JPH0555693U - Magnetic circuit for speaker - Google Patents

Abstract

(57) [Abstract] [Purpose] To provide a magnetic circuit for a speaker in which the weight of the magnetic circuit unit is reduced. A speaker in which magnets 1A and 1B magnetized in the thickness direction are arranged with a predetermined gap so that the same poles face each other and a voice coil is arranged in a magnetic field generated by both magnets. The center plate 2 made of a magnetic material and having a circular outer periphery is arranged so as to be sandwiched between the facing surfaces of the magnets. Outer periphery 22
And an inner peripheral portion 31 of the outer plate are arranged so that a magnetic gap is formed at a predetermined interval.

Description

[Detailed description of the device]

[0001]

[Industrial application]

 The present invention relates to a magnetic circuit for a speaker, and in particular, magnets are arranged at a predetermined interval so that the same poles face each other, and a voice coil is arranged to drive the magnetic field generated by the magnets facing each other. It relates to the magnetic circuit for the constructed speaker.

[0002]

[Prior Art]

 As a magnetic circuit structure of a speaker generally used conventionally, an outer magnet type as shown in FIG. 13 and an inner magnet type as shown in FIG. 14 are often used. Recently, it has been required to reduce the weight of speakers, especially in-vehicle speakers. To reduce the weight of the magnetic circuit components that make up most of the speaker weight, for example, a high-performance magnet such as a neodymium magnet is used to reduce the weight. I am trying to cope with the change. In the figure, 1 is a magnet, Y is a yoke, and P is a top plate, and these constitute a magnetic circuit. Reference numeral 5 is a frame, 7 is a diaphragm, 8 is a voice coil, and 9 is a damper. By reducing the weight of the magnetic circuit constituent portion, the strength of the frame 5 for supporting the magnetic circuit constituent portion, the diaphragm 7 and the like can be fulfilled even if the strength thereof is low. Therefore, the weight reduction of the magnetic circuit component leads to the weight reduction of the frame 5, the overall speaker weight can be reduced, and the lighter the magnetic circuit component weight is, the more effective it is. Lighter weight is required.

However, in the external magnet type magnetic circuit as shown in FIG. 13, the magnet 1 is unavoidably larger in diameter than the outer diameter of the voice coil 8 due to its structure. Has a drawback that the outer diameter becomes large, and the top plate P, the yoke Y, and the like corresponding to the outer diameter are required. Generally, when a neodymium magnet is used, it is possible to reduce the weight by about 60% to 70% as compared with an external magnetic circuit using an ordinary ferrite magnet. Since the outer diameter is large, it is difficult to reduce the weight further.

The inner magnet type magnetic circuit as shown in FIG. 14 does not have the drawback that the outer diameter of the magnet 1 becomes large as described above, but when it is used for a speaker for low-pitched sound in which the amplitude of the diaphragm 7 is large. In order to prevent the bobbin end portion 81 of the voice coil 8 from hitting the bottom surface TY1 of the pot yoke TY and producing an abnormal noise, the stroke allowance portion of the voice coil 8 is sufficiently secured, that is, the pot yoke TY. Must be made deeper, and the amount of yoke material (iron material, etc.) corresponding to the depth must be increased, which inevitably increases the weight. Generally, when a neodymium magnet is used, it is possible to reduce the weight by about 65% to 70% as compared with an inner magnet type magnetic circuit using a normal ferrite magnet, but even if a neodymium magnet is used, the yoke can be reduced. Due to the large amount of material, further weight reduction is difficult.

As is well known, when the magnets are opposed to each other at a predetermined interval so that the same poles are opposed to each other, the repulsive magnetic flows generated from the opposed surfaces of the magnet flow from the opposed surfaces to the outer peripheral portion of the magnet. On the other hand, a magnetic field (hereinafter referred to as a repulsive magnetic field) is generated from the vicinity of the outer peripheral portion toward the outer side. Therefore, a magnetic circuit structure of a type in which a voice coil is arranged in the repulsive magnetic field to drive the voice coil is provided. Various types of magnetic circuit structures of this type and loudspeakers using this magnetic circuit have been proposed (for example, JP-A-56-34298, JP-B-59-31110, and JP-A-59-48). 197, Japanese Examined Patent Publication No. 60-3270, Japanese Utility Model Laid-Open No. 61-128896, Japanese Utility Model Laid-Open No. 62-103395, and JP-A-1-98400). This is roughly divided into (1) a magnet in which the same poles of the magnets are simply opposed to each other at a predetermined interval, and (2) a normal inner magnet type magnetic circuit is improved and developed to provide a magnet on the top plate. (3) As shown in Japanese Patent Application Laid-Open No. 1-98400, a voice coil to be placed in the repulsive magnetic field is devised so that it is not placed outside the voice coil. There are tapes of crystalline metal, etc.

[0006]

[Problems to be solved by the device]

 However, in the case of (1) above, since the magnetic flux distribution is extremely narrow, it is not possible to obtain sufficient magnetic flux to drive the diaphragm of the speaker that is generally used, especially the diaphragm of the in-vehicle speaker, etc. Since it can be used only for a speaker, for example, a headphone or the like in which the diaphragm may have a small amplitude, it has a drawback that the range of use is significantly limited.

Further, in the case of the above (2), (a) the magnetic circuit does not have a symmetrical magnetic flux distribution with respect to the driving direction of the voice coil, and the driving circuit has a different driving force in the amplitude direction of the diaphragm. (B) Basically, since it has the structure of the pot yoke TY as shown in FIG. 14, it has the drawback that there is a limit to the weight reduction of the speaker like the inner magnet type magnetic circuit described above. ing. In order to use this magnetic flux distribution non-symmetrically and the leaked magnetism more efficiently, the pot yokes TY are vertically stacked and the voice coil is arranged inside the pot yoke TY (see Japanese Patent Publication No. 60- 3270), but in this method, a plate is provided on the outer peripheral portion of the voice coil in order to connect the voice coil and the diaphragm, and since the plate and the diaphragm are connected, A slit must be provided on the side surface of the jar yoke TY so that the plate can move up and down, which not only makes the structure extremely complicated, but basically, arranging two jar yokes TY one above the other. Therefore, the stroke allowance of the coil must be provided not only on the lower pot yoke TY side but also on the upper pot yoke TY. Therefore, it is more difficult to reduce the weight of the speaker as compared with the case where the normal pot yoke TY is used.

Further, in the case of the configuration example of the above (3), since the amorphous metal tape or the like is wound around the outside of the voice coil, the weight of the voice coil becomes heavy, and the efficiency becomes worse than that of a normal moving coil type speaker. It is natural to do.

Therefore, an object of the present invention is to solve the above-mentioned drawbacks of the conventional magnetic circuit, and to provide a magnetic circuit for a speaker, which is suitable for reducing the weight of the magnetic circuit section and the entire speaker and which is excellent in performance. ..

[0010]

[Means for Solving the Problems]

 In order to solve this problem, the present invention arranges magnets magnetized in the thickness direction with a predetermined gap so that the same poles face each other, and arranges a voice coil in the magnetic field generated by both magnets. In such a loudspeaker, a ring-shaped magnet is placed so as to hold a ring-shaped center plate made of a magnetic material on the opposite surface of the magnet, and a ring-shaped outer plate made of a magnetic material is placed outside the center plate with a holder. It was supported and arranged so that a magnetic gap with a predetermined interval was formed between the outer peripheral portion of the center plate and the inner peripheral portion of the outer plate. The diameter of the center plate is set to be slightly larger than the diameter of the magnet, preferably about 4 mm or less, and the thickness of the outer plate is set to be larger than that of the center plate. The holder for holding the center plate, outer plate and magnet in place is made of non-magnetic material such as aluminum and is made of conductive material, and the holder should be less than 50% of the outer plate thickness. Form. The holder has a substantially bottomed cylindrical shape and has a center guide portion for inserting the magnet and the center hole of the center plate, and a mounting portion for supporting the magnet and the center plate under the center guide portion. A step portion is formed, and a step portion for disposing the outer plate is formed on the upper surface of the outer peripheral wall. Further, a fin for heat dissipation and a hole for reducing the weight may be formed in the holder, and the hole may be covered with a covering material such as woven cloth or non-woven cloth. In this case, the frame for supporting the diaphragm or the like and the holder may be integrally molded, or the holder may be attached to a conductive frame made of a non-magnetic material such as aluminum.

[0011]

[Action]

 In the conventional case where the same poles of the magnets are simply opposed to each other with a gap, it is not possible to obtain a good magnetic flux density because air, which is a good non-magnetic material, exists between the facing surfaces. Since the center plate made of magnetic material is placed between the facing surfaces on the same pole side of, the magnetic flux can be efficiently guided toward the outer circumference of the magnet, and the diameter of the center plate is smaller than the diameter of the magnet. By making the diameter large, it is possible to more effectively generate the magnetic flux outward from the outer periphery of the center plate. Further, an outer plate made of a magnetic material is provided on the outer side of the center plate, and the outer peripheral portion of the center plate and the inner peripheral portion of the outer plate have a constant gap, and a magnetic gap is formed. It is possible to obtain the magnetic flux of.

[0013]

【Example】

 An embodiment of a magnetic circuit for a speaker according to the present invention will be described with reference to FIGS. 1 to 12. The same components as those of the conventional one described in FIGS. 13 and 14 are designated by the same reference numerals. The detailed description thereof will be omitted. In the speaker magnetic circuit of the present invention, as shown in FIGS. 1 and 2, magnets 1A and 1B magnetized in the thickness direction are arranged at a predetermined interval so that the same poles face each other, and both magnets are arranged. In a loudspeaker in which a voice coil is placed in the magnetic field generated in step 2, place the outer peripheral circular center plate 2 made of magnetic material between the opposing surfaces of both magnets, and place it outside the center plate. A ring-shaped outer plate 3 made of a magnetic material is supported by a holder 4 so that a magnetic gap with a predetermined interval is formed between the outer peripheral portion 22 of the center plate and the inner peripheral portion 31 of the outer plate. It was placed in. Magnets 1A and 1B used in the examples are neodymium-based magnets, and two ring-shaped magnets having an outer diameter of 29 mm, an inner diameter of 12 mm, and a thickness of 6 mm are used. Is magnetized. The center plate 2 is made of iron and has a ring shape with an outer diameter of 29.95 mm, an inner diameter of 11.95 mm, and a thickness of 4 mm, and the outer and inner ridge lines are chamfered with C0.4. The holder 4 for supporting the magnets 1A, 1B, the center plate 2 and the outer plate 3 is made of aluminum and has a substantially bottomed tubular shape. A cylindrical center guide portion 41 projecting upward is provided at the center of the bottom portion 43 of the holder 4, and the magnets 1A, 1B and the center plate 2 are located in the height direction at the lower end portion of the center guide portion 41. A mounting step portion 44 is provided to let out.

After applying an acrylic adhesive to the mounting step portion 44, the magnet inner diameter portion 1B1 of the magnet 1B is inserted into the center guide portion 41 with the N pole facing upward. The outer diameter of the center guide portion 41 is processed to be 11.95 mm in diameter, so that it can be easily inserted. After applying the adhesive to the upper surface of the inserted magnet 1B, press the inner diameter portion 21 of the center plate 2 into the center guide portion 41 until the lower surface of the center plate 2 comes into close contact with the N pole side of the magnet 1B. Push it in and apply an adhesive to the upper surface of the center plate 2 so that the magnet 1A faces the N pole downward and the inner diameter portion 1A1 of the magnet is inserted into the center guide portion 41 so as to be in close contact with the upper surface of the center plate 2. Push it to the desired position. As a result, the N poles of the magnets 1A and 1B are opposed to each other, and the center plate outer peripheral portion 22 is protruded to the outside by about 0.5 mm from the outer peripheral portions 1A2 and 1B2 of the magnets 1A and 1B while sandwiching the center plate 2. It is installed.

In this state, when the magnetic measurement probe was moved up and down about 0.5 mm further from the outer periphery 22 of the center plate, the measurement was performed by moving the probe up and down, and the magnetic distribution as shown in FIG. 3 was obtained. Obtained. Incidentally, the diameter of the center plate 2 is 2 mm smaller than the diameter of the magnets 1A and 1B, that is, the outer peripheral portion 22 of the center plate 2 is recessed inward by 1 mm from the outer peripheral portions 1A2 and 1B2 of the magnets 1A and 1B. Was manufactured and measured in the same manner as above, the numerical value was 4000 to 4500 gauss.

As shown in FIGS. 1 and 2, in the holder 4, the vicinity of the upper end portion of the outer peripheral wall rising from the bottom portion 43 has a height substantially equal to the position of the center plate, and the vicinity of the upper end portion thereof. A guide portion 42 having an inner diameter of 40 mm and a depth of 2 mm is provided on the outer peripheral portion 32 of an outer plate 3 made of iron and having an outer diameter of 39.9 mm, an inner diameter of 32.25 mm and a thickness of 3 mm. After the outer plate 3 is press-fitted, a magnetic gap having a gap of 1.15 mm and a facing width of 3 mm is completed between the inner peripheral portion 22 of the center plate 2 and the outer peripheral portion 32 of the outer plate.

When the magnetic measurement probe was moved up and down in the vicinity of the center of the magnetic gap to measure, a magnetic distribution as shown in FIG. 4 was obtained, and an average value of about 8500 gauss was obtained. The points A to H, which are divided into 8 equal parts, are used as the measurement points, and the measurement is performed by inserting a magnetic measurement probe into the measurement points. A point 8100 (Gauss, the same below), B point 8450, C point The numerical values of 8750, D point 8500, E point 8500, F point 8150, G point 7850, and H point 7900 were obtained, and the magnetic flux in the magnetic circuit was able to obtain a magnetic flux of about 8300 gauss on average. If the increase in the weight of the magnetic circuit portion cannot be taken into consideration, the performance of the magnetic circuit can be improved by making the thickness dimension of the outer plate 3 larger than the thickness dimension of the center plate 2. For example, when the thickness of the outer plate 3 is set to 6 mm, the magnetic distribution as shown in FIG. 5 and the average magnetic flux of about 9000 gauss can be obtained.

The holder 4 has a width of about 2 mm and a thickness of 2. A 5 mm flange portion 45 is provided, and four tongue-shaped attachment portions 46 projecting outward from the flange portion 45 are provided. A 4 mm tap processing is performed near the central portion of the attachment portion 46. is there. After applying a rubber adhesive to the flange portion 45, the bottom portion of the frame 5 is attached. As shown in FIG. 6, the frame 5 is provided with an attachment hole of φ4.5 at a position corresponding to the tap. The magnetic circuit part and the frame 5 are fixed with a 4 mm screw 6. The frame 5 is generally called 6.5 inches with an outer diameter of about 165 mm and a depth of about 20 mm, is made of a 0.7 mmt aluminum plate, is a press frame, and weighs about 40 g. The vibrating plate 7, which is a vibrating system member, uses a pulp cone-shaped vibrating plate having an outer diameter of approximately 134 mm (including edges), a neck diameter of 31 mm, and a depth of approximately 15 mmt. The voice coil 8 is a bobbin material of 0.5 mmt. Made of (PPTA) film with a coil winding width of 8 mm and a DC resistance of 3.43Ω. The damper 9 is generally made of cotton cloth impregnated with phenol and thermoformed such as corrugation. When these vibration system members were assembled into the assembled magnetic circuit part and frame 5 and completed as a speaker and measured, the characteristics shown by the solid line in FIG. 7 were obtained.

For comparison with the loudspeaker manufactured in the above-mentioned embodiment, the same vibration system as that of the loudspeaker is used, the frame has the same shape, and only the material is a 0.7 mmt iron plate which is generally used conventionally. A press frame (approx. 120 g) is used, and a magnetic circuit is also commonly used (top plate has an outer diameter of φ75 mm, inner diameter of φ32.25 mm, thickness of 4.5 mm, magnet has an outer diameter of φ85 mm, A ferrite magnet with an inner diameter of 45 mm and a thickness of 13 mm, a yoke with a pole diameter of 29.95 mm, a bottom outer diameter of 75 mm, and a height of about 20 mm were manufactured and measured. The characteristics shown by the dotted line in FIG. was gotten. As can be seen from this characteristic comparison, the speaker of the present invention can obtain characteristics that are slightly different in the low frequency range but practically no difference, and the impedance curve rises wider than the conventional magnetic circuit. It is confirmed that the sound pressure level is suppressed in the range and the sound pressure level in the high frequency range rises. The weight of the magnetic circuit portion in the above-described embodiment of the present proposal was about 95 g, and the total weight of the speaker was about 146 g. Therefore, when comparing the weight of the magnetic circuit part alone, it is possible to reduce the weight by approximately 84% compared to the conventional magnetic circuit part (603g), and it is approximately 81% lighter than the total weight (780g) of the conventional speaker. Became possible.

In the case of this embodiment, the dimensions of each part were obtained as shown by the solid line in FIG. 7 under the above conditions. However, the dimensions of each part can be freely designed according to the purpose of the speaker. In particular, the performance can be improved by examining the sectional shapes of the center plate 2, the outer plate 3 and the like. Further, in the case of this embodiment, the holder 4 holds the outer plate 3 at a predetermined position. However, as shown in FIG. 11, the magnets 1A and 1B and the center plate 2 are held by the holder 4, and only the outer plate 3 is held. There is no problem even if it is installed on the frame 5 side.

Further, in the case of the present embodiment, the holder 4 is manufactured by cutting the aluminum material in consideration of the uniformity of the magnetic flux distribution above and below the magnetic circuit, the heat radiation effect, etc., but if the design conditions are satisfied, For example, the holder 4 may be formed of a heat-resistant resin such as PPS, but when it is not necessary to consider the uniformity of the magnetic flux distribution, the holder 4 is made of a magnetic material, for example, 0.5 mm t to 0.7 mm t. Almost the same performance can be obtained by manufacturing with an iron plate or the like, and even if a magnetic material having a thickness equal to or greater than the thickness of the outer plate 3 is used, only the portion that secures the strength forming the magnetic circuit is left. The same effect can be obtained by cutting as shown in FIG. 8. In the case of the present embodiment, the holder 4 is manufactured by cutting, so the average thickness of the holder 4 is 1.5 mm t. For thin-wall processing more than this example Further, as shown in FIG. 9, the holder 4 is provided with the holes 47 and the fins 48, and the place where the holes are opened is covered with a covering material 49 such as a woven cloth or a non-woven cloth to reduce the weight and heat dissipation effect. Can be planned. Further, although the holder 4 is fixed by the screw 6 in the case of the present embodiment, it may be fixed to the frame 5 by a method generally used such as caulking or welding, and the frame 5 and the holder 4 are fixed. May be integrally molded as shown in FIG.

[0022]

[Effect of the device]

 According to the magnetic circuit for a speaker of the present invention, if at least the magnetic circuit of the embodiment is used, it is possible to obtain a magnetic flux which is the same as the magnetic flux in the magnetic gap of the conventional speaker. Since the holder for holding the member is made of non-magnetic material such as aluminum, the amount of iron in the magnetic circuit part can be greatly reduced compared to the conventional one, and the weight is reduced by 84% compared to the conventional magnetic circuit part. This is possible. Such a decrease in the iron content in the magnetic circuit section makes it possible to suppress the increase in impedance in a wider range than that of the conventional speaker, and improve the performance in the high range. Especially, it is possible to secure the high-range characteristics and sound quality of 1500 to 7,000 Hz which is preferable for door mounts. Instead of a normal chamber, a feather cone or the like is attached to the upper end of the voice coil bobbin to further extend the high range. Since it is possible to cover the range of the treble tweeter, it is not necessary to use the treble tweeter as in the conventional case, and the door mount speaker and the like can be significantly reduced in weight. In addition, the magnetic circuit of the embodiment is also downsized in volume, and the overall height of the speaker including the magnetic circuit is increased to achieve a thin structure, which is particularly required for the in-vehicle speaker. It is possible to obtain a form very suitable for

The holder is provided with a center guide portion at the center thereof, a mounting step portion at the lower portion thereof, and a step portion on the upper surface of the outer peripheral wall, so that the inner peripheral portion of the magnet or the center plate is provided. By inserting or press-fitting into the center guide portion and inserting or press-fitting the outer peripheral portion of the outer plate into the stepped portion, it is possible to simply install each component at a predetermined position. It eliminates the need for a gap jig or the like used when assembling, and can accurately form magnetic gaps with a predetermined spacing.

Since the magnetic circuit part is significantly lighter than the conventional one, even if the conventional 0.7 mmt iron plate frame is changed to a 0.7 mmt aluminum frame, sufficient strength to support the weight of the magnetic circuit part is secured. As well as being effective for weight reduction, it is possible to reduce the total weight of the speaker by 81% or more. Since the aluminum frame is attached to the aluminum holder, the heat dissipation effect is also the same as that of the conventional iron plate. It is better than the frame, and the cooling effect of the magnetic circuit part is also increased, which is effective as a power measure. Further, in the case of the magnetic circuit according to the present invention, since it is a magnetic circuit utilizing the repulsive magnetic field, there is a leakage magnetic field also on the S pole side as is apparent from FIGS. Therefore, in order for the voice coil to evenly oscillate, it is preferable that the magnetic distribution in the vertical direction about the center plate, that is, in the vibrating plate amplitude direction be symmetrical and uniform. It is extremely effective to use a non-magnetic material such as resin or resin.

Further, for example, the holder can be made thinner than the embodiment of the present invention, and further, by providing the holder with a hole or fin and covering the hole with a covering material, It is possible to prevent foreign matter from entering the magnetic circuit part, in particular, fine iron powder or the like that enters the magnetic gap and causes abnormal noise, and is more effective in reducing the weight and cooling the magnetic circuit. Further, the weight reduction of the magnetic circuit portion by the punching process can promote the weight reduction by thinning the frame, and the weight reduction can be achieved by integrally molding the frame and the holder. Furthermore, for example, the magnetic flux distribution can be changed by changing the thickness of the center plate, outer plate, etc., and by examining the shape of the parts, etc., the magnetic circuit can be freely adapted to the purpose of the speaker. Can be considered and set.

[Brief description of drawings]

FIG. 1 is a partial cross-sectional perspective view of a magnetic circuit for a speaker of the present invention.

FIG. 2 is an exploded perspective view of a magnetic circuit for a speaker of the present invention.

FIG. 3 is a sectional view and a magnetic flux distribution diagram in a state where an outer plate is not arranged.

FIG. 4 is a cross-sectional view and a magnetic flux distribution diagram when the thickness dimension of the outer plate is smaller than the thickness dimension of the center plate.

FIG. 5 is a cross-sectional view and a magnetic flux distribution diagram when the thickness dimension of the outer plate is made larger than the thickness dimension of the center plate.

FIG. 6 is a cross-sectional view of a speaker equipped with the magnetic circuit of the present invention.

FIG. 7 is a comparative characteristic diagram of a speaker equipped with a magnetic circuit of the present invention and a conventional speaker equipped with a magnetic circuit.

FIG. 8 is a sectional view showing another embodiment of the holder.

FIG. 9 is a cross-sectional view showing still another example of the holder.

FIG. 10 is a cross-sectional view showing a state in which the hole of the holder is covered with a covering material.

FIG. 11 is a cross-sectional view of a speaker equipped with a magnetic circuit according to another embodiment of the present invention.

FIG. 12 is a cross-sectional view of a speaker when a holder and a frame are integrally molded.

FIG. 13 is a cross-sectional view of a speaker equipped with a conventional external magnet type magnetic circuit.

FIG. 14 is a cross-sectional view of a speaker equipped with a conventional inner magnet type magnetic circuit.

[Explanation of symbols]

 1A is an upper magnet 1B is a lower magnet 2 is a center plate 21 is a center plate inner peripheral portion 22 is a center plate outer peripheral portion 3 is an outer plate 31 is an outer plate inner peripheral portion 32 is an outer plate outer peripheral portion 4 is a holder 41 Center guide 42 is a stepped portion 43 is a holder bottom portion 44 is a mounting stepped portion 48 is a fin 49 is a covering material 5 is a frame 7 is a diaphragm 8 is a voice coil 9 is a damper

Claims (10)

[Scope of utility model registration request] 1. A magnet (1) magnetized in the thickness direction
(A, 1B) are arranged with a predetermined interval so that the same poles face each other, and a voice coil is placed in the magnetic field generated by both magnets. A circular center plate (2) is arranged so as to be sandwiched, and a ring-shaped outer plate (3) made of a magnetic material is supported by a holder (4) on the outer side of the center plate, and the outer peripheral portion of the center plate ( 22) and a magnetic circuit for a loudspeaker, characterized in that a magnetic gap is formed between the inner peripheral portion (31) of the outer plate (22) and a predetermined gap. 2. The center plate (2) is set to have a diameter dimension larger than that of the magnets (1A, 1B), and the dimension difference is 4 mm or less. Magnetic circuit for speaker. 3. The magnetic circuit for a speaker according to claim 1, wherein the thickness dimension of the outer plate (3) is set to be larger than the thickness dimension of the center plate (2). 4. The holder (4) for holding the center plate (2), the outer plate (3) and the magnets (1A, 1B) in place is made of a non-magnetic material such as aluminum and having conductivity. The magnetic circuit for a speaker according to claim 1, which is molded. 5. The magnetic circuit for a speaker according to claim 1, wherein the holder (4) is made of a magnetic material with a thickness of 50% or less of the thickness of the outer plate (3). 6. The magnet (1A, 1B) and the center plate (2) have a ring shape, and the center hole of the magnet and the center plate is inserted into the central portion of the holder (4) having a substantially bottomed cylindrical shape. Center guide part (4
1) is formed and a mounting step portion 44 for supporting the magnet and the center plate is formed in the lower part of the center guide portion, and the outer plate (3) is arranged on the upper surface of the outer peripheral wall. The magnetic circuit for a speaker according to claim 1, wherein a step portion (42) is formed. 7. The magnetic circuit for a speaker according to claim 1, wherein the holder (4) is provided with fins (48) and holes (47). 8. A fin (48) and a hole (47) are formed in the holder (4) and the hole (47) is covered with a covering material (49) such as woven cloth or non-woven cloth. The magnetic circuit for a speaker according to claim 7, wherein: 9. The magnetic circuit for a speaker according to claim 1, wherein the holder (4) is integrally molded with a frame (5) for supporting the diaphragm (7) and the like. 10. The magnetic circuit for a speaker according to claim 1, wherein the holder (4) is attached to a frame (5) which is made of a non-magnetic material such as aluminum and has conductivity.