Abstract:
The present invention relates to an electromagnetic electro-acoustic transducer to be used for generating an incoming indicator tone for mobile phones and the like. A resin magnet  11  is formed with a hard magnetic material and a soft magnetic material, the lowest resonant frequency can be easily set by changing the magnetic flux density in a magnetic gap by changing the compound ratio of the soft magnetic material, a high magnetic flux density is attained by increasing the magnetic permeability of the resin magnet  11  with the soft magnetic material, thus achieving a smaller size as well as a higher sound pressure through an increase in the driving force exerted to the diaphragm.

Description:
FIELD OF THE INVENTION 
     The present invention relates to electromagnetic electro-acoustic transducers for generating an incoming indicator tone such as in mobile phones. 
     BACKGROUND OF THE INVENTION 
     Referring to FIG. 5, a description will be given on a conventional electromagnetic electro-acoustic transducer. FIG. 5 ( a ) is a top view and FIG. 5 ( b ) is a cross-sectional view. 
     The conventional electromagnetic electro-acoustic transducer comprises a first diaphragm  100 , a second diaphragm  101  being a magnetic material attached on the center of the first diaphragm  100 , a center pole  103  disposed opposite the second diaphragm  101 , a coil  104  wound around the center pole  103 , a ring-shaped resin magnet  105  positioned on the outer periphery of the coil  104 , a yoke  106  in contact with or integrated with the center pole  103 , and a cylindrical housing  107  that circumferentially supports the first diaphragm  100 . A description will be given on the operation of an electromagnetic electro-acoustic transducer of the above configuration. In the initial state when no current is flowing in the coil  104 , a magnetic circuit is formed by the resin magnet  105 , second diaphragm  101 , center pole  103 , and yoke  106 , and the second diaphragm  101  is attracted toward the resin magnet  105  and the center pole  103 . Thereby the first diaphragm  100  is displaced to a position that balances with its elastic force. 
     Next, when an alternating current flows in the coil  104 , an alternating magnetic field is generated by the magnetic circuit where the coil  104  works as a magnetomotive force. The magnetic flux density within the magnetic circuit is determined by the intensity of the alternating magnetic field and the magnetic resistance inside the magnetic circuit. In this case, the magnetic resistance is approximately equal to the combined resistance of the magnetic resistance due to the magnetic gap between the second diaphragm  101  and the center pole  103 , the magnetic resistance due to a magnetic gap between the second diaphragm  101  and the resin magnet  105 , and the magnetic resistance of the resin magnet  105  itself. The specific magnetic permeability of the resin magnet  105  is as low as that of the air and is approximately equal to  1 , so that the magnetic resistance is high. 
     An alternating driving force is generated on the second diaphragm  101  due to a change in the magnetic flux density. As a result, the second diaphragm  101  moves from the initial position together with the attached first diaphragm  100  due to a static attraction force generated by the resin magnet  105  and a change in the alternating driving force generated by an alternating current. And its vibration radiates a sound. 
     The above-mentioned resin magnet  105  is a composite material consisting of a hard ferrite magnetic material, a polyamide resin such as nylon  6 , nylon  12 , and low molecular weight rubber. That is, the resin magnet  105  in the conventional transducer uses a composite material of a hard magnetic material and a resin. 
     By the way, as electromagnetic electro-acoustic transducers are used in mobile phones and the like, their small size and a high sound pressure are required for their alternating driving force. Accordingly, the acuteness of resonance, Q-factor of the mechanical resonant system consisting of the first diaphragm  100  and the second diaphragm  101  is made high, and the lowest resonant frequency of the mechanical resonant system are made close to the regeneration acoustic frequency. 
     Also, while the lowest resonant frequency of the mechanical resonant system is determined by the effective masses of the first diaphragm  100  and the second diaphragm  101  and the stiffness of the first diaphragm  100 , the stiffness of the first diaphragm  100  is not only affected by the modulus of elasticity and thickness of the material used but also by the configuration of deformation due to the static attraction force by the resin magnet  105  and the center pole  103 . 
     The conventional electromagnetic electro-acoustic transducer as described above suffered a problem of resolving the complicated situation as described below in order to obtain an electromagnetic electro-acoustic transducer with a desired characteristic. 
     (1) In order to change the lowest resonant frequency of the mechanical system depending on the regenerated acoustic frequency, it is necessary to change the effective mass and stiffness of the mechanical resonant system. In doing this, it is necessary to change the thickness of the first diaphragm  100  and the thickness and diameter of the second diaphragm  101 , or to change the magnetic gap between the second diaphragm  101  and the center pole  103  in order to change the static attraction force. Such changes in the thickness and diameter of the second diaphragm  101  will result in changes in the magnetic resistance and in various interacting mechanical parameters including effective mass and vibration mode the regeneration acoustic frequency. Consequently such a design change to get a desired characteristic become complicated. 
     (2) As the acuteness of resonance, Q-factor of the mechanical resonant system is high, the ratio of contribution of the lowest resonant frequency to sound pressure is extremely high, making sound pressure variation due to a slight change in a magnetic gap or in the thickness of the diaphragm large. 
     However, in order to make the acuteness of resonance, Q-factor low, although lightening of the mass of the vibratory system or an increase in the driving force (force coefficient) is necessary, lightening of the mass of the vibratory system makes the volume of the second diaphragm  101 , being a magnetic material, small and will result in an increase in the magnetic saturation and magnetic resistance thus resulting in a decrease in the force coefficient. 
     Also, in order to increase the force coefficient, it is necessary to increase either direct current magnetic flux density or alternating current magnetic flux density; and in order to increase the direct current magnetic flux, it is necessary to increase energy of the magnet or upsize the diaphragms thus leading to upsizing of the total size or an increase in the mass of the vibratory system. 
     Although it is necessary to decrease the magnetic resistance in order to increase the alternating current magnetic flux, it is difficult as the magnetic permeability of conventional resin magnet  105  is low. There is other approach such as making the magnetic gaps small but it makes vibration amplitude of the diaphragms small. 
     DISCLOSURE OF THE INVENTION 
     The present invention resolves the above-described problems and provides an electromagnetic electro-acoustic transducer in which the cost is low, the lowest resonant frequency of the mechanical system is variable, and variation of sound pressure at high sound pressures is narrow. 
     A first embodiment of the present invention comprises a first diaphragm, a second diaphragm attached at the center of the first diaphragm and consisting of a magnetic material smaller than the first diaphragm, a center pole provided underneath the center  6 f the second diaphragm with a magnetic gap in between, a coil wound around the outer periphery of the center pole, a ring-shaped resin magnet, and a yoke disposed in a manner such that it comes in contact with the lower parts of the center pole. Here, the magnetic powder of said resin magnet consists of powders of a hard magnetic material and a soft magnetic material, the magnetic flux density and the magnetic permeability can be controlled by changing their compounding ratio, so that lowest resonant frequency and the high sound pressure can be set very easily. 
     A second embodiment of the present invention is one in which the magnetic powder orientation of the resin magnet of the first embodiment is aligned by injection molding in magnetic field. Magnetic energy is enhanced when magnetizing the hard magnetic material in the resin magnet, -thereby obtaining a resin magnet with a high magnetic flux density and a high magnetic permeability jointly by mixing of a soft magnetic material. 
     A third embodiment of the present invention is one in which the hard magnetic material in the resin magnet of the first embodiment is a eutectic composition of a ferrite magnetic material and a rare-earth magnetic material. By employing a eutectic composition of a ferrite and a rare-earth magnetic material as the resin magnet, a higher magnetic flux density is obtained thereby enabling provision of a further superior electromagnetic electro-acoustic transducer. 
     A fourth embodiment of the present invention is one in which the total amount of the magnetic powder within the resin magnet of the first embodiment is in the range 85 to 92% by weight thus enabling fabrication of a resin magnet with superior moldability and magnetic characteristic thereby providing a superior electromagnetic electro-acoustic transducer 
     A fifth embodiment of the present invention is one in which a ring-shaped magnetic plate with an inner diameter smaller than the outer diameter of the second diaphragm is attached on the upper surface of the resin magnet of the electromagnetic electro-acoustic transducer of the first embodiment. As the magnetic resistance can be further reduced with this construction, a higher magnetic flux density can be obtained thereby making it possible to increase the compounding ratio of the soft magnetic material and enabling expansion of controllable ranges of the lowest resonant frequency and the sound pressure. 
     A sixth embodiment of the present invention is one in which the ratio of the soft magnetic material powder in the resin magnet of the electromagnetic electro-acoustic transducer of the first to the fifth embodiments is in the range 15 to 30% by weight and is capable of providing an transducer employing a superior resin magnet with a practical magnetic flux density and a high magnetic permeability. 
     A seventh embodiment of the present invention disposes a first diaphragm, a second diaphragm attached at the center of the first diaphragm and consisting of a magnetic material smaller than the first diaphragm, a center pole provided underneath the center of the second diaphragm with a magnetic gap in between, a coil wound around the outer periphery of the center pole, a ring-shaped magnet disposed on the outside of the coil, a yoke disposed in a manner such that it comes in contact with the lower parts of the center pole, and a ring-shaped magnetic plate having an inner diameter designed in a manner such that the magnetic flux from the second diaphragm enters generally vertically. This embodiment provides an electromagnetic electro-acoustic transducer in which a magnetic circuit is formed in a manner such that the magnetic flux of the magnet returns to the magnet through the yoke, center pole, second diaphragm and the magnetic plate, so that magnetic resistance is reduced by going through the magnetic plate, magnetic flux density is enhanced, and the attraction force toward the second diaphragm is almost maximized thus improving magnetic efficiency. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of an exemplary embodiment of the electromagnetic electro-acoustic transducer of the present invention. FIG. 2 is a cross-sectional view of a molding equipment for molding a resin magnet to be used in the transducer of the present invention. FIG. 3 is the graph plots illustrating energy of magnet, inductance, and lowest resonant frequency of the resin magnet as a function of the compounding ratio of the soft magnetic material in the resin magnet. FIG. 4 is a cross-sectional view of an extended example of the transducer of the present invention. FIG. 5 is a top view (a) and a cross-sectional view (b) of an conventional electromagnetic electro-acoustic transducer. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 1 to  4 , a description will be given on exemplary embodiments of the present invention. 
     FIG. 1 is a cross-sectional view of the electromagnetic electro-acoustic transducer in an exemplary embodiment of the present invention. FIG. 2 is a cross-sectional view of a molding equipment for the resin magnet, which is an essential part of the transducer in an exemplary embodiment of the present invention. FIG. 3 is the characteristics illustrating energy of magnet, inductance, and lowest resonant frequency of the resin magnet as a function of the compounding ratio of hard and soft magnetic materials. FIG. 4 is a cross-sectional view of a transducer in a first exemplary embodiment of the present invention. 
     As the difference between the exemplary embodiments of the present invention and prior art lies in the composition of the resin magnet and the magnetic plate, a description on the difference will be given below. 
     In FIG. 1, a resin magnet  11  is produced through mixing of soft magnetic powder hard magnetic powder and resin as binder, subsequently molding process and magnetizing process. 
     Here, a hard magnetic material generally means a magnetic material that is not easily influenced by an external magnetic field. In this exemplary embodiment, Sr ferrite is used. Ferrite magnetic powder can provide a high magnetic flux density when injection molded while aligning the magnetic orientation as described later. A soft magnetic material generally means a magnetic material that is easily influenced by an external magnetic field. In this exemplary embodiment, MgZn ferrite is used. The particle size being several μm, MgZn ferrite is easily made into a composite, and provides: a high magnetic permeability. 
     As the molding resin, polyamide resins such as nylon  6  and nylon  12  are used as they have a high orientation efficiency during injection molding while aligning the magnetic orientation. From the standpoint of moldability and magnetic characteristic, it is appropriate to set the amount of the total magnetic powder at 85 to 92% by weight and the remainder for the molding resin. 
     Numeral  14  is a magnetic plate attached integrally with the resin magnet  11  on the resin magnet  11  by insertion molding. As is clear from FIG. 1, the inner diameter of the magnetic plate  14  is made smaller than the outer diameter of the second diaphragm  101 . 
     With this configuration, a magnetic circuit is formed in which the magnetic flux of the resin magnet  11  returns to the resin magnet  11  going through a yoke  106 , center plate  103 , second diaphragm  101 , and magnetic plate  14 . By going through the magnetic plate  14 , the magnetic resistance is reduced and the magnetic flux density can be enhanced. Though the inner diameter of the magnetic plate  14  is smaller than the outer diameter of the second diaphragm  101 , when the inner diameter is too small, either the magnetic flux between the second diaphragm  101  and the magnetic plate  14  diffuses, or in the extreme case, the magnetic flux between the center pole  103  and the magnetic plate  14  disappear , and the magnetic force for attracting the second diaphragm  101  to the center pole  103  becomes weak. Consequently, the inner diameter of the magnetic plate  14  is set in a manner such that the magnetic flux between the second diaphragm  101  and the magnetic plate  14  will not diffuse but will generally be in a vertical position (a position in which the attraction force toward the second diaphragm  101  is roughly at the maximum). 
     Also, as the magnetic plate  14  is attached with the resin magnet  11  by insertion molding, its positional relation to the second diaphragm  101  is determined through the housing  107  with a small variation in manufacturing ,thereby attraction force toward the second diaphragm  101  is stabilized and, accordingly, the variation of sound pressure of the electromagnetic electro-acoustic transducer is also well controlled. 
     Next, FIG. 2 is a molding equipment for fabricating the resin magnet  11 , where a molding die  5  is provided with a magnetic material  3  for aligning the magnetic orientation in the direction of the thickness and a non-magnetic material  4 . A resin magnet  2  for aligning is disposed inside the molding die  5 , a compound of a resin, powder of,a hard magnetic material, and powder of a soft magnetic material is filled inside a cavity  6 , and subsequently heated and injection molded to obtain a resin magnet. (During this process, the magnetic plate  14  is set inside the molding die  5  and is integrally molded.) 
     FIG. 3 plots the energy of magnet, inductance, and lowest resonant frequency as a function of the compounding ratio of MgZn ferrite used as the soft magnetic material in the resin magnet  11 . As it is difficult to directly measure the magnetic permeability of the resin magnet, inductance is measured in place of the magnetic permeability (magnetic permeability tends to increase proportionally with increasing inductance ). From the plots, it is found that, with an increase in the compounding ratio of the, soft magnetic material, the energy (BHmax) of the resin magnet decreases, the lowest resonant frequency (f 0 ) decreases, and the inductance increases (magnetic resistance decreases and magnetic permeability increases). 
     It is also confirmed that, as electromagnetic electro-acoustic transducers are small with an approximate size of 12 mm square, and the designed regeneration acoustic frequency is in the range 2.5 KHz to 3.5 KHz, the practical compounding ratio of the soft magnetic material in the resin magnet  11  is in the range 15 to 30%. 
     That is, when the compounding ratio of the soft magnetic material is less than 15%, enough magnetic permeability is not obtained, while, when it is more than 30%, the magnetic flux density becomes low and the lowest resonant frequency becomes too low. From the above, it is possible to continuously change the static attraction force by changing the ratio of the soft magnetic material and to easily control the lowest resonant frequency. Also, by mixing a soft magnetic material, it is possible to increase the magnetic permeability of the resin magnet  11  and to decrease the magnetic resistance of the resin magnet  11 , thereby increasing the magnetic flux density for an alternating magnetic field and increasing the driving force exerted to the diaphragm. 
     It is to be noted that, by further providing the magnetic plate  14 , the magnetic resistance can be reduced and the magnetic flux density can be increased, thus attaining a higher sound pressure. 
     FIG. 4 is an extended example of the present exemplary embodiment. Here, though a resin magnet  11   a  made by mixing a hard magnetic material and a soft magnetic material similar to the electromagnetic electro-acoustic transducer of FIG. 1 is used, a difference lies in that it does not use a magnetic plate  14 . When compared with the transducer of FIG. 1, a decrease in the magnetic resistance between a second diaphragm  101  and the resin magnet  11   a  cannot be obtained as it does not have a magnetic plate  14 . However, it is easy to set the lowest resonant frequency and it is possible to obtain a higher sound pressure and a lower cost by the use of the resin magnet  11   a  prepared by mixing a hard magnetic material and a soft magnetic material. 
     In the above exemplary embodiments, although a description has been made on the use of Sr ferrite as the hard magnetic (powder) material, a resin magnet that has a higher magnetic flux density than that using Sr ferrite alone can be obtained by using a eutectic;compound of a rare-earth magnetic material and Sr ferrite. Here, as the rare-earth magnetic material, a nano-composite magnetic material such as Nd—Fe—B magnetic material can be used. 
     INDUSTRIAL APPLICABILITY 
     By combining a hard magnetic material and a soft magnetic material as the magnetic material of a resin magnet as has been described above, the present invention provides an electromagnetic electro-acoustic transducer of which the magnetic permeability is high, the lowest resonant frequency can be easily set by continuously changing the magnetic flux density depending on the compounding ratio of the soft magnetic material, and the sound pressure is high and narrow variation at high sound pressure range.