Loudspeakers and their production

Abstract

 The straight-sided cone or diaphragm (2) of a moving coil loudspeaker is formed wholly or principally from aluminium which is spun substantially to double its tensile strength, the spinning process being conventional and preferably being effected manually (Fig. 1). Most advantageously, the formed cone (2) is hard anodised on both sides to increase rigidity and heat radiation. A loudspeaker in accordance with the invention has approximately two and one third times the sound volume capacity of a conventional loudspeaker of comparable size without producing any detectable sound distortion.

Description

[0001] This invention relates to loudspeakers and their production and general objects of the invention include the provision of moving coil loudspeakers which are similar in basic construction to conventional loudspeakers but which can be made much more powerful than known loudspeakers of comparable sizes whilst having such low sound distortion characteristics as to be substantially distortion-free for all practical purposes. The vast majority of known loudspeakers employ wood pulp/paper, card or a synthetic plastics material for the formation of their cones (diaphragms) but, unfortunately, when such a cone is subject to relatively large vibratory sound-producing movements (usually known as "excursions"), the cone is temporarily deformed and readily audible distortion of the sound results. Some attempts have been made to use metallic or other more rigid materials to produce loudspeaker cones but, for various reasons, these attempts have met with only very limited success.

[0002] According to one aspect of the present invention, there is provided a method of producing a cone for use in a loudspeaker, the method comprising the steps of forming said cone wholly or principally from aluminium and spinning the cone to harden the same.

[0003] For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:-

Figure 1 is a sectional side elevation illustrating a preferred way of forming the cone of a loudspeaker in accordance with the invention.

Figure 2 is a sectional side elevation somewhat diagrammatically illustrating the formation of a loudspeaker constructed in accordance with the present invention, and,

Figure 3 shows the parts that can be seen in a circle designated A in Figure 2 of the drawings to an enlarged scale and in greater detail.

[0004] Referring to the accompanying drawings, and firstly to Figure 1 thereof, that Figure shows the way in which a sheet metal blank 2 is hardened to form the cone (diaphragm) of a loudspeaker in accordance with the invention. The sheet metal blank 2 is preferably, but not essentially, formed from the almost (99% or more) pure grade aluminium that is known in the United Kingdom as "AA 1200". This material has a modulus of elasticity of 7.05 × 10¹⁰ Newtons per metre² and a density of 2.71 × 10³ per cubic metre. For some purposes, known alloys of aluminium which contain that metal as a major component may be employed but, generally speaking, aluminium with an absolute minimum of impurities gives the best results.

[0005] Returning to Figure 1 of the drawings, that Figure illustrates a metal spinning operation in which the initially circular blank 2 is spun by a lathe or like machine around an axis defined by a lathe shaft 4, a central region of the blank 2 being releasably clamped to a former 1 by a large securing nut 3 so that, when the nut 3 is tightened, the former 1, the sheet metal blank 2, the nut 3 and the shaft 4 will be temporarily united for rapid rotation around the longitudinal axis of the shaft 4. Metal spinning is an art which is known per se and is a particularly effective method of making conically shaped items. Conventional metal spinning techniques may be used entirely successfully in putting the present invention into effect. Thus, the sheet metal blank 2 may be forced into the required conical shape around the former 1 in much the same basic way as a potter moulds clay into a required shape on a potter's wheel. It is noted that, as illustrated, the formed cone will usually be a so-called straight-sided cone since this configuration gives particularly low harmonic distortion characteristics. However, this is not essential and curved/concave sided constructions are within the scope of the invention. Tempered steel or leather-faced wooden or other tools are the usual means by which pressure is applied to the blank 2 and, as shown in Figure 1 of the drawings, these may be employed manually.

[0006] The sheet aluminium or aluminium alloy that affords the blank 2 and, eventually, the cone of the loudspeaker that is being produced, is necessarily thin and, in the case of a cone that is to have a diameter of about 91 millimetres, its thickness may advantageously be of the order of about 0.152 millimetres. Naturally, this thickness is variable having regard to the particular metallic formula in accordance with which the blank 2 is formed, the particular purpose for which the eventual loudspeaker is required, and the diameter of the cone that is being made. A large diameter cone will require a proportionately increased thickness of the aluminium from which it is formed whereas a smaller diameter cone will require a proportionately reduced thickness thereof. The wall thickness of the cone may be graduated from centre to outer edge, the greater thickness being in the central region and the minimum thickness being in the outer peripheral region. Due to the thinness of the blank 2, it is best to shape it around the former 1 in progressive stages using a shaping tool 5 and simultaneously supporting the rear or concave side of the cone that is being produced using, conveniently, a wooden or other stick 6 for this purpose. Once the metal of the blank 2 is sufficiently close to the former 1 to receive automatic support from the latter, the use of the support stick 6 is no longer required. As soon as the sheet metal blank 2 has taken the conical shape of the former 1, it is trimmed to size around its outer and inner peripheries, a relatively large central circular hole being left for the mounting of a voice/sound coil. The smoothly trimmed outer periphery of the cone will be accurately attached, in the finished loudspeaker, to a rubber or synthetic plastics or other foam roll surround as will be described with reference to Figure 2 of the drawings.

[0007] The spinning process increases the tensile strength of the aluminium or aluminium alloy by a factor of approximately two from substantially 80 megapascals to substantially 160 megapascals and this has the considerable advantage that the cone angle of the cone can be much greater or smaller than is conventional, without the metal tending to break, so that the design possibilities for a moving coil loudspeaker of this kind are greatly enhanced. In accordance with a greatly preferred, but not absolutely essential, feature of the invention, the aluminium or aluminium alloy of the formed cone is hard-anodised, advantageously on both sides. In a loudspeaker cone having the dimensions mentioned above by way of example, an anodised layer of aluminium oxide having a thickness of 0.050 mms may be formed on each side of the metal, approximately half this thickness progressively replacing the metal itself so that a total wall thickness of about 0.254 mms is produced.

[0008] It is noted that the method of shaping and tool-­hardening the alloy blank 2 that has been described with reference to Figure 1 of the drawings is a preferred manual method but that this method is by no means the only one that is possible. It will immediately be evident that automatic or semi-automatic mechanical ways of shaping the blank 2 could be substituted for the principally manual method that has been described. If high power handling of the loudspeaker that is to be produced is of great importance, the hard anodic layer of the cone may be permanently coloured black so as further to increase the heat-dissipation which it will effect in relation to the coil to which it is thermally coupled.

[0009] Referring now to Figures 2 and 3 of the drawings, the spun metal cone (diaphragm) of the loudspeaker that is shown in these Figures is again denoted by the reference 2 so as to make it clear that it is the cone that has been formed in the manner already described above with reference to Figure 1 of the drawings. It will be evident to those familiar with the construction of conventional loudspeakers that the loudspeaker of Figure 2 of the drawings is traditionally constructed to a large extent, differing principally from most known loudspeakers in that, as well as the spun aluminium cone 2, it also comprises a spun metal centre dome 7 and advantageously a metallic voice or sound coil former 8. Figure 3 of the drawings shows, to an enlarged scale, a region A in which the outer periphery of the centre dome 7 and the inner periphery of the spun metal aluminium 2 are secured to the voice or sound coil former 8. This attachment is preferably effected by a strong adhesive having good heat conduction properties. An epoxy resin adhesive containing a metallic powder filler is a good example of such an adhesive but is certainly not the only adhesive that could be employed. It is necessary to transfer heat as efficiently as possible from the windings of a voice or sound coil 9 that is arranged around the former 8 to the cone 2 and the centre dome 7 which jointly act as a heat sink, enabling the heat which is produced to be dissipated to the ambient atmosphere as quickly and efficiently as possible since, as is well known, overheated components of loudspeakers seriously distort the sound production, the distortion in known loudspeakers sometimes being irreversible merely by cooling due to the damage caused by the overheating. The metallic components of a loudspeaker in accordance with the invention conduct heat very readily and thus quickly dissipate any temporary heat build-ups that might occur.

[0010] As previously mentioned, much of the construction of a loudspeaker in accordance with the invention is substantially conventional and, referring again to Figure 2 of the drawings, it will be seen that an axial pole piece breather hole 10 extends through a combined pole piece and back plate assembly 18 into the space that is surrounded by the coil former 8 and the centre dome 7. A circular row of ventilation holes 11 extends concentrically around the longitudinal axis of the loudspeaker, said holes 11 being formed through the back plate of the assembly 18 and opening into an annular space bounded internally by the pole piece of the assembly 18 and externally by the loudspeaker magnet which bears the reference 17. An annular front plate 16 is in parallel relationship with the back plate of the assembly 18, its inner periphery closely surrounding the voice or sound coil 9 and its former 8.

[0011] The outer periphery of the spun metal cone 2 is flexibly connected to a very diagrammatically illustrated chassis 13 of the loudspeaker by the previously mentioned rubber or foam or other roll surround which is given the reference 12 in Figure 2 of the drawings. A linen or like suspension of shape-enhanced flexibility 14 has a basically annular shape and is provided in such a position that its inner periphery is connected to the coil former 8 between the cone 2 and the voice or sound coil 9 whilst its outer periphery is connected to an angular rebate in the chassis 13 with the aid of a flexible sealing gasket 15.

[0012] The design of the loudspeaker is such as to maximise the performance of the spun aluminium alloy cone 2 and, to this end, the sizes and shapes of the various components, such as the magnet 17, the voice or sound coil 9 , the linen or like suspension 14 and the roll surround 12 will allow a considerably greater excursion of the cone 2 to take place, without any significant distortion of the produced sound, than occurs in any conventionally constructed loudspeaker of comparable size. The wiring to the coil 9 is not shown but this wiring should also be such as to facilitate the large excursion of the cone 2 that has been referred to. Advantageously, with a view to dissipating heat as quickly and efficiently as possible, the wires that are employed throughout the loudspeaker may be of square or other ploygonal cross-section. Alternatively, a flat ribbon-like or strip-like formation of these "wires" is particularly advantageous in this respect. The connections of the wires or other conductors to the voice coil 9 should preferably be located alongside the linen or other suspension 14 where that suspension 14 is secured to the coil former 8 and, in this area, they should be provided with surrounding electrical insulation in order to ensure that a short circuit cannot occur if they should touch the cone 2 and/or the preferably, but not essentially, metallic coil former 8. Naturally, the construction, position, insulation and length of these wires will take into account the exceptionally large excursions of the cone 2 that are a consequence of a construction in accordance with the invention.

[0013] The spun metal centre dome 7 of the loudspeaker has the shape of an outwardly tapering cone at one end of a cylinder and effectively affords a dust cover which, like the cone 2 itself, is of a substantially rigid construction. This shape and formation has proved to be a significant practical advantage, offering a still further reduction in sound distortion at peak wattage as compared with the use of conventional non-metallic and non-conical centre domes.

[0014] It is noted that the pole piece breather hole 10 is of relatively large diameter, as compared with a conventional construction, and this considerably facilitates heat dissipation and large excursions of the cone 2 in that ambient air is "pumped" into and out of the interior of the loudspeaker by the vibrating cone 2 and the parts to which it is secured, thus keeping the air in substantially constant movement whereby warmed air transports the heat which it receives to the afore-mentioned "heat sink" and is continuously replenished, by admixture, with fresh cool air entering the relatively large diameter breather hole 10.

[0015] If, as will usually be the case, at least one loudspeaker in accordance with the invention is installed in a cabinet or other enclosure, the large excursions of the or each cone 2 will greatly assist heat dissipation by substantially constantly vibrating the ambient air and conducting received heat to the walls of the cabinet or other enclosure which will usually, and advantageously, be ventilated. At least one loudspeaker in accordance with the invention could advantageously be sited in a cast alloy enclosure that is thermally coupled to the or each magnet 17 by, for example, an epoxy resin adhesive containing a metallic powder filler or a cyanoacrylate adhesive. Since an assembly of matched loudspeakers in accordance with the invention, for example, a bass speaker and a "tweeter" speaker will give much more sound volume, without noticeable sound distortion, from a relatively small cabinet or other enclosure because they are of small size as compared with conventional loudspeakers, it would be possible to incorporate into the cabinet or other enclosure itself a transistorised amplifier without producing a bulky unit. The large excursions of the or each loudspeaker cone 2 that are made greatly facilitate the cooling of the transistors and other components of such a "built in" miniaturised amplifier. A guitar or other instrument, a microphone, a tape player, a radio signal receiver, a conventional record player or a compact disc signal producer or other item could then be coupled directly to the amplifier/­loudspeaker unit without the use of a separate intervening amplifier.

[0016] To emphasise the effectiveness of a loudspeaker in accordance with the invention, it is noted that a single loudspeaker of this kind having a substantially 91 millimetre diameter cone contained in a cabinet measuring substantially 159 by 159 by 203 millimetres will operate in a distortion-free manner with an amplifier producing up to 400 watts per channel, the loudspeaker having a sensitivity of 85 decibels per watt at a 1 metre spacing. A commercially available contemporary loudspeaker assembly of modern design and generally comparable overall cabinet size will handle only approximately 150 watts per channel on a substantially continuous basis without significant sound distortion. Considerably larger assemblies of loudspeakers in accordance with the invention than the one discussed above are, of course, entirely possible and offer a readily portable source of substantially distortion-free sound production which source is unobtrusive due to the greatly increased sound production to size ratio characteristics of loudspeakers in accordance with the invention. In particular, the spun aluminium cone 2 offers greatly increased excursion, without noticeable deformation, with the added advantages of significantly increased strength and density.

[0017] Moreover, the aluminium cone 2 has the advantage that it provides a greater resistance to the propagation of reflected sound from the rear of the cone in the cabinet bearing in mind that the reflected sound can emanate through a conventional cone. The aluminium cone 2 therefore provides a less boxy, more open, sound. The cone 2 is also able to respond more quickly to movements of the voice coil 9 as the propagation delay is far shorter than that of a paper cone. Thus, the transient attack is more close to the electrical signal input.

[0018] Sonic velocity in aluminium is 6.32 mms per microsecond and in hard aluminium oxide 10 mms per microsecond. The density of the anodic layer is 2.54 × 10³ and the aluminium itself has a thermal conductivity of 226 watts per metre per degree Celsius/Centigrade. Depending upon the size and particular purpose for which a loudspeaker in accordance with the invention is required, the spun aluminium of its cone 2 may be between a minimum thickness of 0.075 mms and a maximum thickness of 1.015 mms. It is noted that any aluminium alloy having a high modulus of elasticity that may be employed for the cone 2 is advantageously heated by flame during the spinning process that has been described. The large variation in cone angle which has been mentioned is possible following the metal spinning step because this greatly reduces the high notch resistivity of aluminium and many of its alloys which otherwise causes cracking and breakage unless the cone angle is within a relatively narrow range of values. The greatly preferred hard anodising step considerably increases the cone rigidity with a consequent reduction of harmonic and intermodulation distortion. The frequency limit of piston motion of the moving coil of a loudspeaker in accordance with the invention is improved, the speed of heat radiation is increased, the resistance to atmospheric and other corrosion is markedly improved and the thermal coupling with other metal parts of the loudspeaker is extremely good. The operating temperature of the coil 9 is consistently low in relation to the ambient temperature so that increased power handling, without sound distortion, is possible by avoiding a significant increase in temperature-related coil impedance.

Claims

1. A method of producing a cone (2) for use in a loudspeaker, the method being characterised in that it comprises the steps of forming said cone (2) wholly or principally from aluminium and spinning (Fig. 1) the cone (2) to harden the same.

2. A method according to Claim 1, characterised in that it also includes the step of hard anodising the metal of the cone (2).

3. A method according to Claim 2, characterised in that the hard anodising is effected at both opposite sides of the metal of said cone (2).

4. A method according to any preceding claim, characterised in that the spinning process is carried out employing manual shaping tools (Fig. 1).

5. A method according to any preceding claim, characterised in that the metal to form the cone (2) is flame heated during the spinning process.

6. A moving coil loudspeaker, characterised in that it comprises a cone or diaphragm (2) produced in accordance with any preceding claim.

7. A loudspeaker according to Claim 6, characterised in that said cone (2) is movably connected to the coil (9) by means (7, 8) exhibiting relatively high heat conductivity.

8. A loudspeaker according to Claim 6 or 7, characterised in that it comprises a spun metal central dome (7) of part-conical and part-cylindrical configuration.

9. A loudspeaker according to any one of claims 6 to 8, characterised in that a relatively large diameter breather hole (10) extends axially through a combined pole piece and back plate assembly (18) for heat-dissipating purposes.

10. A loudspeaker according to any one of claims 6 to 9, characterised in that its internal electrical conductors are of flat ribbon-like or strip-like configuration.

Drawing