Electroacoustic transducer

Abstract: A electroacoustic transducer converts an electric signal into sound by vibrating a diaphragm (18) magnetically in response to the electric signal input thereto. A movement restricting means (sound emitting cylinder 24; projections 34, 36, 38) for restricting the movement of the diaphragm (18) within an allowable movement range is provided on the inner wall of a resonant chamber (22) disposed at one side of the diaphragm (18). The diaphragm (18) is restricted in moving by the moving restricting means (sound emitting cylinder (24); projections (34, 36, 38) )at a magnetic piece (20) attached to the center of the diaphragm (18).

Description: The present invention relates to an electroacoustic transducer for converting an electric signal input thereto into sound. An electroacoustic transducer is a means for converting an electric signal input thereto into sound. The electroacoustic transducer will produce an acoustic output in response to an input electric signal. Accordingly, the electroacoustic transducer can be employed by electronic devices, etc. as a sounding means such as a buzzer. A prior art electromagnetic type electroacoustic transducer, corresponding to the precharacterising part of claim 1, includes a cylindrical outer casing 102 (see figures 17-21) which is formed of synthetic resins and houses a magnetic driving portion 104 at the rear side t ereof. Input terminals 106 and 108 are formed in the magnetic driving portion 104 for inputting an electric signal to the magnetic driving portion 104. The magnetic driving portion 104 has a columnar core 110 at the center thereof and a coil 114 is wound around the core 110 by way of a bobbin 112. The input terminals 106 and 108 are connected to the ends of the coil 114 which is energized in response to the electric signal input thereto by way of the input terminals 106 and 108. A cylindrical magnet 116 is provided on the inner wall of the cylindrical outer casing 102 and disposed about the coil 114. A diaphragm 118, which is driven by the magnetic driving portion 104, is provided on the peripheral edge of the cylindrical magnet 116 and it is formed of an elastic thin magnetic member. Accordingly, the diaphragm 118 is attracted by the cylindrical magnet 116 and forms a closed magnetic circuit together with the core 110 and the cylindrical magnet 116. A magnetic piece 120 is attached to the center of the diaphragm 118 to establish a close magnetic relation with the core 110 and to add mass to the diaphragm 118. At the front side of the diaphragm 118, there are provided a resonant chamber 122 which is closed by the cylindrical outer casing 102 and serves as a resonant space and a sound emitting cylinder 124 which permits the resonant chamber 122 to be open to the atmosphere. A plurality of ribs 126 for restricting the movement of the diaphragm 118 within an allowable moving range are provided on the wall surface of the resonant chamber 122 at the edge of the diaphragm 118. Fig. 18 is an enlarged cross-sectional view of the diaphragm 118. The diaphragm 118 is formed of a very thin plate member and the disk-like magnetic piece 120 is attached to the center thereof as a rigid member to add mass to the diaphragm 118. The center of the magnetic piece 120 is attached to the center of the diaphragm 118 by spot welding. Denoted at 127 shows the welded portion. It is necessary to sufficiently reduce the size of the welded portion 127 without deterioration of the characteristics of the diaphragm 118 so as to uniform and stabilize the electroacoustic conversion characteristics. Furthermore, it is necessary that the deformation or deterioration of the characteristics of the diaphragm 118 is lessened after the diaphragm 118 and magnetic piece 120 are spot-welded and that they are brought into close contact with each other so as to have stable elasticity as a vibrating member. It is still necessary that the diaphragm 118 is very thin to assure a necessary sound pressure and sounding bandwidth. If the sound pressure or the sounding bandwidth is increased, bonding strength between the diaphragm 118 and the magnetic piece 120 is decreased, which results in deterioration of reliability and stability of the electroacoustic transducer. Meanwhile, such an electroacoustic transducer is provided in a variety of portable electronic devices and is subject to an external force such as a strong vibration, shock, etc. Fig. 19 shows the stationary state of the diaphragm 118 and Fig. 20 shows the vibrating state of the diaphragm 118 when it is normally driven. Fig. 20, (a) shows the movement of the diaphragm 118 toward the core 110 and (b) shows the movement of the diaphragm 118 toward the sound emitting cylinder 124. That is, the diaphragm 118 repeats a vibration to thereby emit a sound depending on the frequency of the input electric signal. Normally, rated input and limited input level corresponding to the allowable moving range are set so that the diaphragm 118 is prevented from moving beyond the allowable moving range. When an external force such as a shock, etc. is applied to the electroacoustic transducer, the diaphragm 118 is liable to be deformed beyond the allowable moving range as illustrated in Fig. 21. Fig. 21(a) shows the movement of the diaphragm 118 toward the core 110 wherein the diaphragm 118 contacts the head of the core 110. That is, the diaphragm 118 is prevented from moving excessively due to the core 110 so that the diaphragm 118 is protected by the core 110. In the case as illustrated in Fig. 21(b) where an external force is applied to the electroacoustic transducer so as to push the diaphragm 118 upward toward the sound emitting cylinder 124, a stress is applied between the diaphragm 118 and the magnetic piece 120 in such a manner to tear the magnetic piece 120 from the diaphragm 118. As a result, there is a possibility that the diaphragm 118 is broken or deformed at the welded portion 127 or its peripheral portion. In case that the external force is strong, there is a possibility that the magnetic piece 120 falls out from the diaphragm 118. There are measures for protecting the diaphragm 118 from the external shock as disclosed in Japanese Utility Model Publication No. 57-28478 entitled "electromagnetic type electroacoustic transducer for wristwatch", in Japanese Utility Model Laid-Open Publication No. 59-159098 entitled "electromagnetic type electroacoustic transducer" and in Japanese Utility Model Laid-Open Publication No. 60-26099 entitled "electromagnetic type sounder", etc. However, there are the following problems. In Japanese Utility Model Publication No. 57-28478, it is difficult to assure a resonant effect since a space in front of the diaphragm is sacrificed so as to prevent an excessive vibration. In Japanese Utility Model Laid-Open Publication No. 59-159098 and also in Japanese Utility Model Laid-Open Publication No. 60-26099, there remains a possibility that the magnetic piece falls out by the shock because the magnetic piece is not restricted in vibration. US-A-4813123 discloses an electroacoustic transducer in which a diaphragm is mounted in a casing which forms a resonant chamber. A sound emitting cylinder is found in the casing which causes the chamber to be open to the atmosphere. The diaphragm carries a magnetic piece fixed to a central part of the membrane for adding mass thereto, and the membrane is driven by a driving means being a core and a coil forming a magnetic circuit. In US-A-4813123 the magnetic piece is integral with a plunger which extends to form the core of the driving means. According to the present invention there is provided an electroacoustic transducer for converting an electric signal input thereto into sound comprising: an outer casing; a diaphragm disposed at an intermediate portion of said outer casing and formed of a magnetic body; said diaphragm having a magnetic piece fixed at a central portion thereof for adding vibrating mass thereto; a resonant chamber formed by said outer casing at said side of said diaphragm where the magnetic piece is fixed; a sound emitting cylinder for permitting said resonant chamber to be open to the atmosphere; magnetic driving means for applying magnetic vibration to said diaphragm in response to said input electric signal; said magnetic driving means comprising: a core, a coil which is wound around said core and is energized in response to said electric signal input thereto through input terminals; and a magnet disposed about said coil constituting a part of closed magnetic circuit with said core to exert a fixed magnetic field upon said diaphragm and said magnetic piece, the magnetic piece and the magnetic driving means being on opposite sides of the diaphragm; characterized in that: the electroacoustic transducer has movement restricting means for restricting the moving range of said diaphragm, said movement restricting means being on an inner surface of said outer casing forming said resonant chamber and projecting toward said magnetic piece of said diaphragm, said movement restricting means having a surface for contacting said magnetic piece upon deflection of said diaphragm beyond a preselected amount, thereby restricting the diaphragm movement sufficiently to prevent separation of the diaphragm and the magnetic piece or permanent deformation or breaking of the diaphragm. The invention prevents an excessive movement of the diaphragm at the center thereof beyond an allowable moving range and that protects the diaphragm from an external force such as a shock without sacrificing a resonant space. The electroacoustic transducer converts an electric signal to sound by vibrating the diaphragm (18) magnetically in response to the input electric signal as illustrated in Figs. 1 to 16, wherein movement restricting means (eg. sound emitting cylinder 24, projections 34, 36 and 38) which is provided on the inner wall of a resonant chamber (22) and disposed at one side of the diaphragm (18) are provided for restricting the movement of the diaphragm (18) within the allowable moving range at a magnetic piece (20) attached to the center of the diaphragm (18). Although a variety of shapes and positions of the movement restricting means are conceived, a single or a plurality of projections (34, 36 and 38) for defining the allowable moving range may be provided on the inner wall of the resonant chamber (22). A sound emitting cylinder (24) for permitting the resonant chamber (22) to be open to the atmosphere can also serve as the movement restricting means. According to the electroacoustic transducer of the invention, it is possible to prevent the generation of peeling force between the diaphragm and the magnetic piece and also prevent deformation, etc., of the diaphragm with assurance even if the shock is applied to the diaphragm (18) since the diaphragm (18) is restricted within the allowable moving range at the magnetic piece (20), i. e., at its central portion. If the movement restricting means is composed of a single projection or a plurality of projections, the resonant space can be less occupied by the movement restricting means, which advantages acoustic characteristics. If the sound emitting cylinder (24) serves also as the movement restricting means, it is not necessary to provide additional projections etc., to thereby simplify its structure. Preferably, the eletroacoustic transducer of the invention has the following features: (a) It is possible to protect the diaphragm against the damage or deformation caused by the shock etc. and to enhance the reliability of the electroacoustic transducer since the excessive movement of the diaphragm due to shock, etc., can be mechanically restricted at its central portion. (b) It is possible to surely prevent a welded portion from being broken and prevent the diaphragm and the magnetic piece from being peeled from each other by an external force. (c) It is possible to prevent deterioration of the characteristics of the diaphragm because the excessive movement of the diaphragm is restricted at the magnetic piece without directly contacting the diaphragm. (d) The electroacoustic transducer of the invention realizes miniaturized construction, comparatively low-frequency sound output, high quality and high reliability. Furthermore, the diaphragm can be more thinned and the magnetic piece can be more weighted due to the restriction of movement of the diaphragm while the reliability of the welded portion is enhanced, although these demands are mutually contradictory in nature. Other objects and features of the invention will be more apparent from embodiments as set forth hereinafter, which will now be described with reference to the accompanying drawings, in which: Fig. 1 is a longitudinal cross-sectional view of the electroacoustic transducer according to a first embodiment of the invention; Fig. 2 is a longitudinal cross-sectional view showing the movement of a diaphragm when an external force is applied to the electroacoustic transducer of Fig. 1; Fig. 3 is a longitudinal cross-sectional view of an electroacoustic transducer according to a second embodiment of the invention; Fig. 4 is a perspective view showing a sound emitting cylinder portion of the electroacoustic transducer of Fig. 3; Fig. 5 is a longitudinal cross-sectional view of an electroacoustic transducer according to a third embodiment of the invention; Fig. 6 is a perspective view showing a sound emitting cylinder portion of the electroacoustic transducer of Fig. 5; Fig. 7 is a longitudinal cross-sectional view of an electroacoustic transducer according to a fourth embodiment of the invention; Fig. 8 is a perspective view showing a sound emitting cylinder portion of the electroacoustic transducer of Fig. 7; Fig. 9 is a perspective view of a sound emitting cylinder portion of an electroacoustic transducer according to a fifth embodiment of the invention; Fig. 10 is a perspective view showing a modification of the sound emitting cylinder portion of the electroacoustic transducer of Fig. 9; Fig. 11 is a longitudinal cross-sectional view of an electroacoustic transducer according to a sixth embodiment of the invention; Fig. 12 is a perspective view showing a sound emitting cylinder and projections viewed from the inner side of an outer casing of the electroacoustic transducer of Fig. 11; Fig. 13 is a longitudinal cross-sectional view of an electroacoustic transducer according to a seventh embodiment of the invention; Fig. 14 is a perspective view showing a sound emitting cylinder viewed from the inner side of an outer casing of the electroacoustic transducer of Fig. 13; Fig. 15 is a longitudinal cross-sectional view of an electroacoustic transducer according to an eighth embodiment of the invention; Fig. 16 is a perspective view showing a sound emitting cylinder viewed from the inner side of an outer casing of the electroacoustic transducer of Fig. 15; Fig. 17 is a longitudinal cross-sectional view of a prior art electroacoustic transducer; Fig. 18 is a cross-sectional view of a diaphragm of the electroacoustic transducer of Fig. 17; Fig. 19 is a cross-sectional view of the electroacoustic transducer of Fig. 17 showing the stationary state of the diagram; Fig. 20 is a cross-sectional view of the electroacoustic transducer of Fig. 17 showing the vibrating state of the diagram when it is normally driven; and Fig. 21 is a cross-sectional view of the electroacoustic transducer of Fig. 17 showing the vibrating state of the diagram when an external force is applied to the electroacoustic transducer of Fig. 17. An electroacoustic transducer according to first to eighth embodiments will be described with reference to Figs. 1 to 16. First Embodiment (Figs. 1 and 2): Figs. 1 and 2 show the electroacoustic transducer according to the first embodiment of the invention. A cylindrical outer casing 2 formed of synthetic resins houses a diaphragm 18, a magnetic driving portion 4 disposed at the rear side of the diaphragm 18 for vibrating the diaphragm 18 in response to an input electric signal and a resonant chamber 22 disposed at the front side (upper side in these figures) of the diaphragm 18 for serving as a resonant space. A sound emitting cylinder 24 is disposed in the resonant chamber 22 for permitting the resonant chamber to be open to the atmosphere. Input terminals for applying an electric signal, not shown, are formed in the magnetic driving portion 4 like the input terminals 106 and 108 as illustrated in Fig. 17. A columnar core 10 is disposed at the center of the magnetic driving portion 4 and a coil 14 is wound around the core 10 by way of a bobbin 112, not shown, like the prior art electroacoustic transducer as illustrated in Fig. 17. The coil 14 is energized in response to the input electric signal through the input terminals like the prior art electroacoustic transducer as illustrated in Fig. 17. A cylindrical magnet 16 is disposed about the coil 14 and constitutes a closed magnetic circuit with the core 10, the diaphragm 18 and a magnetic piece 20. The magnetic piece 20 is fixed to the diaphragm 18 at a welded portion 27. The magnetic piece 20 has, like the prior art electroacoustic transducer, a close magnetic relation with the core 10 and adds mass to the diaphragm 18. In this first embodiment, there is formed a movement restricting means for restricting the movement of the diaphragm 18 within an allowable moving range at the side of the magnetic piece 20 disposed at one side (upper side in Fig. 1) of the diaphragm 18, i.e. at the center of the diaphragm 18. That is, the sound emitting cylinder 24 is designed so long that an interval D between the magnetic piece 20 and itself may be equal to or slightly greater than an ordinary allowable moving range. With such an arrangement, even if the diaphragm 18 moves away from the magnet 16 and moves upward when a strong shock is applied to the electroacoustic transducer, the magnetic piece 20 strikes against the end surface of the sound emitting cylinder 24 so that the diaphragm 18 is prevented from moving excessively as illustrated in Fig. 2. Furthermore, since the movement restriction is performed at the magnetic piece 20, namely, at the center of the diaphragm 18, the influence of the peeling force which was conventionally generated between the diaphragm 18 and the magnetic piece 20 can be removed completely, which enhances the reliability of the electroacoustic transducer. Second Embodiment (Figs. 3 and 4): Figs. 3 and 4 show the electroacoustic transducer according to the second embodiment of the invention. In this embodiment, the sound emitting cylinder 24 of the first embodiment has an inclined surface 30 at its end. With such an arrangement, the average interval between the sound emitting cylinder 24 and the diaphragm 18 can be increased while the interval D for the free movement of the diaphragm 18 is maintained like the first embodiment. As a result, the front surface of the diaphragm 18 can be sufficiently open on the side thereof facing the resonant chamber 22 while the moving range of the diaphragm 18 is restricted, which advantages the acoustic characteristics. Third Embodiment (Figs. 5 and 6): Figs. 5 and 6 show the electroacoustic transducer according to the third embodiment of the invention. In this embodiment, a plurality of U-shaped notches 32 are formed at an end surface of the sound emitting cylinder 24. With such an arrangement, the average interval between the sound emitting cylinder 24 and the diaphragm 18 can be increased while the interval D for the free movement of the diaphragm 18 is maintained like the first embodiment. As a result, the front surface of the diaphragm 18 can be sufficiently open on the side thereof facing the resonant chamber 22 while the moving range of the diaphragm 18 is restricted, which advantages the acoustic characteristics. Fourth Embodiment (Figs. 7 and 8): Figs. 7 and 8 show the electroacoustic transducer according to the fourth embodiment of the invention. In this embodiment, the length of the sound emitting cylinder 24 of the first embodiment is the same as the prior art and a plurality of thin columnar projections 34 serving as a movement restricting means of' the diaphragm 18 are formed on an end surface of the sound emitting cylinder 24. The projections 34 are disposed at equal angular intervals of 120° to restrict the movement of the diaphragm 18 on the average at the center of the diaphragm 18, i.e. at the magnetic piece 20 as illustrated in Fig. 8. With such an arrangement, the movement of the diaphragm 18 due to the application of an external force such as a shock can be restricted and the front surface of the diaphragm 18 can be sufficiently open on the side thereof facing the resonant chamber 22 and also the movement restricting means does not impede the acoustic characteristics. Fifth Embodiment (Figs. 9 and 10): Figs. 9 and 10 show the electroacoustic transducer according to the fifth embodiment of the invention. In this embodiment, a plurality of semicolumnar projections 34 are formed on the sound emitting cylinder 24 by extending portions of the outer peripheral surface of the sound emitting cylinder 24 as illustrated in Fig 9 or a plurality of projections 34 each having the shape of a rectangular pillar are formed on the sound emitting cylinder 24 by extending portions of the wall of the sound emitting cylinder 24 as illustrated in Fig. 10. Sixth Embodiment (Figs. 11 and 12): Figs. 11 and 12 show the electroacoustic transducer according to the sixth embodiment of the invention. In this embodiment, a plurality of plate-like projections 36 which serve as the movement restricting means of the diaphragm 18 are radially extended in the resonant chamber 22 from the sound emitting cylinder 24. That is, each projection 36 constituting a wall plate for dividing the resonant chamber 22 are disposed at equal angular intervals of 120° about the sound emitting cylinder 24 to restrict the movement of the diaphragm 18 on the average at the center of the diaphragm 18, i.e. at the magnetic piece 20. The upper surfaces of the projections 36 are higher than the end surface of the sound emitting cylinder 24. With such projections 36, the same functions and effects as the first to fifth embodiments can be expected too. Seventh Embodiment (Figs. 13 and 14): Figs. 13 and 14 show the electroacoustic transducer according to the seventh embodiment of the invention. In this embodiment, the sound emitting cylinder 24 in the outer casing 2 is displaced in its position and a plurality of projections 38 are formed on the ceiling surface of the resonant chamber 22 at the center thereof. Since the projections 38 serving as a protecting means of the diaphragm 18 are disposed at the center of the resonant chamber 22 and the sound emitting cylinder 24 is displaced therefrom, a resonant sound in the resonant chamber 22 due to the vibration of the diaphragm 18 can be effectively emitted to the atmosphere. Eighth Embodiment (Figs. 15 and 16): Figs. 15 and 16 show the electroacoustic transducer according to the eighth embodiment of the invention. In this embodiment, the sound emitting cylinder 24 is formed on the side wall of the outer casing 2 and a plurality of plate-like projections 38 are formed on the ceiling surface of the resonant chamber 22 at the center thereof. Each projection 38 may be a columnar body. With such an arrangement, the diaphragm 18 can be protected at the magnetic piece 20 against the damage or injure caused by its excessive movement and a resonant sound in the resonant chamber 22 can be emitted from the side surface of the outer casing 2. Although the features of the invention have been described with reference to the first to eighth embodiments, the electroacoustic transducer of the invention is not limited to those embodiments, it is to be understood that the invention includes many embodiments falling within the scope of the claims and having the same effects as the first to eighth embodiments.

Claims: An electroacoustic transducer for converting an electric signal input thereto into sound comprising: an outer casing (2) ; a diaphragm (18) disposed at an intermediate portion of said outer casing (2) and formed of a magnetic body; said diaphragm (18) having a magnetic piece (20) fixed at a central portion thereof for adding vibrating mass thereto; a resonant chamber (22) formed by said outer casing (2) at said side of said diaphragm (18) where the magnetic piece (20) is fixed; a sound emitting cylinder (24) for permitting said resonant chamber (22) to be open to the atmosphere; magnetic driving means (4) for applying magnetic vibration to said diaphragm (18) in response to said input electric signal; said magnetic driving means (4) comprising: a core (10), a coil (14) which is wound around said core (10) and is energized in response to said electric signal input thereto through input terminals; and a magnet (16) disposed about said coil (14) constituting a part of closed magnetic circuit with said core (10) to exert a fixed magnetic field upon said diaphragm (18) and said magnetic piece (20), the magnetic piece (20) and the magnetic driving means (4) being on opposite sides of the diaphragm (18); characterized in that: the electroacoustic transducer has movement restricting means for restricting the moving range of said diaphragm, said movement restricting means being on an inner surface of said outer casing (2) forming said resonant chamber (22) and projecting toward said magnetic piece (20) of said diaphragm (18), said movement restricting means having a surface for contacting said magnetic piece (20) upon deflection of said diaphragm (18) beyond a preselected amount, thereby restricting the diaphragm movement sufficiently to prevent separation of the diaphragm and the magnetic piece or permanent deformation or breaking of the diaphragm. An electroacoustic transducer according to claim 1, wherein said movement restricting means has a plurality of projections (34, 36, 38), each projection being formed on the end surface of said sound emitting cylinder (24) and being directed to said magnetic piece (20) of said diaphragm (18), the movement of said diaphragm (18) being restricted by said projections (34, 36, 38). An electroacoustic transducer according to claim 1, wherein said movement restricting means is formed by extending said sound emitting cylinder (24) toward said magnetic piece (20) of said diaphragm (18). An electroacoustic transducer according to claim 1, wherein said movement restricting means is composed of a plurality of plate-like projections (36), radially formed about said sound emitting cylinder (24) formed in said outer casing (2), the height of said plate-like projections (36) being higher than that of said sound emitting cylinder (24). An electroacoustic transducer according to claim 1, wherein said movement restricting means is composed of a plurality of projections (38) which are provided in said resonant chamber (22) at the position confronting said magnetic piece (20) and wherein said sound emitting cylinder (24) is displaced from the center of said outer casing (2). An electroacoustic transducer according to claim 3, wherein said sound emitting cylinder (24) has an inclined surface (30) at the position confronting said magnetic piece (20) of said diaphragm (18). An electroacoustic transducer according to claim 3, wherein said sound emitting cylinder (24) has notched portions (32) at the position confronting said magnetic piece (20) of said diaphragm (18). An electroacoustic transducer according to claim 2, wherein each said projection (34) is a columnar body. An electroacoustic transducer according to claim 2, wherein each said projection (34) is a pillar body having a semicircular cross-section. An electroacoustic transducer according to claim 2, wherein each said projection (34) is a square pillar body. An electroacoustic transducer according to claim 5, wherein said sound emitting cylinder (24) is formed on the side wall of said outer casing (2).

Inventors: FUSHIMI ISAO, IMAHORI YOSHIO, NITO KATSUTOSHI, FUSHIMI, ISAO, IMAHORI, YOSHIO, NITO, KATSUTOSHI, FUSHIMI, ISAO, C/O STAR MICRONICS CO.,LTD., IMAHORI, YOSHIO, C/O STAR MICRONICS CO.,LTD., NITO, KATSUTOSHI, C/O STAR MICRONICS CO.,LTD.
Applicants: STAR MFG CO, STAR MICRONICS CO., LTD.
Application year: 1993
Main classification: H04R13
IPC codes: H04R17, G10K9, H04R13, G04C21