JP3747267B2 - Piezoelectric speaker - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention has a simple structure, a small size, a light weight, and a piezoelectric speaker that efficiently reproduces ultra-high frequency sound in a wide band with good frequency characteristics and directional characteristics by a shape easy to ride high-order vibration waves. Related to the structure.
[0002]
[Prior art]
Conventionally, piezoelectric type sound generators are roughly classified into two types. As for the electro-mechanical vibration conversion mechanism, as in Conventional Example 1 shown in FIG. 21, one side of a metal plate 2a also serving as an electrode is used. A unimorph type piezoelectric element 2 ′ having a well-known structure in which a piezoelectric ceramic thin plate 2b is attached and the other electrode plate 2c is attached to the other surface, or the metal as in Conventional Example 2 shown in FIG. A bimorph type piezoelectric element 2 in which another piezoelectric ceramic thin plate 2b and an electrode plate 2c are formed on the other surface of the plate 2a is used.
[0003]
An example of the structure of a conventional speaker unit as a sounding body is one in which the piezoelectric element 2 or the piezoelectric element 2 ′ is directly used as a diaphragm. However, since the conversion efficiency is low if the piezoelectric element is used as it is, the conventional structure shown in FIG. In the structure of Example 1, a resonator 5 having a resonance frequency equal to the resonance frequency of the piezoelectric element 2 ′ and having a sound emitting hole 5 a is coupled to the front surface of the piezoelectric element 2 ′. Is amplified by resonance to increase the conversion efficiency.
[0004]
The above-mentioned piezoelectric sounding body is small in size and relatively efficient to radiate sound of a single frequency, but has a narrow reproduction frequency band and is not suitable for reproducing wide-band sound as a speaker. For the purpose of eliminating such drawbacks, the sounding body of Conventional Example 2, that is, a piezoelectric speaker shown in FIG. 22 has been put into practical use. The structure will be described with reference to FIG. 1. A piezoelectric element 2 is attached to the apex of a conical diaphragm 7 whose peripheral edge is supported by a frame 6, and a conductive wire 21a connected to the metal plate 2a. When an electric signal is applied through the conductive wire 21b connected to the electrode plate 2c, the piezoelectric element 2 bends and vibrates in a direction perpendicular to the surface, and the cone-shaped diaphragm 7 vibrates due to the reaction and emits sound. With this structure, since the acoustic energy is directly radiated from the cone-shaped diaphragm 7, the conversion efficiency is slightly higher, and since there is no resonator on the front surface, the reproduction band is widened and can be used as a speaker. Become.
[0005]
[Problems to be solved by the invention]
Of the above, Conventional Example 1 shown in FIG. 21 only emits a single-frequency sound, and thus can be used as a narrow-band signal sound generator, but is not suitable for use as a speaker.
[0006]
The conventional example 2 shown in FIG. 22 can be used as a speaker for sound range reproduction to some extent, but the cone-shaped diaphragm 7 that radiates sound is intended to be used in the conventional mass control band. Therefore, it is relatively rigid and vibrates together with the piezoelectric element 2 so that it reproduces with a certain degree of efficiency at the lowest resonance frequency. However, since the mass of the vibration system increases at higher frequency bands, the efficiency is Since the efficiency is further reduced in the low and higher frequency range, the frequency is reproduced only at the lowest resonance frequency, and the frequency characteristics are not flat in the other bands. There was a disadvantage that it was not wide.
[0007]
Therefore, the present invention is devised to improve the shape of the diaphragm, the coupling structure of the diaphragm and the piezoelectric element, and the structure of attaching the diaphragm and the piezoelectric element to the frame in order to eliminate the disadvantages associated with the conventional example. Another object of the present invention is to provide a piezoelectric speaker that has a flat frequency characteristic and efficiently reproduces a wide band and is inexpensive to manufacture.
[0008]
[Means for Solving the Problems]
The piezoelectric speaker according to the present invention for achieving the above object will be described with reference to FIGS. 1 to 3 showing the first invention and FIGS. 12 to 13 showing the second invention.
The first invention is composed of a plurality of bowl-shaped curved surfaces 1a, 1b,... Joined by joining adjacent sides in the parallel direction, and an outer side 11b opposite to the joined part 11a. , 11b and a film-like diaphragm 1 attached to the frame 3 and a bimorph-type or unimorph-type piezoelectric element 2 supported by adhering the peripheral part to the frame 3, and the diaphragm 1 The piezoelectric speaker is characterized in that the joint portion 11a is composed of a piezoelectric speaker unit 10 which is adhesively bonded to the piezoelectric element 2.
[0009]
The second invention is composed of a plurality of bowl-shaped curved surfaces 1a, 1b,... Joined by joining adjacent sides in the parallel direction, and an outer side part 11b opposite to the joined part 11a. , 11b and a film-like diaphragm 1 attached to the frame 3 and a bimorph-type or unimorph-type piezoelectric element 2 supported by adhering the peripheral part to the frame 3, and the diaphragm 1 The width and the bending depth between the outer edge portion 11b of the plurality of bowl-shaped curved surfaces 1a, 1b,... And the joint portion (11a) and between the joint portions 11a of the bowl-shaped curved surfaces are different from each other. The piezoelectric speaker is characterized in that the joint portion 11 a of the diaphragm 1 includes a piezoelectric speaker unit 20 that is adhesively bonded to the piezoelectric element 2 .
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
Since the piezoelectric speaker having such a configuration employs a rectangular diaphragm 1 having a good area space factor when viewed from the direction of sound wave radiation compared to a normal circular diaphragm, The sound pressure is improved over time, and the band near the lowest resonance frequency can be reproduced with high efficiency.
[0011]
Moreover, since the rising angle α of the joint portion 11a where the diaphragm 1 of the speaker unit 10 of the first embodiment belonging to the first invention shown in FIGS. 1 to 3 is coupled to the piezoelectric element 2 is steep, it is in the vibration direction. On the other hand, it has a large rigidity and has an effect of increasing the high-frequency resonance frequency to widen the reproduction frequency band, and unlike the point bonding in the conical diaphragm, the coupling portion with the piezoelectric element 2 is a line. Therefore, the electroacoustic conversion efficiency is improved, and the reproduction level becomes high even at the high-band resonance frequency, that is, the high-band limit frequency of the reproduction band. In addition, the piezoelectric speaker is excellent in timbre with low manufacturing cost.
[0012]
Further, since the diaphragm 1 has a bowl-shaped curved surface shape, the inclination angle of the diaphragm continuously changes from the rising angle α of the joint portion 11a toward the periphery of the diaphragm, and the longitudinal direction of the bowl-shaped curved surface. Since the part vibrates in the same phase, it is possible to continuously reproduce at a high level even in a band between the lowest resonance frequency and the high-frequency resonance frequency as compared with the conical shape. In the case of a single speaker unit, the directivity is broad in the direction perpendicular to the diaphragm joint 11a.
[0013]
In addition, the speaker unit 20 of the second embodiment belonging to the second invention shown in FIGS . 12 to 13 has a plurality of bowl-shaped curved surfaces 1d, 1e, and 1f formed by joining adjacent sides in the parallel direction to form a joint portion 11a. The film-like diaphragm 1 which is attached to the frame 3 by the outer side parts 11b and 11b opposite to the joined part 11a, and the peripheral part is adhered to the frame 3 and supported. The bimorph-type or unimorph-type piezoelectric element 2 is formed, and the dimensions of the saddle-shaped curved surface 1d and the saddle-shaped curved surface 1f at both ends of the three-segmented saddle-shaped curved surface of the diaphragm 1 are increased, and the central bowl-shaped The dimensions of the curved surface 1e are made smaller than the saddle-shaped curved surface 1d and the saddle-shaped curved surface 1f to change the sound pressure frequency characteristics of the curved surfaces. In this case, the radiation sound of the central bowl-shaped curved surface 1e can be set to a higher sound range than the radiation sounds of the other bowl-shaped curved surfaces 1d and 1f. On the other hand, the bowl-shaped curved surface 1d and the bowl-shaped curved surface 1f at both ends having large dimensions have a large radiation area and are suitable for low-frequency range reproduction, so that a wide-band reproduced sound can be obtained as a whole. In each of the embodiments, the joint portion 11a is directly coupled to the surface of the piezoelectric element 2. However, in the modification of the first embodiment, the diaphragm 1 is connected to the coupling rod 4 at the intermediate point of the joint portion 11a. Thus, the outer peripheral edge is coupled to the center of the piezoelectric element 2 fixed to the frame 3. The peripherally fixed piezoelectric element vibrates in a mode similar to a circular film, and is not constrained along the diameter, so that it vibrates efficiently, and the central portion has a maximum amplitude. Therefore, this embodiment has a high electroacoustic conversion efficiency.
[0014]
In addition, the characteristics of the diaphragm according to the first aspect of the invention described above are the effect of increasing the high frequency resonance frequency to widen the reproduction frequency band, and the band between the lowest resonance frequency and the high frequency resonance frequency. In the case of a single loudspeaker unit that plays back continuously at a high level and has directivity characteristics, it also has the effect of broadening in the direction perpendicular to the joint portion 11a of the diaphragm. In addition, a piezoelectric speaker having a high conversion efficiency is provided.
[0015]
FIG. 20 shows the frequency characteristics of the piezoelectric speaker unit according to the first embodiment of the first invention. As shown, the reproduction frequency band can be reproduced up to a high frequency of 50 KHz, which is far higher than the highest limit of 20 KHz of a normal high-frequency speaker, by the third harmonic shown in the figure. Used for speaker tweeters.
[0016]
【Example】
Configurations of embodiments and modifications of the piezoelectric speaker of the present invention will be described in detail with reference to FIGS. 1 is a top view of Example 1, FIG. 2 is a side sectional view, FIG. 3 is a perspective view of Example 1, FIG. 4 is a partial side sectional view of a modification , FIG. 5 is a top view, and FIG. partial cross-sectional view of another aspect of modification, FIG. 7 is an exploded view showing the manufacturing process of embodiment 1, FIG. 8 example 1, a schematic perspective view of the mold of the diaphragm of example 2, FIG. 9 These are other manufacturing-process exploded views of Example 1. FIG.
[0017]
10 is a top view of another modification of the first embodiment of the present invention, FIG. 11 is a side view thereof, FIG. 12 is a top view of the second embodiment of the speaker of the present invention, and FIG. 13 is a side view thereof. Further, as various application examples using the first embodiment, the second embodiment, and the modified examples thereof, FIG. 14 is an external view and a diagram of a speaker mounted on the front and back surfaces of the plane with their directions changed at right angles. 15 is an exploded perspective view, FIG. 16 is an external view of a speaker mounted on the same plane, and is turned at right angles, FIG. 17 is a top view thereof, and FIG. 18 is a diagram showing the speaker unit of the present invention outside the radial square surface. FIG. 19 is a top view of a speaker in which the speaker unit of the present invention is attached in the same direction outside the radial triangular surface.
[0018]
1 to 3 showing the structure of the piezoelectric speaker unit 10 of Example 1 and FIG. 7 showing the disassembled state, reference numeral 1 denotes a diaphragm of the speaker of the present invention. The outer shape of the polyetherimide film A is cut out in a rectangular shape, bent (B) around the center line serving as the joint 11a, and shaped on both sides adjacent to the bowl-shaped curved surfaces 1a and 1b if necessary. . Thus, the outer side portions 11b and 11b are formed at both end edges (C).
[0019]
Alternatively, the above-described film A is pressed by the hot pressing dies 25 and 26 shown in FIG. 8 and heat-molded, and the two pieces formed by cutting the periphery are used as single bowl-shaped curved surfaces 1a and 1b. As shown in FIG. 9, it can be obtained by bonding the edge that becomes the joint 11a with an adhesive in a separate process and molding the outer edges 11b and 11b at both edges. Further, the pressing shape of the hot pressing dies 25 and 26 is press-molded in one shot on both sides adjacent to the bowl-shaped curved surfaces 1a and 1b as in the shape (C) of the diaphragm 1 in FIG. You can also.
[0020]
The joint 11a of the diaphragm 1 is bonded over the entire length on a line passing through the center of the bimorph type piezoelectric element 2 (or unimorph type piezoelectric element 2 ′) of the speaker of the present invention. . Therefore, since the coupling portion with the piezoelectric element 2 is linear and has a large junction area, the reproduction level is high even at the high-band resonance frequency, that is, the high-band limit frequency of the reproduction band. The piezoelectric element 2 is attached to the frame 3 while being sandwiched between an attachment frame 3b and a presser frame 3c provided inside the outer frame 3a constituting the frame 3.
[0021]
Further, since the diaphragm 1 has a bowl-shaped curved surface shape, the inclination angle of the diaphragm 1 continuously changes from the rising angle α to the top of the joint portion 11a, and the longitudinal portions of the bowl-shaped curved surface are the same. Since it vibrates in phase, it is possible to continuously reproduce at a high level even in the band between the lowest resonance frequency and the high-band resonance frequency as compared with the conical shape. Therefore, the frequency characteristic during this period is flat at a high level. As described above, the rising angle α of the joint portion 11a coupled to the piezoelectric element 2 is steep, so that the rigidity is large in the vibration direction, and the diaphragm 1 is composed of a plurality of bowl-shaped curved surfaces 1a and 1b. Therefore, the high frequency resonance frequency becomes high and the reproduction frequency band becomes wide.
[0022]
In addition, since the rectangular diaphragm 1 is adopted as viewed from the direction of sound wave radiation, the diaphragm area is large in the case of the same outer dimensions (the length of one piece of the square = the diameter of the circle), and is near the lowest resonance frequency. The band can be reproduced with higher efficiency than a conventional circular (conical) diaphragm speaker.
[0023]
Furthermore, the diaphragm 1 used in the speaker of the present invention is easier to manufacture than a conical diaphragm when obtaining a square diaphragm. When the outer dimensions (diameter and one side) are the same, the price per unit area of the acoustic radiation surface viewed from the sound radiation direction is relatively low.
[0024]
4 to 6 the piezoelectric speaker unit variation of the structure metropolitan, diaphragm 1, the piezoelectric element 2, the frame 3 is the same shape as in Example 1, the diaphragm 1, the junction 11a However, it is different in that it is coupled to the center of the piezoelectric element 2 via the coupling rod 4. This modification has a feature that the conversion efficiency is high because the diaphragm 1 is driven by a portion having the largest amplitude of the piezoelectric element 2.
[0025]
10 and 11 are a front view and a side sectional view of another modification. The diaphragm 1 according to this modification includes three bowl-shaped curved surfaces, a bowl-shaped curved surface 1a, a bowl-shaped curved surface 1b, and a bowl-shaped curved surface 1c. The length, width, and radius of curvature of each bowl-shaped curved surface. Are the same, and therefore, the diaphragm 1 has the same sound pressure frequency characteristics on each curved surface, and is suitable as a piezoelectric speaker that emits a large output. The vibration plate 1 is bonded by linear bonding at a position away from the center on the surface of the piezoelectric element 2 and the piezoelectric element 2 by two joint portions 11a and 11a.
[0026]
In the second embodiment shown in FIG. 12 and FIG. 13, the dimensions of the saddle-shaped curved surface 1d and the saddle-shaped curved surface 1f at both ends are large, and the dimensions of the central saddle-shaped curved surface 1e are the same. The sound pressure frequency characteristic of each curved surface is changed by taking it smaller than the curved surface 1f. In this case, the radiation sound of the central bowl-shaped curved surface 1e can be set to a higher sound range than the radiation sounds of the other bowl-shaped curved surfaces 1d and 1f. On the other hand, the bowl-shaped curved surface 1d and the bowl-shaped curved surface 1f at both ends having large dimensions are suitable for low-frequency reproduction because they have a large radiation area. In other modified examples, the vibration plate 1 and the piezoelectric element 2 are adhesively bonded in a linear manner at two points away from the center of the piezoelectric element 2 as in the modified example described above.
[0027]
Thus, in the second embodiment shown in FIGS. 12 and 13, since the bowl-shaped curved surface portions 1d, 1e, and 1f having different shape dimensions are combined, the vibration modes of the respective portions are different, and the frequency characteristics thereof. As a result, the entire reproduction band is wider, the directivity is broader, and the frequency characteristic is flat. In particular, the size of the central bowl-shaped curved surface 1e is small, which is suitable for reproduction in the super high frequency range.
[0028]
In addition, the dimension between the outer side portions 11b and 11b is made larger than the length (diameter) of the joint portion of the piezoelectric element 2, and the surface areas of the diaphragms 1a, 1b and (1c) are increased to increase the conversion efficiency. And can be used for large output. As described above, the speaker using the piezoelectric speaker unit of the present invention has an advantage that the sound pressure frequency characteristic and the directivity can be freely selected and radiated according to the use direction.
[0029]
As described above, each of the embodiments and modifications of the piezoelectric speaker of the present invention has high efficiency in the band near the lowest resonance frequency, and continuously has a high level in the band between the lowest resonance frequency and the high band resonance frequency. Furthermore, since it has a wider playback band and flat frequency characteristics, it has the characteristics of a super tweeter and can be used in combination with woofers, squawkers, and tweeters for super high-pitched sound playback. It can be set as the outstanding multiway speaker.
[0030]
The configurations of the embodiments and modifications of the piezoelectric speaker of the present invention described above are not limited to the structures of the above embodiments. For example, in addition to the polyimide and polyetherimide, any material can be used for the diaphragm 1 as long as it is a moldable material having appropriate rigidity and internal loss, such as plastic film, aluminum or aluminum alloy, paper, and cloth. be able to. Therefore, there is a convenience that can be easily processed. Further, in the case of the modified speaker unit, the size, curvature, etc. of each bowl-shaped curved surface are not necessarily the same, and need not be symmetrical with respect to the center line.
[0031]
Hereinafter, application examples using the piezoelectric speaker unit 10 of the first embodiment, the piezoelectric speaker unit 20 of the second embodiment, and the respective modified speaker units will be described.
[0032]
First, as shown in FIGS. 14 and 15, a speaker using a speaker unit in which the diaphragms 1, 1 having the above-described curved curved surfaces are placed back to back with the piezoelectric element 2 interposed therebetween is a bidirectional speaker. For example, in the case of a speaker as shown in the same figure in which the front and back speaker units are arranged at right angles to each other in the axial direction, the directivity can be changed. Of course, it is also possible to arrange a plurality of elements in the same direction on a plane to sharpen directivity, and to concentrate and emit super high sounds in a specific direction perpendicular to the plane.
[0033]
Further, as shown in FIGS. 16 and 17, a large number of piezoelectric speaker units 10, 10,... (Or piezoelectric speaker units 20, 20,..., Or modified speakers thereof) are arranged on the same plane in a direction perpendicular to each other. The arranged multi-unit speaker can increase the electroacoustic conversion efficiency by increasing the substantial diaphragm area, sharpen directivity, and concentrate the sound at a desired location. As a modification of this application example, if the arrangement of the speaker units is only one column in the vertical direction, for example, a so-called three-dimensional sound radiation system that radiates super high sounds in the horizontal direction with broad directional characteristics, similar to tone zeire. It becomes possible easily.
[0034]
Furthermore, as shown in FIGS. 18 and 19, an assembly in which piezoelectric speaker units 10, 10,... (Or piezoelectric speaker units 20, 20,. Incorporated into a polygonal columnar support 8 such as a triangular prism or a quadrangular prism, or a polyhedral support (not shown), arranged in a polyhedral shape so as to radiate in three directions, four directions or more, and breathing spheres There are many advantages, such as being easy to incorporate so as to constitute a spherical radiator that vibrates.
[0035]
As described above, the speaker of the present invention has a simple structure, a lightweight composition material, and a small number of components. Therefore, the overall cost is low and the assembly is extremely easy. As an example speaker, the advantages of light weight, easy assembly, and low cost are not lost.
[0036]
Further, the shape and size of the bowl-shaped curved surface portion of the diaphragm 1 need not all be the same as shown in the drawing, or symmetrically have the same size and the same shape, and have the constituent elements described in the claims. However, as long as it exhibits the action referred to in the present invention and has the effects described below, it can be implemented with appropriate modifications.
[0037]
【The invention's effect】
As described above, the piezoelectric speaker according to the present invention basically has the effect of a conventional rectangular bowl-shaped curved surface diaphragm as a speaker and also has a unique effect based on the structure unique to the original speaker of the present invention. The following effects can be expected.
[0038]
(1) Since the diaphragm is made of a thin sheet material such as paper, metal, cloth, or synthetic resin, it is easy to obtain and can be easily processed such as cutting, bending, and bonding.
(2) The drive source can be made of a cheaply available material such as a ceramic vibrator and can be manufactured at a low cost as a whole.
(3) In relation to the above (1) and (2), since the speaker unit uses a combination of light materials as the vibration element, the product speaker can be made lightweight and inexpensively mass-produced.
(4) It can be designed freely in a shape that allows easy shaping of the diaphragm.
(5) No equipment such as heating or embossing is required to form the diaphragm.
(6) No large equipment is required for the speaker assembly manufacturing process, and the manufacturing space is small.
(7) Since it can be easily combined with other speakers and does not take up space, it can be easily combined with other speakers as a super tweeter of a multiway speaker.
(8) Since the vertical and horizontal directivity characteristics clearly appear, it can be configured as a speaker having the optimal directivity characteristics for the required service area.
[0039]
Due to the effects described above, the piezoelectric speaker of the present invention has a wide reproduction frequency band as a result, and can obtain a relatively flat frequency characteristic in a wide band between the lowest resonance frequency and the high-band resonance frequency. Thus, a piezoelectric speaker having high efficiency and excellent timbre can be provided at a low cost.
[Brief description of the drawings]
FIG. 1 is a top view showing the structure of Embodiment 1 of the present invention.
FIG. 2 is a side sectional view of the first embodiment.
3 is a perspective view of the first embodiment. FIG.
FIG. 4 is a partial cross-sectional side view of a modified example .
FIG. 5 is a top view of a modified example .
FIG. 6 is a partial cross-sectional view of another side surface of the modified example .
7 is an exploded view showing the manufacturing process of Example 1. FIG.
FIG. 8 is a schematic perspective view of a vibration metal mold according to Example 1 and Example 2. FIG.
9 is another exploded view of the manufacturing process of Example 1. FIG.
FIG. 10 is a top view of another modification of the embodiment of the present invention.
FIG. 11 is a side view of the above.
FIG. 12 is a top view of a second embodiment of the present invention.
FIG. 13 is a side view of the above.
FIG. 14 is an external view of a speaker of an application example that is mounted on the front and back surfaces of a flat surface while changing the direction at right angles.
FIG. 15 is an exploded perspective view of the above.
FIG. 16 is an external view of a speaker of another application example that is mounted on the same plane in a different direction at right angles.
FIG. 17 is a top view of the same.
FIG. 18 is a top view of an application speaker in which the speaker unit of the present invention is attached to the outside of the radial square surface in the same direction.
FIG. 19 is a top view of an application example speaker in which the speaker unit of the present invention is attached to the outside of the radial triangular surface in the same direction.
FIG. 20 is a frequency characteristic diagram of the first embodiment of the present invention.
FIG. 21 is a side sectional view of a conventional speaker 1.
FIG. 22 is a side sectional view of a conventional speaker 2;
[Explanation of symbols]
1 Diaphragm 2 Piezoelectric element (bimorph type)
2 'Piezoelectric element (Unimorph type)
3 Frame 4 Connecting rod 5 Resonator 6 Frame of Conventional Example 2 7 Cone-shaped diaphragm 8 Polygonal columnar support

Claims (2)

  It is composed of a plurality of bowl-shaped curved surfaces (1a), (1b),... Formed by joining adjacent sides in the parallel direction to form a joint portion (11a), and is located outside the joint portion (11a) that is joined. A film-like diaphragm (1) attached to the frame (3) by being attached to the frame (3) at the sides (11b) and (11b), and a bimorph type or unimorph supported by adhering the peripheral part to the frame (3) And a piezoelectric speaker unit (10) in which a joint (11a) of the diaphragm (1) is adhesively bonded to the piezoelectric element (2). Piezoelectric speaker. It is composed of a plurality of bowl-shaped curved surfaces (1a), (1b),... Formed by joining adjacent sides in the parallel direction to form a joint portion (11a), and is located outside the joint portion (11a) that is joined. A film-like diaphragm (1) attached to the frame (3) by being attached to the frame (3) at the sides (11b) and (11b), and a bimorph type or unimorph supported by adhering the peripheral part to the frame (3) Of the plurality of bowl-shaped curved surfaces (1a), (1b),... Of the diaphragm (1) and between the joints (11a) and a bowl-like shape. The width and the bending depth between the joint portions (11a) between the curved surfaces are different from each other, and the joint portion (11a) of the diaphragm (1) is adhesively bonded to the piezoelectric element (2). A piezoelectric speaker comprising a piezoelectric speaker unit (20).

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