Dynamic loudspeaker

Abstract: A loudspeaker (21) comprises a diaphragm (22) having a vibrating portion (215) and an annular conductive portion (28), a current feeding coil (23) facing the conductive portion (28) with a predetermined magnetic gap, and a magnetic circuit (24, 25, 26, 211) comprising a top plate (24), a magnet (25) and a yoke plate (26) to which the current feeding coil (23) is attached. A capacitor which forms a high-pass filter together with the internal resistance of the current feeding coil (23) is connected serially with the current feeding coil (23).

Description: This invention relates to dynamic loudspeakers. In a dynamic type loudspeaker, by allowing an audio signal current to flow through a voice coil in a dc magnetic field, a driving force is obtained. The audio signal current is usually supplied from the exterior to a voice coil through lead wires fixed to a paper cone which forms a diaphragm. There is, therefore, the problem that the lead wires may break due to elastic fatigue or the like caused by the reciprocating motion of the diaphragm. Moreover, the linearity of the reciprocating motion of the diaphragm may be impaired by the spring force of the lead wires, so sound distortion occurs, or the lead wires themselves may resonate and generate an abnormal sound. There is also the problem that in manufacture, since the lead wires must be led out from a narrow gap in the loudspeaker and must be positioned, adhered, and fixed, the assembly thereof is troublesome. To overcome these problems, an induction type loudspeaker from which lead wires are eliminated has been disclosed in Japanese Patent Application publication 56/27039. In this loudspeaker the lead wires are eliminated and a driving coil is arranged near a voice coil wound around a voice coil bobbin. An audio signal current is supplied to the driving coil, and the audio signal is supplied from the driving coil to the voice coil by magnetic induction. That is, when an ac signal of audio frequency flows from an electric power amplifier to the driving coil, an ac magnetic flux corresponding to the input waveform is generated by the driving coil. This ac magnetic flux closely interlinks with the voice coil which is located very close, and since the voice coil itself is short-circuited, a short-circuit current flows through the voice coil due to the ac magnetic flux. Since the voice coil is located in the magnetic field which is produced by a pole piece and peripheral magnetic poles, a force which is proportional to the product of the intensity of the magnetic field and the short-circuit current acts on the voice coil. This force is transferred from the voice coil to the voice coil bobbin, and vibrates a cone-shaped diaphragm, so sound is generated as in an ordinary loudspeaker. There are, however, some residual problems. Since the voice coil is generally fixed to the voice coil bobbin by an adhesive agent, it is difficult for the driving force generated in the voice coil to be directly transferred to the diaphragm. In addition, the voice coil generates heat due to the short-circuit current therein, and it is difficult to radiate the heat satisfactorily. To improve the sensitivity of the loudspeaker, it is required to narrow the gap (the magnetic gap portion) between the coil bobbin and the driving coil, and to wind the voice coil a number of times in the gap. Therefore, the diameter of a metal wire which is used for the voice coil must be small and the heat capacity of the wire is low. Thus, in addition to the problem of heat radiation mentioned above, there is the problem that the voice coil may be damaged by the heat, so the current capacity is limited. Moreover, if the voice coil bobbin is made of paper, it may become carbonized. Therefore, an induction type loudspeaker from which the voice coil is eliminated has been proposed in Japanese Utility Model Registration Application Laid-open 50/105438. That is, an induction type loudspeaker 1 shown in Figure 1 comprises a diaphragm 4 having an annular conductive portion 3 supported to vibrate freely in an annular magnetic gap portion 2 by a damper 10. A current feeding coil 5 fixedly arranged on the side of a magnetic circuit is mechanically separated from the diaphragm 4 as a vibration system, and is electrically coupled with the conductive portion 3 by mutual inductance. The magnetic gap portion 2 is annular and formed between a top plate 7, and a centre pole 9 of a yoke plate 8, which together with a magnet 6, for example of ferrite, form a magnetic circuit. The damper 10 is secured to the top plate 7. The diaphragm 4 may be dome-shaped with the conductive portion 3 at its open edge portion. Therefore, the whole diaphragm 4 is made from a thin plate of a good conductor, such as aluminium, beryllium or magnesium. Moreover, since the current feeding coil 5 is to be mechanically separated from the diaphragm 4 and electrically coupled with the conductive portion 3, the current feeding coil 5 is arranged so as to face the outer or inner periphery of the conductive portion 3. In this case, the current feeding coil 5 is fixed to the outer periphery of the centre pole 9. This loudspeaker 1 operates as follows. When an ac signal corresponding to an audio signal is supplied to the current feeding coil 5, a current of the same frequency is induced in the conductive portion 3 of the diaphragm 4 by mutual inductance, and acts on a dc magnetic field of the magnetic circuit in the magnetic gap portion 2 to drive the diaphragm 4, so that a sound wave is generated. In this loudspeaker not only the lead wires but also the voice coil are eliminated, but there are still problems. Thus, the diaphragm must normally be formed of a metal because it is necessary to develop the induced current in the conductive portion thereof. However, a metal diaphragm is heavy, so the sensitivity of the loudspeaker is reduced. Also, since the mechanical loss is small and the diaphragm is relatively heavy, there is a problem that the frequency characteristic of the loudspeaker is not flat, and sharp resonance peaks appear, as shown in Figure 2, at which it is difficult to brake the diaphragm, and at which the sound quality deteriorates. Moreover, since the conductive portion of the diaphragm and the portions other than the conductive portion in the diaphragm are not insulated, there is a problem of leakage of the induced current. Since the leakage current is not useful in driving the diaphragm, the driving force is weakened, and again the response sensitivity of the loudspeaker deteriorates. In such an induction type loudspeaker, a high-pass filter is equivalently formed on the input side. Therefore, reproduction of low frequencies is impaired. The diaphragm 4 reciprocates in the directions U and D in Figure 3 due to the induced current, and it is assumed that a uniform magnetic field range L1 of the dc magnetic field having a uniform magnetic flux distribution and a length L2 of the conductive portion 3 are substantially equal. Now, consider the case where the diaphragm 4 moves by only a length l in the direction U, and an edge portion 3 a of the conductive portion 3 reaches a point P1 in the uniform magnetic field range L1. In such a state, in the conductive portion 3, only the portion of length corresponding to (L1 - l) lies within the uniform magnetic field range L1. The other portions (that is, the length corresponding to L2 - (L1 - l) of the conductive portion 3 are all outside the uniform magnetic field range L1. When the conductive portion 3 is out of the uniform magnetic field range L1, since the magnetic flux density decreases sharply, if the induced current is constant, the driving force on the diaphragm 4 decreases substantially. Thus, the amplitude of the diaphragm 4 should increase in accordance with the induced current, but when the conductive portion 3 is largely out of the uniform magnetic field range L1, since the driving force is reduced, the amplitude of the diaphragm 4 does not respond accurately to changes in the audio signal, so linearity is lost, and distortion occurs. Patent Specification US-A-2 494 918 discloses an electro dynamic loudspeaker with a paper vibrating cone and two concentric rings of soft iron attached thereto and cooperating with an electric coil mounted on a yoke of a magnetic circuit, the coil lying between the concentric rings and having a width greater than the thickness of a top plate of the magnetic circuit. Patent Specification US-A-4 653 103 discloses a planar loudspeaker having a planar member forming a diaphragm with a voice coil thereon and a magnetic circuit to supply magnetic flux which crosses the voice coil. A dividing network (DNW) including a high pass filter and a low pass filter is provided, and an attenuator is connected between the dividing network and the voice coils. Patent Specification GB-A-803 283 discloses an electro acoustic transducer wherein a diaphragm has a connection piece mounting two axially spaced permanent magnet rings which are disposed in a gap of a magnetic circuit which includes an excitation coil. A spring offsets the magnet rings axially in the gap so that excitation of the magnet attracts one ring and repels the other. According to the invention there is provided a loudspeaker comprising: a diaphragm provided with an annular conductive portion; a current feeding coil facing said conductive portion with a predetermined gap; and a magnetic circuit to which said current feeding coil is attached; characterized in that: a capacitor which forms a high-pass filter together with internal resistance of the current feeding coil is directly serially connected to the current feeding coil such that all of the current passing through the capcitor passes through the current feeding coil. The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: Figure 1 is a cross sectional view of an induction type loudspeaker; Figure 2 is a frequency characteristic diagram; Figure 3 is a partial enlarged cross sectional view of part of the loudspeaker of Figure 1; Figure 4 is a cross sectional view of a first embodiment of a dynamic loudspeaker according to the invention; Figure 5 is a cross sectional view showing a modification; Figures 6 and 7 are perspective views for explaining examples where a conductive portion is integrated by a mechanical coupling; Figures 8 and 9 are cross sectional views for explaining other examples where a conductive portion is integrated by a mechanical coupling; Figure 10 is perspective view for explaining integration of conductive portion by a thin film; Figure 11 is a cross sectional view showing a second embodiment; Figures 12 and 13 are partial enlarged cross sectional views showing first and second modifications respectively; Figure 14 is a cross sectional view of a third embodiment; Figures 15 and 16 are perspective views of examples of a ring-shaped magnetic material; Figures 17 and 18 are partial enlarged cross sectional views of modifications; Figure 19 is an equivalent circuit diagram for explaining the third embodiment; Figure 20 is a frequency characteristic diagram for explaining the third embodiment; Figure 21 is a block diagram of an example of a loudspeaker system to which the present invention is applied; Figure 22 is an equivalent circuit diagram of the loudspeaker system of Figure 21; Figures 23 and 24 are frequency characteristic diagrams for explaining the loudspeaker system of Figure 22. Figure 4 shows the first embodiment in which a cylindrical pole piece 52 is formed at the centre of a disc-shaped yoke plate 51. A ring-shaped magnet 53 is laminated and fixed onto the yoke plate 51. A ring-shaped top plate 54 is laminated and fixed onto the magnet 53. An outer magnet type magnetic circuit is formed by the yoke plate 51, the pole piece 52, the magnet 53, and the top plate 54, A current feeding coil 55 is wound around the inner periphery of the top plate 54. Lead wires 59A and 59B are led out form the current feeding coil 55. As shown in Figure 5, it is also possible to wind a current feeding coil 55A around the inner periphery of the top plate 54, and a current feeding coil 55B around the outer periphery of the pole piece 52, lead wires led out from the current feeding coils 55A and 55B being connected in series or in parallel. It is also possible to use a member formed by winding a wire in a coil shape as the current feeding coil 55 and to attach the current feeding coil 55 to the top plate 54 or the pole piece 52. A dome-shaped diaphragm 56 is formed of a non-conductive material, for example, polymeric film, ceramics, cloth or paper. As will be explained below, a conductive portion 57 is integrally arranged at an edge portion of the diaphragm 56. In this example, a metallic ring is fitted on and attached to the outer periphery of the edge portion of the diaphragm 56, thereby forming the conductive portion 57 which operates as a voice coil of one turn or a few turns as in an ordinary dynamic loudspeaker. A magnetic gap is formed where the outer periphery of the pole piece 52 faces the inner periphery of the top plate 54, and the conductive portion 57 lies in the magnetic gap. The diaphragm 56 is supported by a damper 58 so as to vibrate freely. The damper 58 may alternatively be formed integrally with the diaphragm 56. The loudspeaker is driven by supplying an ac signal corresponding to an audio signal to terminals 510A and 510B of the lead wires 59A and 59B. A magnetic flux is generated in the current feeding coil 55 by the ac signal and interlinks with the conductive portion 57 which faces the current feeding coil 55. Thus, an induced current flows in the conductive portion 57, and a force to move the conductive portion 57 is generated, so the diaphragm 56 is vibrated. In this embodiment, the force generated in the conductive portion 57 is directly transferred to the diaphragm 56. Therefore, a situation in which a coupling portion obstructs the vibration and deteriorates the sound quality as in a known loudspeaker in which the voice coil bobbin is fixed by adhesive, does not occur. Moreover, since the diaphragm 56 is made of a non-conductive material, loss due to a leakage current such as would occur if the whole diaphragm 56 were metallic does not occur. Figures 6 to 9 show the case where a metal ring is fitted as the conductive portion 57 to the diaphragm 56 of non-conductive material, and the diaphragm 56 and the conductive portion 57 are mechanically integrated. A diameter l 1 of the outer periphery of the edge portion of the diaphragm 56 corresponds to a diameter l 2 of the inner periphery of the conductive portion 57 as shown in Figure 6. The metal ring forming the conductive portion 57 is fitted to the edge portion as shown in Figure 7, for example, by heat shrinkage. Figure 8 shows another example in which a metal ring is fitted in and attached to the edge portion of the inner periphery of the diaphragm 56 of non-conductive material to form the conductive portion 57. Figure 9 shows another example in which a metal ring is fitted in and attached to the diaphragm 56 by forming a concave portion 511 having a U-shaped cross section in the conductive portion 57, into which an edge portion of the diaphragm 56 fits. The diaphragm 56 and conductive portion 57 are not limited to such a mechanical coupling. As shown in Figure 10, a conductive thin film may be formed as the conductive portion 57 of the edge portion of the diaphragm 56. The thin film can be formed by electroless plating, chemical vapour deposition (CVD), evaporation deposition, or sputtering, in which cases ceramics, polymeric film, or a resin moulded member may, for example, be used as the diaphragm 56. In addition to such mechanical coupling or the formation of the thin film, it is also possible to provide conductivity in the edge portion of the diaphragm 56 to form the conductive portion 57. For example, when a polymeric film is used for the diaphragm 56, carbon or metal powder can be mixed into the edge portion of the diaphragm 56 to provide conductivity. Moreover, when polyacetylene is used for the diaphragm 56, iodine can be doped into the edge portion of the diaphragm 56 to form the conductive portion 57. On the other hand, in the case where the conductive portion 57 is formed by electroless plating or a thin film is formed by CVD, evaporation deposition or sputtering thereby to form the conductive portion 57, the conductive portion 57 may be formed not only on the outer periphery of the edge portion of the diaphragm 56 but also on the inner periphery of the edge portion, or on the outer and inner peripheries of the edge portion of the diaphragm 56. There is a possibility such that the problems connected with Figure 3 cannot be completely solved by simply having the conductive portion and the current feeding coil facing each other. Measures which can solve the above problems more completely will now be described. Figure 11 shows an embodiment of loudspeaker 71 which comprises a diaphragm 72, a damper 79, a current feeding coil 73, a top plate 74, a magnet 75, a yoke plate 76, and a pole piece 711. The dome-shaped diaphragm 72 comprises a hemispherical vibrating portion 715 and a conductive portion 78 which is annularly formed at an edge portion 77. The diaphragm 72 is supported by the damper 79 so as to vibrate freely with the conductive portion 78 located in a magnetic gap portion 710. The vibrating portion 715 is formed of an insulating material such as a synthetic resin. The whole of the conductive portion 78 is formed of a good conductor like a metal, such as aluminium, beryllium or magnesium. The magnetic gap portion 710 is annularly formed between the top plate 74 and the pole piece 711 of the yoke plate 76. The damper 79 is annular and has a spring characteristic, with its inner peripheral side connected to the periphery of the conductive portion 78 and its outer peripheral side fixed to the top plate 74. The current feeding coil 73 faces the conductive portion 78 with a predetermined gap and is coupled thereto by mutual inductance. In the current feeding coil 73, the winding (winding pitch or the like) and the height are similar to those in known coils. The current feeding coil 73 is arranged to face the outer periphery of the conductive portion 78 and the outer periphery is fixed to one side edge surface 712 of the top plate 74. A magnetic circuit is formed through the magnetic gap portion 710 by the top plate 74, the magnet 75, the yoke plate 76, and the pole piece 711. As shown in Figure 11, the magnet 75 is fixed to the outer peripheral portion of the yoke plate 76, and the top plate 74 is fixed to the outer peripheral portion on the magnet 75. In the region from the top plate 74 to the pole piece 711, the dc magnetic field of the uniform magnetic flux distribution is formed in the uniform magnetic field range L1, which is equal to the height (in the directions U and D) of the top plate 74. The operation of the loudspeaker 71 will now be described. When an audio signal current flows through the current feeding coil 73, an ac magnetic flux corresponding to the audio signal is developed. Since the annular conductive portion 78 closely interlinks the ac magnetic flux, an induced current corresponding to the audio signal is generated in the conductive portion 78 by mutual inductance, and flows mainly in the uniform magnetic field range, and little outside that range. Since the conductive portion 78 is located in the magnetic gap portion 710, a force which is proportional to the product of the intensity of the dc magnetic field in the magnetic gap portion 710 and the magnitude of the induced current acts on the conductive portion 78, directly driving the diaphragm 72 in the directions U-D and generating the sound wave. A force which is proportional to the product of the magnitude of the induced current and the intensity of the dc magnetic field is applied to the conductive portion 78, and, since the induced current accurately corresponds to the audio signal and the intensity of the dc magnetic field does not change, the driving force on the diaphragm 72 corresponds to the audio signal. Therefore, linearity between the audio signal current and the amplitude of the diaphragm 72 is maintained, and no distortion occurs. In the current feeding coil 73 in this embodiment, since the winding method and the length are similar to those of known coils, the impedance does not increase and a good frequency characteristic which does not change even in the high-band region is obtained. Moreover, as compared with a known long voice coil type loudspeaker in which the length of the voice coil wound around the voice coil bobbin is greater than the length of the top plate 74 in the height direction, since in this embodiment no significant induced current flows through the conductive portion 78 outside the uniform magnetic field range L1, electric power is not wasted and the efficiency can be increased. The construction of this embodiment is suitable for a loudspeaker for low frequency sound (a woofer) in which the amplitude of the diaphragm 72 is relatively large. Figure 12 shows an example in which a current feeding coil 730 is formed of flat wire. That is, in place of ordinary wire of circular cross section, a plurality of flat wires 731 of rectangular cross section are laminated and attached onto the inner periphery of the top plate 74. There are various advantages in this, firstly, circular wire comes into point contact with the other wire or the top plate 74, while the flat wire 731 comes into area contact, so that the thermal conductivity is good, and the heat generated in the current feeding coil 730 can easily be conducted away. Secondly, in spite of the fact that the magnetic gap portion 710 between the top plate 74 and the conductive portion 78 is very narrow, there is a requirement to wind the wire as large a number of times as possible. Using circular wire, it comes into point contact, so that gaps are inevitably caused between the conducting wires, or between the wires and top plate 74. However, since the flat wire 731 is in area contact, such gaps do not occur. Therefore, in a space of the same volume, the number of turns can be increased and the narrow magnetic gap portion 710 can be effectively used. Figure 13 shows an example in which a magnetic fluid 740 is arranged in the magnetic gap portion 710. The magnetic fluid 740 may, for example, be formed in a gel state by mixing powder of a magnetic material such as iron into an oil. By inserting the magnetic fluid 740 into the magnetic gap portion 710, various advantages can be expected. Firstly, when the magnetic fluid 740 is in the magnetic gap portion 710, the magnetic gap portion 710 is equivalently narrowed, so that the magnetic flux density is raised and the efficiency is improved. Secondly, since the heat generated in the conductive portion 78 of the diaphragm 72 is transferred through the magnetic fluid 740 and enters the magnetic circuit, an additional cooling effect is obtained. Thirdly, in the case of controlling Q of a resonant circuit of the vibrating system, if the magnetic fluid 740 is present, the characteristic of the vibrating system can more easily be controlled, due to the viscous loss of the fluid. The constructions shown in Figures 12 and 13 can be applied to other embodiments. Figure 14 shows the third embodiment of the invention, in which a cylindrical pole piece 62 is formed at the centre of a disc-shaped yoke plate 61. A ring-shaped magnet 63 is laminated and fixed onto the yoke plate 61. A ring-shaped top plate 64 is laminated and fixed onto the magnet 63. An outer magnet type magnetic circuit is formed by the yoke plate 61, the pole piece 62, the magnet 63, and the top plate 64. A current feeding coil 65 is wound around the inner periphery of the top plate 64. Lead wires 69A and 69B are led out from the current feeding coil 65. Further, a ring-shaped magnetic material 611 is provided on the inner periphery of the current feeding coil 65. It is also possible to use a member formed by winding a wire in a coil shape as the current feeding coil 65, and to attach the current feeding coil 65 to the top plate 64. A dome-shaped diaphragm 66 has a conductive portion 67 formed in the edge portion thereof. The conductive portion 67 operates as a voice coil of one turn. The diaphragm 66 can be formed of a non-conductive material such as polymeric film or ceramics. A magnetic gap is formed where the outer periphery of the pole piece 62 faces the inner periphery of the top plate 64. The conductive portion 67 formed integrally with the diaphragm 66 is located in the magnetic gap. The diaphragm 66 is reciprocably supported by a damper 68, which may be formed integrally with the diaphragm 66. The loudspeaker is driven by supplying an audio signal to terminals 610A and 610B of the lead wires 69A and 69B. That is, an ac audio signal is supplied to the current feeding coil 65 through the lead wires 69A and 69B. A magnetic flux is generated in the current feeding coil 65 corresponding to the audio signal. The magnetic flux interlinks with the conductive portion 67 which is arranged to face the current feeding coil 65, so an induction current flows through the conductive portion 67. Since the conductive portion 67 is located in the magnetic gap when an induced current flows through the conductive portion 67, a force to move the conductive portion 67 is generated, and the diaphragm 66 is vibrated. In this third embodiment, a ring-shaped magnetic material 611 of a high permeability is provided on the inner periphery of the current feeding coil 65, and as shown in Figure 15, it is desirable to use magnetic material the ends of which are cut or insulated, to prevent the induced current flowing in the ring-shaped magnetic material 611. If a magnetic material of high resistance is used, the induced current can alternatively be dissipated. On the other hand, as shown in Figure 16, it is also possible to use a material in which a number of magnetic members 612 are laminated axially. In this third embodiment, the ring-shaped magnetic material 611 is provided on the inner periphery of the current feeding coil 65. Therefore, the coupling coefficient of the current feeding coil 65 and the conductive portion 67 is increased, and hence the response sensitivity of the loudspeaker is improved and the low frequency reproducing limit decreased. The ring-shaped magnetic material 611 may be arranged at any suitable position to raise the coupling coefficient of the current feeding coil 65 and the conductive portion 67. For instance, as shown in Figure 17, the ring-shaped magnetic material 611 may be interposed between the outer periphery of the top plate 64 and the inner periphery of the current feeding coil 65. As shown in Figure 18, it is also possible to wind a current feeding coil 65A around the step portion 62a formed on the outer periphery of the pole piece 62, arrange a ring-shaped magnetic material 611A on the outer periphery of the current feeding coil 65A, wind a current feeding coil 65B around the inner periphery of the top plate 64, and arrange a ring-shaped magnetic material 611B at the inner periphery of the current feeding coil 65B. In the construction of Figure 18, it is possible not only to prevent the current feeding coil 65 from dropping out from the pole piece 62, but also to conduct the heat generated in the current feeding coil 65 into the pole piece 62. Such an induction type loudspeaker is shown by an equivalent circuit in Figure 19, where R denotes an internal resistance of the current feeding coil 65, L is an inductance of the current feeding coil 65, and M is an ideal transformer comprising the current feeding coil 65 and the conductive portion 67. As will be understood from Figure 19, in an input circuit of the induction type loudspeaker, a high-pass filter having a characteristic as shown in Figure 20 is formed by the internal resistance R and the inductance L of the current feeding coil 65. A cut-off frequency ω o of the high-pass filter is determined by R/L. The low frequency reproducing limit is determined in induction type loudspeaker by the high-pass filter, and in known induction type loudspeakers, the reproduction of low frequencies is impaired. Since the cut-off frequency of the high-pass filter is determined by R/L, it can be lowered by reducing the internal resistance R or increasing the inductance L of the current feeding coil 65. Therefore, consideration is given to lowering the low frequency limit by decreasing the internal resistance R. However, this is difficult to do, so consideration is given to increasing the inductance L by increasing the number of turns of the current feeding coil 65. However, in association with an increase in the inductance L the internal resistance R of the current feeding coil 65 increases. Moreover, by adjusting the inductance L of the current feeding coil 65 by the ring-shaped magnetic material 611, the low frequency reproducing limit of the induction type loudspeaker can be freely set, so in a loudspeaker system, the network circuit can be simplified. Although the first to third embodiments have been described with respect to dome-type loudspeakers, the invention also can be applied to cone-type loudspeakers. A loudspeaker system in which such a loudspeaker is applied practically will now be described, with reference to Figure 21. The loudspeaker system comprises a loudspeaker 31 for a high frequency band and a loudspeaker 32 for a low frequency band. An induction type loudspeaker as described above is used as the loudspeaker 31 for the high frequency band. A dynamic type loudspeaker is used as the loudspeaker 32 for the low frequency band. Alternatively, an induction type loudspeaker can also be used as the loudspeaker 32 for the low frequency band. A network circuit 33 is connected between an output amplifier 34 and the loudspeakers 31 and 32, and comprises a capacitor 35 and a low-pass filter 36. The capacitor 35 is arranged at the front stage of the loudspeaker 31, and the low-pass filter 36 is arranged at the front stage of the loudspeaker 32. Connection of the capacitor 35 like this is equivalent to providing a high-pass filter with a steep characteristic of 12 dB/oct at the front stage of the loudspeaker 31. This will now be explained with reference to the Figure 22 which shows an equivalent circuit. The input side of the induction type loudspeaker 31 comprises an inductance L3 of the current feeding coil and an internal resistance R3 of the current feeding coil. The signal on the input side is transferred to the secondary side, comprising the conductive portion through an ideal transformer M3. As shown in Figure 22, on the input side of the induction type loudspeaker 31 for the high frequency band, a high-pass filter of 6 dB/oct as shown in Figure 23 is formed by the inductance L and the internal resistance R of the current feeding coil. A cut-off frequency ω o of the high-pass filter is determined by R3/L3. When the capacitor 35 is connected to the induction type loudspeaker 31, a high-pass filter of 6 dB/oct is further formed by a capacitance C3 of the capacitor 35 and the internal resistance R3 of the current feeding coil. Therefore, when the capacitor 35 is so connected, this is equivalent to a high-pass filter of 6 dB/oct comprising the inductance L3 and the internal resistance R3 of the current feeding coil, and a high-pass filter of 6 dB/oct comprising the capacitance C3 of the capacitor 35 and the internal resistance R3 of the current feeding coil connected in cascade. Therefore, by equalizing the cut-off frequency of the high-pass filter comprising the inductance L3 and the internal resistance R3 of the current feeding coil with the cut-off frequency of the high-pass filter comprising the capacitance C3 of the capacitor 35 and the internal resistance R3 of the current feeding coil as shown in Figure 24, this is equivalent to a high-pass filter of 12 dB/oct at the front stage of the loudspeaker 31 for the high frequency band, and the network circuit 33 is simplified. Even when the induction type loudspeaker is used as a loudspeaker for a middle frequency band, the network circuit can be similarly simplified.

Claims: A loudspeaker (31) comprising: a diaphragm provided with an annular conductive portion; a current feeding coil facing said conductive portion with a predetermined gap; and a magnetic circuit to which said current feeding coil is attached; characterised in that: a capacitor (35) which forms a high-pass filter together with internal resistance of the current feeding coil is directly serially connected to the current feeding coil such that all of the current passing through the capacitor passes through the current feeding coil.

Inventors: INANAGA KIYOFUMI, SOGAWA HIROYUKI, TSUKAMOTO SHIRO, INANAGA, KIYOFUMI, SOGAWA, HIROYUKI, TSUKAMOTO, SHIRO, INANAGA, KIYOFUMI, C/O INTELLECTUAL PROPERTY DIV., SOGAWA, HIROYUKI, C/O INTELLECTUAL PROPERTY DIV., TSUKAMOTO, SHIRO, C/O INTELLECTUAL PROPERTY DIV.
Applicants: SONY CORP, SONY CORPORATION
Application year: 1989
Main classification: H04R9
IPC codes: H04R1, H04R9