Broadband electromagnetic sound source with differently tuned diaphragms
FIELD OF INVENTION 
     The present invention relates to electroacoustic transducers especially for small-sized portable equipment such as a wristwatch and is concerned with broadening the frequency band of the transducer. 
     BACKGROUND OF THE INVENTION 
     Electroacoustic transducers used as a sound source for small-sized portable equipment such as wristwatches, pocket watches and paging systems utilize resonance in order to reduce power consumption but this has a disadvantage that the transducer responds to only a narrow frequency band. Moreover, by reason of manufacturing tolerances, it is difficult to match the frequency of the electroacoustic transducer with the frequency of the energizing circuit. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to broaden the frequency band of an electroacoustic transducer. In contrast with a conventional electroacoustic transducer having only one degree of freedom of mechanical vibration, the electroacoustic transducer of the present invention has two degrees of freedom of mechanical vibration and two mechanical resonance frequencies in order to broaden the frequency band by generating substantially constant acoustic pressure between the two mechanical resonance frequencies. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS: 
     The nature, objects and advantages of the invention will be more fully understood from the following description in conjunction with the accompanying drawings in which: 
     FIG. 1 is a schematic sectional view showing by way of example a conventional electroacoustic transducer, 
     FIG. 2 is a curve showing the drive frequency characteristics of the transducer of FIG. 1, 
     FIG. 3 is a schematic sectional view showing another conventional electroacoustic transducer, 
     FIG. 4 is a curve showing the drive frequency characteristics of the transducer of FIG. 3, 
     FIG. 5 is a schematic sectional view showing the structure of an essential portion of one embodiment of an electroacoustic transducer in accordance with the present invention for the purpose of describing the operation thereof, 
     FIG. 6 is a curve showing the drive frequency characteristics of the structure shown in FIG. 5, 
     FIG. 7 is a schematic view showing the mechanical equivalent of a simplified vibration system in accordance with the invention, 
     FIG. 8 shows curves illustrating the drive frequency characteristics of the structure shown in FIG. 7, 
     FIG. 9 is a schematic section of an electroacoustic transducer in accordance with the invention, and 
     FIG. 10 is a curve showing the drive frequency characteristics of the transducer shown in FIG. 9. 
    
    
     DESCRIPTION OF PRIOR ART 
     In FIG. 1 there is shown by way of example a conventional electroacoustic transducer comprising a magnetic core 1 with a center pole made of soft magnetic material, a coil 2 surrounding the center pole of the core 1, a permanent magnet 3 which surrounds the coil 2, a support 4 supporting a vibrating plate 5, and an armature 6 of soft magnetic material secured to the vibrating plate 5. When an alternating current is passed through the coil 2, attracting and repelling forces are alternately applied to the armature 6 to thereby vibrate the vibrating plate 5 to produce sound. This vibrating system fundamental has only one degree of freedom so that its acoustic pressure-drive frequency characteristic has a single resonance point as shown in FIG. 2 with the result that the frequency band Δf 1  producing a pressure higher than a predetermined acoustic pressure Po is quite narrow as illustrated in FIG. 2. 
     FIG. 3 is a schematic sectional view showing the structure of another conventional-type of electroacoustic transducer which broadens somewhat the frequency band. Components designated by reference numerals 1 to 6 are the same as those in FIG. 1. The electroacoustic transducer shown in FIG. 3 differs from that of FIG. 1 in that it has an acoustic case 7 with a sound radiating port 7a and a space 8 formed by the acoustic case 7 and the vibrating plate 5. The acoustic case 7, sound radiating port 7a and space 8 constitute in effect a Helmholtz resonator which provides the acoustic pressure-drive frequency characteristics shown in FIG. 4. It will be seen that the frequency band Δf 2  of the transducer shown in FIG. 3 is wider than the frequency band Δf 1  obtained by the transducer shown in FIG. 1. The electroacoustic transducer of the structure shown in FIG. 3 is sufficient to be used for one drive frequency within manufacturing tolerances but the structure does not satisfy the requirements in case sounds of different frequency are to be produced, for example sounds differing in frequency by one-half to one octave. 
     DESCRIPTION OF PREFERRED EMBODIMENT 
     The primary object of the present invention is to provide an electroacoustic transducer which will respond to a broad frequency band. The electroacoustic transducer in accordance with the invention will be described with reference to FIGS. 5 to 10. 
     FIG. 5 is a view for explaining the present invention and is a sectional view showing the structure of a mechanical vibration system having two degrees of freedom of mechanical vibration. The electroacoustic transducer shown in FIG. 5 comprises a magnetic core 1 with a center pole made of soft magnetic material, a coil 2 which surrounds the center pole of the core, a permanent magnet 3 which surrounds the coil 2 and a support 4 for a vibrating plate 5 having an armature 6 made of soft magnetic material secured to the vibrating plate 5 as in the electroacoustic transducer shown in FIG. 1. The electroacoustic transducer shown in FIG. 5 differs from that of FIG. 1 by the provision of a second vibrating plate 9 disposed opposite to the vibrating plate 5 and supported by an upwardly extending portion 4a of the support 4. The second vibrating plate 9 is coupled with the first vibrating plate 5 by an air spring comprising an airtight space 10 between the two vibrating plates. 
     In the electroacoustic transducer shown in FIG. 5, when an alternating current is passed through the coil 2, the vibrating plate 5 with armature 6 is electromagnetically driven and vibration of the vibrating plate 5 is transmitted to the second vibrating plate 9 through the air spring comprising the airtight space 10. The acoustic pressure-drive frequency characteristics of the electroacoustic transducer of FIG. 5 has two resonance points as illustrated in FIG. 6. 
     The acoustic pressure-drive characteristic features of the electroacoustic transducer shown in FIG. 5 will be described with reference to FIGS. 7 and 8. The vibration system of the structure shown in FIG. 5 can be converted into an equivalent mechanical vibration system illustrated in FIG. 7. The reference characters M, K and X M  respectively designate an equivalent mass, an equivalent spring constant and an amplitude of the first vibrating plate 5 with the armature 6. The characters &#34;m,&#34; &#34;k&#34; and &#34;x m  &#34; respectively designate an equivalent mass, an equivalent spring constant and the amplitude of the second vibrating plate 9 in FIG. 5. The character ka designates the air-spring constant of the space 10 in FIG. 5. These factors are essential components of the vibration system and although attenuating resistance, acoustic impedance and so forth must actually be considered they are here omitted for the convenience of a simplified description. 
     The solution of forced vibration in case an alternating force Fsinωt is applied to the mass M of such vibration system is indicated by the following equations: ##EQU1## 
     As will be understood from the above equations, two resonance points are produced as seen from the amplitude-drive frequency characteristics illustrated in FIG. 8. 
     It will be obvious to obtain the acoustic pressure in FIG. 6 taking note of the amplitude xm of the second vibrating plate 9 from FIG. 8. The resonance frequencies f 1  and f 2  shown in FIG. 8 can be made to approach one another by properly selecting the constants of the vibration system shown in FIG. 7 but they cannot be completely coincident unless the equivalent mass m becomes 0 (m=0). Hence, two resonance points are always produced. 
     The acoustic pressure is greater at the two resonance points of of such vibration system but only the frequency bands Δf 31  and Δf 32  as shown in FIG. 6 are normally useful and these bands are insufficient to attain the desired objectives. The essential point of the present invention is to compensate this by an acoustic effect in order sufficiently to broaden the frequency band as will be described with reference to FIGS. 9 and 10. 
     FIG. 9 is a schematic sectional view of the structure of an electroacoustic transducer in accordance with the present invention. The electroacoustic transducer shown in FIG. 9 is like that shown in FIG. 5 with the addition of an acoustic case 7 having a sound radiating port 7a and forming with the second vibrating plate 9 a chamber 8 comprising a Helmholtz resonator. 
     When the resonant frequency of the Helmholtz resonator comprising the chamber 8 is provided intermediate the mechanical resonance frequency f 1  and f 2  of the second vibrating plate described with reference to FIG. 8 in the electroacoustic transducer of the structure described above, the acoustic pressure-drive frequency characteristics of the transducer become those shown in FIG. 10 so that the frequency band Δf 4  higher than a predetermined acoustic pressure Po becomes broad as illustrated in FIG. 10. 
     It will be apparent from the foregoing description that although the electro-to-mechanical conversion has been described with reference to an electromagnetic type transducer, the same effect can also be obtained by making use of other electric-to-mechanical transducers such as the piezoelectric type, dynamic type, etc. 
     There is heretofore one type of electroacoustic transducer provided with thin plastic film or the like bonded to the same position as that corresponding to the second vibrating plate 9 of the present invention for the purpose mainly of moisture resistance, dustproofing, etc. but this is clearly different from the object of the present invention and corresponds to the transducer in which the equivalent mass m of the second vibrating plate in FIG. 7 is almost 0 and does not achieve any broadening of the frequency band of the electroacoustic transducer. 
     It is to be noted that although the embodiment of the invention shown by way of example in the drawings utilizes a Helmholtz resonator, the present invention is not limited only to this but may be altered so long as there is provided a resonator having an air chamber including a sound radiating port. 
     It will be understood from the foregoing description that since the electroacoustic transducer constructed according to the present invention positively utilizes resonance in order to obtain a broad frequency band it may produce a variety of tone-colors with low power consumption in order to contribute to multi-functions of small-sized portable equipment such as wristwatches, pocket watches and remote paging units.