GB2104346A - Integral combined core and back plate for loudspeaker and process for fabricating the same - Google Patents

Abstract

The magnet driver for a loudspeaker employs an integral combined core and back plate fabricated from a flat strip of metal as distinguished from casting or forging from a billet. In a first forming step, the center of the strip is formed into the shape of a dome, retaining original thickness, the height and width of the dome being substantially greater than the height and width of the formed core of the magnet motor. Thereafter, the width of the dome is reduced by compressing the walls of the dome and at the same time forming a flat circumferential back plate flange about the base of the dome. Then, the height of the dome is reduced and the top of the dome is flattened by axially compressing the dome while maintaining the walls of the dome vertical. Finally, the formed unit is severed from the metal strip and trimmed. The thickness of the dome walls and flange forming the fabricated core/back plate is essentially that of the original strip of metal. The core portion is hollow. <IMAGE>

Description

SPECIFICATION Integral combined core and back plate for loudspeaker and process for fabricating the same This invention relates to magnet drivers or motors for driving a loudspeaker diaphragm or cone by its voice coil, and in particular to a magnet driver assembly having a fabricated integral core/back plate.

A typical loudspeaker driver motor includes a back plate and an upstanding core as well as a front plate in the magnet circuit. In one common type of driver the back plate is stamped from flat stock and is prided with a hole in the middle, and a solid, cylindrical core which has a plug corresponding to the hole in the back plate is secured to the back plate with the core plug in the hole, the core and back plate being swaged, spot welded, or otherwise permanently joined at the hole in the back plate.

In another type of driver having a combined core and back plate component, the core/back plate is formed of one piece of metal by casting or hot or cold forging processes. Typically, the component is shaped in a series of die stations, culminating in a final product having a solid core and integral back plate.

Often, cores of the prior art have been vented for pressure relief. To this end and also to provide additional convection heat dissipation the core is drilled axially. In addition, by altering the core material by removing some of the metal, the performance characteristics of the driver can be changed as desired, within the satuation limits of the metal.

As loudspeaker drivers have evolved, real driver costs have been somewhat reduced.

However, known structures are still relatively expensive. The two piece core and back plate must be fabricated in a series of steps to join the two parts together. The single piece combined core and back plate must be cast or forged from a relatively expensive billet through a series of die forming steps. Both of these forms of combined core and back plate, due to their solid cores, carry excess metal, inflating their costs.

The present invention provides a process of forming an integral combined core and back plate unit for a magnet driver for a loudspeaker, the core portion of said unit being hollow with a wall thickness essentially the same as the thickness of the back plate, said unit being fabricated from a flat blank or strip of metal in a progressive sequence which draws, bends and compresses the blank or strip in various combinations, the process comprsing the steps of a. raising the center of said blank by drawing and bending the same into the general shape of a dome having outwardly sloping walls while maintaining the thickness of the raised portion of said blank essentially constant, the height and width of said dome being substantially greater than the eventual height and width of the core portion of the fabricated unit, b. reducing the width of said dome by forming the walls of the dome radially inward in a series of steps until the walls of the dome are vertical and cylindrical, at the same time, c. forming a flat circumferential flange about the base of the dome, and reducing the height of said dome and generally flattening the top thereof to complete said core portion while maintaining the thickness of said flange and of said dome and connecting structure substantially constant, and d. trimming said flange to complete said back plate portion of said unit.

The invention also provides an integral core/back plate unit for a magnet driver for a loudspeaker comprisi g a. a flat, annular back plate portion of paramagnetic metal of uniform thickness, and b. a hollow, cylindrical flat-topped core portion of a paramagnetic metal rising vertically from the inner periphery of said annular back plate portion, said core portion having a wall thickness essentially equal to the thickness of said back plate.

The process described above assists in providing a loudspeaker driver or motor of reduced cost and complexity which nevertheless duplicates the performace of prior art driver assemblies.

The core/back plate described above can be fabricated from inexpensive flat metal stock in a novel, low cost, high volume forming process.

Because a major novel aspect of the invention resides in the complete departure from established methods of making combined core/back plates for loudspeaker motors, it will be meanigful to refer to the forming process herein described as "fabrication", involving essentially the forming, drawing and bending of flat, strap metal, as distinguished from casting or forging operations.

In a preferred embodiment, the core/back plate is fabricated from a flat blank or strip of metal as follows. First, the center of the blank is raised by forming the center into the spape of a dome having outwardly sloping walls. The thickness of the raised portions of the blank is maintained essentially constant. The height and width of the dome is substantially greater than the height and width of the core portion of the combined unit.

After the dome is formed, its width is reduced by forcing the walls of the dome inwardly until the walls are essentially vertical and sized. At the same time, a flat, circumferential flange is formed about the base of the dome from the walls of the dome as the width is being reduced.

The flange, after the formed unit is separated from the blank, forms the back plate portion. The thickness of the flange and the dome are maintained essentially constant and equal during this process.

Next the height of the dome is somewhat reduced as the top of the dome is flattened to complete the core by axially applied force while maintaining the size of the walls of the dome and their vertical orientation. Finally, the circumferential flange is trimmed to complete the forming of the back plate portion. If desired, a vent aperture may be formed in the top of the hollow core, either during the process of forming the core/back plate or by drilling after the unit is trimmed.

By forming the integral combined core/back plate in accordance with the process set forth above in which the metal blank is first bent into a dome shape of substantially greater height and width than the formed core portion, the subsequent fabrication steps largely avoid drawing of the metal of the blank and consequent thinning of the wall structure of the unit. The dome and connecting portions therefore have a thickness essentially equal to that of the blank from which it is formed. The highest possible magnetic flux densities are therefore attained.

A preferred embodiment of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional illustration of a prior art magnet driver for a loudspeaker; Figure 2 is a cross-sectional illustration of a prior art magnet driver similar to that of Fig. 1 and having a vented core; Figure 3 is a cross-sectional illustration of a loudspeaker driver comparable to that of Fig. 1 and embodying the invention; Figure 4 is a cross-sectional illustration of a driver comparable to that of Fig. 2 and embodying the invention; Figure 5 is a diagrammatic cross-sectional illustration of the process according to the invention to form the integral core/back plate component of a loudspeaker magnet driver; Figure 6 is an elevational view partly in section of a loudspeaker employing the magnet driver embodying the invention;; Figure 7 is a top plan and side elevational illustration of the integral core/back plate of a loudspeaker magnet driver according to the invention; and Figure 8 is a cross-sectional illustration of an alternative embodiment of the invention.

Prior art core/back plate components of magnet drivers are illustrated in Figs. 1 and 2. The units shown are composed of a back plate 10 and a solid core 12. The back plate 10 and core 1 2 illustrated in Figs. 1 and 2 are combined, integral, components formed from a billet by a conventional hot or cold forging process. The core 1 2 of the unit shown in Fig. 2 has a vent 1 4 drilled at the core axis throughout the length thereof.

The magnet driver is located in the base of a loudspeaket as illustrated in Fig. 6. The loudspeaker driver elements shown include the driving magnet 1 6 and the front plate 1 8 as well as the integral core/back plate 20. The purposes and functions of the magnet 16, the front plate 1 8 and the core and bacle plate are well known and will not be addressed further during this description.

Shown in Fig. 3 is a magnet driver or motor embodying the integral core/back plate 20 according to the invention, again illustrated with a magnet 1 6 and front plate 1 8 of the loudspeaker driver. The core/back plate component 20 includes an annular back plate portion 22 circumferentially disposed about the base of an integral, hollow core 24. The integral core/back plate 20 is fabricated from a paramagnetic metal of high permeability, such as iron or steel, and is essentially uniform in structural thickness and thus permeance throughout the body of the component. Although the height of the core/back plate 20 is shown greater than that of the front plate 18, this is not necessary.

As shown in Fig. 4, the core portion 24 of the core/bacle plate 20 may include a vent 26 to provide pressure relief and allow added convection heat dissipation. Whether vented or not, the top of the core 24 is generally flat, or may include a small center bump or step (not illustrated) to aid in formation. The periphery of the top of the core can be radiused siightly, if desired, as shown at 28 to avoid undesired magnetic flux accumulations. Alternatively, the periphery of the top of the core 24 may be chamfered, at approximately 45, to accomplish the same result.

Likewise, the juncture of the core portion 24 and the back plate portion 22 is radiused, as shown at 30.

Preferably, the core/baele plate unit 20 is fabricated in the course of a series of forming steps in a stamping machine utilizing a progressive die having several stamping stations. The progressive configurations of the core/back plate 20 following each major step of the stamping process are illustrated in Fig. 5. The number of steps shown is illustrative and fewer or a greater number of steps may be employed, depending on the dimensional accuracy required, amount of material to be displaced, final configuration, and type of metal used.

The integral combined core/back plate 20 is formed from a flat blank 32, which may be sized sufficiently for fabrication of the unit or which may be. part of a continuous strip of steel stock which passes through the stamping machine (not illustrated). At the first stamping station, station 1, the flat blank is raised at its center by bending and drawing into the shape generally of a dome 34 having outwardly sloping walls 36. The thickness of the dome is maintained essentially constant, while the height and width of the dome is substantially greater than the height and width of the finally formed core, as shown at station 7, Fig. 5.While the average width of the dome 34 shown at station 1 is approximately twice that of the core illustrated at station 7, and the height of the dome is approximately one and one-quarter times that of the finally formed core, the width and height of the dome formed during the stamping process may be and are altered while the thickness of the core and back plate portions, as shown at station 7, is essentially equal to the thickness of the unformed blank 32.

At station 2, the width of the dome is reduced by forming the walls 36 radially inward and, at the same time, a flat circumferential flange 38 is formed in the base of the dome 34, bent from the walls 36. This process continues and at stations 3 and 4 the height of the dome 34 is reduced and the top of the dome is partially flattened as the width is reduced toward core diameter. At succeeding stations this forming process continues until, as shown at station 7, the dome 34 has been fabricated into the integral combined core/back plate of the invention with the thicknesses of the core and the back plate portions and the structure joining these two portions being essentially equal. Thereafter, the unit is severed from the blank 32 and trimmed, if necessary.

The magnet driver incorporating the core/back plate 20 according to the invention is illustrated in Fig. 6 in its typical orientation in a dynamic loudspeaker 40. The installation and location of the magnet driver in the loudspeaker is conventional, and therefore is not discussed further.

The integral combined core/back plate 20 is shown in top plan and corresponding side elevational views in Fig. 7. The back plate portion 22 is circular, and the core portion 24 is cylindrical.

Fig. 8 illustrates an alternative form of an integral core/back plate 42. Similar to the core/back plate 20, the core/back plate 42 includes an annular back plate portion 44 circumferentially disposed about the base of an integral, hollow core 46. In this embodiment, the top of the core 46 is open and is shaped to include an annular, outwardly turned lip 48 to better concentrate the magnetic flux at the critical portion of the magnetic circuit. Formation of the core/back plate 42 is in accordance with the procedures set forth above in relation to Fig. 5 except, of course, as the core 46 is formed, the lip 48 is shaped simultaneously.

To compare the flux density of the core/back plate of the invention with a comparable prior art cold forged unit, several units according to the invention were fabricated from 9 guage, 10 gauge and 11 gauge steel. The results of this flux density comparison are tabulated in the following Table 1: Table 1 Typical flux density measurements GAP FLUX CORE/BACK PLATE UNIT # (GAUSS x 103) COLD FORGED 1 7.7 2 7.6 FABRICATED 1 7.5 9 Ga. 1010 Steel 2 7.45 3 7.60 FABRICATED 1 7.7 10 Ga. 1010 Steel 2 7.6 3 7.5 4 7.5 5 7.5 6 7.5 FABRICATED 1 7.2 11 Ga. 1010 Steel 2 7.1 3 7.1 From Table 1, it is evident that a core/back plate unit according to the invention fabricated from 10 gauge steel closely matches the flux capacity of a comparable cold-forged unit.The applicant has found that the 10 gauge fabricated unit is near the limit of the steel to conduct flux, since the next thicker formed driver (9 gauge steel) exhibited no greater flux density, while the next thinner formed driver (11 gauge steel) showed a slight loss of flux capacity.

From the foregoing, it will be understood that by careful design of the forming dies at the several stations of the forming process, and by limiting the degree of deformation of the metal at each station, the fabrication process involves essentially the forming of the starting stock with minimum drawing or compression of the metal. The walls of the core portion of the core/back plate unit are essentially as thick as the thickness of the back plate which, in turn, is essentially the unchanged thickness of the starting stock material. In such a structure, maximum flux density capacity is achieved.

The invention herein described provides a very low-cost, high-volume process for the production of integral combined core/back plate units for loudspeaker magnetic driver motors from inexpensive flat steel stock. Unique in this art, the fabrication technique employs forming with minimum drawing to preserve the flux density of the paramagnetic starting material. The units exhibit maximum magnetic circuit system efficiency in that the core portions are hollow and the entire cross sections of the metal forming them serve in the magnetic circuit without the wastage of surplus metal.

The integral core/back plate units of the invention may be designed to meet a wide range of magnet and structural requirements for particular applications. If enhanced dissipation of heat from the magnet coil is desired, the core portions may extend above the normal level of the front plate and/or the tops of the core portions may be apertured for additional convection ventilation.

Claims (9)

1. A process of forming an integral combined core and back plate unit for a magnet driver for a loudspeaker, the core portion of said unit being hollow with a wall thickness essentially the same as the thickness of the back plate, said unit being fabricated from a fiat blank or strip of metal in a progressive sequence which draws, bends and compresses the blank or strip in various combinations, the process comprising the steps of a. raising the center of said blank by drawing and bending the same into the general shape of a dome having outwardly sloping walls while maintaining the thickness of the raised portion of said blank essentially constant, the height and width of said dome being substantially greater than the eventual height and width of the core portion of the fabricated unit, b. reducing the width of said dome by forming the walls of the dome radially inward in a series of steps until the walls of the dome are vertical and cylindrical, at the same time, c. forming a flat circumferential flange about the base of the dome, and reducing the height of said dome and generally flattening the top thereof to complete said core portion while maintaining the thickness of said flange and of said dome and connecting structure substantially constant, and d. trimming said flange to complete said back plate portion of said unit.

2. The process according to claim 1 including the step of forming a vent aperture in the flattened top of the dome.

3. A process of forming an integral combined core and back plate unit for a magnet driver for a loudspeaker substantially as hereinbefore described with reference to the accompanying drawings.

4. An integral combined core and back plate unit for a loudspeaker magnet driver formed according to the process of any one of claims 1 to 3.

5. An integral core/back plate unit for a magnet driver for a loudspeaker comprising a. a flat, annular back plate portion of paramagnetic metal of uniform thickness, and b. a hollow, cylindrical flat-topped core portion of paramagnetic metal rising vertically from the inner periphery of said annular back plate portion, said core portion having a wall thickness essentially equal to the thickness of said back plate.

6. The integral core/back plate unit according to claim 5 and including a vent aperture in said top.

7. The integral core/back plate unit according to claim 5 in which the periphery of the flat top of said core is radiused.

8. The integral core/back plate unit according to claim 5 in which the periphery of the flat top of said core is chamfered.

9. The integral core/back plate unit according to claim 5 in which the top of said core portion is open and includes an annular, outwardly turned lip.

1 0. An integral core back plate unit for a magnet driver for a loudspeaker substantially as hereinbefore described with reference to and as shown in the accompanying drawings.