ELECTROMAGNETIC-ACOUSTIC TRANSDUCER

Abstract: An electromagnetic transducer having a diapragm (11) on at least one face of which there is an audio-frequeny current conductor (15). The conductor (15) is arranged in a wave form pattern to divide the diaphragm (11) into a plurality of acoustically rigid zones (17) and define current paths (16) along the sides of the zones (17). An array of magnets (21) is arranged to produce a magnetic field orthogonal to each current path (16) whereby interaction between the magnetic fields and an audio-frequency current passing along the current paths (16) effects vibration of the diaphragm (11).

Description: ELECTROMAGNETIC-ACOUSTIC TRANSDUCER THIS INVENTION relates to electromagnetic acoustic transducers such as loudspeakers and headphones. One known type of electromagnetic acoustic transducer comprises a diaphragm having on one of its faces an audio¬ frequency current conductor which divides the diaphragm into a series of parallel strips and defines spaced parallel current paths along the longitudinal sides of the strips. A series of permanent magnets are disposed in spaced relation with the diaphragm and are arranged to produce a magnetic field orthogonal to each current path whereby during passage of an audio-frequency current through the conductor interaction between the audio¬ frequency current passing along the current paths and the magnetic fields generates driving forces for effecting vibration of the diaphragm. A transducer of this type has a relatively low mass diaphragm which produces good treble response. However, the transducer does have a disadvantage in that the driving forces for effecting vibration of the diaphragm are only applied to the longitudinal sides of the parallel strips on the diaphragm. The length of the strips in combination with the driving forces being applied only to the longitudinal sides of the strips, allow standing waves to occur on the diaphragm. The formation of standing waves on the diaphragm is undesirable and in an endeavour to prevent this, it has been proposed to position the parallel current paths in closely spaced relationship so as to reduce the width of the strips. This is not altogether satisfactory as it brings about an increase in the cost of the transducer, particularly in the case of a transducer having a OM7I 'fa relatively large diaphragm for good bass response. It is an object of the present invention to provide an electromagnetic acoustic transducer which overcomes or at least reduces the above-mentioned disadvantages. The term "acoustically rigid" when used herein in relation to a zone defined on a diaphragm is taken to mean that the zone is capable of vibrating to generate sound waves up to a predetermined frequency, with break-up free motion (i.e. without the formation of standing waves on the zone of the diaphragm) . In one form the invention resides in an electromagnetic acoustic transducer comprising a diaphragm having on at least one face an audio-frequency current conductor, the conductor being arranged in a waveform pattern to divide the diaphragm into a plurality of acoustically rigid zones and define current paths along the sides of the zones, an array of magnets so arranged that magnetic fields produced thereby interact with the current paths along the sides of the zones whereby during passage of an audio-frequency current through the conductor interaction between the audio-frequency current passing along the current paths and the respective magnetic fields causes vibration of the diaphragm. The waveform pattern may be of a castellated or sawtooth formation. The zones are preferably of rectangular configuration. In one arrangement, the diaphragm may be of flexible material and the zones rendered acoustically rigid by tension on the diaphragm. In a further arrangement, the diaphragm may be of flexible material and the zones IP rendered acoustically rigid by rigidising means; the rigidising means may comprise a compound formation, such as a compound curve, in the diaphragm at each zone. In a still further arrangement, the diaphragm may be of substantially rigid material. The invention will be better understood by reference to the following description of several specific embodiments thereof as shown in the accompanying drawings in which: Figure 1 is a perspective view of a transducer according to the first embodiment, with the transducer being partially cut away for clarity of detail; Figure 2 is a sectional view which is partially cut away and taken along the line of 2-2 of figure 1; Figure 3 is a plan view illustrating an alternative construction of diaphragm for the embodiment of figures 1 and 2; Figure 4 is a plan view illustrating a further altern¬ ative construction of diaphragm for the embodiment of • figures 1 and 2; Figure 5 is a perspective view of a transducer according to a further embodiment, with the transducer being cut away for clarity of detail; Figure 6 is a plan view of the diaphragm of the trans¬ ducer of figure 5; Figure 7 is a sectional view along the line 7-7 of figure 6; Figure 8 is a sectional view along the line 8-8 of figure 6; Figure 9 is a diagrammatic sectional view illustrating the diaphragm and magnet arrangement of a transducer according to a still further embodiment; Figure 10 is a diagrammatic sectional view illustrating the diaphragm and magnet arrangement of a transducer according to a still further embodiment. OMPI Figure 11 is a diagrammatic sectional view illustrating a d iaphragm suspension system of a still further embodiment. Referring to figures 1 and 2 of the accompanying drawings, the electromagnetic transducer according to the first embodiment comprises a flexible diaphragm 11 of relatively light mass which, by way of example only, may be formed of any of a number of film type materials such as polyester film, polycarbonate film or polypropylene film, a paper material or a non-magnetic metal foil such as aluminium. The diaphragm 11 is under tension in both longitudinal and lateral directions and is adhesively bonded or otherwise secured at its periphery to a rigid peripheral frame 13. The frame defines an open area on each side of the diaphragm to accomodate excursions of the diaphragm during vibration thereof. On one face of the diaphragm 11 there is an audio-frequency current conductor 15 arranged in a castellated pattern to divide the diaphragm into a plurality of substantially rectangular zones 17 and define current paths 16 along the sides of the zones. With the castellated arrangement of the conductor, the zones 17 (apart from alternate zones at the periphery of the diaphragm) each have a current path 16 along each of its respective sides. By providing on the other face of the diaphragm, a further audio-frequency conductor (not shown) of similar configuration to, but 180° out of phase with respect to, the conductor 15, a current path can be provided along the outermost side of each peripheral zone. This has the effect of providing a pair of current paths along the other sides of the peripheral zones and along each side of the other zones. While increasing cost, this arrangement is advantageous as it results in the application of an increased driving force to the sides of the zones, apart from the outer sides of the peripheral zones. The outer sides of the peripheral zones do not require an increased driving force as they are adjacent the periphery of the diaphragm which is itself fixed to the peripheral frame 13. By way of example only, the audio frequency conductor 15 may be in the form of a metal foil of aluminium or copper, the former being preferred as it has a higher conductivity per unit weight. The diaphragm may be made by etching from a metal foil plastic film laminate or the metal foil conductor may be applied to the diaphragm by conventional techniques such as printing, silk screening or deposition. In another arrangement, the conductor may be in the form of aluminium or copper wire bonded onto the diaphragm. It will of course be appreciated that in circumstances where the diaphragm is of metal foil or other electrically conductive material, the conductor would be insulated from the diaphragm. On each side of the peripheral frame 13 there is an acoustically transparent backing member 19 of magnetic material such as steel. Each backing member 19 may be formed integral with, or formed separately from and secured to, the peripheral frame 13. An array of permanent magnets 21 is attached to each backing member 19 on the side thereof adjacent the diaphragm 11. The magnets 21 are arranged in a series of rows and columns, with each magnet of both arrays being disposed within the projected plane of a respective zone 17 on the diaphragm. The magnets are in the shape of prisms with the outer or pole faces 23 being of configuration which is geometrically similar to the configuration of the zones 17; in this case, rectangular. The pole faces of adjacent magnets in each array are of opposite polarity, OMPI w h ere b y a magnetic field is produced between adjacen magnets. The pole faces of the magnets are spaced from the diaphrag to an extent that the diaphragm does not contact the pol faces during its excursions. Each backing member 19 provides a low reluctance path fo magnetic fields at the faces of the magnets adjacen thereto and has the effect of concentrating the respectiv magnetic fields extending between the pole faces 23 of th magnets. The effect of this is that the magnetic fields ar orthogonal to the respective current paths 16. As indicated hereinbefore, each backing member 19 i acoustically transparent and, in the illustrate embodiment, this is effected by perforations 25 formed i the backing member in the regions thereof between adjacen magnets. The interaction between an audio-frequency current passin along the current paths 16 and the respective magneti fields produces a driving force on each current pat substantially normal to the diaphragm and this in tur vibrates the diaphragm. A driving force is therefor applied to each side of those zones 17 having current path on all sides thereof. This, in combination with th acoustically rigid nature of the zones, ensures that th zones * of the diaphragm vibrate uniformly and with break-u free motion. Figures 3 and 4 of the drawings illustrate variations i the arrangement of the audio-frequency current conductor o the diaphragm and the reference numerals in these tw figures refer to the same parts as described in relation t the embodiment of figures 1 and 2. In the constructio illustrated in figure 3, the conductor 15 is arranged in sawtooth pattern to divide the diaphragm 11 into plurality of substantially rectangular zones 17. Figure illustrates a construction in which the conductor 15 i arranged in a sawtooth pattern to divide the diaphragm 11 into a plurality of substantially triangular zones. Wit this arrangement the poles faces of the magnets 21 (depicted by the broken lines) are of a configuration whic is geometrically similar to the configuration of the triangular zones. Referring now to figures 5, 6, 7 and 8 of the accompanying drawings, there is shown an electromagnetic acoustic transducer comprising a diaphragm 111 having on each face thereof an audio-frequency conductor 115 (the conductor on the lower face of the diaphragm being omitted in Figure 5 for purposes of clarity) . The audio-frequency conductors 115a and 115b are arranged on the diaphragm in a castellated pattern to divide the diaphragm into a plurality of substantially rectangular zones 117 and define current paths 116 along the sides of the zones. The conductors 115a and 115b are of similar configuration to each other but are 180 out of phase (as best seen in Figure 6 of the drawings), whereby at least one conductor path 116 is along each side of each zone. In fact, a pair of current paths are provided along each side of all zones 117 apart from the zones at the periphery of the diaphragm. It should be appreciated that the conductor 115a and 115b may be arranged on the diaphragm in other suitable patterns, such as for example a sawtooth pattern. It is however preferred that the zones 117 are of substantially rectangular configuration. In addition, it should be appreciated that only one conductor may be provided on the diaphragm if desired (as was the case in the embodiment of figures 1 and 2) . T h e d iaphragm 111 is formed of flexible sheet material and has rigidising means 118 at the zones 117 to render the zones acoustically rigid. In the illustrated arrangement, the rigidising means 118 comprise compound formations in the diaphragm at the respective zones. Specifically, in the illustrated arrangement, the compound formations are each in the form of a hemispherical dome which blends into a rectangular base at or adjacent the periphery of the respective zone. In other constructions, the compound formation may comprise a hollow cone which blends into a rectangular base or a hollow pyramidal shape having a rectangular base. The diaphragm 111 may be of any suitable plastics material such as polyester or polycarbonate, of metal such as copper alloy, aluminium or titanium, or of composite materials. The diaphragm 111 may be formed with the compound formations 118 by any suitable technique such as vacuum moulding, injection moulding or press forming. The diaphragm 111 is supported at its periphery by a suspension system 120 which permits vibratory movement of the diaphragm in a direction substantially perpendicular to the plane of the diaphragm. In the illustrated arrangement, the suspension system is formed integral with the diaphragm and secured to a peripheral frame 113. In an alternative arrangement, the suspension system may be formed separately from the diaphragm and the diaphragm bonded or otherwise secured at its periphery to the suspension system. Spaced from the diaphragm 111 on the side thereof remote from the protruding compound formations 118 is an acoustically transparent backing member 119 of magnetic material such as steel. The backing member 119 may be formed integral with or separately from the peripheral frame 113. OMH An array of permanent magnets 121 are attached to the backing member 119 on the side thereof facing the diaphrag 111. As with the earlier embodiments, the magnets 121 are arranged in a series of rows and columns with each magnet being disposed within the projected plane of a respective zone 117 and with the pole faces of adjacent magnets o opposite polarity. The magnets 121 are in the shape o prisms with the pole faces 123 being of a configuration which is geometrically similar to that of the zones 117. The backing member provides a low reluctance path for magnetic fields at those faces of the magnets adjacent thereto and has the effect of concentrating the magnetic fields extending between the pole faces. The effect of this is that the magnetic fields are orthogonal to the respec¬ tive current paths. ' The transducer according to this embodiment operates in a manner somewhat similar to the embodiments described before¬ hand, except that acoustic rigidity of the zones 117 is maintained by the compounded formation 118 rather than by tension on the diaphragm. The transducers of the embodiments illustrated in figures 9, 10 and 11 are somewhat similar to the transducer illustrated in figures 5 to 8, and the reference numerals used in figures 9, 10 and 11 refer to the same parts as described in relation to the embodiment of figures 5 to 8. Referring to figure 9, the transducer illustrated therein differs from that of figures 5 to 8 in that there is an audio-frequency conductor 115 on only one face of the diaphragm and it is in the form of a ribbon bonded to the diaphragm 111 in an edge-on manner. The respective current paths 116 defined by the ribbon are located within the region 122 between adjacent poles shoes 124 mounted on the pole faces of the permanent magnets 121. With this arrangement, the diaphragm 111 is capable of undergoing relatively large excursions without the conductor moving out of the influence of the magnetic fields. The ribbon may be curved along its length to provide lateral rigidity. In an alternative arrangement, the ribbon 115 may be bonded or otherwise secured to a mount extending outwardly from the plane of the diaphragm. Figure 10 illustrates an arrangement in which a permanent magnet 121 is provided within the projected plane of alter¬ nate zones 117a only, the projected planes of the other zones 117b being occupied by pole shoes 126 integral wit or in contact with the backing- member 119. The permanent magnets 121 are so arranged that their pole faces are o the same polarity and each is fitted with a pole shoe 124. Magnetic fields are established between adjacent pole shoes 124 and 126, and the arrangement eliminates the necessit for a permanent magnet corresponding to each zone 117. In figure 11 there is illustrated an alternative arrangement for suspending the diaphragm 111. Rather tha being supported at its periphery, the diaphragm is supported within its perimeter by a plurality of resilien support members 129. The resilient support members 129 are fitted between the respective apexes 131 of a plurality o the rigidising formations 118 in the diaphragm and a rigi member 133 mounted in spaced relation with the diaphragm. With this form of suspension system, it is possible to utilise a diaphragm in which adjacent zones 117 are capable of limited angular movement with respect to each other. In a further embodiment (not shown) the diaphragm may be formed of rigid material. With this arrangement, the diaphragm may be supported by a peripheral suspensio system similar to that incorporated in the embodiment figures 5 to 8, or by a suspension system similar to th incorporated in the embodiment of figure 11. It should be appreciated that the scope of the invention i not limited to the scope of the embodiments described a that various alterations and variations may be made witho departing from the spirit of the invention.

Claims: THE CLAIMS defining the invention are as follows: 1. An electromagnetic acoustic transducer comprising a diaphragm having on at least one face an audio-frequency current conductor, the conductor being arranged in a wave¬ form pattern to ' divide the diaphragm into a plurality of acoustically rigid zones and define current paths along the sides of the zones, an array of magnets so arranged that magnetic fields produced thereby interact with the current paths along the sides of the zones whereby during passage of an audio-frequency current through the conductor interaction between the audio-frequency current passing along the current paths and the respective magnetic fields causes vibration of the diaphragm. 2. A transducer as claimed in claim 1 wherein the wave¬ form is of castellated formation. 3. A transducer as claimed in claim 1 wherein the wave¬ form is of sawtooth formation. 4. A transducer as claimed in any one of the precedin claims wherein the zones are of rectangular configuration. 5. A transducer as claimed in any one of the precedin claims wherein there is an audio— requency current conductor on each face of the diaphragm, the conductors being so arranged that each zone has at least one current path along each side thereof. 6. A transducer as claimed in any one of the precedin claims wherein the magnets are permanent magnets and are attached to a backing member of magnetic material, the backing member being acoustically transparent. 7. A transducer as claimed in any one of the preceding claims wherein the diaphragm is of flexible material, the zones being rendered acoustically rigid by tension applied to the diaphragm. 8. A transducer as claimed in claim 7 wherein the diaphragm is secured at its periphery to a peripheral frame, the peripheral frame being formed integral with or secured to the backing member. 9. A transducer as claimed in any one of claims 1 to 6 wherein the diaphragm is of flexible material, the zones being rendered acoustically rigid by rigidising means formed in the diaphragm. 10. A transducer as claimed in claim 9 wherein the rigidising means comprises a compound formation in the diaphragm at each zone. 11. A transducer as claimed in claim 10 wherein the compound formation comprises a hemispherical dome merged transitionally with the sides of the respective zone. 12. A transducer as claimed in claim 10 wherein the compound formation comprises a hollow cone merged trans¬ itionally with the sides of the respective zone. 13. A transducer as claimed in claim 10 wherein the compound formation comprises a hollow pyramid merged trans¬ itionally with the sides of the respective zone. 14. A transducer as claimed in any one of claims 1 to 6 wherein the diaphragm is formed of substantially rigid material. -- WIPO 15 A transducer as claimed in any one of the preceding claims wherein the pole faces of the magnets are of configuration which is geometrically similar to the configuration of the zones. 16. A transducer as claimed in any one of the precedin claims wherein there is one said permanent magnet cor¬ responding to each zone on the diaphragm, the pole faces o adjacent magnets being of opposite polarity. 17. A transducer as claimed in any one of claims 1 to 15 wherein there is one said permanent magnet corresponding to each alternate zone on the diaphragm, there being pol shoes corresponding to the other zones, the pole faces o the magnets being of the same polarity. 18. A transducer as claimed in any one of the claims 9 t 17 wherein the diaphragm is supported within its perimete by a plurality of resilient support members which permi said vibration of the diaphragm. 19. A transducer as claimed in claim 18 wherein eac resilient support member is attached to the diaphragm a the apex of a respective compound formation in th diaphragm. 20. An electromechanical acoustic transducer substantiall as herein described with reference to the accompanyin drawings. s y, WIPO

Inventors: CLARKE ANTHONY BERNARD, EVANS GRANT WILLIAM, CLARKE, ANTHONY, BERNARD, EVANS, GRANT, WILLIAM
Applicants: CLARKE ANTHONY BERNARD, EVANS GRANT WILLIAM, CLARKE, ANTHONY, BERNARD, EVANS, GRANT, WILLIAM
Application year: 1983
Main classification: H04R9
IPC codes: H04R9