LOUDSPEAKER WITH COATED VOICE COIL

Abstract: A loudspeaker (1) comprising: a magnet unit (3) which generates a magnetic field; an element suspended for resilient movement round a stable rest position, such as a diaphragm, a cone (5), a dome or other element suitable for radiating sound with selected properties; and an electrical conductor, for instance a speech coil (6), connected to that movable element and having current feed terminals, which conductor (6) is placed in the said magnetic field such that when current is passed through this conductor a force is exerted on the movable element which imparts to the element a deviation from its rest position. In order to effectively prevent degradation and damaging of the conductor (6) the loudspeaker (1) according to the invention has the feature that the conductor (6) is covered by a corrosion and oxidation resistant coating such that entry of the gases stimulating corrosion or oxidation of the conductor (6), particularly oxygen, is prevented.

Description: Loudspeaker with coated voice coll The invention relates to a loudspeaker comprising: a magnet unit which generates a magnetic field; an element suspended for resilient movement round a stable rest position, such as a diaphragm, a cone, a dome or other element suitable for radiating sound with selected properties; and an electrical conductor, for instance a speech coil, connected to that movable element and having current feed terminals, which conductor is placed in the said magnetic field such that when current is passed through this conductor a force is exerted on the movable element which imparts to the element a deviation from its rest position. Such a speaker is generally known. Reference is made for instance to the published Ne- therlands patent application 85.01166. Described herein is a ribbon speaker comprising a diaphragm which is clamped in a frame and in the central region of which is situated a zone containing the conductor arranged in a selected pattern. Due to the high temperatures which can occur in the conductor as a result of passage of strong current, therefore during generating of great sound powers, it can occur, cer¬ tainly after a period of time, that the conductor is subject to degradation. The conductor will generally consist of aluminium. This is in general a very suitable material for loudspeaker purposes in view of the very favourable relation¬ ship between the conductivity of this ' material and its speci¬ fic mass. Compared for instance to gold, silver or a cop¬ per/silver alloy, the sensitivity of the loudspeaker is found to be considerably smaller than with aluminium as conductor material due to the relatively great weight of these mate¬ rials. Titanium is a less suitable material due to its high specific resistance. As material for the conductor aluminium does however have the drawback of being quite strongly sus¬ ceptible to oxidation and corrosion, particularly at the greatly increased temperatures that can occur in a loudspea¬ ker. In order to effectively prevent the said degradation and even damaging of the conductor the loudspeaker according to the invention has the feature that the conductor is co¬ vered by a corrosion and oxidation resistant coating such that entry of the gases stimulating corrosion or oxidation of the conductor, particularly oxygen, is prevented. Since the said degradation is primarily manifest at increased temperatures, the embodiment is recommended which has the characteristic that the coating consists of a heat resistant material. The material of the coating can for instance be a plastic, particularly a thermoplastic material. In this respect a polyimide, for example a polyether- imide, is very strong and resistant. A plastic coating can have a thickness in the order of magnitude of 10 μm. As a result of the extra mass which the coating has, the sensitivity of the loudspeaker will decrease. For a ribbon loudspeaker of the above described type an output decrease has been ascertained in the order of 1 dB. An embodiment can also be used in which the coating consists of ceramic material. Such a loudspeaker can advanta- geously display the feature that the coating has a thickness in the order of magnitude of 0.1 μm. The coating is herein so light that the sensitivity of the loudspeaker does not measu¬ rably decrease due to applying of the coating. Another embodiment has the special feature that the coating comprises an electrically conducting layer, for instance of (precious) metal, which is electrically insulated from the conductor by an electrically insulating layer, for instance of ceramic material. As insulator can be used for instance a ceramic material over which is arranged a coating of precious metal- which serves as oxygen barrier. The loudspeaker according to the invention can also be provided with cooling means in order to effectively drain off the heat generated in the conductor. Although this can be recommended in certain conditions, it is not essential within the scope of the invention. The invention will now be elucidated with reference to the annexed drawings, in which: figure 1 shows a cross section through a cone loud¬ speaker according to the invention in a first embodiment; figure 2 is a view corresponding with figure 1 of a detail of a second embodiment; figure 3 is a view corresponding with figure 2 of a third embodiment; figure 4 is a partly broken away perspective view of a ribbon loudspeaker with horn loading accommodated in a cabinet; figure 5 shows a detail of the loudspeaker of figure 4 ; figure 6 shows the cross section V-V of the loudspeaker unit with loading horn of figure 5; figure 7 shows the section VII-VII of figure 5; figure 8 is a view corresponding with figure 7 of a variant; and figure 9 shows a detail of the loudspeaker according to figure 1 on enlarged scale. Figure 1 shows a loudspeaker 1 of the electrodynamic type comprising a framework 2, a magnet unit 3 coupled there¬ to, a cone 5 connected to the framework via a pliant suspen- sion rim 4 and a cylindrical speech coil 6. The speech coil 6 is axially movable in an annular air gap 7 in which a strong magnetic field prevails. The air gap is bounded on the inner side by a central pole piece 8 and an annular pole piece 9 extending therearound at a distance. A magnetic closing plate 10 is situated on the underside of the central pole piece 8. Arranged between this closing plate 10 and the annular pole piece 9 is an annular permanent magnet 11. As can be seen in figure 1, the central pole piece 8 comprises a cylindrical central portion 12 and a top plate 13 of greater diameter. The annular pole piece 9 has a smaller inner diameter than the permanent magnet 11. This configura¬ tion ensures a strong and concentrated magnetic field in the air gap 7. The speech coil 7 is of known type and comprises a tube 14 serving as a speech coil carrier and a conductor 15 wound thereon in coil-like manner. It will be apparent that when current is sent through the conductor 15 via current feed terminals 16 the speech coil 6 will be displaced counter to the resilience of the pliant suspension rim 4 through interaction with the magnetic field. The loudspeaker 1 is provided with cooling means for cooling the conductor 15. For this purpose the cylindrical centre portion 12 of the central pole piece 8 is provided with annular rows of perforations 17 in the wall, wherein the opening 18 present on the bottom serves for supplying gas, for instance air, under pressure, which gas can enter the cylindrical space 19 through the openings 17, can there serve to cool the conductor 15 and can subsequently leave the space 19 again via openings 20. Also achieved with this configuration and also the configurations according to figures 2 and 3 to be described hereinafter is that the permanent magnet 11 is cooled. This has the particular advantage that when neodymiu is used as magnet material the field strength in the air gap is main¬ tained even in the case of prolonged high electrical load of the loudspeaker 1. Figure 2 shows a second embodiment. Air enters here via the openings 20, passes through the space 19 and leaves the magnet unit 3' via the air gap 7, which ensures an effective cooling of the conductor 15. The central pole piece according to figure 2 differs from that of figure 1 and is therefore designated 8". The associated cylindrical central portion 12' has no perforations. The top plate 13' is an annular widening at the top. The third embodiment according to figure 3 differs from figure 2 in that the through-hole 21 of the central pole piece 8 1 is used here for supplying air from below which enters the air gap 7 from the top, brushes along the conduc¬ tor 15, thus cooling it, and once again leaves the magnet unit 3" via the space 19 and the holes 20. It will otherwise be apparent that combinations of the mentioned embodiment details are also possible. The air flows are also designated in the figures only very generally. and for purposes of orientation. Figure 4 shows a loudspeaker cabinet 22 in which is situated a ribbon loudspeaker to be briefly described herein- below. This ribbon loudspeaker is of per se known type and is described for instance in the Netherlands patent application 85.01166. The cabinet bears a horn 23 which debouches on the front side and in the throat of which is situated a diaphragm Λ 24. This diaphragm is clamped into a frame 25 incorporated in the magnet unit 26 to be further described with reference to figure 6. The cabinet bears at its rear a fan 27 which draws in ambient air 28 and blows it into the rear space 29 of cabinet 22. This air is' blown via perforations 30 in a partition 31 into the forward space 32 of the cabinet 22. Available bet¬ ween the front plate 33 of the magnet unit 26 and the rear plate 34 of the horn 23 is a free gap through which the air can escape into the forward space. The magnet unit 26 is fixed to the partition 31 while the horn 23 is fixed on the side of the mouth to the cabinet 22, thus on the front thereof. As shown clearly in figures 5 and 6 particularly, the horn has plates 35 on the side of the throat, therefore in the region of the diaphragm 24, which plates form an exten¬ sion of the inner walls of horn 23. These plates 35 extend over the outflow openings of the air gaps 36, i.e. the en- tioned free spaces between the magnet unit 26 and the horn 23. Thus achieved in the manner shown in figures 4, 5 and 6 is that the diaphragm 24 is cooled from the long sides of both sides by the air flows brushing past which are desig¬ nated with 36. Situated in the central zone of the diaphragm 24 is a spiral-like conductor 37 which has a free central region 38 as a result of its spiral shape. An effective cooling of the conductor takes place due to the air flows moving in opposing directions from the sides, and these air flows leave the diaphragm in transverse direction thereof through the horn 23. The short sides of the diaphragm are also subjected to such an air cooling, which is of particular importance for the current feed terminals 39 shown in figure 5. The diaphragm 24 is covered by a substantially acous¬ tically transparent plate 40 arranged at some distance there¬ from in the horn. This plate can consist of mesh or foamed material or the like. For the sake of completeness, the magnet unit 26 will also be discussed briefly with specific reference to figure 6. This comprises two elongate permanent magnets 41 consist¬ ing of neodymium. Situated at the bottom is a magnetic clos¬ ing plate 42 which carries a central pole piece 43. At the top each of the permanent magnets 41 carries a pole piece 44 having a tapering form similar to the horn 23. Due to the placing of the diaphragm 24 as shown in figure 6 and in particular the conductor 37 arranged thereon is achieved that this conductor co-acts as well as possible with the magnetic field between the pole piece 43 and the pole pieces 44. Figure 7 shows the diaphragm 24 on which the aluminium conductor 37 is arranged. Applied over the conductor 37 is a coating 101 of a polyimide, particularly polyetherimide, whereby the conductor 37 is wholly protected against the influence of oxygen from the surrounding air. Figure 8 shows a variant in which the conductor 37 is covered by a coating 102 of a ceramic material over which is applied a coating 103 of gold. With respect to the applying of the said coatings it is noted that the application of a polyimide can take place by dissolving the polyimide in a solvent, applying the solution over the conductor and allowing the solvent to evaporate. Application of a ceramic coating can for instance take place by sputtering. The applying of the coating 103 according to figure 8 can for instance take place by vapour deposition. It is self-evident that it must be ensured that the coating layer performs its function correctly in all condi¬ tions. The layer must therefore be completely sealed and integral in all conditions. The insulating layer 102 must also be applied such that the aluminium conductor 37 cannot come into electrical contact with the metal coating 103. Finally, figure 9 shows a detail of the loudspeaker of figure 1 on enlarged scale. In this loudspeaker 1 the speech coil 15 is carried by the cylindrical speech coil carrier 14. As according to figure 7, the aluminium conductor 15 is covered by a coating 104. *****

Claims: CLAIMS 1. Loudspeaker comprising: a magnet unit which generates a magnetic field; an element suspended for resilient movement round a stable rest position, such as a diaphragm, a cone, a dome or other element suitable for radiating sound with selected properties; and an electrical conductor, for instance a speech coil, connected to that movable element and having current feed terminals, which conductor is placed in the said magnetic field such that when current is passed through this conductor a force is exerted on the movable element which imparts to the element a deviation from its rest position; characterized in that the conductor is covered by a corrosion and oxidation resistant coating such that entry of the gases stimulating corrosion or oxidation of the conductor, parti¬ cularly oxygen, is prevented. 2. Loudspeaker as claimed in claim 1, characterized in that the coating consists of a heat resistant material. 3. Loudspeaker as claimed in claim 1, characterized in that the material is a plastic. 4. Loudspeaker as claimed in claim 3, characterized in that the material is a thermoplastic material. 5. Loudspeaker as claimed in claim 4, characterized in that the material is a polyimide, for instance polyether- imide. 6. Loudspeaker as claimed in claim 3, characterized in that the coating has a thickness in the order of magnitude of 10 μm. 7. Loudspeaker as claimed in claim 2, characterized in that the coating consists of ceramic material. 8. Loudspeaker as claimed in claim 7, characterized in that the coating has a thickness in the order of magnitude of 0.1 μm. 9. Loudspeaker as claimed in claim 1, characterized in that the coating comprises an electrically conducting layer, for instance of (precious) metal, which is electrically insulated from the conductor by an electrically insulating layer, for instance of ceramic material. *****

Inventors: WIJNKER EDDY LEON LUCIEN, WIJNKER, EDDY, LEON, LUCIEN
Applicants: STAGE ACCOMPANY B V, STAGE ACCOMPANY B.V.
Application year: 1991
Main classification: H04R9
IPC codes: H01B7, H01F5, H04R9