MOVING COIL DRIVER

Abstract: A moving coil loudspeaker driver has at least one pair of coils (18, 20) on a former (16). The coils move between pole pieces (28, 30) of a permanent magnet system (28, 30, 32, 34). The pair(s) of coils (18, 20) are oppositely wound and closely spaced to reduce the combined inductance of the coils.

Description: TITLE: MOVING COIL DRIVER DESCRIPTION The invention relates to a moving coil driver or motor and is particularly but not exclusively concerned with drivers for distributed mode panel loudspeakers such as are disclosed in our International Patent Application O97/09842. In designing moving-coil loudspeaker drivers, the inductance of the coil windings reduces the available force available to drive a loudspeaker diaphragm. This is because the inductance in series with the DC coil resistance increases the impedance at higher frequencies. U.S. Patent 4,783,824 dated November 8 1988 of KOYBASHI is concerned with increasing the available output from the voice-coil assembly by using a second magnet, which is added to provide magnetic shielding. The distance between the gaps is set "so as not to interact with each other" . The arrangement reduces a large impedance peak at lower frequencies; the data presented however suggest a higher impedance at higher frequencies (>lkHz) characteristic of a higher inductance at those frequencies. More prior art is discussed in an AES conference paper on loudspeakers, held in 1998, entitled "The Dual Coil Loudspeaker" by Douglas Button and Mark Gander. Most of the prior art is related to an apparatus having coil spacing of approximately 25mm which confers a weak mutual inductance and the available inductive coupling is made via the magnetic pole system. In older drive designs the magnetic systems were not run in saturation so that some ferro-magnetic coupling was present, if weakened by eddy current loses in the poles. Modern magnet systems are generally run in saturation when the poles can no longer provide any significant coupling for the coils. It is an object of the invention to improve the performance of a moving coil driver for a loudspeaker. According to the present invention, a moving coil loudspeaker driver, comprising a voice coil former (16) , at least one pair of mutually axially spaced coils (18,20) on the former (16), one coil (18) of each pair being wound in the opposite sense to the other (20) , the pair of coils (18, 20) being spaced apart by a predetermined distance such that the combined inductance of the coils is reduced by at least 5%, in embodiments at least 25% or even at least 40% compared with the coils being spatially separated and a permanent magnet system (28, 30, 32, 34) having pole pieces (28, 30) of opposite polarity arranged adjacent to the respective coils (18, 20) . The spacing, i.e. the axial gap between the facing ends of the coils of each pair, may be sufficiently small to allow direct coupling between the two coils to provide a mutual induction rather than indirect coupling through the pole pieces. By winding the second coil in the opposite direction to that of the first coil some combined (total) induction of the coils may be cancelled. Thus, the anti- phase connection of the coils may achieve the desired reduction in the overall inductance of the system. With advantage the spacing of the coils may be substantially equal to or less than the diameter (width) of the coil . With particular advantage the coil spacing is less than 2mm. This close spacing, compared to the prior art, of the coils may lead to a useful reduction of effective coil inductance. The close spacing of the coils may require the axial motion of the coils to constrained to be small compared with the length of the coils so that one coil does not approach the pole piece intended for the other coil. If the coil were to approach the other pole piece then the force would be the reverse of what was intended which would lead to a very non-linear response. The inventors have realised that drivers for exciting bending waves in plates are in any event constrained in this way. The fixing of the voice coil to the plate holds the voice coil axially and the motion required to impart bending waves to the plate is of much smaller amplitude than the motion required to impart motion to a conventional pistonic loudspeaker. Accordingly, much closer coil separation and hence much greater reductions in combined inductance may be achieved in such bending wave transducers than has hitherto been possible . If desired, only a single layer of wire may be wound in each coil, to reduce the width of the air gap in which the coil moves . More than one pair of oppositely disposed coils may be provided on the coil former, each associated with a respective pole piece of a magnet or magnets. The polarity of the pole piece is preferably determined by the direction of the winding of the associated coil . The magnet system may further comprise one or more non-magnetic spacers. The pole pieces may be no greater in diameter than the magnet itself to reduce unwanted fringing of the magnetic field. The magnet system may have magnetically saturated pole pieces. The spacing of the coils may be less than double preferably less than 1.4 times the axial length of each coil . The invention is diagrammatically illustrated, by way of example, in the accompanying drawings, in which Figure la is a coil with N turns; Figure lb is the coil of Figure la divided into two coils of N/2 turns; Figure 2 is a fragmentary cross-section through a first embodiment of moving coil driver in accordance with the invention; Figure 3 is a fragmentary cross-section through a second embodiment of moving coil driver in accordance with the invention, and Figure 4 is a fragmentary cross-section through a third embodiment of moving coil driver in accordance with the invention. In Figure la, the coil (10) has 100 turns of wire (11). It has the following parameters: N=100, length 3mm, inner radius 12.8mm and outer radius 14mm and thus using the Welsby formula (see below) , the calculated value of the inductance L is 0.439mH. An alternative method for calculating the inductance is to model the system using finite element analysis. A finite element model gives a value of 0.4601mH for the inductance. The inductance (L) of each coil may be calculated using the Welsby formula: t = r 2 - r, A = πr 2 where N is the number of turns in the coil 1 is the length of the coil r λ is the inner radius of the coil r 2 is the outer radius of the coil In Figure lb, the coil of Figure la has been divided into two coils (12,14) of fifty turns of wire (11). Assuming the two coils are sufficiently far apart to prevent coupling, the inductance for the whole system is equal to the sum of the inductance for the individual coils, namely L=2*0. llmH = 0.22mH. If the coils are set 4mm apart, we may expect some mutual coupling. The finite element model predicts the inductance of the system to be 0.349mH. If the current is now reversed in the second coil, for example, by reversing the direction of winding of the coil, the inductance of the second coil may substantially counteract the inductance of the first coil. The model predicts the value of the inductance of the system to be 0.1202mH which is approximately one quarter of the inductance for the undivided coil . The value of the coil inductance becomes significant at high frequency. For example, at 10kHz using the finite element model and the same parameters as in the previous example, the values of the inductance for the single coil and the two half-coils with coupling are 0.694mH and 0.573mH. When the current is reversed in one of the two half-coils the overall inductance of the system is 0.159mH. Once again the inductance of the system is reduced to approximately one quarter. Steel components may be added to the system, for example, if the coil is to function as an electromagnetic force actuator as in Figures 2 to 4. The presence of steel components may cause the value of the inductance to rise but the reduction is expected to be of similar proportion to a system without steel components. Figure 2 shows a moving coil loudspeaker driver which comprises a voice coil former (16) , two mutually axially spaced coils (18,20) on the former (16), one coil (20) being wound in the opposite sense to the other (18) and a permanent magnet assembly (22) . The (•) and (x) symbols indicate different directions of the current flow around the axes of the coils in the respective coils. The coils are connected together in series between terminals (15) as shown in Fig. lb, in such a way that current passing through the coils passes around the coils in opposite directions. The former is glued to a panel (38) capable of supporting resonant bending wave modes . Since the coils (18) and (20) are wound in opposite directions, the magnetic field acting on each section of the moving coil assembly must be reversed. This is achieved by using a single magnet (22) driving two radial air-gaps (24,26), one (24) from the north face, via the top plate or pole piece (28) , and the second (26) from the south face, via the bottom plate or pole piece (30), of a disc magnet (32), magnetised throughout its thickness. The two radial air-gaps share a common outer sleeve (34) which completes the magnetic circuit. The lines of magnet flux are shown as bold arrows. A non-magnetic outer casing (36) holds the outer sleeve concentric with the magnet assembly (22) and its two pole pieces (28,30) . The coils can be arranged with a small axial spacing because a voice coil driving resonant bending waves in a plate does not have a large excursion/ i.e. does not oscillate with a large amplitude of oscillation. Such a close spacing was not possible with prior art coils intended to drive conventional pistonic loudspeaker diaphragms. Such coils needed to have a sufficient spacing so that the significant movement of the coils could not bring one of the coils of a pair into the region of the pole pieces intended to drive the other coils of the pair. Accordingly, large reductions in the inductance were not obtainable using this technique. Although the two air-gaps (24,26) are in series, each one can be reduced by the thickness of the coil-wire used, since it is possible to use only a single layer of coil wire (15) . Of course, more layers may be used, if the magnetic field is of sufficient strength. The magnet, plate and sleeve materials are those commonly found in magnet systems for moving coil loudspeakers. A person skilled in the art will be able to determine the correct dimensions and clearances necessary for optimal magnet performance, but the topology presented herein will deliver a low value for the total coil inductance . It will be seen from the Figure that the spacing of the coils (18, 20) is relatively close. It is envisaged for example that a driver embodying the invention may well be provided in a systems in which the overall height of the driver is some 6mm with the coils (18, 20) being closely spaced - by 2mm or less. Although Figure 2 shows one magnet and two coils, the invention can be extended to multiple coils and magnets. The embodiment of Figure 3 shows a moving coil loudspeaker driver which comprises a voice coil former (38) , two pairs (40,42) of mutually axially spaced coils (40a, 40b) (42a, 42b) on the former (38) and a permanent magnet assembly (44) . The first pair of coils (40) comprises one coil (40b) being wound in the opposite sense to the other (40a) . Similarly, the second pair of coils (42) comprises one coil (42b) being wound in the opposite sense to the other (42a) . Coils (40a) and (42a) are wound in the same direction. The (•) and (x) symbols indicate different directions of the current flow in the respective coils. Since coil (40b) is wound in the opposite direction to coil (40a) and coil (42b) is wound in the opposite direction to coil (42a) , the magnetic field acting on each section of the moving coil assembly must be reversed. This is achieved by using three magnets (46,48,50) and four pole pieces or plates (52,54,56,58), making-up the complete magnet assembly (44) . The magnet assembly comprises alternate layers of pole pieces and magnets. Pole piece (54) is in contact with two magnets (46,48) and the magnets are arranged such that the south pole of both magnets is adjacent to the pole piece. Similarly, pole piece (56) is adjacent to the north faces of two magnets (48,50). The magnet assembly drives four air gaps (60, 62,64,66). The first air gap (60) is driven by the north face of magnet (46) , via the pole piece (52) , the second (62) by the south faces of two magnets (46,48), via pole piece (54) , the third by the north faces of two magnets (48,50) via pole piece (56) and the fourth by the south face of magnet (50) via pole piece (58) . The radial air- gaps share a common outer sleeve (68) which completes the magnetic circuit. The lines of magnetic flux are shown as bold arrows. A non-magnetic outer casing (70) holds the outer sleeve concentric with the magnets and the pole pieces . Figure 4 shows a variation of the assembly of Figure 3 which comprises just two magnets (72,74), separated by a non-magnetic spacer (76) . The assembly comprises a voice coil former (78), two pairs (80,82) of mutually axially spaced coils (80a, 80b), (82a, 82b) on the former (78) and a permanent magnet assembly (84) . The first pair of coils (80) comprises one coil (80b) being wound in the opposite sense to the other (80a) . Similarly, the second pair of coils (82) comprises one coil (82b) being wound in the opposite sense to the other (82a) . Coils (80a) and (82b) are wound in the same direction. The ( • ) and (x) symbols indicate different directions of the current flow in the respective coils. The magnet assembly drives four air gaps (86, 88,90,92). The first air gap (86) is driven by the north face of magnet (72) , via the pole piece (94) , the second (88) by the south faces of magnet (72), via pole piece (96) , the third by the north face of magnet (74) via pole piece (98) and the fourth by the south face of magnet (74) via pole piece (100) . The radial air-gaps share a common outer sleeve (102) which completes the magnetic circuit. The lines of magnetic flux are shown as bold arrows. A nonmagnetic outer casing (104) holds the outer sleeve concentric with the magnets, the non-magnetic spacer and the pole pieces. Instead of arranging the coils in series it is also possible to arrange the individual coils to have separate terminals (15) , and connect the coils so as to commonly drive the coils to give the same result of oppositely directed current flow around the axes of coils of a pair. This may be achieved, for example, by connecting the coils in parallel . The invention thus provides a simple method and means for improving the performance of a moving coil driver for a loudspeaker. The loudspeaker may be generally conventional, i.e. pistonic, or may be a resonant panel, e.g. of the kind described in our co-pending International Application WO97/09842.

Claims: CLAIMS 1. A moving coil loudspeaker driver, comprising a voice coil former (16) , at least one pair of mutually axially spaced coils (18,20) on the former (16), one coil (18) of each pair being wound in the opposite sense to the other (20) , the pair of coils (18, 20) being spaced apart by a predetermined distance such that the combined inductance of the coils is reduced by at least 5% compared with the coils being spatially separated, and a permanent magnet system (28, 30, 32, 34) having pole pieces (28, 30) of opposite polarity arranged adjacent to the respective coils (18, 20) . 2. A moving coil loudspeaker according to claim 1 wherein the coils of the or each pair are arranged sufficiently closely spaced to reduce the combined inductance of the coils by at least 25% compared with the coils being spatially separated. 3. A moving coil loudspeaker driver according to claim 1 or 2 wherein the coils (18, 20) are connected together in series such that current passing through the series connected coils creates opposed magnetic fields in the coils of the or each pair of coils. 4. A driver according to any preceding claim, wherein a single layer of wire (11) is wound in each coil (18, 20) . 5. A driver according to any preceding claim wherein more than one pair (40, 42, 80, 82) of oppositely disposed coils (40a, 40b, 42a, 42b, 80a, 80b, 82a, 82b) is provided on the coil former, each coil being associated with a respective pole piece (52, 54, 56, 58) of a magnet system comprising a plurality of permanent magnets. 6. A driver according to any preceding claim, wherein the spacing of the coils (18, 20, 40a, 40b, 42a, 42b, 80a, 82b) of each pair (18, 20, 40, 42, 80, 82) is substantially equal to or less than their diameter. 7. A driver according to Claim 6, wherein the spacing of the coils is less than 5 mm. 8. A driver according to any preceding claim wherein the spacing of the coils is less than double the axial length of each coil. 9. A driver according to any preceding claim wherein the pole pieces are circular and sandwich at least one permanent magnet having a diameter no smaller than the diameter of the pole pieces. 10. A loudspeaker, comprising a panel capable of supporting bending waves, and a moving-coil loudspeaker driver according to any preceding claim for exciting bending waves in the panel to produce an acoustic output.

Inventors: BANK GRAHAM, ROBERTS MARTIN, BANK, GRAHAM, ROBERTS, MARTIN
Applicants: BANK GRAHAM, NEW TRANSDUCERS LTD, ROBERTS MARTIN, BANK, GRAHAM, NEW TRANSDUCERS LIMITED, ROBERTS, MARTIN
Application year: 2000
Main classification: H04R9
IPC codes: H04R7, H04R9