FR2668018A1 - Method of manufacturing the moving assembly of an electrodynamic loudspeaker, and corresponding moving assembly - Google Patents

Abstract

The field of the invention is that of the design and the improvement of electrodynamic loudspeakers and moving assemblies for such loudspeakers. According to the invention, the method is characterised in that it consists in separately manufacturing at least two component parts of the moving assembly, then in integrating the two components to one another, the said components being made of thermoplastic materials in such a way as to allow the two parts to be integrated together by heat. The integration method falls within the group comprising melting, overmoulding, thermal welding with or without filler material, and multi-injection.

Description

 Method of manufacturing the moving part of an electrodynamic loudspeaker, and corresponding moving part.

 The field of the invention is that of the design and improvement of electrodynamic loudspeakers and mobile equipment for such loudspeakers.

 In a known manner, the mobile assembly of a loudspeaker of this type comprises a mobile membrane (or diaphragm), generally frustoconical, connected to the loudspeaker bowl by two suspensions of flexibility controlled on the one hand at its base by a "front suspension", and on the other hand at its top by a "rear suspension" (or spider). The loudspeaker's excitation voice coil is integral with the membrane at its top.

The main characteristics used to define the performance of such a speaker are
- the efficiency or performance measured in general by the acoustic level obtained at one meter for an electrical power of one Watt input,
- the bandwidth defined by two low and high cutoff frequencies, fmin and fmax,
- the regularity of the response curve (acoustic level as a function of frequency),
- the distortion of the signal for a power at a given frequency and defined by a family of curves which are generally associated with the response curve.

 We can consider three main types of operation of the moving equipment depending on the power and especially the frequencies considered: a type of piston operation and linear speed in the preferential band of use of the loudspeaker, framed at lower frequencies by a type of piston operation at non-linear speed, and at higher frequencies by a type of operation with deformations of the membrane.

 The vibrational modes which appear in this latter type of operation can lead to irregularities in the response curve and significant distortions. They must therefore be properly controlled if the loudspeaker is used in this frequency band. In general, one seeks the extension of the piston operation towards the high frequencies by raising the natural frequencies corresponding to the vibratory modes, and this with a suitable damping.

 The knowledge and mastery of the vibratory modes of the loudspeaker membranes is based on the study of the dynamics of the hulls, the equation of which must integrate the geometry of the hull, the mechanical characteristics of the constituent materials, the connection diameter of the "force" source (here the diameter of the coil) and the boundary conditions characterized by the suspensions. this last point is especially important for the front suspension, whose dynamic behavior is sometimes preponderant for the overall dynamic behavior of the crew.

 The theory shows that the natural frequencies of such hulls are higher the hull is made up of rigid materials of great thickness and having a significant deflection (gap between the top and the base of the hull).

 These constraints are however generally incompatible with other priority constraints in high fidelity.

 Among these constraints, one can in particular note those aiming at minimizing the cost price of the loudspeaker, closely linked to its manufacturing process.

 Conventional methods of manufacturing moving parts generally consist of making the membrane separately, then bringing the rear suspension and the moving coil to it.

We can find two embodiments of the membrane
- either the membrane is formed directly in one piece: central diaphragm + front suspension,
- either the diaphragm and the front suspension are previously manufactured separately, then assembled to each other by gluing.

 This second embodiment has the advantage of allowing a different choice of materials (generally cellulosic) for the diaphragm and the front suspension respectively, which fulfill different functions. However, it has the drawback of requiring an additional bonding operation, which generates additional manufacturing costs (specialized machines, etc.). In addition, the glue joints can generate parasitic geometries, possibly irregularities, detrimental to the quality of sound reproduction of the loudspeaker. This type of bonding bond also encounters reliability problems over time.

 An objective of the present invention is therefore to provide a new method of manufacturing electrodynamic loudspeakers making it possible to eliminate the operations of affixing glue joints for assembling not only the membrane, but the whole of the mobile equipment.

 A complementary objective of the invention is to provide such a manufacturing process ensuring the greatest possible integration of the constituent elements of the movable assembly, so as to minimize the number of assembly operations.

 Among the constraints imposed on loudspeaker manufacturers, there are also those tending to impose reductions in the overall dimensions of the loudspeaker. Indeed, both manufacturers in the field of audio-visual devices, as well as the general public, frequently seek the flattening and miniaturization of elements, in particular loudspeakers. This phenomenon is particularly found, for example, in the case of automobile construction.

 The manufacturers in fact generally provide housing for the installation of loudspeakers (in particular in car doors), which are undersized in relation to the requirements for restored power to cover engine and aerodynamic ambient noise. To optimize the use of the built-in space, interior designers generally work strictly on the standard dimensions of the loudspeaker bowls on the market.

 This tendency to reduce the size of the speakers essentially involves hunting for the depth of the salad bowls, and therefore by reducing the deflection of the membrane.

 The object of the invention is in particular to allow the production of speakers which can be optimized according to this criterion of minimizing the overall dimensions, without harming the acoustic restitution properties. In particular, the method of the invention allows a restructuring of the suspensions and of the membrane for example to reduce the size of the loudspeaker, while retaining good dynamic behavior of the assembly.

 A complementary objective of the invention is to provide a manufacturing process improving the techniques for assembling the mobile assembly to the loudspeaker bowl. Conventionally, this assembly presupposes high operating precision, in order to achieve optimal centering of the moving element in the bowl. However, this type of operation requires the use of specific tools.

The invention makes it possible to considerably simplify this step of assembly, using embodiments of the movable assembly making the latter "self-centering" in the bowl.

 These objectives, as well as others which will appear subsequently, are achieved according to the invention, using a method of manufacturing a movable assembly for an electro-dynamic loudspeaker consisting in separately manufacturing at least two elements. constitutive of the movable assembly, then in securing said elements to one another, said elements being made of thermoplastic materials so as to allow a bonding connection from one to the other by heat.

 Advantageously, these elements are chosen from the same family of materials, advantageously the family of polyolefins, so as to facilitate the heat bonding operations.

 Said constituent elements of the moving assembly are monobloc parts each comprising at least one of the elements belonging to the group comprising the speaker diaphragm, the front suspension, the rear suspension, and the voice coil support.

Thanks to the use of thermoplastic materials there are several advantages
- the possibility of connecting the constituent elements to each other without conventional bonding operation
- the thermoplastic material allows precise and complex conformations of the different elements, making it possible in particular to obtain better performance or reduced dimensions
- The moving team can integrate elastic fixing means to the bowl which solve the problems of centering and joining.

 Thus, according to an advantageous characteristic of the invention, one of said constituent elements of the movable assembly is constituted by the rear suspension and / or the front suspension formed in one piece by injection with the diaphragm.

 In a preferred embodiment, the front suspension of the membrane being fixed to the bowl set back with respect to the upper front plane of said bowl, so that the most advanced portion of the membrane, when it is excited in the maximum position before, at most is flush with said upper front plane of the bowl.

 In this way, the travel of the membrane can no longer be hampered by the existence of a diffusion wall placed directly against the bowl of the speaker.

 The rear suspension of the mobile assembly is preferably constituted by an annular junction portion solisarized approximately at the top of the diaphragm, and formed by a full veil, perforated and / or folded in accordion.

 In an advantageous embodiment, particularly suitable for speakers of small diameter, the rear suspension consists of a junction portion connecting the top of the diaphragm to a central anchoring pad secured to the bowl.

 The joining process of said constituent elements of the movable assembly preferably belongs to the group comprising melting, overmolding, heat sealing with or without addition of material, and multi-injection.

 Advantageously, the coil support is produced separately, then anchored to the diaphragm, the thermoplastic material coating hollow or projecting elements formed in said support.

Other characteristics and advantages of the invention will appear on reading the following description of preferred embodiments of the invention, given by way of illustration and not limitation, and the attached drawings in which
- Figure 1 is a sectional view of a conventional type electrodynamic loudspeaker
- Figure 2 is a half-section view of a first embodiment of a movable assembly according to the invention, with external rear suspension;
- Figure 2A illustrates a variant of fixing the rear suspension or salad bowl by dowelling
- Figure 2B is an enlarged view of the attachment zone of the rear suspension and the high diaphragm coil support of the mobile assembly of Figure 2;
- Figures 2C and 2D show schematically in top view and in perspective view respectively two advantageous embodiments of the rear suspension of the mobile assembly of Figure 2;
- Figure 3 shows a second embodiment of a mobile assembly according to the invention, with internal rear suspension;
- Figure 3A is an enlarged view of the area for securing the rear suspension and the voice coil support to the diaphragm;
- Figure 4 illustrates a third embodiment of the movable assembly of the invention, in which the diaphragm has a rib allowing a reduction in its deflection;
- Figure 4A is a partial top view of the ribbed membrane of Figure 4 ;;
- Figure 5 is a partial cut-away schematic view illustrating an embedding mode before securing the moving coil support at the top of the diaphragm of a moving assembly according to the invention.

 As shown in FIG. 1, an electrodynamic loudspeaker comprises a mobile assembly 12 suspended in a bowl 11. The mobile assembly 12 consists of a membrane 13 formed by a diaphragm 14 terminated in the front peripheral portion by an external suspension 15. At the top of the substantially frustoconical diaphragm 14 is fixed the mobile coil 7 for excitation of the loudspeaker, protected by a core-covering dome 16. The mobile assembly is suspended in its rear part by means of a "spider" (rear suspension) 17.

 The turns of the voice coil move in a magnetic field created between the two pole pieces (formed respectively by the yoke 8, and the field plate 9) of a magnetic circuit equipped with a magnet 10.

 According to the invention, it is advantageous to manufacture the membrane in several elements, then to join these elements together to form the final assembly. This design is based in the invention on the use of thermoformable, and in particular injectable, thermoplastic materials.

 According to the invention, the manufacture of the membrane in several elements makes it possible to choose for each of them a suitable material.

According to the invention, it is thus possible to manufacture separately, in the more sophisticated version Ia
- on the one hand the diaphragm 14, in a semi-rigid thermoplastic material
- on the other hand the front and rear suspensions 15, made of flexible or semi-rigid thermoplastic material;
- Finally, the moving coil support 7 made of rigid material, resistant to the temperature of the turns of the coil, and capable of ensuring mechanical transmission without bending of the driving force of the moving coil.

By appropriately choosing the materials from which these elements are made, the following two cumulative advantages can be obtained
- the possibility of choosing for each separate element a material whose properties correspond exactly to the requested function (generation of sound waves for diaphragm 14; elasticity and resistance to fatigue for the front and rear suspensions; rigidity and temperature resistance for the coil holder 7);
- A very great facility for securing the constituent elements, by a simple hot bonding process.

 This last characteristic makes it possible not only to avoid using, for example, a diaphragm-suspension bonding station, in the production chain, but also to obtain a reliable connection, and therefore good behavior over time of the loudspeaker without fear a degradation or an attack of the glue, as can be the case in existing loudspeakers.

 However, for a number of reasons, in particular to simplify assembly and reduce costs, it may be advantageous to reduce the number of constituent elements of the mobile assembly. It is thus possible, for example, to form the rear suspension and / or the front suspension, in one piece with the diaphragm. This assembly can for example be produced by thermoforming, or by injection molding. Any other combination of the constituent elements of the moving assembly can thus be formed directly in one piece beforehand.

This has the advantage of limiting the number of subsequent assembly operations.

 Thus, in the simplest case, the entire mobile assembly can be formed in one piece, in a single thermoplastic material. This embodiment can be acceptable particularly in the case of small diameter speakers.

 In all configurations, we benefit in any event from the very great ease of shaping of thermoplastic materials.

Advantageously, the following materials can be chosen
- For the diaphragm 14 and the rear suspension 17, a thermoplastic material, loaded or uncharged;
- For the front suspension 15, a compatible thermoplastic elastomer;
- for the coil support 7, a rigid thermoplastic material such as kapton (registered trademark), or any other material (in particular metallic), subject to the constraints well known to those skilled in the art (reduced thermal conduction, rigidity, ...).

 In certain cases, the thermoplastic materials can advantageously belong to the same family of materials, in particular but not exclusively the family of polyolefins. Polyolefins have the advantages of being injectable materials, with sufficient temperature resistance, low density, and a damping coefficient well suited to the dynamic behavior of a loudspeaker membrane, these properties making it possible to produce membranes. thin and low inertia.

 The diaphragm is advantageously made of charged thermoplastic material. Preferably, the material used is a polypropylene filled with hollow glass microbeads, for example of the type sold by the company SOVITEC SA. The microbeads seeming to offer the best characteristics have a diameter of 50 to 60 im. The selection of this type of composite material makes it possible to lighten the membrane, and to obtain an increased flexural modulus compared to uncharged ploypropylenes, and which can reach 2000 to 3000 MPa.

 Of course, other thermoplastic materials, or other fillers (glass or carbon fibers, talc, mica, etc.) can also be used in the context of the invention.

The front suspension 15 is for its part advantageously made of thermoplastic elastomer, injected or thermoformed. The elastomeric material is chosen so as to be able to be joined together hot with the thermoplastic material constituting the diaphragm 14. Advantageously, the elastomer is of the type
Santoprene (registered trademark) marketed by the MONSANTO Company. This material has excellent resistance to residual deformation in tension and compression, and very good resistance to fatigue in bending.

As an indication, other products can be used for the different elements of the mobile equipment.
- Alcrvn (Du Pont de Nemours) halogenated polyolefin: for the front suspension;
- Hytrel (Du Pont de Nemours) polyester: for the membrane with fiber reinforcement, and for the rear suspension with or without fiber reinforcement
- Stanvl (DSM) polyamide: for the membrane and the rear suspension
- Pebax (Atochem) polyamide, polypropylene: for the membrane and the rear suspension.

 Other materials can also be used in the context of the invention, provided that they satisfy both the properties necessary for the function of each of the elements of the moving assembly, and are capable of being assembled one to each other hot.

 Due to the selection of materials recommended by the invention, the final assembly of the entire moving assembly can be carried out hot.

Advantageously, the assembly process consists of a fusion fusion of the elements, in particular of the diaphragm 14 with the suspension 15, for example between two molds heated to an adequate temperature (for example 250 "C.).

 In another embodiment of the invention, the assembly can be carried out by overmolding, by heat sealing with or without the addition of materials, or even by multiinjection. In the case of a bonding by heat sealing, this can for example be carried out by a heat source or by an ultrasonic source. These different assembly methods are made possible by the fact that the materials constituting the elements of the membrane are all thermoplastic. It is this characteristic which makes it possible in particular to envisage a principle of bi-material or even multi-material injection, which is extremely advantageous from the point of view of manufacturing costs.

In the case of a multi-material injection likely to allow the production of a complete mobile assembly at the rate of an injection machine, the following steps can be carried out
- selection of injectable thermoplastics which can be used in the composition of the three parts: membrane, front suspension, rear suspension
- separate production of the previous parts by looking for the best shape or structure of the part, and the best composition of the basic products;
- realization of the complete mold allowing the injection in a single operation of the suspensions in flexible thermoplastics and the membrane in thermoplastic reinforced by fibers.

 The outer rings of the suspensions 15, 17 which must ensure the fixing of the movable assembly on the chassis 11 of the loudspeaker advantageously comprise forms of revolution or pins forming ankles making it possible to obtain self-centering by simple clipping, which makes mounting very quick.

For fixing the coil on the crew, two solutions can be envisaged by way of example
- overmolding of the coil support placed in the mold before injection;
- mounting of the coil on claws formed in the neck of the membrane, and allowing precise positioning of the coil, without the use of a positioning tool. As shown in FIG. 5, these claws 52-53 are for example arranged in two concentric rings separated by a recessing interval of the support 7 of the voice coil 51. The claws 52-53 of the outer and inner rings respectively can be made of so as to be opposite one another in pairs, or in a staggered arrangement or even placed on a crown only. Any other configuration is acceptable, provided that the elasticity of the claws allows the coil support to be embedded mobile. The openings between the claws are designed so as to allow affixing a bonding joint, or equivalent, after positioning of the support 7. The claws 52-53 can also be hot pressed on the coil support 7 to ensure the joining. Each of the crowns can also be produced in the form of a single continuous circular claw.

 Figures 2, 3 and 4 show some possible embodiments of the invention. These examples illustrate some original aspects of conformation, and possibly restructuring of the moving assembly, enabled by the use of thermoplastic materials, and their joining by hot bonding.

More specifically, these examples concern
- a mobile assembly with external rear suspension (fig. 2)
- a mobile unit with interior rear suspension (fig. 3)
- a mobile assembly with interior rear suspension and "flat" ribbed membrane (fig.4).

The embodiment of Figure 2 illustrates the following aspects:
- the connection of the rear suspension and the coil support to the diaphragm, by overmolding (fig.2B)
- the creation of an external rear suspension (fig. 2C, and fig. 2D)
- the attachment of the front or rear suspensions to the chassis 4 by clipping or pegging (fig.2A)
- the arrangement of a fixing recess of the front suspension, relative to the upper plane 21 of the bowl 11.

 As illustrated in FIG. 2B, the mobile assembly is assembled by overmolding the rear suspension 17 and the coil support 7 on the diaphragm 14. This diaphragm may have been previously formed integrally with the front suspension 15.

 The overmolding of the rear suspension 17 on the diaphragm 14 can be carried out for example by hot pressing.

 To secure the coil support 7, holes 22 can be made in the upper part of the support 7. The support 7 is then placed in the injection mold for the membrane 14. During the injection operation, the orifices 22 are crossed by plastic material plugs 23 forming a body with the diaphragm, and ensuring the attachment of the coil support 7 to the diaphragm 14. Any other type of asperity in hollow or relief can play the role of holding, as assured through holes 22.

 The rear suspension 17 is, in the embodiment of Figure 2, an external suspension. Conventionally, it is in fact secured on the one hand in the region of the apex of the diaphragm 14, and on the other hand to a portion of the chassis 4 of greater diameter. The thermoplastic materials allow to foresee any kind of conformation of the rear suspension. This can indeed consist of a full veil, or an open veil (fig.2C), so as to have recesses 24, providing relief, or an adjustment of the elastic properties of the suspension.

 In another embodiment, as illustrated in FIGS. 2 and 2D, the rear suspension may consist of a plurality of radial segments 25, distributed all around the membrane, and for example alternately bent concave and convex.

 In another embodiment, not shown, the rear suspension can also have an accordion-like shape.

 The front 15 and rear 17 suspensions can be joined by elastic embedding links, if necessary removable. Thus, the embedding can be achieved by crowns 18, 27, in one piece with the suspensions 15, 17 respectively. The L-shaped ring 18 has two end returns enclosing the seat 19 of the chassis 11 in the manner of a dovetail, in the same way as the fixing ring 27 of the rear suspension 17.

 In an alternative embodiment, shown in FIG. 2A, the crowns 18 and / or 27 may have pins 28, which can be clipped into holes provided for this purpose in the chassis. Advantageously, an additional claw 29 is provided at the junction zone between the crown 18, 27 and the suspension 15, 17, so as to prevent possible detachment of the membrane at this highly stressed point.

 As presented in the joint patent application in the name of the same applicant, provision may be made for a withdrawal d between the point 20 of immobilization of the external part of the front suspension 15 on the salad bowl, and the upper plane 21 of the salad bowl. This shrinkage is advantageously calculated, in conjunction with the conformation of the front suspension, so as to avoid that, in the extreme front position of the membrane, no point of the latter exceeds the upper plane 21. In this way, we ensures excellence in the qualities of reproduction of the loudspeaker, especially at low frequency, even if it is directly applied against a sound diffusion wall.

 Figures 3 and 3A illustrate a second embodiment of the movable assembly of the invention, in which the rear suspension is mounted internally. More precisely, unlike FIG. 2, the rear suspension 17 in fact connects the zone of the apex of the diaphragm 14 with a central stud 30 anchored in the cylinder head 8 by claws 31. The stud 30 is advantageously made of molded thermoplastic material monobloc with rear suspension 17 and diaphragm 14.

This embodiment is particularly suitable for small speakers.

In this case, corresponding to the reproduction of rather high frequencies, the use of the same material for the whole of the mobile assembly is acceptable.

 It will be understood that the production of such a loudspeaker is extremely simplified, owing to the fact that after one-piece injection, the mobile assembly must simply be clipped into the chassis by the stud 30, then the outer crown 18. This mounting ensures a automatic self-centering of the moving part.

 It will be noted that it is possible to use other types of connection than an elastic claw connection to anchor the stud 30 in the cylinder head 8 (cone at a low angle, suction cup effect, additional bonding allowing in particular vibration damping in certain ranges of frequency,...).

 In the same way as for the embodiment of FIG. 2, the coil support 7 can be attached by overmolding to the monobloc assembly outer ring 18-front suspension 15-diaphragm 14-rear suspension 17plot 30.

 In the embodiment of FIG. 4, the principle of an internal rear suspension 17 has been adopted, cooperating with a diaphragm 14 having a groove 40. The grooves 40 are for example arranged radially, having a projecting portion on the face internal diaphragm, towards the top thereof on the one hand, and a protruding portion on the external face of the same diaphragm, on the side of the front suspension 15. This conformation is only an illustration suggesting the realization of all other preforming or grooving capable of improving the sound reproduction qualities of the membrane, and / or of limiting the deflection thereof so as to reduce the overall size of the loudspeaker.

This freedom of conformation is a direct advantage of the constitution of the mobile assembly in thermoplastic material, in particular injectable thermoplastic. The material of the membrane and its structure are in any event determined so as to obtain on the one hand a vibration "piston" operation as high as possible in frequency, and on the other hand an optimal damping of the own vibratory modes of the moving crew. These conditions, to be fulfilled, require a good adequacy of the front-membrane suspension torque, the adjustment of which is facilitated by the technology of the invention.

 The salad bowl is for example made of metallic material (Zamac, etc.) or else of rigid thermoplastic material, where appropriate of the same family as the membrane. In this case, the mobile-salad bowl assembly can also be carried out by means of a heat connection.

 The tests on the prototype implementation of the manufacturing method according to the invention suggest a saving in manufacturing time of the order of 1 to 10 compared to conventional methods, with a saving in cost of the order of 1 to 2, with equal acoustic quality.

Claims (14)

 1. A method of manufacturing a mobile assembly for an electro-dynamic loudspeaker, characterized in that it consists in separately manufacturing at least two constituent elements of the mobile assembly, then in securing said elements to one another , said elements (7, 14, 15, 17, 18) being made of thermoplastic materials so as to allow a bonding connection from one to the other by heat.  2. Method according to claim 1, characterized in that said constituent elements of the movable assembly are monobloc parts each comprising at least one of the elements belonging to the group comprising the diaphragm (14) of the loudspeaker, the front suspension (15 ), the rear suspension (17), and the support (7) of the voice coil.  3. Method according to any one of claims 1 and 2 characterized in that said thermoplastic materials belong to the same family of materials, in particular the family of polyolefins.  4. Method according to any one of claims 1 to 3 characterized in that said diaphragm (14) is made of charged polypropylene and / or said front suspension (15) is made of elastomeric material.  5. Method according to any one of claims 1 to 4 characterized in that one of said constituent elements of the movable assembly consists of the rear suspension (17) and / or the front suspension (15) formed integrally by injection with the diaphragm (14).  6. Method according to any one of claims 1 to 5 characterized in that the rear suspension (17) consists of an annular junction portion secured in the region of the apex of the diaphragm (14), and formed by a full veil, perforated (24.25) and / or folded in accordion.  7. Method according to any one of claims 1 to 5 characterized in that the rear suspension (17) consists of a junction portion connecting the top of the diaphragm (14) to a central anchoring pad (30) secured to the salad bowl (11).  8. Method according to any one of claims 1 to 7 characterized in that the front suspension (15) and / or rear (17) is fixed to the bowl (11) by elastic pins (28) and / or by circular claw d 'elastic fitting (18, 20; 27; 29).  9. Method according to any one of claims 1 to 8 characterized in that the coil support (7) is produced separately, then anchored to the diaphragm (14), the thermoplastic material coating hollow elements (22) or projecting formed in said support (7).  10. Method according to any one of claims 1 to 9 characterized in that the coil support (7) is previously positioned by elastic embedding in the neck (52,53) of the diaphragm (14), before securing to said diaphragm (14 ) by anchoring.  11. Method according to any one of claims 1 to 10 characterized in that said diaphragm (4) has a rib (40) and / or a preform in relief.  12. Method according to any one of claims 1 to 11 characterized in that the method of connecting said constituent elements of the moving assembly belongs to the group comprising melting, overmolding, heat sealing with or without addition of material, and the multiinjection.  13. Method according to any one of claims 1 to 12 characterized in that the front suspension (15) of the membrane is fixed to the bowl (11) recessed (d) relative to the upper front plane (21) of said bowl ( 11), so that the most advanced portion (22A) of the membrane (13), when it is excited in the maximum front position, at most is flush with said front upper plane (21) of the bowl.  14. Movable assembly produced according to the method of any one of claims 1 to 13.