assembly (100) for a loudspeaker comprising permanent magnets (3) arranged in a ring in central symmetry between two polar plates (1, 2) coupled so as to form a magnetic circuit air gap (4), which polar plates (1, 2) comprise external rings (10, 22) provided with petals (12, 21) in correspondence with the permanent magnets (3). Said magnetic assembly (100) provides ribs (26) on a basis (20) of a first polar plate (2), adapted to at least partially occupy a cavity of the air gap (4) in correspondence with interstices between the adjacent permanent magnets (3). (Figure 1)

Description

[0001] The present invention relates to a magnetic assembly for a loudspeaker.

[0002] Magnetic circuits for a loudspeaker are already known, for example from patents GB709656 and US20110243364, which magnetic circuits comprise a ring-shaped permanent magnet above and beneath which there are two polar plates. These polar plates are shaped with a suitable geometry so as to form a magnetic circuit air gap.

[0003] A coil is provided inside the air gap and can shift inward and outward of it, pushed by the electromotive force, transducing a time-variable electrical signal to a membrane shape diaphragm which produces pressure variations in the air, i.e. sounds.

[0004] The correct reproduction of sound by means of the magnetic circuit of the loudspeaker which serves as a magnetic assembly also depends on the homogeneity and uniformity of the magnetic field flux lines in the air gap. If the magnetic field flux is not homogeneous and uniform in the whole air gap, eddy electric currents are locally generated which distort the electrical signal induced in the moving coil, thus distorting the reproduction of the sound emitted by the loudspeaker.

[0005] In order to make the magnetic field flux lines homogeneous and uniform along the whole air gap, ring-shaped permanent magnets are used, made of a material as homogeneous and uniform as possible. In fact, the material inhomogeneity produces eddy electric currents which, in addition to affecting the correct transduction of the electrical signal in the coil, thus producing parasitic electromotive forces, generate local variations in the heat dispersion which may affect the material magnetization itself.

[0006] It is known from the scientific literature that permanent magnets lose their magnetic properties upon reaching and exceeding the Curie temperature. It is understood that a non homogeneous magnetic material could make only some parts thereof reach the limit temperature and thus lose the magnetic properties. This causes both a decrease in the magnetic flux density in the air gap, measurable in a lower intensity of the magnetic induction along the excursion of the air gap itself, and an inhomogeneity and heterogeneity of the magnetic induction along the whole air gap, thus affecting a good reproduction of sound of the loudspeaker.

[0007] Using magnetic circuits in association with a moving coil in any case produces a strong heat dispersion, so that there are cooling techniques which provide piercing either the magnetic circuit itself or the support of the moving coil to allow an effective cooling by means of a heat flow between the interior and the exterior of the loudspeaker, such as in patents CN2882176, US20110243364 o generally in woofers. Cooling methods are important for preventing the permanent magnets from reaching the Curie temperature limit, thus losing their magnetic properties. In order to obviate such problems, permanent magnets made of ferrite are used with a Curie temperature about 450 °C, instead of neodymium, which instead has a limit temperature about 200 °C.

[0008] The heating of the coil and of the magnetic circuit further causes an increase in the equivalent resistance which affects the electric current transduction in the coil, making the diaphragm lose sound pressure upon the increase of the power applied to the circuit.

[0009] Using magnetic rings made of ferrite in place of neodymium or rare earth elements is advantageous because it allows to reach higher powers, since ferrite withstands higher temperatures than neodymium without losing its magnetic properties, but it has the large disadvantage that ferrite has a lower specific weight than neodymium or other rare earth elements. One of the important factors to be considered for manufacturing a good loudspeaker is the weight of the magnetic assembly which improves the stability of the moving coil. While the cost of a neodymium magnet is much higher than a similar one of ferrite, it certainly is lighter with the same magnetic power, making the loudspeaker more cost-effective, due to the simpler transport and installation, moreover the higher power of neodymium favors the design of some products where reduced overall dimensions are essential.

[0010] In order to make the magnetic field flux lines as homogeneous and uniform as possible along the whole air gap, large sized magnets are needed. Large magnets have a considerable magnetic flux, thus minimizing the magnetic flux dispersion due to other causes. The cost of such large magnets with a large specific weight is one of the factors which limit their common use.

[0011] A permanent magnet with a single ring has a high cost and does not allow an effective dispersion of the heat produced by the magnetic assembly. Some manufacturers opt for reducing the cost of loudspeakers by manufacturing some with magnetic motors having permanent magnet discs spatially arranged in a ring in central symmetry. For example, patent EP1381252 describes a magnetic assembly for a loudspeaker with disc-shaped permanent magnets arranged in a ring in central symmetry. The polar plates of such a magnetic circuit are shaped to follow the profile of this magnetic disc ring as much as possible. Such a magnetic assembly has the advantage of concentrating the magnetic flux density in the air gap through the shaping of the polar plates, but it has the disadvantage of not having a homogeneous magnetic flux density along the whole air gap, since the magnetic material between the disc interstices is missing. In fact, air gaps are formed between the interstices of the magnetic discs where the magnetic field flux lines are dispersed, thus reducing the magnetic flux density in the air gap where the moving coil is present. This results in a lower transduction capability of the electric signal of the coil, and thus in a reduction in the loudspeaker performance.

[0012] It is the object of the present invention to provide a magnetic assembly for a multi-magnet loudspeaker, which concentrates the magnetic flux density in a uniform and homogeneous manner in an air gap where a moving coil is present.

[0013] It is a further object to provide a magnetic assembly for a loudspeaker which operates at high powers.

[0014] It is a still further object to provide a magnetic assembly for a loudspeaker at reasonably low costs with high performance.

[0015] According to the present invention, such objects are achieved by a magnetic assembly for a loudspeaker according to claim 1.

[0016] An advantage of the present invention is to use less magnetic material, considerably saving on the costs of a loudspeaker, and to generate a uniform and homogeneous magnetic field in an air gap of a magnetic assembly for a multi-magnet loudspeaker.

[0017] A more advantageous heat dispersion of the present invention is due to the presence of interstices between the magnetic discs, which allow the loudspeaker to operate at lower temperatures than a single magnetic ring.

[0018] These and other features of the present invention will become more apparent from the following detailed description of an embodiment thereof, shown by way of a non-limiting example with reference to the accompanying drawings, in which:

Figure 1 shows a top plan view of a magnetic assembly according to the present invention;

Figure 2 shows a plan bottom view of the magnetic assembly;

Figure 3 shows a sectional view according to line III-III in Figure 1;

Figure 4 shows a sectional view according to line IV-IV in Figure 2;

Figure 5 shows a perspective exploded view of the magnetic assembly;

Figure 6 shows a plan view of a magnetic assembly for a multi-magnet loudspeaker according to the prior art;

Figure 7 shows a diagram of the intensity of the magnetic field inside the air gap versus an excursion of an air gap of the assembly of figure 6, where the continuous line represents the variation of the intensity of the magnetic field following the line VII-VII in Figure 6, instead the dashed line following the line VIII-VIII in Figure 6;

Figure 8 shows a diagram of the intensity of the magnetic field inside the air gap versus an excursion of an air gap, where the continuous line represents the variation of the intensity of magnetic field following the line III-III in Figure 1, instead the dashed line following the line IV-IV in Figure 2;

Figure 9 shows a first perspective view of the magnetic assembly;

Figure 10 shows a second perspective view of the magnetic assembly;

Figure 11 shows a front view of the magnetic assembly.

[0019] With particular reference to figure 5, a magnetic assembly 100 for a loudspeaker is shown, comprising a sequence of disc-shaped permanent magnets 3 arranged in a ring in central symmetry such as to leave interstices therebetween.

[0020] Said magnetic discs 3 are between discoid shape polar plates 1 and 2.

[0021] The magnetic discs 3 are positioned on a top basis 20 of the underlying polar plate 2. Such basis 20 is flat, but with interlaid ribs 26 against an inner edge of a central ring 25, in correspondence with all the inner interstices between the discs 3 themselves. The edge of the central ring 25 is raised by means of an almost vertical wall 24 with respect to the plane formed by the top basis 20.

[0022] The ribs 26, which are suitably geometrically shaped to almost approximately follow the inlets between the magnetic discs 3 themselves, lean against the central ring 25, beneath wall 24. The central ring 25 slopes down towards the inside via a sloped edge 23 until reaching a cylindrical surface 51 with a central hole 5.

[0023] Each rib 26 consists of a pair of concave surfaces 261 that meet at a line 262 included in a plane (not shown) where the edges of two adjacent magnets 3 meet together (figures 3 and 5). The edges of magnets 3 lean onto said concave surfaces 261.

[0024] An outer ring 22 of the polar plate 2 is provided with a plurality of petals 21 which follow the shape of the ring-like arrangement of the magnetic discs 3. Petals 21 are as many as the discs 3. However, the ring shape of magnetic discs 3 is not exactly followed since a portion with the edges of the discs 3 themselves can be seen in a plan bottom view as in Figure 2.

[0025] The polar plate 1 is placed above the magnetic discs 3 and is suitably geometrically shaped. Petals 12 are on an outer ring 10 of the polar plate 2, which petals almost strictly follow the profile of the ring-shaped arrangement of the magnetic discs 3, but allowing a portion thereof to be seen from a plan top view as in Figure 1. The polar plate 1 provides inside of it two sloped edges 13 and 14 (figures 3 and 4) inclined towards each other in order to decrease the thickness of the polar plate 1 itself to form an internal ring 15 adapted to face in front of the internal ring of the polar plate 2.

[0026] The internal ring 15 has a larger diameter than the internal ring 25 of the polar plate 2, so as to define an air gap 4 therebetween 15, 25.

[0027] Said air gap 4 has an excursion 41 where the intensity of the magnetic induction has been measured, as shown in the diagram in figure 8. The zero position of the axes is indicatively just against the exterior of the excursion 41 of the air gap 4. A moving coil (not shown) for operating the loudspeaker vertically translates inward and outward the air gap 4.

[0028] Figure 6 shows a known magnetic assembly 200 comprising a pair of polar plates 8 and 9 between which permanent magnets 30 are sandwiched. The coupling between the polar plates 8 and 9 defines an air gap 7 with an excursion 71, where a moving coil for operating the loudspeaker vertically translates inward and outward.

[0029] In order to implement a magnetic assembly 100 for a multi-magnet loudspeaker which uniformly and homogeneously concentrates the magnetic flux density in the air gap 4 where the moving coil is provided, it is necessary to suitably shape the geometries of both the external rings 10 and 22 of the two polar plates 1 and 2 so as to concentrate the magnetic flux density in the air gap 4, and to suitably shape the geometry of ribs 26 between the interstices of the magnetic discs 3 so as to make the spatial distribution of the flux lines of the magnetic field in the air gap 4 homogeneous, as shown in figure 8.

[0030] Figure 8 shows the trend of two diagrams of the intensity of the magnetic induction measured in Tesla as a function of the excursion 41 of the air gap 4 in correspondence with a magnetic disc 3 and in correspondence with an interstice between two discs 3. By comparing the continuous line diagram with the discontinuous line one it is seen that the intensity of the magnetic induction is homogeneous and uniform along the whole air gap 4. By comparing the diagram in figure 8 with that in figure 7 it is seen that the geometrical shape of the external rings 10 and 22 of the two polar plates 1 and 2, together with the geometry of the ribs 26, make the magnetic induction uniform and homogeneous inside the air gap 4.

[0031] Contrary to the prior art, the present invention does not attempt to reduce the magnetic flux dispersion in the cavity of the air gap 4, but it advantageously introduces the ribs 26 inside the cavity of the air gap 4 for reducing the length of the path of the magnetic flux lines. The introduction of the suitably geometrically shaped ribs 26 between the interstices of the magnetic discs 3 together with the geometry of the polar plates 1 and 2 makes the magnetic induction in the air gap 4 homogeneous and uniform, thus achieving an exceptional, unexpected and surprising result.

[0032] A further exceptional, unexpected and surprising result for this type of magnetic assembly for a loudspeaker with multiple magnetic discs according to the present invention, which is usually used simply for reducing the manufacturing costs of a single magnetic ring, is also the fact of being apt to be advantageously used at high powers, since it is possible to benefit from the fact that the interstices between discs 3 allow a more efficient and effective heat dispersion to the external environment, also allowing the use of permanent magnets made of neodymium and rare earth elements which, with single-ring or internal disc magnetic assemblies at high powers, would lose instead their magnetic properties due to the limit imposed by the Curie temperature.

Claims

1. Magnetic assembly (100) for a loudspeaker, comprising permanent magnets (3) arranged in a ring in central symmetry between two polar plates (1, 2) coupled so as to form a magnetic circuit air gap (4), which polar plates (1, 2 ) comprise external rings (10, 22) provided with petals (12, 21) in correspondence with the permanent magnets (3), characterized in that it provides ribs (26) on a basis (20) of a first polar plate (2), adapted to at least partially occupy a cavity of the air gap (4) in correspondence with interstices between the adjacent permanent magnets (3).

2. Magnetic assembly according to claim 1, characterized in that each rib (26) consists of a pair of concave surfaces (261) that meet at a line (262) included in a plane where edges of two adjacent permanent magnets (3) meet together.

3. Magnetic assembly according to claim 1 or 2, characterized in that the first polar plate (1) provides inside of it two sloped edges (13, 14) inclined towards each other in order to decrease the thickness of the polar plate (1) itself to form an internal ring (15).

4. Magnetic assembly according to claim 3, characterized in that the second polar plate (2) provides a central ring (25), on which there are the ribs (26), comprising a wall (24) raised with respect to the ribs (26), said central ring (25) leaning to said internal ring (15) to form the air gap (4).

5. Magnetic assembly according to claim 4, characterized in that the central ring (25) slopes down towards the inside via a sloped edge (23) until reaching a cylindrical surface (51) with a central hole (5).

6. Magnetic assembly according to any of the preceding claims, characterized in that the permanent magnets (3) protrude outward from the petals (12, 21) of the polar plates (1, 2).

7. A process for making homogeneous and uniform the magnetic flux lines in a magnetic circuit air gap (4) of a magnetic assembly for a loudspeaker with multi-permanent magnets (3) arranged in a ring in central symmetry between two polar plates (1, 2) forming said air gap (4), characterized in that it provides the step of at least partial filling the inner interstices between two adjacent permanent magnets (3).

Drawing