Abstract:
A loudspeaker is disclosed, comprising a driven body and a suspension for providing a restoring force to the driven body, the suspension having a cup geometry wherein its attachment point on the fixed portion of the loudspeaker is displaced along the axis of motion relative to its attachment point on the driven body and comprising a first concentric region that is extendible to allow reciprocating axial movement of the driven body, a second concentric region which extends transversely from the first region toward one of the attachment points, and a circumferential member affixed to the suspension at a location between the first and second concentric regions, the circumferential member being relatively stiff compared to the material forming the first and second concentric regions.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This Application is a Section 371 National Stage Application of International Application No. PCT/GB2011/000751, filed May 18, 2011 and published as WO/2011/144893 on Nov. 24, 2011, in English, which claims priority of Great Britain Application No. 1008299.8, filed May 19, 2010, the contents of which are hereby incorporated by reference in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to the field of loudspeakers and particularly relates to suspensions, also known as spiders or dampers, and loudspeakers comprising them. 
       BACKGROUND ART 
       [0003]    The suspension is a component on a conventional cone driver. It is also known as the spider or damper. Cone drivers are widely used, particularly for the low (20-500 Hz) and midrange (500-3000 Hz) parts of the audio spectrum. 
         [0004]    The suspension is typically used in conjunction with the surround—a flexible air seal between the cone and chassis. Together these centre the voice coil in the magnet gap, maintain axial travel, and provide a restoring force on the moving parts. Usually the suspension provides the greater portion of this restoring force. Over small excursions this force is fairly linear and influences the resonant frequency of the drive unit. Over larger excursions its behaviour is non-linear and may be characterised by a stiffness-displacement ‘K(x)’ curve. To avoid excessive displacement where moving parts collide with the chassis or magnet assembly, stiffness must increase for both positive and negative displacement. If the stiffness-displacement curve is not symmetrical about zero displacement the restoring force provided will not be equal for forwards and backwards motion and the voice coil will oscillate about a position that is offset from the centre of the magnet gap. At higher excursions much of the coil may be away from the cooling effect of the iron poles and may fail due to over-heating. A smooth symmetrical increase of stiffness with displacement reduces excessive excursions to minimise the distortion caused by motor nonlinearities. 
         [0005]    The suspension is commonly an annular band attached to the voice coil former on its inner edge and the driver chassis on its outer edge. Its structure is often a series of concentric corrugations or ‘rolls’ of material. The number, size, and shape of the rolls greatly affect the stiffness-displacement curve. 
         [0006]    The suspension is typically manufactured from a woven fabric impregnated with resin and moulded into shape. The material needs to be flexible and have some damping properties to minimise resonance in the working bandwidth. It will ideally be porous to avoid radiation of the resonance. 
         [0007]    If the inner and outer edges attach at similar heights in the driver assembly the overall form of the suspension will be planar. In that case, the concentric rolls may be designed to provide a symmetrical stiffness-displacement curve. 
         [0008]    In certain driver designs the inner and outer edges are not attached at similar heights, however. For instance, for manufacturing reasons the mounting surface on the chassis may not be located at the same height as the mounting position on the voice coil former. Most commonly the corrugated part of the suspension would be aligned to the most appropriate position on the voice coil former and the inner edge mounted there. The outer edge would protrude backwards to meet its mounting surface on the chassis. 
         [0009]    An alternative example is a driver in which the suspension attaches to the diaphragm rather than the voice coil former to minimise overall build height. The corrugated part of the suspension is then aligned with the chassis mounting surface and the outer edge mounts there. The inner edge protrudes forward with a generally frusto-conical geometry to mount on the rear of the diaphragm. With this arrangement the rolls will not hit the diaphragm as it moves. 
         [0010]    It would also be possible for both edges to protrude or have a shift in height within the corrugated part of the suspension. 
         [0011]    These non-planar arrangements are sometimes described as cupped suspensions, where the protrusion is the ‘cup’. Such an arrangement may be employed for any number of design reasons. 
         [0012]    The problem with a cupped suspension is that the cup can bend more easily in compression than it can stretch in extension. As a result the stiffness-displacement curve is asymmetric, with the restoring force much lower as the diaphragm moves in the direction of the protruding cup. As mentioned above this is an undesirable characteristic. 
       SUMMARY OF THE INVENTION 
       [0013]    This invention relates to a method of reinforcing a cupped suspension to control bending of the cup and thus obtain a symmetrical stiffness-displacement curve, reducing distortion and increasing power handling. The invention also helps prevent catastrophic suspension collapse further improving power handling. 
         [0014]    A suspension manufactured using conventional techniques is reinforced with a circular loop. This may be located near the point that a cup meets the corrugated part of the suspension. It may be fixed in a specially designed groove, or to the cup, or to the main part. It may be on either side of the suspension, and may be fixed by a suitable adhesive. The loop might be made from metal or plastic. Where the cup is fixed to the moving parts it may be advantageous to use a loop of low mass to minimise additional moving mass, and to avoid resonances of the loop on the suspension. 
         [0015]    The loop should provide greater stiffness than the suspension material to control cup bending. This may be to prevent bending completely in the excursion range of the driver, or limit it by a sufficient amount. The suspension and loop will generally be designed in conjunction to achieve the desired stiffness-displacement curve. 
         [0016]    In some cases additional loops may be used to further control bending. An example is a second loop attached part way up a deep cup. Again, the suspension and loops should ideally be designed in conjunction. 
         [0017]    Thus, the present invention provides a loudspeaker comprising a driven body responsive to electrical signals to undergo excursions from a rest position along an axis of motion, to project acoustic waves from a front of the loudspeaker, and a suspension for providing a restoring force to the driven body towards the rest position, the suspension extending from an attachment point on the driven body to an attachment point on a fixed portion of the loudspeaker, wherein the attachment point on the fixed portion of the loudspeaker is displaced along the axis of motion relative to the attachment point on the driven body when the driven body is at rest, the suspension comprising a first concentric region of the suspension that is extendible to allow reciprocating axial movement of the driven body, a second concentric region which extends transversely from the first region toward one of the attachment points, and a circumferential member affixed to the suspension at a location between the first and second concentric regions, the circumferential member being relatively stiff compared to the material forming the first and second concentric regions. 
         [0018]    The reinforcement solves the asymmetric stiffness-displacement curve problem as the loop controls bending of the cup. The extra stiffness from the loop helps avoid collapse of the cup at high power levels. This allows use of cupped suspensions whilst maintaining control over excursion and power handling, and minimising distortion. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    An embodiment of the present invention will now be described by way of example, with reference to the accompanying figures in which; 
           [0020]      FIG. 1  shows an axial cross-section through a known loudspeaker arrangement having a planar suspension; 
           [0021]      FIG. 2  shows an axial cross-section through a known loudspeaker arrangement having a cupped suspension 
           [0022]      FIG. 3  shows a cross-section through part of a reinforced cupped suspension according to the present invention; 
           [0023]      FIG. 4  shows the deformation under load of the suspension of  FIG. 3 ; 
           [0024]      FIG. 5  shows the stiffness-displacement curve of the suspension of  FIG. 3  compared to the same suspension without the reinforcing loop; and 
           [0025]      FIG. 6  shows a cross-section through the suspension of  FIG. 3  in place within a loudspeaker. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0026]      FIG. 1  is a cross-sectional view of a conventional loudspeaker  10 . 
         [0027]    The loudspeaker  10  comprises a diaphragm  12 , shaped generally in a frusta-conical form. In operation, the diaphragm  12  is driven forwards and backwards to project acoustic pressure waves from the loudspeaker  10 . A surround  22  provides a flexible air seal between the diaphragm  12  and the chassis (not illustrated). The diaphragm  12  extends downwards to connect to a voice coil former  14 , around which is wound a coil of conducting material  16  (i.e. a wire), known as the voice coil. 
         [0028]    A motor system of the loudspeaker  10  comprises an inner magnetic pole piece  18 , a magnet  20 , and an outer pole piece  19  displaced from the inner pole piece  18  to form a magnetic gap  21 . The voice coil  16  is positioned in the gap  21 , such that electrical currents in the coil result in a force that is transferred to the diaphragm  12 . 
         [0029]    As described above, the loudspeaker  10  is further provided with a suspension  24  between the voice coil former  14  and a fixed point  28  on the chassis (not illustrated). The suspension  24  comprises a first (inner) edge, for attachment to the moving parts of the loudspeaker  10 , and a second (outer) edge for attachment to a fixed part of the loudspeaker  10 . In between those two edges is a flexible region, capable of limited expansion and contraction as the diaphragm  12  is driven. The flexible region can be constructed from one or more corrugations or rolls of material, as illustrated. 
         [0030]    Together with the surround  22 , the suspension centres the voice coil in the magnet gap, maintains axial travel, and provides a restoring force on the moving parts (i.e. the voice coil  16 , voice coil former  14  and the diaphragm  12 ). In the illustrated arrangement, the suspension  24  is attached at its inner edge to the voice coil former  14 , and at its outer edge to a mounting surface  28  on the chassis. Both of these attachments are at the same height within the loudspeaker  10  (i.e. the same distance from the front of the loudspeaker  10 ), leading to a generally planar form for the suspension. In this orientation, the suspension is capable of providing a symmetrical stiffness-displacement curve and thus a restoring force to the moving parts of the loudspeaker that is symmetrical about the rest position. 
         [0031]      FIG. 2  shows a further loudspeaker  200 , wherein the suspension is connected differently due to the absence or unavailability of a suitable mounting position  28  on the chassis. The loudspeaker  200  is similar to that described with respect to  FIG. 1  and therefore will not be described in great detail. Similar features have been numbered consistently in the figures. 
         [0032]    Again, the suspension  224  is connected between the moving parts of the loudspeaker and a stationary part, in order to provide a restoring force. The inner edge of the suspension is attached to the voice coil former  14  as before. However, the outer edge is mounted on the outer pole piece  19 , positioned further from the front of the loudspeaker than the inner edge. This results in a non-planar or “cupped” suspension  224 , made up of at least one section  226  which protrudes relative to the corrugated part of the suspension. This protrusion generally has a frusto-conical geometry. Such an arrangement may be employed for any number of design reasons. 
         [0033]    However, in this orientation, it is easier to displace the inner edge of the suspension  224  backwards than forwards as the protruding section may bend, but without rolls requires greater force to stretch. Thus, the stiffness-displacement curve will be asymmetric and the restoring force provided by the suspension for backward travel of the diaphragm  12  less than that for forward travel of the diaphragm. 
         [0034]    The cupped suspension orientation also occurs in other designs, where the central flexible region of the suspension is at a different height (i.e. at a different distance from the front of the loudspeaker) to one or both of the suspension edges. For example, the inner edge may be connected to the diaphragm  12  (or to ribs extending backwards from the diaphragm) and thus be at a greater height (closer to the front of the loudspeaker) than the central flexible region. In another example, the flexible region of the suspension may be at a different height to both of the edges, either above or below the level at which they are attached to the moving or stationary parts of the loudspeaker. In all of these designs, the suspension is non-planar. 
         [0035]    In order to make the stiffness-displacement curve and restoring force of a cupped suspension symmetrical in compression and expansion, according to embodiments of the present invention, we propose a suspension  300  as illustrated in  FIG. 3 . Note that for clarity only half the suspension is shown. In practice, the suspension runs circumferentially about the axis of the acoustic driver. 
         [0036]    The suspension  300  is similar to those previously illustrated, except that it is in a different orientation. It has an inner edge  302  which is attachable to a moving part of the loudspeaker, and an outer edge  304  which is attachable to a fixed part of the loudspeaker. The region  306  between the inner and outer edges has a number of rolls  308  which allow the region to flex, expand and contract. 
         [0037]    The flexible region  306  is at the same height as the outer edge  304 , but the inner edge  302  is at a greater height (i.e. closer to the front of the loudspeaker). Such an orientation may occur, for example, where the outer edge  304  is mounted on the driver chassis, and the inner edge is connected to the diaphragm  12 , or to ribs extending rearwardly from the diaphragm  12 . A frusta-conical ‘cup’ region  312  protrudes up from the flexible region  306  to the inner edge  302 . 
         [0038]    The suspension further comprises a circumferential stiffening member  310 , of relatively high elastic modulus material. That is, the Loop  310  is constructed from a material that provides a greater mechanical stiffness than the material used in the suspension  300  itself. Suitable examples include metal and plastic. However, lighter materials may in some cases be preferable as they will minimize the additional moving mass, and will help prevent the loop resonating on the suspension. The loop  310  is attached to the suspension by any suitable adhesive (not illustrated). 
         [0039]    The loop  310  has a circular cross-section and, in the illustrated embodiment, sits on the inside of the cupped suspension, i.e. on the inside of the curve. In other embodiments the loop could be attached to the outside of the curve. In either orientation, the loop serves to limit the ability of the cup  312  to compress (i.e. to become more curved). Ordinarily, that is an easier direction for the cupped suspension to move in. Thus, the action of the loop  310  is to make the suspension stiffness more symmetrical about the rest position of the moving parts (e.g. the voice coil former  14  and the diaphragm  12 ). The suspension  300  according to embodiments of the present invention is equally difficult to compress as it is to extend (see  FIG. 5 ). 
         [0040]      FIG. 4  shows the suspension  300  undergoing expansion and compression as the diaphragm  12  moves up and down, respectively. In position  300 ″, the inner edge  302  is moved forwards as the diaphragm moves forwards. The loop  310  has little effect on motion in this direction; it does not inhibit the suspension from expanding. In position  300 ′, the diaphragm is moved backwards from its rest position, and the suspension is compressed. The loop  310  provides resistance to this motion, increasing the stiffness of the suspension in compression. 
         [0041]    In some cases additional loops  310  may be used to further control bending. An example is a second loop attached part way up a deep cup. 
         [0042]      FIG. 5  is a graph showing the variation of stiffness in suspensions undergoing compression (negative displacement in the x direction) and expansion (positive displacement in the x direction). 
         [0043]    Line  401  shows the stiffness curve of a conventional suspension, i.e. a cupped suspension without a stiffening loop. It can be seen that the curve is asymmetrical, with a lower stiffness at negative displacements than the corresponding positive displacement. 
         [0044]    Line  402  shows the stiffness curve of a suspension according to embodiments of the present invention, i.e. a cupped suspension with a stiffening loop. The suspension as a whole is stiffer, due to the rigidity of the loop. However, the stiffness in compression is symmetrical with stiffness in expansion. 
         [0045]      FIG. 6  shows the suspension of  FIG. 3  incorporated into a loudspeaker driver  500 , designed as a low build height driver. A magnet assembly  502  carries a permanent magnet  504  and a central pole piece  508 , and has a cylindrical outer pole piece  506  to define a magnetic field gap  510 . A chassis member  512  sits concentrically around the magnet assembly  502  and provides supports for the other parts of the driver  500 . 
         [0046]    These include a voice coil  514  that is supported on a voice coil former  516  so as to lie at least partly within the magnetic field gap  510 . The voice coil former  516  drives a diaphragm  518  which has a planar front surface in order to reduce the overall depth of the driver  500 , as compared to a driver comprising a cone-shaped diaphragm. To provide the necessary rigidity, the diaphragm has stiffening ribs  520  on its rear face, and the voice coil former  516  is attached to these. 
         [0047]    At its radially outermost extent, the diaphragm  518  is attached to a surround  522  which helps to centre the diaphragm  518  relative to the magnetic field gap  510 , acts as an air seal, and provides a restoring force to return the diaphragm  518  to its rest position (illustrated). To increase the restoring force to an adequate level, a suspension  524  is also provided. Suspensions of this type typically comprise an annular band of material, attached on an inner radial edge to the diaphragm or voice coil former, and at an outer radial edge to a chassis component. The tensile properties of the band are controlled by forming a series of corrugations or rolls which tailor the restoring force exerted when the band is stretched to accommodate displacement of the diaphragm. 
         [0048]    In this case, the suspension  524  cannot attach to the voice coil former  516  as there is insufficient room due to the low build height of the driver  500 . Equally, if is it attached to the rear of the diaphragm  518 , it cannot extend radially outwardly as it would then lie immediately behind the diaphragm  518  whose rearward movement would then be obstructed. 
         [0049]    Instead, the suspension  524  comprises a corrugated band  526  as described above, from the radially inner part of which there is a portion  528  which protrudes forward to attach to suitable tabs  530  on the rear of the diaphragm  518 . This allows the corrugated band  526  to extend outwardly, behind and spaced from the diaphragm  518 , to a support  532  provided on the chassis-member 
         [0050]    As described above, a reinforcement  534  is provided in order to control the dynamic behaviour of the suspension. This comprises a circumferential ring of a rigid polymeric material, or a lightweight metallic material such as an alloy of aluminium or the like, and is seated in the inner concave region defined at the join between the protruding portion  528  and the outer band  526 . It can be affixed in this location by a suitable adhesive, or by weaving or other enclosure into the material of the suspension, or otherwise. 
         [0051]    There is thus described a loudspeaker having a cupped suspension comprising a stiffening loop. By action of the loop, the suspension has a stiffness that can be symmetrical in expansion and compression, 
         [0052]    It will of course be understood that many variations may be made to the above-described embodiment without departing from the scope of the present invention. For example, the specific orientations and relative locations of the various elements of the illustrated loudspeakers can be varied as required. The cup can be located on an inner region or an outer region of the suspension, and the suspension itself can be located outside the voice coil or within the interior of the voice coil, as required.