Method of production of an electrodynamic acoustic transducer with a high density coil

Abstract

A method for manufacturing an electrodynamic acoustic transducer is disclosed. The electrodynamic acoustic transducer comprises a frame and/or a housing, a membrane, at least one coil and a magnet system, wherein the coil, in a cross sectional view with a coil axis being part of the sectional plane, comprises a plurality of conductive layers formed by an electrical conductor of the coil. The electrical conductor has a rectangular cross section in said cross sectional view, wherein a longer side of the rectangular cross section is substantially perpendicular to the loop axis. According to this method, a stack of conductive layers is made from the electrical conductor by stacking of separate pieces of the electrical conductor and electrically connecting the stacked separate pieces and/or by folding of the electrical conductor.

Claims

1. A method of manufacturing an electrodynamic acoustic transducer with a frame and/or a housing, a membrane fixed to said frame or said housing, at least one coil, which is attached to the membrane and which has an electrical conductor in the shape of loops running around a coil axis in a loop section, and a magnet system being designed to generate a magnetic field transverse to the conductor in the loop section, comprising the steps of: a) cutting the electrical conductor and a supporting structure out of a metallic foil, wherein the supporting structure is connected to the electrical conductor by a plurality of bars; b) forming an insulation layer on the electrical conductor; c) making a stack of conductive layers from the electrical conductor by stacking of separate pieces of the electrical conductor and electrically connecting the stacked separate pieces; d) connecting the conductive layers to each other by means of an adhesive; and e) removing the supporting structure and bars from the electrical conductor.

2. The method as claimed in claim 1, characterized in that the electrical conductor is cut out of an aluminum foil in step a) and a passivation layer, which is part of the insulation layer, is formed on the electrical conductor by exposing the electrical conductor to hot distilled or de-ionized water and/or to hot vapor of distilled or de-ionized water.

3. The method as claimed in claim 1, characterized in that the conductive layer is cut by means of a laser beam or a water beam in step a).

4. The method as claimed in claim 1, characterized in that the separate pieces of the electrical conductor are electrically connected by means of laser welding or ultrasonic welding in step c).

5. The method as claimed in claim 1, characterized in that first the stack of conductive layers is made from the electrical conductor without an adhesive and then an adhesive is applied to the stacked electrical conductor.

6. The method as claimed in claim 5, characterized in that superfluous adhesive is removed by means of a laser.

7. The method as claimed in claim 1, characterized in that the bars of adjacent conductive layers are located at different positions after step c) when viewed in a direction of the loop axis.

8. The method as claimed in claim 1, characterized in that the coil is coated with an insulating material after step d).

9. The method as claimed in claim 1, characterized in that tensile stress in the electrical conductor is kept below 50 N/mm.sup.2 during steps a) to d).

10. The method as claimed in claim 1, characterized in that an indentation is formed along a tear off line of a bar connecting the electrical conductor to a supporting structure.

11. A method of manufacturing an electrodynamic acoustic transducer with a frame and/or a housing, a membrane fixed to said frame or said housing, at least one coil, which is attached to the membrane and which has an electrical conductor in the shape of loops running around a coil axis in a loop section, and a magnet system being designed to generate a magnetic field transverse to the conductor in the loop section, comprising the steps of: a) cutting the electrical conductor out of a metallic foil; b) forming an insulation layer on the electrical conductor; c) making a stack of conductive layers from the electrical conductor by folding of the electrical conductor; and d) connecting the conductive layers to each other by means of an adhesive.

12. The method as claimed in claim 11, characterized in that an indentation is formed along a folding line, around which the electrical conductor is to be folded, before step c).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other aspects, features, details, utilities, and advantages of the invention will become more fully apparent from the following detailed description, appended claims, and accompanying drawings, wherein the drawings illustrate features in accordance with exemplary embodiments of the invention, and wherein:

(2) FIG. 1 shows a cross sectional side view of an exemplary electrodynamic acoustic transducer.

(3) FIG. 2 shows detailed cross sectional view of an exemplary layer structure of a coil.

(4) FIG. 3 shows the layer structure of FIG. 2 coated with an insulating material.

(5) FIG. 4 shows a cross sectional view of an exemplary layer structure of a coil with thicker outer layers.

(6) FIG. 5 shows a layer structure similar to the one of FIG. 4, but with an additional thicker middle layer.

(7) FIG. 6 shows a perspective view of an exemplary coil with a conductive layer forming a connection to a fixed terminal of the electrodynamic acoustic transducer.

(8) FIG. 7 shows an example how the driving force factor can be flattened by use of the proposed measures.

(9) FIG. 8 shows a perspective view of an exemplary coil built up by separate pieces of a conductive layer.

(10) FIG. 9 shows a top view on a conductive layer with a supporting structure.

(11) FIG. 10 shows a top view on an electrical conductor with a wave like or meander like shape in the unfolded state.

(12) FIG. 11 shows a top view on a protrusion in the corner of an electrical conductor in the unfolded state.

(13) FIG. 12 shows a top view on the electrical conductor of FIG. 11 in the folded state.

(14) FIG. 13 shows a perspective view of the folded electrical conductor of FIG. 12.

(15) FIG. 14 shows a perspective view of an alternative method of folding the electrical conductor of FIG. 11.

(16) FIG. 15 shows a top view of an exemplary supporting structure for an electrical conductor with a wave like or meander like shape.

(17) FIG. 16 shows a detailed top view of the structure depicted in FIG. 15 in the corner region.

(18) FIGS. 17 to 22 show variants of the proposed manufacturing method, in which the contour of the coil is cut out after a number of foil blanks have been stacked.

(19) FIG. 23 shows a perspective view of a prior art drive system in its corner region.

(20) FIG. 24 shows a perspective view of a drive system of the proposed kind in its corner region.

(21) Like reference numbers refer to like or equivalent parts in the several views.

DETAILED DESCRIPTION OF EMBODIMENTS

(22) Various embodiments are described herein to various apparatuses. Numerous specific details are set forth to provide a thorough understanding of the overall structure, function, manufacture, and use of the embodiments as described in the specification and illustrated in the accompanying drawings. It will be understood by those skilled in the art, however, that the embodiments may be practiced without such specific details. In other instances, well-known operations, components, and elements have not been described in detail so as not to obscure the embodiments described in the specification. Those of ordinary skill in the art will understand that the embodiments described and illustrated herein are non-limiting examples, and thus it can be appreciated that the specific structural and functional details disclosed herein may be representative and do not necessarily limit the scope of the embodiments, the scope of which is defined solely by the appended claims.

(23) Reference throughout the specification to “various embodiments,” “some embodiments,” “one embodiment,” or “an embodiment,” or the like, means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, appearances of the phrases “in various embodiments,” “in some embodiments,” “in one embodiment,” or “in an embodiment,” or the like, in places throughout the specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. Thus, the particular features, structures, or characteristics illustrated or described in connection with one embodiment may be combined, in whole or in part, with the features, structures, or characteristics of one or more other embodiments without limitation given that such combination is not illogical or non-functional.

(24) It must be noted that, as used in this specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the content clearly dictates otherwise.

(25) The terms “first,” “second,” and the like in the description and in the claims, if any, are used for distinguishing between similar elements and not necessarily for describing a particular sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in sequences other than those illustrated or otherwise described herein. Furthermore, the terms “include,” “have,” and any variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements is not necessarily limited to those elements, but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.

(26) All directional references (e.g., “plus”, “minus”, “upper”, “lower”, “upward”, “downward”, “left”, “right”, “leftward”, “rightward”, “front”, “rear”, “top”, “bottom”, “over”, “under”, “above”, “below”, “vertical”, “horizontal”, “clockwise”, and “counterclockwise”) are only used for identification purposes to aid the reader's understanding of the present disclosure, and do not create limitations, particularly as to the position, orientation, or use of the any aspect of the disclosure. It is to be understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in other orientations than those illustrated or otherwise described herein.

(27) As used herein, the phrased “configured to,” “configured for,” and similar phrases indicate that the subject device, apparatus, or system is designed and/or constructed (e.g., through appropriate hardware, software, and/or components) to fulfill one or more specific object purposes, not that the subject device, apparatus, or system is merely capable of performing the object purpose.

(28) Joinder references (e.g., “attached”, “coupled”, “connected”, and the like) are to be construed broadly and may include intermediate members between a connection of elements and relative movement between elements. As such, joinder references do not necessarily infer that two elements are directly connected and in fixed relation to each other. It is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative only and not limiting. Changes in detail or structure may be made without departing from the spirit of the invention as defined in the appended claims.

(29) All numbers expressing measurements and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about” or “substantially”, which particularly means a deviation of ±10% from a reference value.

(30) FIG. 1 shows an example of an electrodynamic acoustic transducer 1 in sectional view. The electrodynamic acoustic transducer 1 comprises a housing 2, a membrane 3 fixed to said housing 2, a coil 4 and a magnet system 5. The membrane comprises a bending section 6 and an optional rigid center plate 7. The coil 4 is attached to the membrane 3 and has an electrical conductor 8 in the shape of loops running around a coil axis X in a loop section A. The magnet system 5 comprises a center magnet 9, a pot plate 10 and a top plate 11 and is designed to generate a magnetic field B transverse to the conductor 8 in the loop section A. A current through the conductor 8 of the coil 4 causes the membrane 3 to move according to the electric signal applied to the coil 4.

(31) FIG. 2 shows an example of a coil 4a in more detail. In fact, FIG. 2 shows a cross sectional view with the coil axis X being part of the sectional plane. In other words, the sectional plane is perpendicular to a longitudinal extension of the electrical conductor 8 or perpendicular to a direction of a current flowing through the electrical conductor 8. The coil 4a in this cross sectional view comprises a plurality of conductive layers C1 . . . C3 formed by the electrical conductor 8 with insulation layers D12, D23 in-between. Note that the coil axis X is drawn much narrower to the coil 4a in FIG. 2 than the distance is in reality.

(32) The longer side a of the rectangular cross section of the electrical conductor 8 (that is the width extension of the electrical conductor 8) in said cross sectional view is arranged perpendicular to the loop axis X. In other words, the longer side a is arranged in parallel with a field line of the magnetic field B through said conductor 8 or in parallel with the membrane 3 of the electrodynamic acoustic transducer 1. However, the angle between the longer side a of the rectangular cross section of the electrical conductor 8 and the coil axis X may also be in a range of 80° to 100°.

(33) Preferably, the ratio between the longer side a of the rectangular cross section of the electrical conductor 8 and the smaller side b of the rectangular cross section of the electrical conductor 8 is >4. In other words, the ratio between the width of the electrical conductor 8 and its thickness preferably is >4.

(34) In a further preferred embodiment, the thickness b of a conductive layer C1 . . . C3 is in a range of 10-30 μm. It is also of advantage, if a total thickness c of an insulation layer D12, D23 is in a range of 1-5 μm. In the example of FIG. 2, the insulation layer D12, D23 comprises an optional passivation layer 12, which is about 0.5-1.5 μm thick, and an adhesive 13 with a thickness of about 1-3 μm. Both the passivation 12 and the adhesive 13 form an insulation layer D12, D23.

(35) For the sake of completeness it is noted that the conductive layers C1 . . . C3 are formed by a single electrical conductor 8, which helically runs around the coil axis X. The same counts for the insulation layer D12, D23. That however does not mean, that the electrical conductor 8 is necessarily made of a single piece of metal.

(36) A method of manufacturing an electrodynamic acoustic transducer 1 comprises the steps of: a) cutting the electrical conductor 8 out of a metallic foil, b) forming an insulation layer D12, D23 on the electrical conductor 8, c) making a stack of conductive layers C1 . . . C3 from the electrical conductor 8 and d) (mechanically) connecting the conductive layers C1 . . . C3 to each other by means of an adhesive 13.

(37) The metallic foil may be a copper foil or an aluminum foil or a foil made from an alloy based on copper or aluminum. Cutting in step a) may be done by means of a laser beam, a water jet, plasma cutting, photo etching, a knife or by punching for example. The passivation layer 12 preferably is a Boehmite layer, which is produced by exposing an electrical conductor 8 cut out of an aluminum (alloy) foil in step a) to hot distilled or de-ionized water and/or to hot vapor of distilled or de-ionized water.

(38) Step c) can be done in different ways, which are explained later in more detail. First, making the stack of conductive layers C1 . . . C3 from the electrical conductor 8 may be done by stacking of separate pieces of the electrical conductor 8 and by electrically connecting the stacked separate pieces. Alternatively or in addition, making the stack of conductive layers C1 . . . C3 from the electrical conductor 8 may be done by folding the electrical conductor 8.

(39) In a very advantageous embodiment, first the stack of conductive layers C1 . . . C3 is made from the electrical conductor 8 without an adhesive 13 and then an adhesive 13 is applied to the stacked electrical conductor 8. That means, the adhesive 13 is sucked into the gap between the conductive layers C1 . . . C3 by means of capillary action. In this way, the time for making the stack of conductive layers C1 . . . C3 is not limited by the curing time of the adhesive 13. Moreover, the stack of conductive layers C1 . . . C3 may be made in a very clean way. Superfluous adhesive 13 may be removed by means of a laser.

(40) However, making the stack of conductive layers C1 . . . C3 may also be done by application of glue onto a first layer C1 or onto a passivation layer 12 of the electrical conductor 8, for example by spraying, pad printing or rolling, and by subsequently putting another layer C2 onto the adhesive layer D12. By repeating this sequence, a stack of any desired height can be produced. Alternatively, an insulating foil can be put onto the adhesive, which in turn is wetted with glue itself. Then a conductive layer C2 is put onto the glue of the insulating foil. In a further alternative, a single sided or double sided adhesive plastic foil may be used to build up a stack. If a double sided adhesive plastic foil is used, no further glue is to be applied at all. If a single sided adhesive plastic foil is used, additional glue is used on the non-adhesive side of the foil.

(41) FIG. 3 shows an example of a coil 4b, which is quite similar to the coil 4a shown in FIG. 2. In contrast, the coil 4b is coated with an insulating material 14 after step d). In this way, the coil 4b is protected against short circuits and environmental influences.

(42) In the example of FIG. 2, the thickness b of the electrical conductor 8 is constant along the coil axis X. This however is no necessary condition, and the thickness b of the electrical conductor 8 may also vary along the coil axis X. FIG. 4 shows an example of a coil 4c, wherein the thickness b1 of a conductive layer C1, C4 forming an electrical connection of the coil 4c is thicker than the thickness b2 of an adjacent conductive layer C2, C3. In the example of FIG. 4, the conductive layers C1, C4 forming electrical connections of the coil 4c are the outer conductive layers C1, C4 what means that the coil 4c has two electrical connections. Accordingly, a conductive layer C1, C4 forming an electrical connection of the coil 4c has only one adjacent conductive layer C2, C3.

(43) FIG. 5, shows an example of another coil 4d, which is similar to the coil 4c of FIG. 4. In contrast, the coil 4d has an additional, middle conductive layer C3 forming an electrical connection of the coil 4d, the thickness b1 of which is thicker than the thickness b2 of an adjacent conductive layer C2, C4. In the example of FIG. 5, the conductive layers C1, C3, C5 form electrical connections of the coil 4d what means that the coil 4d has three electrical connections. Accordingly, the conductive layer C3 forming the electrical middle connection of the coil 4d has two adjacent conductive layers C2, C4.

(44) A conductive layer C1 may also (directly) form an electrical connection 15 between the coil 4e (in detail its loop section A) and a non-moving terminal T of the electrodynamic acoustic transducer 1 as this is shown in FIG. 6. The non-moving terminal T may be fixed to the housing 2 or a frame of the electrodynamic acoustic transducer 1 and form an external terminal T. However, the non-moving terminal T may also be connected to an external terminal by means of an additional conductor. Advantageously, no dedicated wires are needed to connect the loop section A of the coil 4e to the non-moving terminal T. Moreover, the conductive layer C1 has excellent bending characteristics in the direction of the loop axis X and thus in the moving direction of the membrane 3. In other words, the conductive layer C1 forming the electrical connection 15 between the coil 4e and a non-moving terminal T is very soft against bending in the moving direction of the membrane 3 and does not much hinder the membrane's movement.

(45) FIG. 7 shows another reason for varying the thickness b of the electrical conductor 8 along the coil axis X. In detail, FIG. 7 shows a coil 4f with constant thickness b and width a of the conductive layers C1 . . . C5 on the left side and a coil 4g with varying thickness b and width a of the conductive layers C1 . . . C5 on the right side. Moreover, the graph of the driving force factor BL over the membrane excursion x is shown in the middle.

(46) In this example, a variation of the thickness b of a conductive layer C1 . . . C5, which corresponds to the length of the shorter side of the rectangular cross section of the conductor 8, is done in a way that the driving force factor BL.sub.4g of a transducer 1 with the right coil 4g is flattened compared to the driving force factor BL.sub.4f of a transducer 1 with the left coil 4f with non-varied thickness b of the conductive layers C1 . . . C5. In fact, the thickness b of the conductive layer C1 . . . C5 (i.e. the shorter side of the rectangular cross section of the electrical conductor 8) of the right coil 4g is larger in a center region of the coil 4g than in a distant region for that reason.

(47) Moreover, a variation of the width a of a conductive layer C1 . . . C5, which corresponds to the length of the longer side of the rectangular cross section of the electrical conductor 8, can be done in a way that the cross sectional area of the electrical conductor 8 and thus the current density in the electrical conductor 8 is kept constant or substantially constant throughout the height of the coil 4g. In fact, the width a of the conductive layer C1 . . . C5 (i.e. the longer side of the rectangular cross section of the electrical conductor 8) of the right coil 4g is smaller in a center region of the coil 4g than in a distant region for that reason.

(48) Alternatively or in addition, the horizontal position of a center of the longer side a of the rectangular cross section of the electrical conductor 8 may vary along the coil axis X. In this way, the coil 4g gets an asymmetrical shape.

(49) As mentioned hereinbefore, making a stack of conductive layers C1 . . . C4 from the electrical conductor 8 may be done by stacking of separate pieces of the electrical conductor 8 and by electrically connecting the stacked separate pieces in step c). An example for such a procedure is shown in FIG. 8. In detail, the separate pieces of the electrical conductor 8 (i.e. foil blanks cut from a foil sheet) are electrically connected by means of laser welding or ultrasonic welding in step c). For that reason, welding joints 16 between the conductive layers C1 . . . C4 are made by use of a laser beam L of a laser 17. Preferably, the laser power is set to a level, at which it cracks a passivation layer 12 or even a complete insulation layer D12, D23 if it is already applied and welds together only two conductive layers C1 . . . C4 without destroying the passivation layer 12 or insulation layer D12, D23 offside the welding joint 16. Moreover, it is advantageous if the welding joints 16 between the different conductive layers C1 . . . C4 are spaced or offset along the course of the electrical conductor 8 as this is shown in FIG. 8.

(50) Because auf the small cross section of the electrical conductor 8, handling a conductive layer C1 . . . C5 may get tricky because of its flimsy structure. For this reason, a supporting structure 18 connected to the electrical conductor 8 by means of bars 19 may be cut out of a metallic foil in step a) as this is shown in the example of FIG. 9. In detail, the supporting structure 18 consists of a comparably broad frame, which is connected to the conductive layer C1 by means of several bars 19. The supporting structure 18 together with the bars 19 is removed from the electrical conductor 8 after step d), i.e. after the conductive layers C1 . . . C5 have been interconnected mechanically by means of an adhesive thus stabilizing the layer structure and making the supporting structure 18 superfluous.

(51) It is of advantage in this context if the bars of adjacent conductive layers C1 . . . C5 are located at different positions after step c) when viewed in a direction of the loop axis X. In other words, the bars 19 are not stacked when the conductive layers C1 . . . C5 are stacked, but the bars 19 of adjacent conductive layers C1 . . . C5 are displaced to each other. In this way, removing the bars 19 after step d) is eased. They may be cut away by means of the laser 17 or may simply be torn off.

(52) Making a stack of conductive layers C1 . . . C4 by stacking of separate pieces of the electrical conductor 8 is not the only possibility. Making a stack of conductive layers C1 . . . C4 from the electrical conductor 8 may also be done by folding the electrical conductor 8. FIG. 10 shows an electrical conductor 8 cut out of a metal foil in the shape of a rectangular wave or in the shape of a meander. In a second step, the electrical conductor 8 is folded in a zigzag fashion or accordion fashion along the folding lines F1 . . . F6. In this way, the electrical conductor 8 in the end helically runs around the coil axis X thus forming the loop section A of a coil 4 . . . 4h.

(53) In this example, the foil blank also comprises an optional section, which later forms the electrical connection 15 or lead between the loop section A of the coil 4 and the non-moving terminal T of the electrodynamic acoustic transducer 1. In other words, the leads 15 of the coil 4 may integrally be formed with the loop section A and may be cut out of the metal foil together with a conductive layer C1 . . . C5 in a single step. In a preferred embodiment, a portion of the metal foil sheet can be covered with a coating prior to cutting the leads 15 to improve performance of the same. For example, a polyamide coating may be deposited on a portion of the metal foil sheet in which the lead 15 are arranged. The polyamide coating improves fatigue performance and/or provides corrosion resistance, which may lead to increased service life of a electrodynamic acoustic transducer 1 incorporating such a coil 4. However, it should be noted that coating the leads 15 prior to cutting is no necessary condition, and the leads 15 may also be coated after the cutting step.

(54) It should be noted that folding the electrical conductor 8 is different to wind an electrical conductor 8. “Folding” means bending the (flat) electrical conductor 8 by 180° so that again a flat structure is formed. “Winding” means bending an electrical conductor 8 continuously so that a round coil is formed or making ongoing bends of <180° in the same direction so that a polygonal coil is formed.

(55) In the example shown in FIG. 10, the bends around the folding lines F1 . . . F6 are arranged in the course of the legs of a polygonal coil 4 . . . 4h. However, the bends may also be arranged outside of the course of the legs of a polygonal coil 4 . . . 4h. In detail, at least two conductive layers C1 . . . C5 or loops can be formed by a single piece of a metallic foil, which comprises a bend between each two conductive layers C1 . . . C5, wherein the bend is arranged in a protrusion or jogged portion of the coil 4 . . . 4h.

(56) FIGS. 11 to 14 show examples of an electrical conductor 8 with such a protrusion 20. FIG. 11 shows the (unbent) corner region of an electrical conductor 8 cut out of a metal foil. FIG. 12 shows a top view of the folded electrical conductor 8. FIG. 13 shows an oblique view of a first example of the folded electrical conductor 8, and FIG. 14 shows an oblique view of a second example of the folded electrical conductor 8.

(57) As is shown in FIGS. 11 to 14, the bend along the folding line F is arranged outside of the course of the legs of the polygonal coil 4 . . . 4h. In detail, the electrical conductor 8 in the region of the protrusion 20 runs out of the plane of the conductive layer C1 . . . C5 by at least the thickness b of the conductive layer C1 . . . C5 in a section from a protrusion 20 start to the folding line F. In the example of FIG. 13, there is a step down out of the plane of the leg coming from the lower left side. In the example of FIG. 14, there is a step up out of the plane of the leg coming from the upper left side.

(58) In addition, the electrical conductor 8 in the region of the protrusion 20 runs along a 180° bending around the folding line F back into the plane of the conductive layer C1 . . . C5. In the example of FIG. 13, electrical conductor 8 is fold upwards back in the plane of the conductive layer C1 . . . C5. In the example of FIG. 14, electrical conductor 8 is fold downwards back in the plane of the conductive layer C1 . . . C5.

(59) However, there may also be a step up out of the plane of the leg coming from the lower left side and a 180° fold downwards back in the plane of the conductive layer C1 . . . C5 in the example of FIG. 13 and a step down out of the plane of the leg coming from the upper left side and a 180° fold upwards back in the plane of the conductive layer C1 . . . C5 in the example of FIG. 14.

(60) In all cases, a portion having twice the thickness b of an electrical conductor 8 is arranged in the protrusion 20 and outside of the course of the legs of the polygonal coil 4 . . . 4h. Accordingly, each conductive layer C1 . . . C5 is an even structure in the course of the legs of the polygonal coil 4 . . . 4h, and the conductive layers C1 . . . C5 can be stacked easily. In this example, said portions having twice the thickness b of an electrical conductor 8 appear in every second corner. However, this is no necessary condition, and other patterns are possible as well.

(61) To provide the above benefits, the dimensions d and e should be equal to or even exceed the width a of the electrical conductor 8. In other words, d≥a and e≥a. When setting the dimension e, also an additional length for enabling the fold should be considered. So, preferably e≥d.

(62) It should be noted that the shape of the protrusions 20 depicted in FIGS. 11 to 14 is just exemplary, and other shapes can provide the above benefits as well. In particular, the protrusions 20 may be rounded or can exclusively be made up from round shapes.

(63) FIGS. 15 and 16 show an example of a supporting structure 18 for the electrical conductor 8 having the shape of a rectangular wave or the shape of a meander like the electrical conductor 8 of FIG. 10 and the protrusions 20 shown in FIGS. 12 to 14. FIG. 15 shows an example with a couple of legs of the wave structure or meander structure, and FIG. 16 shows a detailed view of an protrusion 20. Said supporting structure 18 reduces or eliminates twisting or deformation of the electrical conductor 8 when handling the same, in particular during the folding step.

(64) Again, the electrical conductor 8 is connected to the supporting structure 18 by means of bars 19, and again the supporting structure 18 together with the bars 19 is removed from the electrical conductor 8 after step d), i.e. after the structure has been folded and the conductive layers C1 . . . C5 have been interconnected mechanically by means of an adhesive thus stabilizing the layer structure and making the supporting structure 18 superfluous. To ease folding, a number of cut outs 21 are arranged in the supporting structure 18 along the folding lines F thus forming a perforation. Due to cut outs 21 along the folding lines F in the blank, the electrical conductor 8 folds at the desired folding lines F when lifted. To ease folding, alternatively or in addition, an indentation or groove can be formed along a folding line F before step c). The indentation can be formed with a laser at low laser power, by etching or by embossing.

(65) FIG. 15 furthermore shows, that the bars 19 are located at different positions after step c) when viewed in a direction of the loop axis X after the folding step. In this way, removing the bars 19 after step d) is eased. They may be cut away by means of the laser 17 or may simply be torn off. To ease tearing off the bars 19, a number of cut outs can be arranged along a tear off line R, along which the bar 19 finally is torn off, thus forming a perforation. To ease tearing off the bars 19, alternatively or in addition, also an indentation or groove can be formed along a tear off line R. Again, the indentation can be formed with a laser at low laser power, by etching or by embossing. It should be noted that the perforation and the indentations or grooves equally apply to the bars 19 shown in FIG. 9.

(66) It should be noted at this point that making a stack of conductive layers C1 . . . C5 for a single coil 4 can be done by folding of the electrical conductor 8 and by stacking of separate pieces of the electrical conductor 8, which are electrically connected. That means that separate folded electrical conductors 8 may be stacked and electrically connected or folded electrical conductors 8 may be combined (stacked) with unfolded pieces of the electrical conductor 8.

(67) The folds in the electrical conductors 8 can lead to an increased electrical resistance in the region of the folds which can impact the acoustic performance of the electrodynamic acoustic transducer 1. This resistance increase may be compensated by increasing the width f of the electrical conductors 8 in the region of the folding lines F (see FIG. 11 in this context). In turn, a larger cross-sectional area for the electrical current to flow through is provided, which thus reduces the electrical resistance. However, if aluminum is used for the electrical conductor 8, it may be hardened and locally annealed by the laser 15 in the region of the folds what reduces the electrical resistance as well. In this way, the width f of the electrical conductor 8 in the region of the folding lines F does not need to be increased as there is little to no increase of the resistance as a result of the folding.

(68) FIGS. 17 to 22 show an alternative method of manufacturing the coil 4h being depicted in FIG. 8. The method is similar to the one explained in the context with FIG. 8, but the cutting step a) takes place after step d) here. In detail, a first piece of a metal foil 22a is provided in a first step shown in FIG. 17. The metal foil 22a comprises a cut out 23a at the position, where the electrical conductor 8 is separated later. In FIG. 18 a further piece of a metal foil 22b has been put onto the metal foil 22a. The metal foil 22b comprises a cut out 23a at the position, where the electrical conductor 8 is separated later, too. The laser 17 makes a welding joint 16 to electrically connect the metal foil 22a and the metal foil 22b at the position indicated in FIG. 18. The same sequence is performed for a metal foil 22c in FIG. 19 and a metal foil 22d in FIG. 20. As can be seen, the cut outs 23a . . . 23d in the metal foils 22a . . . 22d are displaced in horizontal direction. As a result, a stack of metal foils 22a . . . 22d, which are electrically connected by welding joints 16 at dedicated positions, is generated. This stack is shown in FIG. 21. In a further step a coil contour E is cut out of the stack of metal foils 22a . . . 22d, e.g. by means of the laser 17, a water jet, plasma cutting, photo etching, a knife or by punching. Hence, a number of conductive layers C1 . . . C5 are cut simultaneously in step a). Finally, the coil 4h, which is already shown in FIG. 8, is generated as depicted in FIG. 22. In FIGS. 17 to 22 the cutting step a) takes place after step d), whereas in the description of FIG. 8 the cutting step a) takes place before step d). In yet another embodiment, the cutting step a) can take place after step c), but before step d).

(69) Generally, the metal foils 22a . . . 22d may have been passivated before they are used to build up a stack. Again, the stack can be build up of “dry” pieces of the metal foils 22a . . . 22d, between which an adhesive 13 is applied and sucked into the gap between the metal foils 22a . . . 22d by means of capillary action. This can be done for each two pieces or once for the whole stack. But, making the stack of the metal foils 22a . . . 22d may also be done by application of glue onto a first metal foil 22a or onto a passivation layer 12 of the metal foil 22a, for example by spraying, pad printing or rolling, and by subsequently putting another metal foil 22b onto the adhesive layer D12. Alternatively, an insulating foil can be put onto the adhesive, which in turn is wetted with glue itself. Then the metal foil 22b is put onto the glue on the insulating foil. In a further alternative, a single sided or double sided adhesive plastic foil may be used to build up the stack. In this embodiment, the adhesive plastic foil is applied onto the first metal foil 22a, and the next metal foil 22b is applied onto the adhesive plastic foil. If a double sided adhesive plastic foil is used, no further glue is to be applied at all. If a single sided adhesive plastic foil is used, additional glue is used on the non-adhesive side of the foil. By repeating the given sequences, a stack of any desired height can be produced.

(70) Finally, FIGS. 23 and 24 illustrate the influence of the coil shape on the output power of the electrodynamic acoustic transducer 1. In detail, FIG. 23 shows the corner region of a prior art drive system, which comprises a center plate 11, separate, linear side magnets 24, 25 and a coil 4′ with rounded corners, and FIG. 24 shows the corner region of a proposed drive system, which comprises a center plate 11, separate, linear side magnets 24, 25 and a coil 4 with sharp corners. When FIGS. 23 and 24 are compared, it gets clear that the air gap g of the proposed drive system in FIG. 24 is substantially smaller in the corner region than the air gap g′ of the prior art drive system of FIG. 23. Accordingly, a transducer 1 using the proposed drive system of FIG. 24 provides more sound power than the prior art drive system of FIG. 23. In other words, the proposed drive system of FIG. 24 is more efficient that the prior art drive system of FIG. 23.

(71) In summary, the proposed method provides coils 4 . . . 4h with a high density of the electrical conductor 8. Preferably, a fill factor, which is the share of all conductive layers C1 . . . C5 on the volume of the coil 4 . . . 4h is >80%. Other solutions, like coils with a coil wire or horizontally stacked layers provide a fill factor which is much lower thus downgrading the power weight ratio of a coil 4 . . . 4h. Moreover, a tensile stress in the electrical conductor 8 preferably can be kept below 50 N/mm.sup.2 during steps a) to d) so as to avoid a belly-shape or bone-shape, which normally occurs when a wire is wound to a coil 4 . . . 4h.

(72) It should be noted that the invention is not limited to the above mentioned embodiments and exemplary working examples. Further developments, modifications and combinations are also within the scope of the patent claims and are placed in the possession of the person skilled in the art from the above disclosure. Accordingly, the techniques and structures described and illustrated herein should be understood to be illustrative and exemplary, and not limiting upon the scope of the present invention.

(73) The scope of the present invention is defined by the appended claims, including known equivalents and unforeseeable equivalents at the time of filing of this application. Although numerous embodiments of this invention have been described above with a certain degree of particularity, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the spirit or scope of this disclosure.

(74) TABLE-US-00001 LIST OF REFERENCES  1 electrodynamic acoustic transducer  2 housing  3 membrane  4, 4′ 4a . . . 4g coil  5 magnet system  6 bending section  7 rigid center plate  8 electrical conductor  9 center magnet 10 pot plate 11 top plate 12 passivation layer 13 adhesive 14 coating 15 electrical connection to non-moving terminal 16 welding joint 17 laser 18 supporting structure 19 bar 20 protrusion/jogged portion 21 cut out 22a . . . 22d metal foil 23a . . . 23d cut out 24 side magnet 25 side magnet a width of the conductive layer (longer side) b, b1, b2 thickness of the conductive layer (shorter side) c (total) thickness of insulation layer d displacement of electrical conductor e displacement of electrical conductor f width of electrical conductor in the fold region g, g′ air gap x excursion A loop section B magnetic field BL driving force factor C1 . . . C5 conductive layer D12, D23 insulation layer E coil contour F, F1 . . . F6 folding line R tear off line T, T1, T2 non-moving terminal X coil axis