Electro acoustic transducer and loudspeaker

Abstract: An electro acoustic transducer for a loudspeaker comprises a magnetic gap between pole pieces 2 and 4 containing a coil 5 inductively coupled to a shorted turn formed by the skirt 13 of dome 11. The diaphragm part of the dome 11 is mounted on the pole piece 4 by a suspension 12. The skirt 13 is inductively coupled to the coil 5 but is spaced from the coil and the pole piece by an air gap, possibly filled with ferrofluid. In order to prevent short circuits between the dome 11 and the pole piece 4, an interior surface of the dome 11 and/or the pole piece 4 is provided with a layer 16, 18 of electrically insulating material.

Description: The invention relates to an electro acoustic transducer, for instance for use in a loudspeaker or audio frequency sound reproduction device, and to a loudspeaker or audio frequency sound reproduction device incorporating such a transducer. In an inductively coupled system of the type shown in GB 545712 and GB 2118398, a moving coil electro acoustic transducer comprises a coil which drives a radiating surface. The coil, which is free to oscillate, is located within a magnetic gap. A shorted turn for driving a radiating dome is located within the coil and in the same magnetic gap. The shorted turn is mechanically independent of the coil and is inductively coupled to the coil. "Mechanically independent" means that, except for residual transfer of momentum between the coil and the shorted turn, for instance passed through the air or any other intervening fluid which lies in the gap between the coil and the shorted turn, there is no coupling of momentum between the coil and the shorted turn. The shorted turn and the radiating dome may be an integral component in the form of a thin cylindrical cup made out of any suitable electrically conductive material, generally metal. The thin cylindrical cup, which will be referred to as a shorted turn dome, is suspended on a magnet assembly pole piece by suspension means. In operation, when an electrical signal is applied to the coil via its input terminals, the shorted turn receives electrical energising signals exclusively from the coil by means of electrical transformer action. The transformer action provides a high pass filtercoupling to the shorted turn. In the transducer shown in GB 545712, the shorted turn dome is provided with a suspension at its skirt. In particular, in Figure 2 of GB 545712, the skirt of the dome is supported on and spaced from the pole piece by a tube of rubber or similar resilient material. Thus, when the shorted turn dome is driven, the shear-resilience of the suspension allows the dome to move. In order to provide a sufficiently compliant suspension, the resilient tube must be relatively thick which, in turn, requires a relatively large air gap. As the air gap is increased, the overall effectiveness of the transducer reduces to the point where it is no longer a viable design. Furthermore, the provision of the suspension at the skirt of the dome inevitably stresses the dome because of the resilience of the suspension. Also, the effective mass of the dome is substantially increased from its actual mass, which may be of the order of 0.1 gm for domes of 2.54 cm (1 inch) diameter, because a substantial portion of the suspension actually moves with the dome. Finally, assembly of the transducer and particularly of the dome is relatively complex and expensive, requiring the use of jigging to ensure that the shorted turn dome is accurately positioned and inserted into the air gap over the pole piece and suspension. In order to maximise the acoustic output from the shorted turn dome and improve the production control and assembly of an inductively coupled system, the clearance gap between the shorted turn and the magnet assembly pole piece should be minimised. However, any contact between the shorted turn dome and the magnet assembly pole piece will cause electrical short circuiting which has an adverse effect on the acoustic output of the shorted turn dome. According to a first aspect of the invention, there is provided an electro-acoustic transducer as defined in the appended Claim 1. Preferred embodiments of the invention are defined in the other appended claims. The provision of an electrically insulating layer allows the clearance gap between the shorted turn and the magnet assembly pole piece to be substantially reduced, thus maximising the acoustic output from the shorted turn dome and improving the production control and ease of assembly of the transducer. At the same time, any possibility of short-circuiting between the pole piece and the shorted turn dome is eliminated. The or each electrically insulating layer may be applied by means of coating, metal finishing, or any other suitable process. An example of a suitable metal finishing process is anodising and an example of a suitable coating process is vapour deposition. The invention will be further described, by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a cross-sectional view of a known inductively coupled electro-acoustic transducer of the type disclosed in GB 2118398; Figure 2 is a cross-sectional view of an inductively coupled electro-acoustic transducer constituting a preferred embodiment of the present invention; and Figure 3 shows a detail of the transducer of Figure 2 to an enlarged scale. The transducer shown in figure 1 is a loudspeaker drive unit for use in a sound reproduction loudspeaker system. The transducer comprises a permanent magnet 1 provided with an annular pole piece 2 and a centre pole piece 4 defining therebetween a magnetic gap. The gap may be an air gap or may contain ferrofluid. A coil 5 is located in the magnetic gap and is wound on a coil former tube 6 which is properly located by a suspension 7 attached to a chassis 8. The forward end of the coil former tube 6 is connected to the centre of an acoustic radiating cone 9 whose outer edge is connected to the chassis 8 by a roll surround 10. A metal dome 11 is suspended on the pole piece 4 by a suspension 12 and has a skirt 13 which extends into the magnetic gap inside the coil 5 and the former tube 6. The mass of the dome is typically of the order of 0.1 gm for a dome of 2.54 cm (1 inch) diameter. The cone 9 driven by the coil 5 provides acoustic output at relatively low frequencies whereas the dome 11 provides acoustic output at relatively high frequencies. The skirt 13 of the dome 11 acts as a shorted turn secondary winding of a transformer whose primary winding is provided by the coil 5. Thus, a signal to be reproduced is supplied to the coil 5 and drives both the cone 9 and the dome 11. The transformer action provides a high pass filtering action and, by appropriate design of the various parts of the transducer, a concentric two-way drive unit is provided without the need for an external cross over filter for dividing the frequency range. The clearance between the dome 11 and the pole piece 4 is shown at 14. This clearance must be sufficiently large to avoid any possible electrical short circuiting, which would adversely effect the acoustic output of the dome 11. Figure 2 shows an electro acoustic transducer of a type similar to that shown in figure 1 but constituting a preferred embodiment of the invention. Like reference numerals refer to like parts and will not be described again. The pole piece 4 is provided with a layer 16 of non-compliant electrically insulating material. The whole internal surface of the dome 11 (including the skirt 13) is also provided with a non-compliant layer 18 of electrically insulating material. The layers 16 and 18 may be formed by any suitable technique, such as coating or metal finishing, so as to provide very thin but rugged electrically insulating layers. In the case of coating, the layers 16 and 18 preferably have a thickness of the order of a few microns. Although electrically insulating layers 16 and 18 are shown on both the pole piece 4 and the internal surface of the dome 11, it is possible to provide only the layer 18. This arrangement allows the clearance gap 14 between the skirt 13 and the pole piece 4 to be minimised while eliminating any risk of electrical short circuits between the dome 11 and the pole piece. The acoustic output of the dome 11 can thus be increased, and there are benefits from improved production control and ease of assembly. For instance, no jigging is necessary in order to assemble the dome 11 to the transducer. Figure 3 shows the centre pole piece 4 and the dome 11 in more detail. The suspension 12 comprises a ring of resilient material of square or rectangular cross-section. The suspension 12 is adhered to an end face of the pole piece 4 and the dome 11 is adhered to the suspension 12 along a circular outer edge thereof. There is, thus, effectively a line contact 20 between the dome and the suspension. Thus, relatively little of the suspension moves with the dome, and the effective mass of the dome is not substantially increased by the contact with the suspension. Although a concentric two-way drive unit has been described, another embodiment provides a single drive unit for high frequencies (a "tweeter"). In this embodiment, the coil is fixed and does not drive a radiating surface, but merely energises the shorted turn dome which provides the only radiating surface.

Claims: An electro-acoustic transducer comprising: a magnetic circuit including a pole piece (4) inside a magnetic gap; a coil (5) for receiving electrical power for driving the transducer, the coil being located at least partly in the magnetic gap; and a high frequency acoustic dome radiating element (11) made of electrically conductive material and comprising a diaphragm, which is mounted on a face of the pole piece (4) facing the diaphragm by a suspension (12), and a skirt (13) which forms a shorted turn extending into the magnetic gap between the coil (5) and the pole piece (4) and spaced from the coil (5) and the pole piece (4) by fluid, the acoustic dome radiating element (11) being mechanically independent of the coil (5), the shorted turn being inductively coupled to the coil (5), characterised by at least one non-compliant electrically insulating layer (16, 18) located between the acoustic dome radiating element (11) and the pole piece (4) and further characterised in that the at least one insulating layer (16, 18) comprises an insulating layer (18) provided on the interior surface of the acoustic dome radiating element (11) and in that the exterior surface of the acoustic dome radiating element (11) is not covered by an insulating layer A transducer as claimed in Claim 1, characterised in that the at least one insulating layer (16, 18) comprises a further insulating layer (16) provided on the pole piece (4). A transducer as claimed in Claim 1 or 2, characterised in that the coil (5) is mechanically connected to a further acoustic radiating element (9). A transducer as claimed in Claim 1, characterised in that the coil (5) is fixed. A transducer as claimed in any one of the preceding claims, characterised in that the or each electrically insulating layer (16, 18) is a surface coating of electrically insulating material. A transducer as claimed in any one of Claims 1 to 4, characterised in that the or each electrically insulating layer (16, 18) is provided by a metal surface finish. A loudspeaker characterised by including a transducer as claimed in any one of the preceding claims.

Inventors: ELIELI BOAZ, ELIELI, BOAZ
Applicants: ELIELI BOAZ, ELIELI, BOAZ
Application year: 1989
Main classification: H04R9
IPC codes: H04R9, H04R1