Abstract
A loudspeaker driver surround 2 comprises a flexible, generally annular element having a central axis 8 along which in use a diaphragm is driven, an outer edge 6 for fitment to an enclosure and an inner edge 4 for fitment to the diaphragm, with a roll surface which extends between the edges and which projects in the direction of the axis, wherein the roll surface has a shape formed by a plurality of axial corrugations 10 extending generally radially with respect to the annular element between the outer and inner edges thereof, the corrugations being shaped and configured such that the roll surface is non-axisymmetric about the axis, and the arrangement being such that cross-sections of the roll surface which extend radially with respect to the annular element between the outer and inner edges thereof have a substantially constant length at all circumferential positions around the annular element and so that the shape of the said cross-section varies continuously between circumferential positions around the annular element, the corrugations giving the projecting roll surface an order of rotational symmetry of at least 30.
Claims
1. A loudspeaker driver surround comprising a generally annular element of resilient material and having a central axis along which in use a diaphragm is driven, a first circumferential edge for fitment to an enclosure and a second circumferential edge for fitment to the diaphragm and/or a voice coil, with a roll surface extending between the edges which projects in the direction of the axis, wherein the roll surface has a shape formed by a plurality of axial corrugations extending generally radially with respect to the annular element between the first and second edges thereof, the corrugations being shaped and configured such that the roll surface is non-axisymmetric about the axis, and the arrangement being such that cross-sections of the roll surface which extend radially with respect to the annular element between the first and second edges thereof have a substantially constant length at all circumferential positions around the annular element and so that the shape of the said cross-section varies continuously between circumferential positions around the annular element, the corrugations giving the projecting roll surface an order of rotational symmetry of at least 30.
2. The loudspeaker driver surround as claimed in claim 1 wherein (when the annular element is viewed axially) points on some of the corrugations, which points are most axially distant from the circumferential edges, form generally linear creases at a first angle to the radial direction between outer and inner edges.
3. The loudspeaker driver surround as claimed in claim 2 wherein (when the annular element is viewed axially) points on others of the corrugations, which points are most axially distant from the circumferential edges, form generally linear creases at a second angle to the radial direction between the outer and inner edges.
4. The loudspeaker driver surround as claimed in claim 3, wherein the first and second angles are equal and opposite.
5. The loudspeaker driver surround as claimed in claim 3, wherein in radial cross section the roll surface comprises a succession of curves alternating to the left and right hand side of a centre line, said curves blending into a uniform roll surface between each curve.
6. The loudspeaker driver surround as claimed in claim 5 wherein the curves on the left and right hand side are similar but reversed.
7. The loudspeaker river surround as claimed in claim 5, wherein the uniform roll surface is a half roll surface.
8. The loudspeaker driver surround as claimed in claim 3, wherein if the parts of the corrugations which are most axially distant from the circumferential edges are used to generate a leading surface, that leading surface would not be planar.
9. The loudspeaker driver surround as claimed in claim 1, wherein the shape and configuration of the corrugations on the roll surface are such that, if the first edge of the annular element were extended axially away from the second edge to the maximum extent possible, the roll surface and the corrugations thereof would unfold to adopt a substantially smooth frusto-conical shape.
10. The loudspeaker driver surround as claimed in claim 1, wherein the roll surface has a sidewall adjacent the first edge which extends substantially axially.
11. The loudspeaker driver surround as claimed in claim 1, wherein the roll surface has a sidewall adjacent the second edge which extends substantially axially.
12. The loudspeaker driver surround as claimed in claim 1, wherein successive corrugations blend smoothly into each other.
13. The loudspeaker driver surround as claimed in claim 1, wherein the corrugations blend smoothly into the first and/or second edges.
14. The loudspeaker driver surround as claimed in claim 1, wherein the thickness of the roll surface is substantially constant.
15. The loudspeaker driver surround as claimed in claim 1, wherein the first circumferential edge is the inner edge of the generally annular surround and the second edge is the outer edge of the surround.
16. A loudspeaker comprising a driver surround, the driver surround comprising a generally annular element of resilient material and having a central axis along which in use a diaphragm is driven, a first circumferential edge for fitment to an enclosure and a second circumferential edge for fitment to the diaphragm and/or a voice coil, with a roll surface extending between the edges which projects in the direction of the axis, wherein the roll surface has a shape formed by a plurality of axial corrugations extending generally radially with respect to the annular element between the first and second edges thereof, the corrugations being shaped and configured such that the roll surface is non-axisymmetric about the axis, and the arrangement being such that cross-sections of the roll surface which extend radially with respect to the annular element between the first and second edges thereof have a substantially constant length at all circumferential positions around the annular element and so that the shape of the said cross-section varies continuously between circumferential positions around the annular element, the corrugations giving the projecting roll surface an order of rotational symmetry of at least 30.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) The invention will now be described by way of example and with reference to the accompanying figures, in which;
(2) FIGS. 1A-1C are schematic views of a prior art surround connected to a diaphragm in various stages of displacement;
(3) FIG. 2 shows the frequency response of two prior art surrounds which are of similar design but of different thicknesses;
(4) FIG. 3 illustrates the change in restoring force for two similar prior art surrounds;
(5) FIG. 4 is a schematic perspective view of an annular loudspeaker driver surround, or suspension, in accordance with the invention;
(6) FIG. 5A is an enlarged, part-sectional view of a part of the surround of FIG. 1;
(7) FIG. 5B is an enlarged, part-sectional view from another direction of the part of FIG. 5A;
(8) FIG. 6A is a schematic part-sectional view of a section of another loudspeaker driver surround, or suspension, in accordance with the invention;
(9) FIG. 6B is an enlarged, part-sectional view from another direction of the part of FIG. 6A;
(10) FIG. 7 is an axial view of the part shown in FIG. 5A, in the direction of the arrow VII-VII;
(11) FIGS. 8A and 8B illustrate the principle behind the number of repetitions of the pattern of the corrugations in the roll surface in surrounds in accordance with the invention;
(12) FIG. 9 illustrates the principle behind the radial cross-sectional shape of the corrugations in the roll surface in surrounds in accordance with the invention, and
(13) FIGS. 10A and 10B are schematic radial cross-section views showing the principle of the shape of the corrugations in the roll surface in surrounds in accordance with the invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS
(14) FIG. 4 shows an annular loudspeaker suspension 2 in its relaxed state (as is the case in all of the subsequent drawings) which has a flat outer circumferential edge 6 for mounting or clamping to the loudspeaker enclosure (not shown) and a flat inner circumferential edge 4 which is configured to be attached to the diaphragm (not shown) or to the voice coil (not shown) of the loudspeaker. The inner and outer edges 4, 6 are in approximately the same plane. In use, the voice coil and the diaphragm vibrate at audio frequencies in the direction of the central axis 8 of the annular surround 2, and the outer edge 6 remains fixed whilst the inner edge 4 reciprocates along axis 8 relative to the outer edge 6 and the loudspeaker enclosure. The suspension 2 is unitary (i.e. formed in one piece) and is formed of a suitably resilient material (such as by being moulded of an elastic material, as is known in the art), and serves to hold the diaphragm/voice coil aligned on the axis 8 throughout the reciprocal motion, and also to urge the diaphragm/voice coil towards a central position where the surround is in its relaxed state, e.g. so that the two edges sit in approximately the same plane along axis 8, counteracting the drive forces produced by the voice coil. Thus far, the surround described has all the attributes of known loudspeaker surrounds, and is as described above in relation to the prior art.
(15) The surround 2 is very generally in the form of a part of a torus, in that it protrudes in the direction of axis 8 away from the general plane of the inner and outer edges 4, 6; however, the protruding portion of the surround (the roll surface) is formed with a plurality of corrugations 10 which give it a complex, non-axisymmetric shape, particularly when viewed along the direction of the axis 8. The roll surface has inner and outer sidewalls 18, 20 (shown in FIG. 5A) which extend generally axially and which are generally cylindrical, and these are connected to the inner and outer edges at a crease 16. The corrugations 10 extend along a part of the sidewalls 18 and blend smoothly into the sidewalls at or before reaching the crease 16.
(16) The important features of the shape of the corrugated surface of the surround 2 between the outer and inner edges 4, 6 are, firstly, that it is not axisymmetric about axis 8 (meaning that if successive radial cross-sections are taken at different positions around axis 8, the shape of those cross-sections does not remain constant (it will be noted from FIGS. 5A and 5B that the corrugations 10 blend smoothly into outer and inner sidewalls 18, 20 which are either cylindrical or frusto conical and extend along the axis 8; sidewalls are not an essential feature of the invention, but where they are present the corrugations 10 must continue onto the sidewall to prevent it from buckling, and could blend smoothly into the crease 16 where the surround turns to form the flat inner and outer edges, as shown in FIG. 5A). Secondly, the corrugations 10 are shaped repetitively and substantially similarly; this gives the projecting roll surface an order of rotational symmetry of at least 30 and, subject to manufacturing constraints, up to 100 or even 200 or any number between these extremes; such a high number of corrugations makes the surround effective in resisting back pressure within the loudspeaker enclosure, whilst they each form the leaves of a hinge that opens or unfolds to allow the driver to move while resisting the pressure from the change in volume of the enclosure. The arrangement is such that there is no part of the roll surface which does not have corrugations. Thirdly, the corrugations are shaped such that, if radial cross-sections of the roll surface are taken at different angular positions around the axis 8, the length of the roll surface in a radial direction between the edges 4, 6 remains constant. Fourthly, the corrugations are at alternate and substantially equal angles to the radial direction in a zigzag pattern, as is best seen in FIG. 7. Fifth, the radial profile of the roll surface varies between a half roll shape and a sharp cornered saw tooth shape (with alternate steep and gentle slopes, as seen in FIGS. 5A-B, 6A-6B and 10B) so as to give a large change in axial position for points on the roll surface at successive circumferential positions. Finally, if the points along the saw tooth pattern which are furthest from the edges 4, 6 in the axial direction 8 were used to generate a leading surface L of the roll surface, this leading surface L is generally annular about the axis 8, but is not planar (although in the drawings it might appear so, it can be seen in FIGS. 6A and 6B that the leading surface L is not planar, but instead is very slightly convexthis is described further below, with reference to FIGS. 10A-10B).
(17) The overall shape of the roll surface permits the roll surface to unfold without buckling as the surround vibrates in use, to the extent that, were the inner edge 6 to be displaced along the axis 8 relative to the outer edge 4 to the maximum extent possible, the roll surface would unroll completely to form a substantially smooth, frusto-conical shape, but without any buckling and without any rotation of the inner edge 6 relative to the outer edge 4; this minimises the mass of the surround for the maximum excursion of the central diaphragm, and allows the restoring force of the surround (the resilience of the material from which it is formed which moves the surround from a driven opposition towards the relaxed position) to be substantially linearised.
(18) FIGS. 5A and 5B are enlarged views of part of the surround 2 shown in FIG. 4, and FIG. 7 is a plan view of that surround, as seen along the axis 8. It can be seen in FIG. 7 that the rounded corrugations axially furthest from the edges 2,4 form a symmetrical zigzag shape which has portions 12, 14 (also shown in FIG. 7) which alternate at similar but opposite angles to the radial direction, and which terminate at rounded knees, or shoulders, 36, 38 (see FIG. 10B) pointing alternately inwards and outwards; these corrugations allow the surround to deform without any rotational movement of the inner edge 6 relative to the outer edge 4. When viewed along the axis, the shoulders 36, 38 lie along two circumferential rings, one towards the inner edge of the annular surround and the other towards its outer edge. The angle of the corrugations to the radial direction is dependent on the size and number of corrugations; in a surround having 50 corrugations, each corrugation subtends about 7.2 and successive portions 12, 14 are angled at about 15 to the radial direction.
(19) FIGS. 6A and 6B show two sections of an alternative form of surround 2 in which features similar in function but not necessarily shape or configuration to those in the surround 2 of FIG. 4 are given the same reference numeral as in FIG. 4 but with the addition of a dash. In these drawings the corrugations 10 clearly extend along the inner and outer axial sidewalls 18, 20 of the roll surface towards the crease 16. The corrugations 10, 10 are preferably smooth, as this facilitates manufacture of the surround (smoothly curved shapes are easily moulded, where sharp corners would make the mould more expensive, and/or make it more complicated and the surround liable to stick in the mould). The inner circumferential edge 4 is shown at a slight angle to the plane of outer edge 6 (in the direction of the leading surface) so as to be suitable to have a conical or domed diaphragm attached thereto.
(20) FIGS. 8A and 8B illustrate the principles for determining the number of corrugations which should be used. When a simple cylindrical half round surround crumples and geometric buckling occurs, when the buckled surround is viewed axially it looks like a many pointed star. The number of points of the star is mainly determined by the ratio of the inside clamp diameter at the cone and the outside clamp diameter at the surround foot. From measurements of surrounds of various sizes in free air, it has been found that the angle the folds make with a radius (fold angle) is between 30 and 50 (rounded for an integer number of repetitions per 360). Adding corrugations gives the surround points at which to fold into a smaller diameter, thus eliminating the abrupt geometric buckling. The number of corrugations must be at least the number of geometric buckling points with a 50 fold angle, and preferably several times more. FIGS. 8A and 8B show how the number of geometric buckling points is determined on a simple half roll surround with a 1:1.175 ratio of inside: outside diameter. FIG. 8A relates to the maximum fold angle and gives the minimum number of geometric buckling points; 15 folds spaced 24 apart, give a fold angle 22 of 47 (predicted minimum number of geometric buckling points), therefore a minimum of 15 corrugations would be required to eliminate geometric buckling. In the example of FIG. 8B, which relates to the minimum fold angle, 26 folds spaced 13.85 apart, give a fold angle of 33 (predicted maximum number of geometric buckling points). Therefore a minimum of 15, and preferably more than 30 corrugations would be required to eliminate geometric buckling in this surround. For resisting pressure deformation, the number of repetitions may need to be higher, as the aim is not only to allow the surround to fold without buckling, but also for it to have the strength to resist the pressure deformation. More corrugations make the surround stronger, and so effectively thicker for the same surround thickness. The exact number of corrugations required to resist the pressure deformation should be greater than the maximum predicted number of geometric buckling points for the surround; this number depends on the surround width, material thickness, and change in cabinet volume, but is typically of the order of 30 or higher. For a large surround the inner:outer diameter is typically around 1:1.3, which would give a minimum of 17 folds, and for very large surrounds, of inner:outer diameter as large as 1:1.45, there would be a minimum of 13 folds, and for such surrounds about 30 corrugations would be suitable.
(21) FIG. 9 illustrates how the radial cross-sectional shape of the roll surface should be chosen. In order to make the surround effectively thick, the change in shape of the surround profile should be large. Varying between a half roll profile and saw teeth profiles of alternating directions gives a large change in position for each point along the surround length, and so increases the effective thickness. The effective thickness is defined as the area of the difference between the middle and extreme profiles divided by the length of the roll. FIG. 9 shows a comparison of the shape, viewed in radial cross-section, where the alternating saw tooth pattern varies between a half roll shape 26 and an alternating parabolic shape 28, and between a half roll shape 26 and a sharp saw tooth shape 30. Both the alternating parabolic shape 28 and the sharp saw tooth shape 30 are of equal length to the half roll 26, which is 20 mm in diameter. The effective thickness is the total area formed by the difference between the extreme surround profiles divided by the length. As can be seen, the effective thickness of the sharp saw tooth 30 is more than twice the parabolic shape 28, so it will be better at resisting pressure deformation.
(22) The effective thickness ratio is the effective thickness divided by the material thickness of the surround. For a surround 0.7 mm thick, this would give an effective thickness ratio of 1.709 for the parabolic profile, and 3.809 for the saw tooth profile.
(23) It is important to ensure that there is no rotational symmetry at any point on the surround other than the edges. FIGS. 10A-10B show two surrounds of the same length with different corrugation profiles. For the surround in FIG. 10A, the centre point 32 is common to all three profiles (the left hand extreme, the half roll and the right hand extreme) so forms a thin circular ring of material that is prone to geometric buckling. The surround in FIG. 10B has no common points between all three profiles, only two spaced points 32 where there are common points between two profiles, so this surround is much less liable to buckle geometrically but instead it unfolds at the corrugations, and also has a greater effective thickness. Although the left and right hand peaks, or shoulders 36, 38 are at the same height above the line 34 (i.e. at the same axial distance from the inner and outer circumferential edges of the surround), they are not at the same height as the half roll peak 40, so that the line of points along the roll surface joining peaks 36, 38, 40 which are axially most distant from the circumferential edges varies in axial position at the same time as it varies in radial and circumferential position: this produces a leading surface (as defined above) which is generally annular about axis 8, but non-planar. The effective thickness and the rotational symmetry can be optimised empirically, subject to the ability of the manufacturing process to accommodate the resulting roll surface shape.
(24) 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 invention has been described with reference to a circular driver surround, but it should be understood that the invention applies equally to non-circular diaphragms, such as elliptical or race track shaped diaphragms, or any shape being symmetrical in two orthogonal directions lying in the general plane of the diaphragm and having a central hole (such as a square or rectangle, with rounded corners). Accordingly, unless clearly indicated otherwise, any use in this description or in the claims of the terms annular, circumference, circumferential, circumferentially or around should not be construed as being restricted to a circular shape, nor as necessarily being centred on a single axis but instead construed broadly as any substantially two-dimensional shape bounded by a closed loop. The invention has been described above in terms of the outer edge of the annular suspension being fixed and the inner edge moving relative thereto, as this is the arrangement in the majority of loudspeakers; however, it will be appreciated that the reverse arrangement (inner edge fixed, outer edge moving) could work equally as well, and so falls within the ambit of this invention. The roll surface can be directed in either axial direction from the outer edges (i.e. a roll or a reverse roll). The corrugations have been described as having a zigzag pattern, of equal and opposite angles which alternate in direction; the zigzag pattern could alternatively be sinusoidal, or in any other repeating waveform. Where different variations or alternative arrangements are described above, it should be understood that embodiments of the invention may incorporate such variations and/or alternatives in any suitable combination.
