Patent Publication Number: US-6907955-B2

Title: Electromagnetic electroacoustic transducer

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an electromagnetic electroacoustic transducer and particularly to a configuration for attaining improvement in frequency characteristic of the electromagnetic electroacoustic transducer. 
   2. Background Art 
   Generally, an electromagnetic electroacoustic transducer includes a diaphragm made of a magnetic material, a magnet for generating a magnetostatic field to make the magnetostatic field act on the diaphragm, an electromagnetic coil for generating an oscillating magnetic field corresponding to an electric signal to make the oscillating magnetic field act on the diaphragm, and a casing for storing the diaphragm, the magnet and the electromagnetic coil therein. The electromagnetic electroacoustic transducer is formed so that an electric signal is converted into an acoustic signal by an electromagnetic transducer function. 
   In the electromagnetic electroacoustic transducer, a sound emitting hole through which a front space on a front surface of the diaphragm communicates with a front outer space in front of the casing is formed in the casing so that sound generated by vibration of the diaphragm is radiated to the front outer space in front of the casing by the sound emitting hole. On this occasion, if a rear space on a rear surface of the diaphragm is closed, sound pressure has a tendency toward decrease because an air damping effect prevents the diaphragm from vibrating sufficiently up to its vibration limit. Particularly when the size of the electromagnetic electroacoustic transducer is reduced, this tendency becomes strong. 
   Therefore, for example, as described in JPA9-149494, there has been heretofore proposed an idea that a second sound emitting hole through which a rear space on a rear surface of the diaphragm communicates with an outer space outside the casing is additionally formed in the casing to reduce air pressure of the rear space to thereby prevent reduction of sound pressure. 
   On this occasion, when the second sound emitting hole is formed so as to communicate with a front outer space in front of the casing, for example, as described in JPY1-16155, improvement in sound pressure can be attained by a resonance effect of the rear space on the rear surface of the diaphragm. 
   In JPY1-16155, no description is made on specific configuration for obtaining the resonance effect of the rear space on the rear surface of the diaphragm. On this occasion, a sufficient resonance effect cannot be obtained by only making the second sound emitting hole communicate with the front outer space in front of the casing, so that improvement in frequency characteristic of the electromagnetic electroacoustic transducer cannot be attained. 
   SUMMARY OF THE INVENTION 
   The invention is developed in consideration of such circumstances and an object of the invention is to provide an electromagnetic electroacoustic transducer effectively using a resonance effect of a rear space on a rear surface of a diaphragm for attaining improvement in frequency characteristic. 
   To achieve the object, the invention provides an electromagnetic electroacoustic transducer, including: a diaphragm made of a magnetic material; a magnet for generating a magnetostatic field to make the magnetostatic field act on said diaphragm; an electromagnetic coil for generating an oscillating magnetic field corresponding to an electric signal to make the oscillating magnetic field act on the diaphragm; and a casing for storing the diaphragm, the magnet and the electromagnetic coil therein; wherein the case has at least one first sound emitting hole through which a front space on a front surface of the diaphragm in the casing communicates with a front outer space in front of the casing and at least one second sound emitting hole through which a rear space on a rear surface of the diaphragm in the casing communicates with the front outer space in front of the casing; and a resonant frequency Fv 2  of the rear space is set at a value in the range: 
     F   0   &lt;Fv   2   ≦Fv   1   
   in which F 0  is a resonant frequency of the diaphragm, and Fv 1  is a resonant frequency of the front space. 
   The specific configuration of the “first sound emitting hole”, such as the place where the first sound emitting hole is formed, the opening shape of the first sound emitting hole, the opening size of the first sound emitting hole and the number of first sound emitting holes to be formed, is not particularly limited if the first sound emitting hole is formed so that the front space on the front surface of the diaphragm in the casing can communicate with the front outer space in front of the casing through the first sound emitting hole. 
   The specific configuration of the “second sound emitting hole”, such as the place where the second sound emitting hole is formed, the opening shape of the second sound emitting hole, the opening size of the second sound emitting hole and the number of second sound emitting holes to be formed, is not particularly limited if the second sound emitting hole is formed so that the rear space on the rear surface of the diaphragm in the casing can communicate with the front outer space in front of the casing through the second sound emitting hole, and that the resonant frequency Fv 2  of the rear space can be set at a value in the aforementioned range. 
   As described in the aforementioned configuration, the electromagnetic electroacoustic transducer according to the invention is formed in the casing in which the diaphragm, the magnet and the electromagnetic coil. In the casing, at least one first sound emitting hole through which a front space on a front surface of the diaphragm communicates with a front outer space in front of the casing and at least one second sound emitting hole through which a rear space on a rear surface of the diaphragm communicates with the front outer space in front of the casing are formed. The resonant frequency Fv 2  of the rear space on the rear surface of the diaphragm is set a value in the range: F 0 &lt;Fv 2 ≦Fv 1  in which F 0  is the resonant frequency of the diaphragm, and Fv 1  is the resonant frequency of the front space on the front surface of the diaphragm. Accordingly, the following operation and effect can be obtained. 
   That is, generally, in the electromagnetic electroacoustic transducer, a frequency slightly higher than the resonant frequency F 0  of the diaphragm is set as a standard frequency Fs which is a standard for activating the electromagnetic electroacoustic transducer. Sound pressure obtained by activating of the electromagnetic electroacoustic transducer at the standard frequency Fs is generated by superposition of a second harmonic of 2×Fs, a third harmonic of 3×Fs and further higher harmonics on a fundamental wave component (first harmonic) of the standard frequency Fs. 
   Generally, in the electromagnetic electroacoustic transducer, the resonant frequency Fv 1  of the front space on the front surface of the diaphragm is set at a value higher by a certain degree than the resonant frequency F 0  of the diaphragm. The resonant frequency Fv 1  may be set at a suitable value so that improvement of sound pressure or band spreading of frequency characteristic at the standard frequency Fs can be attained. 
   Therefore, when the resonant frequency Fv 2  of the rear space on the rear surface of the diaphragm is set at a value higher than the resonant frequency F 0  of the diaphragm but not higher than the resonant frequency Fv 1  of the front space on the front surface of the diaphragm according to the invention, a drop in sound pressure at a frequency band between the resonant frequency F 0  and the resonant frequency Fv 1  can be corrected to attain flattening of frequency characteristic. Furthermore, when the resonant frequency Fv 2  is set as described above, flattening of frequency characteristic in a frequency band lower than the resonant frequency F 0  can also be attained by a function of superposition of harmonics of the resonant frequency Fv 2 . 
   As described above, in accordance with the invention, the resonance effect of the rear space on the rear surface of the diaphragm can be used effectively for attaining improvement in frequency characteristic of the electromagnetic electroacoustic transducer. 
   On this occasion, when the resonant frequency Fv 2  is set at a value in the range Fv 2 ≧1.2×F 0 , a drop in sound pressure in the frequency band between the resonant frequency F 0  and the resonant frequency Fv 1  can be corrected effectively to attain sufficient flattening of frequency characteristic. 
   In this configuration, when the resonant frequency Fv 2  is set at a value near a frequency equal to an integral multiple of the resonant frequency F 0 , sound pressure at the resonant frequency F 0  can be improved by a function of superposition of harmonics of the resonant frequency Fv 2  to thereby improve sound pressure at the standard frequency Fs. 
   In this configuration, when the resonant frequency Fv 1  is set at a value near a frequency three times as high as the resonant frequency F 0  while the resonant frequency Fv 2  is set at a value near a frequency twice as high as the resonant frequency F 0 , sound pressure at the resonant frequency F 0  can be improved greatly by a function of superposition of the third harmonic with the resonant frequency Fv 1  and the second harmonic with the resonant frequency Fv 2  to thereby improve sound pressure at the standard frequency Fs greatly. Furthermore, when the resonant frequencies Fv 1  and Fv 2  are set as described above, a drop in sound pressure in the frequency band between the resonant frequency F 0  and the resonant frequency Fv 1  can be corrected greatly to attain flattening of frequency characteristic effectively. In addition, in this case, flattening of frequency characteristic in a frequency band lower than the resonant frequency F 0  can also be attained effectively by a function of superposition of higher harmonics of the resonant frequency Fv 2 . 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention may be more readily described with reference to the accompanying drawings: 
       FIG. 1  is a front view of an electromagnetic electroacoustic transducer according to an embodiment of the invention in the case where the electromagnetic electroacoustic transducer is disposed so as to face upward. 
       FIG. 2  is a detailed sectional view taken along the line II—II in FIG.  1 . 
       FIG. 3  is a front view of the electromagnetic electroacoustic transducer in the case where a front casing is removed. 
       FIG. 4  is a detailed sectional view showing a first comparative example of the electromagnetic electroacoustic transducer. 
       FIG. 5  is a detailed sectional view showing a second comparative example of the electromagnetic electroacoustic transducer. 
       FIG. 6  is a graph showing a measured result of sound pressure level-frequency characteristic of the electromagnetic electroacoustic transducer in comparison with measured results of sound pressure-frequency characteristics of the first and second comparative examples. 
       FIG. 7  is a graph showing a measured result of sound pressure level-frequency characteristic of the electromagnetic electroacoustic transducer in connection with waveform components of the sound pressure level-frequency characteristic. 
       FIG. 8  is a graph showing a measured result of sound pressure level-frequency characteristic of the first comparative example in connection with waveform components of the sound pressure level-frequency characteristic. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An embodiment of the invention will be described below with reference to the drawings. 
     FIG. 1  is a front view of an electromagnetic electroacoustic transducer  10  according to an embodiment of the invention in the case where the electromagnetic electroacoustic transducer  10  is disposed so as to face upward.  FIG. 2  is a sectional view taken along the line II—II in FIG.  1 .  FIG. 3  is a front view of the electromagnetic electroacoustic transducer  10  in the case where a front casing  18 A is removed. 
   As shown in  FIGS. 1  to  3 , the electromagnetic electroacoustic transducer  10  according to this embodiment includes a diaphragm  12  made of a magnetic material, a magnet  14  for generating a magnetostatic field to make the magnetostatic field act on the diaphragm  12 , an electromagnetic coil  16  for generating an oscillating magnetic field corresponding to an electric signal to make the oscillating magnetic field act on the diaphragm  12 , and a casing  18  in which the diaphragm  12 , the magnet  14  and the electromagnetic coil  16  are stored. The electromagnetic electroacoustic transducer  10  is formed so that an electric signal is converted into an acoustic signal by an electromagnetic transducer function. 
   The casing  18  includes a front casing  18 A, and a rear casing  18 B. The casing  18  is substantially square-shaped, having several millimeters sides but having one chamfered corner in front view. 
   A pole piece  22  is mounted on an inner rear surface of the rear casing  18 B. The pole piece  22  has a plate-shaped base  22 A in the shape of near a circle whose arc is partially cut, and an iron core  22 B formed so as to be integrated with the base  22 A and protrude frontward from the center portion of the base  22 A. The iron core  22 B of the pole piece  22  is wound with a coil  24  to thereby form the electromagnetic coil  16 . 
   The ring-shaped magnet  14  is disposed on the outer circumferential side of the coil  24  on a front surface of the base  22 A of the pole piece  22  so that an annular space is formed between the magnet  14  and the coil  24 . A retaining ring  26  for retaining the magnet  14  concentrically with the iron core  22 B is disposed on the outer circumferential side of the magnet  14 . 
   A concave step portion  26   a  is formed on the whole circumference at an inner circumferential front end portion of the retaining ring  26 . An outer circumferential edge portion of the diaphragm  12  is supported at the concave step portion  26   a.  The diaphragm  12  has a magnetic piece  12 A as an additional mass in its front center portion. The diaphragm  12  is disposed so that the diaphragm  12  is attracted rearward and slightly warped by the action of a magnetostatic field formed on the basis of magnetic flux provided from the magnet  14 . 
   A pin  18   c  for preventing the diaphragm  12  from dropping out because of impact load or other reasons at the time of the fall of the electromagnetic electroacoustic transducer  10  is formed in the front casing  18 A so as to face the magnetic piece  12 A of the diaphragm  12 . An annular wall  18 d for positioning and fixing the retaining ring  26  concentrically with the iron core  22 B is formed in the front casing  18 A. 
   First and second sound emitting holes  18   a  and  18   b  are formed in a front wall of the front casing  18 A. In this embodiment, one first sound emitting hole  18   a  is formed at a place near the pin  18   c  whereas two second sound emitting holes  18   b  are formed in two corner portions respectively. The first sound emitting hole  18   a  is provided so that a front space  2  on a front surface of the diaphragm  12  in the casing  18  communicates with a front outer space  6  in front of the casing  18  through the first sound emitting hole  18   a.  The second sound emitting holes  18   b  are provided so that a rear space  4  on a rear surface of the diaphragm  12  in the casing  18  communicates with the front outer space  6  through the second sound emitting holes  18   b.  Two spaces located in the corner portions on the outer circumferential side of the annular wall  18   d  form communicating spaces  4   a  in the front casing  18 A so that the second sound emitting holes  18   b  communicate with the rear space  4  through the communicating spaces  4   a.  Incidentally, the communicating spaces  4   a  communicate with the rear space  4  through a communicating space  4   b  which is formed on a side of the cut portion of the base  22 A of the pole piece  22  so as to have a thickness equal to the thickness of the base  22 A. 
   Lead terminals  28  are provided in two corner portions of the rear casing  18 B corresponding to the aforementioned two corner portions. The lead terminals  28  are formed so as to be integrated with the rear casing  18 B in a state in which the lead terminals  28  are partially buried in the rear casing  18 B by insert molding. One end portion  28   a  of each lead terminal  28  is formed so as to extend from a rear wall outer surface of the rear casing  18 B to a side wall outer surface of the rear casing  18 B. The other end portion  28   b  of each lead terminal  28  is formed so as to protrude from a rear wall inner surface of the rear casing  18 B toward the communicating space  4   a  in each corner portion of the rear casing  18 B. A pair of coil terminals  24   a  led out from the coil  24  are soldered to the other end portions  28   b  of the lead terminals  28  respectively in a state in which the pair of coil terminals  24   a  are tied to the other end portions  28   b  respectively. Incidentally, a dummy terminal  30  is provided in another corner portion of the rear casing  18 B. 
   In the electromagnetic electroacoustic transducer  10  according to this embodiment, when a current is applied to the coil  24  through the pair of lead terminals  28 , the iron core  22 B serves as an electromagnet for generating a magnetic field at its end. On this occasion, if the magnetic pole generated in the iron core  22 B by the coil  24  is opposite to the magnetic pole generated in the diaphragm  12  by the magnet  14 , the diaphragm  12  is attracted toward the iron core  22 B. On the other hand, if the magnetic pole generated in the iron core  22 B by the coil  24  is equal to the magnetic pole generated in the diaphragm  12  by the magnet  14 , the diaphragm  12  and the iron core  22 B repel each other. Accordingly, when an electric signal intermittent with a predetermined frequency is input into the coil  24 , an intermittent magnetic field is generated at an end of the iron core  22 B to vibrate the diaphragm  12  to thereby produce sound with a sound pressure corresponding to the amplitude of vibration. 
   The electromagnetic electroacoustic transducer  10  is formed so that the sound produced by vibration of the diaphragm  12  is radiated from the front space  2  to the front outer space  6  in front of the casing  18  through the first sound emitting hole  18   a  and from the rear space  4  to the front outer space  6  in front of the casing  18  through the second sound emitting holes  18   b.  In this manner, improvement in sound pressure is attained by the resonance effect of the front space  2  and the resonance effect of the rear space  4 . 
   On this occasion, the resonant frequency Fv 1  of the front space  2  is set at a value three times as high as the resonant frequency F 0  of the diaphragm  12 , and the resonant frequency Fv 2  of the rear space  4  is set at a value twice as high as the resonant frequency F 0  of the diaphragm  12 . Specifically, the resonant frequency F 0  of the diaphragm  12 , the resonant frequency Fv 1  of the front space  2  and the resonant frequency Fv 2  of the rear space  4  are set at 4,000 Hz, 12,000 Hz and 8,000 Hz respectively. 
   The standard frequency Fs of the electromagnetic electroacoustic transducer  10  is set at a value (e.g., about 4,200 Hz) slightly higher than the resonant frequency F 0 . This is based on the following reason. If the standard frequency Fs is selected to be in a frequency band lower than the resonant frequency F 0 , the sound pressure level in the neighborhood of the resonant frequency F 0  is reduced suddenly when the standard frequency Fs becomes slightly lower than the resonant frequency F 0 . On the contrary, if the standard frequency Fs is selected to be in a frequency band higher than the resonant frequency F 0 , a drop in sound pressure level in the neighborhood of the resonant frequency F 0  is gentle. Thus, setting of the standard frequency Fs at a value slightly higher than the resonant frequency F 0  results in reduction of the influence of the shift of the resonant frequency F 0  on the drop in sound pressure. Accordingly, the sound pressure of the electromagnetic electroacoustic transducer  10  can be stabilized to obtain a good yield of products. 
   Incidentally, the resonant frequencies Fv 1  and Fv 2  can be set at required values, for example, by suitable adjustment of opening sizes of the first and second sound emitting holes  18   a  and  18   b.    
     FIG. 6  is a graph showing a measured result of sound pressure level-frequency characteristic of the electromagnetic electroacoustic transducer  10  according to this embodiment in comparison with measured results of sound pressure level-frequency characteristics of first and second comparative examples. The configurations of the first and second comparative examples will be described before the description of the graph. 
   The first comparative example assumes a prior art electromagnetic electroacoustic transducer having a closed rear space. As shown in  FIG. 4 , the electromagnetic electroacoustic transducer  110  according to the first comparative example has the same configuration as the electromagnetic electroacoustic transducer  10  according to this embodiment except that the rear space  4  is closed without formation of any second sound emitting holes  18   b.    
   On the other hand, the second comparative example assumes a prior art electromagnetic electroacoustic transducer having an opened rear space. As shown in  FIG. 5 , in the electromagnetic electroacoustic transducer  210  according to the second comparative example, a second sound emitting hole  18   e  is formed instead of the second sound emitting holes  18   b  of the electromagnetic electroacoustic transducer  10  according to this embodiment. The second sound emitting hole  18   e  is provided for reducing air pressure of the rear space  4  but not for making the rear space  4  communicate with the front outer space  6 . In  FIG. 5 , the casing  18  of the electromagnetic electroacoustic transducer  210  mounted on a board  202  is brought into contact with a housing  204  of an external apparatus (e.g., a cellular phone) through a gasket  206  to thereby prevent the second sound emitting hole  18   e  from communicating with the front outer space  6 . 
   In  FIG. 6 , the thick solid line curve shows sound pressure level-frequency characteristic of the electromagnetic electroacoustic transducer  10  according to this embodiment, the broken line curve shows sound pressure level-frequency characteristic of the electromagnetic electroacoustic transducer  110  according to the first comparative example, and the thin solid line curve shows sound pressure level-frequency characteristic of the electromagnetic electroacoustic transducer  210  according to the second comparative example. 
   As described above, the resonant frequency F 0  of the diaphragm  12  and the resonant frequency Fv 1  of the front space  2  are set at 4,000 Hz and 12,000 Hz respectively. Accordingly, each of the three curves in  FIG. 6  has sound pressure peaks at in the neighborhoods of 4,000 Hz and 12,000 Hz. 
   In the electromagnetic electroacoustic transducer  110  according to the first comparative example, the rear space  4  is however closed so that the resonance effect of the rear space  4  cannot be obtained. For this reason, sound pressure in a frequency band between the resonant frequency F 0  and the resonant frequency Fv 1  is reduced remarkably. Furthermore, sound pressure is reduced as a whole because the air damping effect of the rear space  4  prevents the diaphragm  12  from vibrating sufficiently up to the vibration limit. 
   On the other hand, in the electromagnetic electroacoustic transducer  210  according to the second comparative example, the rear space  4  is opened by the second sound emitting hole  18   e  so that the influence of the air damping effect is eliminated. It is however impossible to obtain the resonance effect of the rear space  4  because the rear space  4  is isolated from the front outer space  6  in front of the casing  18 . For this reason, sound pressure slightly higher than that in the first comparative example as a whole can be obtained but sound pressure in the frequency band between the resonant frequency F 0  and the resonant frequency Fv 1  is reduced remarkably. 
   On the contrary, in the electromagnetic electroacoustic transducer  10  according to this embodiment, the resonance effect of the rear space  4  can be obtained because the rear space  4  communicates with the front outer space  6  through the second sound emitting holes  18   b.  On this occasion, because the resonant frequency Fv 2  of the rear space  4  is set at a median between the resonant frequency F 0  and the resonant frequency Fv 1 , the electromagnetic electroacoustic transducer  10  according to this embodiment has a sound pressure peak in the neighborhood of 8,000 Hz as well as sound pressure peaks in the neighborhoods of 4,000 Hz and 12,000 Hz. For this reason, reduction in sound pressure in the frequency band between the resonant frequency F 0  and the resonant frequency Fv 1  is corrected greatly. 
     FIG. 7  is a graph showing the measured result of sound pressure level-frequency characteristic of the electromagnetic electroacoustic transducer  10  according to this embodiment as shown in  FIG. 6  in connection with waveform components of the sound pressure level-frequency characteristic.  FIG. 8  is a graph showing the measured result of sound pressure level-frequency characteristic of the electromagnetic electroacoustic transducer  110  according to the first comparative example as shown in  FIG. 6  in connection with waveform components of the sound pressure level-frequency characteristic. 
   As shown in each of  FIGS. 7 and 8 , the sound pressure level-frequency characteristic of each electromagnetic electroacoustic transducer  10 ,  110  is obtained by superposition of a fundamental wave component (first harmonic) represented by the broken line, a second harmonic represented by the slightly thin broken line, a third harmonic represented by the thin solid line and further higher harmonics. The sound pressure produced at the time of activating of each electromagnetic electroacoustic transducer  10 ,  110  at the resonant frequency F 0  is obtained by superposition of the second harmonic of 2×F 0 , the third harmonic of 3×F 0  and further higher harmonics on the fundamental wave component of the resonant frequency F 0 . 
   As shown in  FIG. 7 , in the electromagnetic electroacoustic transducer  10  according to this embodiment, because the resonant frequencies Fv 1  and Fv 2  are set at 3×F 0  and 2×F 0  respectively, a sufficiently high sound pressure at the resonant frequency F 0  can be ensured on the basis of the third harmonic with the resonant frequency Fv 1  and the second harmonic with the resonant frequency Fv 2 . Accordingly, when the electromagnetic electroacoustic transducer  10  is activated at the standard frequency Fs slightly higher than the resonant frequency F 0 , a sufficiently high sound pressure can be ensured because the third harmonic with the resonant frequency Fv 1  and the second harmonic with the resonant frequency Fv 2  are superposed on the fundamental wave component. 
   On the contrary, as shown in  FIG. 8 , in the electromagnetic electroacoustic transducer  110  according to the first comparative example, only the third harmonic with the resonant frequency Fv 1  set at 3×F 0  is superposed on the fundamental wave component because the resonance effect of the rear space  4  cannot be obtained. For this reason, a sufficient high sound pressure at the resonant frequency F 0  cannot be ensured. Accordingly, a sufficiently high sound pressure at the standard frequency Fs cannot be ensured. 
   As described above, the electromagnetic electroacoustic transducer  10  according to this embodiment has a sound pressure peak at the resonant frequency Fv 2  set at a median between the resonant frequency F 0  and the resonant frequency Fv 1 , so that reduction in sound pressure in the frequency band between the resonant frequency F 0  and the resonant frequency Fv 1  is corrected greatly. As shown in  FIG. 6 , even in a frequency band lower than the resonant frequency F 0 , flattening of frequency characteristic in a wide range can be attained by superposition of harmonics of the resonant frequency Fv 2 . Accordingly, in the electromagnetic electroacoustic transducer  10  according to this embodiment, when, for example, a melodic alarm is sounded, the melodic alarm can be reproduced smoothly with a small difference between the high level and the low level of sound pressure. 
   On the contrary, in the electromagnetic electroacoustic transducer  110  according to the first comparative example, a frequency band lower than the resonant frequency F 0  is affected by reduction in sound pressure in the frequency band between the resonant frequency F 0  and the resonant frequency Fv 1 . For this reason, the difference between the high level and the low level of sound pressure becomes large. Accordingly, melody reproduction cannot be made smoothly with a small difference between the high level and the low level of sound pressure. 
   In this respect, the electromagnetic electroacoustic transducer  210  according to the second comparative example has substantially the same tendency though the electromagnetic electroacoustic transducer  210  according to the second comparative example is more or less improved compared with the electromagnetic electroacoustic transducer  110  according to the first comparative example. 
   As described above in detail, the electromagnetic electroacoustic transducer  10  according to this embodiment is formed so that the first sound emitting hole  18   a  for making the front space  2  on the front surface of the diaphragm  12  communicate with the front outer space  6  in front of the casing  18  and the second sound emitting holes  18   b  for making the rear space  4  on the rear surface of the diaphragm  12  communicate with the front outer space  6  in front of the casing  18  are formed in the casing  18  in which the diaphragm  12 , the magnet  14  and the electromagnetic coil  16  are stored. The resonant frequency Fv 2  of the rear space  4  on the rear surface of the diaphragm  12  is set at a value in the range F 0 &lt;Fv 2 ≦Fv 1  in which F 0  is the resonant frequency of the diaphragm  12 , and Fv 1  is the resonant frequency of the front space  2  on the front surface of the diaphragm  12 . Accordingly, reduction in sound pressure in the frequency band between the resonant frequency F 0  and the resonant frequency Fv 1  can be corrected to thereby attain flattening of frequency characteristic. Furthermore, when the resonant frequencies are set in this manner, flattening of frequency characteristic even in a frequency band lower than the resonant frequency F 0  can be attained by a function of superposition of harmonics of the resonant frequency Fv 2 . 
   As described above, in accordance with this embodiment, the resonance effect of the rear space  4  on the rear surface of the diaphragm  12  can be effectively used for attaining improvement in frequency characteristic of the electromagnetic electroacoustic transducer  10 . 
   Particularly in this embodiment, the resonant frequency Fv 1  is set at a value three times as high as the resonant frequency F 0 , and the resonant frequency Fv 2  is set at a value twice as high as the resonant frequency F 0 . Accordingly, sound pressure at the resonant frequency F 0  can be improved greatly by a function of superposition of the third harmonic with the resonant frequency Fv 1  and the second harmonic with the resonant frequency Fv 2 . Accordingly, sound pressure at the standard frequency Fs can be improved greatly. Furthermore, when the resonant frequencies are set in this manner, reduction in sound pressure in the frequency band between the resonant frequency F 0  and the resonant frequency Fv 1  can be corrected greatly to thereby attain flattening of frequency characteristic effectively. In addition, flattening of frequency characteristic even in a frequency band lower than the resonant frequency F 0  can be attained effectively by a function of superposition of higher harmonics of the resonant frequency Fv 2 . 
   Particularly when flattening of frequency characteristic of the electromagnetic electroacoustic transducer is attained according to this embodiment, an electroacoustic transducer having the same flat frequency characteristic as an electrodynamic electroacoustic transducer can be achieved while the characteristic of the electromagnetic electroacoustic transducer higher in sound pressure than the electrodynamic electroacoustic transducer is maintained. 
   Although this embodiment has been described on the case where the resonant frequencies Fv 1  and Fv 2  are set at a frequency three times as high as the resonant frequency F 0  and a frequency twice as high as the resonant frequency F 0  respectively, the invention may be also applied to the case where the resonant frequencies Fv 1  and Fv 2  are not accurately set at frequencies equal to integral multiples of F 0 . For example, substantially the same operation and effect as in this embodiment can be obtained if each resonant frequency Fv 1 , Fv 2  is set at a value near a frequency equal to an integral multiple of F 0 , specifically at a value in a range of ±10% as high as a frequency equal to an integral multiple of F 0 . 
   Furthermore, when the resonant frequency Fv 2  is set not at a value near a frequency twice as high as the resonant frequency F 0  but at a value near the resonant frequency F 0  or a value near a frequency three times as high as the resonant frequency F 0 , sound pressure at the resonant frequency F 0  can be improved by a function of superposition of the resonant frequency Fv 2  or harmonics of the resonant frequency Fv 2 . Accordingly, sound pressure at the standard frequency Fs can be improved. 
   Even in the case where the resonant frequency Fv 2  is not set at a value near a frequency equal to an integral multiple of the resonant frequency F 0 , reduction in sound pressure in the frequency band between the resonant frequency F 0  and the resonant frequency Fv 1  can be improved effectively to attain flattening of frequency characteristic sufficiently if the resonant frequency Fv 2  is set at a value in the range Fv 2 ≧1.2×F 0 . 
   Assuming now that the resonant frequency Fv 2  is set at a value satisfying the relation F 0 ≦Fv 2 &lt;1.2×F 0 , then the resonant frequency Fv 2  may superpose on the resonant frequency F 0  or the standard frequency Fs. As a result, frequency characteristic is so peaky that sound pressure is high only in the neighborhood of the resonant frequency F 0 . Accordingly, flattening of frequency characteristic cannot be attained. As described above, this is because sound pressure at the resonant frequency F 0  is made high by the effect of superposition when the resonant frequency Fv 2  is set at a value in a range of ±10% as high as an integral multiple (in this case, Fv 2 =F 0 ) of the resonant frequency F 0 . 
   Sound pressure in a frequency band lower than the resonant frequency F 0  is generated by superposition of harmonics in a frequency band not lower than the resonant frequency F 0  because the sound pressure level of the fundamental wave component is reduced extremely. For this reason, if the resonant frequency Fv 2  is set at a value satisfying the relation Fv 2 &lt;F 0 , flattening of frequency characteristic in all frequency bands cannot be attained because sound pressure of superposed harmonics is reduced when sound pressure in the frequency band between the resonant frequency F 0  and the resonant frequency Fv 1  is reduced remarkably. Furthermore, if the resonant frequency Fv 2  is set at a value in the range Fv 2 &lt;F 0 , the sound pressure level as a whole is finally reduced because the resonance effect at the resonant frequency Fv 2  is not superposed on the standard frequency Fs when the transducer is activated at the standard frequency Fs. 
   In this respect, when the resonant frequency Fv 2  is set at a value in the range Fv 2 ≧1.2×F 0  with respect to the resonant frequency F 0 , the aforementioned operation and effect can be obtained. 
   The relation between the resonant frequency Fv 1  and the resonant frequency Fv 2  may be set as follows. That is, when the resonant frequency Fv 1  is set at a value in a range of ±10% as high as an integral multiple of the resonant frequency F 0 , the resonance effect at the resonant frequency Fv 1  can appear. Accordingly, when the resonant frequency Fv 2  is set at a value in the range Fv 2 &lt;0.8×Fv 1  with respect to the resonant frequency Fv 1 , flattening of frequency characteristic can be attained more effectively. 
   Although the electromagnetic electroacoustic transducer  10  according to this embodiment is formed so that the first and second sound emitting holes  18   a  and  18   b  are formed in the front wall of the front casing  18 A, the first and second sound emitting holes  18   a  and  18   b  may be formed in a side wall of the front casing  18 A if the first and second sound emitting holes  18   a  and  18   b  can be located so as to face the front outer space  6 . Also in this case, the same operation and effect as in the embodiment can be obtained.