Loudspeaker and sound reproduction system employing such a loudspeaker

Abstract: A speaker apparatus capable of reproducing from low-pitched to high-pitched sounds and a voice reproduction system employing the same. The speaker apparatus includes a speaker unit (10) in which a primary coil (1) is mounted in a gap portion between a plate and a centre pole of a magnetic circuit, a secondary coil is disposed within the gap in such a manner as to be fixed to a vibration plate, and a secondary electric current is induced in the secondary coil by a signal current flowing through a primary coil (1), thereby operating the vibration plate; and a speaker driving circuit (520,530) which drives the primary coil of the speaker unit with an analog sound signal, wherein the speaker driving circuit (520,530) interrupts said analog sound signal at a frequency higher than an audible frequency.

Description: The present invention relates to a speaker apparatus for acoustic reproduction and a sound reproduction system employing the same. Various types of speakers for acoustic reproduction have been conceived and made practical. Speaker units have been practically formed as electromagnetically coupled (electromagnetically induced type) speakers in which, for example, a magnet is sandwiched between a centre pole portion provided in a yoke and a plate, forming a magnetic circuit having a gap between the centre pole portion and the plate; within the gap of the magnetic circuit, a primary coil is fixed to the centre pole portion or the plate, and a secondary coil which forms a short coil is disposed within the gap of the magnetic circuit in such a manner as to be fixed to a vibration plate so as to face the primary coil. In this electromagnetically coupled speaker, a secondary electric current is induced in the secondary coil by a signal current flowing through the primary coil. Due to interaction with magnetic flux which occurs in the gap of the magnetic circuit, a driving force responsive to the secondary electric current is produced in the secondary coil in accordance with Fleming's left-hand rule, causing the vibration plate to which the secondary coil is fixed to deflect. In this way, the vibration plate is moved, thereby generating a sound. This electromagnetically coupled speaker has the advantages of having excellent heat dissipation properties and the capability of withstanding a large input because the primary coil through which a signal current flows is fixed to a centre pole portion or a plate formed from a magnetic material, such as iron. Further, if the secondary coil which forms a short coil is formed from a non-magnetic conductive material, for example, a cylindrical member for the length of one turn formed from, for example, aluminum, distortion can be reduced. EP-A-0 605 400 includes a disclosure of such an electromagnetically coupled speaker. Also, a dynamic (electroconductive type) speaker having a voice coil disposed within a gap in a magnetic circuit is practical. In this dynamic speaker, electric power is supplied to a voice coil, and the voice coil is connected to an input terminal provided in a speaker frame by means of a coil extension wire made of tinsel wire so that unwanted vibration and resistance are not applied to the vibration system including the voice coil. Further, in this dynamic speaker, it is considered that the voice coil is divided into portions corresponding to the number of bits of a digital sound signal, and that the respective coils are directly driven by data of the corresponding respective bits of the digital sound signal. As described above, the electromagnetically coupled speaker has the advantages of having excellent heat dissipation properties and the capability of withstanding a large input, and further is capable of reducing distortion. However, if the width of the gap in the magnetic circuit is increased, the magnetic sensitivity of the primary coil and the secondary coil is decreased; therefore, it is not possible to increase the number of turns of the primary coil and the secondary coil. For this reason, it is not possible to increase the inductances of the primary coil and the secondary coil, and the electromagnetic coupling force by which a secondary electric current is induced in the secondary coil by the signal current flowing through the primary coil reduces at a low frequency of below several kHz. Therefore, reproduction of, for example, from 1 kHz to 20 Hz required for sound reproduction cannot be adequately achieved. Due to this, the electromagnetically coupled speaker is used mainly as a speaker for reproducing high-pitched sounds. On the other hand, as described above, in a dynamic speaker, a voice coil is connected to an input terminal provided in the speaker frame by means of a coil extension wire made of tinsel wire. Further, in the dynamic speaker, it is considered that the voice coil is divided into portions for the number of bits of a digital sound signal, and that the respective coils are directly driven by data from each bit of the digital sound signal. However, at present, in a case where a sound signal is digitized, it is common practice to form the digital sound signal with 16 bits for the purpose of faithful sound reproduction. For this reason, in a dynamic speaker, when a voice coil is driven in accordance with a digital sound signal, 16 pairs (i.e., 32 wires) of coil extension wires become necessary for one speaker. However, since the tinsel wire, which is a coil extension wire, swings greatly with the vibration of the voice coil because the tinsel wire is extended from a moving object, namely, a moving voice coil, it is not possible to decrease the distance between them. Therefore, it is very difficult to provide as many as 32 tinsel wires in a speaker. In particular, it is difficult to manufacture a small-size speaker. Alternatively, WO-A-86 03927 discloses a method in which a digital input signal is converted to a discrete sampled analog signal which is then selectively amplified at the sampling frequency of the digitized signal and used to drive a speaker. Accordingly, in the present invention, reproduction down to a low frequency is made possible by an electromagnetically coupled speaker. The present invention provides a speaker apparatus, comprising: a speaker unit having a primary coil in the vicinity of a gap in a magnetic circuit formed with said gap and fixed with respect to said gap, and having a secondary coil disposed within said gap in such a manner as to be fixed to a vibration plate, in use, a secondary electric current being induced in said secondary coil by a signal current flowing through said primary coil, causing said vibration plate to deflect; and a speaker driving circuit which, in use, drives said primary coil of the speaker unit with an analog sound signal, wherein said analog sound signal has been interrupted at a frequency higher than an audible frequency by the speaker driving circuit. In the speaker apparatus of the present invention constructed as described above, since an analog sound signal is interrupted at a frequency higher than an audible frequency and is supplied to the primary coil of the electromagnetically coupled speaker, low-frequency components of the analog sound signal also become high frequencies exceeding 20 kHz as a signal current flowing through the primary coil. Therefore, reproduction down to a low frequency is made possible by an electromagnetically coupled speaker. Further, in the electromagnetically coupled speaker, even if the gap width of a magnetic circuit is decreased, and the number of turns of the primary coil and the secondary coil is decreased so as to prevent sensitivity from decreasing, the electromagnetic coupling force thereof is not decreased when the frequency of the signal current flowing through the primary coil is a high frequency such as exceeding 20 kHz, making sound reproduction possible. The invention will be further described by way of non-limitative example with reference to the accompanying drawings, in which:- Figure 1 is a sectional view illustrating an example of a speaker unit; Figure 2 is a sectional view illustrating another example of the speaker unit; Figure 3 is a sectional view illustrating still another example of the speaker unit; Figure 4 is a block diagram illustrating a sound reproduction system employing another example of the speaker apparatus of the present invention; and Figure 5 is an illustration of the speaker apparatus of Figure 4. Figure 1 shows an example of the speaker unit 10. In the speaker unit 10 of this example, a recess portion 13 is formed around the tip portion of a centre pole portion 12 of a yoke 11 such that a circular cylindrical centre pole portion 12 is integrally provided vertically in the central portion of a circular-plate-shaped flange portion 14, and a primary coil 1 is fitted into the recess portion 13 and thus mounted to the centre pole portion 12. The primary coil 1, in which a plurality of turns of conductors are wound in a ring form, is fitted and bonded to the recess portion 13, and thus mounted to the centre pole portion 12. Alternatively, a plurality of turns of conductors are directly wound around the recess portion 13, and thus the primary coil 1 is mounted to the centre pole portion 12. Alternatively, though not shown, a plurality of turns of conductors are wound around a magnetic bobbin, and the magnetic bobbin is fitted into the recess portion 13, and thus the primary coil 1 is mounted to the centre pole portion 12. An opening (hole) 15 is formed in a flange portion 14 of the yoke 11 at a position continuously adjacent to the centre pole portion 12, and a terminal plate 16 is mounted on the back of the flange portion 14. Then, a coil extension wire 17 made of, for example, tinsel wire, of the primary coil 1 is inserted into the opening 15 in such a manner as to be bonded to the peripheral surface of the centre pole portion 12, and connected by soldering to an input terminal 18 on the terminal plate 16. The coil extension wire 17 is provided for each of the winding beginning and the winding end of the primary coil 1, with each being connected to the separate input terminals. A ring-shaped magnet 21 is bonded to the front of the flange portion 14 of the yoke 11, and a plate 22 is bonded to the front of the ring-shaped magnet 21, forming a magnetic circuit 20 having a gap 23 between the outer peripheral surface of the tip portion of the centre pole portion 12 and the inner peripheral surface of the plate 22. Within the gap 23 of the magnetic circuit 20, a secondary coil 2 which forms a short coil is inserted. In this example, the secondary coil 2 is made into a cylindrical member by moulding a non-magnetic conductive material, for example, aluminum, and is made a coil for the length of one turn. The secondary coil 2 has mounted thereto a cone 32 with an edge 31 on the outer peripheral portion thereof and a damper 34 in such a way that the central openings of the cone and the damper are fitted and bonded. A cap 33 is mounted in such a manner as to cover the central opening of the cone 32 so as to form a lid. Further, a speaker frame 35 is mounted to the plate 22, the edge 31 on the outer peripheral portion of the cone 32 and a gasket 36 are mounted to the speaker frame 35, and the outer peripheral portion of the damper 34 is mounted to the speaker frame 35. As shown in Figure 2, a coil 1a of a part of the primary coil 1 may be mounted to the peripheral surface of the tip portion of the centre pole portion 12, and a coil 1b of the remainder may be mounted to the inner peripheral surface of the plate 22. In this case, the coil extension wire of the coil 1b mounted to the plate 22, though not shown, is inserted, for example, between the plate 22 and the magnet 21, and is connected to the input terminal on the terminal plate mounted to the outer peripheral surface of the plate 22. Further, as shown in Figure 3, the entire primary coil 1 may be mounted to the inner peripheral surface of the plate 22. The coil extension wire in this case also is inserted between the plate 22 and the magnet and is guided out to the outside. As shown in Figures. 1, 2 and 3, the bobbin around which the secondary coil 2 is wound may be omitted by forming the secondary coil 2 from a cylindrical member for one turn. The number of parts can be decreased as a result of forming without a bobbin by omitting the bobbin, and the magnetic sensitivity can be increased by decreasing the width of the gap 23 by an amount corresponding to the thickness of the bobbin. Figure 4 shows a sound reproduction system employing another example of the sound reproduction system of the present invention. In this example, an analog sound signal Ao from an analog sound output apparatus 510, such as a cassette player or an FM tuner, is supplied to a chopper 520 whereby the signal is chopped at a frequency higher than an audible frequency, namely, a frequency fc exceeding 20 kHz, which is said to be the upper limit of audible frequencies, as indicated by an analog sound signal Ac in Figure 5. However, the chopping frequency fc is preferably set at a higher frequency approximately twice 20 kHz, for example, 40 kHz. Further, the time width of the chopping period is made sufficiently shorter than a chopping cycle Tc, for example, 1/10 of the chopping cycle Tc. Then, the chopped analog sound signal Ac from the chopper 520 is amplified by a power amplifier 530 and supplied to the primary coil 1 of the above-described speaker unit 10. However, the speaker unit 10 with one primary coil 1 is used. As described above, in the speaker unit 10 which is an electromagnetically coupled speaker, the electromagnetic coupling force at which a secondary electric current i is induced in the secondary coil 2 by the signal current flowing through the primary coil 1 reduces at the low frequency from several kHz to below 1 kHz. However, according to the example in Figure 4, since the analog sound signal is interrupted at a frequency fc higher than the audible frequencies and is supplied to the primary coil 1 of the speaker unit 10, the lower-frequency components of the analog sound signal also become high frequencies exceeding 20 kHz as a signal current flowing through the primary coil 1. Therefore, it becomes possible for the speaker unit 10 which is an electromagnetically coupled speaker to perform reproduction down to a low frequency. Also, the sound reproduction system of this example is structured so that, for example, the chopper 520 and the power amplifier 530 are formed into one unit and this is connected to the analog sound output apparatus 510, and further, the speaker unit 10 is connected thereto, or components from the chopper 520 to the speaker unit 10 are formed into one unit and this is connected to the analog sound output apparatus 510. As described above, according to the present invention, by interrupting an analog sound signal supplied to a primary coil of an electromagnetically coupled speaker at a frequency higher than an audible frequency, reproduction down to a low frequency becomes possible with an electromagnetically coupled speaker, making it possible to realize a full-range speaker which reproduces from low-pitched to high-pitched sounds. Many different embodiments of the present invention may be constructed without departing from the spirit and scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in this specification. To the contrary, the present invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the invention as hereafter claimed. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications, equivalent structures and functions.

Claims: A speaker apparatus, comprising: a speaker unit (10) having a primary coil (1) in the vicinity of a gap (23) in a magnetic circuit (20) formed with said gap (23) and fixed with respect to said gap (23), and having a secondary coil (2) disposed within said gap (23) in such a manner as to be fixed to a vibration plate (32), in use, a secondary electric current being induced in said secondary coil (2) by a signal current flowing through said primary coil (1), causing said vibration plate (32) to deflect; and a speaker driving circuit (520, 530) which, in use, drives said primary coil (1) of the speaker unit (10) with an analog sound signal (Ac), wherein said analog sound signal has been interrupted at a frequency (fc) higher than an audible frequency by the speaker driving circuit (520, 530). A speaker apparatus according to claim 1, wherein said speaker driving circuit comprises a chopper (520) for interrupting said analog sound signal. A speaker apparatus, according to claim 1 or 2, wherein the interruption frequency (fc) is at least 20 kHz. A speaker apparatus according to claim 3, wherein the interruption frequency is approximately 40 kHz. A speaker apparatus according to any one of the preceding claims, wherein the width of each interruption is substantially shorter than the interruption cycle (Tc = 1/fc). A speaker apparatus according to claim 5, wherein each interruption is approximately 1/10 of the interruption cycle (Tc). A speaker apparatus according to any one of the preceding claims, wherein the speaker driving circuit further comprises a power amplifier (530) for amplifying the interrupted analog sound signal (Ac) and supplying it to the primary coil (1) of the speaker unit (10). A sound reproduction system comprising: an analog sound output apparatus (510) for outputting an analog sound signal (Ao); and a speaker apparatus according to any one of the preceding claims for interrupting said analog sound signal to produce interrupted analog sound signal (Ac) for driving the speaker unit.

Inventors: FUJIHIRA MASAO, KISHIGAMI JUN, MURAGUCHI TAKAHIRO, FUJIHIRA, MASAO, KISHIGAMI, JUN, MURAGUCHI, TAKAHIRO
Applicants: SONY CORP, SONY CORPORATION
Application year: 1997
Main classification: H04R3
IPC codes: H04R3, H04R1